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We report spectroscopic and photometric follow-up of a dormant black hole (BH) candidate from Gaia DR3. The system, which we call Gaia BH2, contains a $\sim 1M_{\odot}$ red giant and a dark companion with mass $M_2 = 8.9\pm 0.3\,M_{\odot}$ that is very likely a BH. The orbital period, $P_{\rm orb} = 1277$ days, is much longer than that of any previously studied BH binary. Our radial velocity (RV) follow-up over a 7-month period spans more than 90% of the orbit's dynamic range in RV and is in excellent agreement with predictions of the Gaia solution. UV imaging and high-resolution optical spectra rule out all plausible luminous companions that could explain the orbit. The star is a bright ($G=12.3$), slightly metal-poor ($\rm [Fe/H]=-0.22$) low-luminosity giant ($T_{\rm eff}=4600\,\rm K$; $R = 7.8\,R_{\odot}$; $\log\left[g/\left({\rm cm\,s^{-2}}\right)\right] = 2.6$). The binary's orbit is moderately eccentric ($e=0.52$). The giant is strongly enhanced in $\alpha-$elements, with $\rm [\alpha/Fe] = +0.26$, but the system's Galactocentric orbit is typical of the thin disk. We obtained X-ray and radio nondetections of the source near periastron, which support BH accretion models in which the net accretion rate at the horizon is much lower than the Bondi-Hoyle-Lyttleton rate. At a distance of 1.16 kpc, Gaia BH2 is the second-nearest known BH, after Gaia BH1. Its orbit -- like that of Gaia BH1 -- seems too wide to have formed through common envelope evolution. Gaia BH1 and BH2 have orbital periods at opposite edges of the Gaia DR3 sensitivity curve, perhaps hinting at a bimodal intrinsic period distribution for wide BH binaries. Dormant BH binaries like Gaia BH1 and Gaia BH2 likely significantly outnumber their close, X-ray bright cousins, but their formation pathways remain uncertain., Comment: Accepted to MNRAS

We constrain the orbital period ($P_{\rm orb}$) distribution of low-mass detached main-sequence eclipsing binaries (EBs) with light curves from the Zwicky Transient Facility (ZTF), which provides a well-understood selection function and sensitivity to faint stars. At short periods ($P_{\rm orb}\lesssim 2$ days), binaries are predicted to evolve significantly due to magnetic braking (MB), which shrinks orbits and ultimately brings detached binaries into contact. The period distribution is thus a sensitive probe of MB. We find that the intrinsic period distribution of low-mass ($0.1\lesssim M_1/M_{\odot} < 0.9$) binaries is basically flat (${\rm d}N/{\rm d}P_{\rm orb} \propto P_{\rm orb}^0$), from $P_{\rm orb}=10$ days down to the contact limit. This is strongly inconsistent with predictions of classical MB models based on the Skumanich relation, which are widely used in binary evolution calculations and predict ${\rm d}N/{\rm d}P_{\rm orb} \propto P_{\rm orb}^{7/3}$ at short periods. The observed distributions are best reproduced by models in which the magnetic field saturates at short periods, with a MB torque that scales roughly as $\dot{J}\propto P_{\rm orb}^{-1}$, as opposed to $\dot{J} \propto P_{\rm orb}^{-3}$ in the standard Skumanich law. We also find no significant difference between the period distributions of binaries containing fully and partially convective stars. Our results confirm that a saturated MB law, which was previously found to describe the spin-down of rapidly rotating isolated M dwarfs, also operates in tidally locked binaries. We advocate using saturated MB models in binary evolution calculations. Our work supports previous suggestions that MB in cataclysmic variables (CVs) is much weaker than assumed in the standard evolutionary model, unless mass transfer leads to significant additional angular momentum loss in CVs., Table 1 included in source files. Accepted to MNRAS

Accurate determination of the rates of nova eruptions in different kinds of galaxies gives us strong constraints on those galaxies’ underlying white dwarf and binary populations, and those stars’ spatial distributions. Until 2016, limitations inherent in ground-based surveys of external galaxies – and dust extinction in the Milky Way – significantly hampered the determination of those rates and how much they differ between different types of galaxies. Infrared Galactic surveys and dense cadence Hubble Space Telescope(HST)-based surveys are overcoming these limitations, leading to sharply increased nova-in-galaxy rates relative to those previously claimed. Here, we present 14 nova candidates that were serendipitously observed during a year-long HST survey of the massive spiral galaxy M51 (the ‘Whirlpool Galaxy’). We use simulations based on observed nova light curves to model the incompleteness of the HST survey in unprecedented detail, determining a nova detection efficiency ϵ = 20.3 per cent. The survey’s M51 area coverage, combined with ϵ, indicates a conservative M51 nova rate of $172^{+46}_{-37}$ novae yr−1, corresponding to a luminosity-specific nova rate (LSNR) of $\sim\!10.4^{+2.8}_{-2.2}$ novae yr−1/1010L⊙,K. Both these rates are approximately an order of magnitude higher than those estimated by ground-based studies, contradicting claims of universal low nova rates in all types of galaxies determined by low cadence, ground-based surveys. They demonstrate that, contrary to theoretical models, the HST-determined LSNR in a giant elliptical galaxy (M87) and a giant spiral galaxy (M51) likely do not differ by an order of magnitude or more, and may in fact be quite similar.

Motivated by recent suggestions that many Be stars form through binary mass transfer, we searched the APOGEE survey for Be stars with bloated, stripped companions. From a well-defined parent sample of 297 Be stars, we identified one mass-transfer binary, HD 15124. The object consists of a main-sequence Be star ($M_{\rm Be}=5.3\pm 0.6 \,M_{\odot}$) with a low-mass ($M_{\rm donor}=0.92\pm 0.22\,M_{\odot}$), subgiant companion on a 5.47-day orbit. The emission lines originate in an accretion disk caused by ongoing mass transfer, not from a decretion disk as in classical Be stars. Both stars have surface abundances bearing imprint of CNO processing in the donor's core: the surface helium fraction is $Y_{\rm He}\approx 0.6$, and the nitrogen-to-carbon ratio is 1000 times the solar value. The system's properties are well-matched by binary evolution models in which mass transfer begins while a $3-5\,M_{\odot}$ donor leaves the main sequence, with the secondary becoming the Be star. These models predict that the system will soon become a detached Be + stripped star binary like HR 6819 and LB-1, with the stripped donor eventually contracting to become a core helium-burning sdOB star. Discovery of one object in this short-lived ($\sim$1 Myr) evolutionary phase implies the existence of many more that have already passed through it and are now Be + sdOB binaries. We infer that $(28_{-16}^{+27})\,\%$ of Be stars have stripped companions, most of which are faint. Together with the dearth of main-sequence companions to Be stars and recent discovery of numerous Be + sdOB binaries in the UV, our results imply that binarity plays an important role in the formation of Be stars., 25 pages, 19 figures, accepted to MNRAS

We analyze two binary systems containing giant stars, V723 Mon ("the Unicorn") and 2M04123153+6738486 ("the Giraffe"). Both giants orbit more massive but less luminous companions, previously proposed to be mass-gap black holes. Spectral disentangling reveals luminous companions with star-like spectra in both systems. Joint modeling of the spectra, light curves, and spectral energy distributions robustly constrains the masses, temperatures, and radii of both components: the primaries are luminous, cool giants ($T_{\rm eff,\,giant} = 3,800\,\rm K$ and $4,000\,\rm K$, $R_{\rm giant}= 22.5\,R_{\odot}$ and $25\,R_{\odot}$) with exceptionally low masses ($M_{\rm giant} \approx 0.4\,M_{\odot}$) that likely fill their Roche lobes. The secondaries are only slightly warmer subgiants ($T_{\rm eff,\,2} = 5,800\,\rm K$ and $5,150\,\rm K$, $R_2= 8.3\,R_{\odot}$ and $9\,R_{\odot}$) and thus are consistent with observed UV limits that would rule out main-sequence stars with similar masses ($M_2 \approx 2.8\,M_{\odot}$ and $\approx 1.8\,M_{\odot}$). In the Unicorn, rapid rotation blurs the spectral lines of the subgiant, making it challenging to detect even at wavelengths where it dominates the total light. Both giants have surface abundances indicative of CNO processing and subsequent envelope stripping. The properties of both systems can be reproduced by binary evolution models in which a $1-2\,M_{\odot}$ primary is stripped by a companion as it ascends the giant branch. The fact that the companions are also evolved implies either that the initial mass ratio was very near unity, or that the companions are temporarily inflated due to rapid accretion. The Unicorn and Giraffe offer a window into into a rarely-observed phase of binary evolution preceding the formation of wide-orbit helium white dwarfs, and eventually, compact binaries containing two helium white dwarfs., 20 pages, 16 figures. Accepted to MNRAS

A major source of uncertainty in the age determination of old ($\sim10$ Gyr) integrated stellar populations is the presence of hot horizontal branch (HB) stars. Here, we describe a simple approach to tackle this problem, and show the performance of this technique that simultaneously models the age, abundances and HB properties of integrated stellar populations. For this we compare the results found during the fits of the integrated spectra of a sample of stellar population benchmarks, against the values obtained from the analysis of their resolved CMDs. We find that the ages derived from our spectral fits for most (26/32) of our targets are within 0.1 dex to their CMDs values. Similarly, for the majority of the targets in our sample we are able to recover successfully the flux contribution from hot HB stars (within $\sim0.15 {\rm ~dex}$ for 18/24 targets) and their mean temperature (14/24 targets within $\sim30 \%$). Finally, we present a diagnostic that can be used to detect spurious solutions in age, that will help identify the few cases when this method fails. These results open a new window for the detailed study of globular clusters beyond the Local Group., Comment: 15 pages, 12 figures and 2 tables. MNRAS in press (Accepted 2021 December 31. Received 2021 December 7; in original form 2021 May 18)

The James Webb Space Telescope (JWST) is expected to observe galaxies at $z>10$ that are presently inaccessible. Here, we use a self-consistent empirical model, the UniverseMachine, to generate mock galaxy catalogues and lightcones over the redshift range $z=0-15$. These data include realistic galaxy properties (stellar masses, star formation rates, and UV luminosities), galaxy-halo relationships, and galaxy-galaxy clustering. Mock observables are also provided for different model parameters spanning observational uncertainties at $z10^7 M_\odot$ and/or $M_{1500}12$ expand dramatically, so efforts to detect $z>12$ galaxies will provide the most valuable constraints on galaxy formation models. The faint-end slopes of the stellar mass/luminosity functions at a given mass/luminosity threshold steepen as redshift increases. This is because observable galaxies are hosted by haloes in the exponentially falling regime of the halo mass function at high redshifts. Hence, these faint-end slopes are robustly predicted to become shallower below current observable limits ($M_\ast < 10^7M_\odot$ or $M_\mathrm{1500}>-17$). For reionization models, extrapolating luminosity functions with a constant faint-end slope from $M_{1500}=-17$ down to $M_{1500}=-12$ gives the most reasonable upper limit for the total UV luminosity and cosmic star formation rate up to $z\sim 12$. We compare to three other empirical models and one semi-analytic model, showing that the range of predicted observables from our approach encompasses predictions from other techniques. Public catalogues and lightcones for common fields are available online., Comment: 17 pages, MNRAS submitted. Catalogs and lightcones available at https://www.peterbehroozi.com/data.html

While we observe a large amount of cold interstellar gas and dust in a subset of the early-type galaxy (ETG) population, the source of this material remains unclear. The two main, competing scenarios are external accretion of lower mass, gas-rich dwarfs and internal production from stellar mass loss and/or cooling from the hot interstellar medium (ISM). We test these hypotheses with measurements of the stellar and nebular metallicities of three ETGs (NGC 2768, NGC 3245, and NGC 4694) from new long-slit, high signal-to-noise ratio spectroscopy from the Multi-Object Double Spectographs (MODs) on the Large Binocular Telescope (LBT). These ETGs have modest star formation rates and minimal evidence of nuclear activity. We model the stellar continuum to derive chemical abundances and measure gas-phase abundances with standard nebular diagnostics. We find that the stellar and gas-phase abundances are very similar, which supports internal production and is very inconsistent with the accretion of smaller, lower metallicity dwarfs. All three of these galaxies are also consistent with an extrapolation of the mass-metallicity relation to higher mass galaxies with lower specific star formation rates. The emission line flux ratios along the long-slit, as well as global line ratios clearly indicate that photoionization dominates and ionization by alternate sources including AGN activity, shocks, cosmic rays, dissipative magnetohydrodynamic waves, and single degenerate Type Ia supernovae progenitors do not significantly affect the line ratios., Comment: 13 pages, 9 figures, 3 tables

We test whether halo age and galaxy age are correlated at fixed halo and galaxy mass. The formation histories, and thus ages, of dark matter halos correlate with their large-scale density $\rho$, an effect known as assembly bias. We test whether this correlation extends to galaxies by measuring the dependence of galaxy stellar age on $\rho$. To clarify the comparison between theory and observation, and to remove the strong environmental effects on satellites, we use galaxy group catalogs to identify central galaxies and measure their quenched fraction, $f_Q$, as a function of large-scale environment. Models that match halo age to central galaxy age predict a strong positive correlation between $f_Q$ and $\rho$. However, we show that the amplitude of this effect depends on the definition of halo age: assembly bias is significantly reduced when removing the effects of splashback halos---those halos that are central but have passed through a larger halo or experienced strong tidal encounters. Defining age using halo mass at its peak value rather than current mass removes these effects. In SDSS data, at M$_{\rm gal}\gtrsim 10^{10.0}$ M_sol/h$^2$, there is a $\sim 5\%$ increase in $f_Q$ from low to high densities, which is in agreement with predictions of dark matter halos using peak halo mass. At lower stellar mass there is little to no correlation of $f_Q$ with $\rho$. For these galaxies, age-matching is inconsistent with the data across the wide range the halo formation metrics that we tested. This implies that halo formation history has a small but statistically significant impact on quenching of star formation at high masses, while the quenching process in low-mass central galaxies is uncorrelated with halo formation history., Comment: 13 pages, 12 figures, submitted to MNRAS

We present results from a blind survey to identify strong gravitational lenses among the population of low-redshift early-type galaxies. The SINFONI Nearby Elliptical Lens Locator Survey (SNELLS) uses integral-field infrared spectroscopy to search for lensed emission line sources behind massive lens candidates at $z$300 km/s) and \alpha-element abundances ([Mg/Fe]>0.3). From the lensing configurations we derive total J-band mass-to-light ratios of 1.8$\pm$0.1, 2.1$\pm$0.1 and 1.9$\pm$0.2 within the $\sim$2 kpc Einstein radius. Correcting for estimated dark-matter contributions, and comparing to stellar population models with a Milky Way (Kroupa) initial mass function (IMF), we determine the "mass excess factor", \alpha. Assuming the lens galaxies have "old" stellar populations (10$\pm$1 Gyr), the average IMF mass factor is $\langle\alpha\rangle$=1.10$\pm$0.08$\pm$0.10, where the first error is random and the second is systematic. If we instead fit the stellar populations from 6dF optical survey spectra, all three galaxies are consistent with being old, but the age errors are 3-4 Gyr, due to limited signal-to-noise. The IMF constraints are therefore looser in this case, with $\langle\alpha\rangle$ = $1.23^{+0.16}_{-0.13}\pm{0.10}$. Our results are thus consistent with a Kroupa IMF (\alpha=1.00) on average, and strongly reject very heavy IMFs with \alpha>2. A Salpeter IMF (\alpha=1.55) is inconsistent at the 3.5$\sigma$ level if the galaxies are old, but cannot be excluded using age constraints derived from the currently-available optical spectra., Comment: Accepted for publication in MNRAS

We investigate the optical spectral region of spectra of ˜1000 stars searching for initial mass function (IMF)-sensitive features to constrain the low-mass end of the IMF slope in elliptical galaxies. The use of indicators bluer than near-infrared features (NaI, CaT, Wing-Ford FeH) is crucial if we want to compare our observations to optical simple stellar population (SSP) models. We use the MILES stellar library (Sánchez-Blázquez et al.) in the wavelength range 3500-7500 Å to select indices that are sensitive to cool dwarf stars and that do not or only weakly depend on age and metallicity. We find several promising indices of molecular TiO and CaH lines. In this wavelength range, the response of a change in the effective temperature of the cool red giant (RGB) population is similar to the response of a change in the number of dwarf stars in the galaxy. We therefore investigate the degeneracy between IMF variation and ΔTeff, RGB, and show that it is possible to break this degeneracy with the new IMF indicators defined here. In particular, we define a CaH1 index around λ6380 Å that arises purely from cool dwarfs, does not strongly depend on age and is anticorrelated with [α/Fe]. This index allows the determination of the low-mass end of the IMF slope from integrated-light measurements when combined with different TiO lines and age- and metallicity-dependent features such as Hβ, Mgb, Fe5270 and Fe5335. The use of several indicators is crucial to break degeneracies between IMF variations, age, abundance pattern and effective temperature of the cool red giant (RGB) population. We measure line-index strengths of our new optical IMF indicators in the Conroy & van Dokkum SSP models and compare these with index strengths of the same spectral features in a sample of stacked Sloan Digital Sky Survey early-type galaxy spectra with varying velocity dispersions. Using different indicators, we find a clear trend of a steepening IMF with increasing velocity dispersion from 150 to 310 km s-1 described by the linear equation x = (2.3 ± 0.1) log σ200 + (2.13 ± 0.15), where x is the IMF slope and σ200 is the central stellar velocity dispersion measured in units of 200 km s-1. We test the robustness of this relation by repeating the analysis with 10 different sets of indicators. We found that the NaD feature has the largest impact on the IMF slope, if we assume solar [Na/Fe] abundance. By including NaD, the slope of the linear relation increases by 0.3 (2.6 ± 0.2). We compute the `IMF mismatch' parameter as the ratio of stellar mass-to-light ratio predicted from the x-σ200 relation to that inferred from SSP models assuming a Salpeter IMF and find good agreement with independent published results.

We present a comprehensive study of 250,000 galaxies targeted by the Baryon Oscillation Spectroscopic Survey (BOSS) up to z ~ 0.7 with the specific goal of identifying and characterising a population of galaxies that has evolved without significant merging. We compute a likelihood that each BOSS galaxy is a progenitor of the Luminous Red Galaxies (LRGs) sample, targeted by SDSS-I/II up z ~ 0.5, by using the fossil record of LRGs and their inferred star-formation histories, metallicity histories and dust content. We determine merger rates, luminosity growth rates and the evolution of the large-scale clustering between the two surveys, and we investigate the effect of using different stellar population synthesis models in our conclusions. We demonstrate that our sample is slowly evolving (of the order of 2 +/- 1.5% per Gyr by merging). Our conclusions refer to the bright and massive end of the galaxy population, with Mi0.55 1E11.2 Msolar, corresponding roughly to 95% and 40% of the LRGs and BOSS galaxy populations, respectively. Our analysis further shows that any possible excess of flux in BOSS galaxies, when compared to LRGs, from potentially unresolved targets at z ~ 0.55 must be less than 1% in the r0.55-band (approximately equivalent to the g-band in the rest-frame of galaxies at z=0.55). When weighting the BOSS galaxies based on the predicted properties of the LRGs, and restricting the analysis to the reddest BOSS galaxies, we find an evolution of the large-scale clustering that is consistent with dynamical passive evolution, assuming a standard cosmology. We conclude that our likelihoods give a weighted sample that is as clean and as close to passive evolution (in dynamical terms, i.e. no or negligible merging) as possible, and that is optimal for cosmological studies.

Using galaxy group/cluster catalogs created from the Sloan Digital Sky Survey Data Release 7, we examine in detail the specific star formation rate (SSFR) distribution of satellite galaxies and its dependence on stellar mass, host halo mass, and halo-centric radius. All galaxies, regardless of central-satellite designation, exhibit a similar bimodal SSFR distribution, with a strong break at SSFR ~ 10^-11 yr^-1 and the same high SSFR peak; in no regime is there ever an excess of galaxies in the `green valley'. Satellite galaxies are simply more likely to lie on the quenched (`red sequence') side of the SSFR distribution. Furthermore, the satellite quenched fraction excess above the field galaxy value is nearly independent of galaxy stellar mass. An enhanced quenched fraction for satellites persists in groups with halo masses down to 3 x 10^11 Msol and increases strongly with halo mass and toward halo center. We find no detectable quenching enhancement for galaxies beyond ~2R_vir around massive clusters once these galaxies have been decomposed into centrals and satellites. These trends imply that (1) galaxies experience no significant environmental effects until they cross within ~R_vir of a more massive host halo, (2) after this, star formation in active satellites continues to evolve in the same manner as active central galaxies for several Gyrs, and (3) once begun, satellite star formation quenching occurs rapidly. These results place strong constraints on satellite-specific quenching mechanisms, as we will discuss further in companion papers.

Using estimates of dark halo masses from satellite kinematics, weak gravitational lensing, and halo abundance matching, combined with the Tully-Fisher and Faber-Jackson relations, we derive the mean relation between the optical, V_opt, and virial, V_200, circular velocities of early- and late-type galaxies at redshift z~0. For late-type galaxies V_opt ~ V_200 over the velocity range V_opt=90-260 km/s, and is consistent with V_opt = V_maxh (the maximum circular velocity of NFW dark matter haloes in the concordance LCDM cosmology). However, for early-type galaxies V_opt \ne V_200, with the exception of early-type galaxies with V_opt simeq 350 km/s. This is inconsistent with early-type galaxies being, in general, globally isothermal. For low mass (V_opt V_maxh, indicating that baryons have modified the potential well, while high mass (V_opt > 400 km/s) early-types have V_opt < V_maxh. Folding in measurements of the black hole mass - velocity dispersion relation, our results imply that the supermassive black hole - halo mass relation has a logarithmic slope which varies from ~1.4 at halo masses of ~10^{12} Msun/h to ~0.65 at halo masses of 10^{13.5} Msun/h. The values of V_opt/V_200 we infer for the Milky Way and M31 are lower than the values currently favored by direct observations and dynamical models. This offset is due to the fact that the Milky Way and M31 have higher V_opt and lower V_200 compared to typical late-type galaxies of the same stellar masses. We show that current high resolution cosmological hydrodynamical simulations are unable to form galaxies which simultaneously reproduce both the V_opt/V_200 ratio and the V_opt-M_star (Tully-Fisher/Faber-Jackson) relation.

We investigate the origin of the relations between stellar mass and optical circular velocity for early-type galaxies (ETGs) and late-type galaxies (LTGs) – the Faber–Jackson (FJ) and Tully–Fisher (TF) relations. We combine measurements of dark halo masses (from satellite kinematics and weak lensing), and the distribution of baryons in galaxies (from a new compilation of galaxy scaling relations), with constraints on dark halo structure from cosmological simulations. The principal unknowns are the halo response to galaxy formation and the stellar initial mass function (IMF). The slopes of the TF and FJ relations are naturally reproduced for a wide range of halo response and IMFs. However, models with a universal IMF and universal halo response cannot simultaneously reproduce the zero-points of both the TF and FJ relations. For a model with a universal Chabrier IMF, LTGs require halo expansion, while ETGs require halo contraction. A Salpeter IMF is permitted for high-mass (σ≳ 180 km s−1) ETGs, but is inconsistent for intermediate masses, unless Vcirc(Re)/σe≳ 1.6. If the IMF is universal and close to Chabrier, we speculate that the presence of a major merger may be responsible for the contraction in ETGs while clumpy accreting streams and/or feedback leads to expansion in LTGs. Alternatively, a recently proposed variation in the IMF disfavours halo contraction in both types of galaxies. Finally we show that our models naturally reproduce flat and featureless circular velocity profiles within the optical regions of galaxies without fine-tuning.

A novel technique is employed for estimating attenuation curves in galaxies where only photometry and spectroscopic redshifts are available. This technique provides a powerful measure of particular extinction features such as the UV bump at 2175\A, which has been observed in environments ranging from the Milky Way to high-redshift star-forming galaxies. Knowledge of the typical strength of the UV bump as a function of environment and redshift is crucial for converting restframe UV flux into star formation rates. The UV bump will impart a unique signature as it moves through various filters due to redshifting; its presence can therefore be disentangled from other stellar population effects. The utility of this technique is demonstrated with a large sample of galaxies drawn from the DEEP2 Galaxy Redshift Survey. The observed B-R color of star-forming galaxies at 0.6