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Katherine E. Whitaker

Publications and source records attributed to Katherine E. Whitaker.

At least 181 records · Page 10Linked to original sources
arXiv2016

The Evolution of the Fractions of Quiescent and Star-forming Galaxies as a Function of Stellar Mass Since z=3: Increasing Importance of Massive, Dusty Star-forming Galaxies in the Early Universe

Using the UltraVISTA DR1 and 3D-HST catalogs, we construct a stellar-mass-complete sample, unique for its combination of surveyed volume and depth, to study the evolution of the fractions of quiescent galaxies, moderately unobscured star-forming galaxies, and dusty star-forming galaxies as a function of stellar mass over the redshift interval $0.2 \le z \le 3.0$. We show that the role of dusty star-forming galaxies within the overall galaxy population becomes more important with increasing stellar mass, and grows rapidly with increasing redshift. Specifically, dusty star-forming galaxies dominate the galaxy population with $\log{(M_{\rm star}/M_{\odot})} \gtrsim 10.3$ at $z\gtrsim2$. The ratio of dusty and non-dusty star-forming galaxies as a function of stellar mass changes little with redshift. Dusty star-forming galaxies dominate the star-forming population at $\log{(M_{\rm star}/M_{\odot})} \gtrsim 10.0-10.5$, being a factor of $\sim$3-5 more common, while unobscured star-forming galaxies dominate at $\log{(M_{\rm star}/M_{\odot})} \lesssim 10$. At $\log{(M_{\rm star}/M_{\odot})} > 10.5$, red galaxies dominate the galaxy population at all redshift $z<3$, either because they are quiescent (at late times) or dusty star-forming (in the early universe).

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arXiv2016

Ultra-deep Ks-band Imaging of the Hubble Frontier Fields

We present an overview of the "KIFF" project, which provides ultra-deep Ks-band imaging of all six of the Hubble Frontier Fields clusters Abell 2744, MACS-0416, Abell S1063, Abell 370, MACS-0717 and MACS-1149. All of these fields have recently been observed with large allocations of Directors' Discretionary Time with the HST and Spitzer telescopes covering 0.4 < lambda < 1.6 microns and 3.6--4.5 microns, respectively. VLT/HAWK-I integrations of the first four fields reach 5-sigma limiting depths of Ks~26.0 (AB, point sources) and have excellent image quality (FWHM ~ 0."4). Shorter Keck/MOSFIRE integrations of the MACS-0717 (MACS-1149) field better observable in the north reach limiting depths Ks=25.5 (25.1) with seeing FWHM ~0."4 (0."5). In all cases the Ks-band mosaics cover the primary cluster and parallel HST/ACS+WFC3 fields. The total area of the Ks-band coverage is 490 arcmin^2. The Ks-band at 2.2 microns crucially fills the gap between the reddest HST filter (1.6 micron ~ H-band) and the IRAC 3.6 micron passband. While reaching the full depths of the space-based imaging is not currently feasible from the ground, the deep Ks-band images provide important constraints on both the redshifts and the stellar population properties of galaxies extending well below the characteristic stellar mass across most of the age of the universe, down to, and including, the redshifts of the targeted galaxy clusters (z < 0.5).

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arXiv2016

Ages of massive galaxies at $0.5 < z < 2.0$ from 3D-HST rest-frame optical spectroscopy

We present low-resolution near-infrared stacked spectra from the 3D-HST survey up to $z=2.0$ and fit them with commonly used stellar population synthesis models: BC03 (Bruzual & Charlot, 2003), FSPS10 (Flexible Stellar Population Synthesis, Conroy & Gunn 2010), and FSPS-C3K (Conroy, Kurucz, Cargile, Castelli, in prep). The accuracy of the grism redshifts allows the unambiguous detection of many emission and absorption features, and thus a first systematic exploration of the rest-frame optical spectra of galaxies up to $z=2$. We select massive galaxies ($\rm log(M_{*} / M_{\odot}) > 10.8$), we divide them into quiescent and star-forming via a rest-frame color-color technique, and we median-stack the samples in 3 redshift bins between $z=0.5$ and $z=2.0$. We find that stellar population models fit the observations well at wavelengths below $\rm 6500 Å$ rest-frame, but show systematic residuals at redder wavelengths. The FSPS-C3K model generally provides the best fits (evaluated with a $χ^2_{red}$ statistics) for quiescent galaxies, while BC03 performs the best for star-forming galaxies. The stellar ages of quiescent galaxies implied by the models, assuming solar metallicity, vary from 4 Gyr at $z \sim 0.75$ to 1.5 Gyr at $z \sim 1.75$, with an uncertainty of a factor of 2 caused by the unknown metallicity. On average the stellar ages are half the age of the Universe at these redshifts. We show that the inferred evolution of ages of quiescent galaxies is in agreement with fundamental plane measurements, assuming an 8 Gyr age for local galaxies. For star-forming galaxies the inferred ages depend strongly on the stellar population model and the shape of the assumed star-formation history.

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arXiv2016

Evidence for non-stellar rest-frame near-IR emission associated with increased star formation in galaxies at $z \sim 1$

We explore the presence of non-stellar rest-frame near-IR ($2-5 \ μ\mathrm{m}$) emission in galaxies at $z \sim 1$. Previous studies identified this excess in relatively small samples and suggested that such non-stellar emission, which could be linked to the $3.3 \ μ\mathrm{m}$ polycyclic aromatic hydrocarbons feature or hot dust emission, is associated with an increased star formation rate (SFR). In this Letter, we confirm and quantify the presence of an IR excess in a significant fraction of galaxies in the 3D-HST GOODS catalogs. By constructing a matched sample of galaxies with and without strong non-stellar near-IR emission, we find that galaxies with such emission are predominantly star-forming galaxies. Moreover, star-forming galaxies with an excess show increased mid- and far-IR and H$α$ emission compared to other star-forming galaxies without. While galaxies with a near-IR excess show a larger fraction of individually detected X-ray active galactic nuclei (AGNs), an X-ray stacking analysis, together with the IR-colors and H$α$ profiles, shows that AGNs are unlikely to be the dominant source of the excess in the majority of galaxies. Our results suggest that non-stellar near-IR emission is linked to increased SFRs and is ubiquitous among star-forming galaxies. As such, the near-IR emission might be a powerful tool to measure SFRs in the era of the James Webb Space Telescope.

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arXiv2015

Spatially-resolved dust maps from Balmer decrements in galaxies at z~1.4

We derive average radial gradients in the dust attenuation towards HII regions in 609 galaxies at z~1.4, using measurements of the Balmer decrement out to r~3kpc. The Balmer decrements are derived from spatially resolved maps of Halpha and Hbeta emission from the 3D-HST survey. We find that with increasing stellar mass (M) both the normalization and strength of the gradient in dust attenuation increases. Galaxies with a mean mass of = 9.2Msun have little dust attenuation at all radii, whereas galaxies with = 10.2Msun have dust attenuation toward Halpha A(Halpha)~2mag in their central regions. We parameterize this as A(Halpha) = b + c log(r), with b = 0.9 + 1.0 log(M10), c = -1.9 - 2.2 log(M10), r in kpc, and M10 the stellar mass in units of 10^10Msun. This expression can be used to correct spatially resolved measurements of Halpha to radial distributions of star formation. When applied to our data, we find that the star formation rates in the central r<1kpc of galaxies in the highest mass bin are ~ 6 Msun/yr, six times higher than before correction and approximately half of the total star formation rate of these galaxies. If this high central star formation rate is maintained for several Gyr, a large fraction of the stars in present-day bulges likely formed in-situ.

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arXiv2015

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.

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arXiv2015

The 3D-HST Survey: Hubble Space Telescope WFC3/G141 grism spectra, redshifts, and emission line measurements for $\sim 100,000$ galaxies

We present reduced data and data products from the 3D-HST survey, a 248-orbit HST Treasury program. The survey obtained WFC3 G141 grism spectroscopy in four of the five CANDELS fields: AEGIS, COSMOS, GOODS-S, and UDS, along with WFC3 $H_{140}$ imaging, parallel ACS G800L spectroscopy, and parallel $I_{814}$ imaging. In a previous paper (Skelton et al. 2014) we presented photometric catalogs in these four fields and in GOODS-N, the fifth CANDELS field. Here we describe and present the WFC3 G141 spectroscopic data, again augmented with data from GO-1600 in GOODS-N. The data analysis is complicated by the fact that no slits are used: all objects in the WFC3 field are dispersed, and many spectra overlap. We developed software to automatically and optimally extract interlaced 2D and 1D spectra for all objects in the Skelton et al. (2014) photometric catalogs. The 2D spectra and the multi-band photometry were fit simultaneously to determine redshifts and emission line strengths, taking the morphology of the galaxies explicitly into account. The resulting catalog has 98,663 measured redshifts and line strengths down to $JH_{IR}\leq 26$ and 22,548 with $JH_{IR}\leq 24$, where we comfortably detect continuum emission. Of this sample 5,459 galaxies are at $z>1.5$ and 9,621 are at $0.7<z<1.5$, where H$α$ falls in the G141 wavelength coverage. Based on comparisons with ground-based spectroscopic redshifts, and on analyses of paired galaxies and repeat observations, the typical redshift error for $JH_{IR}\leq 24$ galaxies in our catalog is $σ_z \approx 0.003 \times (1+z)$, i.e., one native WFC3 pixel. The $3σ$ limit for emission line fluxes of point sources is $1.5\times10^{-17}$ ergs s$^{-1}$ cm$^{-2}$. We show various representations of the full dataset, as well as individual examples that highlight the range of spectra that we find in the survey.

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arXiv2015

Leveraging 3D-HST Grism Redshifts to Quantify Photometric Redshift Performance

We present a study of photometric redshift accuracy in the 3D-HST photometric catalogs, using 3D-HST grism redshifts to quantify and dissect trends in redshift accuracy for galaxies brighter than $H_{F140W}<24$ with an unprecedented and representative high-redshift galaxy sample. We find an average scatter of $0.0197\pm0.0003(1+z)$ in the Skelton et al. (2014) photometric redshifts. Photometric redshift accuracy decreases with magnitude and redshift, but does not vary monotonically with color or stellar mass. The 1-$σ$ scatter lies between $0.01-0.03$(1+z) for galaxies of all masses and colors below $z<2.5$ (for $H_{F140W}{<}24$), with the exception of a population of very red ($U-V > 2$), dusty star-forming galaxies for which the scatter increases to $\sim0.1(1+z)$. Although the overall photometric redshift accuracy for quiescent galaxies is better than for star-forming galaxies, scatter depends more strongly on magnitude and redshift than on galaxy type. We verify these trends using the redshift distributions of close pairs and extend the analysis to fainter objects, where photometric redshift errors further increase to $\sim0.046(1+z)$ at $H_{F160W}=26$. We demonstrate that photometric redshift accuracy is strongly filter-dependent and quantify the contribution of multiple filter combinations. We evaluate the widths of redshift probability distribution functions and find that error estimates are underestimated by a factor of $\sim1.1-1.6$, but that uniformly broadening the distribution does not adequately account for fitting outliers. Finally, we suggest possible applications of these data in planning for current and future surveys and simulate photometric redshift performance in the LSST, DES, and combined DES and VHS surveys.

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arXiv2015

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.

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arXiv2015

Galaxy Structure as a Driver of the Star Formation Sequence Slope and Scatter

It is well established that (1) star-forming galaxies follow a relation between their star formation rate (SFR) and stellar mass (M$_{\star}$), the "star-formation sequence", and (2) the SFRs of galaxies correlate with their structure, where star-forming galaxies are less concentrated than quiescent galaxies at fixed mass. Here, we consider whether the scatter and slope of the star-formation sequence is correlated with systematic variations in the Sersic indices, $n$, of galaxies across the SFR-M$_{\star}$ plane. We use a mass-complete sample of 23,848 galaxies at $0.5 2$ (implying more dominant bulges) have significantly lower SFR/M$_{\star}$ than the main ridgeline of the star-formation sequence. These results suggest that bulges in massive $z\sim2$ galaxies are actively building up, where the stars in the central concentration are relatively young. At $z<1$, the presence of older bulges within star-forming galaxies lowers global SFR/M$_{\star}$, decreasing the slope and contributing significantly to the scatter of the star-formation sequence.

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arXiv2015

Where stars form: inside-out growth and coherent star formation from HST Halpha maps of 2676 galaxies across the main sequence at z~1

We present Ha maps at 1kpc spatial resolution for star-forming galaxies at z~1, made possible by the WFC3 grism on HST. Employing this capability over all five 3D-HST/CANDELS fields provides a sample of 2676 galaxies. By creating deep stacked Halpha (Ha) images, we reach surface brightness limits of 1x10^-18\erg\s\cm^2\arcsec^2, allowing us to map the distribution of ionized gas out to >10kpc for typical L* galaxies at this epoch. We find that the spatial extent of the Ha distribution increases with stellar mass as r(Ha)[kpc]=1.5(Mstars/10^10Msun)^0.23. Furthermore, the Ha emission is more extended than the stellar continuum emission, consistent with inside-out assembly of galactic disks. This effect, however, is mass dependent with r(Ha)/r(stars)=1.1(M/10^10Msun)^0.054, such that at low masses r(Ha)~r(stars). We map the Ha distribution as a function of SFR(IR+UV) and find evidence for `coherent star formation' across the SFR-M plane: above the main sequence, Ha is enhanced at all radii; below the main sequence, Ha is depressed at all radii. This suggests that at all masses the physical processes driving the enhancement or suppression of star formation act throughout the disks of galaxies. It also confirms that the scatter in the star forming main sequence is real and caused by variations in the star formation rate at fixed mass. At high masses (10^10.5<M/Msun<10^11), above the main sequence, Ha is particularly enhanced in the center, plausibly building bulges and/or supermassive black holes. Below the main sequence, the star forming disks are more compact and a strong central dip in the EW(Ha), and the inferred specific star formation rate, appears. Importantly though, across the entirety of the SFR-M plane, the absolute star formation rate as traced by Ha is always centrally peaked, even in galaxies below the main sequence.

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arXiv2015

The MOSFIRE Deep Evolution Field (MOSDEF) Survey: Rest-Frame Optical Spectroscopy for ~1500 H-Selected Galaxies at 1.37 < z < 3.8

In this paper we present the MOSFIRE Deep Evolution Field (MOSDEF) survey. The MOSDEF survey aims to obtain moderate-resolution (R=3000-3650) rest-frame optical spectra (~3700-7000 Angstrom) for ~1500 galaxies at 1.37<z<3.80 in three well-studied CANDELS fields: AEGIS, COSMOS, and GOODS-N. Targets are selected in three redshift intervals: 1.37<z<1.70, 2.09<z<2.61, and 2.95<z<3.80, down to fixed H_AB (F160W) magnitudes of 24.0, 24.5 and 25.0, respectively, using the photometric and spectroscopic catalogs from the 3D-HST survey. We target both strong nebular emission lines (e.g., [OII], Hbeta, [OIII], 5008, Halpha, [NII], and [SII]) and stellar continuum and absorption features (e.g., Balmer lines, Ca-II H and K, Mgb, 4000 Angstrom break). Here we present an overview of our survey, the observational strategy, the data reduction and analysis, and the sample characteristics based on spectra obtained during the first 24 nights. To date, we have completed 21 masks, obtaining spectra for 591 galaxies. For ~80% of the targets we derive a robust redshift from either emission or absorption lines. In addition, we confirm 55 additional galaxies, which were serendipitously detected. The MOSDEF galaxy sample includes unobscured star-forming, dusty star-forming, and quiescent galaxies and spans a wide range in stellar mass (~10^9-10^11.5 Msol) and star formation rate (~10^0-10^3 Msol/yr). The spectroscopically confirmed sample is roughly representative of an H-band limited galaxy sample at these redshifts. With its large sample size, broad diversity in galaxy properties, and wealth of available ancillary data, MOSDEF will transform our understanding of the stellar, gaseous, metal, dust, and black hole content of galaxies during the time when the universe was most active.

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arXiv2015

First results from the VIRIAL survey: the stellar content of $UVJ$-selected quiescent galaxies at $1.5 < z < 2$ from KMOS

We investigate the stellar populations of 25 massive, galaxies ($\log[M_\ast/M_\odot] \geq 10.9$) at $1.5 < z < 2$ using data obtained with the K-band Multi-Object Spectrograph (KMOS) on the ESO VLT. Targets were selected to be quiescent based on their broadband colors and redshifts using data from the 3D-HST grism survey. The mean redshift of our sample is $\bar{z} = 1.75$, where KMOS YJ-band data probe age- and metallicity-sensitive absorption features in the rest-frame optical, including the $G$ band, Fe I, and high-order Balmer lines. Fitting simple stellar population models to a stack of our KMOS spectra, we derive a mean age of $1.03^{+0.13}_{-0.08}$ Gyr. We confirm previous results suggesting a correlation between color and age for quiescent galaxies, finding mean ages of $1.22^{+0.56}_{-0.19}$ Gyr and $0.85^{+0.08}_{-0.05}$ Gyr for the reddest and bluest galaxies in our sample. Combining our KMOS measurements with those obtained from previous studies at $0.2 < z < 2$ we find evidence for a $2-3$ Gyr spread in the formation epoch of massive galaxies. At $z < 1$ the measured stellar ages are consistent with passive evolution, while at $1 < z \lesssim2$ they appear to saturate at $\sim$1 Gyr, which likely reflects changing demographics of the (mean) progenitor population. By comparing to star-formation histories inferred for "normal" star-forming galaxies, we show that the timescales required to form massive galaxies at $z \gtrsim 1.5$ are consistent with the enhanced $α$-element abundances found in massive local early-type galaxies.

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arXiv2014

Reconciling the observed star-forming sequence with the observed stellar mass function

We examine the connection between the observed star-forming sequence (SFR $\propto$ $M^α$) and the observed evolution of the stellar mass function between $0.2 < z < 2.5$. We find the star-forming sequence cannot have a slope $α$ $\lesssim$ 0.9 at all masses and redshifts, as this would result in a much higher number density at $10 < \log(\mathrm{M/M_{\odot}}) < 11$ by $z=1$ than is observed. We show that a transition in the slope of the star-forming sequence, such that $α=1$ at $\log(\mathrm{M/M_{\odot}})<10.5$ and $α=0.7-0.13z$ ({Whitaker} {et~al.} 2012) at $\log(\mathrm{M/M_{\odot}})>10.5$, greatly improves agreement with the evolution of the stellar mass function. We then derive a star-forming sequence which reproduces the evolution of the mass function by design. This star-forming sequence is also well-described by a broken-power law, with a shallow slope at high masses and a steep slope at low masses. At $z=2$, it is offset by $\sim$0.3 dex from the observed star-forming sequence, consistent with the mild disagreement between the cosmic SFR and recent observations of the growth of the stellar mass density. It is unclear whether this problem stems from errors in stellar mass estimates, errors in SFRs, or other effects. We show that a mass-dependent slope is also seen in other self-consistent models of galaxy evolution, including semi-analytical, hydrodynamical, and abundance-matching models. As part of the analysis, we demonstrate that neither mergers nor hidden low-mass quiescent galaxies are likely to reconcile the evolution of the mass function and the star-forming sequence. These results are supported by observations from {Whitaker} {et~al.} (2014).

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arXiv2014

How dead are dead galaxies? Mid-Infrared fluxes of quiescent galaxies at redshift 0.3 < z < 2.5: implications for star formation rates and dust heating

We investigate the star formation rates of quiescent galaxies at high redshift (0.3 < z < 2.5) using 3D-HST WFC3 grism spectroscopy and Spitzer mid-infrared data. We select quiescent galaxies on the basis of the widely used UVJ color-color criteria. Spectral energy distribution fitting (rest frame optical and near-IR) indicates very low star formation rates for quiescent galaxies (sSFR ~ 10^-12 yr^-1). However, SED fitting can miss star formation if it is hidden behind high dust obscuration and ionizing radiation is re-emitted in the mid-infrared. It is therefore fundamental to measure the dust-obscured SFRs with a mid-IR indicator. We stack the MIPS-24um images of quiescent objects in five redshift bins centered on z = 0.5, 0.9, 1.2, 1.7, 2.2 and perform aperture photometry. Including direct 24um detections, we find sSFR ~ 10^-11.9 * (1+z)^4 yr^-1. These values are higher than those indicated by SED fitting, but at each redshift they are 20-40 times lower than those of typical star forming galaxies. The true SFRs of quiescent galaxies might be even lower, as we show that the mid-IR fluxes can be due to processes unrelated to ongoing star formation, such as cirrus dust heated by old stellar populations and circumstellar dust. Our measurements show that star formation quenching is very efficient at every redshift. The measured SFR values are at z > 1.5 marginally consistent with the ones expected from gas recycling (assuming that mass loss from evolved stars refuels star formation) and well above that at lower redshifts.

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arXiv2014

Constraining the Low-Mass Slope of the Star Formation Sequence at 0.5<z<2.5

We constrain the slope of the star formation rate ($\logΨ$) to stellar mass ($\log\mathrm{M_{\star}}$) relation down to $\log(\mathrm{M_{\star}/M_{\odot}})=8.4$ ($\log(\mathrm{M_{\star}/M_{\odot}})=9.2$) at $z=0.5$ ($z=2.5$) with a mass-complete sample of 39,106 star-forming galaxies selected from the 3D-HST photometric catalogs, using deep photometry in the CANDELS fields. For the first time, we find that the slope is dependent on stellar mass, such that it is steeper at low masses ($\log\mathrmΨ\propto\log\mathrm{M_{\star}}$) than at high masses ($\log\mathrmΨ\propto(0.3-0.6)\log\mathrm{M_{\star}}$). These steeper low mass slopes are found for three different star formation indicators: the combination of the ultraviolet (UV) and infrared (IR), calibrated from a stacking analysis of Spitzer/MIPS 24$μ$m imaging; $β$-corrected UV SFRs; and H$α$ SFRs. The normalization of the sequence evolves differently in distinct mass regimes as well: for galaxies less massive than $\log(\mathrm{M_{\star}/M_{\odot}})<10$ the specific SFR ($Ψ/\mathrm{M_{\star}}$) is observed to be roughly self-similar with $Ψ/\mathrm{M_{\star}}\propto(1+z)^{1.9}$, whereas more massive galaxies show a stronger evolution with $Ψ/\mathrm{M_{\star}}\propto(1+z)^{2.2-3.5}$ for $\log(\mathrm{M_{\star}/M_{\odot}})=10.2-11.2$. The fact that we find a steep slope of the star formation sequence for the lower mass galaxies will help reconcile theoretical galaxy formation models with the observations. The results of this study support the analytical conclusions of Leja et al. (2014).

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arXiv2014

Dense cores in galaxies out to z=2.5 in SDSS, UltraVISTA, and the five 3D-HST/CANDELS fields

The dense interiors of massive galaxies are among the most intriguing environments in the Universe. In this paper we ask when these dense cores were formed and determine how galaxies gradually assembled around them. We select galaxies that have a stellar mass >3x10^10 Msun inside r=1 kpc out to z=2.5, using the 3D-HST survey and data at low redshift. Remarkably, the number density of galaxies with dense cores appears to have decreased from z=2.5 to the present. This decrease is probably mostly due to stellar mass loss and the resulting adiabatic expansion, with some contribution from merging. We infer that dense cores were mostly formed at z>2.5, consistent with their largely quiescent stellar populations. While the cores appear to form early, the galaxies in which they reside show strong evolution: their total masses increase by a factor of 2-3 from z=2.5 to z=0 and their effective radii increase by a factor of 5-6. As a result, the contribution of dense cores to the total mass of the galaxies in which they reside decreases from ~50% at z=2.5 to ~15% at z=0. Because of their early formation, the contribution of dense cores to the total stellar mass budget of the Universe is a strong function of redshift. The stars in cores with M_1kpc>3x10^10 Msun make up ~0.1% of the stellar mass density of the Universe today but 10%-20% at z~2, depending on their IMF. The formation of these cores required the conversion of ~10^11 Msun of gas into stars within ~1 kpc, while preventing significant star formation at larger radii.

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