arXiv subjects

Marijn Franx

Publications and source records attributed to Marijn Franx.

At least 109 records · Page 6Linked to original sources

arXiv2016

Stellar Velocity Dispersion for a Strongly-Lensed, Intermediate-Mass Quiescent Galaxy at z=2.8

Measuring stellar velocity dispersions of quiescent galaxies beyond $z\sim2$ is observationally challenging. Such measurements require near-infrared spectra with a continuum detection of at least moderate signal-to-noise, often necessitating long integrations. In this paper, we present deep X-Shooter spectroscopy of one of only two known gravitationally-lensed massive quiescent galaxies at $z>2$. This galaxy is quadruply imaged, with the brightest images magnified by a factor of $\sim5$. The total exposure time of our data is 9.8 hours on-source; however the magnification, and the slit placement encompassing 2 images, provides a total equivalent exposure time of 215 hours. From this deep spectrum we measure a redshift ($z_{\mathrm{spec}}=2.756\pm0.001$), making this one of the highest redshift quiescent galaxies that is spectroscopically confirmed. We simultaneously fit both the spectroscopic and photometric data to determine stellar population parameters and conclude this galaxy is relatively young, intermediate-mass, consistent with low dust content, and has quenched only relatively recently. This recent quenching is confirmed by strong Balmer absorption, particularly $Hδ$. Remarkably, this proves that at least some intermediate-mass galaxies have already quenched as early as $z\sim2.8$. Additionally, we have measured a velocity dispersion ($σ=187\pm43~\mathrm{km/s}$), making this the highest-redshift quiescent galaxy with a dispersion measurement. We confirm that this galaxy falls on the same mass fundamental plane (MFP) as galaxies at z=2.2, consistent with little to no evolution in the MFP up to z=2.8. Overall this galaxy is proof of existence of intermediate-mass quenched galaxies in the distant universe, and that lensing is a powerful tool for determining their properties with improved accuracy.

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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 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

An assessment of the evidence from ATLAS3D for a variable initial mass function

The ATLAS3D Survey has reported evidence for a non-universal stellar initial mass function (IMF) for early type galaxies (ETGs) (Cappellari et al. 2012, 2013b,a). The IMF was constrained by comparing stellar mass measurements from kinematic data with those from spectral energy distribution (SED) fitting. Here we investigate possible effects of scatter in the reported stellar mass measurements and their potential impact on the IMF determination. We find that a trend of the IMF mismatch parameter with the kinematic mass to light ratio, comparable to the trend observed by Cappellari et al. (2012), could arise if the Gaussian errors of the kinematic mass determination are typically 30%. Without additional data, it is hard to separate between the option that the IMF has a true large intrinsic variation or the option that the errors in the determination are larger than anticipated. A correlation of the IMF with other properties would help to make this distinction, but no strong correlation has been found yet. The strongest correlation is with velocity dispersion. However, it has a large scatter and the correlation depends on sample selection and distance measurements. The correlation with velocity dispersion could be partly caused by the colour-dependent calibration of the surface brightness fluctuation distances of Tonry et al. (2001). We find that the K-band luminosity limited ATLAS3D Survey is incomplete for the highest M/L galaxies below 10^10.3 M_sun. There is a significant IMF - velocity dispersion trend for galaxies with SED masses above this limit, but no trend for galaxies with kinematic masses above this limit. We also find an IMF trend with distance, but no correlation between nearest neighbour ETGs, which excludes a large environmental dependence. Our findings do not rule out the reported IMF variations, but they suggest that further study is needed.

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arXiv2014

High-precision Photometric Redshifts from Spitzer/IRAC: Extreme [3.6]-[4.5] Colors Identify Galaxies in the Redshift Range z~6.6-6.9

One of the most challenging aspects of studying galaxies in the z>~7 universe is the infrequent confirmation of their redshifts through spectroscopy, a phenomenon thought to occur from the increasing opacity of the intergalactic medium to Lya photons at z>6.5. The resulting redshift uncertainties inhibit the efficient search for [C II] in z~7 galaxies with sub-mm instruments such as ALMA, given their limited scan speed for faint lines. One means by which to improve the precision of the inferred redshifts is to exploit the potential impact of strong nebular emission lines on the colors of z~4-8 galaxies as observed by Spitzer/IRAC. At z~6.8, galaxies exhibit IRAC colors as blue as [3.6]-[4.5] ~-1, likely due to the contribution of [O III]+Hb to the 3.6 mum flux combined with the absence of line contamination in the 4.5 mum band. In this paper we explore the use of extremely blue [3.6]-[4.5] colors to identify galaxies in the narrow redshift window z~6.6-6.9. When combined with an I-dropout criterion, we demonstrate that we can plausibly select a relatively clean sample of z~6.8 galaxies. Through a systematic application of this selection technique to our catalogs from all five CANDELS fields, we identify 20 probable z~6.6-6.9 galaxies. We estimate that our criteria select the ~50% strongest line emitters at z~6.8 and from the IRAC colors we estimate a typical [O III]+Hb rest-frame equivalent width of 1085A for this sample. The small redshift uncertainties on our sample make it particularly well suited for follow-up studies with facilities such as ALMA.

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arXiv2014

On the importance of using appropriate spectral models to derive physical properties of galaxies at 0.7<z<2.8

Interpreting observations of distant galaxies in terms of constraints on physical parameters - such as stellar mass, star-formation rate (SFR) and dust optical depth - requires spectral synthesis modelling. We analyse the reliability of these physical parameters as determined under commonly adopted `classical' assumptions: star-formation histories assumed to be exponentially declining functions of time, a simple dust law and no emission-line contribution. Improved modelling techniques and data quality now allow us to use a more sophisticated approach, including realistic star-formation histories, combined with modern prescriptions for dust attenuation and nebular emission (Pacifici et al. 2012). We present a Bayesian analysis of the spectra and multi-wavelength photometry of 1048 galaxies from the 3D-HST survey in the redshift range 0.7<z<2.8 and in the stellar mass range 9<log(M/Mo)<12. We find that, using the classical spectral library, stellar masses are systematically overestimated (~0.1 dex) and SFRs are systematically underestimated (~0.6 dex) relative to our more sophisticated approach. We also find that the simultaneous fit of photometric fluxes and emission-line equivalent widths helps break a degeneracy between SFR and optical depth of the dust, reducing the uncertainties on these parameters. Finally, we show how the biases of classical approaches can affect the correlation between stellar mass and SFR for star-forming galaxies (the `Star-Formation Main Sequence'). We conclude that the normalization, slope and scatter of this relation strongly depend on the adopted approach and demonstrate that the classical, oversimplified approach cannot recover the true distribution of stellar mass and SFR.

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arXiv2014

The Relation between Dynamical Mass-to-Light Ratio and Color for Massive Quiescent Galaxies out to z~2 and Comparison with Stellar Population Synthesis Models

We explore the relation between the dynamical mass-to-light ratio ($M/L$) and rest-frame color of massive quiescent galaxies out to z~2. We use a galaxy sample with measured stellar velocity dispersions in combination with Hubble Space Telescope and ground-based multi-band photometry. Our sample spans a large range in $\log M_{dyn}/L_{g}$ (of 1.6~dex) and $\log M_{dyn}/L_{K}$ (of 1.3~dex). There is a strong, approximately linear correlation between the $M/L$ for different wavebands and rest-frame color. The root-mean-scatter scatter in $\log~M_{dyn}/L$ residuals implies that it is possible to estimate the $M/L$ with an accuracy of ~0.25 dex from a single rest-frame optical color. Stellar population synthesis (SPS) models with a Salpeter stellar initial mass function (IMF) can not simultaneously match $M_{dyn}/L_{g}$ vs. $(g-z)_{rest-frame}$ and $M_{dyn}/L_{K}$ vs. $(g-K)_{rest-frame}$. By changing the slope of the IMF we are still unable to explain the M/L of the bluest and reddest galaxies. We find that an IMF with a slope between $α=2.35$ and $α=1.35$ provides the best match. We also explore a broken IMF with a Salpeter slope at $M<1M_{\odot}$ and $M>4M_{\odot}$ and a slope $α$ in the intermediate region. The data favor a slope of $α=1.35$ over $α=2.35$. Nonetheless, our results show that variations between different SPS models are comparable to the IMF variations. In our analysis we assume that the variation in $M/L$ and color is driven by differences in age, and that other contributions (e.g., metallicity evolution, dark matter) are small. These assumptions may be an important source of uncertainty as galaxies evolve in more complex ways.

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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

One Plane for All: Massive Star-Forming and Quiescent Galaxies Lie on the Same Mass Fundamental Plane at z~0 and z~0.7

Scaling relations between galaxy structures and dynamics have been studied extensively for early and late-type galaxies, both in the local universe and at high redshifts. The abundant differences between the properties of disky and elliptical, or star-forming and quiescent, galaxies seem to be characteristic of the local Universe; such clear distinctions begin to disintegrate as observations of massive galaxies probe higher redshifts. In this Paper, we investigate the existence the mass fundamental plane of all massive galaxies ($σ\gtrsim$ 100 km/s). This work includes local galaxies (0.05<z<0.07) from the SDSS, in addition to 31 star-forming and 72 quiescent massive galaxies at intermediate redshift (z~0.7) with absorption line kinematics from deep Keck-DEIMOS spectra and structural parameters from HST imaging. In two parameter scaling relations, star-forming and quiescent galaxies differ structurally and dynamically. However, we show that massive star-forming and quiescent galaxies lie on nearly the same mass fundamental plane, or the relationship between stellar mass surface density, stellar velocity dispersion, and effective radius. The scatter in this relation (measured about $\logσ$) is low: 0.072 dex (0.055 dex intrinsic) at z~0 and 0.10 dex (0.08 dex intrinsic) at z~0.7. This three dimensional surface is not unique: virial relations, with or without a dependence on luminosity profile shapes, can connect galaxy structures and stellar dynamics with similar scatter. This result builds on the recent finding that mass fundamental plane has been stable for early-type galaxies since z~2 (Bezanson et al. 2013). As we now find this also holds for star-forming galaxies to z~0.7, this implies that these scaling relations of galaxies will be minimally susceptible to progenitor biases due to the evolving stellar populations, structures, and dynamics of galaxies through cosmic time.

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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

The Fundamental Plane of massive quiescent galaxies out to z~2

The Fundamental Plane (FP) of early-type galaxies, relating the effective radius, velocity dispersion, and surface brightness, has long been recognized as a unique tool for analyzing galaxy structure and evolution. With the discovery of distant quiescent galaxies and the introduction of high sensitivity near-infrared spectrographs, it is now possible to explore the FP out to z~2. In this Letter we study the evolution of the FP out to z~2 using kinematic measurements of massive quiescent galaxies ($M_{*}>10^{11} M_{\odot}$). We find preliminary evidence for the existence of an FP out to z~2. The scatter of the FP, however, increases from z~0 to z~2, even when taking into account the larger measurement uncertainties at higher redshifts. We find a strong evolution of the zero point from z~2 to z~0: $Δ\log_{10}M/L_g\propto(-0.49\pm0.03)~z$. In order to assess whether our spectroscopic sample is representative of the early-type galaxy population at all redshifts, we compare their rest-frame g-z colors with those from a larger mass complete sample of quiescent galaxies. At z>1 we find that the spectroscopic sample is bluer. We use the color offsets to estimate a mass-to-light ratio (M/L) correction. The implied FP zero point evolution after correction is significantly smaller: $Δ\log_{10}M/L_g\propto(-0.39\pm 0.02)~z$. This is consistent with an apparent formation redshift of $z_{\rm{form}}=6.62^{+3.19}_{-1.44}$ for the underlying population, ignoring the effects of progenitor bias. A more complete spectroscopic sample is required at z~2 to properly measure the M/L evolution from the FP evolution.

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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

The Progenitors of Local Ultra-massive Galaxies Across Cosmic Time: from Dusty Star-bursting to Quiescent Stellar Populations

Using the UltraVISTA catalogs, we investigate the evolution in the 11.4~Gyr since $z=3$ of the progenitors of local ultra-massive galaxies ($\log{(M_{\rm star}/M_{\odot})}\approx11.8$; UMGs), providing a complete and consistent picture of how the most massive galaxies at $z=0$ have assembled. By selecting the progenitors with a semi-empirical approach using abundance matching, we infer a growth in stellar mass of 0.56$^{+0.35}_{-0.25}$ dex, 0.45$^{+0.16}_{-0.20}$~dex, and 0.27$^{+0.08}_{-0.12}$ dex from $z=3$, $z=2$, and $z=1$, respectively, to $z=0$. At $z<1$, the progenitors of UMGs constitute a homogeneous population of only quiescent galaxies with old stellar populations. At $z>1$, the contribution from star-forming galaxies progressively increases, with the progenitors at $2 1$, whereas the remaining was assembled via merging from $z\sim 1$ to the present. Most of the quenching of the star-forming progenitors happened between $z=2.75$ and $z=1.25$, in good agreement with the typical formation redshift and scatter in age of $z=0$ UMGs as derived from their fossil records. The progenitors of local UMGs, including the star-forming ones, never lived on the blue cloud since $z=3$. We propose an alternative path for the formation of local UMGs that refines previously proposed pictures and that is fully consistent with our findings.

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arXiv2014

3D-HST WFC3-selected Photometric Catalogs in the Five CANDELS/3D-HST Fields: Photometry, Photometric Redshifts and Stellar Masses

The 3D-HST and CANDELS programs have provided WFC3 and ACS spectroscopy and photometry over ~900 square arcminutes in five fields: AEGIS, COSMOS, GOODS-North, GOODS-South, and the UKIDSS UDS field. All these fields have a wealth of publicly available imaging datasets in addition to the HST data, which makes it possible to construct the spectral energy distributions (SEDs) of objects over a wide wavelength range. In this paper we describe a photometric analysis of the CANDELS and 3D-HST HST imaging and the ancillary imaging data at wavelengths 0.3um to 8um. Objects were selected in the WFC3 near-IR bands, and their SEDs were determined by carefully taking the effects of the point spread function in each observation into account. A total of 147 distinct imaging datasets were used in the analysis. The photometry is made available in the form of six catalogs: one for each field, as well as a master catalog containing all objects in the entire survey. We also provide derived data products: photometric redshifts, determined with the EAZY code, and stellar population parameters determined with the FAST code. We make all the imaging data that were used in the analysis available, including our reductions of the WFC3 imaging in all five fields. 3D-HST is a spectroscopic survey with the WFC3 and ACS grisms, and the photometric catalogs presented here constitute a necessary first step in the analysis of these grism data. All the data presented in this paper are available through the 3D-HST website.

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