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ABSTRACT: We present a clustering analysis of X-ray selected AGN by compiling X-ray
samples from the literature and re-estimating the dark matter (DM) halo masses
of AGN in a uniform manner. We find that moderate luminosity AGN (Lx(2-10
keV)=10^42-10^44 erg/sec) in the z=0-1.3 Universe are typically found in DM
haloes with masses of ~10^13 Msun. We then compare our findings to the
theoretical predictions of the coupled galaxy and black hole formation model
GALFORM. We find good agreement when our calculation includes the hot-halo mode
of accretion onto the central black hole. This type of accretion, which is
additional to the common cold accretion during disk instabilities and galaxy
mergers, is tightly coupled to the AGN feedback in the model. The hot-halo mode
becomes prominent in DM haloes with masses greater than ~10^12.5 Msun, where
AGN feedback typically operates, giving rise to a distinct class of moderate
luminosity AGN that inhabit rich clusters and superclusters. Cold gas fuelling
of the black hole cannot produce the observationally inferred DM halo masses of
X-ray AGN. Switching off AGN feedback in the model results in a large
population of luminous quasars (Lx(2-10 keV) > 10^44 erg/sec) in DM haloes with
masses up to ~10^14 Msun, which is inconsistent with the observed clustering of
quasars. The abundance of hot-halo AGN decreases significantly in the z~3-4
universe. At such high redshifts, the cold accretion mode is solely responsible
for shaping the environment of moderate luminosity AGN. Our analysis supports
two accretion modes (cold and hot) for the fuelling of supermassive black holes
and strongly underlines the importance of AGN feedback in cosmological models
both of galaxy formation and black hole growth.
05/2013;
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ABSTRACT: The observed stellar mass function (SMF) is very different to the halo mass
function predicted by Lambda-CDM, and it is widely accepted that this is due to
energy feedback from supernovae and black holes. However, the strength and form
of this feedback is not understood. In this paper, we use the phenomenological
model GALFORM to explore how galaxy formation depends on the strength and halo
mass dependence of feedback. We focus on 'expulsion' models in which the wind
mass loading, beta, is proportional to 1/\vdisk^n, with n=0,1,2 and contrast
these models with the successful Bower et al.\ 2008 model (B8W7). A crucial
development is that our code explicitly accounts for the recapture of expelled
gas as the system's halo mass (and thus gravitational potential) increases. We
find that a model with modest wind speed but high mass loading matches the flat
portion of the SMF. When combined with AGN feedback, the model provides a good
description of the observed SMF above 10^9 h^-1 Msol. However, in the expulsion
models, the brightest galaxies are assembled more recently than in B8W7, and
the specific star formation rates of galaxies decrease strongly with decreasing
stellar mass. The expulsion models also tend to have a cosmic star formation
density that is dominated by lower mass galaxies at z=1-3, and dominated high
mass galaxies at low redshift. These trends are in conflict with observational
data, but the comparison highlights some deficiencies of the B8W7 model also.
The experiments in this paper give us important physical insight to the impact
of the feedback process on the formation histories of galaxies, but the strong
mass dependence of feedback adopted in B8W7 still appears to provide the most
promising description of the observed universe.
12/2011;
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ABSTRACT: We use a coupled model of the formation and evolution of galaxies and black holes (BHs) to study the evolution of active galactic nuclei (AGNs) in a cold dark matter universe. The model is embedded in the galaxy formation code galform and predicts the masses, spins and mass accretion histories of BHs in tandem with the formation of their host galaxies. BHs grow by accretion during starbursts, triggered by discs becoming dynamically unstable or by galaxy mergers, and accretion from quasi-hydrostatic hot gas haloes. Using an empirical law for AGN obscuration, our model matches the observed luminosity functions (LFs) of AGNs over a wide range of redshifts. Due to the suppression of cooling in massive haloes by AGN feedback, at low redshift (z≲ 2), the brightest quasars (Lbol≳ 1046 erg s−1) are predicted preferentially to inhabit haloes with masses . The model predicts a hierarchical buildup of BH mass, with the typical mass of actively growing BHs increasing with decreasing redshift. Nevertheless, the model displays clear ‘downsizing’ as reflected in the differential evolution of the space density of faint and bright AGNs. This arises naturally from the interplay between the starburst and hot gas halo accretion modes. The faint end of the LF is dominated by massive BHs accreting at low rates via a thick disc, primarily during the hot-halo mode. The bright end is populated by BHs accreting close to or above the Eddington limit during the starburst mode. Obscuration plays a central role in determining the observed abundance of AGNs and, hence, in their implied cosmic evolution.
Monthly Notices of the Royal Astronomical Society 11/2011; 419(4):2797 - 2820. · 4.90 Impact Factor
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A. S. Font, A. J. Benson,
R. G. Bower,
C. S. Frenk,
A. Cooper,
G. DeLucia,
J. C. Helly,
A. Helmi,
Y.-S. Li,
I. G. McCarthy,
J. F. Navarro,
V. Springel,
E. Starkenburg,
J. Wang,
S. D. M. White
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ABSTRACT: We present a model for the satellites of the Milky Way in which galaxy formation is followed using semi-analytic techniques applied to the six high-resolution N-body simulations of galactic haloes of the Aquarius project. The model, calculated using the galform code, incorporates improved treatments of the relevant physics in the Λ cold dark matter cosmogony, particularly a self-consistent calculation of reionization by ultraviolet (UV) photons emitted by the forming galaxy population, including the progenitors of the central galaxy. Along the merger tree of each halo, the model calculates gas cooling (by Compton scattering off cosmic microwave background photons, molecular hydrogen and atomic processes), gas heating (from hydrogen photoionization and supernova energy), star formation and evolution. The evolution of the intergalactic medium is followed simultaneously with that of the galaxies. Star formation in the more massive progenitor subhaloes is suppressed primarily by supernova feedback, while for smaller subhaloes, it is suppressed primarily by photoionization due to external and internal sources. The model is constrained to match a wide range of properties of the present-day galaxy population as a whole, but at high redshift it requires an escape fraction of UV photons near unity in order to completely reionize the universe by redshift z≳ 8. In the most successful model, the local sources photoionize the pre-galactic region completely by z≃ 10. In addition to the luminosity function of Milky Way satellites, the model matches their observed luminosity–metallicity relation, their radial distribution and the inferred values of the mass within 300 pc, which in the models increase slowly but significantly with luminosity. There is a large variation in satellite properties from halo to halo, with the luminosity function, for example, varying by a factor of ∼2 among the six simulations.
Monthly Notices of the Royal Astronomical Society 09/2011; 417(2):1260 - 1279. · 4.90 Impact Factor
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ABSTRACT: We predict the formation histories, properties and descendants of Lyman-break
galaxies (LBGs) in the Lambda-CDM cosmology. In our model, which incorporates a
top-heavy IMF in starbursts, we find that most LBGs are starbursts triggered by
minor mergers of galaxies. The duration of the LBG phase is predicted to be
quite short, ~20-60 Myr. We investigate the distributions of stellar and halo
masses and morphologies for bright (L_UV > L*_UV) and faint (L_UV > 0.1 L*_UV)
LBGs at z=3, z=6 and z=10 (where we classify LBGs according to their rest-frame
UV luminosities relative the observed characteristic luminosity L*_UV at z
\approx 3). Bright LBGs at z=3 are predicted to have median stellar masses ~
1x10^9 Msun/h and host halo masses ~ 3x10^{11} Msun/h, and to be typically
mildly disk-dominated in stellar mass. On the other hand, faint LBGs at z=10
are predicted to have median stellar masses of only ~ 1x10^7 Msun/h and host
halo masses 2x10^{10} Msun/h, and to be generally bulge-dominated. Bright LBGs
at z=3 evolve to present-day galaxies with median stellar mass ~ 5x^{10} Msun/h
(comparable to the Milky Way), consisting of roughly equal numbers of disk- and
bulge-dominated systems, and hosted by halos with median mass ~2x10^{13} Msun/h
(corresponding to medium-size galaxy groups). The model predicts that 40% of
Milky Way mass galaxies at the present-day have a bright LBG progenitor in the
redshift range 3<z<4, while 95% have a faint LBG progenitor in the same
redshift range, and 7% have a faint LBG progenitor at 10<z<11. With our
multiwavelength model, we also investigate the overlap between the LBG
population and that of submillimetre selected galaxies (SMGs); at z=3, only ~1%
of bright LBGs are also predicted to also be bright SMGs (with an 850 mum flux
in excess of 5 mJy).
05/2011;
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ABSTRACT: We make a detailed investigation of the properties of Lyman-break galaxies (LBGs) in the ΛCDM model. We present predictions for two published variants of the galform semi-analytical model: the Baugh et al. model, which has star formation at high redshifts dominated by merger-driven starbursts with a top-heavy initial mass function (IMF), and the Bower et al. (2006) model, which has active galactic nuclei (AGN) feedback and a standard solar neighbourhood IMF throughout. We show predictions for the evolution of the rest-frame far-UV luminosity function in the redshift range z= 3–20, and compare with the observed luminosity functions of LBGs at z= 3–10. We find that the Baugh et al. model is in excellent agreement with these observations, while the Bower et al. model predicts too many high-luminosity LBGs. Dust extinction, which is predicted self-consistently based on galaxy gas contents, metallicities and sizes, is found to have a large effect on LBG luminosities. We compare predictions for the size evolution of LBGs at different luminosities with observational data for 2 ≲z≲ 7, and find the Baugh et al. model to be in good agreement. We present predictions for stellar, halo and gas masses, star formation rates, circular velocities, bulge-to-disc ratios, gas and stellar metallicities and clustering bias, as functions of far-UV luminosity and redshift. We find a broad consistency with current observational constraints. We then present predictions for the abundance and angular sizes of LBGs out to very high redshift (z≤ 20), finding that planned deep surveys with JWST should detect objects out to z≲ 15. We predict that the effects of dust extinction on the far-UV luminosity density should be large (∼2 mag), even out to high redshifts. The typical UV luminosities of galaxies are predicted to be very low at high redshifts, which has implications for detecting the galaxies responsible for reionizing the intergalactic medium; for example, at z= 10, 50 per cent of the ionizing photons are expected to be produced by galaxies fainter than MAB(1500 Å) − 5 log h∼−15.
Monthly Notices of the Royal Astronomical Society 01/2011; 412(3):1828 - 1852. · 4.90 Impact Factor
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[hide abstract]
ABSTRACT: We use a coupled model of the formation and evolution of galaxies and black holes (BH) to study the evolution of active galactic nuclei (AGN) in a cold dark matter universe. The model predicts the BH mass, spin and mass accretion history. BH mass grows via accretion triggered by discs becoming dynamically unstable or galaxy mergers (called the starburst mode) and accretion from quasi-hydrostatic hot gas haloes (called the hot-halo mode). By taking into account AGN obscuration, we obtain a very good fit to the observed luminosity functions (LF) of AGN (optical, soft and hard X-ray, and bolometric) for a wide range of redshifts (0<z<6). The model predicts a hierarchical build up of BH mass, with the typical mass of actively growing BHs increasing with decreasing redshift. Remarkably, despite this, we find downsizing in the AGN population, in terms of the differential growth with redshift of the space density of faint and bright AGN. This arises naturally from the interplay between the starburst and hot-halo accretion modes. The faint end of the LF is dominated by massive BHs experiencing quiescent accretion via a thick disc, primarily during the hot-halo mode. The bright end of the LF, on the other hand, is dominated by AGN which host BHs accreting close to or in excess of the Eddington limit during the starburst mode. The model predicts that the comoving space density of AGN peaks at z~3, similar to the star formation history. However, when taking into account obscuration, the space density of faint AGN peaks at lower redshift (z<2) than that of bright AGN (z~2-3). This implies that the cosmic evolution of AGN is shaped in part by obscuration. Comment: submitted to MNRAS, comments are welcome
11/2010;
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ABSTRACT: We make a detailed investigation of the properties of Lyman-break galaxies (LBGs) in the LambdaCDM model. We present predictions for two published variants of the GALFORM semi-analytical model: the Baugh et al. (2005) model, which has star formation at high redshifts dominated by merger-driven starbursts with a top-heavy IMF, and the Bower et al. (2006) model, which has AGN feedback and a standard Solar neighbourhood IMF throughout. We show predictions for the evolution of the rest-frame far-UV luminosity function in the redshift range z=3-20, and compare with the observed luminosity functions of LBGs at z=3-10. We find that the Baugh et al. model is in excellent agreement with these observations, while the Bower et al. model predicts too many high-luminosity LBGs. Dust extinction, which is predicted self-consistently based on galaxy gas contents, metallicities and sizes, is found to have a large effect on LBG luminosities. We compare predictions for the size evolution of LBGs at different luminosities with observational data for 2<z<7, and find the Baugh et al. model to be in good agreement. We present predictions for stellar, halo and gas masses, star formation rates, circular velocities, bulge-to-disk ratios, gas and stellar metallicities and clustering bias, as functions of far-UV luminosity and redshift. We find broad consistency with current observational constraints. We then present predictions for the abundance and angular sizes of LBGs out to very high redshift (z<20), finding that planned deep surveys with JWST should detect objects out to z<15. The typical UV luminosities of galaxies are predicted to be very low at high redshifts, which has implications for detecting the galaxies responsible for reionizing the IGM; for example, at z=10, 50% of the ionizing photons are expected to be produced by galaxies fainter than M_AB(1500A)-5logh ~ -15. Comment: 23 pages, 17 figures. Submitted to MNRAS.
04/2010;
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ABSTRACT: Semi-analytic models are a powerful tool for studying the formation of galaxies. However, these models inevitably involve a significant number of poorly constrained parameters that must be adjusted to provide an acceptable match to the observed universe. In this paper, we set out to quantify the degree to which observational data-sets can constrain the model parameters. By revealing degeneracies in the parameter space we can hope to better understand the key physical processes probed by the data. We use novel mathematical techniques to explore the parameter space of the GALFORM semi-analytic model. We base our investigation on the Bower et al. 2006 version of GALFORM, adopting the same methodology of selecting model parameters based on an acceptable match to the local bJ and K luminosity functions. The model contains 16 parameters that are poorly constrained, and we investigate this parameter space using the Model Emulator technique, constructing a Bayesian approximation to the GALFORM model that can be rapidly evaluated at any point in parameter space. By combining successive waves of emulation, we show that only 0.26% of the initial volume is of interest for further exploration. However, within this region we show that the Bower et al. 2006 model is only one choice from an extended sub-space of model parameters that can provide equally acceptable fits. We explore the geometry of this region and begin to explore the physical connections between parameters that are exposed by this analysis. We also consider the impact of adding additional observational data to further constrain the parameter space. Comment: 33 pages, 15 figures. Accepted by MNRAS
04/2010;
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ABSTRACT: The distribution of cold gas in dark matter haloes is driven by key processes in galaxy formation: gas cooling, galaxy mergers, star formation and reheating of gas by supernovae. We compare the predictions of four different galaxy formation models for the spatial distribution of cold gas. We find that satellite galaxies make little contribution to the abundance or clustering strength of cold gas selected samples, and are far less important than they are in optically selected samples. The halo occupation distribution function of present-day central galaxies with cold gas mass > 10^9 h^-1 Msun is peaked around a halo mass of ~ 10^11 h^-1 Msun, a scale that is set by the AGN suppression of gas cooling. The model predictions for the projected correlation function are in good agreement with measurements from the HI Parkes All-Sky Survey. We compare the effective volume of possible surveys with the Square Kilometre Array with those expected for a redshift survey in the near-infrared. Future redshift surveys using neutral hydrogen emission will be competitive with the most ambitious spectroscopic surveys planned in the near-infrared. Comment: 19 pages, 15 figures, To appear in MNRAS
02/2010;
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ABSTRACT: We track the co-evolution of supermassive black holes (SMBHs) and their host galaxies. The calculation is embedded in the GALFORM semi-analytical model which simulates the formation and evolution of galaxies in a cold dark matter (CDM) universe. During the evolution of the host galaxy, hot and cold gas are added to the SMBH by flows triggered by halo gas cooling, disc instabilities and galaxy mergers. This builds up the mass and spin of the BH, and the resulting accretion power regulates the gas cooling and subsequent star formation. The accretion flow is assumed to form a geometrically thin cool disc when the accretion rate exceeds 0.01\dot{M}_Edd, and a geometrically thick, radiatively inefficient hot flow when the accretion rate falls below this value. The resulting quasar optical luminosity function matches observations very well, and the mass of the SMBH correlates with the mass of the galaxy bulge as observed. The BH spin distribution depends strongly on whether the gas in any given accretion episode remains in the same plane (prolonged accretion) or whether, due to self-gravity, it fragments into multiple, randomly aligned accretion episodes (chaotic accretion). In the chaotic accretion model there is a clear correlation of spin with SMBH mass. Massive BHs (M>5\times10^8\Msun) are hosted by giant elliptical galaxies and are rapidly spinning, while lower mass BHs are hosted in spiral galaxies and have much lower spin. Using the Blandford-Znajek mechanism for jet production to calculate the jet power, our model is able to reproduce the radio loudness of radio galaxies, LINERS and Seyferts. This is the first confirmation that a CDM galaxy formation model can reproduce the observed phenomenology of AGN. Comment: Accepted for publication in MNRAS after correcting for revisions suggested by the referee
11/2009;
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ABSTRACT: We use a model for the evolution of galaxies in the far-IR based on the LambdaCDM cosmology to make detailed predictions for upcoming cosmological surveys with the Herschel Space Observatory. We use the combined GALFORM semi-analytical galaxy formation model and GRASIL spectrophotometric code to compute galaxy SEDs including the reprocessing of radiation by dust. The model, which is the same as that in Baugh et al. (2005), assumes two different IMFs: a normal solar neighbourhood IMF for quiescent star formation in disks, and a very top-heavy IMF in starbursts triggered by galaxy mergers. We have shown previously that the top-heavy IMF appears necessary to explain the number counts and redshifts of faint sub-mm galaxies. In this paper, we present predictions for galaxy luminosity functions, number counts and redshift distributions in the Herschel imaging bands. We find that source confusion will be a serious problem in the deepest planned surveys. We also show predictions for physical properties such as star formation rates and stellar, gas and halo masses, together with fluxes at other wavelengths (from the far-UV to the radio) relevant for multi-wavelength follow-up observations. We investigate what fraction of the total IR emission from dust and of the high-mass star formation over the history of the Universe should be resolved by planned surveys with Herschel, and find a fraction ~30-50%, depending on confusion. Finally, we show that galaxies in Herschel surveys should be significantly clustered. Comment: 28 pages, 22 figures, 2 tables. Accepted for publication in MNRAS. Minor changes in response to referee, including new figure comparing to BLAST number counts
09/2009;
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[hide abstract]
ABSTRACT: The clustering amplitude of galaxies depends on their intrinsic luminosity. We compare the properties of publicly available galaxy formation models with clustering measurements from the two-degree field galaxy redshift survey. The model predictions show the same qualitative behaviour as the data but fail to match the observations at the level of accuracy at which current measurements can be made. We demonstrate that this is due to the model producing too many satellite galaxies in massive haloes. We implement simple models to describe two new processes, satellite-satellite mergers and the tidal dissolution of satellites to investigate their impact on the predicted clustering. We find that both processes need to be included in order to produce a model which matches the observations. Comment: 15 pages, 15 figures, submitted to MNRAS
05/2009;
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[hide abstract]
ABSTRACT: The huge size and uniformity of the Sloan Digital Sky Survey makes possible an exacting test of current models of galaxy formation. We compare the predictions of the GALFORM semi-analytical galaxy formation model for the luminosities, morphologies, colours and scale-lengths of local galaxies. GALFORM models the luminosity and size of the disk and bulge components of a galaxy, and so we can compute quantities which can be compared directly with SDSS observations, such as the Petrosian magnitude and the Sersic index. We test the predictions of two published models set in the cold dark matter cosmology: the Baugh et al. (2005) model, which assumes a top-heavy initial mass function (IMF) in starbursts and superwind feedback, and the Bower et al. (2006) model, which uses AGN feedback and a standard IMF. The Bower et al model better reproduces the overall shape of the luminosity function, the morphology-luminosity relation and the colour bimodality observed in the SDSS data, but gives a poor match to the size-luminosity relation. The \Baugh et al. model successfully predicts the size-luminosity relation for late-type galaxies. Both models fail to reproduce the sizes of bright early-type galaxies. These problems highlight the need to understand better both the role of feedback processes in determining galaxy sizes, in particular the treatment of the angular momentum of gas reheated by supernovae, and the sizes of the stellar spheroids formed by galaxy mergers and disk instabilities. Comment: Accepted for publication in MNRAS. 23 pages, 24 figures. Small changes following referee's report, including more extended discussion of physical parameters affecting disk size vs luminosity relation (new Sec.4.5.4 and new Fig.20)
12/2008;
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ABSTRACT: Only ∼10 per cent of baryons in the Universe are in the form of stars, yet most models of luminous structure formation have concentrated on the properties of the luminous stellar matter. Such models are now largely successful at reproducing the observed properties of galaxies, including the galaxy luminosity function and the star formation history of the universe. In this paper we focus on the ‘flip side’ of galaxy formation and investigate the properties of the material that is not presently locked up in galaxies. This ‘by-product’ of galaxy formation can be observed as an X-ray emitting plasma [the intracluster medium (ICM)] in groups and clusters. Since much of this material has been processed through galaxies, observations of the ICM represent an orthogonal set of constraints on galaxy formation models. In this paper, we attempt to self-consistently model the formation of galaxies and the heating of the ICM. We set out the challenges for such a combined model and demonstrate a possible means of bringing the model into line with both sets of constraints.In this paper, we present a version of the Durham semi-analytic galaxy formation model galform that allows us to investigate the properties of the ICM. As we would expect on the basis of gravitational scaling arguments, the previous model fails to reproduce even the most basic observed properties of the ICM. We present a simple modification to the model to allow for heat input into the ICM from the active galactic nucleus (AGN) ‘radio-mode’ feedback. This heating acts to expel gas from the X-ray luminous central regions of the host halo. With this modification, the model reproduces the observed gas mass fractions and luminosity–temperature (L–T) relation of groups and clusters. In contrast to simple ‘pre-heating’ models of the ICM, the model predicts mildly positive evolution of the L–T relation, particularly at low temperatures. The model is energetically plausible, but seems to exceed the observed heating rates of intermediate-temperature clusters. Introducing the heating process into the model requires changes to a number of model parameters in order to retain a good match to the observed galaxy properties. With the revised parameters, the best-fitting luminosity function is comparable to that presented in Bower et al. The new model makes a fundamental step forward, providing a unified model of galaxy and cluster ICM formation. However, the detailed comparison with the data is not completely satisfactory, and we highlight key areas for improvement.
Monthly Notices of the Royal Astronomical Society 11/2008; 390(4):1399 - 1410. · 4.90 Impact Factor
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ABSTRACT: Current models of galaxy formation predict satellite galaxies in groups and clusters that are redder than observed. We investigate the effect on the colours of satellite galaxies produced by the ram-pressure stripping of their hot-gaseous atmospheres as the satellites orbit within their parent halo. We incorporate a model of the stripping process based on detailed hydrodynamic simulations within the Durham semi-analytic model of galaxy formation. The simulations show that the environment in groups and clusters is less aggressive than previously assumed. The main uncertainty in the model is the treatment of gas expelled by supernovae. With reasonable assumptions for the stripping of this material, we find that satellite galaxies are able to retain a significant fraction of their hot gas for several Gyr, thereby replenishing their reservoirs of cold, star-forming gas and remaining blue for a relatively long period of time. A bimodal distribution of galaxy colours, similar to that observed in Sloan Digital Sky Survey data, is established and the colours of the satellite galaxies are in good agreement with the data. In addition, our model naturally accounts for the observed dependence of satellite colours on environment, from small groups to high-mass clusters.
Monthly Notices of the Royal Astronomical Society 09/2008; 389(4):1619 - 1629. · 4.90 Impact Factor
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[hide abstract]
ABSTRACT: We present a hierarchical galaxy formation model which can account for the number counts of sources detected through their emission at sub-millimetre wavelengths. The first stage in our approach is an ab initio calculation of the star formation histories for a representative sample of galaxies, which is carried out using the semi-analytical galaxy formation model GALFORM. These star formation histories are then input into the spectro-photometric code GRASIL, to produce a spectral energy distribution for each galaxy. Dust extinction and emission are treated self consistently in our model, without having to resort to ad-hoc assumptions about the amount of attenuation by dust or the temperature at which the dust radiates. We argue that it is necessary to modify the form of the stellar initial mass function in starbursts in order to match the observed number of sub-mm sources, if we are to retain the previous good matches enjoyed between observations and model predictions in the local universe. We also list some other observational tests that have been passed by our model.
04/2006;
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ABSTRACT: We have modelled the process of reionization of the IGM by photoionization by galaxies, in order to predict the epoch of reionization. We use a sophisticated semi-analytic model to track the formation of galaxies. Our study represents a much more complete and physically consistent modelling of reionization than has been conducted in the past, containing significant improvements in the modelling of the collapse of baryons into dark matter halos, and in the model for the cooling and condensation of gas within halos (including photoheating from a self-consistently computed ionizing background and cooling due to H2). We find that reionization can be achieved by z~10-20 in a LCDM cosmological model with sigma8~0.9. However, a cosmological model with a running spectral index is only able to achieve reionization before z~9 if extreme assumptions are made about the physics of feedback at high redshifts. We also consider the galaxy formation model discussed by Baugh et al., and find that it is able to reionize the Universe by z~12. The previous results assume that all of the ionizing photons produced by stars in galaxies are able to escape and ionize the IGM. If this is not the case, then the redshift of reionization could be substantially reduced. We find that extended periods of partial reionization and double reionizations can occur in models in which the first stars formed via cooling by H2 molecules, are very massive, and in which the escape fraction of ionizing photons ~10-30%. Such models do not fully reionize until z~6-7, but predict an electron scattering optical depth as large as 0.15. Models with lower sigma8=0.7-0.8 as suggested by the recent WMAP three year data have reduced redshifts of reionization, but can be consistent with the lower optical depth also suggested by the WMAP three year data. Comment: 21 pages, submitted to MNRAS
12/2005;
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ABSTRACT: Recent observations of the distant Universe suggest that much of the stellar mass of bright galaxies was already in place at $z>1$. This presents a challenge for models of galaxy formation because massive halos are assembled late in hierarchical cosmologies such as cold dark matter (CDM). In this paper, we discuss a new implementation of the Durham semi-analytic model in which feedback due to active galactic nuclei (AGN) is assumed to quench cooling flows in massive halos. This mechanism naturally creates a break in the local galaxy luminosity function at bright magnitudes. The model is implemented within the Millennium N-body simulation; the accurate dark matter merger trees and large number of realizations of the galaxy formation process that the simulation provides results in highly accurate statistics. After adjusting the values of the physical parameters in the model by reference to the properties of local galaxies, we use it to investigate the evolution of the K-band luminosity and galaxy stellar mass functions. We also calculate the volume averaged star formation rate density of the Universe as a function of redshift and the way in which this is apportioned amongst galaxies of different mass. The model robustly predicts a substantial population of massive galaxies out to redshift $z\sim 5$ and a star formation rate density which rises with increasing redshift in objects of all masses. Although observational data on these properties have been cited as evidence for ``anti-hierarchical'' galaxy formation, we find that when AGN feedback is taken into account, the fundamentally hierachical CDM model provides a very good match to these observations. Comment: 11 pages, 8 colour figures. Accepted for publication in MNRAS. Galaxy catalogues are available for down load from http://www.icc.dur.ac.uk/
11/2005;
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ABSTRACT: We develop an analytic model to calculate the rate at which galaxy discs are heated by dark matter substructures orbiting in their haloes. The model takes into account the internal structure, mass function and accretion rate of satellites expected in the ΛCDM cosmology, as well as the growth of the disc by accretion and mergers, but it ignores resonant heating of the disc 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 disc scaleheights for galaxies of different luminosities. For L* spiral galaxies, it predicts a median disc thickness of only 5 per cent of the radial scalelength if substructure is the only source of heating. The median disc thickness increases to nearly 20 per cent of the radial scalelength 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 disc of the Milky Way galaxy. The distribution of disc 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 discs of spiral galaxies seems to be entirely compatible with the abundance of substructure in dark matter haloes predicted by the standard Λ-dominated cold dark matter model of structure formation. In an Ω0= 1 universe, our best model of galaxy formation produces similar scaleheights, a consequence of the fact that similar amounts of substructure are accreted by haloes during the lifetime of the disc in Ω0= 1 and 0.3, Λ0= 0.7 cold dark matter cosmologies.
Monthly Notices of the Royal Astronomical Society 06/2004; 351(4):1215 - 1236. · 4.90 Impact Factor
