Article

# Insights on the Formation, Evolution, and Activity of Massive Galaxies From Ultra-Compact and Disky Galaxies at z=2-3

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(Impact Factor: 5.99). 12/2011; 743(1). DOI: 10.1088/0004-637X/743/1/87

ABSTRACT

We present our results on the structure and activity of massive galaxies at z = 1-3 using one of the largest (166 with M-star >= 5 x 10(10) M-circle dot) and most diverse samples of massive galaxies derived from the GOODS-NICMOS survey: (1) Sersic fits to deep NIC3 F160W images indicate that the rest-frame optical structures of massive galaxies are very different at z = 2-3 compared to z similar to 0. Approximately 40% of massive galaxies are ultracompact (r(e) <= 2 kpc), compared to less than 1% at z similar to 0. Furthermore, most (similar to 65%) systems at z = 2-3 have a low Sersic index n <= 2, compared to similar to 13% at z similar to 0. We present evidence that the n <= 2 systems at z = 2-3 likely contain prominent disks, unlike most massive z similar to 0 systems. (2) There is a correlation between structure and star formation rates (SFRs). The majority (similar to 85%) of non-active galactic nucleus (AGN) massive galaxies at z = 2-3, with SFR high enough to yield a 5 sigma (30 mu Jy) 24 mu m Spitzer detection, have low n <= 2. Such n <= 2 systems host the highest SFR. (3) The frequency of AGNs is similar to 40% at z = 2-3. Most (similar to 65%) AGN hosts have disky (n <= 2) morphologies. Ultracompact galaxies appear quiescent in terms of both AGN activity and star formation. (4) Large stellar surface densities imply massive galaxies at z = 2-3 formed via rapid, highly dissipative events at z > 2. The large fraction of n <= 2 disky systems suggests cold mode accretion complements gas-rich major mergers at z > 2. In order for massive galaxies at z = 2-3 to evolve into present-day massive E/S0s, they need to significantly increase (n, r(e)). Dry minor and major mergers may play an important role in this process.

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##### Article: The cosmological size and velocity dispersion evolution of massive early-type galaxies
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ABSTRACT: We analyze 40 cosmological re-simulations of individual massive galaxies with present-day stellar masses of $M_{*} > 6.3 \times 10^{10} M_{\odot}$ in order to investigate the physical origin of the observed strong increase in galaxy sizes and the decrease of the stellar velocity dispersions since redshift $z \approx 2$. At present 25 out of 40 galaxies are quiescent with structural parameters (sizes and velocity dispersions) in agreement with local early type galaxies. At z=2 all simulated galaxies with $M_* \gtrsim 10^{11}M_{\odot}$ (11 out of 40) at z=2 are compact with projected half-mass radii of $\approx$ 0.77 ($\pm$0.24) kpc and line-of-sight velocity dispersions within the projected half-mass radius of $\approx$ 262 ($\pm$28) kms$^{-1}$ (3 out of 11 are already quiescent). Similar to observed compact early-type galaxies at high redshift the simulated galaxies are clearly offset from the local mass-size and mass-velocity dispersion relations. Towards redshift zero the sizes increase by a factor of $\sim 5-6$, following $R_{1/2} \propto (1+z)^{\alpha}$ with $\alpha = -1.44$ for quiescent galaxies ($\alpha = -1.12$ for all galaxies). The velocity dispersions drop by about one-third since $z \approx 2$, following $\sigma_{1/2} \propto (1+z)^{\beta}$ with $\beta = 0.44$ for the quiescent galaxies ($\beta = 0.37$ for all galaxies). The simulated size and dispersion evolution is in good agreement with observations and results from the subsequent accretion and merging of stellar systems at $z\lesssim 2$ which is a natural consequence of the hierarchical structure formation. A significant number of the simulated massive galaxies (7 out of 40) experience no merger more massive than 1:4 (usually considered as major mergers). On average, the dominant accretion mode is stellar minor mergers with a mass-weighted mass-ratio of 1:5. (abridged)
The Astrophysical Journal 06/2011; 744(1). DOI:10.1088/0004-637X/744/1/63 · 5.99 Impact Factor
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##### Article: The relationship between star formation rates, local density and stellar mass up to z ~ 3 in the GOODS NICMOS Survey
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ABSTRACT: We investigate the relation between star formation rates and local galaxy environment for a stellar mass selected galaxy sample in the redshift range 1.5 < z < 3. We use near-infra-red imaging from an extremely deep Hubble Space Telescope survey, the GOODS-NICMOS Survey (GNS) to measure local galaxy densities based on the nearest neighbour approach, while star-formation rates are estimated from rest-frame UV-fluxes. Due to our imaging depth we can examine galaxies down to a colour-independent stellar mass completeness limit of log M\ast = 9.5 M\odot at z ~ 3. We find a strong dependence of star formation activity on galaxy stellar mass over the whole redshift range, which does not depend on local environment. The average star formation rates are largely independent of local environment apart from in the highest relative over-densities. Galaxies in over-densities of a factor of > 5 have on average lower star formation rates by a factor of 2 - 3, but only up to redshifts of z ~ 2. We do not see any evidence for AGN activity influencing these relations. We also investigate the influence of the very local environment on star-formation activity by counting neighbours within 30 kpc radius. This shows that galaxies with two or more close neighbours have on average significantly lower star formation rates as well as lower specific star formation rates up to z ~ 2.5. We suggest that this might be due to star formation quenching induced by galaxy merging processes.
Monthly Notices of the Royal Astronomical Society 08/2011; 418(2). DOI:10.1111/j.1365-2966.2011.19559.x · 5.11 Impact Factor
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##### Article: First Results from the 3D-HST Survey: The Striking Diversity of Massive Galaxies at z>1
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ABSTRACT: We present first results from the 3D-HST program, a near-IR spectroscopic survey performed with the Wide Field Camera 3 on the Hubble Space Telescope. We have used 3D-HST spectra to measure redshifts and Halpha equivalent widths for a stellar mass-limited sample of 34 galaxies at 1<z<1.5 with M(stellar)>10^11 M(sun) in the COSMOS, GOODS, and AEGIS fields. We find that a substantial fraction of massive galaxies at this epoch are forming stars at a high rate: the fraction of galaxies with Halpha equivalent widths >10 A is 59%, compared to 10% among SDSS galaxies of similar masses at z=0.1. Galaxies with weak Halpha emission show absorption lines typical of 2-4 Gyr old stellar populations. The structural parameters of the galaxies, derived from the associated WFC3 F140W imaging data, correlate with the presence of Halpha: quiescent galaxies are compact with high Sersic index and high inferred velocity dispersion, whereas star-forming galaxies are typically large two-armed spiral galaxies, with low Sersic index. Some of these star forming galaxies might be progenitors of the most massive S0 and Sa galaxies. Our results challenge the idea that galaxies at fixed mass form a homogeneous population with small scatter in their properties. Instead we find that massive galaxies form a highly diverse population at z>1, in marked contrast to the local Universe.
The Astrophysical Journal Letters 08/2011; 743. DOI:10.1088/2041-8205/743/1/L15 · 5.34 Impact Factor
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