arXiv subjects

Richard G. Bower

Publications and source records attributed to Richard G. Bower.

At least 127 records · Page 7Linked to original sources

arXiv2009

Hot gas halos around disk galaxies: Confronting cosmological simulations with observations

Models of disk galaxy formation commonly predict the existence of an extended reservoir of accreted hot gas surrounding massive spirals at low redshift. As a test of these models, we use X-ray and H-alpha data of the two massive, quiescent edge-on spirals NGC 5746 and NGC 5170 to investigate the amount and origin of any hot gas in their halos. Contrary to our earlier claim, the Chandra analysis of NGC 5746, employing more recent calibration data, does not reveal any significant evidence for diffuse X-ray emission outside the optical disk, with a 3-sigma upper limit to the halo X-ray luminosity of 4e39 erg/s. An identical study of the less massive NGC 5170 also fails to detect any extraplanar X-ray emission. By extracting hot halo properties of disk galaxies formed in cosmological hydrodynamical simulations, we compare these results to expectations for cosmological accretion of hot gas by spirals. For Milky Way-sized galaxies, these high-resolution simulations predict hot halo X-ray luminosities which are lower by a factor of ~2 compared to our earlier results reported by Toft et al. (2002). We find the new simulation predictions to be consistent with our observational constraints for both NGC 5746 and NGC 5170, while also confirming that the hot gas detected so far around more actively star-forming spirals is in general probably associated with stellar activity in the disk. Observational results on quiescent disk galaxies at the high-mass end are nevertheless providing powerful constraints on theoretical predictions, and hence on the assumed input physics in numerical studies of disk galaxy formation and evolution.

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arXiv2008

The flip-side of galaxy formation: A combined model of Galaxy Formation and Cluster Heating

Only ~10% 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. 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, hereafter ICM) in groups and clusters, and 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 (presented in Bower et al. 2006) fails to reproduce even the most basic observed properties of the ICM; however, we present a simple modification to the model to allow for heat input into the ICM from the 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 relation of groups and 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. (2006). 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.

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arXiv2008

The Colours of Satellite Galaxies in Groups and Clusters

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 Gigayears, 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 SDSS 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.

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arXiv2008

Evolution in the Disks and Bulges of Group Galaxies since z=0.4

We present quantitative morphology measurements of a sample of optically selected group galaxies at 0.3 < z < 0.55 using the Hubble Space Telescope (HST) Advanced Camera for Surveys (ACS) and the GIM2D surface brightness--fitting software package. The group sample is derived from the Canadian Network for Observational Cosmology Field Redshift survey (CNOC2) and follow-up Magellan spectroscopy. We compare these measurements to a similarly selected group sample from the Millennium Galaxy Catalogue (MGC) at 0.05 < z < 0.12. We find that, at both epochs, the group and field fractional bulge luminosity (B/T) distributions differ significantly, with the dominant difference being a deficit of disk--dominated (B/T < 0.2) galaxies in the group samples. At fixed luminosity, z=0.4 groups have ~ 5.5 +/- 2 % fewer disk--dominated galaxies than the field, while by z=0.1 this difference has increased to ~ 19 +/- 6 %. Despite the morphological evolution we see no evidence that the group environment is actively perturbing or otherwise affecting the entire existing disk population. At both redshifts, the disks of group galaxies have similar scaling relations and show similar median asymmetries as the disks of field galaxies. We do find evidence that the fraction of highly asymmetric, bulge--dominated galaxies is 6 +/- 3 % higher in groups than in the field, suggesting there may be enhanced merging in group environments. We replicate our group samples at z=0.4 and z=0 using the semi-analytic galaxy catalogues of Bower et al (2006). This model accurately reproduces the B/T distributions of the group and field at z=0.1. However, the model does not reproduce our finding that the deficit of disks in groups has increased significantly since z=0.4.

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arXiv2008

Towards a Holistic View of the Heating and Cooling of the Intracluster Medium

(Abridged) X-ray clusters are conventionally divided into two classes: "cool core" (CC) clusters and "non-cool core" (NCC) clusters. Yet relatively little attention has been given to the origins of this dichotomy and, in particular, to the energetics and thermal histories of the two classes. We develop a model for the entropy profiles of clusters starting from the configuration established by gravitational shock heating and radiative cooling. At large radii, gravitational heating accounts for the observed profiles and their scalings well. However, at small and intermediate radii, radiative cooling and gravitational heating cannot be combined to explain the observed profiles of either type of cluster. The inferred entropy profiles of NCC clusters require that material is preheated prior to cluster collapse in order to explain the absence of low entropy (cool) material in these systems. We show that a similar modification is also required in CC clusters in order to match their properties at intermediate radii. In CC clusters, this modification is unstable, and an additional process is required to prevent cooling below a temperature of a few keV. We show that this can be achieved by adding a self-consistent AGN feedback loop in which the lowest-entropy, most rapidly cooling material is heated so that it rises buoyantly to mix with material at larger radii. The resulting model does not require fine tuning and is in excellent agreement with a wide variety of observational data. Some of the other implications of this model are briefly discussed.

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arXiv2008

Testing Cold Dark Matter with the hierarchical buildup of stellar light

(Abridged) We demonstrate that the tenet of hierarchical structure growth leads directly to a robust, falsifiable prediction for the correlation between stellar fraction (fstar) and total system mass (M500) of galaxy groups and clusters. This prediction is relatively insensitive to the details of baryonic physics or cosmological parameters. In particular, if the fstar-M500 relation is fixed and does not evolve with redshift, CDM models predict the logarithmic slope of this relation to be b>-0.3. This constraint can be weakened if the fstar-M500 relation evolves strongly, but this implies more stars must be formed in situ in groups at low redshift. Conservatively requiring that at least half the stars in groups were formed by z=1, the constraint from evolution models is b>-0.35. Since the most massive clusters (M500=1E15 Msun) are observed to have fstar=0.01, this means that groups with M500=5E13 Msun must have fstar<0.03. Recent observations by Gonzalez et al. (2007) indicate a much steeper relation, with fstar>0.04 in groups, leading to b=-0.64. If confirmed, this would rule out hierarchical structure formation models: today's clusters could not have been built from today's groups, or even from the higher-redshift progenitors of those groups. We perform a careful analysis of these and other data to identify the most important systematic uncertainties in their measurements. Although correlated uncertainties on stellar and total masses might explain the steep observed relation, the data are only consistent with theory if the observed group masses are systematically underestimated.

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arXiv2007

The impact of radio feedback from active galactic nuclei in cosmological simulations: Formation of disk galaxies

In this paper, we present a new implementation of feedback due to active galactic nuclei (AGN) in cosmological simulations of galaxy formation. We assume that a fraction of jet energy, which is generated by an AGN, is transferred to the surrounding gas as thermal energy. Combining a theoretical model of mass accretion onto black holes with a multiphase description of star-forming gas, we self-consistently follow evolution of both galaxies and their central black holes. The novelty in our model is that we consider two distinct accretion modes: standard radiatively efficient thin accretion disks and radiatively inefficient accretion flows which we will generically refer to as RIAFs; motivated by theoretical models for jet production in accretion disks, we assume that only the RIAF is responsible for the AGN feedback. We find that, after an initial episode of bursting star formation, the accretion rate onto the central black hole drops so that the accretion disk switches to a RIAF structure. At this point, the feedback from the AGN becomes efficient and slightly suppresses star formation in the galactic disk and almost completely halts star formation in the bulge. As a result, the nucleus becomes a stochastically fuelled low-luminosity AGN (Seyfert galaxy) with recurrent short-lived episodes of activity after the star bursts. Our model predicts several properties of the low-luminosity AGN including the bolometric luminosity, jet powers, the effect on kpc-scale of the radio jet and the AGN lifetime, which are in broad agreement with observations of Seyfert galaxies and their radio activity. We also find that the mass ratios between the central black hole and the the host spheroid at z = 0 are ~10^{-3} regardless of the strength of either supernova feedback or AGN feedback. (abridged)

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arXiv2007

Ram pressure stripping the hot gaseous halos of galaxies in groups and clusters

We use a large suite of carefully controlled full hydrodynamic simulations to study the ram pressure stripping of the hot gaseous halos of galaxies as they fall into massive groups and clusters. The sensitivity of the results to the orbit, total galaxy mass, and galaxy structural properties is explored. For typical structural and orbital parameters, we find that ~30% of the initial hot galactic halo gas can remain in place after 10 Gyr. We propose a physically simple analytic model that describes the stripping seen in the simulations remarkably well. The model is analogous to the original formulation of Gunn & Gott (1972), except that it is appropriate for the case of a spherical (hot) gas distribution (as opposed to a face-on cold disk) and takes into account that stripping is not instantaneous but occurs on a characteristic timescale. The model reproduces the results of the simulations to within approximately 10% at almost all times for all the orbits, mass ratios, and galaxy structural properties we have explored. The one exception involves unlikely systems where the orbit of the galaxy is highly non-radial and its mass exceeds about 10% of the group or cluster into which it is falling (in which case the model under-predicts the stripping following pericentric passage). The proposed model has several interesting applications, including modelling the ram pressure stripping of both observed and cosmologically-simulated galaxies and as a way to improve current semi-analytic models of galaxy formation. One immediate consequence is that the colours and morphologies of satellite galaxies in groups and clusters will differ significantly from those predicted with the standard assumption of complete stripping of the hot coronae.

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arXiv2007

Models for jet power in elliptical galaxies: A case for rapidly spinning black holes

The power of jets from black holes are expected to depend on both the spin of the black hole and the structure of the accretion disk in the region of the last stable orbit. We investigate these dependencies using two different physical models for the jet power: the classical Blandford-Znajek (BZ) model and a hybrid model developed by Meier. In the BZ case, the jets are powered by magnetic fields directly threading the spinning black hole while in the hybrid model, the jet energy is extracted from both the accretion disk as well as the black hole via magnetic fields anchored to the accretion flow inside and outside the hole's ergosphere. The hybrid model takes advantage of the strengths of both the Blandford-Payne and BZ mechanisms, while avoiding the more controversial features of the latter. We develop these models more fully to account for general relativistic effects and to focus on advection-dominated accretion flows (ADAF) for which the jet power is expected to be a significant fraction of the accreted rest mass energy. We apply the models to elliptical galaxies, in order to see if these models can explain the observed correlation between the Bondi accretion rates and the total jet powers. For typical values of the disk viscosity parameter alpha~0.04-0.3 and mass accretion rates consistent with ADAF model expectations, we find that the observed correlation requires j>0.9; i.e., it implies that the black holes are rapidly spinning. Our results suggest that the central black holes in the cores of clusters of galaxies must be rapidly rotating in order to drive jets powerful enough to heat the intracluster medium and quench cooling flows.

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arXiv2007

Revisiting the Baryon Fractions of Galaxy Clusters: A Comparison with WMAP 3-year Results

The universal baryonic mass fraction (Omega_b/Omega_m) can be sensitively constrained using X-ray observations of galaxy clusters. In this paper, we compare the baryonic mass fraction inferred from measurements of the cosmic microwave background with the gas mass fractions (f_gas) of a large sample of clusters taken from the recent literature. In systems cooler than 4 keV, f_gas declines as the system temperature decreases. However, in higher temperature systems, f_gas(r500) converges to approx. (0.12 +/- 0.02)(h/0.72)^{-1.5}, where the uncertainty reflects the systematic variations between clusters at r500. This is significantly lower than the maximum-likelihood value of the baryon fraction from the recently released WMAP 3-year results. We investigate possible reasons for this discrepancy, including the effects of radiative cooling and non-gravitational heating, and conclude that the most likely solution is that Omega_m is higher than the best-fit WMAP value (we find Omega_m = 0.36^{+0.11}_{-0.08}), but consistent at the 2-sigma level. Degeneracies within the WMAP data require that sigma_8 must also be greater than the maximum likelihood value for consistency between the data sets.

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arXiv2007

Modelling Shock Heating in Cluster Mergers: I. Moving Beyond the Spherical Accretion Model

(Abridged) The thermal history of the intracluster medium (ICM) is complex. Heat input from cluster mergers, AGN, and galaxy winds offsets and may even halt the cooling of the ICM. Consequently, the processes that set the properties of the ICM play a key role in determining how galaxies form. In this paper we focus on the shock heating of the ICM during cluster mergers, with the eventual aim of incorporating this mechanism into semi-analytic models of galaxy formation. We use a suite of hydrodynamic simulations to track the evolution of the ICM in idealised two-body mergers. We find the heating of the ICM can be understood relatively simply by considering the evolution of the gas entropy during the mergers. We examine the processes that generate the entropy in order to understand why previous analytic shock heating models failed. We find that: (1) The energy that is thermalised in the collision greatly exceeds the kinetic energy available when the systems first touch. The smaller system penetrates deep into the potential well before it is disrupted. (2) For unequal mass mergers, most of the energy is thermalised in the more massive component. The heating of the smaller system is minor and its gas sinks to the centre of the final system. (3) The bulk of the entropy generation occurs in two distinct episodes. The first episode occurs following core collision, when a shock wave is generated that propagates outwards from the centre. This causes the combined system to expand rapidly and overshoot hydrostatic equilibrium. The second episode occurs as this material is shock heated as it re-collapses. This revised model for entropy generation significantly improves our physical understanding of cosmological gas simulations.

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arXiv2006

Modelling the jet power in elliptical galaxies: support for rapidly spinning black holes

Recently, Allen et al. measured a tight correlation between the Bondi accretion rates and jet powers of the nuclei of nearby X-ray luminous elliptical galaxies. We employ two models of jet powering to understand the above correlation and derive constraints on the spin and accretion rate of the central black holes. The first is the Blandford-Znajek model, in which the spin energy of the hole is extracted as jet power; the second model is an hybrid version of the Blandford-Payne and Blandford-Znajek processes, in which the outflow is generated in the inner parts of the accretion disk. We assume advection-dominated accretion flows (ADAF) and account for general relativistic effects. Our modelling implies that for typical values of the disk viscosity parameter alpha~0.01-1 the tight correlation implies the narrow range of spins j~0.7-1 and accretion rates Mdot(R_ms)~(0.01-1) Mdot_Bondi. Our results provide support for the "spin paradigm" scenario and suggest that the central black holes in the cores of clusters of galaxies must be rapidly rotating in order to drive radio jets powerful enough to quench the cooling flows.

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arXiv2006

Investigating hot gas in the halos of two massive spirals: Observations and cosmological simulations

Models of disk galaxy formation commonly predict the existence of an extended reservoir of hot gas surrounding massive spirals at low redshift. As a test of these models, we have obtained X-ray and optical data of the two massive edge-on spirals NGC 5746 and NGC 5170, in order to investigate the amount and origin of hot gas in their disks and halos. Chandra observations of NGC 5746 reveal evidence for diffuse X-ray emission with a total luminosity of ~7 x 10^39 erg/s surrounding this galaxy out to at least ~20 kpc from the disk, whereas an identical study of the less massive NGC 5170 fails to detect any extraplanar X-ray emission. Unlike the case for other disk galaxies with detected X-ray halos, the halo emission around NGC 5746 is not accompanied by extraplanar H-alpha or radio emission, and there is no evidence for significant nuclear or starburst activity in the disk. In contrast to these other cases, the emission around NGC 5746 therefore appears to arise from the cooling of externally accreted material rather than from disk outflows. To verify this idea, we present results of cosmological simulations of galaxy formation and evolution, showing our observations to be in good agreement with expectations for cosmological accretion, while also confirming that the X-ray halos of other spirals do not fit well into an accretion scenario. We find that the estimated cooling rate of hot halo gas around NGC 5746 would provide sufficient material for star formation in the disk to proceed at its present rate. This lends support to the idea that a supply of hot ambient gas is potentially available as fuel for star formation in massive, nearby spirals, and suggests that accretion of hot gas could be important for maintaining the stellar disks of such galaxies.

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arXiv2006

The stellar mass content of distant galaxy groups

We have obtained near-infrared imaging of 58 galaxy groups, in the redshift range 0.1<z<0.6, from the William Herschel Telescope and from the Spitzer IRAC data archive. The groups are selected from the CNOC2 redshift survey, with additional spectroscopy from the Baade telescope (Magellan). Our group samples are statistically complete to K(Vega}=17.7 (INGRID) and [3.6](AB)=19.9 (IRAC). From these data we construct near-infrared luminosity functions, for groups in bins of velocity dispersion, up to 800 km/s, and redshift. The total amount of near-infrared luminosity per group is compared with the dynamical mass, estimated from the velocity dispersion, to compute the mass-to-light ratio, M/Lk. We find that the M/Lk values in these groups are in good agreement with those of their statistical descendants at z=0, with no evidence for evolution beyond that expected for a passively evolving population. There is a trend of M/Lk with group mass, which increases from M/Lk~10 for groups with sigma<250 km/s to M/Lk~100 for sigma=425-800 km/s. This trend is weaker, but still present, if we estimate the total mass from weak lensing measurements. In terms of stellar mass, stars make up ~2 per cent of the mass in the smallest groups, and ~1 per cent in the most massive groups. We also use the near-infrared data to consider the correlations between stellar populations and stellar masses, for group and field galaxies at 0.1<z<0.6. We find that fewer group galaxies show strong [OII] emission compared with field galaxies of the same stellar mass and at the same redshift. We conclude that most of the stellar mass in these groups was already in place by z~0.4, with little environment-driven evolution to the present day.

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arXiv2006

Discovery of a very extended X-ray halo around a quiescent spiral galaxy - the "missing link" of galaxy formation

Hot gaseous haloes surrounding galaxies and extending well beyond the distribution of stars are a ubiquitous prediction of galaxy formation scenarios. The haloes are believed to consist of gravitationally trapped gas with a temperature of millions of Kelvin. The existence of such hot haloes around massive elliptical galaxies has been established through their X-ray emission. While gas out-flowing from starburst spiral galaxies has been detected, searches for hot haloes around normal, quiescent spiral galaxies have so far failed, casting doubts on the fundamental physics in galaxy formation models. Here we present the first detection of a hot, large-scale gaseous halo surrounding a normal, quiescent spiral galaxy, NGC 5746, alleviating a long-standing problem for galaxy formation models. In contrast to starburst galaxies, where the X-ray halo can be powered by the supernova energy, there is no such power source in NGC 5746. The only compelling explanation is that we are here witnessing a galaxy forming from gradually in-flowing hot and dilute halo gas.

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arXiv2005

On the evolutionary status of early-type galaxies in clusters at z~0.2 - I. The Fundamental Plane

We investigate a spectroscopic sample of 48 early-type (E+S0) galaxies in the rich cluster Abell 2390 (z=0.23) and 48 E+S0 galaxies from a previously published survey of Abell 2218 (z=0.18). The A2390 spectra are based on Multi Object Spectroscopy with MOSCA at the Calar Alto 3.5-m, complemented by ground-based and HST F555W and F814W observations. Our investigation spans a broad range in luminosity (-20.5>M_r>-23.0) and a wide FOV of 1.5x1.5 Mpc^2. Since the A2218 and A2390 samples are very similar, we can combine them and analyse a total of 96 E+S0 galaxies at z~0.2. We construct the Faber-Jackson relation (FJR) for all E+S0s and detect a modest luminosity evolution. The average offset from the local FJR in Gunn r-band is (DeltaM_r)=0.32+-0.22^m. Less-massive galaxies show a trend for a larger evolution than more-massive galaxies. HST/WFPC2 surface brightness profile fits were used to derive structural parameters. For 34 E+S0 galaxies which enter the Fundamental Plane (FP) we deduce a mild evolution in Gunn r with a zero-point offset of 0.10+-0.06, corresponding to 0.31+-0.18^m. Elliptical and lenticular galaxies are uniformly distributed along the FP with a similar scatter of 0.1 dex. We find little evidence for differences between the populations of ellipticals and S0s. Lenticulars induce on average a larger evolution of 0.44+-0.18^m than ellipticals with 0.02+-0.21^m. The M/L ratios at z=0.2 are offset by (Deltalog(M/L_r))=-0.12+-0.06 dex compared to those of Coma. Our results can be reconciled with a passive evolution of the stellar populations and a high formation redshift for the bulk of the stars in E+S0 galaxies. However, our findings are also consistent with the hierarchical formation picture for rich clusters. [abridged]

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arXiv2004

The Evolution of [OII] Emission from Cluster Galaxies

We investigate the evolution of the star formation rate in cluster galaxies. We complement data from the CNOC1 cluster survey (0.15<z<0.6) with measurements from galaxy clusters in the 2dF galaxy redshift survey (0.05<z<0.1) and measurements from recently published work on higher redshift clusters, up to almost z=1. We focus our attention on galaxies in the cluster core, ie. galaxies with r<0.7h^{-1}_{70}Mpc. Averaging over clusters in redshift bins, we find that the fraction of galaxies with strong [OII] emission is < 20% in cluster cores, and the fraction evolves little with redshift. In contrast, field galaxies from the survey show a very strong increase over the same redshift range. It thus appears that the environment in the cores of rich clusters is hostile to star formation at all the redshifts studied. We compare this result with the evolution of the colours of galaxies in cluster cores, first reported by Butcher & Oemler (1984). Using the same galaxies for our analysis of the [OII] emission, we confirm that the fraction of blue galaxies, which are defined as galaxies 0.2 mag bluer in the rest frame B-V than the red sequence of each cluster, increases strongly with redshift. Since the colours of galaxies retain a memory of their recent star formation history, while emission from the [OII] line does not, we suggest that these two results can best be reconciled if the rate at which the clusters are being assembled is higher in the past, and the galaxies from which it is being assembled are typically bluer.

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arXiv2004

The Difference Between Clusters and Groups: A Journey from Cluster Cores to their Outskirts and Beyond

In this review, we take the reader on a journey. We start by looking at the properties of galaxies in the cores of rich clusters. We have focused on the overall picture: star formation in clusters is strongly suppressed relative to field galaxies at the same redshift. We will argue that the increasing activity and blue populations of clusters with redshift results from a greater level of activity in field galaxies rather than a change in the transformation imposed by the cluster environment. With this in mind, we travel out from the cluster focussing firstly on the properties of galaxies in the outskirts of clusters and then on galaxies in isolated groups. At low redshift, we are able to efficiently probe these environments using the Sloan and 2DF redshift surveys. These allow an accurate comparison of galaxy star formation rates in different regions. The current results show a strong suppression of star formation above a critical threshold in local density. The threshold seems similar regardless of the overall mass of the system. At low redshift at least, only galaxies in close, isolated pairs have their star formation rate boosted above the global average. At higher redshift, work on constructing homogeneous catalogues of galaxies in groups and in the infall regions of clusters is still at an early stage. In the final section, we draw these strands together, summarising what we can deduce about the mechanisms that transform star forming field galaxies into their quiescent cluster counter-parts. We discuss what we can learn about the impact of environment on the global star formation history of the universe.

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