C. S. Crawford

Chandra imaging of the complex X-ray core of the Perseus cluster

ABSTRACT We report subarcsec-resolution X-ray imaging of the core ofthe Perseus cluster around thegalaxy NGC1275 with the Chandra X-ray Observatory. The ROSAT-discovered holes asso-ciated with the radio lobes have X-ray bright rims which are cooler than the surrounding gasand not due to shocks. The holes themselves may contain some hotter gas. We map strongphotoelectric absorption across the Northern lobe and rim due to a small infalling irregulargalaxy, known as the high velocity system. Two outer holes, one of which was previouslyknown, are identified with recently found spurs of low-frequency radio emission. The spiralappearance of the X-ray cooler gas and the outer optical parts of NGC1275 may be due toangular momentum in the cooling flow.Key words: galaxies: individual: Perseus – cooling flows – galaxies: in dividual: NGC1275– X-rays: galaxies 1 INTRODUCTIONThe Perseus cluster, Abell426, at a redshift z = 0.0183 or dis-tance about 100 Mpc is the brightest cluster in the sky in X-rays. Ithosts the nearest large cooling flow (e.g. Fabian et al 1981; A llenet al 1990; Fabian et al 1994). X-ray analysis of ASCA spectrain-dicates that the mass deposition rate is about 300M

A very deep Chandra observation of the Perseus cluster: shocks, ripples and conduction

We present the first results from a very deep Chandra X-ray observation of the core of the Perseus cluster of galaxies. A pressure map reveals a clear thick band of high pressure around the inner radio bubbles. The gas in the band must be expanding outward and the sharp front to it is identified as a shock front, yet we see no temperature jump across it; indeed there is more soft emission behind the shock than in front of it. We conclude that in this inner region either thermal conduction operates efficiently or the co-existing relativistic plasma seen as the radio mini-halo is mediating the shock. If common, isothermal shocks in cluster cores mean that we cannot diagnose the expansion speed of radio bubbles from temperature measurements alone. They can at times expand more rapidly than currently assumed without producing significant regions of hot gas. Bubbles may also be significantly more energetic. The pressure ripples found in earlier images are identified as isothermal sound waves. A simple estimate based on their amplitude confirms that they can be an effective distributed heat source able to balance radiative cooling.We see multiphase gas with about 10{sup 9}M{sub {circle_dot}} at a temperature of about 0.5 keV. Much, but not all, of this cooler gas is spatially associated with the optical filamentary nebula around the central galaxy, NGC1275. A residual cooling flow of about 50M{sub {circle_dot}} yr{sup -1} may be taking place. A channel is found in the pressure map along the path of the bubbles, with indications found of outer bubbles. The channel connects in the S with a curious cold front.

A deep Chandra observation of the Perseus cluster: shocks and ripples

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 north-east, 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 around 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.

The ROSAT Brightest Cluster Sample – III. Optical spectra of the central cluster galaxies

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. 1998), and 18 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 (NII)��6548,6584 with Hin 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 H�-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 Hluminosity. Galaxies with the more luminous line emission (L(H�)> 10 41 ergs 1 ) show a significantly bluer continuum, whereas lower-luminosity and (NII)-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 theobserved Hluminosity. The large number of lower-mass stars relative to the O star population suggests that this anomalous population is due to a series of starbursts in the central galaxy. The lower H�-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 (NII) is very similar to low-level LINER activity commonly found in many normal elliptical galaxies.

The relationship between the optical Hα filaments and the X-ray emission in the core of the Perseus cluster

NGC 1275 in the centre of the Perseus cluster of galaxies, Abell 426, is surrounded by a spectacular filamentary Hα nebula. Deep Chandra X-ray imaging has revealed that the brighter outer filaments are also detected in soft X-rays. This can be due to conduction and mixing of the cold gas in the filaments with the hot, dense intracluster medium. We show the correspondence of the filaments in both wavebands and draw attention to the relationship of two prominent curved north-west filaments to an outer, buoyant radio bubble seen as a hole in the X-ray image. There is a strong resemblance in the shape of the hole and the disposition of the filaments to the behaviour of a large air bubble rising in water. If this is a correct analogy, then the flow is laminar and the intracluster gas around this radio source is not turbulent. We obtain a limit on the viscosity of this gas.

The ROSAT Brightest Cluster Sample (BCS): The Cluster X-Ray Luminosity Function within z = 0.3

We present and discuss the X-ray luminosity function (XLF) of the ROSAT Brightest Cluster sample (BCS), an X-ray flux limited sample of clusters of galaxies in the northern hemisphere compiled from ROSAT All-Sky Survey data. The BCS allows the local cluster XLF to be determined with unprecedented accuracy over almost three decades in X-ray luminosity and provides an important reference for searches for cluster evolution at higher redshifts. We find the significance of evolution in both the XLF amplitude and in the characteristic cluster luminosity L -->X to be less than 1.8 ? within the redshift range covered by our sample thereby disproving previous claims of strong evolution within z 0.2.

Chandra imaging of the X-ray core of Abell 1795

We report the discovery of a 40 arcsec long X-ray filament in th e core of the cluster of galaxies A 1795. The feature coincides with an H�+NII filament found by Cowie et al in the early 1980s and resolved into at least 2 U-band filaments by McNamar a et al in the mid 1990s. The (emission-weighted) temperature of the X-ray emitting gas along the filament is 2.5 3 keV, as revealed by X-ray colour ratios. The deprojected temperature will be less. A detailed temperature map of the core of the cluster presented. The cD galaxy at the head of the filament is probably moving through or oscillating in the cluster core. The radiative cooling time of the X-ray emitting gas in the filament is about 3×10 8 yr which is similar to the age of the filament obtained from its length and velocity. This suggests that th e filament is produced by cooling of the gas from the intracluster medium. The filament, much of which is well separated from the body of the cD galaxy and its radio source, is potentially of great importance in helping to understand the energy and ionization source of the optical nebulosity common in cooling flows.

Magnetic support of the optical emission line filaments in NGC 1275

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, which are plausibly in excess of 108 years old. The filaments are dragged out from the centre of the galaxy by radio-emitting ‘bubbles’ rising buoyantly in the hot intracluster gas, before later falling back. They act as 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 extremely hot (4 × 107 K) gas has been unclear. Here we report 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 XMM-Newton serendipitous survey - IV. Optical identification of the XMM-Newton medium sensitivity survey (XMS)

Aims. X-ray sources at intermediate fluxes (a few x 10(-14) erg cm(-2) s(-1)) with a sky density of similar to 100 deg(-2) are responsible for a significant fraction of the cosmic X-ray background at various energies below 10 keV. The aim of this paper is to provide an unbiased and quantitative description of the X-ray source population at these fluxes and in various X-ray energy bands. Methods. We present the XMM-Newton Medium sensitivity Survey (XMS), including a total of 318 X-ray sources found among the serendipitous content of 25 XMM-Newton target fields. The XMS comprises four largely overlapping source samples selected at soft (0.5-2 keV), intermediate (0.5-4.5 keV), hard (2-10 keV) and ultra-hard (4.5-7.5 keV) bands, the first three of them being flux-limited. Results. We report on the optical identification of the XMS samples, complete to 85-95%. At the flux levels sampled by the XMS we find that the X-ray sky is largely dominated by Active Galactic Nuclei. The fraction of stars in soft X-ray selected samples is below 10%, and only a few per cent for hard selected samples. We find that the fraction of optically obscured objects in the AGN population stays constant at around 15-20% for soft and intermediate band selected X-ray sources, over 2 decades of flux. The fraction of obscured objects amongst the AGN population is larger (similar to 35-45%) in the hard or ultra-hard selected samples, and constant across a similarly wide flux range. The distribution in X-ray-to-optical flux ratio is a strong function of the selection band, with a larger fraction of sources with high values in hard selected samples. Sources with X-ray-to-optical flux ratios in excess of 10 are dominated by obscured AGN, but with a significant contribution from unobscured AGN.

On the origin and excitation of the extended nebula surrounding NGC 1275

We use line-of-sight velocity information on the filamentary emission-line nebula of NGC 1275 to infer a dynamical model of the nebulas flow through the surrounding intracluster gas. We detect outflowing gas and flow patterns that match simulations of buoyantly rising bubbles from which we deduce that some of the nebula filaments have been drawn out of NGC 1275. We find a radial gradient of the ratio [N II]λ6584/Hα which may be due to a variation in metallicity, interactions with the surrounding intracluster medium or a hardening of the excitation mechanism. We find no preferred spatial correlation of stellar clusters within the filaments and there is a notable lack of [O III]λ5007 emission, therefore it is unlikely that the filaments are ionized by stellar ultraviolet.