Forming Compact Massive Galaxies

In this paper we study a key phase in the formation of massive galaxies: the transition of star forming galaxies into massive (M_stars~10^11 Msun), compact (r_e~1 kpc) quiescent galaxies, which takes place from z~3 to z~1.5. We use HST grism redshifts and extensive photometry in all five 3D-HST/CANDELS fields, more than doubling the area used previously for such studies, and combine these data with Keck MOSFIRE and NIRSPEC spectroscopy. We first confirm that a population of massive, compact, star forming galaxies exists at z~2, using K-band spectroscopy of 25 of these objects at 2.0<z<2.5. They have a median NII/Halpha ratio of 0.6, are highly obscured with SFR(tot)/SFR(Halpha)~10, and have a large range of observed line widths. We infer from the kinematics and spatial distribution of Halpha that the galaxies have rotating disks of ionized gas that are a factor of ~2 more extended than the stellar distribution. By combining measurements of individual galaxies, we find that the kinematics are consistent with a nearly Keplerian fall-off from V_rot~500 km/s at 1 kpc to V_rot~250 km/s at 7 kpc, and that the total mass out to this radius is dominated by the dense stellar component. Next, we study the size and mass evolution of the progenitors of compact massive galaxies. Even though individual galaxies may have had complex histories with periods of compaction and mergers, we show that the population of progenitors likely followed a simple inside-out growth track in the size-mass plane of d(log r_e) ~ 0.3 d(log M_stars). This mode of growth gradually increases the stellar mass within a fixed physical radius, and galaxies quench when they reach a stellar density or velocity dispersion threshold. As shown in other studies, the mode of growth changes after quenching, as dry mergers take the galaxies on a relatively steep track in the size-mass plane.

Tight Correlations Between Massive Galaxy Structural Properties and Dynamics: The Mass Fundamental Plane Was in Place by z~2

The Fundamental Plane (FP) is an empirical relation between the size, surface brightness, and velocity dispersion of early-type galaxies. This relation has been studied extensively for early-type galaxies in the local universe to constrain galaxy formation mechanisms. The evolution of the zeropoint of this plane has been extended to high redshifts to study the luminosity evolution of massive galaxies, under the assumption of structural homology. In this work, we assess this assumption by replacing surface brightness with stellar mass density and present the evolution of the mass FP for massive, quiescent galaxies since z~2. By accounting for stellar populations, we thereby isolate and trace structural and dynamical evolution. Despite the observed dramatic evolution in the sizes and morphologies of massive galaxies since z~3, we find that quiescent galaxies lie on the mass FP out to z~2. In contrast with ~1.4 dex evolution in the luminosity FP, average residuals from the z~0 mass FP are less than ~0.15 dex since z~2. Assuming the Hyde & Bernardi (2009) mass FP slope, we find that this minimal offset scales as (1+z)^{-0.095+/-0.043}. This result lends credence to previous studies that derived luminosity evolution from the FP. Therefore, despite their compact sizes and suggestions that massive galaxies are more disk-like at z~2, the relationship between their dynamics and structural properties are consistent with local early-type galaxies. Finally, we find no strong evidence for a tilt of the mass FP relative to the Virial plane, but emphasize the need for full models including selection biases to fully investigate this issue.

Galaxy environments over cosmic time: the non-evolving radial galaxy distributions around massive galaxies since z=1.6

We present a statistical study of the environments of massive galaxies in four redshift bins between z=0.04 and z=1.6, using data from the Sloan Digital Sky Survey (SDSS) and the NEWFIRM Medium Band Survey (NMBS). We measure the projected radial distribution of galaxies in cylinders around a constant number density selected sample of massive galaxies and utilize a statistical subtraction of contaminating sources. Our analysis shows that massive primary galaxies typically live in group halos and are surrounded by 2 to 3 satellites with masses more than one-tenth of the primary galaxy mass. The cumulative stellar mass in these satellites roughly equals the mass of the primary galaxy itself. We further find that the radial number density profile of galaxies around massive primaries has not evolved significantly in either slope or overall normalization in the past 9.5 Gyr. A simplistic interpretation of this result can be taken as evidence for a lack of mergers in the studied groups and as support for a static evolution model of halos containing massive primaries. Alternatively, there exists a tight balance between mergers and accretion of new satellites such that the overall distribution of galaxies in and around the halo is preserved. The latter interpretation is supported by a comparison to a semi-analytic model, which shows a similar constant average satellite distribution over the same redshift range.

HST/WFC3 Confirmation of the Inside-Out Growth of Massive Galaxies at 0<z<2 and Identification of their Star Forming Progenitors at z~3

We study the structural evolution of massive galaxies by linking progenitors and descendants at a constant cumulative number density of n_c=1.4x10^{-4} Mpc^{-3} to z~3. Structural parameters were measured by fitting Sersic 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 Sersic 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 Sersic 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.

The Star-formation Mass Sequence out to z=2.5

We study the star formation rate (SFR) - stellar mass (M*) relation in a self-consistent manner from 0 < z < 2.5 with a sample of galaxies selected from the NEWFIRM Medium-Band Survey. We find a significant non-linear slope of the relation, SFR propto M*^0.6, and a constant observed scatter of 0.34 dex, independent of redshift and M*. However, if we select only blue galaxies we find a linear relation SFR propto M*, similar to previous results at z = 0 by Peng et al. (2010). This selection excludes red, dusty, star-forming galaxies with higher masses, which brings down the slope. By selecting on L_IR/L_UV (a proxy for dust obscuration) and the rest-frame U-V colors, we show that star-forming galaxies fall in three distinct regions of the log(SFR)-log(M*) plane: 1) actively star-forming galaxies with normal dust obscuration and associated colors (54% for log(M*) > 10 at 1 < z < 1.5), 2) red star-forming galaxies with low levels of dust obscuration and low specific SFRs (11%), and 3) dusty, blue star-forming galaxies with high specific SFRs (7%). The remaining 28% comprises quiescent galaxies. Galaxies on the normal star formation sequence show strong trends of increasing dust attenuation with stellar mass and a decreasing specific SFR, with an observed scatter of 0.25 dex (0.17 dex intrinsic scatter). The dusty, blue galaxies reside in the upper envelope of the star formation sequence with remarkably similar spectral shapes at all masses, suggesting that the same physical process is dominating the stellar light. The red, low-dust star-forming galaxies may be in the process of shutting off and migrating to the quiescent population.

Spatially resolved Halpha maps and sizes of 57 strongly star-forming galaxies at z~1 from 3D-HST: evidence for rapid inside-out assembly of disk galaxies

We investigate the build-up of galaxies at z~1 using maps of Halpha and stellar continuum emission for a sample of 57 galaxies with rest-frame Halpha equivalent widths >100 Angstroms in the 3D-HST grism survey. We find that the Halpha emission broadly follows the rest-frame R-band light but that it is typically somewhat more extended and clumpy. We quantify the spatial distribution with the half-light radius. The median Halpha effective radius r_e(Halpha) is 4.2+-0.1 kpc but the sizes span a large range, from compact objects with r_e(Halpha) ~ 1.0 kpc to extended disks with r_e(Halpha) ~ 15 kpc. Comparing Halpha sizes to continuum sizes, we find <r_e(Halpha)/r_e(R)>=1.3+-0.1 for the full sample. That is, star formation, as traced by Halpha, typically occurs out to larger radii than the rest-frame R-band stellar continuum; galaxies are growing their radii and building up from the inside out. This effect appears to be somewhat more pronounced for the largest galaxies. Using the measured Halpha sizes, we derive star formation rate surface densities. We find that they range from ~0.05 Msun yr^{-1} kpc^{-2} for the largest galaxies to ~5 Msun yr^{-1} kpc^{-2} for the smallest galaxies, implying a large range in physical conditions in rapidly star-forming z~1 galaxies. Finally, we infer that all galaxies in the sample have very high gas mass fractions and stellar mass doubling times < 500 Myr. Although other explanations are also possible, a straightforward interpretation is that we are simultaneously witnessing the rapid formation of compact bulges and large disks at z~1.

Observations of dark and luminous matter: the radial distribution of satellite galaxies around massive red galaxies

We study the projected radial distribution of satellite galaxies around more than 28,000 Luminous Red Galaxies (LRGs) at 0.28<z<0.40 and trace the gravitational potential of LRG groups in the range 15<r/kpc<700. We show that at large radii the satellite number density profile is well fitted by a projected NFW profile with r_s~270 kpc and that at small radii this model underestimates the number of satellite galaxies. Utilizing the previously measured stellar light distribution of LRGs from deep imaging stacks we demonstrate that this small scale excess is consistent with a non-negligible baryonic mass contribution to the gravitational potential of massive groups and clusters. The combined NFW+scaled stellar profile provides an excellent fit to the satellite number density profile all the way from 15 kpc to 700 kpc. Dark matter dominates the total mass profile of LRG halos at r>25 kpc whereas baryons account for more than 50% of the mass at smaller radii. We calculate the total dark-to-baryonic mass ratio and show that it is consistent with measurements from weak lensing for environments dominated by massive early type galaxies. Finally, we divide the satellite galaxies in our sample into three luminosity bins and show that the satellite light profiles of all brightness levels are consistent with each other outside of roughly 25 kpc. At smaller radii we find evidence for a mild mass segregation with an increasing fraction of bright satellites close to the central LRG.

First Results from the 3D-HST Survey: The Striking Diversity of Massive Galaxies at z>1

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.

Mass growth and mergers: direct observations of the luminosity function of LRG satellite galaxies out to z=0.7 from SDSS and BOSS images

We present a statistical study of the luminosity functions of galaxies surrounding luminous red galaxies (LRGs) at average redshifts <z>=0.34 and <z>=0.65. The luminosity functions are derived by extracting source photometry around more than 40,000 LRGs and subtracting foreground and background contamination using randomly selected control fields. We show that at both studied redshifts the average luminosity functions of the LRGs and their satellite galaxies are poorly fitted by a Schechter function due to a luminosity gap between the centrals and their most luminous satellites. We utilize a two-component fit of a Schechter function plus a log-normal distribution to demonstrate that LRGs are typically brighter than their most luminous satellite by roughly 1.3 magnitudes. This luminosity gap implies that interactions within LRG environments are typically restricted to minor mergers with mass ratios of 1:4 or lower. The luminosity functions further imply that roughly 35% of the mass in the environment is locked in the LRG itself, supporting the idea that mass growth through major mergers within the environment is unlikely. Lastly, we show that the luminosity gap may be at least partially explained by the selection of LRGs as the gap can be reproduced by sparsely sampling a Schechter function. In that case LRGs may represent only a small fraction of central galaxies in similar mass halos.

Halpha and 4000 Angstrom Break Measurements for ~3500 K-selected Galaxies at 0.5<z<2.0

We measure spectral features of ~3500 K-selected galaxies at 0.5<z<2.0 from high quality medium-band photometry using a new technique. First, we divide the galaxy sample in 32 subsamples based on the similarities between the full spectral energy distributions (SEDs) of the galaxies. For each of these 32 galaxy types we construct a composite SED by de-redshifting and scaling the observed photometry. This approach increases the signal-to-noise ratio and sampling of galaxy SEDs and allows for model-independent stellar population studies. The composite SEDs are of spectroscopic quality, and facilitate -- for the first time -- Halpha measurement for a large magnitude-limited sample of distant galaxies. The linewidths indicate a photometric redshift uncertainty of dz<0.02x(1+z). The composite SEDs also show the Balmer and 4000 Angstrom breaks, MgII absorption at ~2800 Angstrom, the dust absorption feature at 2175 Angstrom, and blended [OIII]+Hbeta emission. We compare the total equivalent width of Halpha, [NII], and [SII] (W_Halpha+) with the strength of the 4000 Angstrom break (D(4000)) and the best-fit specific star formation rate, and find that all these properties are strongly correlated. This is a reassuring result, as currently most distant stellar population studies are based on just continuum emission. Furthermore, the relation between W_Halpha+ and Dn(4000) provides interesting clues to the SFHs of galaxies, as these features are sensitive to different stellar ages. We find that the correlation between W_Halpha+ and D(4000) at 0.5<z<2.0 is similar to z~0, and that the suppression of star formation in galaxies at z<2 is generally not abrupt, but a gradual process.