We present a 90 per cent flux‐complete sample of the 201 X‐ray‐brightest clusters of galaxies in the northern hemisphere (δ ≥ 0°), at high Galactic latitudes (|b| ≥ 20°), with measured redshifts z ≤ 0.3 and fluxes higher than 4.4 × 10−12 erg cm−2 s−1 in the 0.1–2.4 keV band. The sample, called the ROSAT Brightest Cluster Sample (BCS), is selected from ROSAT All‐Sky Survey data and is the largest X‐ray‐selected cluster sample compiled to date. In addition to Abell clusters, which form the bulk of the sample, the BCS also contains the X‐ray‐brightest Zwicky clusters and other clusters selected from their X‐ray properties alone. Effort has been made to ensure the highest possible completeness of the sample and the smallest possible contamination by non‐cluster X‐ray sources. X‐ray fluxes are computed using an algorithm tailored for the detection and characterization of X‐ray emission from galaxy clusters. These fluxes are accurate to better than 15 per cent (mean 1σ error).  We find the cumulative log N–log S distribution of clusters to follow a power law κSα with α = 1.31+0.06−0.03 (errors are the 10th and 90th percentiles) down to fluxes of 2 × 10−12 erg cm−2 s−1, i.e. considerably below the BCS flux limit. Although our best‐fitting slope disagrees formally with the canonical value of −1.5 for a Euclidean distribution, the BCS log N–log S distribution is consistent with a non‐evolving cluster population if cosmological effects are taken into account.  Our sample will allow us to examine large‐scale structure in the northern hemisphere, determine the spatial cluster–cluster correlation function, investigate correlations between the X‐ray and optical properties of the clusters, establish the X‐ray luminosity function for galaxy clusters, and discuss the implications of the results for cluster evolution.

We present new spectra of dominant galaxies in X‐ray‐selected clusters of galaxies, which combine with our previously published spectra to form a sample of 256 dominant galaxies in 215 clusters. 177 of the clusters are members of the ROSAT Brightest Cluster Sample (BCS; Ebeling et al.), and 17 have no previous measured redshift. This is the first paper in a series correlating the properties of brightest cluster galaxies and their host clusters in the radio, optical and X‐ray wavebands. 27 per cent of the central dominant galaxies have emission‐line spectra, all but five with line intensity ratios typical of cooling flow nebulae. A further 6 per cent show only [N ii]λλ6548,6584 with Hα in absorption. We find no evidence for an increase in the frequency of line emission with X‐ray luminosity. Purely X‐ray‐selected clusters at low redshift have a higher probability of containing line emission. The projected separation between the optical position of the dominant galaxy and its host cluster X‐ray centroid is less for the line‐emitting galaxies than for those without line emission, consistent with a closer association of the central galaxy and the gravitational centre in cooling flow clusters. The more Hα‐luminous galaxies have larger emission‐line regions and show a higher ratio of Balmer to forbidden line emission, although there is a continuous trend of ionization behaviour across four decades in Hα luminosity. Galaxies with the more luminous line emission [L(Hα)> 1041ergs−1] show a significantly bluer continuum, whereas lower luminosity and [N ii]‐only line emitters have continua that differ little from those of non‐line‐emitting dominant galaxies. Values of the Balmer decrement in the more luminous systems commonly imply intrinsic reddening of E(B−V)∼0.3 and, when this is corrected for, the excess blue light can be characterized by a population of massive young stars. Several of the galaxies require a large population of O stars, which also provide sufficient photoionization to produce the observed Hα luminosity. The large number of lower mass stars relative to the O‐star population suggests that this anomalous population is caused by a series of starbursts in the central galaxy. The lower Hα‐luminosity systems show a higher ionization state and few massive stars, requiring instead the introduction of a harder source of photoionization, such as turbulent mixing layers, or low‐level nuclear activity. The line emission from the systems showing only [N ii] is very similar to low‐level LINER activity commonly found in many normal elliptical galaxies.

We report subarcsec-resolution X-ray imaging of the core of the Perseus cluster around the galaxy NGC 1275 with the Chandra X-ray Observatory. The ROSAT-discovered holes associated with the radio lobes have X-ray bright rims which are cooler than the surrounding gas and not due to shocks. The holes themselves may contain some hotter gas. We map strong photoelectric absorption across the Northern lobe and rim due to a small infalling irregular galaxy, known as the high velocity system. Two outer holes, one of which was previously known, are identified with recently found spurs of low-frequency radio emission. The spiral appearance of the X-ray cooler gas and the outer optical parts of NGC 1275 may be due to angular momentum in the cooling flow.Comment: 5 pages, 7 figures (6 colour), accepted by MNRAS, high resolution version at http://www-xray.ast.cam.ac.uk/papers/per_chandra.ps.g

We present a low‐flux extension of the X‐ray‐selected ROSAT Brightest Cluster Sample (BCS) published in Paper I of this series. Like the original BCS and employing an identical selection procedure, the BCS extension is compiled from ROSAT All‐Sky Survey (RASS) data in the northern hemisphere (δ≥0°) and at high Galactic latitudes (|b|≥20°). It comprises 99 X‐ray‐selected clusters of galaxies with measured redshifts z≤0.3 (as well as eight more at z>0.3) and total fluxes between 2.8×10−12 and 4.4×10−12 erg cm−2 s−1 in the 0.1–2.4 keV band (the latter value being the flux limit of the original BCS). The extension can be combined with the main sample published in 1998 to form the homogeneously selected extended BCS (eBCS), the largest and statistically best understood cluster sample to emerge from the RASS to date. The nominal completeness of the combined sample (defined with respect to a power‐law fit to the bright end of the BCS log N–log S distribution) is relatively low at 75 per cent (compared with 90 per cent for the high‐flux sample of Paper I). However, just as for the original BCS, this incompleteness can be accurately quantified, and thus statistically corrected for, as a function of X‐ray luminosity and redshift. In addition to its importance for improved statistical studies of the properties of clusters in the local Universe, the low‐flux extension of the BCS is also intended to serve as a finding list for X‐ray‐bright clusters in the northern hemisphere which we hope will prove useful in the preparation of cluster observations with the next generation of X‐ray telescopes such as Chandra and XMM‐Newton. An electronic version of the eBCS can be obtained from the following URL: http://www.ifa.hawaii.edu/~ebeling/clusters/BCS.html.

We present preliminary results from a deep observation lasting almost 200 ks, of the centre of the Perseus cluster of galaxies around NGC 1275. The X-ray surface brightness of the intracluster gas beyond the inner 20 kpc, which contains the inner radio bubbles, is very smooth apart from some low amplitude quasi-periodic ripples. A clear density jump at a radius of 24 kpc to the NE, about 10 kpc out from the bubble rim, appears to be due to a weak shock driven by the northern radio bubble. A similar front may exist round both inner bubbles but is masked elsewhere by rim emission from bright cooler gas. The continuous blowing of bubbles by the central radio source, leading to the propagation of weak shocks seen as the observed fronts and ripples, gives a rate of working which balances the radiative cooling within the inner 50 kpc of the cluster core.

We present preliminary results from a deep observation lasting almost 200 ks, of the centre of the Perseus cluster of galaxies around NGC 1275. The X-ray surface brightness of the intracluster gas beyond the inner 20 kpc, which contains the inner radio bubbles, is very smooth apart from some low amplitude quasi-periodic ripples. A clear density jump at a radius of 24 kpc to the NE, about 10 kpc out from the bubble rim, appears to be due to a weak shock driven by the northern radio bubble. A similar front may exist round both inner bubbles but is masked elsewhere by rim emission from bright cooler gas. The continuous blowing of bubbles by the central radio source, leading to the propagation of weak shocks and viscously-dissipating sound waves seen as the observed fronts and ripples, gives a rate of working which balances the radiative cooling within the inner 50 kpc of the cluster core.Comment: Accepted for publication in MNRAS (minor changes) Higher picture quality available from http://www-xray.ast.cam.ac.uk/papers/per_200ks.pd

Cold molecular gas has recently been detected in several cooling flow clusters of galaxies containing huge optical nebula. These optical filaments are tightly linked to cooling flows and related phenomena, such as rising bubbles of relativistic plasma fed by radio jets. We present here a map, in the CO(2-1) rotational line, of the cold molecular gas associated with some of the Hα filaments surrounding the central galaxy of the Perseus cluster: NGC 1275. The map, extending to about 50 kpc (135 arcsec) from the center of the galaxy, has been made with the 18-receiver array HERA at the focus of the IRAM 30 m telescope. Although most of the cold gas is concentrated to the center of the galaxy, the CO emission is also clearly associated with the extended filaments conspicuous in ionised gas, and could trace a possible reservoir fueling the star formation there. Some of the CO emission is also found where the X-ray gas could cool down more efficiently at the rims of the central X-ray cavities (where the hot gas is thought to have been pushed out and compressed by the expanding radio lobes of the central AGN). The CO global kinematics do not show any rotation in NGC 1275. The cold gas is probably a mixture of gas falling down on the central galaxy and of uplifted gas dragged out by a rising bubble in the intracluster medium. As recently suggested in other cluster cores, the cold gas peculiar morphology and kinematics argue for the picture of an intermittent cooling flow scenario where the central AGN plays an important role.

The giant elliptical galaxy NGC 1275, at the centre of the Perseus cluster, is surrounded by a well-known giant nebulosity of emission-line filaments 1,2 , which are plausibly about >10 8 yr old 3 . The filaments are dragged out from the centre of the galaxy by the radio bubbles rising buoyantly in the hot intracluster gas 4 before later falling back. They act as dramatic markers of the feedback process by which energy is transferred from the central massive black hole to the surrounding gas. The mechanism by which the filaments are stabilized against tidal shear and dissipation into the surrounding 4×10 7 K gas has been unclear. Here we report new observations that resolve thread-like structures in the filaments. Some threads extend over 6 kpc, yet are only 70 pc wide. We conclude that magnetic fields in the threads, in pressure balance with the surrounding gas, stabilize the filaments, so allowing a large mass of cold gas to accumulate and delay star formation.The images presented here (Figs. 1-4) were taken with the Advanced Camera for Surveys (ACS) on the NASA Hubble Space Telescope (HST) using three filters; F625W in the red contains the Hα line, F550M is mostly continuum and F435W in the blue which highlights young stars. In Fig. 2 we show part of the Northern filament ~27 kpc from the nucleus (we adopt H 0 =71 km s -1 Mpc -1 which at a redshift 0.0176 for NGC 1275 gives 352 pc arcsec -1 ). The filaments seen in the WIYN groundbased image (right) are just resolved into narrow threads with the HST ACS (see Supplementary Information). This also occurs in many other filaments including the north-west "horseshoe" filament ( Fig. 3) which lies immediately interior to the outer ghost bubble in X-ray images 5 . A fine thread of emission is seen in the Northern filament system extending about 16 arcsec or 5.8 kpc. Averaged over kpc strips it is about 4 pixels (0.2 arcsec) or about 70 pc wide. (This is an upper limit as the point spread function of the ACS is about one half this value.) The aspect ratio (length / thickness) therefore approaches 100. The top of the horseshoe which is about 6 kpc across is similar, as are many other relatively isolated filaments.In order to estimate the required magnetic field we need to know the properties of a filament and its surroundings. We shall concentrate on a thread of radius 35 pc and length 6 kpc at a distance of 25 kpc from the nucleus of NGC 1275 (Fig. 2) as a basic structural unit typical of what is now resolved in the filaments. To estimate the mass for such a thread we scale from the total gas mass of 10 8 M⊙ inferred from CO emission 6 observed in a 22 arcsec IRAM beam on the same Northern filament complex. Assuming that the mass scales with Hα emission, which is the case for the H 2 emission measured with Spitzer 7 , then our fiducial thread has a mass of about 10 6 M⊙. Its mean density is then ~2 cm -3 and perpendicular column density N ~ 4×10 20 cm -2 or Σ ┴ ~ 7×10 -4 g cm -2 . The lengthwise column density, Σ∥, is l/2r times larger.The variation in projected rad...