CANDELS: The progenitors of compact quiescent galaxies at z~2

The Astrophysical Journal (Impact Factor: 5.99). 06/2012; 765(2). DOI: 10.1088/0004-637X/765/2/104
Source: arXiv


We combine high-resolution HST/WFC3 images with multi-wavelength photometry
to track the evolution of structure and activity of massive (log(M*) > 10)
galaxies at redshifts z = 1.4 - 3 in two fields of the Cosmic Assembly
Near-infrared Deep Extragalactic Legacy Survey (CANDELS). We detect compact,
star-forming galaxies (cSFGs) whose number densities, masses, sizes, and star
formation rates qualify them as likely progenitors of compact, quiescent,
massive galaxies (cQGs) at z = 1.5 - 3. At z > 2 most cSFGs have specific
star-formation rates (sSFR = 10^-9 yr^-1) half that of typical, massive SFGs at
the same epoch, and host X-ray luminous AGN 30 times (~30%) more frequently.
These properties suggest that cSFGs are formed by gas-rich processes (mergers
or disk-instabilities) that induce a compact starburst and feed an AGN, which,
in turn, quench the star formation on dynamical timescales (few 10^8 yr). The
cSFGs are continuously being formed at z = 2 - 3 and fade to cQGs by z = 1.5.
After this epoch, cSFGs are rare, thereby truncating the formation of new cQGs.
Meanwhile, down to z = 1, existing cQGs continue to enlarge to match local QGs
in size, while less-gas-rich mergers and other secular mechanisms shepherd
(larger) SFGs as later arrivals to the red sequence. In summary, we propose two
evolutionary scenarios of QG formation: an early (z > 2), fast-formation path
of rapidly-quenched cSFGs that evolve into cQGs that later enlarge within the
quiescent phase, and a slow, late-arrival (z < 2) path for SFGs to form QGs
without passing through a compact state.

Download full-text


Available from: Nimish P. Hathi, Dec 23, 2013
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We study the (M,sigma) and (M,Re) projections of the thin Mass Plane (MP) (M,sigma,Re) which describes the distribution of the galaxy population. The distribution of galaxy properties on the MP is characterized by: (i) a zone of exclusion described by two power-laws joined by a break at M 3e10 Msun. This results in a break in the mean M-sigma relation with Msigma^2.3 and M sigma^4.7 at small and large sigma respectively; (ii) a mass M 2e11 Msun which separates a population dominated by flat fast rotator with disks and spiral galaxies at lower masses, from one dominated by quite round slow rotators at larger masses; (iii) below that mass the distribution of ETGs properties tends to be constant along lines of constant sigma; (iv) it forms a parallel sequence with the distribution of spiral galaxies; (v) at even lower masses, the distribution of fast rotator ETGs and late spirals naturally extends to that of dwarf ETGs (Sph) and dwarf irregulars (Im) respectively. We show via dynamical models that sigma traces the bulge fraction, which drives the observed trends in M/L, Hbeta, colour, IMF and molecular gas fraction. We interpret this as due to a combination of two main effects: (i) an increase of the bulge fraction which increases sigma and greatly enhances the likelihood for a galaxy to have its star formation quenched, and (ii) dry merging, increasing galaxy mass along lines of nearly constant sigma, while leaving the population unchanged. [Abriged]
    Monthly Notices of the Royal Astronomical Society 08/2012; 432(3). DOI:10.1093/mnras/stt644 · 5.11 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Abridged. We use COSMOS to study in a self-consistent way the change in the number densities of quenched early-type galaxies (Q-ETGs) of a given size over the interval 0.2 < z < 1.0 to study the claimed size evolution of these galaxies. At 10^10.5<Mgalaxy<10^11 Msun, we see no change in the number density of compact Q-ETGs, while at >10^11 Msun we find a decrease by 30%. In both mass bins, the increase of the median sizes of Q-ETGs with time is primarily caused by the addition to the size function of larger and more diffuse Q-ETGs. At all masses, compact Q-ETGs become systematically redder towards later epochs, with a (U-V) difference consistent with passive evolution of their stellar populations, indicating that they are a population that does not appreciably evolve in size. At all epochs, the larger Q-ETGs (at least in the lower mass bin) have average rest-frame colors systematically bluer than those of the more compact Q-ETGs, suggesting that the former are younger than the latter. The idea that new, large, Q-ETGs are responsible for the observed growth in the median size of the population at a given mass is supported by the sizes and number of the star-forming galaxies that are expected to be progenitors of the new Q-ETGs over the same period. In the low mass bin, the new Q-ETG have 30% smaller sizes than their star-forming progenitors. This is likely due to the fading of their disks after they cease star-formation. Comparison with higher z shows that the median size of newly-quenched galaxies roughly scales, at constant mass, as (1+z)^-1. The dominant cause of the size evolution seen in the Q-ETG population is thus that the average sizes of individual Q-ETGs scale with the average density of the Universe at the time when they were quenched, with subsequent size changes in individual objects through eg merging of secondary importance, especially at masses <10^11 Msun.
    The Astrophysical Journal 02/2013; 773(2). DOI:10.1088/0004-637X/773/2/112 · 5.99 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We measure the evolution of the galaxy stellar mass function from z=1.3 to z=0.5 using the first 53,608 redshifts of the ongoing VIMOS Public Extragalactic Survey (VIPERS). We estimate the galaxy stellar mass function at several epochs discussing in detail the amount of cosmic variance affecting our estimate. We find that Poisson noise and cosmic variance of the galaxy mass function in the VIPERS survey are comparable with the statistical uncertainties of large surveys in the local universe. VIPERS data allow us to determine with unprecedented accuracy the high-mass tail of the galaxy stellar mass function, which includes a significant number of galaxies that are usually too rare to detect with any of the past spectroscopic surveys. At the epochs sampled by VIPERS, massive galaxies had already assembled most of their stellar mass. We apply a photometric classification in the (U-V) rest-frame colour to compute the mass function of blue and red galaxies, finding evidence for the evolution of their contribution to the total number density budget: the transition mass above which red galaxies dominate is found to be about 10^10.4 M_sun at z=0.55 and evolves proportionally to (1+z)^3. We are able to trace separately the evolution of the number density of blue and red galaxies with masses above 10^11.4 M_sun, in a mass range barely studied in previous work. We find that for such large masses, red galaxies show a milder evolution with redshift, when compared to objects at lower masses. At the same time, we detect a population of similarly massive blue galaxies, which are no longer detectable below z=0.7. These results show the improved statistical power of VIPERS data, and give initial promising indications of mass-dependent quenching of galaxies at z~1. [Abridged]
    Astronomy and Astrophysics 03/2013; 558. DOI:10.1051/0004-6361/201321511 · 4.38 Impact Factor
Show more