J. S. Sanders

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.

Baryons at the Edge of the X-ray–Brightest Galaxy Cluster

The Suzaku satellite provides a census of the gas, metals, and dark matter out to the outskirts of the Perseus Cluster. Studies of the diffuse x-ray–emitting gas in galaxy clusters have provided powerful constraints on cosmological parameters and insights into plasma astrophysics. However, measurements of the faint cluster outskirts have become possible only recently. Using data from the Suzaku x-ray telescope, we determined an accurate, spatially resolved census of the gas, metals, and dark matter out to the edge of the Perseus Cluster. Contrary to previous results, our measurements of the cluster baryon fraction are consistent with the expected universal value at half of the virial radius. The apparent baryon fraction exceeds the cosmic mean at larger radii, suggesting a clumpy distribution of the gas, which is important for understanding the ongoing growth of clusters from the surrounding cosmic web.

A deeper X-ray study of the core of the Perseus galaxy cluster: the power of sound waves and the distribution of metals and cosmic rays

We make a further study of the very deep Chandra observation of the X-ray brightest galaxy cluster, A 426 in Perseus. We examine the radial distribution of energy flux inferred by the quasi-concentric ripples in surface brightness, assuming they are due to sound waves, and show that it is a significant fraction of the energy lost by radiative cooling within the inner 75‐ 100 kpc, where the cooling time is 4‐5 Gyr, respectively. The wave flux decreases outward with radius, consistent with energy being dissipated. Some newly discovered large ripples beyond 100 kpc, and a possible intact bubble at 170 kpc radius, may indicate a larger level of activity by the nucleus a few 100 Myr ago. The distribution of metals in the intracluster gas peaks at a radius of about 40 kpc and is significantly clumpy on scales of 5 kpc. The temperature distribution of the soft X-ray filaments and the hard X-ray emission component found within the inner 50 kpc are analysed in detail. The pressure due to the non-thermal electrons, responsible for a spectral component interpreted as inverse Compton emission, is high within 40 kpc of the centre and boosts the power in sound waves there; it drops steeply beyond 40 kpc. We find no thermal emission from the radio bubbles; in order for any thermal gas to have a filling factor within the bubbles exceeding 50 per cent, the temperature of that gas has to exceed 50 keV.

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.

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.

Mapping small-scale temperature and abundance structures in the core of the Perseus cluster

We report further results from a 191-ks Chandra observation of the core of the Perseus cluster, Abell 426. The emission-weighted temperature and abundance structures are mapped in detail. There are temperature variations down to ∼ 1 kpc in the brightest regions. Globally, the strongest X-ray surface brightness features appear to be caused by temperature changes. Density and temperature changes conspire to give approximate azimuthal balance in pressure showing that the gas is in hydrostatic equilibrium. Si, S, Ar, Ca, Fe and Ni abundance profiles rise inwards from about 100 kpc, peaking at about 30-40 kpc. Most of these abundances drop inwards of the peak, but Ne shows a central peak, all of which may be explained by resonance scattering. There is no evidence for a widespread additional cooler temperature component in the cluster with a temperature greater than a factor of 2 from the local temperature. There is, however, evidence for a widespread hard component which may be non-thermal. The temperature and abundance of gas in the cluster are observed to be correlated in a manner similar to that found between clusters.

Spatially-resolved X-ray spectroscopy of the core of the Centaurus cluster

ABSTRA C T We present Chandra data from a 31.7-ks observation of the Centaurus cluster, using the ACIS-S detector. Images of the X-ray emission show a plume-like feature at the centre of the cluster, of extent 60 arcsec (20 kpc in projection). The feature has the same metallicity as gas at a similar radius, but is cooler. Using adaptive binning, we generate temperature, abundance and absorption maps of the cluster core. The radial abundance profile shows that the previously known, steep abundance gradient peaks with a metallicity of 1: 3‐1 : 8Z ( at a radius of about 45 arcsec (15 kpc), before falling back to 0.4 Z( at the centre of the cluster. A radial temperature profile shows that the temperature decreases inwards. We determine the spatial distributions of each of two temperature components, where applicable. The radiative cooling time of the cooler component within the inner 10 arcsec (3 kpc) is less than 2 £ 10 7 yr. X-ray holes in the image coincident with the radio lobes are seen, as well as two outer sharp temperature drops, or cold fronts. The origin of the plume is unclear. The existence of the strong abundance gradient is a strong constraint on extensive convection or gas motion driven by a central radio source.

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.