Gerard Lemson

From dwarf spheroidals to cD galaxies: simulating the galaxy population in a ΛCDM cosmology

We have updated and extended our semi-analytic galaxy formation modelling capabilities and applied them simultaneously to the stored halo/subhalo merger trees of the Millennium and Millennium-II simulations. These differ by a factor of 125 in mass resolution, allowing explicit testing of resolution effects on predicted galaxy properties. We have revised the treatments of the transition between the rapid infall and cooling flow regimes of gas accretion, of the sizes of bulges and of gaseous and stellar disks, of supernova feedback, of the transition between central and satellite status as galaxies fall into larger systems, and of gas and star stripping once they become satellites. Plausible values of efficiency and scaling parameters yield an excellent fit not only to the observed abundance of low-redshift galaxies over 5 orders of magnitude in stellar mass and 9 magnitudes in luminosity, but also to the observed abundance of Milky Way satellites. This suggests that reionisation effects may not be needed to solve the “missing satellite” problem except, perhaps, for the faintest objects. The same model matches the observed large-scale clustering of galaxies as a function of stellar mass and colour. The fit remains excellent down to � 30 kpc for massive galaxies. For M∗ < 6×10 10 M⊙, however, the model overpredicts clustering at scales below � 1 Mpc, suggesting that the assumed fluctuation amplitude, σ8 = 0.9, is too high. The observed difference in clustering between active and passive galaxies is matched quite well for all masses. Galaxy distributions within rich clusters agree between the simulations and match those observed, but only if galaxies without dark matter subhalos (so-called orphans) are included. Even at MS-II resolution, schemes which assign galaxies only to resolved dark matter subhalos cannot match observed clusters. Our model predicts a larger passive fraction among low-mass galaxies than is observed, as well as an overabundance of � 10 10 M⊙ galaxies beyond z � 0.6. (The abundance of � 10 11 M⊙ galaxies is matched out to z � 3.) These discrepancies appear to reflect deficiencies in the way star-formation rates are modelled.

Galaxy formation in the Planck Cosmology I - Matching the observed evolution of star-formation rates, colours and stellar masses

We have updated the Munich galaxy formation model to the Planck first-year cosmology, while modifying the treatment of baryonic processes to reproduce recent data on the abundance and passive fractions of galaxies from z = 3 down to z = 0. Matching these more extensive and more precise observational results requires us to delay the reincorporation of wind ejecta, to lower the threshold for turning cold gas into stars, to eliminate ram-pressure stripping in halos less massive than 10 14 M⊙, and to modify our model for radio-mode feedback. These changes cure the most obvious failings of our previous models, namely the overly early formation of low-mass galaxies and the overly large fraction of them that are passive at late times. The new model reproduces the observed evolution both of the stellar mass function and of the distribution of star-formation rate at each stellar mass. Massive galaxies assemble most of their mass before z = 1 and are predominantly old and passive at z = 0, while lower mass galaxies assemble later and are predominantly blue and star-forming at z = 0. This phenomenological but physically based model allows the observations to be interpreted in terms of the eciency of the various processes t hat control the formation and evolution of galaxies as a function of their stellar mass, gas content, environment and time.

Galaxy formation in WMAP1 and WMAP7 cosmologies

Using the technique of Angulo & White (2010) we scale the Millennium and Millennium-II simulations of structure growth in aCDM universe from the cosmo- logical parameters with which they were carried out (based on first-year results from the Wilkinson Microwave Anisotropy Probe, WMAP1) to parameters consistent with the seven-year WMAP data (WMAP7). We implement semi-analytic galaxy formation modelling on both simulations in both cosmologies to investigate how the formation, evolution and clustering of galaxies are predicted to vary with cosmological parame- ters. The increased matter density m and decreased linear fluctuation amplitude σ8 in WMAP7 have compensating effects, so that the abundance and clustering of dark halos are predicted to be very similar to those in WMAP1 for z 6 3. As a result, local galaxy properties can be reproduced equally well in the two cosmologies by slightly altering galaxy formation parameters. The evolution of the galaxy populations is then also similar. In WMAP7, structure forms slightly later. This shifts the peak in cosmic star formation rate to lower redshift, resulting in slightly bluer galaxies at z = 0. Nev- ertheless, the model still predicts more passive low-mass galaxies than are observed. For rp 1 galaxies are predicted to be more strongly clustered for WMAP7. Differences in galaxy properties, including, clustering, in these two cosmologies are rather small out to z � 3. Given that there are still considerable residual uncertainties in galaxy formation models, it is very difficult to distinguish WMAP1 from WMAP7 through observations of galaxy properties or their evolution.

Radio-loud Narrow-Line Type 1 Quasars

We present the first systematic study of (non-radio-selected) radio-loud narrow-line Seyfert 1 (NLS1) galaxies. Cross-correlation of the Catalogue of Quasars and Active Nuclei with several radio and optical catalogs led to the identification of ~11 radio-loud NLS1 candidates, including four previously known ones. This study almost triples the number of known radio-loud NLS1 galaxies if all candidates are confirmed. Most of the radio-loud NLS1 galaxies are compact, steep-spectrum sources accreting close to or above the Eddington limit. The radio-loud NLS1 galaxies of our sample are remarkable in that they occupy a previously rarely populated regime in NLS1 multiwavelength parameter space. While their [O III]/Hβ and Fe II/Hβ intensity ratios almost cover the whole range observed in NLS1 galaxies, their radio properties extend the range of radio-loud objects to those with small widths of the broad Balmer lines. Their black hole masses are generally at the upper observed end among NLS1 galaxies but are still unusually small in view of the radio loudness of the sources. Among the radio-detected NLS1 galaxies, the radio index R is distributed quite smoothly up to the critical value of R 10 and covers about 4 orders of magnitude in total. Statistics show that ~7% of the NLS1 galaxies are formally radio-loud, while only 2.5% exceed a radio index R > 100. Implications for NLS1 models are discussed. Several mechanisms are considered as explanations for the radio loudness of the NLS1 galaxies and for the lower frequency of radio-loud galaxies among NLS1 galaxies than among quasars. While properties of most sources (with two to three exceptions) generally do not favor relativistic beaming, the combination of accretion mode and spin may explain the observations.

Simulations of the galaxy population constrained by observations from z = 3 to the present day: implications for galactic winds and the fate of their ejecta

We apply Monte Carlo Markov Chain (MCMC) methods to large-scale simulations of galaxy formation in a LambdaCDM cosmology in order to explore how star formation and feedback are constrained by the observed luminosity and stellar mass functions of galaxies. We build models jointly on the Millennium and Millennium-II simulations, applying fast sampling techniques which allow observed galaxy abundances over the ranges 7<log(M*/Msun)<12 and z=0 to z=3 to be used simultaneously as constraints in the MCMC analysis. When z=0 constraints alone are imposed, we reproduce the results of previous modelling by Guo et al. (2012), but no single set of parameters can reproduce observed galaxy abundances at all redshifts simultaneously, reflecting the fact that low-mass galaxies form too early and thus are overabundant at high redshift in this model. The data require the efficiency with which galactic wind ejecta are reaccreted to vary with redshift and halo mass quite differently than previously assumed, but in a similar way as in some recent hydrodynamic simulations of galaxy formation. We propose a specific model in which reincorporation timescales vary inversely with halo mass and are independent of redshift. This produces an evolving galaxy population which fits observed abundances as a function of stellar mass, B- and K-band luminosity at all redshifts simultaneously. It also produces a significant improvement in two other areas where previous models were deficient. It leads to present day dwarf galaxy populations which are younger, bluer, more strongly star-forming and more weakly clustered on small scales than before, although the passive fraction of faint dwarfs remains too high.

Bursty stellar populations and obscured active galactic nuclei in galaxy bulges

We investigate trends between the recent star formation history and black hole growth in galaxy bulges in the Sloan Digital Sky Survey (SDSS). The galaxies lie at 0.01 60%) is contributed by bulges with more moderate recent or ongoing star formation. The strongest accreting black holes reside in bulges with a wide range in recent SFH. We conclude that our results support the popular hypothesis for black hole growth occurring through gas inflow into the central regions of galaxies, foll owed by a starburst and triggering of the AGN. However, while this is a significant pathway for th e growth of black holes, it is not the dominant one in the present-day Universe. More unspectacular processes are apparently responsible for the majority of this growth. In order to arrive at these conclusions we have developed a set of new high signal-to-noise ratio (SNR) optical spectral indicators, designed to allow a detailed study of stellar populations which have undergone recent enhanced star formation. Working in the rest-frame wavelength range 3750-4150 ˚ A, ideally suited to many recent and ongoing spectroscopic surveys at low and high redshift, the first two indices are equivalent to the previously well studied 4000 ˚ A break strength and Hδ equivalent width. The primary advantage of this new method is a greatly improved SNR for the latter index, allowing the present study to use spectra with SNR-perpixel as low as 8. The third index measures the excess strength of Ca II (H&K), which is particularly sensitive to the transition of a post-starbur st spectrum from A to F stars, and allows the degeneracy between time of burst and strength of burst to be broken.

The MultiDark Database: Release of the Bolshoi and MultiDark cosmological simulations

We present the online MultiDark Database -- a Virtual Observatory-oriented, relational database for hosting various cosmological simulations. The data is accessible via an SQL (Structured Query Language) query interface, which also allows users to directly pose scientific questions, as shown in a number of examples in this paper. Further examples for the usage of the database are given in its extensive online documentation (www.multidark.org). The database is based on the same technology as the Millennium Database, a fact that will greatly facilitate the usage of both suites of cosmological simulations. The first release of the MultiDark Database hosts two 8.6 billion particle cosmological N-body simulations: the Bolshoi (250/h Mpc simulation box, 1/h kpc resolution) and MultiDark Run1 simulation (MDR1, or BigBolshoi, 1000/h Mpc simulation box, 7/h kpc resolution). The extraction methods for halos/subhalos from the raw simulation data, and how this data is structured in the database are explained in this paper. With the first data release, users get full access to halo/subhalo catalogs, various profiles of the halos at redshifts z=0-15, and raw dark matter data for one time-step of the Bolshoi and four time-steps of the MultiDark simulation. Later releases will also include galaxy mock catalogs and additional merging trees for both simulations as well as new large volume simulations with high resolution. This project is further proof of the viability to store and present complex data using relational database technology. We encourage other simulators to publish their results in a similar manner.

Confronting theoretical models with the observed evolution of the galaxy population out to z= 4

We construct lightcones for the semi-analytic galaxy formation simulation of Guo et al. (2011) and make mock catalogues for comparison with deep high-redshift surveys. Photometric properties are calculated with two different stellar population synthesis codes (Bruzual & Charlot 2003; Maraston 2005) in order to study sensitivity to this aspect of the modelling. The catalogues are publicly available and include photometry for a large number of observed bands from 4000u to 6µm, as well as rest-frame photometry and other intrinsic properties of the galaxies (e.g positions, peculiar velocities, stellar masses, sizes, morphologies, gas fractions, star formation rates, metallicities, halo properties). Guo et al. (2011) tuned their model to fit the low-redshift galaxy population but noted that at z > 1 it overpredicts the abundance of galaxies below the “knee” of the stellar mass function. Here we extend the comparison to deep galaxy counts in the B, i, J, K and IRAC 3.6µm, 4.5µm and 5.8µm bands, to the redshift distributions of K and 5.8µm selected galaxies, and to the evolution of rest-frame luminosity functions in the B and K bands. The B, i and J counts are well reproduced, but at longer wavelengths the overabundant high-redshift galaxies produce excess faint counts. At bright magnitudes, counts in the IRAC bands are underpredicted, reflecting overly low stellar metallicities and the neglect of PAH emission. The predicted redshift distributions for K and 5.8µm selected samples highlight the effect of emission from thermally pulsing AGB stars. The full treatment of Maraston (2005) predicts three times as many z � 2 galaxies in faint 5.8µm selected samples as the model of Bruzual & Charlot (2003), whereas the two models give similar predictions for Kband selected samples. Although luminosity functions are adequately reproduced out to z � 3 in rest-frame B, the same is true at rest-frame K only if TP-AGB emission is included, and then only at high luminosity. Fainter than L⋆ the two synthesis models agree but overpredict the number of galaxies, another reflection of the overabundance of � 10 10 M⊙ model galaxies at z > 1.

Local gravity versus local velocity: solutions for β and non-linear bias

We perform a reconstruction of the cosmological large scale flows in the nearby Universe using two complementary observational sets. The first, the SFI++ sample of Tully-Fisher (TF) measurements of galaxies, provides a direct probe of the flows. The second, the whole sky distribution of galaxies in the 2MASS redshift survey (2MRS), yields a prediction of the flows given the cosmological density parameter, , and a biasing relation between mass and galaxies. We aim at an unbiased comparison between the peculiar velocity fields extracted from the two data sets and its implication on the cosmological parameters and the biasing relation. We expand the fields in a set of orthonormal basis functions, each representing a plausible realization of a cosmological velocity field smoothed in such a way as to give a nearly constant error on the derived SFI++ velocities. The statistical analysis is done on the coefficients of the modal expansion of the fields by means of the basis functions. Our analysis completely avoids the strong error covariance in the smoothed TF velocities by the use of orthonormal basis functions and employs elaborate mock data sets to extensively calibrate the errors in 2MRS predicted velocities. We relate the 2MRS galaxy distribution to the mass density field by a linear bias factor, b, and include a luminosity dependent, / L � , galaxy weighting. We assess the agreement between the fields as a �

The Varied Fates of z ~ 2 Star-forming Galaxies

Star-forming galaxies constitute the majority of galaxies with stellar masses 1010 h?2 M? at -->z ~ 2. It is thus critical to understand their origins, evolution, and connection to the underlying dark matter distribution. To this end, we identify the dark matter halos (including subhalos) that are likely to contain star-forming galaxies at -->z ~ 2 (z2SFGs) within a large dissipationless cosmological simulation and then use halo merger histories to follow the evolution of z2SFG descendants to -->z ~ 1 and -->z ~ 0. The evolved halos at these epochs are then confronted with an array of observational data in order to uncover the likely descendants of z2SFGs. Although the evolved halos have clustering strengths comparable to red galaxies at -->z ~ 1 and -->z ~ 0, we find that the bulk of z2SFGs do not evolve into red galaxies, at either epoch. This conclusion is based primarily on the fact that the space density of z2SFGs is much higher than that of lower redshift red galaxies, even when accounting for the merging of z2SFG descendants, which decreases the number density of z2SFG descendants by at most a factor of two by -->z ~ 0. Of the ~50% of z2SFGs that survive to -->z ~ 0, ~70% reside at the center of -->z ~ 0 dark matter halos with -->M > 1012 h?1 M?. Halo occupation modeling of -->z ~ 0 galaxies suggests that such halos are occupied by galaxies with -->Mr ? 20.5, implying that these z2SFGs evolve into typical ~L* galaxies today, including our own Galaxy. The remaining ~30% become satellite galaxies by -->z ~ 0, and comparison to halo occupation modeling suggests that they are rather faint, with -->Mr ? 19.5. These conclusions are qualitatively generic in the sense that any halo-mass-selected sample of galaxies at one epoch will evolve into a more complex and heterogeneous sample of galaxies at a later epoch.