arXiv subjects

Simon D. M. White

Publications and source records attributed to Simon D. M. White.

At least 199 records · Page 11Linked to original sources

arXiv2007

Assembly bias in the clustering of dark matter haloes

We use a very large simulation of structure growth in a LCDM universe -- the Millennium Simulation -- to study assembly bias, the fact that the large-scale clustering of haloes of given mass varies significantly with their assembly history. We extend earlier work based on the same simulation by superposing results for redshifts from 0 to 3, by defining a less noisy estimator of clustering amplitude, and by considering halo concentration, substructure mass fraction and spin, as well as formation time, as additional parameters. These improvements lead to results with less noise than previous studies and covering a wider range of halo masses and structural properties. We find significant and significantly different assembly bias effects for all the halo properties we consider, although in all cases the dependences on halo mass and on redshift are adequately described as a dependence on equivalent peak height nu(M,z). The nu-dependences for different halo properties differ qualitatively and are not related as might naively be expected given the relations between formation time, concentration, substructure fraction and spin found for the halo population as a whole. These results suggest that it will be difficult to build models for the galaxy populations of dark haloes which can robustly relate the amplitude of large-scale galaxy clustering to that for mass clustering at better than the 10% level.

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arXiv2006

Halo assembly bias and its effects on galaxy clustering

The clustering of dark halos depends not only on their mass but also on their assembly history, a dependence we term `assembly bias'. Using a galaxy formation model grafted onto the Millennium Simulation of the LCDM cosmogony, we study how assembly bias affects galaxy clustering. We compare the original simulation to `shuffled' versions where the galaxy populations are randomly swapped among halos of similar mass, thus isolating the effects of correlations between assembly history and environment at fixed mass. Such correlations are ignored in the halo occupation distribution models often used populate dark matter simulations with galaxies, but they are significant in our more realistic simulation. Assembly bias enhances 2-point correlations by 10% for galaxies with M_bJ-5logh brighter than -17, but suppresses them by a similar amount for galaxies brighter than -20. When such samples are split by colour, assembly bias is 5% stronger for red galaxies and 5% weaker for blue ones. Halo central galaxies are differently affected by assembly bias than are galaxies of all types. It almost doubles the correlation amplitude for faint red central galaxies. Shuffling galaxies among halos of fixed formation redshift or concentration in addition to fixed mass produces biases which are not much smaller than when mass alone is fixed. Assembly bias must reflect a correlation of environment with aspects of halo assembly which are not encoded in either of these parameters. It induces effects which could compromise precision measurements of cosmological parameters from large galaxy surveys.

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arXiv2006

The build-up of the colour-magnitude relation in galaxy clusters since z~0.8

Using galaxy clusters from the ESO Distant Cluster Survey, we study how the distribution of galaxies along the colour-magnitude relation has evolved since z~0.8. While red-sequence galaxies in all these clusters are well described by an old, passively evolving population, we confirm our previous finding of a significant evolution in their luminosity distribution as a function of redshift. When compared to galaxy clusters in the local Universe, the high redshift EDisCS clusters exhibit a significant "deficit" of faint red galaxies. Combining clusters in three different redshift bins, and defining as `faint' all galaxies in the range 0.4 > L/L* > 0.1, we find a clear decrease in the luminous-to-faint ratio of red galaxies from z~0.8 to z~0.4. The amount of such a decrease appears to be in qualitative agreement with predictions of a model where the blue bright galaxies that populate the colour-magnitude diagram of high redshift clusters, have their star formation suppressed by the hostile cluster environment. Although model results need to be interpreted with caution, our findings clearly indicate that the red-sequence population of high-redshift clusters does not contain all progenitors of nearby red-sequence cluster galaxies. A significant fraction of these must have moved onto the red-sequence below z~0.8.

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arXiv2006

The high redshift galaxy population in hierarchical galaxy formation models

We compare observations of the high redshift galaxy population to the predictions of the galaxy formation model of Croton et al. (2006) and De Lucia & Blaizot (2006). This model, implemented on the Millennium Simulation of the concordance LCDM cosmogony, introduces "radio mode" feedback from the central galaxies of groups and clusters in order to obtain quantitative agreement with the luminosity, colour, morphology and clustering properties of the present-day galaxy population. Here we construct deep light cone surveys in order to compare model predictions to the observed counts and redshift distributions of distant galaxies, as well as to their inferred luminosity and mass functions out to redshift 5. With the exception of the mass functions, all these properties are sensitive to modelling of dust obscuration. A simple but plausible treatment agrees moderately well with most of the data. The predicted abundance of relatively massive (~M*) galaxies appears systematically high at high redshift, suggesting that such galaxies assemble earlier in this model than in the real Universe. An independent galaxy formation model implemented on the same simulation matches the observed mass functions slightly better, so the discrepancy probably reflects incomplete galaxy formation physics rather than problems with the underlying cosmogony.

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arXiv2006

Asymmetries in the inner regions of LCDM haloes

Many galaxies display warps, lopsided images, asymmetric rotation curves or other features which suggest that their immediate dynamical environment is neither static nor in equilibrium. In Cold Dark Matter (CDM) theories, such non-equilibrium features are expected in the inner regions of many dark haloes as a result of recent hierarchical growth. We used the excellent statistics provided by the very large Millennium Simulation to study (i) how the distribution of position and velocity asymmetries predicted for halo cores by the concordance LCDM cosmogony depends on halo mass, and (ii) how much of the dark matter in the inner core has been added at relatively recent times. Asymmetries are typically larger in more massive haloes. Thus 20% of cluster halos have density centre separated from barycentre by more than 20% of the virial radius, while only 7% of Milky Way halos have such large asymmetries. About 40% of all cluster halos have a mean core velocity which differs from the barycentre velocity by more than a quarter of the characteristic halo circular velocity, whereas only 10% of Milky Way halos have such large velocity offsets. About 25% of all cluster haloes have acquired more than a quarter of the mass currently in their inner 10kpc through mergers since z=1. The corresponding percentage of Milky Way haloes is 15%. These numbers seem quite compatible with the levels of asymmetry seen in the observable regions of galaxies, but quantitative comparison requires more detailed modelling of the observable components.

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arXiv2006

The clustering of narrow-line AGN in the local Universe

We have analyzed the clustering of ~ 90,000 narrow-line AGN drawn from the Data Release 4 (DR4) of the SDSS. We compute the cross-correlation between AGN and a reference sample of galaxies, and compare this to results for control samples of inactive galaxies matched simultaneously in redshift,stellar mass,concentration, velocity dispersion and the 4000A break strength. We also compare near-neighbour counts around AGN and around the control galaxies. On scales larger than a few Mpc, AGN have almost the same clustering amplitude as the control sample. This demonstrates that AGN host galaxies and inactive galaxies populate dark matter halos of similar mass.On scales between 100kpc and 1Mpc,AGN are clustered more weakly than the control galaxies. We use mock catalogues constructed from high-resolution N-body simulations to interpret this anti-bias, showing that the observed effect is easily understood if AGN are preferentially located at the centres of their dark matter halos. On scales less than 70 kpc, AGN cluster marginally more strongly than the control sample, but the effect is weak. When compared to the control sample, we find that only one in a hundred AGN has an extra neighbour within a radius of 70 kpc. This excess increases as a function of the accretion rate onto the black hole, but it does not rise above the few percent level. Although interactions between galaxies may be responsible for triggering nuclear activity in a minority of nearby AGN, some other mechanism is required to explain the activity seen in the majority of the objects in our sample. (abridged)

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arXiv2006

Feedback and metal enrichment in cosmological SPH simulations - II. A multiphase model with supernova energy feedback

We have developed a new scheme to treat a multiphase interstellar medium in smoothed particle hydrodynamics simulations of galaxy formation. This scheme can represent a co-spatial mixture of cold and hot ISM components, and is formulated without scale-dependent parameters. It is thus particularly suited to studies of cosmological structure formation where galaxies with a wide range of masses form simultaneously. We also present new algorithms for energy and heavy element injection by supernovae, and show that together these schemes can reproduce several important observed effects in galaxy evolution. Both in collapsing systems and in quiescent galaxies our codes can reproduce the Kennicutt relation between the surface densities of gas and of star formation. Strongly metal-enhanced winds are generated in both cases with ratios of mass-loss to star formation which are similar to those observed. This leads to a self-regulated cycle for star formation activity. The overall impact of feedback depends on galaxy mass. Star formation is suppressed at most by a factor of a few in massive galaxies, but in low-mass systems the effects can be much larger, giving star formation an episodic, bursty character. The larger the energy fraction assumed available in feedback, the more massive the outflows and the lower the final stellar masses. Winds from forming disks are collimated perpendicular to the disk plane, reach velocities up to 1000 km/s, and efficiently transport metals out of the galaxies. The asymptotically unbound baryon fraction drops from >95 per cent to ~30 per cent from the least to the most massive of our idealised galaxies, but the fraction of all metals ejected with this component exceeds 60 per cent regardless of mass. Such winds could plausibly enrich the intergalactic medium to observed levels.

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arXiv2006

The many lives of active galactic nuclei: cooling flows, black holes and the luminosities and colours of galaxies

We simulate the growth of galaxies and their central supermassive black holes by implementing a suite of semi-analytic models on the output of the Millennium Run, a very large simulation of the concordance LCDM cosmogony. Our procedures follow the detailed assembly history of each object and are able to track the evolution of all galaxies more massive than the Small Magellanic Cloud throughout a volume comparable to that of large modern redshift surveys. In this first paper we supplement previous treatments of the growth and activity of central black holes with a new model for `radio' feedback from those AGN that lie at the centre of a quasistatic X-ray emitting atmosphere in a galaxy group or cluster. We show that for energetically and observationally plausible parameters such a model can simultaneously explain: (i) the low observed mass drop-out rate in cooling flows; (ii) the exponential cut-off at the bright end of the galaxy luminosity function; and (iii) the fact that the most massive galaxies tend to be bulge-dominated systems in clusters and to contain systematically older stars than lower mass galaxies. This success occurs because static hot atmospheres form only in the most massive structures, and radio feedback (in contrast, for example, to supernova or starburst feedback) can suppress further cooling and star formation without itself requiring star formation. We discuss possible physical models which might explain the accretion rate scalings required for our phenomenological `radio mode' model to be successful.

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arXiv2006

The large-scale structure of the Universe

Research over the past 25 years has led to the view that the rich tapestry of present-day cosmic structure arose during the first instants of creation, where weak ripples were imposed on the otherwise uniform and rapidly expanding primordial soup. Over 14 billion years of evolution, these ripples have been amplified to enormous proportions by gravitational forces, producing ever-growing concentrations of dark matter in which ordinary gases cool, condense and fragment to make galaxies. This process can be faithfully mimicked in large computer simulations, and tested by a variety of observations that probe the history of the Universe starting from just 400,000 yr after the Big Bang.

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arXiv2006

How Rare is the Bullet Cluster?

The galaxy cluster 1E 0657-56 has a bullet-like subcluster that is moving away from the centre of the main cluster at high speed. Markevitch et al. (2004) recently estimated a relative velocity of V_bullet = 4500 +1100/-800 km/s, based on observations of the bow shock in front of the subcluster. The weak lensing analysis of Clowe et al. (2004) indicates that a substantial secondary mass peak is associated with this subcluster. We estimate the likelihood of such a configuration by examining the distribution of subhalo velocities for clusters in the Millennium Run, a large LCDM cosmological simulation. We find that the most massive subhalo has a velocity as high as that of the bullet subcluster in only about 1 out of every 100 cluster-sized halos. This estimate is strongly dependent on the precise velocity adopted for the bullet. One of the ten most massive subhalos has such a high velocity about 40% of the time. We conclude that the velocity of the bullet subcluster is not exceptionally high for a cluster substructure, and can be accommodated within the currently favoured LCDM comogony.

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arXiv2006

The dependence of clustering on galaxy properties

(abridged)We use a sample of ~200,000 galaxies drawn from the Sloan Digital Sky Survey to study how clustering depends on properties such as stellar mass (M*), colour (g-r), 4000A break strength (D4000), concentration index (C), and stellar surface mass density (μ_*). We find that more massive galaxies cluster more strongly than less massive galaxies, with the difference increasing above the characteristic stellar mass of the Schechter mass function. When divided by physical quantities, galaxies with redder colours, larger D4000, higher C and larger μ_* cluster more strongly. The clustering differences are largest on small scales and for low mass galaxies. At fixed stellar mass,the dependences of clustering on colour and 4000A break strength are similar. Different results are obtained when galaxies are split by concentration or surface density. The dependence of w(r_p) on g-r and D4000 extends out to physical scales that are significantly larger than those of individual dark matter haloes (> 5 Mpc/h). This large-scale clustering dependence is not seen for the parameters C or μ_*. On small scales (< 1 Mpc/h), the amplitude of the correlation function is constant for ``young'' galaxies with 1.1 < D4000< 1.5 and a steeply rising function of age for ``older'' galaxies with D4000>1.5. In contrast, the dependence of the amplitude of w(r_p) on concentration on scales less than 1 Mpc/h is strongest for disk-dominated galaxies with C<2.6. This demonstrates that different processes are required to explain environmental trends in the structure and in star formation history of galaxies.

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arXiv2006

The dependence of the pairwise velocity dispersion on galaxy properties

(abridged) We present measurements of the pairwise velocity dispersion (PVD) for different classes of galaxies in the Sloan Digital Sky Survey. For a sample of about 200,000 galaxies, we study the dependence of the PVD on galaxy properties such as luminosity, stellar mass (M_*), colour (g-r), 4000A break strength (D4000), concentration index (C), and stellar surface mass density (μ_*). The luminosity dependence of the PVD is in good agreement with the results of Jing & Börner (2004) for the 2dFGRS catalog. The value of σ_{12} measured at k=1 h/Mpc decreases as a function of increasing galaxy luminosity for galaxies fainter than L*, before increasing again for the most luminous galaxies in our sample. Each of the galaxy subsamples selected according to luminosity or stellar mass is divided into two further subsamples according to colour, D4000, C and μ_*. We find that galaxies with redder colours and higher D4000, C, and μ_* values have larger PVDs on all scales and at all luminosities/stellar masses. The dependence of the PVD on parameters related to recent star formation(colour, D4000) is stronger than on parameters related to galaxy structure (C, μ_*), especially on small scales and for faint galaxies. The reddest galaxies and galaxies with high surface mass densities and intermediate concentrations have the highest pairwise peculiar velocities, i.e. these move in the strongest gravitational fields. We conclude that the faint red population located in rich clusters is responsible for the high PVD values that are measured for low-luminosity galaxies on small scales.

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arXiv2006

The soft X-ray properties of quasars in the Sloan Digital Sky Survey

We use the ROSAT All Sky Survey (RASS) to study the soft X-ray properties of a homogeneous sample of 46,420 quasars selected from the third data release of the Sloan Digital Sky Survey (SDSS). Optical luminosities, both at rest-frame 2500Å($L_{2500}$) and in [OIII] ($L_{[\rm{OIII}]}$) span more than three orders of magnitude, while redshifts range over $0.1<z<5.4$. We detect 3366 quasars directly in the observed 0.1--2.4 keV band. Sub-samples of radio-loud and radio-quiet objects (RLQs and RQQs) are obtained by cross-matching with the FIRST catalogue. We study the distribution of X-ray luminosity as a function of optical luminosity, redshift and radio power using both individual detections and stacks of complete sets of similar quasars. At every optical luminosity and redshift $\log L_{2\kev}$ is, to a good approximation, normally distributed with dispersion $\sim 0.40$, at least brightwards of the median X-ray luminosity. This median X-ray luminosity of quasars is a power law of optical luminosity with index $\sim 0.53$ for $L_{2500}$ and $\sim 0.30$ for $L_{[\rm{OIII}]}$. RLQs are systematically brighter than RQQs by about a factor of 2 at given optical luminosity. The zero-points of these relations increase systematically with redshift, possibly in different ways for RLQs and RQQs. Evolution is particularly strong at low redshift and if the optical luminosity is characterised by $L_{[\rm{OIII}]}$. At low redshift and at given $L_{[\rm{OIII}]}$ the soft X-ray emission from type II AGN is more than 100 times weaker than that from type I AGN.

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arXiv2006

Erratum - The many lives of AGN: cooling flows, black holes and the luminosities and colours of galaxies

In Figure 6 we inadvertently labeled the proxy circular velocity as the virial velocity of the dark matter halo instead of what is actually plotted, the maximum circular velocity of the dark matter halo. The maximum halo circular velocity is a much better estimate of the disk V_c than is V_vir. This confusion influenced the discussion of the Tully-Fisher relation in our paper. In fact, Figure 6 demonstrates that it is possible to simultaneously reproduce both the local Tully-Fisher relation and luminosity function using semi-analytic techniques applied to the standard LCDM cosmology, thus contradicting previous studies of this issue and our own discussion in Section 3.6.

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arXiv2005

The First Generation of Star-Forming Haloes

We model gas cooling in high-resolution N-body simulations in order to investigate the formation of the first generation of stars. We follow a region of a LCDM universe especially selected to contain a rich cluster by the present day. The properties of the dark haloes that form in these sub-solar mass resolution simulations are presented in a companion paper by Gao et al. The first gas clouds able to cool by molecular hydrogen line emission collapse at extremely high redshift, z ~ 47, when the mass of the dark halo is 2.4 x 10^5 Msun/h. By z ~ 30, a substantial population of haloes are capable of undergoing molecular hydrogen cooling although their ability to form stars is dependent on the efficiency of feedback processes such as dissociating Lyman-Werner radiation. The mass of the main halo grows extremely rapidly and, by z ~ 36, its virial temperature has reached 10^4K, at which point gas cooling becomes dominated by more effective atomic line processes. By z ~ 30, a small ``group'' of such potential galaxies will have formed unless prevented from doing so by feedback processes. By this redshift, massive (>~ 100 Msun) population III stars are able to ionise gas well beyond their own host halo and neighbouring HII regions can percolate to form an ionized superbubble. Such patches would be too widely separated to contribute significantly to reionisation at this time. The large number density of early cooling haloes in the pre-reionised universe raises the exciting prospect that this ultra-early generation of stars may be observable as gamma-ray bursts or supernovae.

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arXiv2005

How do galactic winds affect the Lyalpha forest?

We investigate the effect of galactic winds on the Lyalpha forest in cosmological simulations of structure and galaxy formation. We combine high resolution N-body simulations of the evolution of the dark matter with a semi-analytic model for the formation and evolution of galaxies which includes detailed prescriptions for the long-term evolution of galactic winds. This model is the first to describe the evolution of outflows as a two-phase process (an adiabatic bubble followed by a momentum--driven shell) and to include metal--dependent cooling of the outflowing material. We find that the main statistical properties of the Lyalpha forest, namely the flux power spectrum P(k) and the flux probability distribution function (PDF), are not significantly affected by winds and so do not significantly constrain wind models. Winds around galaxies do, however, produce detectable signatures in the forest, in particular, increased flux transmissivity inside hot bubbles, and narrow, saturated absorption lines caused by dense cooled shells. We find that the Lyalpha flux transmissivity is highly enhanced near strongly wind-blowing galaxies, almost half of all high-redshift galaxies in our sample, in agreement with the results of Adelberger et al. (2005). Finally, we propose a new method to identify absorption lines potentially due to wind shells in the Lyalpha forest: we calculate the abundance of saturated regions in spectra as a function of region width and we find that the number with widths smaller than about 1 Angstrom at z=3 and 0.6 Angstrom at z=2 may be more than doubled. This should be detectable in real spectra.

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arXiv2005

The formation history of elliptical galaxies

We take advantage of the largest high-resolution simulation of cosmic structure growth ever carried out -- the Millennium Simulation of the concordance LambdaCDM cosmogony -- to study how the star formation histories, ages and metallicities of elliptical galaxies depend on environment and on stellar mass. We concentrate on a galaxy formation model which is tuned to fit the joint luminosity/colour/morphology distribution of low redshift galaxies. Massive ellipticals in this model have higher metal abundances, older luminosity-weighted ages, shorter star formation timescales, but lower assembly redshifts than less massive systems. Within clusters the typical masses, ages and metal abundances of ellipticals are predicted to decrease, on average, with increasing distance from the cluster centre. We also quantify the effective number of progenitors of ellipticals as a function of present stellar mass, finding typical numbers below 2 for M* < 10^{11} Msun, rising to about 5 for the most massive systems. These findings are consistent with recent observational results that suggest ``down-sizing'' or ``anti-hierarchical'' behaviour for the star formation history of the elliptical galaxy population, despite the fact that our model includes all the standard elements of hierarchical galaxy formation and is implemented on the standard, LambdaCDM cosmogony.

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arXiv2005

Gas Infall and Stochastic Star Formation in Galaxies in the Local Universe

We study the recent star formation histories of local galaxies by analyzing the scatter in their colours and spectral properties. We present evidence that the distribution of star formation histories changes qualitatively above a characteristic stellar surface mass density of 3x10^8 M_sol kpc^-2, corresponding to the transition between disk-dominated and bulge-dominated galaxies. When we average over subpopulations of galaxies with densities below this value, we find that subpopulations of all masses form their stars at the same average rate per unit stellar mass. However, the scatter in galaxy colours and emission line strengths is larger for more compact galaxies of a given mass. This suggests that star formation occurs in shorter, higher amplitude events in galaxies with smaller sizes. Above the characteristic density, galaxy growth through star formation shuts down. We propose that star formation events are triggered when cold gas is accreted onto a galaxy. We have used the new high resolution Millennium Simulation of structure formation in a concordance LCDM Universe to quantify the incidence of these accretion events and we show that the observational data are well fit by a model in which the consumption time of accreted gas decreases with the surface density of the galaxy as t_cons \propto mu*^-1. The dark matter halos hosting massive galaxies with high densities are also expected to grow through accretion, but the observations indicate that star formation in bulge- dominated galaxies is no longer coupled to the hierarchical build-up of their surrounding dark matter halos.

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