Kim-Vy H. Tran

Galaxy stellar mass functions from ZFOURGE/CANDELS: an excess of low-mass galaxies since z=2 and the rapid buildup of quiescent galaxies

Using observations from the FourStar Galaxy Evolution Survey (ZFOURGE), we obtain the deepest measurements to date of the galaxy stellar mass function (SMF) at 0.2 < z < 3. ZFOURGE provides well-constrained photometric redshifts made possible through deep medium-bandwidth imaging at 1-2 μm. We combine this with Hubble Space Telescope imaging from the Cosmic Assembly Near-IR Deep Extragalactic Legacy Survey, allowing for the efficient selection of both blue and red galaxies down to stellar masses of ~109.5 M ☉ at z ~ 2.5. The total surveyed area is 316 arcmin2 distributed over three independent fields. We supplement these data with the wider and shallower NEWFIRM Medium-Band Survey to provide stronger constraints at high masses. Several studies at z ≤ 1.5 have revealed a steepening of the slope at the low-mass end of the SMF, leading to an upturn at masses <1010 M ☉ that is not well described by a standard single-Schechter function. We find evidence that this feature extends to at least z ~ 2 and that it can be found in both the star-forming and quiescent populations individually. The characteristic mass (M*) and slope at the lowest masses (α) of a double-Schechter function fit to the SMF stay roughly constant at Log(M/M ☉) ~ 10.65 and ~ – 1.5, respectively. The SMF of star-forming galaxies has evolved primarily in normalization, while the change in shape is relatively minor. Our data allow us, for the first time, to observe a rapid buildup at the low-mass end of the quiescent SMF. Since z = 2.5, the total stellar mass density of quiescent galaxies (down to 109 M ☉) has increased by a factor of ~12, whereas the mass density of star-forming galaxies only increases by a factor of ~2.2.

A pair of lensed galaxies at z = 4.92 in the field of CL 1358+62

The cluster CL 1358+62 displays a prominent red arc in WFPC2 images obtained with the Hubble Space Telescope. Keck spectra of the arc show Lyα emission at 7204 A, a continuum drop blueward of the line, and several absorption lines to the red. We identify the arc as a gravitationally lensed galaxy at a redshift of z = 4.92. It is the highest redshift object currently known. A gravitational lens model was used to reconstruct images of the high-redshift galaxy. The reconstructed image is asymmetric, containing a bright knot and a patch of extended emission 04 from the knot. The effective radius of the bright knot is 0022 or 130 h−150 pc. The extended patch is partially resolved into compact regions of star formation. The reconstructed galaxy has IAB = 24, giving a bolometric luminosity of about 3 × 1011 L☉. This can be produced by a star formation rate of 36 h−250 M☉ yr-1 (q0 = 0.5) or by an instantaneous starburst of 3 × 108 M☉. The spectral lines show velocity variations on the order of 300 km s-1 along the arc. The Si II line is blueshifted with respect to the Lyα emission, and the Lyα emission line is asymmetric with a red tail. These spectral features are naturally explained by an outflow model, in which the blue side of the Lyα line has been absorbed by outflowing neutral H I. Evidence from other sources indicates that outflows are common in starburst galaxies at high and low redshift. We have discovered a companion galaxy with a radial velocity only 450 km s-1 different than that of the arc. The serendipitous discovery of these two galaxies suggests that systematic searches may uncover galaxies at even higher redshifts.

Reversal of Fortune: Confirmation of an Increasing Star Formation-Density Relation in a Cluster at z=1.62

We measure the rest-frame colors (dust-corrected), infrared luminosities, star formation rates, and stellar masses of 92 galaxies in a Spitzer-selected cluster at z = 1.62. By fitting spectral energy distributions (SEDs) to 10-band photometry (0.4µm 10 11 L� , and these IR luminous members follow the same trend of increasing star formation with stellar mass that is observed in the field at z � 2. Using rates derived from both the 24µm imaging and SED fitting, we find that the relative fraction of star-forming members triples from the lowest to highest galaxy density regions, e .g. the IR luminous fraction increases from � 8% at �� 10 gal Mpc -2 to � 25% at �& 100 gal Mpc -2 . The observed increase is a reversal of the well-documented trend at z < 1 and signals that we have reached the epoch when massive cluster galaxies are still forming a substantial fraction of their stars. Subject headings:galaxies: evolution - galaxies: starburst - galaxies: clus ters: individual (ClG J0218.3-0510) - infrared: galaxies

Clusters at Half Hubble Time: Galaxy Structure and Colors in RX J0152.7–1357 and MS 1054–03

We study the photometric and structural properties of spectroscopically confirmed members in the two massive X-ray‐selected z ≈ 0.83 galaxy clusters MS 1054‐03 and RX J0152.7‐1357 using three-band mosaic imaging with the Hubble Space TelescopeAdvanced Camera for Surveys. The samples include 105 and 140 members of RX J0152.7‐1357 and MS 1054‐03, respectively, with ACS F775W magnitude i775 < 24.0. We fit the 2-D galaxy light profiles to determine effective radii and Se rsic indices; deviations from the smooth profiles are quantified by the ratio of the rms residuals to the mean of t he galaxy model. Galaxies are then classified according to a combination of this rms/mean ratio and the Sersic index; the resulting classes correlate well with visually classified morphological types, but are less affec ted by orientation. We find the size‐surface brightness relations in the two clusters to be very similar, supporting recent results on the evolution of this relationship with redshift. We examine in detail the color‐magnitude relations in these clusters and systematic effects on the residuals with respect to these relations. The color-ma gnitude residuals correlate with the local density, as measured from both galaxy numbers and weak lensing. These correlations are strongest for the full galaxy samples (commensurate with the morphology‐density relation), but are also present at lower significance levels for the early- and late-type samples individually. Weaker correlations are found with cluster radius, resulting from the more fundamental dependence on local density. We identify a threshold surface mass density of � ≈ 0.1, in units of the critical density, above which there are rel atively few blue (star-forming) galaxies. In RX J0152.7‐1357, there is an offset of 0.006 ± 0.002 in the mean redshifts of the early- and late-type galaxies, which produces a trend in the color residuals with velocity and may result from an infalling foreground association of late-type galaxies. Comparison of the color‐color diagrams for these clusters to stellar population models implies that a range of star formation time-scales are needed to reproduce the locus of galaxy colors. We also identify two galaxies, one in each cluster, whose colors can only be explained by large amounts, AV ∼ 1 mag, of internal dust extinction. Converting to rest-frame bandpasses, we find elliptical galaxy color scatters of 0.03 ± 0.01 mag in (U-B) and 0.07 ± 0.01 mag in (U-V), indicating mean ages of ∼ 3.5 Gyr, similar to the estimates from the mean colors and corresponding to formation at z ≈ 2.2. Thus, when the universe was half its present age, cluster ellipticals were half the age of the universe at that epoch; the same is coincidentally true of the median ages of ellipticals today. However, the most massive local cluster ellipticals have ages & 10 Gyr, consistent with our results for their likely progenitors at z & 0.8.

The HETDEX pilot survey. III. The low metallicities of high-redshift lyα galaxies

We present the results of Keck/NIRSPEC spectroscopic observations of three Lyα emitting galaxies (LAEs) at z~ 2.3 discovered with the HETDEX pilot survey. We detect Hα, [O III], and Hβ emission from two galaxies at z= 2.29 and 2.49, designated HPS194 and HPS256, respectively, representing the first detection of multiple rest-frame optical emission lines in galaxies at high redshift selected on the basis of their Lyα emission. We find that the redshifts of the Lyα emission from these galaxies are offset redward of the systemic redshifts (derived from the Hα and [O III] emission) by Δv = 162 ± 37 (photometric) ± 42 (systematic) km s–1 for HPS194 and Δv = 36 ± 35 ± 18 km s–1 for HPS256. An interpretation for HPS194 is that a large-scale outflow may be occurring in its interstellar medium. This outflow is likely powered by star-formation activity, as examining emission line ratios implies that neither LAE hosts an active galactic nucleus. Using the upper limits on the [N II] emission, we place meaningful constraints on the gas-phase metallicities in these two LAEs of Z< 0.17 and < 0.28 Z ☉ (1σ). Measuring the stellar masses of these objects via spectral energy distribution (SED) fitting (~1010 and 6 × 108 M ☉, respectively), we study the nature of LAEs in a mass-metallicity plane. At least one of these two LAEs appears to be more metal poor than continuum-selected star-forming galaxies at the same redshift and stellar mass, implying that objects exhibiting Lyα emission may be systematically less chemically enriched than the general galaxy population. We use the SEDs of these two galaxies to show that neglecting the contribution of the measured emission line fluxes when fitting stellar population models to the observed photometry can result in overestimates of the population age by orders of magnitude and the stellar mass by a factor of ~2. This effect is particularly important at z 7, where similarly strong emission lines may masquerade in the photometry as a 4000 A break.

The galaxy populations of X-ray-detected, poor groups

We determine the quantitative morphology and star formation properties of galaxies in six nearby X-ray-detected, poor groups using multiobject spectroscopy and wide-field R imaging. The mean recessional velocities of the galaxy groups range from 2843 to 7558 km s-1. Each group has 15-38 confirmed members ranging in luminosity from dwarfs to giants (-13.7 ≥ MR - 5 log h ≥ -21.9). We measure structural parameters for each galaxy by fitting a PSF-convolved, two-component model to their surface brightness profiles. To compare the samples directly, we fade, smooth, and rebin each galaxy image so that we effectively observe each galaxy at the same redshift (9000 km s-1) and physical resolution (0.87 h-1 kpc). The structural parameters are combined with [O II] measurements to test for correlations between morphological characteristics and current star formation in these galaxies. We compare results for the groups to a sample of field galaxies. We find that: (1) Galaxies spanning a wide range in morphological type and luminosity are fit well by a de Vaucouleurs bulge with exponential disk profile. (2) Morphologically classifying these nearby group galaxies by their bulge fraction (B/T) is fairly robust on average, even when their redshift has increased by up to a factor of 4 and the effective resolution of the images is degraded by up to a factor of 5. (3) The fraction of bulge-dominated systems in these groups is higher than in the field (~50% versus ~20%). (4) The fraction of bulge-dominated systems in groups decreases with increasing radius, similar to the morphology-radius (~density) relation observed in galaxy clusters. (5) Current star formation in group galaxies is correlated with significant morphological asymmetry for disk-dominated systems (B/T < 0.4). (6) The group galaxies that are most disk dominated (B/T < 0.2) are less star forming and asymmetric on average than their counterparts in the field.

The SFR-M* Relation and Empirical Star-Formation Histories from ZFOURGE at 0.5 < z < 4

We explore star-formation histories (SFHs) of galaxies based on the evolution of the star-formation rate stellar mass relation (SFR-M*). Using data from the FourStar Galaxy Evolution Survey (ZFOURGE) in combination with far-IR imaging from the Spitzer and Herschel observatories we measure the SFR-M* relation at 0.5 < z < 4. Similar to recent works we find that the average infrared SEDs of galaxies are roughly consistent with a single infrared template across a broad range of redshifts and stellar masses, with evidence for only weak deviations. We find that the SFR-M* relation is not consistent with a single power-law of the form SFR ~ M*^a at any redshift; it has a power-law slope of a~1 at low masses, and becomes shallower above a turnover mass (M_0) that ranges from 10^9.5 - 10^10.8 Msol, with evidence that M_0 increases with redshift. We compare our measurements to results from state-of-the-art cosmological simulations, and find general agreement in the slope of the SFR-M* relation albeit with systematic offsets. We use the evolving SFR-M* sequence to generate SFHs, finding that typical SFRs of individual galaxies rise at early times and decline after reaching a peak. This peak occurs earlier for more massive galaxies. We integrate these SFHs to generate mass-growth histories and compare to the implied mass-growth from the evolution of the stellar mass function. We find that these two estimates are in broad qualitative agreement, but that there is room for improvement at a more detailed level. At early times the SFHs suggest mass-growth rates that are as much as 10x higher than inferred from the stellar mass function. However, at later times the SFHs under-predict the inferred evolution, as is expected in the case of additional growth due to mergers.

IR Observations of MS 1054–03: Star Formation and Its Evolution in Rich Galaxy Clusters

We study the IR properties of galaxies in the cluster MS 1054-03 at z = 0.83 by combining MIPS 24 μm data with spectra of more than 400 galaxies and a very deep K-band-selected catalog. Nineteen IR cluster members are selected spectroscopically, and an additional 15 are selected by their photometric redshifts. We derive the IR luminosity function of the cluster and find strong evolution compared to the similar-mass Coma Cluster. The best-fitting Schechter function gives L = 11.49 L☉ with a fixed faint-end slope, about 1 order of magnitude larger than that in Coma. The rate of evolution of the IR luminosity from Coma to MS 1054-03 is consistent with that found in field galaxies, and it suggests that some internal mechanism, e.g., the consumption of the gas fuel, is responsible for the general decline of the cosmic SFR in different environments. The mass-normalized integrated SFR within 0.5R200 in MS 1054-03 also shows evolution compared with other rich clusters at lower redshifts, but the trend is less conclusive if the mass selection effect is considered. A nonnegligible fraction (13% ± 3%) of cluster members are forming stars actively, and the overdensity of IR galaxies is about 20 compared to the field. It is unlikely that clusters only passively accrete star-forming galaxies from the surrounding fields and have their star formation quenched quickly afterward; instead, many cluster galaxies still have large amounts of gas, and their star formation may be enhanced by the interaction with the cluster.

Spectroscopic confirmation of multiple red galaxy-galaxy mergers in MS 1054-03 (z = 0.83)

We present follow-up spectroscopy of the galaxy cluster MS 1054-03 (z = 0.83) confirming that at least six of the nine merging galaxy pairs identified by van Dokkum et al. in 1999 are indeed bound systems: they have projected separations of Rs L*, σ1D > 200 km s-1); these bound galaxy pairs must evolve into E/S0 members by z ~ 0.7. These results, combined with MS 1054s high merger fraction and reservoir of likely future mergers, indicates that most, if not all, of its early-type members evolved from (passive) galaxy-galaxy mergers at z 1.

The Velocity Dispersion of MS 1054–03: A Massive Galaxy Cluster at High Redshift

We present results from a dynamical study of the high-redshift, massive, X-ray-luminous galaxy cluster MS 1054-03. We significantly increase the number of confirmed cluster members by adding 20 to an existing set of 12; using the confirmed members, we estimate MS 1054-03s redshift, velocity dispersion, and mass. We find that z = 0.8329 ± 0.0017, σ = 1170 ± 150 km s-1, and the central mass is approximately 1.9 ± 0.5 × 1015 h-1 M☉ (within R = 1 h-1 Mpc; H0 = 100 h km s-1 Mpc-1, q0 = 0.5). MS 1054-03 is one of a handful of high-redshift (z > 0.5) clusters known that also has X-ray and weak-lensing observations; we find our dynamical mass agrees with mass estimates from both studies. The confirmation of MS 1054-03 as a massive cluster at z ~ 0.8 is consistent with an open (ΩM ~ 0.3) or flat, Λ-dominated (ΩM + ΩΛ = 1) universe. In addition, we compare MS 1054-03s velocity dispersion and X-ray temperature to a sample of low- and intermediate-redshift galaxy clusters to test for evolution in the σ-TX relation; we find no evidence for evolution in this relation to z ~ 0.8.