We study the scaling relations between global properties of dwarf galaxies in the Local Group. In addition to quantifying the correlations between pairs of variables, we explore the "shape" of the ...distribution of galaxies in log parameter space using standardised Principal Component Analysis (PCA), the analysis is performed first in the 3-D structural parameter space of stellar mass M*, internal velocity V and characteristic radius R* (or surface brightness mu*). It is then extended to a 4-D space that includes a stellar-population parameter such as metallicity Z or star formation rate SFR. We find that the Local-Group dwarfs basically define a one-parameter "Fundamental Line" (FL), primarily driven by stellar mass, M*. A more detailed inspection reveals differences between the star-formation properties of dwarf irregulars (dI's) and dwarf ellipticals (dE's), beyond the tendency of the latter to be more massive. In particular, the metallicities of dI's are typically lower by a factor of 3 at a given M* and they grow faster with increasing M*, showing a tighter FL in the 4-D space for the dE's. The structural scaling relations of dI's resemble those of the more massive spirals, but the dI's have lower star formation rates for a given M* which also grow faster with increasing M*. On the other hand, the FL of the dE's departs from the fundamental plane of bigger ellipticals. While the one-parameter nature of the FL and the associated slopes of the scaling relations are consistent with the general predictions of supernova feedback (Dekel & Woo 2003), the differences between the FL's of the dE's and the dI's remain a challenge and should serve as a guide for the secondary physical processes responsible for these two types.
We examine the fraction of massive (\(M_{*}>10^{10} M_{\odot}\)), compact star-forming galaxies (cSFGs) that host an active galactic nucleus (AGN) at \(z\sim2\). These cSFGs are likely the direct ...progenitors of the compact quiescent galaxies observed at this epoch, which are the first population of passive galaxies to appear in large numbers in the early Universe. We identify cSFGs that host an AGN using a combination of Hubble WFC3 imaging and Chandra X-ray observations in four fields: the Chandra Deep Fields, the Extended Groth Strip, and the UKIDSS Ultra Deep Survey field. We find that \(39.2^{+3.9}_{-3.6}\)\% (65/166) of cSFGs at \(1.4<z<3.0\) host an X-ray detected AGN. This fraction is 3.2 times higher than the incidence of AGN in extended star-forming galaxies with similar masses at these redshifts. This difference is significant at the \(6.2\sigma\) level. Our results are consistent with models in which cSFGs are formed through a dissipative contraction that triggers a compact starburst and concurrent growth of the central black hole. We also discuss our findings in the context of cosmological galaxy evolution simulations that require feedback energy to rapidly quench cSFGs. We show that the AGN fraction peaks precisely where energy injection is needed to reproduce the decline in the number density of cSFGs with redshift. Our results suggest that the first abundant population of massive, quenched galaxies emerged directly following a phase of elevated supermassive black hole growth and further hints at a possible connection between AGN and the rapid quenching of star formation in these galaxies.
This is the first in a series of papers examining the demographics of
star-forming galaxies at $0.2<z<2.5$ in CANDELS. We study 9,100 galaxies from
GOODS-S and UDS having published values of ...redshifts, masses, star-formation
rates (SFRs), and dust attenuation ($A_V$) derived from UV-optical SED fitting.
In agreement with previous works, we find that the $UVJ$ colors of a galaxy are
closely correlated with its specific star-formation rate (SSFR) and $A_V$. We
define rotated $UVJ$ coordinate axes, termed $S_\mathrm{SED}$ and
$C_\mathrm{SED}$, that are parallel and perpendicular to the star-forming
sequence and derive a quantitative calibration that predicts SSFR from
$C_\mathrm{SED}$ with an accuracy of ~0.2 dex. SFRs from UV-optical fitting and
from UV+IR values based on Spitzer/MIPS 24 $\mu\mathrm{m}$ agree well overall,
but systematic differences of order 0.2 dex exist at high and low redshifts. A
novel plotting scheme conveys the evolution of multiple galaxy properties
simultaneously, and dust growth, as well as star-formation decline and
quenching, exhibit "mass-accelerated evolution" ("downsizing"). A population of
transition galaxies below the star-forming main sequence is identified. These
objects are located between star-forming and quiescent galaxies in $UVJ$ space
and have lower $A_V$ and smaller radii than galaxies on the main sequence.
Their properties are consistent with their being in transit between the two
regions. The relative numbers of quenched, transition, and star-forming
galaxies are given as a function of mass and redshift.
We study the correlation of galaxy structural properties with their location relative to the SFR-M* correlation, also known as the star formation "main sequence" (SFMS), in the CANDELS and GAMA ...surveys and in a semi-analytic model (SAM) of galaxy formation. We first study the distribution of median Sersic index, effective radius, star formation rate (SFR) density and stellar mass density in the SFR-M* plane. We then define a redshift dependent main sequence and examine the medians of these quantities as a function of distance from this main sequence, both above (higher SFRs) and below (lower SFRs). Finally, we examine the distributions of distance from the main sequence in bins of these quantities. We find strong correlations between all of these galaxy structural properties and the distance from the SFMS, such that as we move from galaxies above the SFMS to those below it, we see a nearly monotonic trend towards higher median Sersic index, smaller radius, lower SFR density, and higher stellar density. In the semi-analytic model, bulge growth is driven by mergers and disk instabilities, and is accompanied by the growth of a supermassive black hole which can regulate or quench star formation via Active Galactic Nucleus (AGN) feedback. We find that our model qualitatively reproduces the trends described above, supporting a picture in which black holes and bulges co-evolve, and AGN feedback plays a critical role in moving galaxies off of the SFMS.
We challenge a widely accepted assumption of observational cosmology: that successful reconstruction of observed galaxy density fields from measured galaxy velocity fields (or vice versa), using the ...methods of gravitational instability theory, implies that the observed large-scale structures and large-scale flows were produced by the action of gravity. This assumption is false, in that there exist nongravitational theories that pass the reconstruction tests and gravitational theories with certain forms of biased galaxy formation that fail them. Gravitational instability theory predicts specific correlations between large-scale velocity and mass density fields, but the same correlations arise in any model where (a) structures in the galaxy distribution grow from homogeneous initial conditions in a way that satisfies the continuity equation, and (b) the present-day velocity field is irrotational and proportional to the time-averaged velocity field. We demonstrate these assertions using analytical arguments and N-body simulations. If large-scale structure is formed by gravitational instability, then the ratio of the galaxy density contrast to the divergence of the velocity field yields an estimate of the density parameter Omega (or, more generally, an estimate of beta identically equal to Omega(exp 0.6)/b, where b is an assumed constant of proportionality between galaxy and mass density fluctuations. In nongravitational scenarios, the values of Omega or beta estimated in this way may fail to represent the true cosmological values. However, even if nongravitational forces initiate and shape the growth of structure, gravitationally induced accelerations can dominate the velocity field at late times, long after the action of any nongravitational impulses. The estimated beta approaches the true value in such cases, and in our numerical simulations the estimated beta values are reasonably accurate for both gravitational and nongravitational models. Reconstruction tests that show correlations between galaxy density and velocity fields can rule out some physically interesting models of large-scale structure. In particular, successful reconstructions constrain the nature of any bias between the galaxy and mass distributions, since processes that modulate the efficiency of galaxy formation on large scales in a way that violates the continuity equation also produce a mismatch between the observed galaxy density and the density inferred from the peculiar velocity field. We obtain successful reconstructions for a gravitational model with peaks biasing, but we also show examples of gravitational and nongravitational models that fail reconstruction tests because of more complicated modulations of galaxy formation.
We analyze a suite of 33 cosmological simulations of the evolution of Milky Way-mass galaxies in low-density environments. Our sample spans a broad range of Hubble types at z=0, from nearly bulgeless ...disks to bulge-dominated galaxies. Despite the fact that a large fraction of the bulge is typically in place by z=1, we find no significant correlation between the morphology at z=1 and at z=0. The z=1 progenitors of disk galaxies span a range of morphologies, including smooth disks, unstable disks, interacting galaxies and bulge-dominated systems. By z=0.5, spiral arms and bars are largely in place and the progenitor morphology is correlated with the final morphology. We next focus on late-type galaxies with a bulge-to-total ratio B/T<0.3 at z=0. These show a correlation between B/T at z=0 and the mass ratio of the largest merger at z<2, as well as with the gas accretion rate at z>1. We find that the galaxies with the lowest B/T tend to have a quiet baryon input history, with no major mergers at z<2, and with a low and constant gas accretion rate that keeps a stable angular-momentum direction. More violent merger or gas accretion histories lead to galaxies with more prominent bulges. Most disk galaxies have a bulge Sersic index n<2. The galaxies with the highest bulge Sersic index tend to have histories of intense gas accretion and disk instability rather than active mergers.
Mon.Not.Roy.Astron.Soc.345:349-364,2003 We investigate the conditions for the existence of an expanding virial shock
in the gas falling within a spherical dark-matter halo. The shock relies on
...pressure support by the shock-heated gas behind it. When the radiative cooling
is efficient compared to the infall rate the post-shock gas becomes unstable;
it collapses inwards and cannot support the shock.
We find for a monoatomic gas that the shock is stable when the post-shock
pressure and density obey gamma effective>10/7, with gamma effective begin the
time depended equivalent to the adiabatic index. We express the effective gamma
in terms of r, u and rho at the shock to obtain a simple condition for shock
stability. This result is confirmed by hydrodynamical simulations, using an
accurate spheri-symmetric Lagrangian code. When the stability analysis is
applied in cosmology, we find that a virial shock does not develop in most
haloes that form before z ~ 2, and it never forms in haloes less massive than a
few 10^11 solar masses. In such haloes the infalling gas is never heated to the
virial temperature, and it does not need to cool radiatively before it drops
into a disc. Instead, the gas collapses at T ~ 10^4K directly into the disc.
This should have nontrivial effects on the star-formation rate and on the gas
removal by supernova-driven winds. Instead of radiating soft x rays, this gas
would emit lyman alpha thus helping explain the low flux of soft x-ray
background and the lyman alpha emitters observed at high redshift.
I address the following issues: All bulk velocity measurements (but one) are consistent with our standard gravitational instability theory. New accurate data and reconstruction methods allow ...high-resolution dynamical analysis nearby, revealing Virgo, Ursa Major and Fornax as attractors. Large peculiar-velocity surveys enable robust reconstruction of the dynamical fields on the Great-Attractor scale. A decomposition of the velocity field into its local and tidal components indicates the presence of big perturbations further away. Cluster velocities start exploring very large scales, revealing Coma, Shapely and other mass enhancements, and constraining a possible local Hubble bubble. Supernovae type Ia (SNIa) are very promising for cosmic flow analysis. Peculiar velocities do provide unique valuable constraints on cosmological parameters, e.g., 0.3<Omega_m<1 (95% confidence) independent of biasing. Jointly with other data they can confine other parameters such as Omega_Lambda, h, sigma_8, n, and the biasing. Nontrivial features of the biasing scheme can explain much of the span of estimates for beta. Quantitative error analysis is essential in our maturing field; every method ought to be calibrated with suitable mock catalogs, that are offered as benchmarks.
2003, MNRAS, 344, 1131 We study in simple terms the role of feedback in establishing the scaling
relations of low-surface-brightness and dwarf galaxies with stellar masses in
the range 6x10^5 <M*< ...3x10^10 Msun. These galaxies, as measured from SDSS and
in the Local Group, show tight correlations of internal velocity, metallicity
and surface brightness (or radius) with M*. They define a fundamental line
distinguishing them from the brighter galaxies of high surface brightness and
metallicity. The idealized model assumes spherical collapse of CDM haloes to
virial equilibrium and angular-momentum conservation. The relations for bright
galaxies are reproduced by assuming that M* is a constant fraction of the halo
mass M. The upper bound to the low-luminosity LSBs coincides with the virial
velocity of haloes in which supernova feedback could significantly suppress
star formation, V<100km/s (Dekel & Silk 1986). We argue that the energy fed to
the gas obeys E_sn \prop M* despite the radiative losses, and equate it with
the binding energy of the gas to obtain M*/M \prop V^2. This idealized model
provides surprisingly good fits to the scaling relations of low-luminosity LSBs
and dwarfs, which indicates that supernova feedback had a primary role in
determining the fundamental line. The apparent lower bound for galaxies at
V~10km/s may be due to the cooling barrier at T~10^4 K. Some fraction of the
dark haloes may show no stars due to complete gas removal either by supernova
winds from neighboring galaxies or by radiative feedback after cosmological
reionization at z_ion. Radiative feedback may also explain the distinction
between dwarf spheroidals (dE) and irregulars (dI), where the dEs of V<30km/s
form stars before z_ion and are then cleaned out of gas, while the dIs with
V>30km/s retain gas-rich discs with feedback-regulated star formation.
We examine the spheroid growth and star formation quenching experienced by galaxies from z~3 to the present by studying the evolution with redshift of the quiescent and spheroid-dominated fractions ...of galaxies from the CANDELS and GAMA surveys. We compare the observed fractions with predictions from a semi-analytic model which includes prescriptions for bulge growth and AGN feedback due to mergers and disk instabilities. We facilitate direct morphological comparison by converting our model bulge-to-total stellar mass ratios to Sersic indices. We then subdivide our population into the four quadrants of the sSFR-Sersic index plane and study the buildup of each of these subpopulations. We find that the fraction of star forming disks declines steadily, while the fraction of quiescent spheroids builds up over cosmic time. The fractions of star forming spheroids and quiescent disks are both non-negligible, and stay nearly constant over the period we have studied, at about 10% and 15-20% respectively. Our model is qualitatively successful at reproducing the evolution of the two "main" populations (star forming disk-dominated galaxies and quiescent spheroid-dominated galaxies), and approximately reproduces the relative fractions of all four types, but predicts a stronger decline in star forming spheroids, and increase in quiescent disks, than seen in the observations. A model with an additional channel for bulge growth via disk instabilities agrees better overall with the observations than a model in which bulges may grow only through mergers. We study evolutionary tracks of some individual galaxies as they experience morphological transformation and quenching, and examine the importance of different physical drivers of this transformation (major and minor mergers and disk instabilities). We find that complex histories with multiple transformative events are the norm.
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