arXiv subjects

Michael L. Balogh

Publications and source records attributed to Michael L. Balogh.

86 records · Page 5Linked to original sources

arXiv2003

On the Relationship between Cooling Flows and Bubbles

A common feature of the X-ray bubbles observed in Chandra images of some "cooling flow" clusters is that they appear to be surrounded by bright, cool shells. Temperature maps of a few nearby luminous clusters reveal that the shells consist of the coolest gas in the clusters -- much cooler than the surrounding medium. Using simple models, we study the effects of this cool emission on the inferred cooling flow properties of clusters. We find that the introduction of bubbles into model clusters that do NOT have cooling flows results in temperature and surface brightness profiles that resemble those seen in nearby "cooling flow" clusters. They also approximately reproduce the recent XMM-Newton and Chandra observations of a high minimum temperature of 1-3 keV. Hence, bubbles, if present, must be taken into account when inferring the physical properties of the ICM. In the case of some clusters, bubbles may account entirely for these observed features, calling into question their designation as clusters with cooling flows. However, since not all nearby "cooling flow" clusters show bubble-like features, we suggest that there may be a diverse range of physical phenomena that give rise to the same observed features.

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arXiv2003

Cluster Sunyaev-Zeldovich Effect Scaling Relations

X-ray observations of an "entropy floor" in nearby groups and clusters of galaxies offer evidence that important non-gravitational processes, such as radiative cooling and/or "preheating", have strongly influenced the evolution of the intracluster medium (ICM). We examine how the presence of an entropy floor modifies the thermal Sunyaev-Zeldovich (SZ) effect. A detailed analysis of scaling relations between X-ray and SZ effect observables and also between the two primary SZ effect observables is presented. We find that relationships between the central Compton parameter and the temperature or mass of a cluster are extremely sensitive to the presence of an entropy floor. The same is true for correlations between the integrated Compton parameter and the X-ray luminosity or the central Compton parameter. In fact, if the entropy floor is as high as inferred in recent analyses of X-ray data, a comparison of these correlations with both current and future SZ effect observations should show a clear signature of this excess entropy. Moreover, because the SZ effect is redshift-independent, the relations can potentially be used to track the evolution of the cluster gas and possibly discriminate between the possible sources of the excess entropy. To facilitate comparisons with observations, we provide analytic fits to these scaling relations.

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arXiv2002

Galaxy Properties in Low X-Ray Luminosity Clusters at z=0.25

We present the first spectroscopic survey of intrinsically low Lx clusters at z>>0, with HST WFPC2 imaging and ground-based spectroscopy. We study 172 confirmed cluster members in a sample of ten clusters at z=0.23-0.3, with Lx<4.E43 h^{-2} ergs/s. The core of each cluster is imaged with WFPC2 in the F702W filter, and the spectroscopic sample is statistically complete to Mr-19.0+5log(h), within an 11' field. The clusters are dynamically well-separated from the surrounding field and the velocity dispersions range from ~350-850 km/s. Emission line galaxies in these clusters are relatively rare, comprising only 22 +/- 4% of the sample. There is no evidence that these emission-line galaxies are dynamically distinct from the majority of the cluster population, though our sample is too small to rule out the ~30% difference that has been observed in more massive clusters. We find eleven galaxies, comprising 6% of the cluster members, which are disk-dominated but show no sign of emission in their spectrum. Most of these are relatively isolated, spiral galaxies with smooth disks. We find no cluster members with a starburst or post-starburst spectrum. The striking similarity between the spectral and morphological properties of galaxies in these clusters and those of galaxies in more massive systems at similar redshifts implies that the physical processes responsible for truncating star formation in galaxies are not restricted to the rare, rich cluster environment, but are viable in much more common environments. In particular, we conclude that ram pressure stripping or cluster-induced starbursts cannot be solely responsible for the low star formation rates in these systems.

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arXiv2002

New Tests of the Cluster Entropy Floor Hypothesis

Recent efforts to account for the observed X-ray luminosity - temperature relation of galaxy clusters has led to suggestions that the ICM has an apparent ``entropy floor'' at or above the level of 300 keV cm^2. Here, we propose new tests based on the thermal Sunyaev-Zeldovich effect and on the cluster gas mass - temperature trend (from X-ray data) to probe the level of excess entropy in the ICM. We show that these new tests lend further support to the case for a high entropy floor in massive clusters.

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arXiv2002

Modified-Entropy Models for the Intracluster Medium

We present a set of cluster models that link the present-day properties of clusters to the processes that govern galaxy formation. These models treat the entropy distribution of the intracluster medium as its most fundamental property. Because convection strives to establish an entropy gradient that rises with radius, the observable properties of a relaxed cluster depend entirely on its dark-matter potential and the entropy distribution of its uncondensed gas. Guided by simulations, we compute the intracluster entropy distribution that arises in the absence of radiative cooling and supernova heating by assuming that the gas-density distribution would be identical to that of the dark matter. The lowest-entropy gas would then fall below a critical entropy threshold at which the cooling time equals a Hubble time. Radiative cooling and whatever feedback is associated with it must modify the entropy of that low-entropy gas, changing the overall entropy distribution function and thereby altering the observable properties of the cluster. Using some phenomenological prescriptions for entropy modification based on the existence of this cooling threshold, we construct a remarkably realistic set of cluster models. The surface-brightness profiles, mass-temperature relation, and luminosity-temperature relation of observed clusters all naturally emerge from these models. By introducing a single adjustable parameter related to the amount of intracluster gas that can cool within a Hubble time, we can also reproduce the observed temperature gradients of clusters and the deviations of cooling-flow clusters from the standard luminosity-temperature relation.

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arXiv2002

The Cluster Gas Mass - Temperature Relation: Evidence for a High Level of Preheating

Recent X-ray observations have been used to demonstrate that the cluster gas mass-temperature relation is steeper than theoretical self-similar predictions drawn from numerical simulations that consider the evolution of the cluster gas through the effects of gravity and shock heating alone. One possible explanation for this is that the gas mass fraction is not constant across clusters of different temperature, as is usually assumed. Observationally, however, there is no compelling evidence for gas mass fraction variation, especially in the case of hot clusters. Seeking an alternative physical explanation for the observed trends, we investigate the role in the cluster gas mass - temperature relation of the preheating of the intracluster medium by some arbitrary source for clusters with emission-weighted mean temperatures of greater than about 3 keV. Making use of the physically-motivated, analytic models developed in 2002 by Babul and coworkers, we find that preheating does, indeed, lead to a steeper relation. This is in agreement with previous theoretical studies on the relation. However, in apparent conflict with these studies, we argue that a ``high'' level of entropy injection is required to match observations. In particular, an entropy floor of greater than about 300 keV cm^2 is required. We also present a new test, namely, the study of the relation within different fixed radii. This allows one to indirectly probe the density profiles of clusters, since it samples different fractions of the virial radius for clusters of different temperature. This test also confirms that a high level of preheating is required to match observations.

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arXiv2001

Physical Implications of the X-ray Properties of Galaxy Groups and Clusters

Within the standard framework of structure formation, where clusters and groups of galaxies are built up from the merging of smaller systems, the physical properties of the intracluster medium, such as the gas temperature and the total X-ray luminosity, are predicted to possess well defined self-similar scaling relations. Observed clusters and groups, however, show strong deviations from these predicted relations. We argue that these deviations are unlikely to be entirely due to observational biasses; we assume they are physically based, due to the presence of excess entropy in the intracluster medium in addition to that generated by accretion shocks during the formation of the cluster. Several mechanisms have been suggested as a means of generating this entropy. Focussing on those mechanisms that preheat the gas before it becomes a constituent of the virialized cluster environment, we present a simple, intuitive, physically motivated, analytic model that successfully captures the important physics associated with the accretion of high entropy gas onto group and cluster-scale systems. We use the model to derive the new relationships between the observable properties of clusters and groups of galaxies, as well as the evolution of these relations. These include the luminosity-temperature and luminosity-$σ$ relations, as well as the temperature distribution function and X-ray luminosity function. These properties are found to be a more accurate description of the observations than those predicted from the standard framework. Future observations will that further test the efficacy of the preheated gas scenario are also discussed.

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arXiv2001

Bubbles, feedback and the intra-cluster medium: three-dimensional hydrodynamic simulations

We use a three dimensional hydrodynamical code to simulate the effect of energy injection on cooling flows in the intracluster medium. Specifically, we compare a simulation of a 10$^{15}$ $M_\odot$ cluster with radiative cooling only, with a second simulation in which thermal energy is injected 31 $\kpc$ off-centre, over 64 kpc$^{3}$ at a rate of $4.9\times 10^{44} \ergs$ for 50 Myr. The heat injection forms a hot, low density bubble which quickly rises, dragging behind it material from the cluster core. The rising bubble pushes with it a shell of gas which expands and cools. We find the appearance of the bubble in X-ray temperature and luminosity to be in good qualitative agreement with recent {\it Chandra} observations of cluster cores. Toward the end of the simulation, at 600 Myr, the displaced gas begins to fall back toward the core, and the subsequent turbulence is very efficient at mixing the low and high entropy gas. The result is that the cooling flow is disrupted for up to $\sim 50$ Myr after the injection of energy ceases. Thus, this mechanism provides a very efficient method for regulating cooling flows, if the injection events occur with a 1:1 duty cycle.

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arXiv2001

The Star Formation Histories of Local Galaxies: Continuous or Intermittent?

We explore a model in which efficient star formation in galaxies is triggered by merging satellites. We show that the merger/interaction rates predicted by hierarchical galaxy formation models depend strongly on galaxy mass. If a merger of a satellite larger than 1% the mass of the primary triggers efficient star formation, low-mass dwarf galaxies experience strong bursts separated by quiescent periods lasting several Gyr. Massive galaxies, such as our own Milky Way, are perturbed by a 1% satellite every few hundred million years, and thus have fluctuating, but relatively continuous star formation histories. We study the spectral signatures of a population of galaxies undergoing intermittent star formation by combining the models with the latest version of the Bruzual & Charlot spectral synthesis code. We concentrate on spectral indicators that are sensitive primarily to stellar age, rather than to metallicity and extinction. We show that if a population undergoes intermittent rather than continuous star formation, the signatures of the bursts should be evident from the observed dispersion in the star formation rates, H-delta equivalent widths and gas mass fractions of galaxies with 4000 Angstrom break strengths indicative of recent or ongoing star formation.

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arXiv2000

A low global star formation rate in the rich galaxy cluster AC114 at z=0.32

We present the results of a wide-field survey for H-alpha emitting galaxies in the cluster AC114 at z=0.32. Spectra centred on H-alpha at the cluster redshift have been obtained for 586 galaxies to I~22 out to a radius around 2Mpc. At most, only ~10% of these were found to be H-alpha-emitting cluster members. These objects are predominantly blue and of late-type spiral morphology, consistent with them hosting star formation. However, ~65% of the cluster members classified morphologically as spirals (with HST), have no detectable H-alpha emission; star-formation and morphological evolution in cluster galaxies appear to be largely decoupled. Changes in the H-alpha detection rate and the strength of H-alpha emission with environment are found to be weak within the region studied. Star formation within the cluster members is also found to be strongly and uniformly suppressed, with the rates inferred from the H-alpha emission not exceeding 4Mo/yr, and AC114's H-alpha luminosity function being an order of magnitude below that observed for field galaxies at the same redshift. None of the galaxies detected have the high star formation rates associated with `starburst' galaxies; however, this may still be reconciliable with the known (8+/-3%) fraction of `post-starburst' galaxies within AC114, given the poorly determined but short lifetimes of starbursts and the possibility that much of the associated star formation is obscured by dust.

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arXiv2000

H-alpha Photometry of Abell 2390

We present the results of a search for strong H-alpha emission line galaxies (rest frame equivalent widths greater than 50 A) in the z=0.23 cluster Abell 2390. The survey contains 1189 galaxies over 270 square arcminutes, and is 50% complete at M_r=-17.5+5 log h. The fraction of galaxies in which H-alpha is detected at the 2sigma level rises from 0.0 in the central regions (excluding the cD galaxy) to 12.5+/-8% at R200. For 165 of the galaxies in our catalogue, we compare the H-alpha equivalent widths with their [OII] equivalent widths, from the CNOC1 spectra. The fraction of strong H-alpha emission line galaxies is consistent with the fraction of strong [OII] emission galaxies in the CNOC1 sample: only 2+/-1% have no detectable [OII] emission and yet significant (>2sigma) H-alpha equivalent widths. Dust obscuration, non-thermal ionization, and aperture effects are all likely to contribute to this non-correspondence of emission lines. We identify six spectroscopically 'secure' k+a galaxies (W(OII)<5 A and W(Hdelta)>5 A); at least two of these show strong signs in H-alpha of star formation in regions that are covered by the slit from which the spectra were obtained. Thus, some fraction of galaxies classified k+a based on spectra shortward of 6000 A are likely to be undergoing significant star formation. These results are consistent with a 'strangulation' model for cluster galaxy evolution, in which star formation in cluster galaxies is gradually decreased, and is neither enhanced nor abruptly terminated by the cluster environment.

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arXiv2000

The Spatial and Kinematic Distributions of Cluster Galaxies in a LCDM Universe -- Comparison with Observations

We combine dissipationless N-body simulations and semi-analytic models of galaxy formation to study the spatial and kinematic distributions of cluster galaxies in a LCDM cosmology. We investigate how the star formation rates, colours and morphologies of galaxies vary as a function of distance from the cluster centre and compare our results with the CNOC1 survey of galaxies from 15 X-ray luminous clusters in the redshift range 0.18 to 0.55. In our model, gas no longer cools onto galaxies after they fall into the cluster and their star formation rates decline on timescales of 1-2 Gyr. Galaxies in cluster cores have lower star formation rates and redder colours than galaxies in the outer regions because they were accreted earlier. Our colour and star formation gradients agree with those those derived from the data. The difference in velocity dispersions between red and blue galaxies observed in the CNOC1 clusters is also well reproduced by the model. We assume that the morphologies of cluster galaxies are determined solely by their merging histories. Morphology gradients in clusters arise naturally, with the fraction of bulge- dominated galaxies highest in cluster cores. We compare these gradients with the CNOC1 data and find excellent agreement for bulge-dominated galaxies. The simulated clusters contain too few galaxies of intermediate bulge-to-disk ratio, suggesting that additional processes may influence the morphological evolution of disk-dominated galaxies in clusters. Although the properties of the cluster galaxies in our model agree extremely well with the data, the same is not true of field galaxies. Both the star formation rates and the colours of bright field galaxies appear to evolve much more strongly from redshift 0.2 to 0.4 in the CNOC1 field sample than in our simulations.

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arXiv2000

The Origin of Star Formation Gradients in Rich Galaxy Clusters

We examine the origin of clustercentric gradients in the star formation rates and colors of rich cluster galaxies within the context of a simple model where clusters are built through the ongoing accretion of field galaxies. The model assumes that after galaxies enter the cluster their star formation rates decline on a timescale of a few Gyrs, the typical gas consumption timescale of disk galaxies in the field. Such behaviour might be expected if tides and ram pressure strip off the gaseous envelopes that normally fuel star formation in spirals over a Hubble time. Combining these timescales with mass accretion histories derived from N-body simulations of cluster formation in a Lambda-CDM universe, we reproduce the systematic differences observed in the color distribution of cluster and field galaxies, as well as the strong suppression of star formation in cluster galaxies and its dependence on clustercentric radius. The simulations also indicate that a significant fraction of galaxies beyond the virial radius of the cluster may have been within the main body of the cluster in the past, a result that explains naturally why star formation in the outskirts of clusters (and as far out as two virial radii) is systematically suppressed relative to the field. The agreement with the data beyond the cluster virial radius is also improved if we assume that stripping happens within lower mass systems, before the galaxy is accreted into the main body of the cluster. We conclude that the star formation rates of cluster galaxies depend primarily on the time elapsed since their accretion onto massive virialized systems, and that the cessation of star formation may have taken place gradually over a few Gyrs.

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arXiv1999

Differential Galaxy Evolution in Cluster and Field Galaxies at z=0.3

(abridged) We measure spectral indices for 1823 galaxies in the CNOC1 sample of fifteen X-ray luminous clusters at 0.18<z<0.55, to investigate the mechanisms responsible for differential evolution between the galaxy cluster and field environments. The radial trends of D4000, Hdelta and [OII] are all consistent with an age sequence, in the sense that the last episode of star formation occurred more recently in galaxies farthest from the cluster center. Throughout the cluster environment, galaxies show evidence for older mean stellar populations than field galaxies. From the subsample of galaxies more luminous than M_r=-18.8 + 5log h, we identify a sample of K+A galaxies, which may result from recently terminated star formation. Corrected for a systematic effect which results from the large uncertainties on individual spectral index measurements, we estimate that K+A galaxies make up only 1.5 +/- 0.8 % of the cluster sample, and 1.2 +/- 0.8 % of the field. We compare our data with spectrophotometric models and conclude that up to 1.9 +/- 0.8 % of the cluster population may have had its star formation recently truncated without a starburst. However, this is still not significantly greater than the fraction of such galaxies in the field, 3.1 +/- 1.0 %. Furthermore, we do not detect an excess of cluster galaxies that have unambiguously undergone a starburst within the last 1 Gyr. Our results imply that these cluster environments are not responsible for inducing starbursts; thus, the increase in cluster blue galaxy fraction with redshift may not be a strictly cluster--specific phenomenon. We suggest that the truncation of star formation in clusters may largely be a gradual process, perhaps due to the exhaustion of gas in the galactic disk over fairly long timescales.

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