Assembly History Of Galaxies Research Articles

THE METALLICITY BIMODALITY OF GLOBULAR CLUSTER SYSTEMS: A TEST OF GALAXY ASSEMBLY AND OF THE EVOLUTION OF THE GALAXY MASS-METALLICITY RELATION

(Abridged) We build a theoretical model to study the origin of the globular cluster metallicity bimodality in the hierarchical galaxy assembly scenario, based on the observed galaxy mass-[O/H] relation and the galaxy stellar mass function up to z ~4, and on theoretical merger rates. We derive a new galaxy [Fe/H]-M(star) relation as a function of z, and by assuming that GCs share the metallicity of their parent galaxy when they form, we populate the merger tree with GCs. We perform a series of Monte-Carlo simulations of the galaxy assembly, and study the properties of the final GC population as a function of galaxy mass, assembly and star formation history, and under different assumptions for the evolution of the galaxy mass-[Fe/H] relation. The main results are: 1) The hierarchical clustering scenario naturally predicts a metallicity bimodality in the galaxy GC population: the metal-rich GCs are formed in the galaxy main progenitor around z~2, and the metal-poor GCs are accreted from satellites and formed at z~3-4. 2) The model reproduces the observed relations for the metallicity of the metal-rich and metal-poor GCs as a function of galaxy mass. The positions of the metal-poor and metal-rich peaks depend exclusively on the evolution of the galaxy mass-[Fe/H] relation and the [O/Fe], both of which can be constrained by this method. We find that the galaxy [O/Fe] evolves linearly with z from a value of ~0.5 at z~4 to a value of ~0.1 at z=0. 3) Given a galaxy mass, the relative strength of the metal-rich and metal-poor peaks depends exclusively on the galaxy assembly and star formation history: galaxies in denser environments and/or early types galaxies show a larger fraction of metal-poor GCs, while galaxies with a sparse merger history and/or late type galaxies are dominated by metal-rich GCs. 4) The GC metallicity bimodality disappears for galaxy masses below M(star)~1e9, and for z>2.

UNCOVERING DRIVERS OF DISK ASSEMBLY: BULGELESS GALAXIES AND THE STELLAR MASS TULLY-FISHER RELATION

In order to determine what processes govern the assembly history of galaxies with rotating disks, we examine the stellar mass Tully-Fisher relation over a wide range in redshift partitioned according to whether or not galaxies contain a prominent bulge. Using our earlier Keck spectroscopic sample, for which bulge/total parameters are available from analyses of HST images, we find that bulgeless disk galaxies with z > 0.8 present a significant offset from the local Tully-Fisher relation whereas, at all redshifts probed, those with significant bulges fall along the local relation. Our results support the suggestion that bulge growth may somehow expedite the maturing of disk galaxies onto the Tully-Fisher relation. We discuss a variety of physical hypotheses that may explain this result in the context of kinematic observations of star-forming galaxies at redshifts z = 0 and z > 2.

Galactic star formation and accretion histories from matching galaxies to dark matter haloes

We present a new statistical method to determine the relationship between the stellar masses of galaxies and the masses of their host dark matter haloes over the entire cosmic history from z~4 to the present. This multi-epoch abundance matching (MEAM) model self-consistently takes into account that satellite galaxies first become satellites at times earlier than they are observed. We employ a redshift-dependent parameterization of the stellar-to-halo mass relation to populate haloes and subhaloes in the Millennium simulations with galaxies, requiring that the observed stellar mass functions at several redshifts be reproduced simultaneously. Using merger trees extracted from the dark matter simulations in combination with MEAM, we predict the average assembly histories of galaxies, separating into star formation within the galaxies (in-situ) and accretion of stars (ex-situ). The peak star formation efficiency decreases with redshift from 23% at z=0 to 9% at z=4 while the corresponding halo mass increases from 10^11.8M\odot to 10^12.5M\odot. The star formation rate of central galaxies peaks at a redshift which depends on halo mass; for massive haloes this peak is at early cosmic times while for low-mass galaxies the peak has not been reached yet. In haloes similar to that of the Milky-Way about half of the central stellar mass is assembled after z=0.7. In low-mass haloes, the accretion of satellites contributes little to the assembly of their central galaxies, while in massive haloes more than half of the central stellar mass is formed ex-situ with significant accretion of satellites at z<2. We find that our method implies a cosmic star formation history and an evolution of specific star formation rates which are consistent with those inferred directly. We present convenient fitting functions for stellar masses, star formation rates, and accretion rates as functions of halo mass and redshift.

THE SLUGGS SURVEY: NGC 3115, A CRITICAL TEST CASE FOR METALLICITY BIMODALITY IN GLOBULAR CLUSTER SYSTEMS

Due to its proximity (9 Mpc) and the strongly bimodal color distribution of its spectroscopically well-sampled globular cluster (GC) system, the early-type galaxy NGC 3115 provides one of the best available tests of whether the color bimodality widely observed in GC systems generally reflects a true metallicity bimodality. Color bimodality has alternatively been attributed to a strongly nonlinear color--metallicity relation reflecting the influence of hot horizontal branch stars. Here we couple Subaru Suprime-Cam gi photometry with Keck/DEIMOS spectroscopy to accurately measure GC colors and a CaT index that measures the CaII triplet. We find the NGC 3115 GC system to be unambiguously bimodal in both color and the CaT index. Using simple stellar population models, we show that the CaT index is essentially unaffected by variations in horizontal branch morphology over the range of metallicities relevant to GC systems (and is thus a robust indicator of metallicity) and confirm bimodality in the metallicity distribution. We assess the existing evidence for and against multiple metallicity subpopulations in early and late-type galaxies and conclude that metallicity bi/multimodality is common. We briefly discuss how this fundamental characteristic links directly to the star formation and assembly histories of galaxies.

Assembly Histories and Observational Properties of Simulated Early-type Galaxies

AbstractWe demonstrate that massive simulated galaxies assemble in two phases, with the initial growth dominated by compact in situ star formation, whereas the late growth is dominated by accretion of old stars formed in subunits outside the main galaxy. We also show that 1) gravitational feedback strongly suppresses late star formation in massive galaxies contributing to the observed galaxy colour bimodality that 2) the observed galaxy downsizing can be explained naturally in the two-phased model and finally that 3) the details of the assembly histories of massive galaxies are directly connected to their observed kinematic properties.

The influence of halo evolution on galaxy structure

AbstractIf Einstein-Newton gravity holds on galactic and larger scales, then current observations demonstrate that the stars and interstellar gas of a typical bright galaxy account for only a few percent of its total nonlinear mass. Dark matter makes up the rest and cannot be faint stars or any other baryonic form because it was already present and decoupled from the radiation plasma at z = 1000, long before any nonlinear object formed. The weak gravito-sonic waves so precisely measured by CMB observations are detected again at z = 4 as order unity fluctuations in intergalactic matter. These subsequently collapse to form today's galaxy/halo systems, whose mean mass profiles can be accurately determined through gravitational lensing. High-resolution simulations link the observed dark matter structures seen at all these epochs, demonstrating that they are consistent and providing detailed predictions for all aspects of halo structure and growth. Requiring consistency with the abundance and clustering of real galaxies strongly constrains the galaxy-halo relation, both today and at high redshift. This results in detailed predictions for galaxy assembly histories and for the gravitational arena in which galaxies live. Dark halos are not expected to be passive or symmetric but to have a rich and continually evolving structure which will drive evolution in the central galaxy over its full life, exciting warps, spiral patterns and tidal arms, thickening disks, producing rings, bars and bulges. Their growth is closely related to the provision of new gas for galaxy building.

THE ASSEMBLY HISTORY OF DISK GALAXIES. I. THE TULLY-FISHER RELATION TO z {approx_equal} 1.3 FROM DEEP EXPOSURES WITH DEIMOS

We present new measures of the evolving scaling relations between stellar mass, luminosity and rotational velocity for a morphologically inclusive sample of 129 disk-like galaxies with z{sub AB} < 22.5 in the redshift range 0.2 <z < 1.3, based on spectra from DEep Imaging Multi-Object Spectrograph on the Keck II telescope, multi-color Hubble Space Telescope (HST) Advanced Camera for Surveys photometry, and ground-based K{sub s} -band imaging. A unique feature of our survey is the extended spectroscopic integration times, leading to significant improvements in determining characteristic rotational velocities for each galaxy. Rotation curves are reliably traced to the radius where they begin to flatten for {approx}90% of our sample, and we model the HST-resolved bulge and disk components in order to accurately de-project our measured velocities, accounting for seeing and dispersion. We demonstrate the merit of these advances by recovering an intrinsic scatter on the stellar mass Tully-Fisher relation a factor of two to three less than in previous studies at intermediate redshift and comparable to that of locally determined relations. With our increased precision, we find that the relation is well established by (z) {approx} 1, with no significant evolution to (z) {approx} 0.3, {Delta}M{sub *} {approx} 0.04 {+-}more » 0.07 dex. A clearer trend of evolution is seen in the B-band Tully-Fisher relation corresponding to a decline in luminosity of {Delta}M{sub B} {approx} 0.85 {+-} 0.28 magnitudes at fixed velocity over the same redshift range, reflecting the changes in star formation over this period. As an illustration of the opportunities possible when gas masses are available for a sample such as ours, we show how our dynamical and stellar mass data can be used to evaluate the likely contributions of baryons and dark matter to the assembly history of spiral galaxies.« less

THE ASSEMBLY HISTORY OF DISK GALAXIES. I. THE TULLY-FISHER RELATION TOz≃ 1.3 FROM DEEP EXPOSURES WITH DEIMOS

We present new measures of the evolving scaling relations between stellar mass, luminosity and rotational velocity for a morphologically-inclusive sample of 129 disk-like galaxies with z_AB<22.5 in the redshift range 0.2<z<1.3, based on spectra from DEIMOS on the Keck II telescope, multi-color HST ACS photometry, and ground-based Ks-band imaging. A unique feature of our survey is the extended spectroscopic integration times, leading to significant improvements in determining characteristic rotational velocities for each galaxy. Rotation curves are reliably traced to the radius where they begin to flatten for ~90% of our sample, and we model the HST-resolved bulge and disk components in order to accurately de-project our measured velocities, accounting for seeing and dispersion. We demonstrate the merit of these advances by recovering an intrinsic scatter on the stellar mass Tully-Fisher relation a factor of 2-3 less than in previous studies at intermediate redshift and comparable to that of locally-determined relations. With our increased precision, we find the relation is well-established by <z>~1, with no significant evolution to <z>~0.3, \DeltaM_stellar ~ 0.04+/-0.07 dex. A clearer trend of evolution is seen in the B-band Tully-Fisher relation corresponding to a decline in luminosity of \DeltaM_B ~ 0.85+/-0.28 magnitudes at fixed velocity over the same redshift range, reflecting the changes in star formation over this period. As an illustration of the opportunities possible when gas masses are available for a sample such as ours, we show how our dynamical and stellar mass data can be used to evaluate the likely contributions of baryons and dark matter to the assembly history of spiral galaxies.

Large-scale gas dynamics in the adhesion model: implications for the two-phase massive galaxy formation scenario

The mass assembly and star formation histories of massive galaxies identified at low redshift z in different cosmological hydrodynamical simulations, have been studied through a detailed follow-up backwards in time of their constituent mass elements (sampled by particles) of different types. Then, the configurations they depict at progressively higher zs have been analysed. The analyses show that these histories share common generic patterns, irrespective of particular circumstances. In any case, the results we have found are different depending on the particle type. The most outstanding differences follow. We have found that by z ~ 3.5 - 6, mass elements identified as stellar particles at z=0 exhibit a gaseous cosmic-web-like morphology with scales of ~ 1 physical Mpc, where the densest mass elements have already turned into stars by z ~ 6. These settings are in fact the densest pieces of the cosmic web, where no hot particles show up, and dynamically organized as a hierarchy of flow convergence regions, that is, attraction basins for mass flows. On the other hand, mass elements identified at the diffuse hot coronae surrounding massive galaxies at z = 0, do not display a clear web-like morphology at any z. Diffuse gas is heated when flow convergence regions go through contractive deformations, and most of it keeps hot and with low density along the evolution. To shed light on the physical foundations of the behaviour our analyses show up, as well as on their possible observational implications, these patterns have been confronted with some generic properties of singular flows as described by the adhesion model. We have found that these common patterns simulations show can be interpreted as a consequence of flow properties, that, moreover, could explain different generic observational results on massive galaxies or their samples. We briefly discuss some of them.[Abridged]

Evidence of major dry mergers at M* &amp;gt; 2 × 1011M⊙ from curvature in early-type galaxy scaling relations?

Abstract For early-type galaxies, the correlations between stellar mass and size, velocity dispersion, surface brightness, colour, axial ratio and colour gradient all indicate that two mass scales, M*= 3 × 1010 and 2 × 1011 M⊙, are special. The smaller scale could mark the transition between wet and dry mergers, or it could be related to the interplay between supernovae (SNe) and active galactic nuclei (AGNs) feedback, although quantitative measures of this transition may be affected by morphological contamination. At the more massive scale, mean axial ratios and colour gradients are maximal, and above it, the colours are redder, the sizes larger and the velocity dispersions smaller than expected based on the scaling at lower M*. In contrast, the colour–σ relation, and indeed, most scaling relations with σ, are not curved: they are well described by a single power law, or in some cases, are almost completely flat. When major dry mergers change masses, sizes, axial ratios and colour gradients, they are expected to change the colours or velocity dispersions much less. Therefore, the fact that scaling relations at σ &amp;gt; 150 km s −1 show no features, whereas the size–M*, b/a–M*, colour–M* and colour gradient–M* relations do, suggests that M*= 2 × 1011 M⊙ is the scale above which major mergers dominate the assembly histories of early-type galaxies.

Constraining the star formation and the assembly histories of normal and compact early-type galaxies at 1 &lt; z &lt; 2

[Abridged]We present a study based on a sample of 62 early-type galaxies (ETGs) at 0.9<z_spec<2 aimed at constraining their past star formation and mass assembly histories. The sample is composed of normal ETGs having effective radii comparable to the mean radius of local ones and of compact ETGs having effective radii from two to six times smaller. We do not find evidence of a dependence of the compactness of ETGs on their stellar mass. We find that the stellar mass of normal ETGs formed at z_form<3 while the stellar content of compact ETGs formed at 2<z_form<10 with a large fraction of them characterized by z_form>5. Earlier stars formed at z_form>5 are assembled in compact and more massive (M_*>10^11 M_sun) ETGs while stars later formed (z_form<3) or resulting from subsequent episodes of star formation are assembled both in compact and normal ETGs. Thus, the older the stellar population the higher the mass of the hosting galaxy but not vice versa. This suggests that the epoch of formation may play a role in the formation of massive ETGs rather than the mass itself. The possible general scheme in which normal <z>~1.5 ETGs are descendants of high-z compact spheroids enlarged through subsequent dry mergers is not compatible with the current models which predict a number of dry mergers two orders of magnitude lower than the one needed. Moreover, we do not find evidence supporting a dependence of the compactness of galaxies on their redshift of assembly. Finally, we propose a simple scheme of formation and assembly of the stellar mass of ETGs based on dissipative gas-rich merger which can qualitatively account for the co-existence of normal and compact ETGs observed at <z>~1.5 in spite of the same stellar mass, the lack of normal ETGs with high z_form and the absence of correlation between compactness, stellar mass and formation redshift.

Galaxy-induced transformation of dark matter haloes

We use N-body/gasdynamical LambdaCDM cosmological simulations to examine the effect of the assembly of a central galaxy on the shape and mass profile of its dark halo. Two series of simulations are compared; one that follows only the evolution of the dark matter component and a second one where a baryonic component is added. These simulations include radiative cooling but neglect star formation and feedback, leading most baryons to collect at the halo center in a disk which is too small and too massive when compared with typical spiral. This unrealistic model allows us, nevertheless, to gauge the maximum effect that galaxies may have in transforming their dark halos. We find that the shape of the halo becomes more axisymmetric: halos are transformed from triaxial into essentially oblate systems, with well-aligned isopotential contours of roughly constant flattening (c/a ~ 0.85). Halos always contract as a result of galaxy assembly, but the effect is substantially less pronounced than predicted by the "adiabatic contraction" hypothesis. The reduced contraction helps to reconcile LambdaCDM halos with constraints on the dark matter content inside the solar circle and should alleviate the long-standing difficulty of matching simultaneously the scaling properties of galaxy disks and the luminosity function. The halo contraction is also less pronounced than found in earlier simulations, a disagreement that suggests that halo contraction is not solely a function of the initial and final distribution of baryons. Not only how much baryonic mass has been deposited at the center of a halo matters, but also the mode of its deposition. It might prove impossible to predict the halo response without a detailed understanding of a galaxy's assembly history. (Abriged)

Structures in the fundamental plane of early-type galaxies

The fundamental plane of early-type galaxies is a rather tight three-parameter correlation discovered more than twenty years ago. It has resisted a both global and precise physical interpretation despite a consequent number of works, observational, theoretical or using numerical simulations. It appears that its precise properties depend on the population of galaxies in study. Instead of selecting a priori these populations, we propose to objectively construct homologous populations from multivariate analyses. We have undertaken multivariate cluster and cladistic analyses of a sample of 56 low-redshift galaxy clusters containing 699 early-type galaxies, using four parameters: effective radius, velocity dispersion, surface brightness averaged over effective radius, and Mg2 index. All our analyses are consistent with seven groups that define separate regions on the global fundamental plane, not across its thickness. In fact, each group shows its own fundamental plane, which is more loosely defined for less diversified groups. We conclude that the global fundamental plane is not a bent surface, but made of a collection of several groups characterizing several fundamental planes with different thicknesses and orientations in the parameter space. Our diversification scenario probably indicates that the level of diversity is linked to the number and the nature of transforming events and that the fundamental plane is the result of several transforming events. We also show that our classification, not the fundamental planes, is universal within our redshift range (0.007 - 0.053). We find that the three groups with the thinnest fundamental planes presumably formed through dissipative (wet) mergers. In one of them, this(ese) merger(s) must have been quite ancient because of the relatively low metallicity of its galaxies, Two of these groups have subsequently undergone dry mergers to increase their masses. In the k-space, the third one clearly occupies the region where bulges (of lenticular or spiral galaxies) lie and might also have formed through minor mergers and accretions. The two least diversified groups probably did not form by major mergers and must have been strongly affected by interactions, some of the gas in the objects of one of these groups having possibly been swept out. The interpretation, based on specific assembly histories of galaxies of our seven groups, shows that they are truly homologous. They were obtained directly from several observables, thus independently of any a priori classification. The diversification scenario relating these groups does not depend on models or numerical simulations, but is objectively provided by the cladistic analysis. Consequently, our classification is more easily compared to models and numerical simulations, and our work can be readily repeated with additional observables.

MODELING THE METALLICITY DISTRIBUTION OF GLOBULAR CLUSTERS

Observed metallicities of globular clusters reflect physical conditions in the interstellar medium of their high-redshift host galaxies. Globular cluster systems in most large galaxies display bimodal color and metallicity distributions, which are often interpreted as indicating two distinct modes of cluster formation. The metal-rich and metal-poor clusters have systematically different locations and kinematics in their host galaxies. However, the red and blue clusters have similar internal properties, such as the masses, sizes, and ages. It is therefore interesting to explore whether both metal-rich and metal-poor clusters could form by a common mechanism and still be consistent with the bimodal distribution. We present such a model, which prescribes the formation of globular clusters semi-analytically using galaxy assembly history from cosmological simulations coupled with observed scaling relations for the amount and metallicity of cold gas available for star formation. We assume that massive star clusters form only during mergers of massive gas-rich galaxies and tune the model parameters to reproduce the observed distribution in the Galaxy. A wide, but not entire, range of model realizations produces metallicity distributions consistent with the data. We find that early mergers of smaller hosts create exclusively blue clusters, whereas subsequent mergers of more massive galaxies create both red and blue clusters. Thus bimodality arises naturally as the result of a small number of late massive merger events. This conclusion is not significantly affected by the large uncertainties in our knowledge of the stellar mass and cold gas mass in high-redshift galaxies. The fraction of galactic stellar mass locked in globular clusters declines from over 10% at z>3 to 0.1% at present.

A census of nuclear stellar discs in early-type galaxies

Nuclear stellar discs (NSDs), of a few tens to hundreds of parsec across, are a common and yet poorly studied feature of early-type galaxies. Still, such small discs represent a powerful tool to constrain the assembling history of galaxies, since they can be used to trace to the epoch when galaxies experienced their last major merger event. By studying the fraction and stellar age of NSDs, it is thus possible to test the predictions for the assembly history of early-type galaxies according to the current hierarchical paradigm for galaxy formation. In this paper we have produced the most comprehensive census of NSDs in nearby early-type galaxies by searching for such discs in objects within 100 Mpc and by using archival images from the Hubble Space Telescope. We found that NSDs are present in approximately 20 per cent of early-type galaxies, and that the fraction of galaxies with NSDs depends neither on their Hubble type nor on their galactic environment, whereas the incidence of NSDs appears to decline in the most massive systems. Furthermore, we have separated the light contribution of 12 such discs from that of their surrounding stellar bulge in order to extract their physical properties. This doubles the number of decomposed NSDs and although the derived values for their central surface brightness and scalelength are consistent with previous studies, they also give a hint of possible different characteristics due to different formation scenario between nuclear discs and other kinds of large galactic discs.

On the origin of globular cluster bimodality

AbstractGlobular cluster systems in most large galaxies display bimodal color and metallicity distributions, which are frequently interpreted as indicating two distinct modes of cluster formation. The metal-rich (red) and metal-poor (blue) clusters have systematically different locations and kinematics in their host galaxies. However, the red and blue clusters have similar internal properties, such as their masses, sizes, and ages. It is therefore interesting to explore whether both metal-rich and metal-poor clusters could form by a common mechanism and still be consistent with the bimodal distribution. We show that if all globular clusters form only during mergers of massive, gas-rich protogalactic disks, their metallicity distribution could be statistically consistent with that of the Galactic globulars. We take the galaxy assembly history from cosmological dark-matter simulations and couple it with the observed scaling relations for the amount of cold gas available for star formation. In the best-fitting model, early mergers of smaller hosts create exclusively blue clusters, while subsequent mergers of progenitor galaxies with a range of masses create both red and blue clusters. Thus, bimodality arises naturally as the result of a small number of late, massive merger events. We calculate cluster mass loss, including the effects of two-body scattering and stellar evolution, and find that more blue than red clusters are disrupted by the present time because of their lower initial masses and older ages. The present-day mass function in the best-fitting model is consistent with the Galactic distribution. However, the spatial distribution of model clusters is much more extended than observed and is independent of the parameters of our model.

STELLAR KINEMATIC CONSTRAINTS ON GALACTIC STRUCTURE MODELS REVISITED: BAR AND SPIRAL ARM RESONANCES

We study the phase space available to the local stellar distribution using a Galactic potential consistent with several recent observational constraints. We find that the induced phase space structure has several observable consequences. The spiral arm contribution to the kinematic structure in the solar neighborhood may be as important as the one produced by the Galactic bar. We suggest that some of the stellar kinematic groups in the solar neighborhood, such as the Hercules structure and the kinematic branches, can be created by the dynamical resonances of self-gravitating spiral arms and not exclusively by the Galactic bar. A structure coincident with the Arcturus kinematic group is developed when a hot stellar disk population is considered, which introduces a new perspective on the interpretation of its extragalactic origin. A bar-related resonant mechanism can modify this kinematic structure. We show that particles in the dark matter disk-like structure predicted by recent lambda cold dark matter galaxy formation experiments, with similar kinematics to the thick disk, are affected by the same resonances, developing phase space structures or dark kinematic groups that are independent of the Galaxy assembly history and substructure abundance. We discuss the possibility of using the stellar phase space groups as constraints to nonaxisymmetric models of the Milky Way structure.

COMMISSION 37: STAR CLUSTERS AND ASSOCIATIONS

Star clusters are valuable tools for theoretical and observational astronomy across a wide range of disciplines from cosmology to stellar spectroscopy. For example, properties of globular clusters are used to constrain stellar evolutionary models, nucleosynthesis and chemical evolution, as well as the star formation and assembly histories of galaxies and the distribution of dark matter in present-day galaxies. Open clusters are widely used as stellar laboratories for the study of specific stellar phenomena (e.g., various emission-line stars, pulsating pre-MS stars, magnetic massive stars, binarity, stellar rotation, etc.). They also provide observational constraints on models of massive star evolution and of Galactic disk formation and chemical evolution.

The galaxy stellar mass-star formation rate relation: evidence for an evolving stellar initial mass function?

The evolution of the galaxy stellar mass--star formation rate relationship (M*-SFR) provides key constraints on the stellar mass assembly histories of galaxies. For star-forming galaxies, M*-SFR is observed to be fairly tight with a slope close to unity from z~0-2. Simulations of galaxy formation reproduce these trends owing to the generic dominance of smooth and steady cold accretion in these systems. In contrast, the amplitude of the M*-SFR relation evolves markedly differently than in models. Stated in terms of a star formation activity parameter alpha=(M*/SFR)/(t_H-1 Gyr), models predict a constant alpha~1 out to redshifts z=4+, while the observed M*-SFR relation indicates that alpha increases by X3 from z~2 until today. The low alpha at high-z not only conflicts with models, but is also difficult to reconcile with other observations of high-z galaxies. Systematic biases could significantly affect measurements of M* and SFR, but detailed considerations suggest that none are obvious candidates to reconcile the discrepancy. A speculative solution is considered in which the stellar initial mass function (IMF) evolves towards more high-mass star formation at earlier epochs. Following Larson, a model is investigated in which the characteristic mass Mhat where the IMF turns over increases with redshift. The observed and predicted M*-SFR evolution may be brought into agreement if Mhat=0.5(1+z)^2 Mo out to z~2. Such evolution broadly matches recent observations of cosmic stellar mass growth, and the resulting z=0 cumulative IMF is similar to the paunchy IMF favored by Fardal et al to reconcile the observed cosmic star formation history with present-day fossil light measures. [abridged]

The Mass Assembly History of Spheroidal Galaxies: Did Newly Formed Systems Arise via Major Mergers?

We examine the properties of a morphologically selected sample of 0.4 < z < 1.0 spheroidal galaxies in the GOODS fields in order to ascertain whether or not their increase in abundance with time arises primarily from major mergers. To address this question, we determine the scaling relations between the dynamical mass M_(dyn), determined from stellar velocity dispersions, and the stellar mass M_*, determined from optical and infrared photometry. We exploit these relations across the larger sample for which we have stellar masses in order to construct the first statistically robust estimate of the evolving dynamical mass function (MF) over 0 < z < 1. The trends observed match those seen in the stellar mass functions of Bundy et al. regarding the top-down growth in the abundance of spheroidal galaxies. By referencing our dynamical masses to the halo virial mass, M_(vir), we compare the growth rate in the abundance of spheroidals to that predicted by the assembly of dark matter halos. Our comparisons demonstrate that major mergers do not fully account for the appearance of new spheroidals since z ~ 1 and that additional mechanisms, such as morphological transformations, are required to drive the observed evolution.