Chandra observations of the interacting galaxies NGC 3395/3396 (Arp 270)

In this paper we present the results of a 20-ks high-resolution Chandra X-ray observation of the peculiar galaxy pair NGC 3395/3396, a system at a very early stage of merging, and less evolved than the famous Antennae and Mice merging systems. Previously unpublished ROSAT High-Resolution Imager data are also presented. The point-source population and the hot diffuse gas in this system are investigated and compared with other merging galaxy pairs. 16 X-ray point sources are detected in Arp 270, seven of which are classified as ultraluminous X-ray sources (ULXs, LX>= 1039 erg s-1). From spectral fits and the age of the system it seems likely that these are predominantly high-mass X-ray binaries. The diffuse gas emits at a global temperature of ~0.5 keV, consistent with temperatures observed in other interacting systems, and we see no evidence of the starburst-driven hot gaseous outflows seen in more evolved systems such as The Mice and The Antennae. It is likely that these features are absent from Arp 270 as the gas has had insufficient time to break out of the galaxy discs. 32 per cent of the luminosity of Arp 270 arises from the diffuse gas in the system, this is low when compared with later stage merging systems and gives further credence that this is an early-stage merger. Comparing the ULX population of Arp 270 to other merging systems, we derive a relationship between the star formation rate of the system, indicated by LFIR, and the number [N(ULX)] and luminosity (LULX) of its ULX population. We find N(ULX) ~L0.18FIR and LULX~L0.54FIR. These relationships, coupled with the relation of the point-source X-ray luminosity (LXP) to LK and LFIR+UV (Colbert et al. 2003), indicate that the ULX sources in an interacting system have contributions from both the old and young stellar populations.Peer reviewe

Metallicity structure in X-ray bright galaxy groups

Using Chandra X-ray data of a sample of 15 X-ray bright galaxy groups, we present preliminary results of a coherent study of the radial distribution of metal abundances in the hot gas in groups. The iron content in group outskirts is found to be lower than in clusters by a factor of ~2, despite showing mean levels in the central regions comparable to those of clusters. The abundance profiles are used to constrain the contribution from supernovae type Ia and II to the chemical enrichment and thermal energy of the intragroup medium at different group radii. The results suggest a scenario in which a substantial fraction of the chemical enrichment of groups took place in filaments prior to group collapse.Comment: 5 pages, 2 figures. To appear in the proceedings of ESO Astrophysics Symposia: "Groups of Galaxies in the Nearby Universe", eds. I. Saviane, V. Ivanov, J. Burissova (Springer

Constraining the Role of SN Ia and SN II in Galaxy Groups by Spatially Resolved Analysis of ROSAT and ASCA Observations

We present the results of modelling the distribution of gas properties in the galaxy groups HCG51, HCG62 and NGC5044, and in the poor cluster AWM7, using both ASCA SIS and ROSAT data. The spectral quality of the ASCA data allows the radial distribution in the abundances of several elements to be resolved. In all systems apart from HCG51, we see both central cooling flows, and a general decline in metal abundances with radius. The ratio of iron to alpha-element abundances varies significantly, and in comparison with theoretical supernova yields, indicates a significant contribution to the metal abundance of the intergalactic medium (IGM) from type Ia supernovae. This is seen both within the groups, and also throughout much of the cluster AWM7. The total energy input into the IGM from supernovae can be calculated from our results, and is typically 20-40 per cent of the thermal energy of the gas, mostly from SNe II. Our results support the idea that the SN II ejecta have been more widely distributed in the IGM, probably due to the action of galaxy winds, and the lower iron mass to light ratio in groups suggests that some of this enriched gas has been lost altogether from the shallower potential wells of the smaller systems.Comment: 14 pages, accepted for MNRA

ROSAT PSPC observations of nearby spiral galaxies - II. Statistical properties

We present a statistical analysis of the largest X-ray survey of nearby spiral galaxies in which diffuse emission has been separated from discrete source contributions. Regression and rank-order correlation analyses are used to compare X-ray properties such as total, source and diffuse luminosities, and diffuse emission temperature, with a variety of physical and multi-wavelength properties, such as galaxy mass, type and activity, and optical and infrared luminosity. The results are discussed in terms of the way in which hot gas and discrete X-ray sources scale with the mass and activity of galaxies, and with the star formation rate. We find that the X-ray properties of starburst galaxies are dependent primarily on their star-forming activity, whilst for more quiescent galaxies, galaxy mass is the more important parameter. One of the most intriguing results is the tight linear scaling between far-infrared and diffuse X-ray luminosity across the sample, even though the hot gas changes from a hydrostatic corona to a free wind across the activity range sampled here.Comment: 13 pages, latex file, 18 postscript figures, to appear in MNRA

Cluster scaling and its redshift evolution from XMM-Newton

We put together the results of XMM-Newton observations of a number of representative group and cluster samples at low and high redshifts. These results confirm the entropy ramp as an explanation of the observed scaling relations. We observe a mild evolution in the entropy of clusters. The observed degree of evolution is consistent with expectations of the shock heating at a fixed overdensity (500) with respect to the critical density in LCDM. The study of the evolution in the pressure scaling imposes strong requirements in the definition of the average temperature of the cluster. The scaling temperature should be consistent to better than the 10% level. Once such a consistency is achieved, no additional evolution in the pressure has been detected in addition to the prediction of the shock heating in the LCDM Universe.Comment: 8 pages, Adv. in Space Research, in pres

Cosmological parameters from Galaxy Clusters: an Introduction

This lecture is an introduction to cosmological tests with clusters of galaxies. Here I do not intend to provide a complete review of the subject, but rather to describe the basic procedures to set up the fitting machinery to constrain cosmological parameters from clusters, and to show how to handle data with a critical insight. I will focus mainly on the properties of X-ray clusters of galaxies, showing their success as cosmological tools, to end up discussing the complex thermodynamics of the diffuse intracluster medium and its impact on the cosmological tests.Comment: 32 pages, 16 figures, conference proceedings for the 3rd Aegean Summer School, Chios, 26 September - 1 October, 200

The Chandra view of galaxy mergers

From a Chandra survey of nine interacting galaxy systems the evolution of X-ray emission during the merger process has been investigated. It is found that the X-ray luminosity peaks ~300Myr before nuclear coalescence, and then dips, even though we know that rapid and increasing activity is still taking place at this time. It is likely that this drop in X-ray luminosity is a consequence of outflows breaking out of the galactic discs of these systems. In this work it is also shown that, for the systems close to the point of nuclear coalescence, LFIR becomes massively enhanced compared to the X-ray luminosity of these systems. We suggest that this enhancement may indicate a `top heavy' initial mass function (IMF), with an enhanced fraction of massive stars. At a time ~1Gyr after coalescence, the merger remnants in our sample are X-ray faint when compared to typical mature elliptical galaxies. However, we do see evidence that these systems will start to resemble typical elliptical galaxies at a greater dynamical age, given the properties of the 3-Gyr system within our sample, supporting the idea that halo regeneration will take place within low LX merger remnants. As a part of this survey, detailed Chandra observations for the double nucleus merger system Markarian266 (Mrk266) and the merger remnant Arp222 are presented for the first time. With the Mrk266 observation, in contrast to previous studies, we now have good spectral information of the individual components part seen with the ROSAT High Resolution Imager (HRI). Additionally, the structure of the emission to the north of the system can clearly be distinguished and there is also a suggestion of some extension of X-ray emission to the south-east of the nuclear region, indicating that this galaxy could just be on the verge of large-scale galactic winds breaking out. Within Arp222 an X-ray luminosity of 1.46 × 1040ergs-1 has been detected, this is the lowest value of LX within our sample. The diffuse gas of Arp222 has been modelled with a temperature of 0.6keV and, from CO observations it has been found to host very little molecular gas, indicating that, from current observations, Arp222 does not resemble a mature elliptical.Peer reviewe

The cool wake around 4c 34.16 as seen by XMM-Newton

Peer reviewe

On optical mass estimation methods for galaxy groups

We examine the performance of a variety of different estimators for the mass of galaxy groups, based on their galaxy distribution alone. We draw galaxies from the Sloan Digital Sky Survey for a set of groups and clusters for which hydrostatic mass estimates based on high-quality Chandra X-ray data are available. These are used to calibrate the galaxy-based mass proxies, and to test their performance. Richness, luminosity, galaxy overdensity, rms radius and dynamical mass proxies are all explored. These different mass indicators all have their merits, and we argue that using them in combination can provide protection against being misled by the effects of dynamical disturbance or variations in star formation efficiency. Using them in this way leads us to infer the presence of significant non-statistical scatter in the X-ray based mass estimates we employ. We apply a similar analysis to a set of mock groups derived from applying a semi-analytic galaxy formation code to the Millennium dark matter simulation. The relations between halo mass and the mass proxies differ significantly in some cases from those seen in the observational groups, and we discuss possible reasons for this

A Heating Model for the Millennium Gas Run

Summary. The comparison between observations of galaxy clusters thermo-dynamical properties and theoretical predictions suggests that non-gravitational heating needs to be added into the models. We implement an internally self-consistent heating scheme into GADGET-2 for the third (and fourth) run of the Millennium gas project (Pearce et al. in preparation), a set of four hydrodynamical cosmological simulations with N = 2×(5×10 8) particles and with the same volume (L = 500h −1 Mpc) and structures as the the N-body Millennium Simulation (Springel et al. 2005). Our aim is to reproduce the observed thermo-dynamical properties of galaxy clusters.