Total Halo Mass Research Articles

Comparison of simple mass estimators for slowly rotating elliptical galaxies

We compare the performance of mass estimators for elliptical galaxies that rely on the directly observable surface brightness and velocity dispersion profiles, without invoking computationally expensive detailed modeling. These methods recover the mass at a specific radius where the mass estimate is expected to be least sensitive to the anisotropy of stellar orbits. One method (Wolf et al. 2010) uses the total luminosity-weighted velocity dispersion and evaluates the mass at a 3D half-light radius $r_{1/2}$, i.e., it depends on the GLOBAL galaxy properties. Another approach (Churazov et al. 2010) estimates the mass from the velocity dispersion at a radius $R_2$ where the surface brightness declines as $R^{-2}$, i.e., it depends on the LOCAL properties. We evaluate the accuracy of the two methods for analytical models, simulated galaxies and real elliptical galaxies that have already been modeled by the Schwarzschild's orbit-superposition technique. Both estimators recover an almost unbiased circular speed estimate with a modest RMS scatter ($\lesssim 10 \%$). Tests on analytical models and simulated galaxies indicate that the local estimator has a smaller RMS scatter than the global one. We show by examination of simulated galaxies that the projected velocity dispersion at $R_2$ could serve as a good proxy for the virial galaxy mass. For simulated galaxies the total halo mass scales with $\sigma_p(R_2)$ as $M_{vir} \left[M_{\odot}h^{-1}\right] \approx 6\cdot 10^{12} \left( \frac{\sigma_p(R_2)}{200\, \rm km\, s^{-1}} \right)^{4}$ with RMS scatter $\approx 40 \%$.

THE STELLAR MASS–HALO MASS RELATION FOR LOW-MASS X-RAY GROUPS AT 0.5< z < 1 IN THE CDFS WITH CSI

Since z~1, the stellar mass density locked in low mass groups and clusters has grown by a factor of ~8. Here we make the first statistical measurements of the stellar mass content of low mass X-ray groups at 0.5<z<1, enabling the calibration of stellar-to-halo mass scales for wide-field optical and infrared surveys. Groups are selected from combined Chandra and XMM-Newton X-ray observations in the Chandra Deep Field South (CDFS). These ultra-deep observations allow us to identify bona fide low mass groups at high redshift and enable measurements of their total halo masses. We compute aggregate stellar masses for these halos using galaxies from the Carnegie-Spitzer-IMACS (CSI) spectroscopic redshift survey. Stars comprise ~3-4% of the total mass of group halos with masses 10^{12.8}<M200/Msun<10^{13.5} (about the mass of Fornax and 1/50th the mass of Virgo). Complementing our sample with higher mass halos at these redshifts, we find that the stellar-to-halo mass ratio decreases toward higher halo masses, consistent with other work in the local and high redshift universe. The observed scatter about the stellar-halo mass relation is ~0.25 dex, which is relatively small and suggests that total group stellar mass can serve as a rough proxy for halo mass. We find no evidence for any significant evolution in the stellar-halo mass relation since z<1. Quantifying the stellar content in groups since this epoch is critical given that hierarchical assembly leads to such halos growing in number density and hosting increasing shares of quiescent galaxies.

Dynamical signatures of a ΛCDM-halo and the distribution of the baryons in M 33

We map the neutral atomic gas content of M33 using high resolution VLA and GBT observations and fit a tilted ring model to determine the orientation of the extended gaseous disk and its rotation curve. The disk of M33 warps from 8 kpc outwards without substantial change of its inclination with respect to the line of sight. Rotational velocities rise steeply with radius in the inner disk, reaching 100 km/s in 4 kpc, then the rotation curve becomes more perturbed and flatter with velocities as high as 120-130 km/s out to 23 kpc. We derive the stellar mass surface density map of M33's optical disk, via pixel -SED fitting methods based on population synthesis models, which highlights variations in the mass-to-light ratio. The stellar mass surface further out is estimated from deep images of outer disk fields. Stellar and gas maps are then used in the dynamical analysis of the rotation curve to constrain the dark matter distribution which is relevant at all radii. A dark matter halo with a Navarro-Frenk-White density profile in a LCDM cosmology, provides the best fit to the rotation curve for a dark halo concentration C=10 and a total halo mass of 4.3 10^{11}Msun. This imples a baryonic fraction of order 0.02 and the evolutionary history of this galaxy should account for loss of a large fraction of its original baryonic content.

Halo mass and assembly history exposed in the faint outskirts: the stellar and dark matter haloes of Illustris galaxies

We use the Illustris simulations to gain insight into the build-up of the outer, low-surface brightness regions which surround galaxies. We characterize the stellar haloes by means of the logarithmic slope of the spherically averaged stellar density profiles, αSTARS at z = 0, and we relate these slopes to the properties of the underlying dark matter (DM) haloes, their central galaxies, and their assembly histories. We analyse a sample of ∼5000 galaxies resolved with more than 5 × 104 particles each, and spanning a variety of morphologies and halo masses (3 × 1011 ≤ Mvir ≲ 1014 M⊙). We find a strong trend between stellar halo slope and total halo mass, where more massive objects have shallower stellar haloes than the less massive ones (−5.5 ± 0.5 < αSTARS < −3.5 ± 0.2 in the studied mass range). At fixed halo mass, we show that disc-like, blue, young, and more massive galaxies are surrounded by significantly steeper stellar haloes than elliptical, red, older, and less massive galaxies. Overall, the stellar density profiles fall off much more steeply than the underlying DM, and no clear trend holds between stellar slope and DM halo concentration. However, DM haloes which formed more recently, or which accreted larger fractions of stellar mass from infalling satellites, exhibit shallower stellar haloes than their older analogues with similar masses, by up to ΔαSTARS ∼ 0.5–0.7. Our findings, combined with the most recent measurements of the strikingly different stellar power-law indices for M31 and the Milky Way, appear to favour a massive M31, and a Milky Way characterized by a much quieter accretion history over the past 10 Gyr than its companion.

The impact of galaxy formation on the total mass, mass profile and abundance of haloes

We use cosmological hydrodynamical simulations to investigate how the inclusion of physical processes relevant to galaxy formation (star formation, metal-line cooling, stellar winds, supernovae and feedback from Active Galactic Nuclei, AGN) change the properties of haloes, over four orders of magnitude in mass. We find that gas expulsion and the associated dark matter (DM) expansion induced by supernova-driven winds are important for haloes with masses M200 < 10^13 Msun, lowering their masses by up to 20% relative to a DM-only model. AGN feedback, which is required to prevent overcooling, has a significant impact on halo masses all the way up to cluster scales (M200 ~ 10^15 Msun). Baryonic physics changes the total mass profiles of haloes out to several times the virial radius, a modification that cannot be captured by a change in the halo concentration. The decrease in the total halo mass causes a decrease in the halo mass function of about 20%. This effect can have important consequences for abundance matching technique as well as for most semi-analytic models of galaxy formation. We provide analytic fitting formulae, derived from simulations that reproduce the observed baryon fractions, to correct halo masses and mass functions from DM-only simulations. The effect of baryonic physics (AGN feedback in particular) on cluster number counts is about as large as changing the cosmology from WMAP7 to Planck, even when a moderately high mass limit of M500 ~ 10^14 Msun is adopted. Thus, for precision cosmology the effects of baryons must be accounted for.

CENTRAL GALAXIES IN DIFFERENT ENVIRONMENTS: DO THEY HAVE SIMILAR PROPERTIES?

We perform an exhaustive comparison among central galaxies from SDSS catalogs in different local environments at 0.01<=z<=0.08. The central galaxies are separated into two categories: group centrals (host halos containing satellites) and field centrals (host halos without satellites). From the latter, we select other two subsamples: isolated centrals and bright field centrals, both with the same magnitude limit. The stellar mass (Ms) distributions of the field and group central galaxies are different, which explains why in general the field central galaxies are mainly located in the blue cloud/star forming regions, whereas the group central galaxies are strongly biased to the red sequence/passive regions. The isolated centrals occupy the same regions as the bright field centrals since both populations have similar Ms distributions. At parity of Ms, the color and specific star formation rate (sSFR) distributions of the samples are similar, specially between field and group centrals. Furthermore, we find that the stellar-to-halo mass (Ms-Mh) relation of isolated galaxies does not depend on the color, sSFR and morphological type. For systems without satellites, the Ms-Mh relation steepens at high halo masses compared to group centrals, which is a consequence of assuming a one-to-one relation between group total stellar mass and halo mass. Under the same assumption, the scatter around the Ms-Mh relation of centrals with satellites increases with halo mass. Our results suggest that the mass growth of central galaxies is mostly driven by the halo mass, with environment and mergers playing a secondary role.

A tight scaling relation of dark matter in galaxy clusters

Abstract Recent studies in different types of galaxies reveal that the product of the central density and the core radius (ρcrc) is a constant. However, some empirical studies involving galaxy clusters suggest that the product ρcrc depends weakly on the total dark halo mass. In this Letter, we re-analyse the hot gas data from 106 clusters and obtain a surprisingly tight scaling relation: $\rho _{\rm c} \propto r_{\rm c}^{-1.46 \pm 0.16}$. This result generally agrees with the claims that ρcrc is not a constant for all scales of structure. Moreover, this relation does not support the velocity-dependent cross-section of dark matter if the core formation is due to the self-interaction of dark matter.

A census of stellar mass in ten massive haloes atz~ 1 from the GCLASS Survey

We study the stellar mass content of massive haloes in the redshift range 0.86<z<1.34, by measuring: (1) The stellar mass in the central galaxy versus total dynamical halo mass. (2) The total stellar mass (including satellites) versus total halo mass. (3) The radial stellar mass and number density profiles for the ensemble halo. We use a Ks-band selected catalogue for the 10 clusters in the Gemini Cluster Astrophysics Spectroscopic Survey (GCLASS), with photometric redshifts and stellar masses measured from 11-band SED fitting. Combining the photometric catalogues with the deep spectroscopic component of GCLASS, we correct the cluster galaxy sample for interlopers. We also perform a dynamical analysis of the cluster galaxies to estimate the halo mass M200 for each cluster based on a measurement of its velocity dispersion. (1) We find that the central galaxy stellar mass fraction decreases with total halo mass, and that this is in reasonable quantitative agreement with measurements from abundance matching studies at z~1. (2) The total stellar mass fractions of these systems decrease with halo mass, indicating that lower mass systems are more efficient at transforming baryons into stars. We find the total stellar mass to be a good proxy for total halo mass, with a small intrinsic scatter. When we compare these results with literature measurements, we find that the stellar mass fraction at fixed halo mass shows no significant evolution in the range 0<z<1. (3) We measure a relatively high NFW concentration parameter c ~ 7 for the stellar mass distribution in these clusters. A simple model shows that the stellar mass content of GCLASS can evolve in typical distributions observed at lower redshifts if the clusters primarily accrete stellar mass onto the outskirts. [Abridged]

Imprints of dark energy on cosmic structure formation – III. Sparsity of dark matter halo profiles

We study the imprint of Dark Energy on the density profile of Dark Matter halos using a set of high-resolution large volume cosmological N-body simulations from the Dark Energy Universe Simulation Series (DEUSS). We first focus on the analysis of the goodness-of-fit of the Navarro-Frenk-White (NFW) profile which we find to vary with halo mass and redshift. We also find that the fraction of halos ill-fitted by NFW varies with cosmology, thus indicating that the mass assembly of halos with perturbed density profiles carries a characteristic signature of Dark Energy. To access this information independently of any parametric profile, we introduce a new observable quantity: the halo sparsity $s_\Delta$. This is defined as the mass ratio $M_{200}/M_\Delta$, i.e. the ratio of mass inside a sphere of radius $r_{200}$ to that contained within a radius $r_\Delta$, enclosing 200 and $\Delta$ times the mean matter density respectively. We find the average sparsity to be nearly independent of the total halo mass, while its value can be inferred to better than a few percent from the ratio of the integrated halo mass functions at overdensities $\Delta$ and 200 respectively. This provides a consistency relation that can validate observational measurements of the halo sparsity. Most importantly, the sparsity significantly varies with the underlying Dark Energy model, thus providing an alternative cosmological probe.

ULTRA-COMPACT HIGH VELOCITY CLOUDS AS MINIHALOS AND DWARF GALAXIES

We present dark-matter minihalo models for the Ultra-Compact High Velocity HI Clouds (UCHVCs) recently discovered in the 21 cm ALFALFA survey. We assume gravitational confinement of 10^4 K HI gas by flat-cored dark-matter subhalos within the Local Group. We show that for flat cores, typical (median) tidally-stripped cosmological subhalos at redshift z=0 have dark-matter masses of ~10^7 M_{sun} within the central 300 pc (independent of total halo mass), consistent with the "Strigari mass scale" observed in low-luminosity dwarf galaxies. Flat-cored subhalos also resolve the mass-discrepancy between simulated and observed satellites around the Milky Way. For the UCHVCs we calculate the photoionization-limited hydrostatic gas profiles for any distance-dependent total observed HI mass and predict the associated (projected) HI half-mass radii, assuming the clouds are embedded in distant (d > 300 kpc) and unstripped subhalos. For a typical UCHVC (0.9 Jy km/s) we predict physical HI half-mass radii of 0.18 to 0.35 kpc (or angular sizes of 0.6 to 2.1 arcmin) for distances ranging from 300 kpc to 2 Mpc. As a consistency check we model the gas rich dwarf galaxy Leo T, for which there is a well-resolved HI column density profile and a known distance (420 kpc). For Leo T we find that a subhalo with M_{300} ~ 8 (+/-0.2) 10^6 M_{sun} best fits the observed HI profile. We derive an upper limit of P_{HIM} < 150 K/cm^3 for the pressure of any enveloping hot IGM gas at the distance of Leo T. Our analysis suggests that some of the UCHVCs may in fact constitute a population of 21-cm-selected but optically-faint dwarf galaxies in the Local Group.

The strong environmental dependence of black hole scaling relations

We investigate how the scaling relations between central black hole mass (Mbh) and host galaxy properties (velocity dispersion, bulge stellar mass and bulge luminosity) depend on the large scale environment. For each of a sample of 69 galaxies with dynamical black hole measurements we compile four environmental measures (nearest neighbor distance, fixed aperture number density, total halo mass, and central/satellite). We find that central and satellite galaxies follow distinctly separate scalings in each of the three relations we have examined. The Mbh - sigma relation of central galaxies is significantly steeper (6.38 +/- 0.49) than that of satellite galaxies (4.91 +/- 0.49), but has a similar intercept. This behavior remains even after restricting to a sample of only early type galaxies or after removing the 8 brightest cluster galaxies. The Mbh - sigma relation shows more modest differences when splitting the sample based on the other environmental indicators, suggesting that they are driven by the underlying satellite/central fractions. Separate relations for centrals and satellites are also seen in the power law scaling between black hole mass and bulge stellar mass or bulge luminosity. We suggest that gas rich, low mass galaxies undergo a period of rapid black hole growth in the process of becoming satellites. If central galaxies on the current Mbh - sigma relation are representative progenitors of the satellite population, the observations imply that a sigma = 120 km/s galaxy must nearly triple its central black hole mass. The elevated black hole masses of massive central galaxies are then a natural consequence of the accretion of satellites.

REPRODUCING THE STELLAR MASS/HALO MASS RELATION IN SIMULATED ΛCDM GALAXIES: THEORY VERSUS OBSERVATIONAL ESTIMATES

We examine the present-day total stellar-to-halo mass (SHM) ratio as a function of halo mass for a new sample of simulated field galaxies using fully cosmological, LCDM, high resolution SPH + N-Body simulations.These simulations include an explicit treatment of metal line cooling, dust and self-shielding, H2 based star formation and supernova driven gas outflows. The 18 simulated halos have masses ranging from a few times 10^8 to nearly 10^12 solar masses. At z=0 our simulated galaxies have a baryon content and morphology typical of field galaxies. Over a stellar mass range of 2.2 x 10^3 to 4.5 x 10^10 solar masses, we find extremely good agreement between the SHM ratio in simulations and the present-day predictions from the statistical Abundance Matching Technique presented in Moster et al. (2012). This improvement over past simulations is due to a number systematic factors, each decreasing the SHM ratios: 1) gas outflows that reduce the overall SF efficiency but allow for the formation of a cold gas component 2) estimating the stellar masses of simulated galaxies using artificial observations and photometric techniques similar to those used in observations and 3) accounting for a systematic, up to 30 percent overestimate in total halo masses in DM-only simulations, due to the neglect of baryon loss over cosmic times. Our analysis suggests that stellar mass estimates based on photometric magnitudes can underestimate the contribution of old stellar populations to the total stellar mass, leading to stellar mass errors of up to 50 percent for individual galaxies. These results highlight the importance of using proper techniques to compare simulations with observations and reduce the perceived tension between the star formation efficiency in galaxy formation models and in real galaxies.

Predictions for ASKAP neutral hydrogen surveys

The Australian Square Kilometer Array Pathfinder (ASKAP) will revolutionise our knowledge of gas-rich galaxies in the Universe. Here we present predictions for two proposed extragalactic ASKAP neutral hydrogen (HI) emission-line surveys, based on semi-analytic models applied to cosmological N-body simulations. The ASKAP HI All-Sky Survey, known as WALLABY, is a shallow 3 Pi survey (z = 0 - 0.26) which will probe the mass and dynamics of over 600,000 galaxies. A much deeper small-area HI survey, called DINGO, aims to trace the evolution of HI from z = 0 - 0.43, a cosmological volume of 40 million Mpc^3, detecting potentially 100,000 galaxies. The high-sensitivity 30 antenna ASKAP core (diameter ~2 km) will provide an angular resolution of 30 arcsec (at z=0). Our simulations show that the majority of galaxies detected in WALLABY (87.5%) will be resolved. About 5000 galaxies will be well resolved, i.e. more than five beams (2.5 arcmin) across the major axis, enabling kinematic studies of their gaseous disks. This number would rise to 160,000 galaxies if all 36 ASKAP antennas could be used; the additional six antennas provide baselines up to 6 km, resulting in an angular resolution of 10 arcsec. For DINGO this increased resolution is highly desirable to minimise source confusion; reducing confusion rates from a maximum of 10% of sources at the survey edge to 3%. We estimate that the sources detected by WALLABY and DINGO will span four orders of magnitude in total halo mass (from 10^{11} to 10^{15} Msol) and nearly seven orders of magnitude in stellar mass (from 10^{5} to 10^{12} Msol), allowing us to investigate the process of galaxy formation across the last four billion years.

THE SPIN ALIGNMENTS IN GALAXY PAIRS AS A TEST OF BOUNCING COUPLED DARK ENERGY

We investigate the effect of coupled dark energy (cDE) on the spin alignments in isolated pairs of galactic halos, using the publicly available data from the hydrodynamical cDE simulations (H-CoDECs) which were run for various cDE models such as EXP001, EXP002, EXP003 (with exponential potential and constant coupling), EXP008e3 (with exponential potential and exponential coupling) and SUGRA003 (with supergravity potential and negative constant coupling) as well as for a standard LCDM cosmology (with the WMAP7 parameters). Measuring the cosines of the angles between the spin axes in isolated pairs of galactic halos for each model and determining its probability density distribution, we show that for the SUGRA003 model with bouncing cDE the null hypothesis of no spin alignment in pairs of galactic halos is rejected at 99.999% confidence level. In contrast, the LCDM cosmology yields no significant signal of spin alignment and the other four cDE models also exhibit only weak signals of spin-alignments. The strength of the spin alignment signal is found to be almost independent of the total halo mass and separation distance in galaxy pairs. Showing also that no signal is detected from the Sloan Digital Sky Survey DR 7, we conclude that the spin alignments in galaxy pairs is in principle a unique test of bouncing cDE models.

The radial distribution of galaxies in groups and clusters

We present a new catalogue of 55,121 groups and clusters centred on Luminous Red Galaxies from SDSS DR7 in the redshift range 0.15<z<0.4. We provide halo mass estimates for each of these groups derived from a calibration between the optical richness of bright galaxies (M_r<-20.5) within 1 Mpc, and X-ray-derived mass for a small subset of 129 groups and clusters with X-ray measurements. We derive the mean (stacked) surface number density profiles of galaxies as a function of total halo mass in different mass bins. We find that derived profiles can be well-described by a projected NFW profile with a concentration parameter (<c>~2.6) that is approximately a factor of two lower than that of the dark matter (as predicted by N-body cosmological simulations) and nearly independent of halo mass. Interestingly, in spite of the difference in shape between the galaxy and dark matter radial distributions, both exhibit a high degree of self-similarity. A self-consistent comparison to several recent semi-analytic models of galaxy formation indicates that: (1) beyond ~0.3 r_500 current models are able to reproduce both the shape and normalisation of the satellite profiles; and (2) within ~0.3 r_500 the predicted profiles are sensitive to the details of the satellite-BCG merger timescale calculation. The former is a direct result of the models being tuned to match the global galaxy luminosity function combined with the assumption that the satellite galaxies do not suffer significant tidal stripping, even though their surrounding dark matter haloes can be removed through this process. Combining our results with measurements of the intracluster light should provide a way to inform theoretical models on the efficacy of the tidal stripping and merging processes.

The evolution of the star formation activity per halo mass up to redshift ~1.6 as seen byHerschel

Star formation in massive galaxies is quenched at some point during hierarchical mass assembly. To understand where and when the quenching processes takes place, we study the evolution of the total star formation rate per unit total halo mass (\Sigma(SFR/M)) in three different mass scales: low mass halos (field galaxies), groups, and clusters, up to a redshift ~1.6. We use deep far-infrared PACS data at 100 and 160 um to accurately estimate the total star formation rate of the Luminous Infrared Galaxy population of 9 clusters with mass ~10^{15} M_{\odot}, and 9 groups/poor clusters with mass ~ 5 x 10^{13} M_{\odot}. Estimates of the field \Sigma(SFR/M) are derived from the literature, by dividing the star formation rate density by the mean comoving matter density of the universe. The field \Sigma(SFR/M) increases with redshift up to z~1 and it is constant thereafter. The evolution of the \Sigma(SFR/M)-z relation in galaxy systems is much faster than in the field. Up to redshift z~0.2, the field has a higher \Sigma(SFR/M) than galaxy groups and galaxy clusters. At higher redshifts, galaxy groups and the field have similar \Sigma(SFR/M), while massive clusters have significantly lower \Sigma(SFR/M) than both groups and the field. There is a hint of a reversal of the SFR activity vs. environment at z~1.6, where the group \Sigma(SFR/M) lies above the field \Sigma(SFR/M)-z relation. We discuss possible interpretations of our results in terms of the processes of downsizing, and star-formation quenching.

Predicting the merger fraction of Lyα emitters from redshift z ∼ 3 to 7

Rapid mass assembly, likely from mergers or smooth accretion, has been predicted to play a vital role in star formation in high-redshift Lyα emitters. Here, we predict the major merger, minor merger and smooth accreting Lyα emitter fraction from z ≈ 3 to 7 using a large dark matter simulation and a simple physical model that is successful in reproducing many observations over this large redshift range. The central tenet of this model, different from many of the earlier models, is that the star formation in Lyα emitters is proportional to the mass accretion rate rather than the total halo mass. We find that at z ≈ 3 nearly 35 per cent of the Lyα emitters accrete their mass through major (3:1) mergers, and this fraction increases to about 50 per cent at z ≈ 7. This implies that the star formation in a large fraction of high-redshift Lyα emitters is driven by mergers. Comparing our predictions with observations, we find that the model-predicted fractions are relatively larger. While there is discrepancy between the model predictions and observed merger fractions, some of this difference (˜15 per cent) can be attributed to the mass-ratio used to define a merger in the simulation. We predict that future, deeper observations which use a 3:1 definition of major mergers will find ≥30 per cent major merger fraction of Lyα emitters at redshifts ≥3.

RADIAL DISTRIBUTION OF STARS, GAS, AND DUST IN SINGS GALAXIES. III. MODELING THE EVOLUTION OF THE STELLAR COMPONENT IN GALAXY DISKS

(Abridged) We analyze the evolution of 42 spiral galaxies in the Spitzer Infrared Nearby Galaxies Survey, using extinction-corrected UV, optical and near-infrared radial profiles to probe the emission of stars of different ages as a function of radius. We fit these profiles with models that describe the chemical and spectro-photometric evolution of spiral disks within a self-consistent framework. These backward models succesfully reproduce the multi-wavelength profiles of our galaxies, except the UV profiles of some early-type disks. From the model fitting we infer the maximum circular velocity of the rotation curve (Vc) and the dimensionless spin parameter (lambda). The values of Vc are in good agreement with the velocities measured in HI rotation curves. While our sample is not volume-limited, the resulting distribution of spins is close to the lognormal function obtained in cosmological N-body simulations, peaking at ~0.03 regardless of the total halo mass. We do not find any evident trend between spin and Hubble type, besides an increase in the scatter for the latest types. According to the model, galaxies evolve along a roughly constant mass-size relation, increasing their scale-lengths as they become more massive. The radial scale-length of most disks in our sample seems to have increased at a rate of 0.05-0.06 kpc/Gyr, being now 20-25% larger than at z=1.

THE GROWTH OF DARK MATTER HALOS: EVIDENCE FOR SIGNIFICANT SMOOTH ACCRETION

We study the growth of dark matter halos in the concordance LCDM cosmology using several N-body simulations of large cosmological volumes. We build merger trees from the Millennium and Millennium-II simulations, covering a range 10^9-10^15 Msun in halo mass and 1-10^5 in merger mass ratio. Our algorithm takes special care of halo fragmentation and ensures that the mass contribution of each merger to halo growth is only counted once. This way the integrated merger rate converges and we can consistently determine the contribution of mergers of different mass ratios to halo growth. We find that all resolved mergers, up to mass ratios of 10^5, contribute only ~60% of the total halo mass growth, while major mergers are subdominant, e.g. mergers with mass ratios smaller than 3:1 (10:1) contribute only ~20% (~30%). This is verified with an analysis of two additional simulation boxes, where we follow all particles individually throughout cosmic time. Our results are also robust against using several halo definitions. Under the assumption that the power-law behaviour of the merger rate at large mass ratios can be extrapolated to arbitrarily large mass ratios, it is found that, independently of halo mass, ~40% of the mass in halos comes from genuinely smooth accretion of dark matter that was never bound in smaller halos. We discuss possible implications of our findings for galaxy formation. One implication, assuming as is standard that the pristine intergalactic medium is heated and photoionized by UV photons, is that all halos accrete >40% of their baryons in smooth "cold" T>~10^4K gas, rather than as warm, enriched or clumpy gas or as stars.

Globular cluster systems in nearby dwarf galaxies - III. Formation efficiencies of old globular clusters★

We investigate the globular cluster (GC) system scaling parameters as a function of galaxy mass, i.e. specific frequency (S_N), specific luminosity (S_L), specific mass (S_M), and specific number (^T) of GCs. We sample the entire range in galaxy luminosity (Mv = -11 to -23 mag = 10^6 - 10^11 L_sol), environment, and morphology. Irrespective of galaxy type, we confirm the increase of the S_N-value above and below a galaxy magnitude of Mv = -20 mag. Over the full mass range, the S_L-value of early-type galaxies is, on average, twice that of late-types. To investigate the observed trends we derive theoretical predictions of GC system scaling parameters as a function of host galaxy mass based on the models of Dekel & Birnboim (2006) in which star-formation processes are regulated by stellar and supernova feedback below a stellar mass of 3x10^10 M_sol, and by virial shocks above it. We find that the analytical model describes remarkably well the shape of the GC system scaling parameter distributions with a universal specific GC formation efficiency, eta, which relates the total mass in GCs to the total galaxy halo mass. Early-type and late-type galaxies show a similar mean value of eta = 5.5e-5, with an increasing scatter towards lower galaxy masses. This can be due to the enhanced stochastic nature of the star and star-cluster formation processes for such systems. Some massive galaxies have excess eta values compared to what is expected from the mean model prediction for galaxies more luminous than Mv = -20 mag (Lv=10^10L_sol). This may be attributed to a very efficient early GC formation, less efficient production of field stars or accretion of predominantly low-mass/luminosity high-eta galaxies, or a mixture of all these effects. (Abridged)