The census of nuclear activity of late-type galaxies in the Virgo cluster

The first spectroscopic census of active galactic nuclei (AGNs) associated with late-type galaxies in the Virgo cluster was carried out by observing 213 out of a complete set of 237 galaxies more massive than   M _dyn > 10^8.5 M_⊙  . Among them, 77 are classified as AGNs [including 21 transition objects, 47 low-ionization nuclear emission regions (LINERs) and nine Seyferts] and comprise 32 per cent of the late-type galaxies in Virgo. Due to spectroscopic incompleteness, at most 21 AGNs are missed in the survey, so that the fraction would increase up to 41 per cent. Using corollary near-infrared observations that enable us to estimate galaxy dynamical masses, it is found that AGNs are hosted exclusively in massive galaxies, i.e.   M _dyn≳ 10¹⁰ M_⊙  . Their frequency increases steeply with the dynamical mass from zero at   M _dyn≈ 10^9.5 M_⊙  to virtually 1 at   M _dyn > 10^11.5 M_⊙  . These frequencies are consistent with those of low-luminosity AGNs found in the general field by the Sloan Digital Sky Survey. Massive galaxies that harbour AGNs commonly show conspicuous r -band star-like nuclear enhancements. Conversely, they often, but not necessarily, contain massive bulges. A few well-known AGNs (e.g. M61, M100, NGC 4535) are found in massive Sc galaxies with little or no bulge. The AGN fraction seems to be only marginally sensitive to galaxy environment. We infer the black hole masses using the known scaling relations of quiescent black holes. No black holes lighter than  ∼10⁶ M_⊙  are found active in our sample.     

The missing soft X-ray luminosity in cluster cooling flows

The gas temperature in the cores of many clusters of galaxies drops inward by about a factor of 3 or more within the central 100-kpc radius. The radiative cooling time drops over the same region from 5 or more Gyr down to below a few 10⁸ yr. Although this indicates that cooling flows are taking place, XMM-Newton spectra show no evidence for strong mass cooling rates of gas below  1–2 keV  . The soft X-ray luminosity expected from steady cooling flows is missing. Here we outline and test the energetics of a cold mixing model in which gas below  1–2 keV  falls from the flow and is rapidly cooled by mixing with cold gas. The missing X-ray luminosity can emerge in the ultraviolet, optical and infrared bands, where strong emission nebulosities are commonly seen. We explore further the requirements for any heat sources that balance the radiative cooling in cluster cores.     

Tracing gas motions in the Centaurus cluster

We apply the stochastic model of iron transport developed by Rebusco et al. to the Centaurus cluster. Using this model, we find that an effective diffusion coefficient D in the range  2 × 10²⁸–4 × 10²⁸ cm² s⁻¹  can approximately reproduce the observed abundance distribution. Reproducing the flat central profile and sharp drop around  30–70 kpc  , however, requires a diffusion coefficient that drops rapidly with radius so that   D > 4 × 10²⁸ cm² s⁻¹  only inside about  25 kpc  . Assuming that all transport is due to fully developed turbulence, which is also responsible for offsetting cooling in the cluster core, we calculate the length- and velocity-scales of energy injection. These length-scales are found to be up to a factor of ∼10 larger than expected if the turbulence is due to the inflation and rising of a bubble. We also calculate the turbulent thermal conductivity and find it is unlikely to be significant in preventing cooling.     

Stochastic heating of cooling flows

It is generally accepted that the heating of gas in clusters of galaxies by active galactic nuclei is a form of feedback. Feedback is required to ensure a long-term, sustainable balance between heating and cooling. This work investigates the impact of proportional stochastic feedback on the energy balance in the intracluster medium. Using a generalized analytical model for a cluster atmosphere, it is shown that an energy equilibrium can be reached exponentially quickly. Applying the tools of stochastic calculus, it is demonstrated that the result is robust with regard to the model parameters, even though they affect the amount of variability in the system.     

Late-type dwarf galaxies in the Virgo cluster — I. The samples

We selected samples of late-type dwarf galaxies in the Virgo cluster with H  i information. The galaxies were observed at the Wise Observatory using several broad-band and Hα bandpasses. UV measurements were carried out with the IUE Observatory from VILSPA, and with the FAUST shuttle-borne UV telescope.   We describe the observations in detail, paying particular attention to the determination of measurement errors, and present the observational results together with published data and far-infrared information from IRAS . The sample will be analysed in subsequent papers, in order to study star formation mechanisms in galaxies.     

ASCA and ROSAT observations of nearby cluster cooling flows

We present a detailed analysis of the X-ray properties of the cooling flows in a sample of nearby, X-ray-bright clusters of galaxies using high-quality ASCA spectra and ROSAT X-ray images. We demonstrate the need for multiphase models to consistently explain the spectral and imaging X-ray data for the clusters. The mass deposition rates of the cooling flows, independently determined from the ASCA spectra and ROSAT images, exhibit reasonable agreement. We confirm the presence of intrinsic X-ray absorption in the clusters using a variety of spectral models. We also report detections of 100-μm infrared emission, spatially coincident with the cooling flows, in several of the systems studied. The observed infrared fluxes and flux limits are in good agreement with the predicted values owing to reprocessed X-ray emission from the cooling flows. We present precise measurements of the abundances of iron, magnesium, silicon and sulphur in the central regions of the Virgo and Centaurus clusters. Our results firmly favour models in which a high mass fraction (70–80 per cent) of the iron in the X-ray gas in these regions originates from Type Ia supernovae. Finally, we present a series of methods which may be used to estimate the ages of cooling flows from X-ray data. The results for the present sample of clusters indicate ages of between 2.5 and 7 Gyr. If the ages of cooling flows are primarily set by subcluster merger events, then our results suggest that in the largest clusters, mergers with subclusters with masses of ∼30 per cent of the final cluster mass are likely to disrupt cooling flows.     

Simulation of radio plasma in clusters of galaxies

We present three-dimensional hydrodynamical simulations of buoyant gas in a typical cluster environment. In these simulations, hot matter was injected continuously into a small region offset from the cluster centre. In agreement with previous analytic estimates, we found that the bubbles evolve very differently depending on the rate of energy injection. Using tracer particles we computed the efficiency of the bubbles to stir the intracluster medium (ICM) and find that recurrent low-power sources are more effective in mixing the inner cluster region than rarer large outbursts. Moreover, we computed radio maps of the bubbles based on different assumptions about the magnetic field. In the radio band the bubbles closely resemble FR I sources. For the bubbles to be detectable for long enough to account for FR I sources, we found that reacceleration has to take place. The bubbles are generally difficult to detect, both in the radio and in the X-ray band. Thus it is possible to hide a significant amount of energy in the form of bubbles in clusters.     

Numerical simulations of rotating cooling flows in galaxy cluster environments

We have used 2D numerical simulations to study the evolution of galaxy cluster cooling flows undergoing a rotational perturbation. We show that such rotations in the intracluster medium may arise from cluster/subcluster mergers. Our galaxy cluster initial conditions involve spherically symmetric, steady-state cooling flows with varying mass-dropout strengths. The rotational perturbation serves to break the symmetry for each of the initial cooling flows, resulting in the formation of thin, gaseous disc-like structure extending radially out to ∼10 kpc. Disc-like structure formed for low mass-dropout strength simulations appears to contain cooling condensations whereas disc-like structure in higher mass-dropout strength simulations appears smooth. This is due to the influence of mass-dropout on the degree of cooling, which serves to reduce the strength of thermal instabilities by the removal of 'cold' gas from the flow. Morphological comparisons of the disc-like structure formed in our simulations are made to structure observed in the X-ray emitting gas of A4059. Comparisons of the gas dynamics within the disc-like structure are also made to the solid-body rotation profile observed from emission-line gas within the central galaxy of Hydra A. The influence of grid effects on the simulations is also discussed.     

Impact of stochastic gas motions on galaxy cluster abundance profiles

The impact of stochastic gas motions on the metal distribution in cluster cores is evaluated. Peaked abundance profiles are a characteristic feature of clusters with cool cores, and abundance peaks are probably associated with the brightest cluster galaxies (BCGs), which dwell in cluster cores. However, the width of the abundance peaks is significantly broader than the BCG light distribution, suggesting that some gas motions are transporting metals originating from within the BCG. Assuming that this process can be treated as diffusive, and using the brightest X-ray cluster A426 (Perseus) as an example, we estimate that a diffusion coefficient of the order of  2 × 10²⁹ cm² s⁻¹  is needed to explain the width of the observed abundance profiles. Much lower (higher) diffusion coefficients would result in too peaked (too shallow) profiles. Such diffusion could be produced by stochastic gas motions, and our analysis provides constraints on the product of their characteristic velocity and their spatial coherence scale. We speculate that the activity of the supermassive black hole of the BCG is driving the stochastic gas motions in cluster cores. When combined with the assumption that the dissipation of the same motions is a key gas heating mechanism, one can estimate both the velocity and the spatial scale of such diffusive processes.     

The nature of a dusty ring in Virgo

We detected a ring-like distribution of far-infrared (FIR) emission in the direction of the centre of the Virgo cluster (VC). We studied this feature in the FIR, radio and optical domains, and deduced that the dust within the feature reddens the galaxies in the direction of the VC but does not affect stars within the Milky Way. This is likely to be a dusty feature in the foreground of the VC, presumably in the Galactic halo. The H  i distribution follows the morphology of the FIR emission and shows peculiar kinematic behaviour. We propose that a highly supersonic past collision between an H  i cloud and the Galactic H  i formed a shock that heated the interface gas to soft X-ray temperatures. H  i remnants from the projectile and from the shocked Galactic H  i rain down on to the disc as intermediate-velocity gas.
Our finding emphasizes that extragalactic astronomy must consider the possibility of extinction by dust at high galactic latitude and far from the Galactic plane, which may show structure on 1° and smaller scales. This is particularly important for studies of the VC, e.g. in the determination of the Hubble constant from Cepheids in cluster galaxies.     

The 6.4-keV fluorescent iron line from cluster cooling flows

The fate of the cooling gas in the central regions of rich clusters of galaxies is not well understood. In one plausible scenario clouds of atomic or molecular gas are formed. However the mass of the cold gas, inferred from measurements of low-energy X-ray absorption, is hardly consistent with the absence of powerful CO or 21-cm emission lines from the cooling flow region. Among the factors which may affect the detectability of the cold clouds are their optical depth, shape and covering fraction. Thus, alternative methods to determine the mass in cold clouds, which are less sensitive to these parameters, are important.   For the inner region of the cooling flow (e.g. within a radius of ∼50–100 kpc) the Thomson optical depth of the hot gas in a massive cooling flow can be as large as ∼ 0.01. Assuming that the cooling time in the inner region is few times shorter than the lifetime of the cluster, the Thomson depth of the accumulated cold gas can be accordingly higher (if most of the gas remains in the form of clouds). The illumination of the cold clouds by the X-ray emission of the hot gas should lead to the appearance of a 6.4-keV iron fluorescent line, with an equivalent width proportional to τ_T. The equivalent width only weakly depends on the detailed properties of the clouds, e.g. on the column density of individual clouds, as long as the column density is less than a few 10²³ cm⁻². Another effect also associated exclusively with the cold gas is a flux in the Compton shoulder of bright X-ray emission lines. It also scales linearly with the Thomson optical depth of the cold gas. With the new generation of X-ray telescopes, combining large effective area and high spectral resolution, the mass of the cold gas in cooling flows (and its distribution) can be measured.