arXiv subjects

Richard G. Bower

Publications and source records attributed to Richard G. Bower.

At least 109 records · Page 6Linked to original sources

arXiv2011

The population of Milky Way satellites in the LambdaCDM cosmology

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 halos of the Aquarius project. The model, calculated using the Galform code, incorporates improved treatments of the relevant physics in the LambdaCDM cosmogony, particularly a self-consistent calculation of reionization by 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 subhalos is suppressed primarily by supernova feedback, while for smaller subhalos 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 completely to 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.

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arXiv2011

Numerical overcooling in shocks

We present a study of cooling in radiative shocks simulated with smoothed particle hydrodynamics (SPH) and adaptive mesh refinement codes. We obtain a similarity solution for a shock-tube problem in the presence of radiative cooling, and test how well the solution is reproduced in Gadget and Flash. Shock broadening governed by the details of the numerical scheme (artificial viscosity or Riemann solvers) leads to potentially significant overcooling in both codes. We interpret our findings in terms of a resolution criterion, and apply it to realistic simulations of cosmological accretion shocks onto galaxy haloes, cold accretion and thermal feedback from supernovae or active galactic nuclei. To avoid numerical overcooling of accretion shocks onto haloes that should develop a hot corona requires a particle or cell mass resolution of 10^6 M_sun, which is within reach of current state-of-the-art simulations. At this mass resolution, thermal feedback in the interstellar medium of a galaxy requires temperatures of supernova or AGN driven bubbles to be in excess of 10^7 K at densities of n_H=1.0 cm^-3, in order to avoid spurious suppression of the feedback by numerical overcooling.

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arXiv2011

On the impact of empirical and theoretical star formation laws on galaxy formation

We investigate the consequences of applying different star formation laws in the galaxy formation model GALFORM. Three broad star formation laws are implemented: the empirical relations of Kennicutt and Schmidt and Blitz & Rosolowsky and the theoretical model of Krumholz, McKee & Tumlinson. These laws have no free parameters once calibrated against observations of the star formation rate (SFR) and gas surface density in nearby galaxies. We start from published models, and investigate which observables are sensitive to a change in the star formation law, without altering any other model parameters. We show that changing the star formation law (i) does not significantly affect either the star formation history of the universe or the galaxy luminosity functions in the optical and near-IR, due to an effective balance between the quiescent and burst star formation modes; (ii) greatly affects the cold gas contents of galaxies; (iii) changes the location of galaxies in the SFR versus stellar mass plane, so that a second sequence of "passive" galaxies arises, in addition to the known "active" sequence. We show that this plane can be used to discriminate between the star formation laws.

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arXiv2011

The local star-formation rate density: assessing calibrations using [OII], Ha and UV luminosities

We explore the use of simple star-formation rate (SFR) indicators (such as may be used in high-redshift galaxy surveys) in the local Universe using [OII], Ha, and u-band luminosities from the deeper 275 deg^2 Stripe 82 subsample of the Sloan Digital Sky Survey (SDSS) coupled with UV data from the Galaxy Evolution EXplorer satellite (GALEX). We examine the consistency of such methods using the star-formation rate density (SFRD) as a function of stellar mass in this local volume, and quantify the accuracy of corrections for dust and metallicity on the various indicators. Rest-frame u-band promises to be a particularly good SFR estimator for high redshift studies since it does not require a particularly large or sensitive extinction correction, yet yields results broadly consistent with more observationally expensive methods. We suggest that the [OII]-derived SFR, commonly used at higher redshifts (z~1), can be used to reliably estimate SFRs for ensembles of galaxies, but for high mass galaxies (log(M*/Msun)>10), a larger correction than is typically used is required to compensate for the effects of metallicity dependence and dust extinction. We provide a new empirical mass-dependent correction for the [OII]-SFR.

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arXiv2011

4098 galaxy clusters to z~0.6 in the Sloan Digital Sky Survey equatorial Stripe 82

We present a catalogue of 4098 photometrically selected galaxy clusters with a median redshift = 0.32 in the 270 square degree 'Stripe 82' region of the Sloan Digital Sky Survey (SDSS), covering the celestial equator in the Southern Galactic Cap (-50 < RA < 59 deg, |Dec| < 1.25 deg). Owing to the multi-epoch SDSS coverage of this region, the ugriz photometry is ~2 magnitudes deeper than single scans within the main SDSS footprint. We exploit this to detect clusters of galaxies using an algorithm that searches for statistically significant overdensities of galaxies in a Voronoi tessellation of the projected sky. 32% of the clusters have at least one member with a spectroscopic redshift from existing public data (SDSS Data Release 7, 2SLAQ & WiggleZ), and the remainder have a robust photometric redshift (accurate to ~5-9% at the median redshift of the sample). The weighted average of the member galaxies' redshifts provides a reasonably accurate estimate of the cluster redshift. The cluster catalogue is publicly available for exploitation by the community to pursue a range of science objectives. In addition to the cluster catalogue, we provide a linked catalogue of 18,295 V<21 mag quasar sight-lines with impact parameters within <3 Mpc of the cluster cores selected from the catalogue of Veron et al. (2010). The background quasars cover 0.25 < z < 2, where MgII absorption-line systems associated with the clusters are detectable in optical spectra.

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arXiv2011

A spectroscopic measurement of galaxy formation timescales with ROLES

We present measurements of the specific star-formation rate (SSFR)-stellar mass relation for star-forming galaxies. Our deep spectroscopic samples are based on the Redshift One LDSS3 Emission line Survey, ROLES, and European Southern Observatory, ESO, public spectroscopy at z=1, and on the Sloan Digital Sky Survey (SDSS) at z=0.1. These datasets cover an equally deep mass range of 8.5<~log(M*/Msun)<~11 at both epochs. We find that the SSFR--mass relation evolves in a way which is remarkably independent of stellar mass, as we previously found for the star-formation rate density (SFRD)--mass relation. At higher masses, such as those probed by previous surveys, the evolution in SSFR--mass is almost independent of stellar mass. At higher masses (log(M*/Msun)>10) the shapes of the cumulative cosmic SFRDs are very similar at both z=0.1 and z=1.0, both showing 70% of the total SFRD above a mass of log(M*/Msun)>10. Mass functions are constructed for star-forming galaxies and found to evolve by only <35% between z=1 and z=0.1 over the whole mass range. The evolution is such that the mass function decreases with increasing cosmic time, confirming that galaxies are leaving the star-forming sequence/blue cloud. The observational results are extended to z~2 by adding two recent Lyman break galaxy samples, and data at these three epochs (z=0.1, 1, 2) are compared with the GALFORM semi-analytic model of galaxy formation. GALFORM predicts an overall SFR density (SFRD) as a function of stellar mass in reasonable agreement with the observations. The star formation timescales inferred from 1/SSFR also give reasonable overall agreement, with the agreement becoming worse at the lowest and highest masses. [abridged]

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arXiv2011

Direct observational evidence for a large transient galaxy population in groups at 0.85<z<1

(abridged) We introduce our survey of galaxy groups at 0.85 15 members. The dynamical mass estimates are in good agreement with the masses estimated from the X-ray luminosity, with most of the groups having 13 1E10.1 Msun, and for blue galaxies we sample masses as low as Mstar=1E8.8 Msun. Like lower-redshift groups, these systems are dominated by red galaxies, at all stellar masses Mstar>1E10.1 Msun. Few group galaxies inhabit the "blue cloud" that dominates the surrounding field; instead, we find a large and possibly distinct population of galaxies with intermediate colours. The "green valley" that exists at low redshift is instead well-populated in these groups, containing ~30 per cent of galaxies. These do not appear to be exceptionally dusty galaxies, and about half show prominent Balmer-absorption lines. Furthermore, their HST morphologies appear to be intermediate between those of red-sequence and blue-cloud galaxies of the same stellar mass. We postulate that these are a transient population, migrating from the blue cloud to the red sequence, with a star formation rate that declines with an exponential timescale 0.6 Gyr< tau < 2 Gyr. Their prominence among the group galaxy population, and the marked lack of blue, star-forming galaxies, provides evidence that the group environment either directly reduces star formation in member galaxies, or at least prevents its rejuvenation during the normal cycle of galaxy evolution.

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arXiv2010

The stellar and hot gas content of low-mass galaxy clusters

We analyse the stellar and hot gas content of 18 nearby, low-mass galaxy clusters, detected in redshift space and selected to have a dynamical mass 3E14<M/Msun<6E14, as measured from the 2dF Galaxy Redshift Survey. We combine X-ray measurements from both Chandra and XMM with ground-based near-infrared observations from CTIO, AAT and CFHT to compare the mass in hot gas and stars to the dynamical mass and state of the clusters. Only 13 of the clusters are detected in X-ray emission, and for these systems we find that a range of 7-20 per cent of their baryonic mass, and <3 per cent of their dynamical mass, is detected in starlight, similar to what is observed in more massive clusters. In contrast, the five undetected clusters are underluminous in X-ray emission, by up to a factor 10, given their stellar mass. Although the velocity distribution of cluster members in these systems is indistinguishable from a Gaussian, all show subtle signs of being unrelaxed: either they lack a central, dominant galaxy, or the bright galaxy distribution is less concentrated and/or more elongated than the rest of the sample. Thus we conclude that low-mass clusters and groups selected from the velocity distribution of their galaxies exhibit a dichotomy in their hot gas properties. Either they are detected in X-ray, in which case they generally lie on the usual scaling relations, or they are completely undetected in X-ray emission. The non-detections may be partly related to the apparently young dynamical state of the clusters, but it remains a distinct possibility that some of these systems are exceptionally devoid of hot emitting gas as the result of its expulsion or rarefaction.

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arXiv2010

The Dawn of the Red: Star formation histories of group galaxies over the past 5 billion years

We examine the star formation properties of group and field galaxies in two surveys, the Sloan Digital Sky Survey (SDSS; at z ~ 0.08) and the Group Environment and Evolution Collaboration (GEEC; at z ~ 0.4). Using UV imaging from the GALEX space telescope, along with optical and, for GEEC, near infrared photometry, we compare the observed spectral energy distributions to large suites of stellar population synthesis models. This allows us to accurately determine star formation rates and stellar masses. We find that star forming galaxies of all environments undergo a systematic lowering of their star formation rate between z=0.4 and z=0.08 regardless of mass. Nonetheless, the fraction of passive galaxies is higher in groups than the field at both redshifts. Moreover, the difference between the group and field grows with time and is mass-dependent, in the sense the the difference is larger at low masses. However, the star formation properties of star forming galaxies, as measured by their average specific star formation rates, are consistent within the errors in the group and field environment at fixed redshift. The evolution of passive fraction in groups between z=0.4 and z=0 is consistent with a simple accretion model, in which galaxies are environmentally affected 3 Gyrs after falling into a ~ 10E13 Msun group. This long timescale appears to be inconsistent with the need to transform galaxies quickly enough to ensure that star forming galaxies appear similar in both the group and field, as observed.

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arXiv2010

Gas expulsion by quasar-driven winds as a solution to the over-cooling problem in galaxy groups and clusters

Galaxy groups are not scaled down versions of massive galaxy clusters - the hot gas in groups (known as the intragroup medium, IGrM for short) is, on average, less dense than the intracluster medium, implying that one or more non-gravitational processes (e.g., radiative cooling, star formation, and/or feedback) has had a relatively larger effect on groups. In the present study, we compare a number of cosmological hydrodynamic simulations that form part of the OverWhelmingly Large Simulations project to isolate and quantify the effects of cooling and feedback from supernovae (SNe) and active galactic nuclei (AGN) on the gas. This is achieved by comparing Lagrangian thermal histories of the gas in the different runs, which were all started from identical initial conditions. While radiative cooling, star formation, and SN feedback are all necessary ingredients, only runs that also include AGN feedback are able to successfully reproduce the optical and X-ray properties of groups and low-mass clusters. We isolate how, when, and exactly what gas is heated by AGN. Interestingly, we find that the gas that constitutes the present-day IGrM is that which was not strongly heated by AGN. Instead, the low median density/high median entropy of the gas in present-day groups is achieved by the ejection of lower entropy gas from low-mass progenitor galaxies at high redshift (primarily 2 < z < 4). This corresponds to the epoch when supermassive black holes accreted most of their mass, typically at a rate that is close to the Eddington limit (i.e., when the black holes are in a `quasar mode').

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arXiv2010

Mass transport by buoyant bubbles in galaxy clusters

We investigate the effect of three important processes by which AGN-blown bubbles transport material: drift, wake transport and entrainment. The first of these, drift, occurs because a buoyant bubble pushes aside the adjacent material, giving rise to a net upward displacement of the fluid behind the bubble. For a spherical bubble, the mass of upwardly displaced material is roughly equal to half the mass displaced by the bubble, and should be ~ 10^{7-9} solar masses depending on the local ICM and bubble parameters. We show that in classical cool core clusters, the upward displacement by drift may be a key process in explaining the presence of filaments behind bubbles. A bubble also carries a parcel of material in a region at its rear, known as the wake. The mass of the wake is comparable to the drift mass and increases the average density of the bubble, trapping it closer to the cluster centre and reducing the amount of heating it can do during its ascent. Moreover, material dropping out of the wake will also contribute to the trailing filaments. Mass transport by the bubble wake can effectively prevent the build-up of cool material in the central galaxy, even if AGN heating does not balance ICM cooling. Finally, we consider entrainment, the process by which ambient material is incorporated into the bubble. Abridged

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arXiv2010

The case for AGN feedback in galaxy groups

[Abridged] The relatively recent insight that energy input from supermassive black holes (BHs) can have a substantial effect on the star formation rates (SFRs) of galaxies motivates us to examine its effects on the scale of galaxy groups. At present, groups contain most of the galaxies and a significant fraction of the overall baryon content of the universe. To explore the effects of BH feedback on groups, we analyse two high resolution cosmological hydro simulations from the OverWhelmingly Large Simulations project. While both include galactic winds driven by supernovae, only one includes feedback from BHs. We compare the properties of the simulated groups to a wide range of observational data, including hot gas radial profiles and gas mass fractions (fgas), luminosity-mass-temperature (L-M-T) scaling relations, K-band luminosity of the group and its central brightest galaxy (CBG), SFRs and ages of the CBG, and gas/stellar metallicities. Both runs yield entropy profiles similar to the data, while the run without AGN feedback yields highly peaked temperature profiles, in discord with the observations. Energy input from BHs significantly reduces fgas for groups with masses less than ~10^14 Msun, yielding fgas-T and L-T relations that are in agreement with the data. The run without AGN feedback suffers from the well known overcooling problem; the resulting K-band luminosities are much larger than observed. By contrast, the run that includes BH feedback yields K-band luminosities and CBG SFRs and ages in agreement with current estimates. Both runs yield very similar gas-phase metallicities that match X-ray data, but they predict very different stellar metallicities. Based on the above, galaxy groups provide a compelling case that BH feedback is a crucial ingredient in the formation of massive galaxies.

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arXiv2010

Galaxy Formation Spanning Cosmic History

Over the past several decades, galaxy formation theory has met with significant successes. In order to test current theories thoroughly we require predictions for as yet unprobed regimes. To this end, we describe a new implementation of the Galform semi-analytic model of galaxy formation. Our motivation is the success of the model described by Bower et al. in explaining many aspects of galaxy formation. Despite this success, the Bower et al. model fails to match some observational constraints and certain aspects of its physical implementation are not as realistic as we would like. The model described in this work includes substantially updated physics, taking into account developments in our understanding over the past decade, and removes certain limiting assumptions made by this (and most other) semi-analytic models. This allows it to be exploited reliably in high-redshift and low mass regimes. Furthermore, we have performed an exhaustive search of model parameter space to find a particular set of model parameters which produce results in good agreement with a wide range of observational data (luminosity functions, galaxy sizes and dynamics, clustering, colours, metal content) over a wide range of redshifts. This model represents a solid basis on which to perform calculations of galaxy formation in as yet unprobed regimes.

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arXiv2010

The mass assembly of galaxy groups and the evolution of the magnitude gap

We investigate the assembly of groups and clusters of galaxies using the Millennium dark matter simulation and the associated gas simulations and semi-analytic catalogues of galaxies. In particular, in order to find an observable quantity that could be used to identify early-formed groups, we study the development of the difference in magnitude between their brightest galaxies to assess the use of magnitude gaps as possible indicators. We select galaxy groups and clusters at redshift z=1 with dark matter halo mass M(R200) > 1E13/h Msun, and trace their properties until the present time (z=0). We consider only the systems with X-ray luminosity L_X> 0.25E42/h^2 erg/s at z=0. While it is true that a large magnitude gap between the two brightest galaxies of a particular group often indicates that a large fraction of its mass was assembled at an early epoch, it is not a necessary condition. More than 90% of fossil groups defined on the basis of their magnitude gaps (at any epoch between 0 2 (in the R-band, within 0.5R200 of the centre of the group), an alternative criterion Delta m(14)>2.5 (within the same radius) finds 50% more early-formed systems, and those that on average retain their fossil phase longer. However, the conventional criterion performs marginally better at finding early-formed groups at the high-mass end of groups. Nevertheless, both criteria fail to identify a majority of the early-formed systems.

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arXiv2010

The Redshift One LDSS-3 Emission line Survey (ROLES) II: Survey method and z~1 mass-dependent star-formation rate density

Motivated by suggestions of 'cosmic downsizing', in which the dominant contribution to the cosmic star formation rate density (SFRD) proceeds from higher to lower mass galaxies with increasing cosmic time, we describe the design and implementation of the Redshift One LDSS3 Emission line Survey (ROLES). ROLES is a K-selected (22.5 < K_AB < 24.0) survey for dwarf galaxies [8.5<log(M*/Msun)< 9.5] at 0.89 < z < 1.15 drawn from two extremely deep fields (GOODS-S and MS1054-FIRES). Using the [OII]3727 emission line, we obtain redshifts and star-formation rates (SFRs) for star-forming galaxies down to a limit of ~0.3 Msun/yr. We present the [OII] luminosity function measured in ROLES and find a faint end slope of alpha_faint ~ -1.5, similar to that measured at z~0.1 in the SDSS. By combining ROLES with higher mass surveys, we measure the SFRD as a function of stellar mass using [OII] (with and without various empirical corrections), and using SED-fitting to obtain the SFR from the rest-frame UV luminosity for galaxies with spectroscopic redshifts. Our best estimate of the corrected [OII]-SFRD and UV SFRD both independently show that the SFRD evolves equally for galaxies of all masses between z~1 and z~0.1. The exact evolution in normalisation depends on the indicator used, with the [OII]-based estimate showing a change of a factor of ~2.6 and the UV-based a factor of ~6. We discuss possible reasons for the discrepancy in normalisation between the indicators, but note that the magnitude of this uncertainty is comparable to the discrepancy between indicators seen in other z~1 works. Our result that the shape of the SFRD as a function of stellar mass (and hence the mass range of galaxies dominating the SFRD) does not evolve between z~1 and z~0.1 is robust to the choice of indicator. [abridged]

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arXiv2009

Large-scale Structure and Dynamics of the Most X-ray Luminous Galaxy Cluster Known -- RX J1347-1145

We present photometric, spectroscopic and weak lensing analysis of the large-scale structure and dynamics of the most X-ray luminous galaxy cluster known, RX J1347-1145, at z=0.451. We spectroscopically confirmed 47 new members with LDSS3 on the Magellan telescope. Together with previously known members, we measure a new velocity dispersion of 1163(+/-97) km/s. The mass inferred from our velocity dispersion is M200 = 1.16^{+0.32}_{-0.27}x10^{15} solar mass, with r200=1.85Mpc, under the assumption of a singular isothermal sphere. We also present a weak lensing analysis using deep CFHT data on this cluster, and find a deprojected mass of 1.47^{+0.46}_{-0.43}x10^{15} solar mass within r200, in excellent agreement with our dynamical estimate. Thus, our new dynamical mass estimate is consistent with that from weak lensing and X-ray studies in the literature, resolving a previously claimed discrepancy. We photometrically detect and spectroscopically confirm another massive cluster with sigma=780(+/-100) km/s and M200=3.4^{+1.4}_{-1.1}x10^{14} solar mass ~7Mpc south-west of RX J1347-1145, which we refer to as RXJ1347-SW. Our spectroscopic survey reveals a possible excess of galaxies in velocity space in the region between RX J1347-1145 and RXJ1347-SW; comparing with simulations, this excess appears consistent with that expected from a large filamentary structure traced by galaxies connecting these two clusters.

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arXiv2009

The accretion of galaxies into groups and clusters

We use the galaxy stellar mass and halo merger tree information from the semi-analytic model galaxy catalogue of Font et al. (2009) to examine the accretion of galaxies into a large sample of groups and clusters, covering a wide range in halo mass (10E12.9 to 10E15.3 Msun/h), and selected from each of four redshift epochs (z=0, 0.5, 1.0 and 1.5). We find that clusters at all examined redshifts have accreted a significant fraction of their final galaxy populations through galaxy groups. A 10E14.5 Msun/h mass cluster at z=0 has, on average, accreted ~ 40% of its galaxies (Mstellar > 10E9 Msun/h) from halos with masses greater than 10E13 Msun/h. Further, the galaxies which are accreted through groups are more massive, on average, than galaxies accreted through smaller halos or from the field population. We find that at a given epoch, the fraction of galaxies accreted from isolated environments is independent of the final cluster or group mass. In contrast, we find that observing a cluster of the same halo mass at each redshift epoch implies different accretion rates of isolated galaxies, from 5-6 % per Gyr at z=0 to 15% per Gyr at z=1.5. We find that combining the existence of a Butcher Oemler effect at z=0.5 and the observations that galaxies within groups display significant environmental effects with galaxy accretion histories justifies striking conclusions. Namely, that the dominant environmental process must begin to occur in halos of 10E12 -- 10E13 Msun/h, and act over timescales of > 2 Gyrs. This argues in favor of a mechanism like "strangulation", in which the hot halo of a galaxy is stripped upon infalling into a more massive halo . This simple model predicts that by z=1.5 galaxy groups and clusters will display little to no environmental effects.

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arXiv2009

The colour of galaxies in distant groups

(Abridged) We present new optical and near-infrared imaging for a sample of 98 spectroscopically-selected galaxy groups at 0.25<z<0.55. We measure accurate colours for group members and the surrounding field population, statistically complete above a stellar mass limit of M=1E10 Msun. The overall colour distribution is bimodal in both the field and group samples; but at fixed luminosity the fraction of group galaxies populating the red peak is larger, by 20+/-7 per cent, than that of the field. In particular, group members with early-type morphologies, as identified in HST imaging, exhibit a tight red sequence, similar to that seen for more massive clusters. We show that approximately 20-30 per cent of galaxies on the red sequence may be dust-reddened galaxies with non-negligible star formation and early-spiral morphologies. This is true of both the field and group sample, and shows little dependence on near infrared luminosity. Thus, the fraction of bright group members with no sign of star formation or AGN activity is 54+/-6 per cent. Our field sample, which includes galaxies in all environments, contains 35+/-3 per cent of such inactive galaxies, consistent with the amount expected if all such galaxies are located in groups and clusters. This reinforces our earlier conclusions, that dense environments at z<0.5 are associated with a premature cessation of star formation in some galaxies; in particular we find no evidence for significantly enhanced star formation in these environments. Simple galaxy formation models predict a quenching of star formation in groups that is too efficient, overpopulating the red sequence. Attempts to fix this by increasing the timescale of this quenching equally for all group members distorts the colour distribution in a way that is inconsistent with observations.

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