Search

Large photometric surveys provide a rich source of observations of quiescent galaxies, including a surprisingly large population at z > 1. However, identifying large, but clean, samples of quiescent galaxies has proven difficult because of their near-degeneracy with interlopers such as dusty, star-forming galaxies. We describe a new technique for selecting quiescent galaxies based upon t-distributed stochastic neighbor embedding (t-SNE), an unsupervised machine-learning algorithm for dimensionality reduction. This t-SNE selection provides an improvement both over UVJ, removing interlopers that otherwise would pass color selection, and over photometric template fitting, more strongly toward high redshift. Due to the similarity between the colors of high- and low-redshift quiescent galaxies, under our assumptions, t-SNE outperforms template fitting in 63% of trials at redshifts where a large training sample already exists. It also may be able to select quiescent galaxies more efficiently at higher redshifts than the training sample. [ABSTRACT FROM AUTHOR]

We report two secure () and one tentative (z ≈ 3.767) spectroscopic confirmations of massive and quiescent galaxies through K-band observations with Keck/MOSFIRE and Very Large Telescope/X-Shooter. The stellar continuum emission, absence of strong nebular emission lines, and lack of significant far-infrared detections confirm the passive nature of these objects, disfavoring the alternative solution of low-redshift dusty star-forming interlopers. We derive stellar masses of log(M⋆/M⊙) ∼ 11 and ongoing star formation rates placing these galaxies ≳1–2 dex below the main sequence at their redshifts. The adopted parameterization of the star formation history suggests that these sources experienced a strong ( M⊙ yr−1) and short (∼50 Myr) burst of star formation, peaking ∼150–500 Myr before the time of observation, all properties reminiscent of the characteristics of submillimeter galaxies (SMGs) at z > 4. We investigate this connection by comparing the comoving number densities and the properties of these two populations. We find a fair agreement only with the deepest submillimeter surveys detecting not only the most extreme starbursts but also more normal galaxies. We support these findings by further exploring the Illustris TNG cosmological simulation, retrieving populations of both fully quenched massive galaxies at z ∼ 3–4 and SMGs at z ∼ 4−5, with number densities and properties in agreement with the observations at z ∼ 3 but in increasing tension at higher redshift. Nevertheless, as suggested by the observations, not all of the progenitors of quiescent galaxies at these redshifts shine as bright SMGs in their past, and, similarly, not all bright SMGs quench by z ∼ 3, both fractions depending on the threshold assumed to define the SMGs themselves. [ABSTRACT FROM AUTHOR]

We present a detailed analysis of a large sample of spectroscopically confirmed massive quiescent galaxies (MQGs; log(M*/M⊙) ∼ 11.5) at z ≳ 2. This sample comprises 15 galaxies selected in the COSMOS and UDS fields by their bright K-band magnitudes and followed up with Very Large Telescope (VLT) X-shooter spectroscopy and Hubble Space Telescope (HST)/WFC3 HF160W imaging. These observations allow us to unambiguously confirm their redshifts, ascertain their quiescent nature and stellar ages, and reliably assess their internal kinematics and effective radii. We find that these galaxies are compact, consistent with the high-mass end of the stellar mass–size relation for quiescent galaxies at z = 2. Moreover, the distribution of the measured stellar velocity dispersions of the sample is consistent with the most massive local early-type galaxies from the MASSIVE Survey, showing that evolution in these galaxies is dominated by changes in size. The HST images reveal, as surprisingly high, that 40% of the sample has tidal features suggestive of mergers and companions in close proximity, including three galaxies experiencing ongoing major mergers. The absence of velocity dispersion evolution from z = 2 to 0, coupled with a doubling of the stellar mass, with a factor of 4 size increase and the observed disturbed stellar morphologies, supports dry minor mergers as the primary drivers of the evolution of the MQGs over the last 10 billion yr. [ABSTRACT FROM AUTHOR]

Few detailed investigations of stellar populations in passive galaxies beyond z ≈ 1 are based on deep spectroscopic observations, due to the difficulty in obtaining such data. We present a study of stellar populations, structure, and mass-to-light ratios (M/L) of a large sample of bulge-dominated galaxies in the two z = 1.27 clusters Lynx E and Lynx W, based on deep ground-based optical spectroscopy combined with imaging from the Hubble Space Telescope. We find that Lynx E has a well-defined core of red passive galaxies, while Lynx W lacks such a core. If all the sample galaxies evolve similarly in size from z = 1.27 to the present, the data would allow only 0.1 dex size growth at a fixed dynamical mass. However, to link the Lynx central galaxies to brightest cluster galaxies similar to those of low-redshift clusters, the Lynx galaxies would have to grow by at least a factor 5, possibly through major merging. The M/L ratios and the Balmer absorption lines of the Lynx galaxies are consistent with passive evolution of the stellar populations from z = 1.27 to the present and support ages of 1–3 Gyr. The galaxies in the outskirts of the clusters contain younger stellar populations than found in the cluster cores. However, when evolved passively to z ≈ 0 both populations are consistent with the observed populations in the Coma cluster galaxies. The bulge-dominated emission line galaxies in the clusters are dominated by stellar populations with subsolar metallicities. Thus, additional enrichment of these is required to produce Coma-like stellar populations by z ≈ 0. [ABSTRACT FROM AUTHOR]

We present the rest-frame optical sizes of massive quiescent galaxies (QGs) at z ∼ 4 measured at the K′-band with the Infrared Camera and Spectrograph and adaptive optics (AO) facility, AO188, on the Subaru telescope. Based on a deep multiwavelength catalog in the Subaru XMM-Newton Deep Survey Field, covering a wide wavelength range from the u-band to the IRAC 8.0 μm over 0.7 deg2, we evaluate the photometric redshift to identify massive (M⋆ ∼ 1011M⊙) galaxies with suppressed star formation. These galaxies show a prominent Balmer break feature at z ∼ 4, suggestive of an evolved stellar population. We then conduct follow-up K′-band imaging with AO for the five brightest galaxies (KAB,total = 22.5 ∼ 23.4). Compared to lower redshift ones, QGs at z ∼ 4 have smaller physical sizes of effective radii reff = 0.2–1.7 kpc. The mean size measured by stacking the four brightest objects, a more robust measurement, is reff = 0.5 kpc. This is the first measurement of the rest-frame optical sizes of QGs at z ∼ 4. We evaluate the robustness of our size measurements using simulations and find that our size estimates are reasonably accurate with an expected systematic bias of ∼0.2 kpc. If we account for the stellar mass evolution, massive QGs at z ∼ 4 are likely to evolve into the most massive galaxies today. We find their size evolution with cosmic time in the form of . Their size growth is proportional to the square of stellar mass, indicating that size–stellar mass growth is driven by minor dry mergers. [ABSTRACT FROM AUTHOR]

We present morphological properties of dusty star-forming galaxies at z = 1–3 determined with the high-resolution (FWHM ∼ 0.″19) Atacama Large Milllimeter/submillimeter Array (ALMA) 1 mm map of our ASAGAO survey covering a 26 arcmin2 area in GOODS-S. In conjunction with the ALMA archival data, our sample consists of 45 ALMA sources with infrared luminosity (LIR) range of ∼1011–1013L⊙. To obtain an average rest-frame far-infrared (FIR) profile, we perform individual measurements and careful stacking of the ALMA sources using the uv-visibility method that includes positional-uncertainty and smoothing-effect evaluations through Monte Carlo simulations. We find that our sample has an average FIR-wavelength Sérsic index and effective radius of nFIR = 1.2 ± 0.2 and Re,FIR = 1.0–1.3 kpc, respectively, additionally with a point-source component at the center, indicative of the existence of active galactic nuclei. The average FIR profile agrees with a morphology of an exponential disk clearly distinguished from a de Vaucouleurs spheroidal profile (Sérsic index of 4). We also examine the rest-frame optical Sérsic index nopt and effective radius Re,opt with deep Hubble SpaceTelescope (HST) images. Interestingly, we obtain nopt = 0.9 ± 0.3 (≃nFIR) and Re,opt = 3.2 ± 0.6 kpc (>Re,FIR), suggesting that the dusty disk-like structure is embedded within a larger stellar disk. The rest-frame UV and FIR data of HST and ALMA provide us with a radial profile of the total star formation rate (SFR), where the infrared SFR dominates over the UV SFR at the center. Under the assumption of a constant SFR, a compact stellar distribution in z ∼ 1–2 compact quiescent galaxies (cQGs) is well reproduced, while a spheroidal stellar morphology of cQGs (nopt = 4) is not, suggestive of other important mechanism(s) such as dynamical dissipation. [ABSTRACT FROM AUTHOR]

We measure the merger fraction of massive galaxies using the UltraVISTA/COSMOS catalog, complemented with the deeper, higher resolution 3DHST+CANDELS catalog, presenting the largest mass-complete photometric merger sample up to . We find that the variation in the mass ratio probe can explain the discrepant redshift evolution of the merger fraction in the literature: selecting mergers using the H160-band flux ratio leads to an increasing merger fraction with redshift, while selecting mergers using the stellar mass ratio reveals a merger fraction with little redshift dependence at . Defining major and minor mergers as having stellar mass ratios of 1:1–4:1 and 4:1–10:1, respectively, the results imply ∼1 major merger and ∼0.7 minor merger on average for a massive (log ) galaxy during . There may be an additional major (minor) merger if we use the H-band flux ratio selection. The observed amount of major merging alone is sufficient to explain the observed number density evolution for the very massive (log ) galaxies. The observed number of major and minor mergers can increase the size of a massive quiescent galaxy by a factor of two at most. This amount of merging is enough to bring the compact quiescent galaxies formed at to lie at below the mean of the stellar mass–size relation as measured in some works (e.g., Newman et al.), but additional mechanisms are needed to fully explain the evolution, and to be consistent with works suggesting stronger evolution. [ABSTRACT FROM AUTHOR]

We performed a comprehensive stacking analysis on ∼14,200 quiescent galaxy (QG) candidates at z = 0–3 across mid-, far-infrared (MIR and FIR), and radio wavelengths. Identified via their rest-frame NUV − r and r − J colors, the QG candidates () have drastically different IR and radio properties depending on their 24 μm emission strength. The fraction of QG candidates with strong 24 μm emission (equivalent to inferred star formation rates SFR, hereafter “IR-bright”) increases with redshift and peaks at 15%, and their stacked MIPS 24 μm, Herschel (PACS and SPIRE) and VLA emissions are consistent with being star-forming galaxies (SFGs). In contrast, the majority of QG candidates are faint or undetected at 24 μm individually (i.e., SFR24 < 100 M⊙ yr−1, hereafter “IR-faint”). Their low dust-obscured SFRs derived from Herschel stacking (SFRH ≲ 3, 15, 50 M⊙ yr−1 out to z ∼ 1, 2, 3) are >2.5–12.5× lower than compared to SFGs. This is consistent with the quiescence, as expected from their low unobscured SFRs, as inferred from modeling their ultraviolet-to-NIR photometry. The discrepancy between the LIR derived from stacking Herschel and 24 μm indicates that IR-faint QGs have dust SEDs that are different from those of SFGs. For the most massive () IR-faint QGs at z < 1.5, the stacked 1.4 GHz emission is in excess of that expected from other SFR indicators, suggesting a widespread presence of low-luminosity active galactic nuclei. Our results reaffirm the existence of a significant population of QGs out to z = 3, thus corroborating the need to quench star formation in galaxies at early epochs. [ABSTRACT FROM AUTHOR]

We present SÍGAME simulations of the 157.7 μm fine structure line emission from cosmological smoothed particle hydrodynamics simulations of seven main sequence galaxies at z = 2. Using sub-grid physics prescriptions the gas in our simulations is modeled as a multi-phased interstellar medium comprised of molecular gas residing in giant molecular clouds, an atomic gas phase associated with photo-dissociation regions (PDRs) at the cloud surfaces, and a diffuse, ionized gas phase. Adopting logotropic cloud density profiles and accounting for heating by the local FUV radiation field and cosmic rays by scaling both with local star formation rate (SFR) volume density, we calculate the emission using a photon escape probability formalism. The emission peaks in the central of our galaxies as do the SFR radial profiles, with most () originating in the molecular gas phase, whereas further out (), the atomic/PDR gas dominates () the emission, no longer tracing ongoing star formation. Throughout, the ionized gas contribution is negligible (). The luminosity versus SFR (–SFR) relationship, integrated as well as spatially resolved (on scales of 1 kpc), delineated by our simulated galaxies is in good agreement with the corresponding relations observed locally and at high redshifts. In our simulations, the molecular gas dominates the budget at ( ), while atomic/PDR gas takes over at lower SFRs, suggesting a picture in which predominantly traces the molecular gas in high-density/pressure regions where star formation is ongoing, and otherwise reveals the atomic/PDR gas phase. [ABSTRACT FROM AUTHOR]

Two millimeter observations of the MACS J1149.6+2223 cluster have detected a source that was consistent with the location of the lensed MACS 1149-JD galaxy at z = 9.6. A positive identification would have rendered this galaxy as the youngest dust forming galaxy in the universe. Follow up observation with the AzTEC 1.1 mm camera and the IRAM NOrthern Extended Millimeter Array (NOEMA) at 1.3 mm have not confirmed this association. In this paper we show that the NOEMA observations associate the 2 mm source with [PCB2012] 2882,12 source number 2882 in the Cluster Lensing And Supernova survey with Hubble (CLASH) catalog of MACS J1149.6+2223. This source, hereafter referred to as CLASH 2882, is a gravitationally lensed spiral galaxy at z = 0.99. We combine the Goddard IRAM Superconducting 2-Millimeter Observer (GISMO) 2 mm and NOEMA 1.3 mm fluxes with other (rest frame) UV to far-IR observations to construct the full spectral energy distribution of this galaxy, and derive its star formation history, and stellar and interstellar dust content. The current star formation rate of the galaxy is M⊙ yr−1, and its dust mass is about 5 × 107M⊙, where μ is the lensing magnification factor for this source, which has a mean value of 2.7. The inferred dust mass is higher than the maximum dust mass that can be produced by core collapse supernovae and evolved AGB stars. As with many other star forming galaxies, most of the dust mass in CLASH 2882 must have been accreted in the dense phases of the interstellar medium. [ABSTRACT FROM AUTHOR]

We present the first spectroscopic confirmation of an ultramassive galaxy at redshift using data from Keck-NIRSPEC, VLT-X-shooter, and GTC-Osiris. We detect strong [O iii] and Lyα emission, and weak [O ii], C iv, and He ii, placing C1-23152 at a spectroscopic redshift of . The modeling of the emission-line-corrected spectral energy distribution (SED) results in a best-fit stellar mass of , a star formation rate of <7 yr−1, and negligible dust extinction. The stars appear to have formed in a short intense burst ∼300–500 Myr prior to the observation epoch, setting the formation redshift of this galaxy at z ∼ 4.1. From the analysis of the line ratios and widths and the observed flux at 24 μm, we confirm the presence of a luminous hidden active galactic nucleus (AGN), with bolometric luminosity of erg s−1. Potential contamination of the observed SED from the AGN continuum is constrained, placing a lower limit on the stellar mass of . HST/WFC3 and ACS images are modeled, resulting in an effective radius of kpc in the band and a Sérsic index . This object may be a prototype of the progenitors of local most massive elliptical galaxies in the first 2 Gyr of cosmic history, having formed most of its stars at in a highly dissipative, intense, and short burst of star formation. C1-23152 is completing its transition to a post-starburst phase while hosting a powerful AGN, potentially responsible for the quenching of the star formation activity. [ABSTRACT FROM AUTHOR]