arXiv subjects

C. G. Lacey

Publications and source records attributed to C. G. Lacey.

At least 73 records · Page 4Linked to original sources

arXiv2004

Heating of Galactic Disks by Infalling Satellites

We develop an analytic model to calculate the rate at which galaxy disks are heated by dark matter substructures orbiting in their halos. The model takes into account the internal structure, mass function and accretion rate of satellites expected in the LambdaCDM cosmology, as well as the growth of the disk by accretion and mergers, but it ignores resonant heating of the disk and the dynamical effects of spiral arms and bars. We calibrate this model against N-body simulations and demonstrate that it is able to reproduce the N-body heating rates to within a factor of 3 in the majority of cases. Our model gives the distribution of disk scale-heights for galaxies of different luminosities. For L* spiral galaxies, it predicts a median disk thickness of only 5% of the radial scale-length if substructure is the only source of heating. The median disk thickness increases to nearly 20% of the radial scale-length when heating due to gravitational scattering of stars by molecular clouds is also included. The latter value is close to the thickness estimated observationally for the disk of the Milky Way galaxy. The distribution of disk thickness predicted by the model is also consistent with a recent observational determination for sub-L* galaxies by Bizyaev & Mitronova. Thus, the observed thickness of the stellar disks of spiral galaxies seems to be entirely compatible with the abundance of substructure in dark matter halos predicted by the standard Lambda-dominated cold dark matter model of structure formation. In an Omega_0=1 universe, our best model of galaxy formation produces similar scale-heights, a consequence of the fact that similar amounts of substructure are accreted by halos during the lifetime of the disk in Omega_0=1 and Omega_0=0.3, Lambda_0=0.7 cold dark matter cosmologies.

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arXiv2003

What Shapes the Luminosity Function of Galaxies?

We investigate the physical mechanisms that shape the luminosity function. Beginning with the mass function of dark matter halos, we show how gas cooling, photoionization, feedback, merging and thermal conduction affect the shape of the luminosity function. We consider three processes whereby supernovae can affect the forming galaxy: (1) reheating of disk gas to the halo temperature; (2) expansion of the diffuse halo gas; (3) expulsion of cold disk gas from the halo. While feedback of form (1) is able to flatten the faint end of the luminosity function, this alone does not produce the sharp cut-off observed at large luminosities. Feedback of form (2) is also unable to solve this problem. The relative paucity of very bright galaxies can only be explained if cooling in massive halos is strongly suppressed. Conduction is a promising mechanism, but an uncomfortably high efficiency is required to suppress cooling to the desired level. If, instead, superwinds are responsible for the lack of bright galaxies, then the total energy budget required to obtain a good match to the galaxy luminosity function greatly exceeds the energy available from supernova explosions. The mechanism is only viable if the formation of central supermassive black holes play a crucial role in limiting the amount of stars that form in the host galaxy. (abridged)

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arXiv2002

The Effects of Photoionization on Galaxy Formation -- III: Environmental Dependence in the Luminosity Function

Using semi-analytic modeling techniques, we calculate the luminosity function of galaxy populations residing in cold dark matter halos of different mass. We pay particular attention to the influence of the reionization of the Universe on the number of faint galaxies and to the effects of dynamical friction and tidal limitation of satellites on the number of bright galaxies. We find substantial differences in the shapes of the galaxy luminosity functions in halos of different mass which reflect generic features of the cold dark matter model of galaxy formation and thus offer the opportunity to test it. We then consider how the individual halo luminosity functions combine together to produce the global luminosity function. Surprisingly, the global function ends up having a shallower faint end slope than those of the constituent halo luminosity functions. We compare our model predictions with the limited datasets compiled by Trentham & Hodgkin. We find good agreement with the luminosity functions measured in the Virgo and Coma clusters but significant disagreement with the luminosity functions measured in the Local Group and Ursa Minor cluster. We speculate on possible inadequacies in our modeling and in the existing observational samples. The luminosity functions of galaxies in groups and clusters identified in the 2dF and SDSS galaxy redshift surveys offer the prospect of testing galaxy formation models in detail.

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arXiv2001

The Effects of Photoionization on Galaxy Formation - I: Model and Results at z=0

We develop a coupled model for the evolution of the global properties of the intergalactic medium (IGM) and the formation of galaxies, in the presence of a photoionizing background due to stars and quasars. We use this model to predict the thermodynamic history of the IGM when photoionized by galaxies forming in a cold dark matter (CDM) universe. The evolution of the galaxies is calculated using a semi-analytical model, including a detailed treatment of the effects of tidal stripping and dynamical friction on satellite galaxies orbiting inside larger dark matter halos. We include in the model the negative feedback on galaxy formation from the photoionizing background. Photoionization inhibits galaxy formation in low-mass dark matter halos in two ways: (i) heating of the IGM and inhibition of the collapse of gas into dark halos by the IGM pressure, and (ii) reduction in the radiative cooling of gas within halos. The result of our method is a self-consistent model of galaxy formation and the IGM. The IGM is reheated twice (during reionization of HI and HeII), and we find that the star formation rate per unit volume is slightly suppressed after each episode of reheating. We find that galaxies brighter than L_* are mostly unaffected by reionization, while the abundance of faint galaxies is significantly reduced, leading to present-day galaxy luminosity functions with shallow faint end slopes, in good agreement with recent observational data. Reionization also affects other properties of these faint galaxies, in a readily understandable way.

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arXiv2001

The Effects of Photoionization on Galaxy Formation - II: Satellites in the Local Group

We use a self-consistent model of galaxy formation and the evolution of the intergalactic medium to study the effects of the reionization of the universe at high redshift on the properties of satellite galaxies like those seen around the Milky Way. Photoionization suppresses the formation of small galaxies, so that surviving satellites are preferentially those that formed before the universe reionized. As a result, the number of satellites expected today is about an order of magnitude smaller than the number inferred by identifying satellites with subhalos in high-resolution simulations of the dark matter. The resulting satellite population has an abundance and a distribution of circular velocities similar to those observed in the Local Group. We explore many other properties of satellite galaxies, including their gas content, metallicity and star formation rate, and find generally good agreement with available data. Our model predicts the existence of many as yet undetected satellites in the Local Group. We quantify their observability in terms of their apparent magnitude and surface brightness and also in terms of their constituent stars. A near-complete census of the Milky Way's satellites would require imaging to V~20 and to a surface brightness fainter than 26 V-band magnitudes per square arcsecond. Satellites with integrated luminosity V=15 should contain of order 100 stars brighter than B=26, with central stellar densities of a few tens per square arcminute. Discovery of a large population of faint satellites would provide a strong test of current models of galaxy formation.

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arXiv2001

Polarization of the Cosmic Microwave Background from Non-Uniform Reionization

We study the signal in the Cosmic Microwave Background (CMB) polarization anisotropy resulting from patchy reionization. It is well known that the primordial polarization of the CMB is very sensitive to the details of reionization. Combining a semi-analytic model of galaxy formation, in which the optical depth to the reionization epoch is in the range 0.014 to 0.048, with a high resolution N-body simulation we find that reionization generates a peak with amplitude 0.05~0.15 μK at large angular scales. The position of this peak reveals the size of the horizon at reionization, whilst its amplitude is a measure of the optical depth to reionization. On small scales, ionized patches prior to full reionization create a second order polarization signal due to the coupling of the free electron density fluctuation with the quadrupole moment of the temperature anisotropy. Careful study reveals that the coupling generates the same power spectra for electric and magnetic modes, whose amplitudes of polarization anisotropies from this process are predicted to be ~10nK. The amplitude depends strongly on the total baryon density $Ω_b$ and on the spatial correlations of the free electron density in the ionized regions, and weakly on the fraction of ionizing photons able to escape their source galaxy, $f_{esc}$. The first- and second-order signals are therefore sensitive to the details of how the reionization occurred. Detection of these signals will place important constraints on the reionization history of the Universe.

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arXiv2001

Semi-analytic galaxy formation: understanding the high redshift universe

There is now compelling evidence in favour of the hierarchical structure formation paradigm. Semi-analytic modelling is a powerful tool which allows the formation and evolution of galaxies to be followed in a hierarchical framework. We review some of the latest developments in this area before discussing how such models can help us to interpret observations of the high redshift Universe.

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arXiv2001

The evolution of the galaxy distribution

We follow the evolution of the galaxy population in a Lambda-CDM cosmology by means of high-resolution N-body simulations in which the formation of galaxies and their observable properties are calculated using a semi-analytic model. We display images of the spatial distribution of galaxies in the simulations that illustrate its evolution and provide a qualitative understanding of the processes responsible for various biases that develop. We consider three specific statistical measures of clustering at z=1 and z=0: the correlation length (in real- and redshift-space) of galaxies of different luminosity, the morphology-density relation and the genus curve of the topology of galaxy isodensity surfaces. For galaxies with luminosity below L*, the z=0 correlation length depends very little on the luminosity of the sample, but for brighter galaxies it increases very rapidly, reaching values in excess of 10Mpc/h. The "accelerated" dynamical evolution experienced by galaxies in rich clusters, which is partly responsible for this effect, also results in a strong morphology-density relation. Remarkably, this relation is already well-established at z=1. The genus curves of the galaxies are significantly different from the genus curves of the dark matter but this is not due to genuine topological differences but rather to the sparse sampling of the density field provided by galaxies. The predictions of our model at z=0 will be tested by forthcoming data from the 2dF and Sloan galaxy surveys, and those at z=1 by the DEEP and VIRMOS surveys.

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arXiv2001

The Impact of Galaxy Formation on the X-ray Evolution of Clusters

We present a new model for the X-ray properties of the intracluster medium that explicitly includes heating of the gas by the energy released during the evolution of cluster galaxies. We calculate the evolution of clusters by combining the semi-analytic model of galaxy formation of Cole et al. with a simple model for the radial profile of the intracluster gas. We focus on the cluster X-ray luminosity function and on the relation between X-ray temperature and luminosity. We show that cooling alone is not enough to account for the flatness of the observed T-L relation or for the lack of strong redshift evolution in the observed X-ray luminosity function. Gas heating, on the other hand, can solve these two problems: in the Λ-CDM cosmology, our model reproduces fairly well the T-L relation and the X-ray luminosity function. Furthermore, it predicts only weak evolution in these two properties out to z=0.5, in agreement with recent observational data. A successful model requires an energy input of 1--2 x 10^49 ergs per solar mass of stars formed. This is comparable to the total energy released by the supernovae associated with the formation of the cluster galaxies. Thus, unless the transfer of supernovae energy to the intracluster gas is very (perhaps unrealistically) efficient, additional sources of energy, such as mechanical energy from AGN winds are required. However, the amplification of an initial energy input by the response of the intracluster medium to protocluster mergers might ease the energy requirements. Our model makes definite predictions for the X-ray properties of groups and clusters at high redshift which may soon be tested with data from the Chandra and Newton satellites.

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arXiv2000

The Dependence of Velocity and Clustering Statistics on Galaxy Properties

We use a combination of N-body simulations of the hierarchical clustering of dark matter and semi-analytic modelling of the physics of galaxy formation to probe the relationship between the galaxy distribution and the mass distribution. The pairwise line-of-sight velocity dispersion of galaxies is sensitive to the number of galaxies present in halos of different mass. The galaxy velocity dispersion is ~40% lower than that of the dark matter and is in reasonable agreement with the values measured in the Las Campanas redshift survey by Jing et al. over two decades in pair separation. The origin of this offset is statistical rather than dynamical, and depends upon the relative efficiency of galaxy formation in dark matter halos of different mass. Although the model galaxies and the dark matter have markedly different correlation functions in real space, such biases conspire to cause the redshift space correlation functions to be remarkably similar to each other. The predicted redshift-space galaxy correlation function agrees well with observations. We find no evidence in the model for a dependence of clustering strength on intrinsic galaxy luminosity, unless extremely bright galaxies, two magnitudes brighter than L*, are considered. However, there are significant differences when model galaxies are selected either by morphology or by colour. Early type or red galaxies show a much stronger clustering amplitude than late type or blue galaxies, particularly on small scales, again in good agreement with observations.

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arXiv2000

Non-uniform reionization by galaxies and its effect on the cosmic microwave backgroun

We present predictions for the reionization of the intergalactic medium by stars in high-redshift galaxies. We calculate ionizing luminosities of galaxies, including the effects of absorption by interstellar gas and dust on the escape fraction, in order to calculate the filling factor of ionized hydrogen in the IGM. For a $Λ{\rm CDM}$ cosmology and a physical calculation of the escape fraction, we find that the hydrogen in the IGM will be reionized at redshift z=6.1 if the IGM has uniform density, but only by z=4.5 if the IGM is clumped. We combine our model with an N-body simulation of the distribution of dark matter in the universe in order to calculate the secondary temperature anisotropies induced in the cosmic microwave background (CMB) by scattering off free electrons. The models predict a spectrum of secondary anisotropies covering a broad range of angular scales, with fractional temperature fluctuations $\sim 10^{-7}-10^{-6}$ on arcminute scales. The amplitude depends somewhat on the geometry of reionization, with models in which the regions of highest gas density are reionized first giving larger CMB fluctuations than the case where galaxies ionize surrounding spherical regions, and models where low density regions reionize first giving the smallest fluctuations. Measurement of these anisotropies can therefore put important constraints on the reionization process, in particular, the redshift evolution of the filling factor, and should be a primary objective of a next generation submillimeter telescope such as the Atacama Large Millimeter Array. (abrdiged)

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arXiv2000

Modelling the Extinction Properties of Galaxies

Recently (Granato, Lacey, Silva et al. 2000, astro-ph/0001308) we have combined our spectrophotometric galaxy evolution code which includes dust reprocessing (GRASIL, Silva et al. 1998) with semi-analytical galaxy formation models (GALFORM, Cole et al. 1999). One of the most characteristic features of the former is that the dust is divided in two main phases: molecular cloud complexes, where stars are assumed to be born, and the diffuse interstellar medium. As a consequence, stellar populations of different ages have different geometrical relationships with the two phases, which is essential in understanding several observed properties of galaxies, in particular those undergoing major episodes of star formation at any redshift. Indeed, our merged GRASIL+GALFORM model reproduces fairly well the SEDs of normal spirals and starbursts from the far-UV to the sub-mm and their internal extinction properties. In particular in the model the observed starburst attenuation law (Calzetti 1999) is accounted for as an effect of geometry of stars and dust, and has nothing to do with the optical properties of dust grains.

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arXiv2000

The infrared side of galaxy formation. I. The local universe in the semi-analytical framework

We present a new evolutionary model for the far-UV to sub-mm properties of the galaxy population. This combines a semi-analytic galaxy formation model based on hierarchical clustering (GALFORM) with a spectro-photometric code which includes dust reprocessing (GRASIL). The former provides the star formation and metal enrichment histories, together with the gas mass and various geometrical parameters, for a representative sample of galaxies formed in different density environments. These quantities allow us to model the SEDs of galaxies, taking into account stellar emission and also dust extinction and re-emission. Two phases are considered for the dust: molecular cloud complexes, where stars are assumed to be born, and the diffuse interstellar medium. The model includes both galaxies forming stars quiescently in disks, and starbursts triggered by galaxy mergers. We test our models against the observed spectro-photometric properties of galaxies in the local Universe. The models reproduce fairly well the SEDs of normal spirals and starbursts, and their internal extinction properties. The starbursts follow the observed relationship between the FIR to UV luminosity ratio and the slope of the UV continuum. They also reproduce the observed starburst attenuation law (Calzetti et al 99). This result is remarkable, because we use a dust mixture which reproduces the Milky Way extinction law. It suggests that the observed attenuation law is related to the geometry of the stars and dust. We compute galaxy luminosity functions over our wide range of wavelengths, which turn out to be in good agreement with observational data. The UV continuum turns out to be a poor star formation indicator for our models, whilst the infrared luminosity is much more reliable.

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arXiv1999

The Nature of Galaxy Bias and Clustering

We have used a combination of high resolution cosmological N-body simulations and semi-analytic modelling of galaxy formation to investigate the processes that determine the spatial distribution of galaxies in cold dark matter (CDM) models. The galaxy distribution depends sensitively on the efficiency with which galaxies form in halos of different mass. In small mass halos, galaxy formation is inhibited by the reheating of cooled gas by feedback processes, whereas in large mass halos, it is inhibited by the long cooling time of the gas. As a result, the mass-to-light ratio of halos has a deep minimum at the halo mass associated with L* galaxies. This leads to a scale dependent bias in the distribution of galaxies relative to the distribution of mass. On large scales, the bias in the galaxy distribution is related in a simple way to the bias in the distribution of massive halos. On small scales, the correlation function is determined by the interplay between various effects including the spatial exclusion of dark matter halos, the distribution function of the number of galaxies occupying a single dark matter halo and, to a lesser extent, dynamical friction. Remarkably, these processes conspire to produce a correlation function in a flat, Omega_0=0.3, CDM model that is close to a power-law over nearly four orders of magnitude in amplitude. This model agrees well with the correlation function of galaxies measured in the APM survey. On small scales, the model galaxies are less strongly clustered than the dark matter whereas on large scales, they trace the occupied halos. Our clustering predictions are robust to changes in the parameters of the galaxy formation model, provided only those models that match the bright end of the galaxy luminosity function are considered. (abridged)

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arXiv1999

Ab initio galaxy formation

The formation and evolution of galaxies can be followed in the context of cosmological structure formation using the technique of semi-analytic modelling. We give a brief outline of the features incorporated into the semi-analytic model of Cole etal (1999). We present two examples of model predictions that can be tested using photometric redshift techniques. The first prediction, of the star formation history of the universe, has already been shown to be in broad argeement with the observational estimates. The second prediction, of the evolution of galaxy clustering with redshift, will be addressed with some of the forthcoming deep, multi-filter imaging surveys discussed at this meeting.

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arXiv1999

Modelling the evolution of galaxy clustering

Measurements of galaxy clustering are now becoming possible over a range of redshifts out to z=3. We use a semi-analytic model of galaxy formation to compute the expected evolution of the galaxy correlation function with redshift. We illustrate how the degree of clustering evolution is sensitive to the details of the sample selection. For a fixed apparent magnitude limit, galaxies selected at higher redshifts are located in progressively rarer dark matter haloes, compared with the general population of galaxies in place at each redshift. As a result these galaxies are highly biased tracers of the underlying dark matter distribution and exhibit stronger clustering than the dark matter. In general, the correlation length measured in comoving units, decreases at first with increasing redshift, before increasing again at higher redshift. We show that the epsilon-model often used to interpret the angular correlation function of faint galaxies gives an inadequate description of the evolution of clustering, and offers no physical insight into the clustering process. We compare our predictions with those of a simple, popular model in which a one-to-one correspondence between galaxies and dark halos is assumed. Qualitatively, this model reproduces the correct evolutionary behaviour at high redshift, but the quantitative results can be significantly in error. Our theoretical expectations are in good agreement with the high redshift clustering data of Carlberg etal and Postman etal but are higher than the measurements of Le Fevre etal.

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arXiv1999

Modeling Dust on Galactic SED: Application to Semi-Analytical Galaxy Formation Models

We present the basic features and preliminary results of the interface between our spectro-photometric model GRASIL (that calculates galactic SED from the UV to the sub-mm with a detailed computation of dust extinction and thermal reemission) with the semi-analytical galaxy formation model GALFORM (that computes galaxy formation and evolution in the hierarchical scenario, providing the star formation history as an input to our model). With these two models we are able to synthetize simulated samples of a few thousands galaxies suited for statistical studies of galaxy properties to investigate on galaxy formation and evolution. We find good agreement with the available data of SED and luminosity functions.

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arXiv1998

Hybrid galaxy formation

We model the two-point correlation function of galaxies in a cold dark matter Universe by combining two powerful theoretical tools - dissipationless N-body simulations of dark matter clustering (specifically the GIF simulations carried out by MPIA and the Virgo Consortium) and semi-analytic modelling of galaxy formation. We construct catalogues of galaxies containing a wide range of information for each galaxy, including magnitudes in various bands, star formation rates, disk and bulge sizes (from the semi-analytic model described by Cole et al 1998) and, importantly, spatial positions and peculiar velocities from the N-body simulation. We then use this information to study the clustering properties of galaxies.

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