R. M. Johnstone

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.

A ROSAT study of the cores of clusters of galaxies - I. Cooling flows in an X-ray flux-limited sample

ABSTRA C T This is the first part of a study of the detailed X-ray properties of the cores of nearby clusters. We have used the flux-limited sample of 55 clusters listed by Edge et al., and archival and proprietary data from the ROSAT observatory. In this paper an X-ray spatial analysis based on the surface-brightness-deprojection technique is applied to the clusters in the sample with the aim of studying their cooling flow properties. We determine the fraction of cooling flows in this sample to be 70‐90 per cent, and estimate the contribution of the flow region to the cluster Xray luminosity. We show that the luminosity within a strong cooling flow can account for up to 70 per cent of a cluster X-ray bolometric luminosity. Our analysis indicates that about 40 per cent of the clusters in the sample have flows depositing more than 100 M( yr π1 throughout the cooling region, and that these possibly have been undisturbed for many Gyr, confirming that cooling flows are the natural state of cluster cores. New cooling flows in the sample are presented, and previously ambiguous ones are clarified. We have constructed a catalogue of some intracluster medium properties for the clusters in this sample. The profiles of the mass deposited from cooling flows are analysed, and evidence is presented for the existence of breaks in some of the profiles. Comparison is made to recent optical and radio data. We crosscorrelate our sample with the Green Bank, NVSS and FIRST surveys, and with the volumelimited sample of brightest cluster galaxies presented by Lauer & Postman. Although weak trends exist, no strong correlation between optical magnitude or radio power of the brightest cluster galaxy and the strength of the flow is found.

Chandra observations of Abell 2199

We present results from an analysis of two Chandra observations of the rich, nearby galaxy cluster Abell 2199. We find evidence (having corrected for projection effects) for radial gradients in temperature and metallicity in the X-ray emitting gas: the temperature drops from kT ∼ 4.2 keV at R = 200 kpc to 1.6 keV within R = 5 kpc of the centre. The metallicity rises from ∼0.3 solar at R = 200 kpc to ∼0.7 solar at R = 30 kpc before dropping to 0.3 solar within the central 5 kpc. We find evidence for structure in the surface brightness distribution associated with the central radio source 3C 338. No evidence is found for the gas having a large spread in temperature at any particular location despite the cooling time being short (<10 9 yr) within the central ∼15 kpc. Heating and mass cooling rates are calculated for various assumptions about the state of the gas.

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 prevalence of cooling cores in clusters of galaxies at z≈ 0.15–0.4

We present a Chandra study of 38 X-ray-luminous clusters of galaxies in the ROSAT Brightest Cluster Sample (BCS) that lie at moderate redshifts (z 0.15-0.4). Based primarily on power ratios and temperature maps, we find that the majority of clusters at moderate redshift generally have smooth, relaxed morphologies with some evidence for mild substructure perhaps indicative of recent minor merger activity. Using spatially resolved spectral analyses, we find that cool cores appear still to be common at moderate redshift. At a radius of 50 kpc, we find that at least 55 per cent of the clusters in our sample exhibit signs of mild cooling (t cool < 10 Gyr), while in the central bin at least 34 per cent demonstrate signs of strong cooling (t cool < 2 Gyr). These percentages are nearly identical to those found for luminous, low-redshift clusters of galaxies, indicating that there appears to be little evolution in cluster cores since z 0.4 and suggesting that heating and cooling mechanisms may already have stabilized by this epoch. Comparing the central cooling times to catalogues of central Ha emission in BCS clusters, we find a strong correspondence between the detection of Hα and central cooling time. We also confirm a strong correlation between the central cooling time and cluster power ratios, indicating that crude morphological measures can be used as a proxy for more rigorous analysis in the face of limited signal-to-noise ratio data. Finally, we find that the central temperatures for our sample typically drop by no more than a factor of ∼3-4 from the peak cluster temperatures, similar to those of many nearby clusters.

ASCA and ROSAT observations of nearby cluster cooling flows

ABSTRA C T 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-mm 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 Xray 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.

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.