Abstract
We present a new cosmological, magnetohydrodynamical simulation for galaxy formation: TNG50, the third and final instalment of the IllustrisTNG project. TNG50 evolves 2 × 21603 dark matter ...particles and gas cells in a volume 50 comoving Mpc across. It hence reaches a numerical resolution typical of zoom-in simulations, with a baryonic element mass of $8.5\times 10^4\, {\rm M}_{\odot }$ and an average cell size of 70–140 pc in the star-forming regions of galaxies. Simultaneously, TNG50 samples ∼700 (6500) galaxies with stellar masses above $10^{10} \, (10^8)\, {\rm M}_{\odot }$ at $z$ = 1. Here we investigate the structural and kinematical evolution of star-forming galaxies across cosmic time (0 ≲ $z$ ≲ 6). We quantify their sizes, disc heights, 3D shapes, and degree of rotational versus dispersion-supported motions as traced by rest-frame V-band light (i.e. roughly stellar mass) and by $\rm H\,\alpha$ light (i.e. star-forming and dense gas). The unprecedented resolution of TNG50 enables us to model galaxies with sub-kpc half-light radii and with ≲300-pc disc heights. Coupled with the large-volume statistics, we characterize a diverse, redshift- and mass-dependent structural and kinematical morphological mix of galaxies all the way to early epochs. Our model predicts that for star-forming galaxies the fraction of disc-like morphologies, based on 3D stellar shapes, increases with both cosmic time and galaxy stellar mass. Gas kinematics reveal that the vast majority of $10^{9-11.5}\, {\rm M}_{\odot }$ star-forming galaxies are rotationally supported discs for most cosmic epochs (Vrot/σ > 2–3, $z$ ≲ 5), being dynamically hotter at earlier epochs ($z$ ≳ 1.5). Despite large velocity dispersion at high redshift, cold and dense gas in galaxies predominantly arranges in disky or elongated shapes at all times and masses; these gaseous components exhibit rotationally dominated motions far exceeding the collisionless stellar bodies.
Research over the past decade has shown diminishing evidence for major galaxy mergers being a dominant mechanism for the growth of supermassive black holes (BHs) in galaxies and the triggering of ...optically or X-ray-selected active galactic nuclei (AGNs). For the first time we test whether such a connection exists at least in the most "plausible" part of parameter space: the highest specific accretion rate broad-line AGNs at the peak epoch of BH activity around z = 2. To that end we analyze two samples-21 AGNs with L/Ledd > 0.7 and 92 stellar mass- and redshift-matched inactive galaxies-observed with HST/WFC3. We remove the AGN point sources from their host galaxies and avoid bias in visual classification by adding and then subtracting mock point sources to and from the comparison galaxies, producing matched residual structures for both sets. The resulting samples are joined and visually ranked according to distortion strength by 10 experts. The ensuing individual rankings are combined into a consensus sequence and from this we derive the merger fractions for both samples. With the merger fractions fm,agn = 0.24 0.09 for the AGN host galaxy sample and fm,ina = 0.19 0.04 for the inactive galaxies, we find no significant difference between the samples. This finding is consistent with previous studies for different AGN populations, and we conclude that even BH growth at the highest specific accretion rates and at the peak of cosmic AGN activity is not predominantly caused by major mergers.
ABSTRACT
We use mock images of z = 0.1 galaxies in the 100 Mpc EAGLE simulation to establish the differences between the sizes and morphologies inferred from the stellar mass distributions and the ...optical light distributions. The optical, r-band images used were constructed with a radiative transfer method to account for the effects of dust, and we measure galaxy structural parameters by fitting Sérsic models to the images with galfit. We find that the derived half-light radii differ systematically from the stellar half-mass radii, as the r-band sizes are typically 0.1 dex larger, and can deviate by as much as ${\approx}0.5\,$ dex, depending on the dust attenuation and star formation activity, as well as the measurement method used. Consequently, we demonstrate that the r-band sizes significantly improve the agreement between the simulated and observed stellar mass–size relation: star-forming and quiescent galaxies in EAGLE are typically only slightly larger than observed (by 0.1 dex), and the slope and scatter of the local relation are reproduced well for both populations. Finally, we compare the obtained morphologies with measurements from the GAMA survey, finding that too few EAGLE galaxies have bulge-like light profiles (Sérsic indices of n ∼ 4). Despite the presence of a significant population of triaxial systems among the simulated galaxies, the surface brightness and stellar mass density profiles tend to be closer to exponential discs (n ∼ 1–2). Our results highlight the need to measure the sizes and morphologies of simulated galaxies using common observational methods in order to perform a meaningful comparison with observations.
We analyze the stellar age indicators (Dn4000 and EW(Hδ)) and sizes of 467 quiescent galaxies with M* ≥ 1010 M 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
We model the projected b/a–log a distributions of CANDELS star-forming main-sequence galaxies, where a (b) is the half-light semimajor (semiminor) axis of the galaxy images measured by ...galfit. We find that smaller a galaxies are rounder at all stellar masses M* and redshifts, so we include a when analysing b/a distributions. Approximating intrinsic shapes of the galaxies as triaxial ellipsoids and assuming a multivariate normal distribution of galaxy size and two shape parameters, we construct their intrinsic shape and size distributions to obtain the fractions of elongated (prolate), discy (oblate), and spheroidal galaxies in each redshift and mass bin. We find that galaxies tend to be prolate at low M* and high redshifts, and discy at high M* and low redshifts, qualitatively consistent with van der Wel et al., implying that galaxies tend to evolve from prolate to discy. These results are consistent with the predictions from simulations that the transition from prolate to oblate is caused by a compaction event at a characteristic mass range, making the galaxy centre baryon dominated. We give probabilities of a galaxy’s being elongated, discy, or spheroidal as a function of its M*, redshift, and projected b/a and a, which can facilitate target selections of galaxies with specific shapes at high redshifts.
ABSTRACT In this paper we study a key phase in the formation of massive galaxies: the transition of star-forming galaxies into massive (Mstars ∼ 1011M ), compact (re ∼ 1 kpc) quiescent galaxies, ...which takes place from z ∼ 3 to z ∼ 1.5. We use HST grism redshifts and extensive photometry in all five 3D-HST/CANDELS fields, more than doubling the area used previously for such studies, and combine these data with Keck MOSFIRE and NIRSPEC spectroscopy. We first confirm that a population of massive, compact, star-forming galaxies exists at z 2, using K-band spectroscopy of 25 of these objects at 2.0 < z < 2.5. They have a median N ii/H ratio of 0.6, are highly obscured with SFR(tot)/SFR(H ) ∼10, and have a large range of observed line widths. We infer from the kinematics and spatial distribution of H that the galaxies have rotating disks of ionized gas that are a factor of ∼2 more extended than the stellar distribution. By combining measurements of individual galaxies, we find that the kinematics are consistent with a nearly Keplerian fall-off from Vrot ∼ 500 km s−1 at 1 kpc to Vrot ∼ 250 km s−1 at 7 kpc, and that the total mass out to this radius is dominated by the dense stellar component. Next, we study the size and mass evolution of the progenitors of compact massive galaxies. Even though individual galaxies may have had complex histories with periods of compaction and mergers, we show that the population of progenitors likely followed a simple inside-out growth track in the size-mass plane of This mode of growth gradually increases the stellar mass within a fixed physical radius, and galaxies quench when they reach a stellar density or velocity dispersion threshold. As shown in other studies, the mode of growth changes after quenching, as dry mergers take the galaxies on a relatively steep track in the size-mass plane.
ABSTRACT
We reliably extend the stellar mass–size relation over 0.2 ≤ z ≤ 2 to low stellar mass galaxies by combining the depth of Hubble Frontier Fields with the large volume covered by CANDELS. ...Galaxies are simultaneously modelled in multiple bands using the tools developed by the MegaMorph project, allowing robust size (i.e. half-light radius) estimates even for small, faint, and high redshift galaxies. We show that above 107 M⊙, star-forming galaxies are well represented by a single power law on the mass–size plane over our entire redshift range. Conversely, the stellar mass–size relation is steep for quiescent galaxies with stellar masses $\ge 10^{10.3}\, {\rm M}_\odot$ and flattens at lower masses, regardless of whether quiescence is selected based on star-formation activity, rest-frame colours, or structural characteristics. This flattening occurs at sizes of ∼1 kpc at z ≤ 1. As a result, a double power law is preferred for the stellar mass–size relation of quiescent galaxies, at least above 10$^7\, {\rm M}_\odot$. We find no strong redshift dependence in the slope of the relation of star-forming galaxies as well as of high mass quiescent galaxies. We also show that star-forming galaxies with stellar masses $\ge 10^{9.5}\, {\rm M}_\odot$ and quiescent galaxies with stellar masses $\ge 10^{10.3}\, {\rm M}_\odot$ have undergone significant size growth since z ∼ 2, as expected; however, low mass galaxies have not. Finally, we supplement our data with predominantly quiescent dwarf galaxies from the core of the Fornax cluster, showing that the stellar mass–size relation is continuous below 10$^7\, {\rm M}_\odot$, but a more complicated functional form is necessary to describe the relation.
Abstract
Dynamical models for 673 galaxies at
z
= 0.6–1.0 with spatially resolved (long-slit) stellar kinematic data from LEGA-C are used to calibrate virial mass estimates defined as
M
vir
=
K
σ
′
⋆
...,
int
2
R
, with
K
a scaling factor,
σ
′
⋆
,
int
the spatially integrated stellar velocity second moment from the LEGA-C survey, and
R
the effective radius measured from a Sérsic profile fit to Hubble Space Telescope imaging. The sample is representative for
M
⋆
> 3 × 10
10
M
⊙
and includes all types of galaxies, irrespective of morphology and color. We demonstrate that using
R
=
R
sma
(the semimajor axis length of the ellipse that encloses 50% of the light) in combination with an inclination correction on
σ
′
⋆
,
int
produces an unbiased
M
vir
. We confirm the importance of projection effects on
σ
′
⋆
,
int
by showing the existence of a similar residual trend between virial mass estimates and inclination for the nearby early-type galaxies in the ATLAS
3D
survey. Also, as previously shown, when using a Sérsic profile-based
R
estimate, a Sérsic index-dependent correction to account for nonhomology in the radial profiles is required. With respect to analogous dynamical models for low-redshift galaxies from the ATLAS
3D
survey we find a systematic offset of 0.1 dex in the calibrated virial constant for LEGA-C, which may be due to physical differences between the galaxy samples or an unknown systematic error. Either way, with our work we establish a common mass scale for galaxies across 8 Gyr of cosmic time with a systematic uncertainty of at most 0.1 dex.
We analyze the colors and sizes of 32 quiescent (UVJ-selected) galaxies with strong Balmer absorption (EW(Hδ) ≥ 4 ) 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
We present H
α
maps at 1 kpc spatial resolution for star-forming galaxies at
z
∼ 1, made possible by the Wide Field Camera 3 grism on
Hubble Space Telescope
(
HST
). Employing this ...capability over all five 3D-
HST
/CANDELS fields provides a sample of 3200 galaxies enabling a division into subsamples based on stellar mass and star formation rate (SFR). By creating deep stacked H
α
images, we reach surface brightness limits of 1 × 10
−18
erg s
−1
cm
−2
arcsec
−2
, allowing us to map the distribution of ionized gas to ∼10 kpc for typical
L
* galaxies at this epoch. We find that the spatial extent of the H
α
distribution increases with stellar mass as
kpc. The H
α
emission is more extended than the stellar continuum emission, consistent with inside-out assembly of galactic disks. This effect grows stronger with mass as
. We map the H
α
distribution as a function of SFR(IR+UV) and find evidence for “coherent star formation” across the SFR–
M
*
plane: above the main sequence (MS), H
α
is enhanced at all radii; below the MS, H
α
is depressed at all radii. This suggests that at all masses the physical processes driving the enhancement or suppression of star formation act throughout the disks of galaxies. At high masses (
), above the MS, H
α
is particularly enhanced in the center, potentially building bulges and/or supermassive black holes. Below the MS, a strong central dip in the EW(H
α
), as well as the inferred specific SFR, appears. Importantly, though, across the entirety of the SFR–
M
*
plane, the absolute SFR as traced by H
α
is always centrally peaked, even in galaxies below the MS.
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