Sabine Schindler

Observations of Extended Radio Emission in Clusters

We review observations of extended regions of radio emission in clusters; these include diffuse emission in ‘relics’, and the large central regions commonly referred to as ‘halos’. The spectral observations, as well as Faraday rotation measurements of background and cluster radio sources, provide the main evidence for large-scale intracluster magnetic fields and significant densities of relativistic electrons. Implications from these observations on acceleration mechanisms of these electrons are reviewed, including turbulent and shock acceleration, and also the origin of some of the electrons in collisions of relativistic protons by ambient protons in the (thermal) gas. Improved knowledge of non-thermal phenomena in clusters requires more extensive and detailed radio measurements; we briefly review prospects for future observations.

The ROSAT-ESO flux limited X-ray (REFLEX) galaxy cluster survey. I. The construction of the cluster sample ?

We discuss the construction of an X-ray flux-limited sample of galaxy clusters, the REFLEX survey catalogue, to be used for cosmological studies. This cluster identication and redshift survey was conducted in the frame of an ESO key programme and is based on candidates selected from the southern part of the ROSAT All-Sky Survey (RASS). For the rst cluster candidate selection from a flux-limited RASS source list, we make use of optical data from the COSMOS digital catalogue produced from the scans of the UK-Schmidt plates. To ensure homogeneity of the sample construction process, this selection is based only on this one well-dened optical data base. The nature of the candidates selected in this process is subsequently checked by a more detailed evaluation of the X-ray and optical source properties and available literature data. The nal identication and the redshift is then based on optical spectroscopic follow-up observations. In this paper we document the process by which the primary cluster candidate catalogue is constructed prior to the optical follow-up observations. We describe the reanalysis of the RASS source catalogue which enables us to impose a proper flux limit cut to the X-ray source list without introducing a severe bias against extended sources. We discuss the correlation of the X-ray and optical (COSMOS) data to nd galaxy density enhancements at the RASS X-ray source positions and the further evaluation of the nature of these cluster candidates. Based also on the results of the follow-up observations we provide a statistical analysis of the completeness and contamination of the nal cluster sample and show results on the cluster number counts. The nal sample of identied X-ray clusters reaches a flux limit of 3 10 12 erg s 1 cm 2 in the 0.1{2.4 keV band and comprises 452 clusters in an area of 4.24 ster. The results imply a completeness of the REFLEX cluster sample well in excess of 90%. We also derive for the rst time an upper limit of less than 9% for the number of clusters which may feature a dominant contribution to the X-ray emission from AGN. This accuracy is sucient for the use of this cluster sample for cosmological tests.

A Flux-limited Sample of Bright Clusters of Galaxies from the Southern Part of the ROSAT All-Sky Survey: The Catalog and log N-log S

We describe the selection of an X-ray flux-limited sample of bright clusters of galaxies in the southern hemisphere, based on the first analysis of the ROSAT All-Sky Survey data (RASS1). The sample is constructed starting from an identification of candidate clusters in RASS1, after which their X-ray fluxes are remeasured using the steepness-ratio technique. This method is better suited than the RASS1 standard algorithm to measuring flux from extended sources. The final sample is count-rate-limited in the ROSAT hard band (0.5-2.0 keV), so that as a result of the distribution of NH, its effective flux limit varies between ~3 and 4×10−12 ergs cm-2 s-1 over the selected area. This covers the δ<25 part of the south Galactic cap region (b<-20°), excluding patches of low RASS1 exposure time and of the Magellanic Clouds area, for a total of 8235 deg2. One hundred and thirty candidate sources fulfill our selection criteria for bona fide clusters of galaxies in this area. Of these, 101 are Abell/ACO clusters, while 29 do not have a counterpart in these catalogs. Of these clusters, 126 (97%) have a redshift, and for these we compute an X-ray luminosity. 20% of the cluster redshifts come from new observations, as part of the ESO Key Program survey of RASS clusters that is currently under completion. Considering the intrinsic biases and incompletenesses introduced by the flux selection and source identification processes, we estimate the overall completeness to be better than 90%. The observed number count distribution, log N-log S, is well fitted by a power law with slope α=1.34±0.15 and normalization A=11.87±1.04 sr-1 (10-11 ergs cm-2 s-1)α, in good agreement with other measurements.

On the influence of ram-pressure stripping on the star formation of simulated spiral galaxies

Aims. We investigate the influence of ram-pressure stripping on the star formation and the mass distribution in simulated spiral galaxies. Special emphasis is put on the question of where the newly formed stars are located. The stripping radius from the simulation is compared to analytical estimates. Methods. Disc galaxies are modelled in combined N-body/hydrodynamic simulations (GADGET-2) with prescriptions for cooling, star formation, stellar feedback, and galactic winds. These model galaxies move through a constant density and temperature gas, which has parameters comparable to the intra-cluster medium (ICM) in the outskirts of a galaxy cluster (T = 3k eV≈ 3.6 × 10 7 K and ρ = 10 −28 g/cm 3 ). With this numerical setup we analyse the influence of ram-pressure stripping on the star formation rate of the model galaxy. Results. We find that the star formation rate is significantly enhanced by the ram-pressure effect (up to a factor of 3). Stars form in the compressed central region of the galaxy, as well as in the stripped gas behind the galaxy. Newly formed stars can be found up to hundred kpc behind the disc, forming structures with sizes of roughly 1 kpc in diameter and with masses of up to 10 7 M� .A s they do not possess a dark matter halo due to their formation history, we name them “stripped baryonic dwarf” galaxies. We also find that the analytical estimate for the stripping radius from a Gunn & Gott (1972) criterion agrees well with the numerical value from the simulation. Like in former investigations, edge-on systems lose less gas than face-on systems, and the resulting spatial distribution of the gas and the newly formed stars is different.

Correlation of the magnetic field and the intra-cluster gas density in galaxy clusters

We present a correlation between X-ray surface brightness and Faraday rotation measure in galaxy clusters, both, from radio and X-ray observations as well as from modeling of the intra-cluster medium. The ob- served correlation rules out a magnetic eld of constant strength throughout the cluster. Cosmological, magneto- hydrodynamic simulations of galaxy clusters are used to show that for a magnetic eld of cosmic origin this correlation is expected and excellently reproduces the observations showing that the rms scatter of the Faraday rotation increases linearly with the X-ray surface brightness. From the correlation between the observable quanti- ties, rotation measure and X-ray surface brightness, we infer a relation between the physical quantities: magnetic eld and gas density. For the best available observations, those of A119, we nd B/ n 0:9 e .

The ROSAT-ESO Flux-Limited X-ray (REFLEX) galaxy cluster survey — II. The spatial correlation function

We report the results of the spatial two-point correlation functioncc(r) for the new X- ray galaxy cluster survey REFLEX, which comprises of 452 X-ray selected clusters (449 with redshifts) detected by the ROSAT satellite during the ROSAT All-Sky-Survey (RASS). The REFLEX cluster sample is flux limited to 3 × 10 12 erg s 1 cm 2 in the ROSAT energy band (0.1 − 2.4 keV) and spans 3 decades in X-ray luminosity (10 42 −10 45 h 2 erg s 1 ), containing galaxy groups and rich clusters out to a redshift z ≤ 0.3. Covering a contiguous area of 4.24 sr REFLEX is the largest X-ray cluster sample to date for which spatial clustering has been analysed. Correlation studies using clusters selected on the basis of their X-ray emission are particularly interesting as they are largely free from the projection biases inherent to optical studies. For the entire flux-limited sample we find that the correlation length (the scale at which the correlation amplitude passes through unity) r0 ≃ 20h 1 Mpc. For example, if a power-law fit is made to �(r) over the range 4 − 40h 1 Mpc then r0 = 18.8 ± 0.9. An indication of the robustness of this result comes from the high degree of isotropy seen in the clustering pattern on scales close to the correlation length. On larger scalescc(r) deviates from a power-law, crossing zero at ≃ 45h 1 Mpc. From an examination of 5 volume-limited cluster sub-samples we find no significant trend of r0 with limiting X-ray luminosity. A comparison with recent model predictions for the clustering properties of X-ray flux-limited samples, indicates that Cold Dark Matter models with the matter density m = 1 fail to produce sufficient clustering to account for the data, while m ≃ 0.3 models provide an excellent fit.

The ROSAT-ESO Flux-Limited X-ray (REFLEX) Galaxy Cluster Survey. IV. The X-ray luminosity function

The X-ray galaxy cluster sample from the REFLEX Cluster Survey, which covers the X-ray brightest galaxy clusters detected in the ROSAT All-Sky Survey in the southern sky, is used to construct the X-ray luminosity function of clusters in the local universe. With 452 clusters detected above an X-ray flux limit of 3 × 10-12 ergs s-1 cm-2 in 4.24 sr of the sky, this sample is the most comprehensive X-ray cluster sample with a well-documented selection function, providing the best current census of the local X-ray galaxy cluster population. In this paper we discuss the construction of the luminosity function and the effects of flux measurement errors and of variations with sample region, and we compare the results with those from previous surveys.

Metal Enrichment Processes in the Intra-Cluster Medium

Institut fu¨r Astronomie, Universit¨at Wien, Tu¨rkenschanzstr. 17, 1180 Vienna, AustriaReceived / AcceptedAbstract. We present numerical simulations of galaxy clusters which include interaction processes between thegalaxies and the intra-cluster gas. The considered interaction processes are galactic winds and ram-pressurestripping, which both transfer metal-enriched interstellar medium into the intra-cluster gas and hence increase itsmetallicity. We investigate the efficiency and time evolution of the interaction processes by simulated metallicitymaps, which are directly comparable to those obtained from X-ray observations. We find that ram-pressurestripping is more efficient than quiet (i.e. non-starburst driven) galactic winds in the redshift interval between 1and 0. The expelled metals are not mixed immediately with the intra-cluster gas, but inhomogeneities are visiblein the metallicity maps. Even stripes of higher metallicity that a single galaxy has left behind can be seen. Thespatial distribution of the metals transported by ram-pressure stripping and by galactic winds are very differentfor massive clusters: the former process yields a centrally concentrated metal distribution while the latter resultsin an extended metal distribution.Key words. Galaxies:clusters:general – Galaxies:abundances – Galaxies:interactions – Galaxies:ISM – X-ray:galaxies:clusters

Observations of Metals in the Intra-Cluster Medium

Because of their deep gravitational potential wells, clusters of galaxies retain all the metals produced by the stellar populations of the member galaxies. Most of these metals reside in the hot plasma which dominates the baryon content of clusters. This makes them excellent laboratories for the study of the nucleosynthesis and chemical enrichment history of the Universe. Here we review the history, current possibilities and limitations of the abundance studies, and the present observational status of X-ray measurements of the chemical composition of the intra-cluster medium. We summarise the latest progress in using the abundance patterns in clusters to put constraints on theoretical models of supernovae and we show how cluster abundances provide new insights into the star-formation history of the Universe.

Discovery of diffuse radio emission at the center of the most X-ray-luminous cluster RX J1347.5-1145

Context. We report on new VLA radio observations of the distant cluster RX J1347.5-1145, which is the most luminous in X-rays. Aims. We aim at investigating the possible presence of diffuse and extended radio emission in this very peculiar system which shows both a massive cooling flow and merging signatures. Methods. New low resolution (∼18 arcsec) VLA radio observations of this cluster are combined with higher resolution (∼2 arcsec) data available in the VLA archive. Results. We discover the presence of a diffuse and extended (∼500 kpc) radio source centered on the cluster, unrelated to the radio emission of the central AGN. The properties of the radio source, in particular a) its occurrence at the center of a massive cooling flow cluster, b) its total size comparable to that of the cooling region, c) its agreement with the observational trend between radio luminosity and cooling flow power, indicate that RX J1347.5-1145 hosts a radio mini-halo. We suggest that the radio emission of this mini-halo, which is the most distant object of its class discovered up to now, is due to electron re-acceleration triggered by the central cooling flow. However, we also note that the morphology of the diffuse radio emission shows an elongation coincident with the position of a hot subclump detected in X-rays, thus suggesting that additional energy for the electron re-acceleration might be provided by the submerger event.