Showing papers by "Joel R. Primack published in 2014"

Bulge growth and quenching since z = 2.5 in CANDELS/3D-HST

Abstract: Exploiting the deep high-resolution imaging of all five CANDELS fields, and accurate redshift information provided by 3D-HST, we investigate the relation between structure and stellar populations for a mass-selected sample of 6764 galaxies above 1010 M ☉, spanning the redshift range 0.5 < z < 2.5. For the first time, we fit two-dimensional models comprising a single Sersic fit and two-component (i.e., bulge + disk) decompositions not only to the H-band light distributions, but also to the stellar mass maps reconstructed from resolved stellar population modeling. We confirm that the increased bulge prominence among quiescent galaxies, as reported previously based on rest-optical observations, remains in place when considering the distributions of stellar mass. Moreover, we observe an increase of the typical Sersic index and bulge-to-total ratio (with median B/T reaching 40%-50%) among star-forming galaxies above 1011 M ☉. Given that quenching for these most massive systems is likely to be imminent, our findings suggest that significant bulge growth precedes a departure from the star-forming main sequence. We demonstrate that the bulge mass (and ideally knowledge of the bulge and total mass) is a more reliable predictor of the star-forming versus quiescent state of a galaxy than the total stellar mass. The same trends are predicted by the state-of-the-art, semi-analytic model by Somerville et al. In this model, bulges and black holes grow hand in hand through merging and/or disk instabilities, and feedback from active galactic nuclei shuts off star formation. Further observations will be required to pin down star formation quenching mechanisms, but our results imply that they must be internal to the galaxies and closely associated with bulge growth.

Radiative feedback and the low efficiency of galaxy formation in low-mass haloes at high redshift

Abstract: Any successful model of galaxy formation needs to explain the low rate of star formation in the small progenitors of today's galaxies. This inefficiency is necessary for reproducing the low stellar-to-virial mass fractions, suggested by current abundance matching models. A possible driver of this low efficiency is the radiation pressure exerted by ionizing photons from massive stars. The effect of radiation pressure in cosmological, zoom-in galaxy formation simulations is modeled as a non-thermal pressure that acts only in dense and optically thick star-forming regions. We also include photoionization and photoheating by massive stars. The full photoionization of hydrogen reduces the radiative cooling in the $10^{4-4.5}$ K regime. The main effect of radiation pressure is to regulate and limit the high values of gas density and the amount of gas available for star formation. This maintains a low star formation rate of $\sim 1 \ {\rm M_\odot} \ {\rm yr}^{-1}$ in halos with masses about $10^{11} \ {M_\odot}$ at $z\simeq3$. Infrared trapping and photoionization/photoheating processes are secondary effects in this mass range. The galaxies residing in these low-mass halos contain only $\sim0.6\%$ of the total virial mass in stars, roughly consistent with abundance matching. Radiative feedback maintains an extended galaxy with a rising circular velocity profile.

Candels+3D-HST: compact SFGs at z ∼ 2-3, the progenitors of the first quiescent galaxies

Abstract: We analyze the star-forming and structural properties of 45 massive (log(M/M_) > 10) compact star-forming galaxies (SFGs) at 2 < z < 3 to explore whether they are progenitors of compact quiescent galaxies at z ∼ 2. The optical/NIR and far-IR Spitzer/Herschel colors indicate that most compact SFGs are heavily obscured. Nearly half (47%) host an X-ray-bright active galactic nucleus (AGN). In contrast, only about 10% of other massive galaxies at that time host AGNs. Compact SFGs have centrally concentrated light profiles and spheroidal morphologies similar to quiescent galaxies and are thus strikingly different from other SFGs, which typically are disk-like and sometimes clumpy or irregular.Most compact SFGs lie either within the star formation rate (SFR)–mass main sequence (65%) or below it (30%), on the expected evolutionary path toward quiescent galaxies. These results show conclusively that galaxies become more compact before they lose their gas and dust, quenching star formation. Using extensive HST photometry from CANDELS and grism spectroscopy from the 3D-HST survey, we model their stellar populations with either exponentially declining (τ ) star formation histories (SFHs) or physically motivated SFHs drawn from semianalytic models (SAMs). SAMs predict longer formation timescales and older ages ∼2 Gyr, which are nearly twice as old as the estimates of the τ models. Both models yield good spectral energy distribution fits, indicating that the systematic uncertainty in the age due to degeneracies in the SFH is of that order of magnitude. However, SAM SFHs better match the observed slope and zero point of the SFR–mass main sequence. Contrary to expectations, some low-mass compact SFGs (log(M/M_) = 10–10.6) have younger ages but lower specific SFRs than that of more massive galaxies, suggesting that the low-mass galaxies reach the red sequence faster. If the progenitors of compact SFGs are extended SFGs, state-of-the-art SAMs show that mergers and disk instabilities (DIs) are both able to shrink galaxies, but DIs are more frequent (60% versus 40%) and form more concentrated galaxies. We confirm this result via high-resolution hydrodynamic simulations.

The population of giant clumps in simulated high-z galaxies: in situ and ex situ migration and survival

Abstract: We study the properties of giant clumps and their radial gradients in high-$z$ disc galaxies using AMR cosmological simulations. Our sample consists of 770 snapshots in the redshift range $z=4-1$ from 29 galaxies that at $z=2$ span the stellar mass range $(0.2-3)\times 10^{11}M_{\odot}$. Extended gas discs exist in 83% of the snapshots. Clumps are identified by gas density in 3D and their stellar and dark matter components are considered thereafter. While most of the overdensities are diffuse and elongated, 91% of their mass and 83% of their star-fromation rate (SFR) are in compact round clumps. Nearly all galaxies have a central, massive bulge clump, while 70% of the discs show off-center clumps, 3-4 per galaxy. The fraction of clumpy discs peaks at intermediate disc masses. Clumps are divided based on dark-matter content into $\textit{in-situ}$ and $\textit{ex-situ}$, originating from violent disc instability (VDI) and minor mergers respectively. 60% of the discs are in a VDI phase showing off-center $\textit{in-situ}$ clumps, which contribute 1-7% of the disc mass and 5-45% of its SFR. The $\textit{in-situ}$ clumps constitute 75% of the off-center clumps in terms of number and SFR but only half the mass, each clump containing on average 1% of the disc mass and 6% of its SFR. They have young stellar ages, $100-400 {\rm Myr}$, and high specific SFR (sSFR), $1-10 {\rm Gyr}^{-1}$. They exhibit gradients resulting from inward clump migration, where the inner clumps are somewhat more massive and older, with lower gas fraction and sSFR and higher metallicity. Similar observed gradients indicate that clumps survive outflows. The \exsitu clumps have stellar ages $0.5-3 {\rm Gyr}$ and sSFR $\sim 0.1-2 {\rm Gyr}^{-1}$, and they exhibit weaker gradients. Massive clumps of old stars at large radii are likely \exsitu mergers, though half of them share the disc rotation.

Geometry of Star-Forming Galaxies from SDSS, 3D-HST, and CANDELS

Abstract: We determine the intrinsic, three-dimensional shape distribution of star-forming galaxies at 0 10(10) M-circle dot) disks are the most common geometric shape at all z less than or similar to 2. Lower-mass galaxies at z > 1 possess a broad range of geometric shapes: the fraction of elongated (prolate) galaxies increases toward higher redshifts and lower masses. Galaxies with stellar mass 10(9) M-circle dot (10(10) M-circle dot) are a mix of roughly equal numbers of elongated and disk galaxies at z similar to 1 (z similar to 2). This suggests that galaxies in this mass range do not yet have disks that are sustained over many orbital periods, implying that galaxies with present-day stellar mass comparable to that of the Milky Way typically first formed such sustained stellar disks at redshift z similar to 1.5-2. Combined with constraints on the evolution of the star formation rate density and the distribution of star formation over galaxies with different masses, our findings imply that, averaged over cosmic time, the majority of stars formed in disks.

Mergers and mass accretion for infalling halos both end well outside cluster virial radii

Abstract: We find that infalling dark matter halos (i.e., the progenitors of satellite halos) begin losing mass well outside the virial radius of their eventual host halos. The peak mass occurs at a range of clustercentric distances, with median and 68th percentile range of for progenitors of z = 0 satellites. The peak circular velocity for infalling halos occurs at significantly larger distances ( at z = 0). This difference arises because different physical processes set peak circular velocity (typically, ~1:5 and larger mergers which cause transient circular velocity spikes) and peak mass (typically, smooth accretion) for infalling halos. We find that infalling halos also stop having significant mergers well before they enter the virial radius of their eventual hosts. Mergers larger than a 1:40 ratio in halo mass end for infalling halos at similar clustercentric distances (~1.9 R vir, host) as the end of overall mass accretion. However, mergers larger than 1:3 typically end for infalling halos at more than four virial radial away from their eventual hosts. This limits the ability of mergers to affect quenching and morphology changes in clusters. We also note that the transient spikes which set peak circular velocity may lead to issues with abundance matching on that parameter, including unphysical galaxy stellar mass growth profiles near clusters; we propose a simple observational test to check if a better halo proxy for galaxy stellar mass exists.

Semi-analytic Models for the CANDELS Survey: Comparison of Predictions for Intrinsic Galaxy Properties

Abstract: We compare the predictions of three independently developed semi-analytic galaxy formation models (SAMs) that are being used to aid in the interpretation of results from the CANDELS survey. These models are each applied to the same set of halo merger trees extracted from the 'Bolshoi' high-resolution cosmological N-body simulation and are carefully tuned to match the local galaxy stellar mass function using the powerful method of Bayesian Inference coupled with Markov Chain Monte Carlo or by hand. The comparisons reveal that in spite of the significantly different parameterizations for star formation and feedback processes, the three models yield qualitatively similar predictions for the assembly histories of galaxy stellar mass and star formation over cosmic time. Comparing SAM predictions with existing estimates of the stellar mass function from z = 0-8, we show that the SAMs generally require strong outflows to suppress star formation in low-mass halos to match the present-day stellar mass function, as is the present common wisdom. However, all of the models considered produce predictions for the star formation rates (SFRs) and metallicities of low-mass galaxies that are inconsistent with existing data. The predictions for metallicity-stellar mass relations and their evolution clearly diverge between the models. We suggestmore » that large differences in the metallicity relations and small differences in the stellar mass assembly histories of model galaxies stem from different assumptions for the outflow mass-loading factor produced by feedback. Importantly, while more accurate observational measurements for stellar mass, SFR and metallicity of galaxies at 1 < z < 5 will discriminate between models, the discrepancies between the constrained models and existing data of these observables have already revealed challenging problems in understanding star formation and its feedback in galaxy formation. The three sets of models are being used to construct catalogs of mock galaxies on light cones that have the same geometry as the CANDELS survey, which should be particularly useful for quantifying the biases and uncertainties on measurements and inferences from the real observations.« less

Understanding the structural scaling relations of early-type galaxies

Abstract: We use a large suite of hydrodynamical simulations of binary galaxy mergers to construct and calibrate a physical prescription for computing the effective radii and velocity dispersions of spheroids. We implement this prescription within a semi-analytic model embedded in merger trees extracted from the Bolshoi Lambda-CDM N-body simulation, accounting for spheroid growth via major and minor mergers as well as disk instabilities. We find that without disk instabilities, our model does not predict sufficient numbers of intermediate mass early-type galaxies in the local universe. Spheroids also form earlier in models with spheroid growth via disk instabilities. Our model correctly predicts the normalization, slope, and scatter of the low-redshift size-mass and Fundamental Plane relations for early type galaxies. It predicts a degree of curvature in the Faber-Jackson relation that is not seen in local observations, but this could be alleviated if higher mass spheroids have more bottom-heavy initial mass functions. The model also correctly predicts the observed strong evolution of the size-mass relation for spheroids out to higher redshifts, as well as the slower evolution in the normalization of the Faber-Jackson relation. We emphasize that these are genuine predictions of the model since it was tuned to match hydrodynamical simulations and not these observations.

Clumpy Galaxies in CANDELS. I. The Definition of UV Clumps and the Fraction of Clumpy Galaxies at 0.5<z<3

Abstract: Although giant clumps of stars are crucial to galaxy formation and evolution, the most basic demographics of clumps are still uncertain, mainly because the definition of clumps has not been thoroughly discussed. In this paper, we study the basic demographics of clumps in star-forming galaxies (SFGs) at 0.5 0.5. The redshift evolution of Fclumpy changes with the stellar mass (M*) of the galaxies. Low-mass (log(M*/Msun)<9.8) galaxies keep an almost constant Fclumpy of about 60% from z~3.0 to z~0.5. Intermediate-mass and massive galaxies drop their Fclumpy from 55% at z~3.0 to 40% and 15%, respectively, at z~0.5. We find that (1) the trend of disk stabilization predicted by violent disk instability matches the Fclumpy trend of massive galaxies; (2) minor mergers are a viable explanation of the Fclumpy trend of intermediate-mass galaxies at z<1.5, given a realistic observability timescale; and (3) major mergers are unlikely responsible for the Fclumpy trend in all masses at z<1.5. The clump contribution to the rest-frame UV light of SFGs shows a broad peak around galaxies with log(M*/Msun)~10.5 at all redshifts, possibly linked to the molecular gas fraction of the galaxies. (Abridged)

Keck-I MOSFIRE Spectroscopy of Compact Star- Forming Galaxies at z≳ 2: High Velocity Dispersions in Progenitors of Compact Quiescent Galaxies

Abstract: We present Keck-I MOSFIRE near-infrared spectroscopy for a sample of 13 compact star-forming galaxies (SFGs) at redshift 2 ≤ z ≤ 25 with star formation rates of SFR ~ 100 M_☉ yr^(–1) and masses of log(M/M_☉) ~108 Their high integrated gas velocity dispersions of σ_(int_ =230^(+40)_(-30) km s^(–1), as measured from emission lines of Hα and [O III], and the resultant M_* -σ_(int) relation and M_*-M_(dyn) all match well to those of compact quiescent galaxies at z ~ 2, as measured from stellar absorption lines Since log(M*/M_(dyn)) =–006 ± 02 dex, these compact SFGs appear to be dynamically relaxed and evolved, ie, depleted in gas and dark matter (<13^(+17)_(-13)%), and present larger σ_(int) than their non-compact SFG counterparts at the same epoch Without infusion of external gas, depletion timescales are short, less than ~300 Myr This discovery adds another link to our new dynamical chain of evidence that compact SFGs at z ≳ 2 are already losing gas to become the immediate progenitors of compact quiescent galaxies by z ~ 2

Star formation and clumps in cosmological galaxy simulations with radiation pressure feedback

Abstract: Cosmological simulations of galaxies have typically produced too many stars at early times. We study the global and morphological eects of radiation pressure (RP) in eight pairs of high-resolution cosmological galaxy formation simulations. We nd that the additional feedback suppresses star formation globally by a factor of 2. Despite this reduction, the simulations still overproduce stars by a factor of 2 with respect to the predictions provided by abundance matching methods for halos more massive than 5 10 11 M h 1 (Behroozi, Wechsler & Conroy 2013). We also study the morphological impact of radiation pressure on our simulations. In simulations with RP the average number of low mass clumps falls dramatically. Only clumps with stellar masses Mclump=Mdisk 6 5% are impacted by the inclusion of RP, and RP and no-RP clump counts above this range are comparable. The inclusion of RP depresses the contrast ratios of clumps by factors of a few for clump masses less than 5% of the disk masses. For more massive clumps, the dierences between and RP and no-RP simulations diminish. We note however, that the simulations analysed have disk stellar masses below about 2 10 10 M h 1 . By creating mock Hubble Space Telescope observations we

Simulating multiple merger pathways to the central kinematics of early-type galaxies

Abstract: Two-dimensional integral field surveys such as ATLAS^3D are producing rich observational data sets yielding insights into galaxy formation. These new kinematic observations have highlighted the need to understand the evolutionary mechanisms leading to a spectrum of fast rotators and slow rotators in early-type galaxies. We address the formation of slow and fast rotators through a series of controlled, comprehensive hydrodynamical simulations, sampling idealized galaxy merger scenarios constructed from model spiral galaxies. Idealized and controlled simulations of this sort complement the more ‘realistic’ cosmological simulations by isolating and analysing the effects of specific parameters, as we do in this paper. We recreate minor and major binary mergers, binary merger trees with multiple progenitors, and multiple sequential mergers. Within each of these categories of formation history, we correlate progenitor gas fraction, mass ratio, orbital pericentre, orbital ellipticity, and spin with remnant kinematic properties. We create kinematic profiles of these 95 simulations comparable to ATLAS^3D data. By constructing remnant profiles of the projected specific angular momentum ($\lambda _R= \langle R|V|\rangle / \langle R \sqrt{V^2+\sigma ^2}\rangle$), triaxiality, and measuring the incidences of kinematic twists and kinematically decoupled cores, we distinguish between varying formation scenarios. We find that binary mergers nearly always form fast rotators. Slow rotators can be formed from zero initial angular momentum configurations and gas-poor mergers, but are not as round as the ATLAS^3D galaxies. Remnants of binary merger trees are triaxial slow rotators. Sequential mergers form round slow rotators that most resemble the ATLAS^3D rotators.

Modelling the ages and metallicities of early-type galaxies in Fundamental Plane space

Abstract: Recent observations have probed the formation histories of nearby elliptical galaxies by tracking correlations between the stellar population parameters, age and metallicity, and the structural parameters that enter the Fundamental Plane, size Re and velocity dispersion . These studies have found intriguing correlations between these four parameters. In this work, we make use of a semi-analytic model, based on halo merger trees extracted from the Bolshoi cosmological simulation, that predicts the structural properties of spheroid-dominated galaxies based on an analytic model that has been tested and calibrated against an extensive suite of hydrodynamic+N-body binary merger simulations. We predict the Re, , luminosity, age, and metallicity of spheroid-dominated galaxies, enabling us to compare directly to observations. Our model predicts a strong correlation between age and for early-type galaxies, and no signicant correlation between age and radius, in agreement with observations. In addition we predict a strong correlation between metallicity and , and a weak correlation between metallicity and Re, in qualitative agreement with observations. We nd that the correlations with arise as a result of the strong link between and the galaxy’s assembly time. Minor mergers produce a large change in radius while leaving nearly the same, which explains the weaker trends with radius.

Geometry of Star-Forming Galaxies from SDSS, 3D-HST and CANDELS

Abstract: We determine the intrinsic, 3-dimensional shape distribution of star-forming galaxies at 0 1e10 Msol) disks are the most common geometric shape at all z 1 possess a broad range of geometric shapes: the fraction of elongated (prolate) galaxies increases toward higher redshifts and lower masses. Galaxies with stellar mass 1e9 Msol (1e10 Msol) are a mix of roughly equal numbers of elongated and disk galaxies at z~1 (z~2). This suggests that galaxies in this mass range do not yet have disks that are sustained over many orbital periods, implying that galaxies with present-day stellar mass comparable to that of the Milky Way typically first formed such sustained stellar disks at redshift z~1.5-2. Combined with constraints on the evolution of the star formation rate density and the distribution of star formation over galaxies with different masses, our findings imply that, averaged over cosmic time, the majority of stars formed in disks.

Bulge Growth and Quenching since z = 2.5 in CANDELS/3D-HST

Abstract: Exploiting the deep high-resolution imaging of all 5 CANDELS fields, and accurate redshift information provided by 3D-HST, we investigate the relation between structure and stellar populations for a mass-selected sample of 6764 galaxies above 10^10 Msun, spanning the redshift range 0.5 < z < 2.5. For the first time, we fit 2-dimensional models comprising a single Sersic fit and two-component (i.e., bulge + disk) decompositions not only to the H-band light distributions, but also to the stellar mass maps reconstructed from resolved stellar population modeling. We confirm that the increased bulge prominence among quiescent galaxies, as reported previously based on rest-optical observations, remains in place when considering the distributions of stellar mass. Moreover, we observe an increase of the typical Sersic index and bulge-to-total ratio (with median B/T reaching 40-50%) among star-forming galaxies above 10^11 Msun. Given that quenching for these most massive systems is likely to be imminent, our findings suggest that significant bulge growth precedes a departure from the star-forming main sequence. We demonstrate that the bulge mass (and ideally knowledge of the bulge and total mass) is a more reliable predictor of the star-forming versus quiescent state of a galaxy than the total stellar mass. The same trends are predicted by the state-of-the-art semi-analytic model by Somerville et al. In the latter, bulges and black holes grow hand in hand through merging and/or disk instabilities, and AGN-feedback shuts off star formation. Further observations will be required to pin down star formation quenching mechanisms, but our results imply they must be internal to the galaxies and closely associated with bulge growth.