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Dark matter (DM) bars that shadow stellar bars have been previously shown to form in idealized simulations of isolated disk galaxies, but have yet to be studied in a fully cosmological context. In this work, we analyze a population of disk galaxies within the TNG50 simulation to determine the characteristics of their dark bars. We estimate bar strength and orientation using both the in-plane Fourier A _2 density moments and the quadrupolar coefficients of the spherical harmonic basis function expansions of the density. We additionally present two novel methods for measuring the bar pattern speed Ω _p and rotation axis orientation using these coefficients, and apply them to one sample galaxy located in a TNG50 subbox. Consistent with isolated simulations, DM bars are shorter than their stellar counterparts and are 75% weaker in A _2 . DM bars dominate the shape of the inner halo potential and are apparent in the time series of quadrupolar coefficients. In our selected subbox galaxy, the stellar and dark bars remain co-aligned throughout the last 8 Gyr and have identical Ω _p . Pattern speed Ω _p evolves considerably over the last 8 Gyr, consistent with torques on the bars due to dynamical friction and gas accretion, and is seen to increase following a merger at t _lb = 1.5 Gyr. Rather than remaining static in time, the bar rotation axis displays both precession and nutation possibly caused by torques outside the plane of rotation. We find that the shape of the stellar and DM mass distributions are tightly correlated with Ω _p .

Mergers of and interactions between galaxies imprint a wide diversity of morphological, dynamical, and chemical characteristics in stellar halos and tidal streams. Measuring these characteristics elucidates aspects of the progenitors of the galaxies we observe today. The M81 group is the perfect galaxy group to understand the past, present, and future of a group of galaxies in the process of merging. Here, we measure the end of star formation ( t _90 ) and metallicity ([M/H]) of the stellar halo of M82 and the eastern tidal stream of NGC 3077 to: (1) test the idea that M82 possesses a genuine stellar halo, formed before any interaction with M81; (2) determine if NGC 3077's tidal disruption is related to the star formation history in its tails; and (3) create a timeline of the assembly history of the central trio in the M81 group. We argue that M82 possesses a genuine, metal-poor ([M/H] ∼ −1.62 dex) stellar halo, formed from the merger of a small satellite galaxy roughly 6.6 Gyr ago. We also find that the stars present in NGC 3077's tails formed before tidal disruption with M81, and possess a roughly uniform metallicity as shown in S. Okamoto et al., implying that NGC 3077's progenitor had significant population gradients. Finally, we present a timeline of the central trio’s merger/interaction history.

We present the galaxy stellar mass–size relation in the rest-frame near-IR (1.5 μ m) and its evolution with redshift up to z = 2.5. Sérsic profiles are measured for ∼26,000 galaxies with stellar masses M _⋆ > 10 ^9 M _⊙ from JWST/NIRCam F277W and F444W imaging provided by the COSMOS-WEB and PRIMER surveys using coordinates, redshifts, colors, and stellar mass estimates from the COSMOS2020 catalog. The new rest-frame near-IR effective radii are generally smaller than previously measured rest-frame optical sizes, on average by 0.14 dex, with no significant dependence on redshift. For quiescent galaxies, this size offset does not depend on stellar mass, but for star-forming galaxies, the offset increases from −0.1 dex at M _⋆ = 10 ^9.5 M _⊙ to −0.25 dex at M _⋆ > 10 ^11 M _⊙ . That is, we find that the near-IR stellar mass–size relation for star-forming galaxies is flatter in the rest-frame near-IR than in the rest-frame optical at all redshifts 0.5 < z < 2.5. The general pace of size evolution is the same in the near-IR as previously demonstrated in the optical, with slower evolution ( R _e ∝ (1 + z ) ^−0.7 ) for L * star-forming galaxies and faster evolution ( R _e ∝ (1 + z ) ^−1.3 ) for L * quiescent galaxies. Massive ( M _⋆ > 10 ^11 M _⊙ ) star-forming galaxies evolve in size almost as fast as quiescent galaxies. Low-mass ( M _⋆ < 10 ^10 M _⊙ ) quiescent galaxies evolve as slow as star-forming galaxies. Our main conclusion is that the size evolution narrative as it has emerged over the past two decades does not radically change when accessing the rest-frame near-IR with JWST, a better proxy of the underlying stellar mass distribution.

We measure the high-mass stellar initial mass function (IMF) from resolved stars in M33 young stellar clusters. Leveraging the Hubble Space Telescope’s high resolving power, we fully model the IMF probabilistically. We first model the optical color–magnitude diagram of each cluster to constrain its power-law slope Γ, marginalized over other cluster parameters in the fit (e.g., cluster age, mass, and radius). We then probabilistically model the distribution of mass function (MF) slopes for a highly strict cluster sample of nine clusters more massive than log(Mass/ M _⊙ ) = 3.6; above this mass, all clusters have well-populated main sequences of massive stars and should have accurate recovery of their MF slopes, based on extensive tests with artificial clusters. We find that the ensemble IMF is best described by a mean high-mass slope of $\overline{{\rm{\Gamma }}}=1.49\pm 0.18$ , with an intrinsic scatter of ${\sigma }_{{\rm{\Gamma }}}^{2}={0.02}_{0.00}^{+0.16}$ , consistent with a universal IMF. We find no dependence of the IMF on environmental impacts such as the local star formation rate (SFR) or galactocentric radius within M33, which serves as a proxy for metallicity. This $\overline{{\rm{\Gamma }}}$ measurement is consistent with similar measurements in M31, despite M33 having a much higher SFR intensity. While this measurement is formally consistent with the canonical Kroupa (Γ = 1.30) IMF, as well as the Salpeter (Γ = 1.35) value, it is the second Local Group cluster sample to show evidence for a somewhat steeper high-mass IMF slope. We explore the impacts a steeper IMF slope has on a number of astronomical subfields.

We test the relationship between UV-derived star formation rates (SFRs) and the 7.7 μ m polycyclic aromatic hydrocarbon luminosities from the integrated emission of galaxies at z ∼ 0–2. We utilize multiband photometry covering 0.2–160 μ m from the Hubble Space Telescope, CFHT, JWST, Spitzer, and Herschel for galaxies in the Cosmic Evolution Early Release Science (CEERS) Survey. We perform spectral energy distribution (SED) modeling of these data to measure dust-corrected far-UV (FUV) luminosities, L _FUV , and UV-derived SFRs. We then fit SED models to the JWST/MIRI 7.7–21 μ m CEERS data to derive rest-frame 7.7 μ m luminosities, L _770 , using the average flux density in the rest-frame MIRI F770W bandpass. We observe a correlation between L _770 and L _FUV , where $\mathrm{log}{L}_{770}\propto (1.27\pm 0.04)\mathrm{log}{L}_{\mathrm{FUV}}$ . L _770 diverges from this relation for galaxies at lower metallicities, lower dust obscuration, and for galaxies dominated by evolved stellar populations. We derive a “single-wavelength” SFR calibration for L _770 that has a scatter from model estimated SFRs ( σ _ΔSFR ) of 0.24 dex. We derive a “multiwavelength” calibration for the linear combination of the observed FUV luminosity (uncorrected for dust) and the rest-frame 7.7 μ m luminosity, which has a scatter of σ _ΔSFR = 0.21 dex. The relatively small decrease in σ suggests this is near the systematic accuracy of the total SFRs using either calibration. These results demonstrate that the rest-frame 7.7 μ m emission constrained by JWST/MIRI is a tracer of the SFR for distant galaxies to this accuracy, provided the galaxies are dominated by star formation with moderate-to-high levels of attenuation and metallicity.

The massive end of the gas-phase mass–metallicity relation (MZR) is a sensitive probe of active galactic nuclei (AGN) feedback that is a crucial but highly uncertain component of galaxy evolution models. In this paper, we extend the z ∼ 0.7 MZR by ∼0.5 dex up to log ( M _⋆ / M _⊙ ) ∼ 11.1. We use extremely deep VLT VIMOS spectra from the Large Early Galaxy Astrophysics Census (LEGA-C) survey to measure metallicities for 145 galaxies. The LEGA-C MZR matches the normalization of the z ∼ 0.8 DEEP2 MZR where they overlap, so we combine the two to create an MZR spanning from 9.3 to 11.1 log ( M _⋆ / M _⊙ ). The LEGA-C+DEEP2 MZR at z ∼ 0.7 is offset to slightly lower metallicities (0.05–0.13 dex) than the z ∼ 0 MZR, but it otherwise mirrors the established power-law rise at low/intermediate stellar masses and asymptotic flattening at high stellar masses. We compare the LEGA-C+DEEP2 MZR to the MZR from two cosmological simulations (IllustrisTNG and SIMBA), which predict qualitatively different metallicity trends for high-mass galaxies. This comparison highlights that our extended MZR provides a crucial observational constraint for galaxy evolution models in a mass regime where the MZR is very sensitive to choices about the implementation of AGN feedback.

The 3D geometries of high-redshift galaxies remain poorly understood. We build a differentiable Bayesian model and use Hamiltonian Monte Carlo to efficiently and robustly infer the 3D shapes of star-forming galaxies in James Webb Space Telescope Cosmic Evolution Early Release Science observations with $\mathrm{log}{M}_{* }/{M}_{\odot }=9.0\mbox{--}10.5$ at z = 0.5–8.0. We reproduce previous results from the Hubble Space Telescope Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey in a fraction of the computing time and constrain the mean ellipticity, triaxiality, size, and covariances with samples as small as ∼50 galaxies. We find high 3D ellipticities for all mass–redshift bins, suggesting oblate (disky) or prolate (elongated) geometries. We break that degeneracy by constraining the mean triaxiality to be ∼1 for $\mathrm{log}{M}_{* }/{M}_{\odot }=9.0\mbox{--}9.5$ dwarfs at z > 1 (favoring the prolate scenario), with significantly lower triaxialities for higher masses and lower redshifts indicating the emergence of disks. The prolate population traces out a “banana” in the projected $b/a\mbox{--}\mathrm{log}a$ diagram with an excess of low- b / a , large- $\mathrm{log}a$ galaxies. The dwarf prolate fraction rises from ∼25% at z = 0.5–1.0 to ∼50%–80% at z = 3–8. Our results imply a second kind of disk settling from oval (triaxial) to more circular (axisymmetric) shapes with time. We simultaneously constrain the 3D size–mass relation and its dependence on 3D geometry. High-probability prolate and oblate candidates show remarkably similar Sérsic indices ( n ∼ 1), nonparametric morphological properties, and specific star formation rates. Both tend to be visually classified as disks or irregular, but edge-on oblate candidates show more dust attenuation. We discuss selection effects, follow-up prospects, and theoretical implications.

We present individual star formation histories (SFHs) of ∼3000 massive galaxies (log( M _* / M _⊙ ) > 10.5) from the Large Early Galaxy Astrophysics Census spectroscopic survey at a lookback time of ∼7 billion yr and quantify the population trends leveraging 20 hr deep-integrated spectra of these ∼1800 star-forming and ∼1200 quiescent galaxies at 0.6 < z < 1.0. Essentially all galaxies at this epoch contain stars of age

Visual inspections of the first optical rest-frame images from JWST have indicated a surprisingly high fraction of disk galaxies at high redshifts. Here, we alternatively apply self-supervised machine learning to explore the morphological diversity at z ≥ 3. Our proposed data-driven representation scheme of galaxy morphologies, calibrated on mock images from the TNG50 simulation, is shown to be robust to noise and to correlate well with the physical properties of the simulated galaxies, including their 3D structure. We apply the method simultaneously to F200W and F356W galaxy images of a mass-complete sample ( M _* / M _⊙ > 10 ^9 ) at 3 ≤ z ≤ 6 from the first JWST/NIRCam CEERS data release. We find that the simulated and observed galaxies do not exactly populate the same manifold in the representation space from contrastive learning. We also find that half the galaxies classified as disks—either convolutional neural network-based or visually—populate a similar region of the representation space as TNG50 galaxies with low stellar specific angular momentum and nonoblate structure. Although our data-driven study does not allow us to firmly conclude on the true nature of these galaxies, it suggests that the disk fraction at z ≥ 3 remains uncertain and possibly overestimated by traditional supervised classifications. Deeper imaging and spectroscopic follow-ups as well as comparisons with other simulations will help to unambiguously determine the true nature of these galaxies, and establish more robust constraints on the emergence of disks at very high redshift.

The Cosmic Evolution Early Release Science program observed the Extended Groth Strip (EGS) with the Mid-Infrared Instrument (MIRI) on the James Webb Space Telescope (JWST) in 2022. In this paper, we discuss the four MIRI pointings that observed with longer-wavelength filters, including F770W, F1000W, F1280W, F1500W, F1800W, and F2100W. We compare the MIRI galaxies with the Spitzer/MIPS 24 μ m population in the EGS field. We find that MIRI can observe an order of magnitude deeper than MIPS in significantly shorter integration times, attributable to JWST's much larger aperture and MIRI’s improved sensitivity. MIRI is exceptionally good at finding faint ( L _IR < 10 ^10 L _⊙ ) galaxies at z ∼ 1–2. We find that a significant portion of MIRI galaxies are “mid-IR weak”—they have strong near-IR emission and relatively weaker mid-IR emission, and most of the star formation is unobscured. We present new IR templates that capture how the mid-to-near-IR emission changes with increasing infrared luminosity. We present two color–color diagrams to separate mid-IR weak galaxies and active galactic nuclei (AGN) from dusty star-forming galaxies and find that these color diagrams are most effective when used in conjunction with each other. We present the first number counts of 10 μ m sources and find that there are ≲10 IR AGN per MIRI pointing, possibly due to the difficulty of distinguishing AGN from intrinsically mid-IR weak galaxies (due to low metallicities or dust content). We conclude that MIRI is most effective at observing moderate-luminosity ( L _IR = 10 ^9 –10 ^10 L _⊙ ) galaxies at z = 1–2, and that photometry alone is not effective at identifying AGN within this faint population.

We report on the discovery of two low-luminosity, broad-line active galactic nuclei (AGNs) at z > 5 identified using JWST NIRSpec spectroscopy from the Cosmic Evolution Early Release Science (CEERS) survey. We detect broad H α emission in the spectra of both sources, with FWHM of 2060 ± 290 km s ^−1 and 1800 ± 200 km s ^−1 , resulting in virial black hole (BH) masses that are 1–2 dex below those of existing samples of luminous quasars at z > 5. The first source, CEERS 2782 at z = 5.242, is 2–3 dex fainter than known quasars at similar redshifts and was previously identified as a candidate low-luminosity AGN based on its morphology and rest-frame optical spectral energy distribution (SED). We measure a BH mass of M _BH = (1.3 ± 0.4) × 10 ^7 M _⊙ , confirming that this AGN is powered by the least massive BH known in the Universe at the end of cosmic reionization. The second source, CEERS 746 at z = 5.624, is inferred to be a heavily obscured, broad-line AGN caught in a transition phase between a dust-obscured starburst and an unobscured quasar. We estimate its BH mass to be in the range of M _BH ≃ (0.9–4.7) × 10 ^7 M _⊙ , depending on the level of dust obscuration assumed. We perform SED fitting to derive host stellar masses, M _⋆ , allowing us to place constraints on the BH–galaxy mass relationship in the lowest mass range yet probed in the early Universe. The M _BH / M _⋆ ratio for CEERS 2782, in particular, is consistent with or higher than the empirical relationship seen in massive galaxies at z = 0. We examine the narrow emission line ratios of both sources and find that their location on the BPT and OHNO diagrams is consistent with model predictions for moderately low metallicity AGNs with Z / Z _⊙ ≃ 0.2–0.4. The spectroscopic identification of low-luminosity, broad-line AGNs at z > 5 with M _BH ≃ 10 ^7 M _⊙ demonstrates the capability of JWST to push BH masses closer to the range predicted for the BH seed population and provides a unique opportunity to study the early stages of BH–galaxy assembly.

M64, often called the “Evil Eye” galaxy, is unique among local galaxies. Beyond its dramatic, dusty nucleus, it also hosts an outer gas disk that counter-rotates relative to its stars. The mass of this outer disk is comparable to the gas content of the Small Magellanic Cloud (SMC), prompting the idea that it was likely accreted in a recent minor merger. Yet, detailed follow-up studies of M64's outer disk have shown no evidence of such an event, leading to other interpretations, such as a “flyby” interaction with the distant diffuse satellite Coma P. We present Subaru Hyper Suprime-Cam observations of M64's stellar halo, which resolve its stellar populations and reveal a spectacular radial shell feature, oriented ∼30° relative to the major axis and along the rotation axis of the outer gas disk. The shell is ∼45 kpc southeast of M64, while a similar but more diffuse plume to the northwest extends to >100 kpc. We estimate a stellar mass and metallicity for the southern shell of M _⋆ = 1.80 ± 0.54 × 10 ^8 M _⊙ and [M/H] = −1.0, respectively, and a similar mass of 1.42 ± 0.71 × 10 ^8 M _⊙ for the northern plume. Taking into account the accreted material in M64's inner disk, we estimate a total stellar mass for the progenitor satellite of M _⋆,prog ≃ 5 × 10 ^8 M _⊙ . These results suggest that M64 is in the final stages of a minor merger with a gas-rich satellite strikingly similar to the SMC, in which M64's accreted counter-rotating gas originated, and which is responsible for the formation of its dusty inner star-forming disk.

We present results from the Cosmic Evolution Early Release Survey on the stellar population parameters for 28 galaxies with redshifts 4 < z < 9 using imaging data from the James Webb Space Telescope (JWST) Mid-Infrared Instrument (MIRI) combined with data from the Hubble Space Telescope and the Spitzer Space Telescope. The JWST/MIRI 5.6 and 7.7 μ m data extend the coverage of the rest-frame spectral energy distribution to nearly 1 μ m for galaxies in this redshift range. By modeling the galaxies’ SEDs the MIRI data show that the galaxies have, on average, rest-frame UV (1600 Å)— I -band colors 0.4 mag bluer than derived when using photometry that lacks MIRI. Therefore, the galaxies have lower ratios of stellar mass to light. The MIRI data reduce the stellar masses by $\langle {\rm{\Delta }}\mathrm{log}{M}_{* }\rangle =0.25$ dex at 4 < z < 6 and 0.37 dex at 6 < z < 9. This also reduces the star formation rates (SFRs) by 〈ΔlogSFR〉 = 0.14 dex at 4 < z < 6 and 0.27 dex at 6 < z < 9. The MIRI data also improve constraints on the allowable stellar mass formed in early star formation. We model this using a star formation history that includes both a “burst” at z _f = 100 and a slowly varying (“delayed- τ ”) model. The MIRI data reduce the allowable stellar mass by 0.6 dex at 4 < z < 6 and by ≈1 dex at 6 < z < 9. Applying these results globally, this reduces the cosmic stellar-mass density by an order of magnitude in the early Universe ( z ≈ 9). Therefore, observations of rest-frame ≳1 μ m are paramount for constraining the stellar-mass buildup in galaxies at very high redshifts.

Lyman-break galaxy (LBG) candidates at z ≳ 10 are rapidly being identified in James Webb Space Telescope (JWST)/NIRCam observations. Due to the (redshifted) break produced by neutral hydrogen absorption of rest-frame UV photons, these sources are expected to drop out in the bluer filters while being well detected in redder filters. However, here we show that dust-enshrouded star-forming galaxies at lower redshifts ( z ≲ 7) may also mimic the near-infrared (near-IR) colors of z > 10 LBGs, representing potential contaminants in LBG candidate samples. First, we analyze CEERS-DSFG-1, a NIRCam dropout undetected in the F115W and F150W filters but detected at longer wavelengths. Combining the JWST data with (sub)millimeter constraints, including deep NOEMA interferometric observations, we show that this source is a dusty star-forming galaxy (DSFG) at z ≈ 5.1. We also present a tentative 2.6 σ SCUBA-2 detection at 850 μ m around a recently identified z ≈ 16 LBG candidate in the same field and show that, if the emission is real and associated with this candidate, the available photometry is consistent with a z ∼ 5 dusty galaxy with strong nebular emission lines despite its blue near-IR colors. Further observations on this candidate are imperative to mitigate the low confidence of this tentative submillimeter emission and its positional uncertainty. Our analysis shows that robust (sub)millimeter detections of NIRCam dropout galaxies likely imply z ∼ 4–6 redshift solutions, where the observed near-IR break would be the result of a strong rest-frame optical Balmer break combined with high dust attenuation and strong nebular line emission, rather than the rest-frame UV Lyman break. This provides evidence that DSFGs may contaminate searches for ultra-high redshift LBG candidates from JWST observations.

Stellar bars are key drivers of secular evolution in galaxies and can be effectively studied using rest-frame near-infrared (NIR) images, which trace the underlying stellar mass and are less impacted by dust and star formation than rest-frame UV or optical images. We leverage the power of JWST CEERS NIRCam images to present the first quantitative identification and characterization of stellar bars at z > 1 based on rest-frame NIR F444W images of high resolution (∼1.3 kpc at z ∼ 1–3). We identify stellar bars in these images using quantitative criteria based on ellipse fits. For this pilot study, we present six examples of robustly identified bars at z > 1 with spectroscopic redshifts, including the two highest-redshift bars at z ∼ 2.136 and 2.312 quantitatively identified and characterized to date. The stellar bars at z ∼ 1.1–2.3 presented in our study have projected semimajor axes of ∼2.9–4.3 kpc and projected ellipticities of ∼0.41–0.53 in the rest-frame NIR. The barred host galaxies have stellar masses ∼1 × 10 ^10 to 2 × 10 ^11 M _⊙ and star formation rates of ∼21–295 M _⊙ yr ^−1 , and several have potential nearby companions. Our finding of bars at z ∼ 1.1–2.3 demonstrates the early onset of such instabilities and supports simulations where bars form early in massive dynamically cold disks. It also suggests that if these bars at lookback times of 8–11 Gyr survive out to present epochs, bar-driven secular processes may operate over a long time and have a significant impact on some galaxies by z ∼ 0.

We report on the host properties of five X-ray-luminous active galactic nuclei (AGN) identified at 3 < z < 5 in the first epoch of imaging from the Cosmic Evolution Early Release Science Survey. Each galaxy has been imaged with the JWST Near-Infrared Camera, which provides rest-frame optical morphologies at these redshifts. We also derive stellar masses and star formation rates for each host by fitting its spectral energy distribution using a combination of galaxy and AGN templates. We find that three of the AGN hosts have spheroidal morphologies, one is a bulge-dominated disk, and one is dominated by pointlike emission. None are found to show strong morphological disturbances that might indicate a recent interaction or merger event. When compared to a sample of mass-matched inactive galaxies, we find that the AGN hosts have morphologies that are less disturbed and more bulge-dominated. Notably, all four of the resolved hosts have rest-frame optical colors consistent with a quenched or poststarburst stellar population. The presence of AGN in passively evolving galaxies at z > 3 is significant because a rapid feedback mechanism is required in most semianalytic models and cosmological simulations to explain the growing population of massive quiescent galaxies observed at these redshifts. Our findings show that AGN can continue to inject energy into these systems after their star formation is curtailed, potentially heating their halos and preventing renewed star formation. Additional observations will be needed to determine what role this feedback may play in helping to quench these systems and/or maintain their quiescent state.

The new capabilities that JWST offers in the near- and mid-infrared (IR) are used to investigate in unprecedented detail the nature of optical/near-IR-faint, mid-IR-bright sources, with HST-dark galaxies among them. We gather JWST data from the CEERS survey in the Extended Groth Strip, jointly with HST data, and analyze spatially resolved optical-to-mid-IR spectral energy distributions to estimate photometric redshifts in two dimensions and stellar population properties on a pixel-by-pixel basis for red galaxies detected by NIRCam. We select 138 galaxies with F150W − F356W > 1.5 mag and F356W < 27.5 mag. The nature of these sources is threefold: (1) 71% are dusty star-forming galaxies (SFGs) at 2 < z < 6 with $9\lt \mathrm{log}{M}_{\star }/{M}_{\odot }\lt 11$ and a variety of specific SFRs (100 Gyr ^−1 ); (2) 18% are quiescent/dormant (i.e., subject to reignition/rejuvenation) galaxies (QGs) at 3 < z < 5, with $\mathrm{log}{M}_{\star }/{M}_{\odot }\sim 10$ and poststarburst mass-weighted ages (0.5–1.0 Gyr); and (3) 11% are strong young starbursts with indications of high equivalent width emission lines (typically, [O iii ]+H β ) at 6 < z < 7 (XELG- z 6) and $\mathrm{log}{M}_{\star }/{M}_{\odot }\sim 9.5$ . The sample is dominated by disk-like galaxies with remarkable compactness for XELG- z 6 (effective radii smaller than 0.4 kpc). Large attenuations in SFGs, 2 < A ( V ) < 5 mag, are found within 1.5 times the effective radius, approximately 2 kpc, while QGs present A ( V ) ∼ 0.2 mag. Our SED-fitting technique reproduces the expected dust emission luminosities of IR-bright and submillimeter galaxies. This study implies high levels of star formation activity between z ∼ 20 and z ∼ 10, where virtually 100% of our galaxies had already formed 10 ^8 M _⊙ , 60% had assembled 10 ^9 M _⊙ , and 10% up to 10 ^10 M _⊙ (in situ or ex situ).

We report the discovery of a candidate galaxy with a photo- z of z ∼ 12 in the first epoch of the James Webb Space Telescope (JWST) Cosmic Evolution Early Release Science Survey. Following conservative selection criteria, we identify a source with a robust z _phot = ${11.8}_{-0.2}^{+0.3}$ (1 σ uncertainty) with m _F200W = 27.3 and ≳7 σ detections in five filters. The source is not detected at λ < 1.4 μ m in deep imaging from both Hubble Space Telescope (HST) and JWST and has faint ∼3 σ detections in JWST F150W and HST F160W, which signal a Ly α break near the red edge of both filters, implying z ∼ 12. This object (Maisie’s Galaxy) exhibits F115W − F200W > 1.9 mag (2 σ lower limit) with a blue continuum slope, resulting in 99.6% of the photo- z probability distribution function favoring z > 11. All data-quality images show no artifacts at the candidate’s position, and independent analyses consistently find a strong preference for z > 11. Its colors are inconsistent with Galactic stars, and it is resolved ( r _h = 340 ± 14 pc). Maisie’s Galaxy has log M _* / M _⊙ ∼ 8.5 and is highly star-forming (log sSFR ∼ −8.2 yr ^−1 ), with a blue rest-UV color ( β ∼ −2.5) indicating little dust, though not extremely low metallicity. While the presence of this source is in tension with most predictions, it agrees with empirical extrapolations assuming UV luminosity functions that smoothly decline with increasing redshift. Should follow-up spectroscopy validate this redshift, our universe was already aglow with galaxies less than 400 Myr after the Big Bang.

We use CEERS JWST/NIRCam imaging to measure rest-frame near-IR light profiles of 435 M _⋆ > 10 ^10 M _⊙ galaxies in the redshift range of 0.5 < z < 2.3. We compare the resulting rest-frame 1.5–2 μ m half-light radii ( R _NIR ) with stellar half-mass radii ( ${R}_{{{M}}_{\star }}$ ) derived with multicolor light profiles from CANDELS Hubble Space Telescope imaging. In general agreement with previous work, we find that R _NIR and ${R}_{{{M}}_{\star }}$ are up to 40% smaller than the rest-frame optical half-light radius R _opt . The agreement between R _NIR and ${R}_{{{M}}_{\star }}$ is excellent, with a negligible systematic offset ( 10 ^11 M _⊙ ) quiescent galaxies between z = 0.5 and z = 2.3, again comparing R _opt , ${R}_{{{M}}_{\star }}$ , and ${R}_{{{M}}_{\star },3{\rm{D}}}$ . We conclude that the main tenets of the evolution of the size narrative established over the past 20 yr, based on rest-frame optical light profile analysis, still hold in the era of JWST/NIRCam observations in the rest-frame near-IR.

Quenching of star formation in the central galaxies of cosmological halos is thought to result from energy released as gas accretes onto a supermassive black hole. The same energy source also appears to lower the central density and raise the cooling time of baryonic atmospheres in massive halos, thereby limiting both star formation and black hole growth, by lifting the baryons in those halos to greater altitudes. One predicted signature of that feedback mechanism is a nearly linear relationship between the central black hole’s mass ( M _BH ) and the original binding energy of the halo’s baryons. We present the increasingly strong observational evidence supporting a such a relationship, showing that it extends up to halos of mass M _halo ∼ 10 ^14 M _⊙ . We then compare current observational constraints on the M _BH – M _halo relation with numerical simulations, finding that black hole masses in IllustrisTNG appear to exceed those constraints at M _halo < 10 ^13 M _⊙ and that black hole masses in EAGLE fall short of observations at M _halo ∼ 10 ^14 M _⊙ . A closer look at IllustrisTNG shows that quenching of star formation and suppression of black hole growth do indeed coincide with black hole energy input that lifts the halo’s baryons. However, IllustrisTNG does not reproduce the observed M _BH – M _halo relation because its black holes gain mass primarily through accretion that does not contribute to baryon lifting. We suggest adjustments to some of the parameters in the IllustrisTNG feedback algorithm that may allow the resulting black hole masses to reflect the inherent links between black hole growth, baryon lifting, and star formation among the massive galaxies in those simulations.

We examine the wavelength dependence of radial light profiles based on Sérsic index n measurements of 1067 galaxies with M _* ≥ 10 ^9.5 M _⊙ and in the redshift range 0.5 < z < 3. The sample and rest-frame optical light profiles are drawn from the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS) and 3D Hubble Space Telescope (HST); rest-frame near-infrared light profiles are inferred from images collected for the Cosmic Evolution Early Release Science (CEERS) program with the Near Infrared Camera (NIRCam) on board of the James Webb Space Telescope (JWST). n shows only a weak dependence on the wavelength, regardless of the redshift, galaxy mass, and type. On average, star-forming galaxies have n = 1–1.5 and quiescent galaxies have n = 3–4 in the rest-frame optical and near-infrared. The strong correlation at all wavelengths between n and star formation activity implies a physical connection between the radial stellar mass profile and star formation activity. The main caveat is that the current sample is too small to discern trends for the most massive galaxies ( M _* > 10 ^11 M _⊙ ).

We present a detailed analysis of the structure of the Local Group flocculent spiral galaxy M33, as measured using the Panchromatic Hubble Andromeda Treasury Triangulum Extended Region (PHATTER) survey. Leveraging the multiwavelength coverage of PHATTER, we find that the oldest populations are dominated by a smooth exponential disk with two distinct spiral arms and a classical central bar—completely distinct from what is seen in broadband optical imaging, and the first-ever confirmation of a bar in M33. We estimate a bar extent of ∼1 kpc. The two spiral arms are asymmetric in orientation and strength, and likely represent the innermost impact of the recent tidal interaction responsible for M33's warp at larger scales. The flocculent multiarmed morphology for which M33 is known is only visible in the young upper main-sequence population, which closely tracks the morphology of the interstellar medium. We investigate the stability of M33's disk, finding Q ∼ 1 over the majority of the disk. We fit multiple components to the old stellar density distribution and find that, when considering recent stellar kinematics, M33's bulk structure favors the inclusion of an accreted halo component, modeled as a broken power law. The best-fit halo has an outer power-law index of −3 and accurately describes observational evidence of M33's stellar halo from both resolved stellar spectroscopy in the disk and its stellar populations at large radius. Integrating this profile yields a total halo stellar mass of ∼5 × 10 ^8 M _⊙ , for a stellar halo mass fraction of 16%, most of which resides in the innermost 2.5 kpc.

We present the final legacy version of stellar photometry for the Panchromatic Hubble Andromeda Treasury (PHAT) survey. We have reprocessed all of the Hubble Space Telescope Wide Field Camera 3 and Advanced Camera for Surveys near-ultraviolet (F275W, F336W), optical (F475W, F814W), and near-infrared (F110W, F160W) imaging from the PHAT survey using an improved method that optimized the survey depth and chip-gap coverage by including all overlapping exposures in all bands in the photometry. An additional improvement was gained through the use of charge transfer efficiency (CTE)–corrected input images, which provide more complete star finding as well as more reliable photometry for the NUV bands, which had no CTE correction in the previous version of the PHAT photometry. While this method requires significantly more computing resources and time than earlier versions where the photometry was performed on individual pointings, it results in smaller systematic instrumental completeness variations as demonstrated by cleaner maps in stellar density, and it results in optimal constraints on stellar fluxes in all bands from the survey data. Our resulting catalog has 138 million stars, 18% more than the previous catalog, with lower density regions gaining as much as 40% more stars. The new catalog produces nearly seamless population maps that show relatively well-mixed distributions for populations associated with ages older than 1–2 Gyr and highly structured distributions for the younger populations.

We present a new approach to measuring the thickness of a partially face-on stellar disk, using dust geometry. In a moderately-inclined disk galaxy, the fraction of reddened stars is expected to be 50% everywhere, assuming that dust lies in a thin midplane. In a thickened disk, however, a wide range of radii project onto the line of sight. Assuming stellar density declines with radius, this geometrical projection leads to differences in the numbers of stars on the near and far sides of the thin dust layer. The fraction of reddened stars will thus differ from the 50% prediction, with a deviation that becomes larger for puffier disks. We map the fraction of reddened red giant branch (RGB) stars across M31, which shows prominent dust lanes on only one side of the major axis. The fraction of reddened stars varies systematically from 20% to 80%, which requires that these stars have an exponential scale height h _z that is 0.14 ± 0.015 times the exponential scale length ( h _r ≈ 5.5 kpc). M31's RGB stars must therefore have h _z = 770 ± 80 pc, which is far thicker than the Milky Way’s thin disk, but comparable to its thick disk. The lack of a significant thin disk in M31 is unexpected, but consistent with its interaction history and high disk velocity dispersion. We suggest that asymmetric reddening be used as a generic criterion for identifying “thick disk”-dominated systems, and discuss prospects for future 3D tomographic mapping of the gas and stars in M31.

We present the mid-infrared (MIR) morphologies for 64 star-forming galaxies (SFGs) at 0.2 < z < 2.5 with stellar mass M _* > 10 ^9 M _⊙ using James Webb Space Telescope (JWST) Mid-Infrared Instrument (MIRI) observations from the Cosmic Evolution Early Release Science survey. The MIRI bands span the MIR (7.7–21 μ m), enabling us to measure the effective radii ( R _eff ) and Sérsic indexes of these SFGs at rest-frame 6.2 and 7.7 μ m, which contains strong emission from Polycyclic aromatic hydrocarbon (PAH) features, a well-established tracer of star formation in galaxies. We define a “PAH band” as the MIRI bandpass that contains these features at the redshift of the galaxy. We then compare the galaxy morphologies in the PAH bands to those in the rest-frame near-ultraviolet (NUV) using Hubble Space Telescope (HST) Advanced Camera for Surveys (ACS)/F435W or ACS/F606W and optical/near-IR using HST WFC3/F160W imaging from UVCANDELS and CANDELS. The R _eff of galaxies in the PAH band are slightly smaller (∼10%) than those in F160W for galaxies with M _* ≳ 10 ^9.5 M _⊙ at z ≤ 1.2, but the PAH band and F160W have similar fractions of light within 1 kpc. In contrast, the R _eff of galaxies in the NUV band are larger, with lower fractions of light within 1 kpc compared to F160W for galaxies at z ≤ 1.2. Using the MIRI data to estimate the SFR _IR surface density, we find that the correlation between the SFR _IR surface density and stellar mass has a steeper slope than that of the SFR _UV surface density and stellar mass, suggesting more massive galaxies having increasing amounts of obscured fraction of star formation in their inner regions. This paper demonstrates how the high-angular resolution data from JWST/MIRI can reveal new information about the morphology of obscured star formation.

We present Dark Energy Spectroscopic Instrument (DESI) observations of the inner halo of M31, which reveal the kinematics of a recent merger—a galactic immigration event—in exquisite detail. Of the 11,416 sources studied in 3.75 hr of on-sky exposure time, 7438 are M31 sources with well-measured radial velocities. The observations reveal intricate coherent kinematic structure in the positions and velocities of individual stars: streams, wedges, and chevrons. While hints of coherent structures have been previously detected in M31, this is the first time they have been seen with such detail and clarity in a galaxy beyond the Milky Way. We find clear kinematic evidence for shell structures in the Giant Stellar Stream, the Northeast Shelf, and Western Shelf regions. The kinematics are remarkably similar to the predictions of dynamical models constructed to explain the spatial morphology of the inner halo. The results are consistent with the interpretation that much of the substructure in the inner halo of M31 is produced by a single galactic immigration event 1–2 Gyr ago. Significant numbers of metal-rich stars ([Fe/H] > − 0.5) are present in all of the detected substructures, suggesting that the immigrating galaxy had an extended star formation history. We also investigate the ability of the shells and Giant Stellar Stream to constrain the gravitational potential of M31, and estimate the mass within a projected radius of 125 kpc to be ${\mathrm{log}}_{10}\,{M}_{\mathrm{NFW}}(\lt 125\,\mathrm{kpc})/{M}_{\odot }={11.80}_{-0.10}^{+0.12}$ . The results herald a new era in our ability to study stars on a galactic scale and the immigration histories of galaxies.

It is not yet settled how the combination of secular processes and merging gives rise to the bulges and pseudobulges of galaxies. The nearby ( D ∼ 4.2 Mpc) disk galaxy M94 (NGC 4736) has the largest pseudobulge in the local universe, and offers a unique opportunity for investigating the role of merging in the formation of its pseudobulge. We present a first ever look at M94's stellar halo, which we expect to contain a fossil record of M94's past mergers. Using Subaru's Hyper Suprime-Cam, we resolve and identify red giant branch (RGB) stars in M94's halo, finding two distinct populations. After correcting for completeness through artificial star tests, we can measure the radial profile of each RGB population. The metal-rich RGB stars show an unbroken exponential profile to a radius of 30 kpc that is a clear continuation of M94's outer disk. M94's metal-poor stellar halo is detectable over a wider area and clearly separates from its metal-rich disk. By integrating the halo density profile, we infer a total accreted stellar mass of ∼2.8 × 10 ^8 M _⊙ , with a median metallicity of [M/H] = −1.4. This indicates that M94's most-massive past merger was with a galaxy similar to, or less massive than, the Small Magellanic Cloud. Few nearby galaxies have had such a low-mass dominant merger; therefore we suggest that M94's pseudobulge was not significantly impacted by merging.

Galaxy mergers are expected to influence galaxy properties, yet measurements of individual merger histories are lacking. Models predict that merger histories can be measured using stellar halos and that these halos can be quantified using observations of resolved stars along their minor axis. Such observations reveal that Milky Way-mass galaxies have a wide range of stellar halo properties and show a correlation between their stellar halo masses and metallicities. This correlation agrees with merger-driven models where stellar halos are formed by satellite galaxy disruption. In these models, the largest accreted satellite dominates the stellar halo properties. Consequently, the observed diversity in the stellar halos of Milky Way-mass galaxies implies a large range in the masses of their largest merger partners. In particular, the Milky Way’s low mass halo implies an unusually quiet merger history. We used these measurements to seek predicted correlations between the bulge and central black hole (BH) mass and the mass of the largest merger partner. We found no significant correlations: while some galaxies with large bulges and BHs have large stellar halos and thus experienced a major or minor merger, half have small stellar halos and never experienced a significant merger event. These results indicate that bulge and BH growth is not solely driven by merger-related processes.

A galaxy’s stellar mass-to-light ratio () is a useful tool for converting luminosity to stellar mass (). However, the practical utility of inferred from stellar population synthesis (SPS) models is limited by mismatches between the real and assumed models for star-formation history (SFH) and dust geometry, both of which vary within galaxies. Here, we measure spatial variations in and their dependence on color, SFH, and dust across the disk of M31, using a map of derived from color–magnitude diagrams of resolved stars in the Panchromatic Hubble Andromeda Treasury survey. First, we find comparable scatter in for the optical and mid-IR, contrary to the common idea that is less variable in the IR. Second, we confirm that is correlated with color for both the optical and mid-IR and report color versus relations (CMLRs) in M31 for filters used in the Sloan Digital Sky Survey and Widefield Infrared Survey Explorer. Third, we show that the CMLR residuals correlate with recent SFH, such that quiescent regions are offset to higher than star-forming regions at a fixed color. The mid-IR CMLR, however, is not linear due to the high scatter of in star-forming regions. Finally, we find a flatter optical CMLR than any SPS-based CMLRs in the literature. We show that this is an effect of dust geometry, which is typically neglected but should be accounted for when using optical data to map . [ABSTRACT FROM AUTHOR]

We analyze the colors and sizes of 32 quiescent (UVJ-selected) galaxies with strong Balmer absorption (EW(Hδ) ≥ 4 Å) at z ∼ 0.8 drawn from DR2 of the LEGA-C survey to test the hypothesis that these galaxies experienced compact, central starbursts before quenching. These recently quenched galaxies, usually referred to as post-starburst galaxies, span a wide range of colors, and we find a clear correlation between color and half-light radius, such that bluer galaxies are smaller. We build simple toy models to explain this correlation: a normal star-forming disk plus a central, compact starburst component. Bursts with exponential decay timescale of ∼100 Myr that produce ∼10% to more than 100% of the preexisting masses can reproduce the observed correlation. More significant bursts also produce bluer and smaller descendants. Our findings imply that when galaxies shut down star formation rapidly, they generally had experienced compact, starburst events and that the large, observed spread in sizes and colors mostly reflects a variety of burst strengths. Recently quenched galaxies should have younger stellar ages in the centers; multiwavelength data with high spatial resolution are required to reveal the age gradient. Highly dissipative processes should be responsible for this type of formation history. While determining the mechanisms for individual galaxies is challenging, some recently quenched galaxies show signs of gravitational interactions, suggesting that mergers are likely an important mechanism in triggering the rapid shutdown of star formation activities at z ∼ 0.8. [ABSTRACT FROM AUTHOR]

We present the detection of very extended stellar populations around the Large Magellanic Cloud (LMC) out to R ∼ 21°, or ∼18.5 kpc at the LMC distance of 50 kpc, as detected in the Survey of the Magellanic Stellar History (SMASH) performed with the Dark Energy Camera on the NOAO Blanco 4 m Telescope. The deep (g ∼ 24) SMASH color–magnitude diagrams (CMDs) clearly reveal old (∼9 Gyr), metal-poor ([Fe/H] ≈ −0.8 dex) main-sequence stars at a distance of ∼50 kpc. The surface brightness of these detections is extremely low with our most distant detection at Σg ≈ 34 mag arcsec−2. The SMASH radial density profile breaks from the inner LMC exponential decline at ∼13°–15° and a second component at larger radii has a shallower slope with power-law index α = −2.2 that contributes ∼0.4% of the LMC’s total stellar mass. In addition, the SMASH densities exhibit large scatter around our best-fit model of ∼70% indicating that the envelope of stellar material in the LMC periphery is highly disturbed. We also use data from the NOAO Source catalog to map the LMC main-sequence populations at intermediate radii and detect a steep dropoff in density on the eastern side of the LMC (at R ≈ 8°) as well as an extended structure to the far northeast. These combined results confirm the existence of a very extended, low-density envelope of stellar material with a disturbed shape around the LMC. The exact origin of this structure remains unclear, but the leading options include an accreted halo or tidally stripped outer disk material. [ABSTRACT FROM AUTHOR]

We explore the stellar structure of the Large Magellanic Cloud (LMC) disk using data from the Survey of the MAgellanic Stellar History and the Dark Energy Survey. We detect a ring-like stellar overdensity in the red clump star count map at a radius of ∼6° (∼5.2 kpc at the LMC distance) that is continuous over ∼270° in position angle and is only limited by the current data coverage. The overdensity shows an amplitude up to 2.5 times higher than that of the underlying smooth disk. This structure might be related to the multiple arms found by de Vaucouleurs. We find that the overdensity shows spatial correlation with intermediate-age star clusters, but not with young (<1 Gyr) main-sequence stars, indicating the stellar populations associated with the overdensity are intermediate in age or older. Our findings on the LMC overdensity can be explained by either of two distinct formation mechanisms of a ring-like overdensity: (1) the overdensity formed out of an asymmetric one-armed spiral wrapping around the LMC main body, which is induced by repeated encounters with the Small Magellanic Cloud (SMC) over the last Gyr, or (2) the overdensity formed very recently as a tidal response to a direct collision with the SMC. Although the measured properties of the overdensity alone cannot distinguish between the two candidate scenarios, the consistency with both scenarios suggests that the ring-like overdensity is likely a product of tidal interaction with the SMC, but not with the Milky Way halo. [ABSTRACT FROM AUTHOR]

We analyze the stellar age indicators (Dn4000 and EW(Hδ)) and sizes of 467 quiescent galaxies with M* ≥ 1010M⊙ at z ∼ 0.7 drawn from DR2 of the LEGA-C survey. Interpreting index variations in terms of equivalent single stellar population age, we find that the median stellar population is younger for larger galaxies at fixed stellar mass. The effect is significant, yet small; the ages of the larger and smaller subsets differ by only <500 Myr, much less than the age variation among individual galaxies (∼1.5 Gyr). At the same time, post-starburst galaxies—those that experienced recent and rapid quenching events—are much smaller than expected based on the global correlation between age and size of normal quiescent galaxies. These coexisting trends unify seemingly contradictory results in the literature; the complex correlations between size and age indicators revealed by our large sample of galaxies with high-quality spectra suggest that there are multiple evolutionary pathways to quiescence. Regardless of the specific physical mechanisms responsible for the cessation of star formation in massive galaxies, the large scatter in Dn4000 and EW(Hδ) immediately implies that galaxies follow a large variety of evolutionary pathways. On the one hand, we see evidence for a process that slowly shuts off star formation and transforms star-forming galaxies to quiescent galaxies without necessarily changing their structures. On the other hand, there is likely a mechanism that rapidly quenches galaxies, an event that coincides with dramatic structural changes, producing post-starburst galaxies that can be smaller than their progenitors. [ABSTRACT FROM AUTHOR]

We present a study of the three-dimensional (3D) structure of the Large Magellanic Cloud (LMC) using ∼2.2 million red clump (RC) stars selected from the Survey of the MAgellanic Stellar History. To correct for line-of-sight dust extinction, the intrinsic RC color and magnitude and their radial dependence are carefully measured by using internal nearly dust-free regions. These are then used to construct an accurate 2D reddening map (165 deg2 area with ∼10′ resolution) of the LMC disk and the 3D spatial distribution of RC stars. An inclined disk model is fit to the 2D distance map, yielding a best-fit inclination angle degrees with random errors of ±0.°19 and line-of-nodes position angle degrees with random errors of ±0.°49. These angles vary with galactic radius, indicating that the LMC disk is warped and twisted likely due to the repeated tidal interactions with the Small Magellanic Cloud (SMC). For the first time, our data reveal a significant warp in the southwestern part of the outer disk starting at ρ ∼ 7° that departs from the defined LMC plane up to ∼4 kpc toward the SMC, suggesting that it originated from a strong interaction with the SMC. In addition, the inner disk encompassing the off-centered bar appears to be tilted up to 5°–15° relative to the rest of the LMC disk. These findings on the outer warp and the tilted bar are consistent with the predictions from the Besla et al. simulation of a recent direct collision with the SMC. [ABSTRACT FROM AUTHOR]

We report the discovery of 30 stars with extreme space velocities (≳480 ) in the Gaia-DR2 archive. These stars are a subset of 1743 stars with high-precision parallax, large tangential velocity (vtan > 300 ), and measured line-of-sight velocity in DR2. By tracing the orbits of the stars back in time, we find at least one of them is consistent with having been ejected by the supermassive black hole at the Galactic Center. Another star has an orbit that passed near the Large Magellanic Cloud about 200 Myr ago. Unlike previously discovered blue hypervelocity stars, our sample is metal-poor (−1.5 < [Fe/H] < −1.0) and quite old (>1 ). We discuss possible mechanisms for accelerating old stars to such extreme velocities. The high observed space density of this population, relative to potential acceleration mechanisms, implies that these stars are probably bound to the Milky Way (MW). If they are bound, the discovery of this population would require a local escape speed of around ∼600 and consequently imply a virial mass of M200 ∼ 1.4 × 1012M⊙ for the MW. [ABSTRACT FROM AUTHOR]

The dwarf satellites of “giant” Milky Way (MW)–mass galaxies are our primary probes of low-mass dark matter halos. The number and velocities of the satellite galaxies of the MW and M31 initially puzzled galaxy formation theorists, but are now reproduced well by many models. Yet, are the MW’s and M31's satellites representative? Were galaxy formation models “overfit”? These questions motivate deep searches for satellite galaxies outside the Local Group. We present a deep survey of the “classical” satellites (M⋆ ≥ 4 × 105M⊙) of the MW-mass galaxy M94 out to a 150 kpc projected distance. We find only two satellites, each with M⋆ ∼ 106M⊙, compared with 6–12 such satellites in the four other MW-mass systems with comparable data (MW, M31, M81, and M101). Using a “standard” prescription for occupying dark matter halos (halos were taken from the fully hydrodynamical EAGLE simulation) with galaxies, we find that such a sparse satellite population occurs in <0.2% of MW-mass systems—a <1% probability among a sample of five (known systems + M94). In order to produce an M94-like system more frequently we make satellite galaxy formation much more stochastic than is currently predicted by dramatically increasing the slope and scatter of the stellar mass–halo mass (SMHM) relation. Surprisingly, the SMHM relation must be altered even for halos masses up to 1011M⊙—significantly above the mass scales predicted to have increased scatter from current hydrodynamical models. The sparse satellite population of this “lonely giant” thus advocates for an important modification to ideas of how the satellites around MW-mass galaxies form. [ABSTRACT FROM AUTHOR]

Using high-resolution spectra from the VLT Large Early Galaxy Astrophysics Census (LEGA-C) program, we reconstruct the star formation histories (SFHs) of 607 galaxies at redshifts z = 0.6–1.0 and stellar masses ≳1010M⊙ using a custom full spectrum fitting algorithm that incorporates the emcee and FSPS packages. We show that the mass-weighted age of a galaxy correlates strongly with stellar velocity dispersion (σ*) and ongoing star formation (SF) activity, with the stellar content in higher-σ* galaxies having formed earlier and faster. The SFHs of quiescent galaxies are generally consistent with passive evolution since their main SF epoch, but a minority show clear evidence of a rejuvenation event in their recent past. The mean age of stars in galaxies that are star-forming is generally significantly younger, with SF peaking after z < 1.5 for almost all star-forming galaxies in the sample: many of these still have either constant or rising SFRs on timescales >100 Myr. This indicates that z > 2 progenitors of z ∼ 1 star-forming galaxies are generally far less massive. Finally, despite considerable variance in the individual SFHs, we show that the current SF activity of massive galaxies (>L*) at z ∼ 1 correlates with SF levels at least 3 Gyr prior: SFHs retain “memory” on a large fraction of the Hubble time. Our results illustrate a novel approach to resolve the formation phase of galaxies, and, by identifying their individual evolutionary paths, one can connect progenitors and descendants across cosmic time. This is uniquely enabled by the high-quality continuum spectroscopy provided by the LEGA-C survey. [ABSTRACT FROM AUTHOR]

We present stellar rotation curves and velocity dispersion profiles for 104 quiescent galaxies at z = 0.6–1 from the Large Early Galaxy Astrophysics Census (LEGA-C) spectroscopic survey. Rotation is typically probed across 10–20 kpc, or to an average of 2.7Re. Combined with central stellar velocity dispersions (σ0) this provides the first determination of the dynamical state of a sample selected by a lack of star formation activity at large lookback time. The most massive galaxies (M⋆ > 2 × 1011M⊙) generally show no or little rotation measured at 5 kpc ( in eight of ten cases), while ∼64% of less massive galaxies show significant rotation. This is reminiscent of local fast- and slow-rotating ellipticals and implies that low- and high-redshift quiescent galaxies have qualitatively similar dynamical structures. We compare distributions at z ∼ 0.8 and the present day by re-binning and smoothing the kinematic maps of 91 low-redshift quiescent galaxies from the Calar Alto Legacy Integral Field Area (CALIFA) survey and find evidence for a decrease in rotational support since z ∼ 1. This result is especially strong when galaxies are compared at fixed velocity dispersion; if velocity dispersion does not evolve for individual galaxies then the rotational velocity at 5 kpc was an average of 94 ± 22% higher in z ∼ 0.8 quiescent galaxies than today. Considering that the number of quiescent galaxies grows with time and that new additions to the population descend from rotationally supported star-forming galaxies, our results imply that quiescent galaxies must lose angular momentum between z ∼ 1 and the present, presumably through dissipationless merging, and/or that the mechanism that transforms star-forming galaxies also reduces their rotational support. [ABSTRACT FROM AUTHOR]

Drawing from the LEGA-C data set, we present the spectroscopic view of the stellar population across a large volume- and mass-selected sample of galaxies at large look-back time. We measure the 4000 Å break (Dn4000) and Balmer absorption line strengths (probed by Hδ) from 1019 high-quality spectra of z = 0.6–1.0 galaxies with M* = 2 × 1010M⊙ to 3 × 1011M⊙. Our analysis serves as a first illustration of the power of high-resolution, high signal-to-noise ratio continuum spectroscopy at intermediate redshifts as a qualitatively new tool to constrain galaxy formation models. The observed Dn4000–EW(Hδ) distribution of our sample overlaps with the distribution traced by present-day galaxies, but z ∼ 0.8 galaxies populate that locus in a fundamentally different manner. While old galaxies dominate the present-day population at all stellar masses >2 × 1010M⊙, we see a bimodal Dn4000–EW(Hδ) distribution at z ∼ 0.8, implying a bimodal light-weighted age distribution. The light-weighted age depends strongly on stellar mass, with the most massive galaxies >1 × 1011M⊙ being almost all older than 2 Gyr. At the same time, we estimate that galaxies in this high-mass range are only ∼3 Gyr younger than their z ∼ 0.1 counterparts, at odds with purely passive evolution given a difference in look-back time of >5 Gyr; younger galaxies must grow to >1011M⊙ in the meantime, or small amounts of young stars must keep the light-weighted ages young. Star-forming galaxies at z ∼ 0.8 have stronger Hδ absorption than present-day galaxies with the same Dn4000, implying larger short-term variations in star formation activity. [ABSTRACT FROM AUTHOR]

Studying giant star-forming clumps in distant galaxies is important to understand galaxy formation and evolution. At present, however, observers and theorists have not reached a consensus on whether the observed “clumps” in distant galaxies are the same phenomenon that is seen in simulations. In this paper, as a step to establish a benchmark of direct comparisons between observations and theories, we publish a sample of clumps constructed to represent the commonly observed “clumps” in the literature. This sample contains 3193 clumps detected from 1270 galaxies at . The clumps are detected from rest-frame UV images, as described in our previous paper. Their physical properties (e.g., rest-frame color, stellar mass (), star formation rate (SFR), age, and dust extinction) are measured by fitting the spectral energy distribution (SED) to synthetic stellar population models. We carefully test the procedures of measuring clump properties, especially the method of subtracting background fluxes from the diffuse component of galaxies. With our fiducial background subtraction, we find a radial clump U − V color variation, where clumps close to galactic centers are redder than those in outskirts. The slope of the color gradient (clump color as a function of their galactocentric distance scaled by the semimajor axis of galaxies) changes with redshift and of the host galaxies: at a fixed , the slope becomes steeper toward low redshift, and at a fixed redshift, it becomes slightly steeper with . Based on our SED fitting, this observed color gradient can be explained by a combination of a negative age gradient, a negative E(B − V) gradient, and a positive specific SFR gradient of the clumps. We also find that the color gradients of clumps are steeper than those of intra-clump regions. Correspondingly, the radial gradients of the derived physical properties of clumps are different from those of the diffuse component or intra-clump regions. [ABSTRACT FROM AUTHOR]

The velocity anisotropy parameter, β, is a measure of the kinematic state of orbits in the stellar halo, which holds promise for constraining the merger history of the Milky Way (MW). We determine global trends for β as a function of radius from three suites of simulations, including accretion-only and cosmological hydrodynamic simulations. We find that the two types of simulations are consistent and predict strong radial anisotropy () for Galactocentric radii greater than 10 kpc. Previous observations of β for the MW’s stellar halo claim a detection of an isotropic or tangential “dip” at r ∼ 20 kpc. Using the N-body+SPH simulations, we investigate the temporal persistence, population origin, and severity of “dips” in β. We find that dips in the in situ stellar halo are long-lived, while dips in the accreted stellar halo are short-lived and tied to the recent accretion of satellite material. We also find that a major merger as early as z ∼ 1 can result in a present-day low (isotropic to tangential) value of β over a broad range of radii and angles. While all of these mechanisms are plausible drivers for the β dip observed in the MW, each mechanism in the simulations has a unique metallicity signature associated with it, implying that future spectroscopic surveys could distinguish between them. Since an accurate knowledge of β(r) is required for measuring the mass of the MW halo, we note that significant transient dips in β could cause an overestimate of the halo’s mass when using spherical Jeans equation modeling. [ABSTRACT FROM AUTHOR]

We study galactic star formation activity as a function of environment and stellar mass over 0.5 < z < 2.0 using the FourStar Galaxy Evolution (ZFOURGE) survey. We estimate the galaxy environment using a Bayesian-motivated measure of the distance to the third nearest neighbor for galaxies to the stellar mass completeness of our survey, at z = 1.3 (2.0). This method, when applied to a mock catalog with the photometric-redshift precision () of ZFOURGE, accurately recovers galaxies in low- and high-density environments. We quantify the environmental quenching efficiency and show that at , it depends on galaxy stellar mass, demonstrating that the effects of quenching related to (stellar) mass and environment are not separable. In high-density environments, the mass and environmental quenching efficiencies are comparable for massive galaxies () at all redshifts. For lower-mass galaxies (), the environmental quenching efficiency is very low at , but increases rapidly with decreasing redshift. Environmental quenching can account for nearly all quiescent lower-mass galaxies (), which appear primarily at . The morphologies of lower-mass quiescent galaxies are inconsistent with those expected of recently quenched star-forming galaxies. Some environmental process must transform the morphologies on similar timescales as the environmental quenching itself. The evolution of the environmental quenching favors models that combine gas starvation (as galaxies become satellites) with gas exhaustion through star formation and outflows (“overconsumption”), and additional processes such as galaxy interactions, tidal stripping, and disk fading to account for the morphological differences between the quiescent and star-forming galaxy populations. [ABSTRACT FROM AUTHOR]

We investigate the stellar kinematics and stellar populations of 58 radio-loud galaxies of intermediate luminosities (L3 GHz > 1023 W Hz−1) at 0.6 < z < 1. This sample is constructed by cross-matching galaxies from the deep VLT/VIMOS LEGA-C spectroscopic survey with the VLA 3 GHz data set. The LEGA-C continuum spectra reveal for the first time stellar velocity dispersions and age indicators of z ∼ 1 radio galaxies. We find that z ∼ 1 radio-loud active galactic nucleus (AGN) occur exclusively in predominantly old galaxies with high velocity dispersions: σ* > 175 km s−1, corresponding to black hole masses in excess of 108M⊙. Furthermore, we confirm that at a fixed stellar mass the fraction of radio-loud AGN at z ∼ 1 is five to 10 times higher than in the local universe, suggesting that quiescent, massive galaxies at z ∼ 1 switch on as radio AGN on average once every Gyr. Our results strengthen the existing evidence for a link between high black hole masses, radio loudness, and quiescence at z ∼ 1. [ABSTRACT FROM AUTHOR]

We map the star formation history across M31 by fitting stellar evolution models to color–magnitude diagrams of each 83″ × 83″ (0.3 × 1.4 kpc, deprojected) region of the Panchromatic Hubble Andromeda Treasury (PHAT) survey outside of the innermost portion. We find that most of the star formation occurred prior to ∼8 Gyr ago, followed by a relatively quiescent period until ∼4 Gyr ago, a subsequent star formation episode about 2 Gyr ago, and a return to relative quiescence. There appears to be little, if any, structure visible for populations with ages older than 2 Gyr, suggesting significant mixing since that epoch. Finally, assuming a Kroupa initial mass function from 0.1 to 100 M⊙, we find that the total amount of star formation over the past 14 Gyr in the area over which we have fit models is 5 × 1010M⊙. Fitting the radial distribution of this star formation and assuming azimuthal symmetry, (1.5 ± 0.2) × 1011M⊙ of stars has formed in the M31 disk as a whole, (9 ± 2) × 1010M⊙ of which has likely survived to the present after accounting for evolutionary effects. This mass is about one-fifth of the total dynamical mass of M31. [ABSTRACT FROM AUTHOR]