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Register a new userCapturing a star formation burst in galaxies infalling onto the cluster Abell 1367
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The discovery of a striking astrophysical laboratory in the cluster of galaxies Abell 1367 (Sakai et al. 2002) is confirmed with independent imaging and spectroscopic observations and further investigated in the present analysis. Two giant and ten dwarf/HII galaxies, members to a group, are simultaneously undergoing a burst of star formation. Redshift measurements suggest that the group galaxies are in the process of falling into the cluster at very high speed. We explore two possible mechanisms that could have triggered the short-lived stellar burst we are witnessing: the first, internal to the group itself, via tidal interactions among its members, the hypothesis favoured by Sakai et al. (2002); the second associated with the high velocity infall of the group galaxies into the cluster intergalactic medium. We present evidences in favour and against the two hypotheses.
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We present a dynamical analysis of the central ~1.3 square degrees of the cluster of galaxies Abell 1367, based on 273 redshift measurements (of which 119 are news). From the analysis of the 146 confirmed cluster members we derive a significantly non-Gaussian velocity distribution, with a mean location C_{BI} = 6484+/-81 km/s and a scale S_{BI} = 891+/-58 km/s. The cluster appears elongated from the North-West to the South-East with two main density peaks associated with two substructures. The North-West subcluster is probably in the early phase of merging into the South-East substructure (~ 0.2 Gyr before core crossing). A dynamical study of the two subclouds points out the existence of a group of star-forming galaxies infalling into the core of the South-East subcloud and suggests that two other groups are infalling into the NW and SE subclusters respectively. These three subgroups contain a higher fraction of star-forming galaxies than the cluster core, as expected during merging events. Abell 1367 appears as a young cluster currently forming at the intersection of two filaments.
We present the GALEX NUV (2310 A) and FUV (1530 A) galaxy luminosity functions of the nearby cluster of galaxies A1367 in the magnitude range -20.3< M_AB < -13.3. The luminosity functions are consistent with previous (~ 2 mag shallower) estimates based on the FOCA and FAUST experiments, but display a steeper faint-end slope than the GALEX luminosity function for local field galaxies. Using spectro-photometric optical data we select out star-forming systems from quiescent galaxies and study their separate contributions to the cluster luminosity function. We find that the UV luminosity function of cluster star-forming galaxies is consistent with the field. The difference between the cluster and field LF is entirely due to the contribution at low luminosities (M_AB >-16 mag) of non star-forming, early-type galaxies that are significantly over dense in clusters.
Multi-wavelength observations show that Abell 1367 (A1367) is a dynamically young cluster, with at least two subclusters merging along the SE-NW direction. With the wide-field XMM-Newton mosaic of A1367, we discover a previously unknown merger shock at the NW edge of the cluster. We estimate the shock Mach number from the density and temperature jumps as $M_{rho}=1.21pm0.08$ and $M_T=1.60pm0.07$, respectively. This shock region also corresponds to a radio relic discovered with the VLA and GBT, which could be produced by the shock re-acceleration of pre-existing seed relativistic electrons. We suggest that some of the seed relativistic electrons originate from late-type, star-forming galaxies in this region.
We investigate the star formation rate and its location in the major merger cluster Abell 2465 at $z$ = 0.245. Optical properties of the cluster are described in Paper I. Measurements of the H$alpha$ and infrared dust emission of galaxies in the cluster were made with an interference filter centred on the redshifted line at a wavelength of 817 nm and utilized data from the WISE satellite 12 $mu$m band. Imaging in the Johnson $U$ and $B$ bands was obtained, and along with SDSS $u$ and $r$ was used to study the blue fraction, which appears enhanced, as a further signatures of star formation in the cluster. Star formation rates were calculated using standard calibrations. The total star formation rate normalized by the cluster mass, $Sigma SFR/M_{cl}$ compared to compilations for other clusters indicate that the components of Abell 2465 lie above the mean $z$ and $M_{cl}$ relations, suggestive that interacting galaxy clusters have enhanced star formation. The projected radial distribution of the star forming galaxies does not follow a NFW profile and is relatively flat indicating that fewer star forming galaxies are in the cluster centre. The morphologies of the H$alpha$ sources within $R_{200}$ for the cluster as a whole indicate that many are disturbed or merging, suggesting that a combination of merging or harassment is working.
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