HST/WFC3 CONFIRMATION OF THE INSIDE-OUT GROWTH OF MASSIVE GALAXIES AT 0 <z< 2 AND IDENTIFICATION OF THEIR STAR-FORMING PROGENITORS ATz∼ 3

Abstract

We study the structural evolution of massive galaxies by linking progenitors and descendants at a constant cumulative number density of n c = 1.4 × 10 −4 Mpc −3 to z ∼ 3. Structural parameters were measured by fitting Sérsic profiles to high-resolution CANDELS HST WFC3 J 125 and H 160 imaging in the UKIDSS-UDS at 1 < z < 3 and ACS I 814 imaging in COSMOS at 0.25 < z < 1. At a given redshift, we selected the HST band that most closely samples a common rest-frame wavelength so as to minimize systematics from color gradients in galaxies. At fixed n c , galaxies grow in stellar mass by a factor of ∼ 3 from z ∼ 3 to z ∼ 0. The size evolution is complex: galaxies appear roughly constant in size from z ∼ 3 to z ∼ 2 and then grow rapidly to lower redshifts. The evolution in the surface mass density profiles indicates that most of the mass at r < 2 kpc was in place by z ∼ 2, and that most of the new mass growth occurred at larger radii. This inside-out mass growth is therefore responsible for the larger sizes and higher Sérsic indices of the descendants toward low redshift. At z < 2, the effective radius evolves with the stellar mass as r e ∝ M 2.0 , consistent with scenarios that find dissipationless minor mergers to be a key driver of size evolution. The progenitors at z ∼ 3 were likely star forming disks with r e ∼ 2 kpc, based on their low Sérsic index of n ∼ 1, low median axis ratio of b/a ∼ 0.52, and typical location in the star-forming region of the U − V versus V − J diagram. By z ∼ 1.5, many of these star-forming disks disappeared, giving rise to compact quiescent galaxies. Toward lower redshifts, these galaxies continued to assemble mass at larger radii and became the local ellipticals that dominate the high-mass end of the mass function at the present epoch.

Sizes, colour gradients and resolved stellar mass distributions for the massive cluster galaxies in XMMUJ2235-2557 atz= 1.39

“…It is consistent with the observed inside-out growth as seen from the evolution of the stellar mass surface density profiles of passive field galaxies over redshift (e.g. van Dokkum et al 2010;Patel et al 2013). Nevertheless, traditionally this type of merger activity is believed to be suppressed in virialized clusters because of the high velocity dispersion, resulting in high relative velocities between cluster members (e.g.…”

“…This progressive growth appears to happen mainly at the outer envelopes, as several works have shown that massive (M * 1 × 10 11 M ) passive galaxies at high-redshift have comparable central densities to local ellipticals, suggesting the mass assemble took place mainly at outer radii over cosmic time (i.e. the "inside-out" growth scenario, Bezanson et al 2009;van Dokkum et al 2010;Patel et al 2013).…”

Early formation of massive, compact, spheroidal galaxies with classical profiles by violent disc instability or mergers

“…This time-scale is very similar to the halo growth, due to the fact that the stellar-to-virial mass ratio is almost time-independent in this mass and redshift range. This fraction could evolve by a factor of ∼2-3 if The dash-dotted line is the observed fit in Patel et al (2013). Bottom: Evolution of the spheroid mass fraction.…”

“…These galaxies with stellar masses of ∼10 11 M ⊙ at z = 1.5 − 2 have typical effective radii of ∼2 kpc, a factor ∼3 smaller than present-day spheroids and a factor ∼2 lower than extended discs with similar mass at the same redshift (Patel et al 2013). Therefore, the spheroids are significantly denser than other high-z galaxies.…”

“…Patel et al (2013) reported a mass growth by a factor ∼2 between z ≃ 3 and z ≃ 1. van found a similar growth in another sample of slightly more massive, and better resolved, galaxies. Therefore, observations suggest a significant and continuous mass growth for the high-z progenitors of massive galaxies before z ≃ 1.…”