Massive Star-forming Galaxies Research Articles

Environmental quenching and galactic conformity in the galaxy cross-correlation signal

It has long been known that environment has a large effect on star formation in galaxies. There are several known plausible mechanisms to remove the cool gas needed for star formation, such as strangulation, harassment and ram-pressure stripping.It is unclear which process is dominant, and over what range of stellar mass. In this paper, we find evidence for suppression of the cross-correlation function between massive galaxies and less massive star-forming galaxies, giving a measure of how less likely a galaxy is to be star-forming in the vicinity of a more massive galaxy. We develop a formalism for modelling environmental quenching mechanisms within the Halo Occupation Distribution scheme. We find that at $z \sim 2$ environment is not a significant factor in determining quenching of star-forming galaxies, and that galaxies are quenched with similar probabilities when they are satellites in sub-group environments, as they are globally. However, by $z \sim 0.5$ galaxies are much less likely to be star forming when in a high density (group or low-mass cluster) environment than when not. This increased probability of being quenched does not appear to have significant radial dependence within the halo at lower redshifts, supportive of the quenching being caused by the halting of fresh inflows of pristine gas, as opposed to by tidal stripping. Furthermore, by separating the massive sample into passive and star-forming, we see that this effect is further enhanced when the central galaxy is passive, a manifestation of galactic conformity.

The Properties of the Massive Star-forming Galaxies with an Outside-in Assembly Mode

Abstract Previous findings show that massive ( ) star-forming (SF) galaxies usually have an “inside-out” stellar mass assembly mode. In this paper, we have for the first time selected a sample of 77 massive SF galaxies with an “outside-in” assembly mode (called the “targeted sample”) from the Mapping Nearby Galaxies at the Apache Point Observatory (MaNGA) survey. For comparison, two control samples are constructed from the MaNGA sample matched in stellar mass: a sample of 154 normal SF galaxies and a sample of 62 quiescent galaxies. In contrast to normal SF galaxies, the targeted galaxies appear to be smoother and more bulge-dominated and have a smaller size and higher concentration, star formation rate, and gas-phase metallicity as a whole. However, they have a larger size and lower concentration than quiescent galaxies. Unlike the normal SF sample, the targeted sample exhibits a slightly positive gradient of the 4000 Å break and a pronounced negative gradient of Hα equivalent width. Furthermore, the median surface mass density profile is between those of the normal SF and quiescent samples, indicating that the gas accretion of quiescent galaxies is not likely to be the main approach for the outside-in assembly mode. Our results suggest that the targeted galaxies are likely in the transitional phase from normal SF galaxies to quiescent galaxies, with rapid ongoing central stellar mass assembly (or bulge growth). We discuss several possible formation mechanisms for the outside-in mass assembly mode.

Cosmic evolution of stellar quenching by AGN feedback: clues from the Horizon-AGN simulation

Abstract The observed massive end of the galaxy stellar mass function is steeper than its predicted dark matter halo counterpart in the standard Λ cold dark matter paradigm. In this paper, we investigate the impact of active galactic nuclei (AGN) feedback on star formation in massive galaxies. We isolate the impact of AGN by comparing two simulations from the HORIZON suite, which are identical except that one also includes supermassive black holes (SMBHs) and related feedback models. This allows us to cross-identify individual galaxies between simulations and quantify the effect of AGN feedback on their properties, including stellar mass and gas outflows. We find that massive galaxies (M* ≥ 1011 M⊙) are quenched by AGN feedback to the extent that their stellar masses decrease by up to 80 per cent at z = 0. SMBHs affect their host halo through a combination of outflows that reduce their baryonic mass, particularly for galaxies in the mass range 109 M⊙ ≤ M* ≤ 1011 M⊙, and a disruption of central gas inflows, which limits in situ star formation. As a result, net gas inflows on to massive galaxies, M* ≥ 1011 M⊙, drop by up to 70 per cent. We measure a redshift evolution in the stellar mass ratio of twin galaxies with and without AGN feedback, with galaxies of a given stellar mass showing stronger signs of quenching earlier on. This evolution is driven by a progressive flattening of the MSMBH–M* relation with redshift, particularly for galaxies with M* ≤ 1010 M⊙. MSMBH/M* ratios decrease over time, as falling average gas densities in galaxies curb SMBH growth.

Mufasa: the assembly of the red sequence

We examine the growth and evolution of quenched galaxies in the Mufasa cosmological hydrodynamic simulations that include an evolving halo mass-based quenching prescription, with galaxy colours computed accounting for line-of-sight extinction to individual star particles. Mufasa reproduces the observed present-day red sequence reasonably well, including its slope, amplitude, and scatter. In Mufasa, the red sequence slope is driven entirely by the steep stellar mass-stellar metallicity relation, which independently agrees with observations. High-mass star-forming galaxies blend smoothly onto the red sequence, indicating the lack of a well-defined green valley at M*>10^10.5 Mo. The most massive galaxies quench the earliest and then grow very little in mass via dry merging; they attain their high masses at earlier epochs when cold inflows more effectively penetrate hot halos. To higher redshifts, the red sequence becomes increasingly contaminated with massive dusty star-forming galaxies; UVJ selection subtly but effectively separates these populations. We then examine the evolution of the mass functions of central and satellite galaxies split into passive and star-forming via UVJ. Massive quenched systems show good agreement with observations out to z~2, despite not including a rapid early quenching mode associated with mergers. However, low-mass quenched galaxies are far too numerous at z<1 in Mufasa, indicating that Mufasa strongly over-quenches satellites. A challenge for hydrodynamic simulations is to devise a quenching model that produces enough early massive quenched galaxies and keeps them quenched to z=0, while not being so strong as to over-quench satellites; Mufasa's current scheme fails at the latter.

Herschel and Hubble Study of a Lensed Massive Dusty Starbursting Galaxy at z ∼ 3∗

Abstract We present the results of combined deep Keck/NIRC2, HST/WFC3 near-infrared, and Herschel far-infrared observations of an extremely star-forming dusty lensed galaxy identified from the Herschel Astrophysical Terahertz Large Area Survey (H-ATLAS J133542.9+300401). The galaxy is gravitationally lensed by a massive WISE-identified galaxy cluster at z ∼ 1. The lensed galaxy is spectroscopically confirmed at z = 2.685 from detection of by GBT and from detection of obtained with CARMA. We use the combined spectroscopic and imaging observations to construct a detailed model of the background dusty lensed submillimeter galaxy (SMG), which allows us to study the source plane properties of the target. The best-fit lens model provides magnifications of μ star = 2.10 ± 0.11 and μ dust = 2.02 ± 0.06 for the stellar and dust components, respectively. Multiband data yield a magnification-corrected star formation rate of 1900(±200) M ⊙ yr−1 and a stellar mass of , consistent with a main sequence of star formation at z ∼ 2.6. The CO observations yield a molecular gas mass of 8.3(±1.0) × 1010 M ⊙, similar to the most massive star-forming galaxies, which together with the high star formation efficiency, are responsible for the intense observed star formation rates. The lensed SMG has a very short gas depletion timescale of ∼40 Myr. The high stellar mass and small gas fractions observed indicate that the lensed SMG likely has already formed most of its stellar mass and could be a progenitor of the most massive elliptical galaxies found in the local universe.

The Origins of UV–optical Color Gradients in Star-forming Galaxies at z ∼ 2: Predominant Dust Gradients but Negligible sSFR Gradients

Abstract The rest-frame UV–optical (i.e., NUV − B) color is sensitive to both low-level recent star formation (specific star formation rate—sSFR) and dust. In this Letter, we extend our previous work on the origins of NUV − B color gradients in star-forming galaxies (SFGs) at to those at . We use a sample of 1335 large (semimajor axis radius ) SFGs with extended UV emission out to in the mass range at in the CANDELS/GOODS-S and UDS fields. We show that these SFGs generally have negative NUV − B color gradients (redder centers), and their color gradients strongly increase with galaxy mass. We also show that the global rest-frame FUV − NUV color is approximately linear with , which is derived by modeling the observed integrated FUV to NIR spectral energy distributions of the galaxies. Applying this integrated calibration to our spatially resolved data, we find a negative dust gradient (more dust extinguished in the centers), which steadily becomes steeper with galaxy mass. We further find that the NUV − B color gradients become nearly zero after correcting for dust gradients regardless of galaxy mass. This indicates that the sSFR gradients are negligible and dust reddening is likely the principal cause of negative UV–optical color gradients in these SFGs. Our findings support that the buildup of the stellar mass in SFGs at Cosmic Noon is self-similar inside .

Z ∼ 2: An Epoch of Disk Assembly

Abstract We explore the evolution of the internal gas kinematics of star-forming galaxies from the peak of cosmic star formation at z ∼ 2 to today. Measurements of galaxy rotation velocity V rot, which quantify ordered motions, and gas velocity dispersion σ g , which quantify disordered motions, are adopted from the DEEP2 and SIGMA surveys. This sample covers a continuous baseline in redshift over 0.1 &lt; z &lt; 2.5 , spanning 10 Gyr. At low redshift, nearly all sufficiently massive star-forming galaxies are rotationally supported ( V rot &gt; σ g ). By z = 2, 50% and 70% of galaxies are rotationally supported at low ( 10 9 – 10 10 M ⊙ ) and high ( 10 10 – 10 11 M ⊙ ) stellar mass, respectively. For V rot &gt; 3 σ g , the percentage drops below 35% for all masses. From z = 2 to now, galaxies exhibit remarkably smooth kinematic evolution on average. All galaxies tend toward rotational support with time, and higher-mass systems reach it earlier. This is largely due to a mass-independent decline in σ g by a factor of 3 since z = 2. Over the same time period, V rot increases by a factor of 1.5 in low-mass systems but does not evolve at high mass. These trends in V rot and σ g are at a fixed stellar mass and therefore should not be interpreted as evolutionary tracks for galaxy populations. When populations are linked in time via abundance matching, σ g declines as before and V rot strongly increases with time for all galaxy populations, enhancing the evolution in V rot / σ g . These results indicate that z = 2 is a period of disk assembly, during which strong rotational support is only just beginning to emerge.

The Evolution of the Tully–Fisher Relation between z ∼ 2.3 and z ∼ 0.9 with KMOS3D ∗

Abstract We investigate the stellar mass and baryonic mass Tully–Fisher relations (TFRs) of massive star-forming disk galaxies at redshift and as part of the integral field spectroscopy survey. Our spatially resolved data allow reliable modeling of individual galaxies, including the effect of pressure support on the inferred gravitational potential. At fixed circular velocity, we find higher baryonic masses and similar stellar masses at as compared to . Together with the decreasing gas-to-stellar mass ratios with decreasing redshift, this implies that the contribution of dark matter to the dynamical mass on the galaxy scale increases toward lower redshift. A comparison to local relations reveals a negative evolution of the stellar and baryonic TFR zero points from z = 0 to , no evolution of the stellar TFR zero point from to , and a positive evolution of the baryonic TFR zero point from to . We discuss a toy model of disk galaxy evolution to explain the observed nonmonotonic TFR evolution, taking into account the empirically motivated redshift dependencies of galactic gas fractions and the relative amount of baryons to dark matter on galaxy and halo scales.

Survival of Massive Star-forming Galaxies in Cluster Cores Drives Gas-phase Metallicity Gradients: The Effects of Ram Pressure Stripping

Abstract Recent observations of galaxies in a cluster at z = 0.35 show that their integrated gas-phase metallicities increase with decreasing cluster-centric distance. To test whether ram pressure stripping (RPS) is the underlying cause, we use a semianalytic model to quantify the “observational bias” that RPS introduces into the aperture-based metallicity measurements. We take integral field spectroscopy of local galaxies, remove gas from their outer galactic disks via RPS, and then conduct mock slit observations of cluster galaxies at z = 0.35. Our RPS model predicts a typical cluster-scale metallicity gradient of −0.03 dex/Mpc. By removing gas from the outer galactic disks, RPS introduces a mean metallicity enhancement of + 0.02 dex at a fixed stellar mass. This gas removal and subsequent quenching of star formation preferentially removes low-mass cluster galaxies from the observed star-forming population. As only the more massive star-forming galaxies survive to reach the cluster core, RPS produces a cluster-scale stellar mass gradient of − 0.05 log ( M * / M ⊙ ) /Mpc. This mass segregation drives the predicted cluster-scale metallicity gradient of −0.03 dex/Mpc. However, the effects of RPS alone cannot explain the higher metallicities measured in cluster galaxies at z = 0.35. We hypothesize that additional mechanisms including steep internal metallicity gradients and self-enrichment due to gas strangulation are needed to reproduce our observations at z = 0.35.

Systematic Survey for [O ii], [O iii], and Hα Blobs at z = 0.1–1.5: The Implication for Evolution of Galactic-scale Outflow

Abstract We conduct a systematic search for galaxies at with [O ii] , [O iii] , or Hα emission lines extended over at least 30 kpc by using deep narrowband and broadband imaging in the Subaru-XMM Deep Survey field. These extended emission-line galaxies are dubbed [O ii], [O iii], or Hα blobs. Based on a new selection method that securely selects extended emission-line galaxies, we find 77 blobs at with the isophotal area of emission lines down to erg s−1 cm−2 kpc−2. Four of them are spectroscopically confirmed to be [O iii] blobs at z = 0.83. We identify AGN activities in eight blobs with X-ray and radio data, and find that the fraction of AGN contribution increases with increasing isophotal area of the extended emission. With the Kolmogorov–Smirnov (KS) and Anderson–Darling tests, we confirm that the stellar-mass distributions of Hα and [O ii] blobs are not drawn from those of the emitters at the % confidence level in that Hα and [O ii] blobs are located at the massive end of the distributions, but cannot reject a null hypothesis of being the same distributions in terms of the specific star formation rates. It is suggested that galactic-scale outflows tend to be more prominent in more massive star-forming galaxies. Exploiting our sample homogeneously selected over the large area, we derive the number densities of blobs at each epoch. The number densities of blobs decrease drastically with redshifts at a rate that is larger than that of the decrease of cosmic star formation densities.

The VIMOS Ultra Deep Survey first data release: Spectra and spectroscopic redshifts of 698 objects up to zspec ~ 6 in CANDELS

This paper describes the first data release (DR1) of the VIMOS Ultra Deep Survey (VUDS). The VUDS-DR1 is the release of all low-resolution spectroscopic data obtained in 276.9 arcmin2 of the CANDELS-COSMOS and CANDELS-ECDFS survey areas, including accurate spectroscopic redshifts zspec and individual spectra obtained with VIMOS on the ESO-VLT. A total of 698 objects have a measured redshift, with 677 galaxies, two type-I AGN, and a small number of 19 contaminating stars. The targets of the spectroscopic survey are selected primarily on the basis of their photometric redshifts to ensure a broad population coverage. About 500 galaxies have zspec &gt; 2, 48of which have zspec &gt; 4; the highest reliable redshifts reach beyond zspec = 6. This data set approximately doubles the number of galaxies with spectroscopic redshifts at z &gt; 3 in these fields. We discuss the general properties of the VUDS–DR1 sample in terms of the spectroscopic redshift distribution, the distribution of Lyman-α equivalent widths, and physical properties including stellar masses M⋆ and star formation rates derived from spectral energy distribution fitting with the knowledge of zspec. We highlight the properties of the most massive star-forming galaxies, noting the wide range in spectral properties, with Lyman-α in emission or in absorption, and in imaging properties with compact, multi-component, or pair morphologies. We present the catalogue database and data products. All VUDS-DR1 data are publicly available and can be retrieved from a dedicated query-based database. Future VUDS data releases will follow this VUDS-DR1 to give access to the spectra and associated measurement of ~8000 objects in the full ~1 square degree of the VUDS survey.

Strongly baryon-dominated disk galaxies at the peak of galaxy formation ten billion years ago.

In the cold dark matter cosmology, the baryonic components of galaxies-stars and gas-are thought to be mixed with and embedded in non-baryonic and non-relativistic dark matter, which dominates the total mass of the galaxy and its dark-matter halo. In the local (low-redshift) Universe, the mass of dark matter within a galactic disk increases with disk radius, becoming appreciable and then dominant in the outer, baryonic regions of the disks of star-forming galaxies. This results in rotation velocities of the visible matter within the disk that are constant or increasing with disk radius-a hallmark of the dark-matter model. Comparisons between the dynamical mass, inferred from these velocities in rotational equilibrium, and the sum of the stellar and cold-gas mass at the peak epoch of galaxy formation ten billion years ago, inferred from ancillary data, suggest high baryon fractions in the inner, star-forming regions of the disks. Although this implied baryon fraction may be larger than in the local Universe, the systematic uncertainties (owing to the chosen stellar initial-mass function and the calibration of gas masses) render such comparisons inconclusive in terms of the mass of dark matter. Here we report rotation curves (showing rotation velocity as a function of disk radius) for the outer disks of six massive star-forming galaxies, and find that the rotation velocities are not constant, but decrease with radius. We propose that this trend arises because of a combination of two main factors: first, a large fraction of the massive high-redshift galaxy population was strongly baryon-dominated, with dark matter playing a smaller part than in the local Universe; and second, the large velocity dispersion in high-redshift disks introduces a substantial pressure term that leads to a decrease in rotation velocity with increasing radius. The effect of both factors appears to increase with redshift. Qualitatively, the observations suggest that baryons in the early (high-redshift) Universe efficiently condensed at the centres of dark-matter haloes when gas fractions were high and dark matter was less concentrated.

New analytical solutions for chemical evolution models: characterizing the population of star-forming and passive galaxies

Analytical models of chemical evolution, including inflow and outflow of gas, are important tools to study how the metal content in galaxies evolves as a function of time. In this work, we present new analytical solutions for the evolution of the gas mass, total mass and metallicity of a galactic system, when a decaying exponential infall rate of gas and galactic winds are assumed. We apply our model to characterize a sample of local star-forming and passive galaxies from the Sloan Digital Sky Survey data, with the aim of reproducing their observed mass-metallicity relation; in this way, we can derive how the two populations of star-forming and passive galaxies differ in their particular distribution of ages, formation time scales, infall masses and mass loading factors. We find that the local passive galaxies are on average older and assembled on shorter typical time-scales than the local star-forming ones; on the other hand, the larger mass star-forming galaxies show generally older ages and longer typical formation time-scales compared with the smaller mass star-forming galaxies. Finally, we conclude that the local star-forming galaxies experience stronger galactic winds than the passive galaxy population. We explore the effect of assuming different initial mass functions in our model, showing that to reproduce the observed mass-metallicity relation stronger winds are requested if the initial mass function is top-heavy. Finally, our analytical models predict the assumed sample of local galaxies to lie on a tight surface in the 3D space defined by stellar metallicity, star formation rate and stellar mass, thus mimicking the well-known "fundamental relation".

PHIBSS: exploring the dependence of the CO–H2 conversion factor on total mass surface density at z&lt;1.5

We present an analysis of the relationship between the CO-H$_{2}$ conversion factor ($\alpha_{\rm CO}$) and total mass surface density ($\Sigma_{\rm tot}$) in star-forming galaxies at $z < 1.5$. Our sample, which is drawn from the IRAM Plateau de Bure HIgh-$z$ Blue Sequence Survey (PHIBSS) and the CO Legacy Database for GASS (COLD GASS), includes 'normal,' massive star-forming galaxies that dominate the evolution of the cosmic star formation rate (SFR) at this epoch and probe the $\Sigma_{\rm tot}$ regime where the strongest variation in $\alpha_{\rm CO}$ is observed. We constrain $\alpha_{\rm CO}$ via existing CO observations, measurements of the star formation rate, and an assumed molecular gas depletion time ($t_{\rm dep}$=$M_{\rm gas}$/SFR) --- the latter two of which establish the total molecular gas mass independent of the observed CO luminosity. For a broad range of adopted depletion times, we find that $\alpha_{\rm CO}$ is independent of total mass surface density, with little deviation from the canonical Milky Way value. This runs contrary to a scenario in which $\alpha_{\rm CO}$ decreases as surface density increases within the extended clouds of molecular gas that potentially fuel clumps of star formation in $z\sim1$ galaxies, similar to those observed in local ULIRGs. Instead, our results suggest that molecular gas, both at $z\sim0$ and $z\sim1$, is primarily in the form of self-gravitating molecular clouds. While CO observations suggest a factor of $\sim3$ reduction in the average molecular gas depletion time between $z \sim 0$ and $z\sim1$, we find that, for typical galaxies, the structure of molecular gas and the process of star formation at $z \sim 1$ is otherwise remarkably similar to that observed in local star-forming systems.

The Tarantula Massive Binary Monitoring

Massive binaries (MBs) play a crucial role in the Universe. Knowing the distributions of their orbital parameters (OPs) is important for a wide range of topics, from stellar feedback to binary evolution channels, from the distribution of supernova types to gravitational wave progenitors, yet, no direct measurements exist outside the Milky Way. The Tarantula Massive Binary Monitoring was designed to help fill this gap by obtaining multi-epoch radial velocity monitoring of 102 MBs in the 30 Dor. In this paper, we analyse 32 VLT/FLAMES observations of 93 O- and 7 B-type binaries. We performed a Fourier analysis and obtained orbital solutions for 82 systems: 51 single- and 31 double-lined spectroscopic binaries. Overall, the OPs and binary fraction are remarkably similar across the 30 Dor region and compared to existing Galactic samples (GSs). This indicates that within these domains environmental effects are of second order in shaping the properties of MBs. A small difference is found in the distribution of orbital periods (OrbPs), which is slightly flatter (in log space) in 30 Dor than in the Galaxy, although this may be compatible within error estimates and differences in the fitting methodology. Also, OrbPs in 30 Dor can be as short as 1.1 d; somewhat shorter than seen in GSs. Equal mass binaries q>0.95 in 30 Dor are all found outside NGC 2070 the very young and massive cluster at 30 Dor's core. One outstanding exception however is the fact that earliest spectral types (O2-O7) tend to have shorter OrbPs than latter (O9.2-O9.7). Our results point to a relative universality of the incidence rate of MBs and their OPs in the metallicity range from solar ($Z_{\odot}$) to about $0.5Z_{\odot}$. This provides the first direct constraints on MB properties in massive star-forming galaxies at the Universes peak of star formation at redshifts z~1 to 2, which are estimated to have $Z~0.5Z_{\odot}$.

The structural and size evolution of star-forming galaxies over the last 11 Gyr

We present new results on the evolution of rest-frame blue/UV sizes and Sersic indices of H$\alpha$-selected star-forming galaxies over the last 11 Gyrs. We investigate how the perceived evolution can be affected by a range of biases and systematics such as cosmological dimming and resolution effects. We use GALFIT and an artificial redshifting technique, which includes the luminosity evolution of H$\alpha$-selected galaxies, to quantify the change on the measured structural parameters with redshift. We find typical sizes of 2 to 3 kpc and Sersic indices of n~1.2, close to pure exponential disks all the way from z=2.23 to z=0.4. At z=0 we find typical sizes of 4-5 kpc. Our results show that, when using GALFIT, cosmological dimming has a negligible impact on the derived effective radius for galaxies with <10 kpc, but we find a ~20% bias on the estimate of the median Sersic indices, rendering galaxies more disk-like. Star-forming galaxies have grown on average by a factor of 2-3 in the last 11 Gyrs with $r_e\propto(1+z)^{-0.75}$. By exploring the evolution of the stellar mass-size relation we find evidence for a stronger size evolution of the most massive star-forming galaxies since z~2, as they grow faster towards z~0 when compared to the lower stellar mass counterparts. As we are tracing the rest-frame blue/UV, we are likely witnessing the growth of disks where star formation is ongoing in galaxies while their profiles remain close to exponential disks, n<1.5, across the same period.

Giant clumps in the FIRE simulations: a case study of a massive high-redshift galaxy

The morphology of massive star-forming galaxies at high redshift is often dominated by giant clumps of mass ~10^8-10^9 Msun and size ~100-1000 pc. Previous studies have proposed that giant clumps might have an important role in the evolution of their host galaxy, particularly in building the central bulge. However, this depends on whether clumps live long enough to migrate from their original location in the disc or whether they get disrupted by their own stellar feedback before reaching the centre of the galaxy. We use cosmological hydrodynamical simulations from the FIRE (Feedback in Realistic Environments) project that implement explicit treatments of stellar feedback and ISM physics to study the properties of these clumps. We follow the evolution of giant clumps in a massive (stellar mass ~10^10.8 Msun at z=1), discy, gas-rich galaxy from redshift z>2 to z=1. Even though the clumpy phase of this galaxy lasts over a gigayear, individual gas clumps are short-lived, with mean lifetime of massive clumps of ~20 Myr. During that time, they turn between 0.1% and 20% of their gas into stars before being disrupted, similar to local GMCs. Clumps with M>10^7 Msun account for ~20% of the total star formation in the galaxy during the clumpy phase, producing ~10^10 Msun of stars. We do not find evidence for net inward migration of clumps within the galaxy. The number of giant clumps and their mass decrease at lower redshifts, following the decrease in the overall gas fraction and star-formation rate.

The metal enrichment of passive galaxies in cosmological simulations of galaxy formation

Massive early-type galaxies have higher metallicities and higher ratios of $\alpha$ elements to iron than their less massive counterparts. Reproducing these correlations has long been a problem for hierarchical galaxy formation theory, both in semi-analytic models and cosmological hydrodynamic simulations. We show that a simulation in which gas cooling in massive dark haloes is quenched by radio-mode active galactic nuclei (AGNs) feedback naturally reproduces the observed trend between $\alpha$/Fe and the velocity dispersion of galaxies, $\sigma$. The quenching occurs earlier for more massive galaxies. Consequently, these galaxies complete their star formation before $\alpha$/Fe is diluted by the contribution from type Ia supernovae. For galaxies more massive than $\sim 10^{11}~M_\odot$ whose $\alpha$/Fe correlates positively with stellar mass, we find an inversely correlated mass-metallicity relation. This is a common problem in simulations in which star formation in massive galaxies is quenched either by quasar- or radio-mode AGN feedback. The early suppression of gas cooling in progenitors of massive galaxies prevents them from recapturing enriched gas ejected as winds. Simultaneously reproducing the [$\alpha$/Fe]-$\sigma$ relation and the mass-metallicity relation is, thus, difficult in the current framework of galaxy formation.

KINEMATIC DOWNSIZING AT z ∼ 2

ABSTRACT We present results from a survey of the internal kinematics of 49 star-forming galaxies at in the CANDELS fields with the Keck/MOSFIRE spectrograph, Survey in the near-Infrared of Galaxies with Multiple position Angles (SIGMA). Kinematics (rotation velocity V rot and gas velocity dispersion ) are measured from nebular emission lines which trace the hot ionized gas surrounding star-forming regions. We find that by , massive star-forming galaxies ( ) have assembled primitive disks: their kinematics are dominated by rotation, they are consistent with a marginally stable disk model, and they form a Tully–Fisher relation. These massive galaxies have values of that are factors of 2–5 lower than local well-ordered galaxies at similar masses. Such results are consistent with findings by other studies. We find that low-mass galaxies ( ) at this epoch are still in the early stages of disk assembly: their kinematics are often dominated by gas velocity dispersion and they fall from the Tully–Fisher relation to significantly low values of V rot. This “kinematic downsizing” implies that the process(es) responsible for disrupting disks at have a stronger effect and/or are more active in low-mass systems. In conclusion, we find that the period of rapid stellar mass growth at is coincident with the nascent assembly of low-mass disks and the assembly and settling of high-mass disks.

THE ANGULAR MOMENTUM DISTRIBUTION AND BARYON CONTENT OF STAR-FORMING GALAXIES AT z ∼ 1–3*

ABSTRACT We analyze the angular momenta of massive star-forming galaxies (SFGs) at the peak of the cosmic star formation epoch (z ∼ 0.8–2.6). Our sample of ∼360 log(M */M ⊙) ∼ 9.3–11.8 SFGs is mainly based on the KMOS3D and SINS/zC-SINF surveys of Hα kinematics, and collectively provides a representative subset of the massive star-forming population. The inferred halo scale angular momentum distribution is broadly consistent with that theoretically predicted for their dark matter halos, in terms of mean spin parameter ∼ 0.037 and its dispersion (σ logλ ∼ 0.2). Spin parameters correlate with the disk radial scale and with their stellar surface density, but do not depend significantly on halo mass, stellar mass, or redshift. Our data thus support the long-standing assumption that on average, even at high redshifts, the specific angular momentum of disk galaxies reflects that of their dark matter halos (j d = j DM). The lack of correlation between λ × (j d /j DM) and the nuclear stellar density Σ*(1 kpc) favors a scenario where disk-internal angular momentum redistribution leads to “compaction” inside massive high-redshift disks. For our sample, the inferred average stellar to dark matter mass ratio is ∼2%, consistent with abundance matching results. Including the molecular gas, the total baryonic disk to dark matter mass ratio is ∼5% for halos near 1012 M ⊙, which corresponds to 31% of the cosmologically available baryons, implying that high-redshift disks are strongly baryon dominated.