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Register a new userAn X-ray underluminous cluster of galaxies in the 4Ms CDFS observations
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[Abridged] We use the large public spectroscopic database available in the GOODS-South field to estimate the dynamical mass and the virialization status of cluster ClG 0332-2747 at z=0.734. Cluster members selected from their photometric redshift are used with spectroscopic ones to analyse the galaxy population of the cluster. In the newly released Chandra 4Ms observations we detect a faint extended X-ray emission associated to the cluster. Finally, we compare the optical and X-ray properties of ClG 0332-2747 with the predictions of a well tested semianalytic model. We estimate the velocity dispersion and the virial mass considering all 44 spectroscopic members, or 20 red-sequence members only. We obtain sigma_v=634 +/- 105 Km/s, M_200=3.07 ^{+1.57}_{-1.16}~10^{14} M_sun in the former case, and slightly lower values in the latter case. The cluster appears to have reached the virial equilibrium: it shows a perfectly Gaussian velocity distribution and no evidence for substructures. ClG 0332-2747 contains a high fraction of bright red galaxies, and is dominated by a very massive (1.1 x 10^{12} M_sun) old brightest cluster galaxy (BCG), suggesting that it formed at an early epoch. We detect a faint extended X-ray source centered on the BCG, with a total X-ray luminosity of L_X ~ 2 x 10^{42} erg s^-1 (0.1-2.4 keV). This L_X is lower by a factor of ~10-20 than expected according to the M-L_X relation. We provide a possible explanation of this discrepancy as due to the effects of AGN feedback on the ICM: the semianalytic model reproduces the M-L_X relation measured from X-ray bright clusters, and it predicts a high scatter at low masses due to heating and expulsion of the cluster gas. Interestingly, in the model clusters with an evolved galaxy population like ClG 0332-2747 present the largest scatter in X-ray luminosity.
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Multiwavelength studies of radio relics at merger shocks set powerful constraints on the relics origin and formation mechanism. However, for X-ray observations, a main difficulty is represented by the low X-ray surface brightness far out in the cluster outskirts, where relics are typically found. Here, we present XMM-Newton results from a 130-ks observation of CIZA J2242.8+5301, a cluster at z=0.19 that hosts a double radio relic. We focus on the well-defined northern relic. There is a difference of ~55% between the temperature we measure behind the relic, and the temperature measured with Suzaku. We analyse the reasons for this large discrepancy, and discuss the possibility of reliably measuring the temperature beyond the northern relic.
We have performed a pilot Chandra survey of an off-center region of the Coma cluster to explore the X-ray properties and Luminosity Function of normal galaxies. We present results on 13 Chandra-detected galaxies with optical photometric matches, including four spectroscopically-confirmed Coma-member galaxies. All seven spectroscopically confirmed giant Coma galaxies in this field have detections or limits consistent with low X-ray to optical flux ratios (fX/fR < 10^-3). We do not have sufficient numbers of X-ray detected galaxies to directly measure the galaxy X-ray Luminosity Function (XLF). However, since we have a well-measured optical LF, we take this low X-ray to optical flux ratio for the 7 spectroscopically confirmed galaxies to translate the optical LF to an XLF. We find good agreement with Finoguenov et al. (2004), indicating that the X-ray emission per unit optical flux per galaxy is suppressed in clusters of galaxies, but extends this work to a specific off-center environment in the Coma cluster. Finally, we report the discovery of a region of diffuse X-ray flux which might correspond to a small group interacting with the Coma Intra-Cluster Medium (ICM).
We investigate the star formation history of the universe using FIREWORKS, a multiwavelength survey of the CDFS. We study the evolution of the specific star formation rate (sSFR) with redshift in different mass bins from z = 0 to z ~ 3. We find that the sSFR increases with redshift for all masses. The logarithmic increase of the sSFR with redshift is nearly independent of mass, but this cannot yet be verified at the lowest-mass bins at z > 0.8, due to incompleteness. We convert the sSFRs to a dimensionless growth rate to facilitate a comparison with a semi-analytic galaxy formation model that was implemented on the Millennium Simulation. The model predicts that the growth rates and sSFRs increase similarly with redshift for all masses, consistent with the observations. However, we find that for all masses, the inferred observed growth rates increase more rapidly with redshift than the model predictions. We discuss several possible causes for this discrepancy, ranging from field-to-field variance,
Based on Suzaku X-ray observations, we study the hot gas around the NGC4839 group of galaxies and the radio relic in the outskirts of the Coma cluster. We find a gradual decline in the gas temperature from 5 keV around NGC4839 to 3.6 keV at the radio relic, across which there is a further, steeper drop down to 1.5 keV. This drop as well as the observed surface brightness profile are consistent with a shock with Mach number M = 2.2 pm 0.5 and velocity vs = (1410 pm 110) km s^-1. A lower limit of B > 0.33 mu G is derived on the magnetic field strength around the relic from upper limits to inverse Compton X-ray emission. Although this suggests that the non-thermal electrons responsible for the relic are generated by diffusive shock acceleration (DSA), the relation between the measured Mach number and the electron spectrum inferred from radio observations are inconsistent with that expected from the simplest, test-particle theory of DSA. Nevertheless, DSA is still viable if it is initiated by the injection of a pre-existing population of non-thermal electrons. Combined with previous measurements, the temperature profile of Coma in the southwest direction is shallower outside NGC4839 and also slightly shallower in the outermost region. The metal abundance around NGC4839 is confirmed to be higher than in its vicinity, implying a significant peak in the abundance profile that decreases to 0.2 solar toward the outskirts. We interpret these facts as due to ram pressure stripping of metal-enriched gas from NGC4839 as it falls into Coma. The relic shock may result from the combined interaction of pre-existing intracluster gas, gas associated with NGC 4839, and cooler gas flowing in from the large-scale structure filament in the southwest.
We present an analysis of the colour-magnitude relation for a sample of 56 X-ray underluminous Abell clusters, aiming to unveil properties that may elucidate the evolutionary stages of the galaxy populations that compose such systems. To do so, we compared the parameters of their colour-magnitude relations with the ones found for another sample of 50 normal X-ray emitting Abell clusters, both selected in an objective way. The $g$ and $r$ magnitudes from the SDSS-DR7 were used for constructing the colour-magnitude relations. We found that both samples show the same trend: the red sequence slopes change with redshift, but the slopes for X-ray underluminous clusters are always flatter than those for the normal clusters, by a difference of about 69% along the surveyed redshift range of 0.05 $le z <$ 0.20. Also, the intrinsic scatter of the colour-magnitude relation was found to grow with redshift for both samples but, for the X-ray underluminous clusters, this is systematically larger by about 28%. By applying the Cramer test to the result of this comparison between X-ray normal and underluminous cluster samples, we get probabilities of 92% and 99% that the red sequence slope and intrinsic scatter distributions, respectively, differ, in the sense that X-ray underluminous clusters red sequences show flatter slopes and higher scatters in their relations. No significant differences in the distributions of red-sequence median colours are found between the two cluster samples. This points to X-ray underluminous clusters being younger systems than normal clusters, possibly in the process of accreting groups of galaxies, individual galaxies and gas.
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