Rachel S. Somerville

Profiles of dark haloes. Evolution, scatter, and environment

We study dark-matter halo density profiles in a high-resolution N-body simulation of aCDM cosmology. Our statistical sample contains �5000 haloes in the range 10 11 10 14 h −1 M⊙ and the resolution allows a study of subhaloes inside host haloes. The profiles are parameterized by an NFW form with two parameters, an inner radius rs and a virial radius Rvir, and we define the halo concentration cvirRvir/rs. We find that, for a given halo mass, the redshift dependence of the median concentration is cvir / (1 + z) −1 . This corresponds to rs(z) � constant, and is contrary to earlier suspicions that cvir does not vary much with redshift. The implications are that high- redshift galaxies are predicted to be more extended and dimmer than expected before. Second, we find that the scatter in halo profiles is large, with a 1� �(logcvir) = 0.18 at a given mass, corresponding to a scatter in maximum rotation velocities of �Vmax/Vmax = 0.12. We discuss implications for modelling the Tully-Fisher relation, which has a smaller reported intrinsic scatter. Third, subhaloes and haloes in dense environments tend to be more concentrated than isolated haloes, and show a larger scatter. These results suggest that cvir is an essential parameter for the theory of galaxy modelling, and we briefly discuss implications for the universality of the Tully- Fisher relation, the formation of low surface brightness galaxies, and the origin of the Hubble sequence. We present an improved analytic treatment of halo formation that fits the measured relations between halo parameters and their redshift dependence, and can thus serve semi-analytic studies of galaxy formation.

Star Formation in AEGIS Field Galaxies since z = 1.1: The Dominance of Gradually Declining Star Formation, and the Main Sequence of Star-forming Galaxies

We analyze star formation (SF) as a function of stellar mass (M☉) and redshift z in the All-Wavelength Extended Groth Strip International Survey. For 2905 field galaxies, complete to 10^10(10^10.8 )M at z < 0.7(1), with Keck spectroscopic redshifts out to z = 1.1, we compile SF rates (SFRs) from emission lines, GALEX, and Spitzer MIPS 24 µm photometry, optical-NIR M* measurements, and HST morphologies. Galaxies with reliable signs of SF form a distinct “main sequence” (MS), with a limited range of SFRs at a given M* and z (1 σ ≾ ±0.3 dex), and log (SFR) approximately proportional to log M*. The range of log (SFR) remains constant to z > 1, while the MS as a whole moves to higher SFR as z increases. The range of the SFR along the MS constrains the amplitude of episodic variations of SF and the effect of mergers on the SFR. Typical galaxies spend ∼67%(95%) of their lifetime since z = 1 within a factor of ≾2(4) of their average SFR at a given M* and z. The dominant mode of the evolution of SF since z ∼ 1 is apparently a gradual decline of the average SFR in most individual galaxies, not a decreasing frequency of starburst episodes, or a decreasing factor by which SFRs are enhanced in starbursts. LIRGs at z ∼ 1 seem to mostly reflect the high SFR typical for massive galaxies at that epoch. The smooth MS may reflect that the same set of few physical processes governs SF prior to additional quenching processes. A gradual process like gas exhaustion may play a dominant role.

The Great Observatories Origins Deep Survey: Initial Results from Optical and Near-Infrared Imaging

This special issue of the Astrophysical Journal Letters is dedicated to presenting initial results from the Great Observatories Origins Deep Survey (GOODS) that are primarily, but not exclusively, based on multiband imaging data obtained with the Hubble Space Telescope and the Advanced Camera for Surveys (ACS). The survey covers roughly 320 arcmin2 in the ACS F435W, F606W, F814W, and F850LP bands, divided into two well-studied fields. Existing deep observations from the Chandra X-Ray Observatory and ground-based facilities are supplemented with new, deep imaging in the optical and near-infrared from the European Southern Observatory and from the Kitt Peak National Observatory. Deep observations with the Space Infrared Telescope Facility are scheduled. Reduced data from all facilities are being released worldwide within 3-6 months of acquisition. Together, this data set provides two deep reference fields for studies of distant normal and active galaxies, supernovae, and faint stars in our own Galaxy. This Letter serves to outline the survey strategy and describe the specific data that have been used in the accompanying letters, summarizing the reduction procedures and sensitivity limits.

Semi-analytic modelling of galaxy formation: the local Universe

Using semi-analytic models of galaxy formation, we investigate galaxy properties such as the Tully-Fisher relation, the B and K-band luminosity functions, cold gas contents, sizes, metallicities, and colours, and compare our results with observations of local galaxies. We investigate several different recipes for star formation and supernova feedback, including choices that are similar to the treatment in Kauffmann, White & Guiderdoni (1993) and Cole et al. (1994) as well as some new recipes. We obtain good agreement with all of the key local observations mentioned above. In particular, in our best models, we simultaneously produce good agreement with both the observed B and K-band luminosity functions and the I-band Tully-Fisher relation. Improved cooling and supernova feedback modelling, inclusion of dust extinction, and an improved Press-Schechter model all contribute to this success. We present results for several variants of the CDM family of cosmologies, and find that models with values of 0 ≃ 0.3–0.5 give the best agreement with observations.

Constraints on the relationship between stellar mass and halo mass at low and high redshift

We use a statistical approach to determine the relationship between the stellar masses of galaxies and the masses of the dark matter halos in which they reside. We obtain a parameterized stellar-to-halo mass (SHM) relation by populating halos and subhalos in an N-body simulation with galaxies and requiring that the observed stellar mass function be reproduced. We find good agreement w ith constraints from galaxy-galaxy lensing and predictions of semi-analytic models. Using this mapping, and the positions of the halos and subhalos obtained from the simulation, we find that our model predictions for th e galaxy two-point correlation function (CF) as a function of stellar mass are in excellent agreement with the observed clustering properties in the SDSS at z = 0. We show that the clustering data do not provide additional strong constraints on the SHM function and conclude that our model can therefore predict clustering as a functio n of stellar mass. We compute the conditional mass

A semi-analytic model for the co-evolution of galaxies, black holes and active galactic nuclei

We present a new semi-analytic model that self-consistently traces the growth of supermassive black holes (BH) and their host galaxies within the context of the Lambda cold dark matter (� CDM) cosmological framework. In our model, the energy emitted by accreting black holes regulates the growth of the black holes themselves, drives galactic scale winds that can remove cold gas from galaxies, and produces powerful jets that heat the hot gas atmospheres surrounding groups and clusters. We present a comprehensive comparison of our model predictions with observational measurements of key physical properties of low-redshift galaxies, such as cold gas fractions, stellar metallicities and ages, and specific star formation rates. We find that our new models successfully reproduce the exponential cut-off in the stellar mass function and the stellar and cold gas mass densities at z ∼ 0, and predict that star formation should be largely, but not entirely, quenched in massive galaxies at the present day. We also find that our model of self-regulated BH growth naturally reproduces the observed relation between BH mass and bulge mass. We explore the global formation history of galaxies and black holes in our models, presenting predictions for the cosmic histories of star formation, stellar mass assembly, cold gas and metals. We find that models assuming the ‘concordance’ � CDM cosmology overproduce star formation and stellar mass at high redshift (z 2). A model with less small-scale power predicts less star formation at high redshift, and excellent agreement with the observed stellar mass assembly history, but may have difficulty accounting for the cold gas in quasar absorption systems at high redshift (z ∼ 3–4).

The nature of high-redshift galaxies

Using semi-analytic models of galaxy formation set within the cold dark matter (CDM) merging hierarchy, we investigate several scenarios for the nature of the high-redshift ) Lyman-break galaxies (LBGs). We consider a ‘collisional starburst’ model in which bursts of star formation are triggered by galaxy–galaxy mergers, and find that a significant fraction of LBGs are predicted to be starbursts. This model reproduces the observed comoving number density of bright LBGs as a function of redshift and the observed luminosity function at and with a reasonable amount of dust extinction. Model galaxies at have star formation rates, half-light radii, colours and internal velocity dispersions that are in good agreement with the data. Global quantities such as the star formation rate density and cold gas and metal content of the Universe as a function of redshift also agree well. Two ‘quiescent’ models without starbursts are also investigated. In one, the star formation efficiency in galaxies remains constant with redshift, while in the other, it scales inversely with disc dynamical time, and thus increases rapidly with redshift. The first quiescent model is strongly ruled out, as it does not produce enough high-redshift galaxies once realistic dust extinction is accounted for. The second quiescent model fits marginally, but underproduces cold gas and very bright galaxies at high redshift. A general conclusion is that star formation at high redshift must be more efficient than locally. The collisional starburst model appears to accomplish this naturally without violating other observational constraints.

ΛCDM-based Models for the Milky Way and M31. I. Dynamical Models

We apply standard disk formation theory with adiabatic contraction within cuspy halo models predicted by the standard cold dark matter (?CDM) cosmology. The resulting models are confronted with the broad range of observational data available for the Milky Way and M31 galaxies. We find that there is a narrow range of parameters that can satisfy the observational constraints, but within this range, the models score remarkably well. Our favored models have virial masses of 1012 and 1.6 ? 1012 M? for the Galaxy and for M31, respectively, average spin parameters ? ? 0.03-0.05, and concentrations Cvir = 10-17, typical for halos of this mass in the standard ?CDM cosmology. The models require neither dark matter modifications nor flat cores to fit the observational data. We explore two types of models, with and without the exchange of angular momentum between the dark matter and the baryons. The models without exchange give reasonable rotation curves, fulfill constraints in the solar neighborhood, and satisfy constraints at larger radii, but they may be problematic for fast rotating central bars. We explore models in which the baryons experience additional contraction due to loss of angular momentum to the surrounding dark matter. These models produce similar global properties, but the dark matter is only a 25% of the total mass in the central 3 kpc region, allowing a fast rotating bar to persist. According to preliminary calculations, our model galaxies probably have sufficient baryonic mass in the central ~3.5 kpc to reproduce recent observational values of the optical depth to microlensing events toward the Galactic center. Our dynamical models unequivocally require that about 50% of all the gas inside the virial radius must not be in the disk or in the bulge, a result that is obtained naturally in standard semianalytic models. Assuming that the Milky Way is typical, we investigate whether the range of virial masses allowed by our dynamical models is compatible with constraints from the galaxy luminosity function. We find that if the Milky Way has a luminosity MK = -24.0, then these constraints are satisfied, but if it is more luminous (as expected if it lies on the Tully-Fisher relation), then the predicted space density is larger than the observed space density of galaxies of the corresponding luminosity by a factor of 1.5-2. We conclude that observed rotation curves and dynamical properties of normal spiral galaxies appear to be consistent with standard ?CDM.

The Hubble Ultra Deep Field

This paper presents the Hubble Ultra Deep Field (HUDF), a 1 million s exposure of an 11 arcmin2 region in the southern sky with the Advanced Camera for Surveys on the Hubble Space Telescope using Directors Discretionary Time. The exposure time was divided among four filters, F435W (B435), F606W (V606), F775W (i 775), and F850LP (z850), to give approximately uniform limiting magnitudes mAB ~ 29 for point sources. The image contains at least 10,000 objects, presented here as a catalog, the vast majority of which are galaxies. Visual inspection of the images shows few if any galaxies at redshifts greater than ~4 that resemble present-day spiral or elliptical galaxies. The image reinforces the conclusion from the original Hubble Deep Field that galaxies evolved strongly during the first few billion years in the infancy of the universe. Using the Lyman break dropout method to derive samples of galaxies at redshifts between 4 and 7, it is possible to study the apparent evolution of the galaxy luminosity function and number density. Examination of the catalog for dropout sources yields 504 B435 dropouts, 204 V 606 dropouts, and 54 i775 dropouts. The i775 dropouts are most likely galaxies at redshifts between 6 and 7. Using these samples, which are at different redshifts but derived from the same data, we find no evidence for a change in the characteristic luminosity of galaxies but some evidence for a decrease in their number densities between redshifts of 4 and 7. Assessing the factors needed to derive the luminosity function from the data suggests that there is considerable uncertainty in parameters from samples discovered with different instruments and derived using independent assumptions about the source populations. This assessment calls into question some of the strong conclusions of recently published work on distant galaxies. The ultraviolet luminosity density of these samples is dominated by galaxies fainter than the characteristic luminosity, and the HUDF reveals considerably more luminosity than shallower surveys. The apparent ultraviolet luminosity density of galaxies appears to decrease from redshifts of a few to redshifts greater than 6, although this decrease may be the result of faint-end incompleteness in the most distant samples. The highest redshift samples show that star formation was already vigorous at the earliest epochs at which galaxies have been observed, less than 1 billion years after the big bang.

The effect of galaxy mass ratio on merger-driven starbursts

We employ numerical simulations of galaxy mergers to explore the effect of galaxy mass ratio on merger-driven starbursts. Our numerical simulations include radiative cooling of gas, star formation, and stellar feedback to follow the interaction and merger of four disc galaxies. The galaxy models span a factor of 23 in total mass and are designed to be representative of typical galaxies in the local universe. We find that the merger-driven star formation is a strong function of merger mass ratio, with very little, if any, induced star formation for large mass ratio mergers. We define a burst efficiency that is useful to characterize the merger-driven star formation and test that it is insensitive to uncertainties in the feedback parametrization. In accord with previous work we find that the burst efficiency depends on the structure of the primary galaxy. In particular, the presence of a massive stellar bulge stabilizes the disc and suppresses merger-driven star formation for large mass ratio mergers. Direct, coplanar merging orbits produce the largest tidal disturbance and yield the most intense burst of star formation. Contrary to naive expectations, a more compact distribution of gas or an increased gas fraction both decrease the burst efficiency. Owing to the efficient feedback model and the newer version of smoothed particle hydrodynamics employed here, the burst efficiencies of the mergers presented here are smaller than in previous studies.