Size Growth Of Early-type Galaxies Research Articles

Evidence for radially independent size growth of early-type galaxies in clusters

It is not well understood whether the growth of early-type cluster galaxies proceeds inside-out, outside-in, or at the same pace at all radii. In this work we measured the galaxy size, defined by the radius including 80% of the galaxy light, non-parametrically. We also determined a non-parametric estimate of galaxy light concentration, which measures the curvature of the surface brightness profile in the galaxy outskirts. We used an almost random sampling of a mass-limited sample formed by 128 morphologically early-type galaxies in clusters with log M/M⊙ ≳ 10.7 spanning the wide range 0.17 < z < 1.81. From these data we derived the size-mass and concentration-mass relations, as well as their evolution. At 80% light radius, early-type galaxies in clusters are about 2.7 times larger than at 50% radius at all redshifts, and close to de Vaucouleurs profiles in the last 10 Gyr. While between z = 2 and z = 0 both half-light and 80% light sizes increase by a factor of 1.7, concentration stays constant within 2%, that is to say the size growth of early-type galaxies in cluster environments proceeds at the same pace at both radii. Existing physical explanations proposed in the literature are inconsistent with our results, demonstrating the need for dedicated numerical simulations to identify the physical mechanism affecting the galaxy structure.

The cosmic epoch dependence of environmental effects on size evolution of red-sequence early-type galaxies

This work aims to observationally investigate the history of size growth of early-type galaxies and how the growth depends on cosmic epoch and the mass of the halo in which they are embedded. We carried out a photometric and structural analysis in the rest-frameVband of a mass-selected (logM/M⊙> 10.7) sample of red-sequence early-type galaxies with spectroscopic/grism redshift in the general field up toz= 2 to complement a previous work presenting an identical analysis but in halos 100 times more massive and 1000 times denser. We homogeneously derived sizes (effective radii) fully accounting for the multi-component nature of galaxies and the common presence of isophote twists and ellipticity gradients. By using these mass-selected samples, composed of 170 red-sequence early-type galaxies in the general field and 224 identically selected and analyzed in clusters, we isolate the effect on galaxy sizes of the halo in which galaxies are embedded and its dependence on epoch. We find that the log of the galaxy size at a fixed stellar mass, logM/M⊙= 11, has increased with epoch at a rate twice as fast in the field than in cluster in the last 10 Gyr (0.26 ± 0.03 versus 0.13 ± 0.02 dex per unit redshift). Red-sequence early-type galaxies in the general field reached the size of their cousins in denser environment byz= 0.25 ± 0.13 in spite of being three times smaller atz∼ 2. Data point toward a model where size growth is epoch-independent (i.e., ∂ logre/∂z=c), but with a ratecdepending on environment, ∂c/∂ logMhalo≈ 0.05. Environment determines the growth rate (d logre/dz) at all redshifts, indicating an external origin for the galaxy growth without any clear epoch where it ceases to have an effect. The larger size of early-type galaxies in massive halos at high redshift indicates that their size grew buildup earlier (atz> 2) at an accelerated rate, slowing down at some still unidentifiedz> 2 redshift. Instead, the size growth rate of red-sequence early-type galaxies in low-mass halos is reversed: it proceeds at an increased rate at late epochs after an early period (z> 2) of reduced growth, in agreement with the qualitative hierarchical picture of galaxy evolution. We found similar values of scatter around the mass-size relation independently of environment and epoch, indicating that the amount of dissipation in the system forming the observed galaxy does not vary greatly with epoch or environment.

Mass and size growth of early-type galaxies by dry mergers in cluster environments

We perform dry merger simulations to investigate the role of dry mergers in the size growth of early-type galaxies in high density environments. We replace the virialized dark matter haloes obtained by a large cosmological $N$-body simulation with $N$-body galaxy models consisting of two components, a stellar bulge and a dark matter halo, which have higher mass resolution than the cosmological simulation. We then re-simulate nine cluster forming regions, whose masses range from 1e+14 Msun to 5e+14 Msun. Masses and sizes of stellar bulges are also assumed to satisfy the stellar mass--size relation of high-z compact massive early-type galaxies. We find that dry major mergers considerably contribute to the mass and size growth of central massive galaxies. One or two dry major mergers double the average stellar mass and quadruple the average size between $z=2$ and $z=0$. These growths favorably agree with observations. Moreover, the density distributions of our simulated central massive galaxies grow from the inside-out, which is consistent with recent observations. The mass--size evolution is approximated as R propto M_{*}^{alpha}, with alpha ~ 2.24. Most of our simulated galaxies are efficiently grown by dry mergers, and their stellar mass--size relations match the ones observed in the local Universe. Our results show that the central galaxies in the cluster haloes are potential descendants of high-z (z ~ 2-3) compact massive early-type galaxies. This conclusion is consistent with previous numerical studies which investigate the formation and evolution of compact massive early-type galaxies.

FORMING EARLY-TYPE GALAXIES IN ΛCDM SIMULATIONS. I. ASSEMBLY HISTORIES

We present a sample of nine high resolution cosmological simulations in the mass range of M_vir=7x10^11-4x10^12 M_sun starting from LambdaCDM initial conditions. Our simulations include primordial radiative cooling, photoionization, star formation, supernova II feedback, but exclude supernova driven winds and AGN feedback. The simulated galaxies assemble in two phases, with the initial growth dominated by compact (r<r_eff) in situ star formation fueled by cold, low entropy gas streams resulting in a very similar mean assembly redshift of z_{f,ins}~2.5 for the in situ stellar component in all galaxies. The late growth is dominated by accretion of old stars formed in subunits outside the main galaxy (r>r_eff) resulting in an assembly redshift of z_{f,acc}~0.5-1.5 with much larger scatter. We find a positive correlation between the fraction of accreted stars and the final mass of our galaxies. We show that gravitational feedback strongly suppresses late star formation in massive galaxies contributing to the observed galaxy color bimodality. The accretion of stellar material is also responsible for the observed size growth of early-type galaxies. In addition, we find that the dark matter fractions within the stellar half-mass radii continuously increase towards lower redshift from about f_DM~0.05 at z~3 to f_DM~0.1-0.3 at z=0. Furthermore, the logarithmic slope of the total density profile is nearly isothermal at the present-day (gamma'~1.9-2.2). Finally, the input of gravitational heating lowers the central dark matter densities in the galaxies, with the effect being smaller compared to simulations without supernova feedback.