The connection between X-ray Clusters and Star Formation

The properties of X-ray clusters of galaxies can be well understood in terms of a competition between shock heating and adiabatic compression. Strong shocks are expected to be important for massive clusters, while adiabatic compression is dominant for small clusters and groups. The scale of the shock/adiabatic transition is marked by a change of slope of the L-T relation and in the global properties of the emitting plasma. This scale is connected to star formation processes. Two quantities are crucial: the average energy injected in the IGM from stars and SNe, and the epoch of the energy injection. We show how these quantities can be synthesized in terms of specific entropy, which ultimately determines the X-ray emission from groups and clusters.Comment: 4 pages, including 2 figures, LaTex2e. To be published in the Proceedings of the ``VLT Opening Symposium'', Antofagasta (Chile), 1-4 March 1999. Typos changed in eq. (3

Emission Lines in X-ray Spectra of Clusters of Galaxies

Emission lines in X-ray spectra of clusters of galaxies reveal the presence of heavy elements in the diffuse hot plasma (the Intra Cluster Medium, or ICM) in virial equilibrium in the dark matter potential well. The relatively simple physical state of the ICM allows us to estimate, with good accuracy, its thermodynamical properties and chemical abundances. These measures put strong constraints on the interaction processes between the galaxies and the surrounding medium, and have significant impact on models of galaxy formation as well. This field is rapidly evolving thanks to the X-ray satellites Chandra and XMM-Newton. Among the most relevant progresses in the last years, we briefly discuss the nature of cool cores and the measure of the Iron abundance in high redshift clusters. Future X-ray missions with bolometers promise to provide a substantial step forward to a more comprehensive understanding of the complex physics of the ICM.Comment: 8 pages, 3 figures, Proceedings of the VI Serbian Conference on Spectral Line Shapes in Astrophysics, Sremski Karlovci, Serbia June 11-15 200

Searching for bulk motions in the ICM of massive, merging clusters with Chandra CCD data

We search for bulk motions in the intracluster medium (ICM) of massive clusters showing evidence of an ongoing or recent major merger with spatially resolved spectroscopy in {\sl Chandra} CCD data. We identify a sample of 6 merging clusters with $>$150 ks {\sl Chandra} exposure in the redshift range $0.1 < z < 0.3$. By performing X-ray spectral analysis of projected ICM regions selected according to their surface brightness, we obtain the projected redshift maps for all of these clusters. After performing a robust analysis of the statistical and systematic uncertainties in the measured X-ray redshift $z_{\rm X}$, we check whether or not the global $z_{\rm X}$ distribution differs from that expected when the ICM is at rest. We find evidence of significant bulk motions at more than 3$\sigma$ in A2142 and A115, and less than 2$\sigma$ in A2034 and A520. Focusing on single regions, we identify significant localized velocity differences in all of the merging clusters. We also perform the same analysis on two relaxed clusters with no signatures of recent mergers, finding no signs of bulk motions, as expected. Our results indicate that deep {\sl Chandra} CCD data enable us to identify the presence of bulk motions at the level of $v_{\rm BM} >$ 1000\ ${\rm km\ s^{-1}}$ in the ICM of massive merging clusters at $0.1<z<0.3$. Although the CCD spectral resolution is not sufficient for a detailed analysis of the ICM dynamics, {\sl Chandra} CCD data constitute a key diagnostic tool complementing X-ray bolometers on board future X-ray missions

The unrelaxed dynamical structure of the galaxy cluster Abell 85

For the first time, we explore the dynamics of the central region of a galaxy cluster within $r_{500}\sim 600h^{-1}$~kpc from its center by combining optical and X-ray spectroscopy. We use (1) the caustic technique that identifies the cluster substructures and their galaxy members with optical spectroscopic data, and (2) the X-ray redshift fitting procedure that estimates the redshift distribution of the intracluster medium (ICM). We use the spatial and redshift distributions of the galaxies and of the X-ray emitting gas to associate the optical substructures to the X-ray regions. When we apply this approach to Abell 85 (A85), a complex dynamical structure of A85 emerges from our analysis: a galaxy group, with redshift $z=0.0509 \pm 0.0021$ is passing through the cluster center along the line of sight dragging part of the ICM present in the cluster core; two additional groups, at redshift $z=0.0547 \pm 0.0022$ and $z=0.0570 \pm 0.0020$, are going through the cluster in opposite directions, almost perpendicularly to the line of sight, and have substantially perturbed the dynamics of the ICM. An additional group in the outskirts of A85, at redshift $z=0.0561 \pm 0.0023$, is associated to a secondary peak of the X-ray emission, at redshift $z=0.0583^{+0.0039}_{-0.0047}$. Although our analysis and results on A85 need to be confirmed by high-resolution spectroscopy, they demonstrate how our new approach can be a powerful tool to constrain the formation history of galaxy clusters by unveiling their central and surrounding structures.Comment: 12 pages, 11 figures, accepted by Ap

ALMA-SZ Detection of a Galaxy Cluster Merger Shock at Half the Age of the Universe

We present ALMA measurements of a merger shock using the thermal Sunyaev-Zel'dovich (SZ) effect signal, at the location of a radio relic in the famous El Gordo galaxy cluster at $z \approx 0.9$. Multi-wavelength analysis in combination with the archival Chandra data and a high-resolution radio image provides a consistent picture of the thermal and non-thermal signal variation across the shock front and helps to put robust constraints on the shock Mach number as well as the relic magnetic field. We employ a Bayesian analysis technique for modeling the SZ and X-ray data self-consistently, illustrating respective parameter degeneracies. Combined results indicate a shock with Mach number ${\cal M} = 2.4^{+1.3}_{-0.6}$, which in turn suggests a high value of the magnetic field (of the order of $4-10 ~\mu$G) to account for the observed relic width at 2 GHz. At roughly half the current age of the universe, this is the highest-redshift direct detection of a cluster shock to date, and one of the first instances of an ALMA-SZ observation in a galaxy cluster. It shows the tremendous potential for future ALMA-SZ observations to detect merger shocks and other cluster substructures out to the highest redshifts.Comment: Matched to the ApJL published version (2016 September 22), minor grammar and typo fixe