Multiple merging in the Abell cluster 1367

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.Comment: 15 pages, 13 figures, 7 tables. Accepted for publication on A&A. High resolution figures at http://goldmine.mib.infn.it/papers/a1367.htm

Optical spectroscopy and the UV luminosity function of galaxies in the Abell 1367, Coma and Virgo clusters

Optical spectroscopy of 93 galaxies, 60 projected in the direction of Abell 1367, 21 onto the Coma cluster and 12 on Virgo, is reported. The targets were selected either because they were detected in previous H\alpha, UV or r' surveys. The present observations bring to 100% the redshift completeness of H\alpha selected galaxies in the Coma region and to 75% in Abell 1367. All observed galaxies except one show H\alpha emission and belong to the clusters. This confirms previous determinations of the H\alpha luminosity function of the two clusters that were based on the assumption that all H\alpha detected galaxies were cluster members. Using the newly obtained data we re-determine the UV luminosity function of Coma and we compute for the first time the UV luminosity function of A1367. Their faint end slopes remain uncertain (-2.00 < \alpha < -1.35) due to insufficient knowledge of the background counts. If 90% of the UV selected galaxies without redshift will be found in the background (as our survey indicates), the slope of UV luminosity function will be \alpha ~ -1.35, in agreement with the UV luminosity function of the field (Sullivan et al. 2000) and with the H$\alpha$ luminosity functions of the two clusters (Iglesias-Paramo et al. 2002). We discover a point like H\alpha source in the Virgo cluster, associated with the giant galaxy VCC873, possibly an extragalactic HII region similar to the one recently observed in Virgo by Gerhard et al. (2002).Comment: 8 pages, 6 figures, 2 tables. Accepted for publication on A&

AKARI/IRC Broadband Mid-infrared data as an indicator of Star Formation Rate

AKARI/Infrared Camera (IRC) Point Source Catalog provides a large amount of flux data at {\it S9W} ($9\ {\rm \mu m}$) and {\it L18W} ($18\ {\rm \mu m}$) bands. With the goal of constructing Star-Formation Rate(SFR) calculations using IRC data, we analyzed an IR selected GALEX-SDSS-2MASS-AKARI(IRC/Far-Infrared Surveyor) sample of 153 nearby galaxies. The far-infrared fluxes were obtained from AKARI diffuse maps to correct the underestimation for extended sources raised by the point-spread function photometry. SFRs of these galaxies were derived by the spectral energy distribution fitting program CIGALE. In spite of complicated features contained in these bands, both the {\it S9W} and {\it L18W} emission correlate with the SFR of galaxies. The SFR calibrations using {\it S9W} and {\it L18W} are presented for the first time. These calibrations agree well with previous works based on Spitzer data within the scatters, and should be applicable to dust-rich galaxies.Comment: PASJ, in pres

The UV luminosity function of nearby clusters of galaxies

We present the UV composite luminosity function for galaxies in the Virgo, Coma and Abell 1367 clusters. The luminosity function (LF) is well fitted by a Schechter function with M*(UV} - 5*log h(75) = -20.75 +/- 0.40 and alpha = -1.50 +/- 0.10 and does not differ significantly from the local UV luminosity function of the field. This result is in agreement with recent studies carried out in the Halpha and B-bands which find no difference between the LFs of star forming galaxies in clusters and in the field. This indicates that, whatever mechanisms are responsible for quenching the star formation in clusters, they influence similarly the giant and the dwarf populations, leaving the shape of the LF unchanged and only modifying its normalization.Comment: 5 pages, 4 figures, 2 tables.Accepted for publication on A&A. Data avaliable at http://goldmine.mib.infn.it/papers/LF_UV.htm

An extragalactic HII region in the Virgo cluster

We present spectroscopic observations for six emission-line objects projected onto the Virgo cluster. These sources have been selected from narrow band (H\alpha+[NII]) images showing faint detectable continuum emission and EW>100 Angstrom. Five of these sources result [OIII]\lambda 5007 emitters at z ~ 0.31, while one 122603+130724 is confirmed to be an HII region belonging to the Virgo cluster. This point-like source has a recessional velocity of ~ 200 km/s, and is associated with the giant galaxy VCC873 (NGC 4402). It has a higher luminosity, star formation rate and metallicity than the extragalactic HII region recently discovered near the Virgo galaxy VCC836 by Gerhard et al. (2002).Comment: 7 pages, 2 figures, 4 tables. Accepted for publication on A&A. High resolution figures and FITS images available at http://goldmine.mib.infn.it/papers/vcc873.htm

Fossil group origins V. The dependence of the luminosity function on the magnitude gap

In nature we observe galaxy aggregations that span a wide range of magnitude gaps between the two first-ranked galaxies of a system ($\Delta m_{12}$). There are systems with gaps close to zero (e.g., the Coma cluster), and at the other extreme of the distribution, the largest gaps are found among the so-called fossil systems. Fossil and non-fossil systems could have different galaxy populations that should be reflected in their luminosity functions. In this work we study, for the first time, the dependence of the luminosity function parameters on $\Delta m_{12}$ using data obtained by the fossil group origins (FOGO) project. We constructed a hybrid luminosity function for 102 groups and clusters at $z \le 0.25$. We stacked all the individual luminosity functions, dividing them into bins of $\Delta m_{12}$, and studied their best-fit Schechter parameters. We additionally computed a relative luminosity function, expressed as a function of the central galaxy luminosity, which boosts our capacity to detect differences, especially at the bright end. We find trends as a function of $\Delta m_{12}$ at both the bright and faint ends of the luminosity function. In particular, at the bright end, the larger the magnitude gap, the fainter the characteristic magnitude $M^\ast$. We also find differences at the faint end. In this region, the larger the gap, the flatter the faint-end slope $\alpha$. The differences found at the bright end support a dissipationless, dynamical friction-driven merging model for the growth of the central galaxy in group- and cluster-sized halos. The differences in the faint end cannot be explained by this mechanism. Other processes, such as enhanced tidal disruption due to early infall and/or prevalence of eccentric orbits, may play a role. However, a larger sample of systems with $\Delta m_{12} > 1.5$ is needed to establish the differences at the faint end.Comment: 11 pages, 10 figures, accepted for publication in A&

Fossil Groups Origins III. The relation between optical and X-ray luminosities

This study is part of the FOssil Groups Origin (FOGO) project which aims at carrying out a systematic and multiwavelength study of a large sample of fossil systems. Here we focus on the relation between the optical luminosity (Lopt) and X-ray luminosity (Lx). Out of a sample of 28 candidate fossil systems, we consider a sample of 12 systems whose fossil classification has been confirmed by a companion study. They are compared with the complementary sample of 16 systems whose fossil nature is not confirmed and with a subsample of 102 galaxy systems from the RASS-SDSS galaxy cluster survey. Fossil and normal systems span the same redshift range 0<z<0.5 and have the same Lx distribution. For each fossil system, the Lx in the 0.1-2.4 keV band is computed using data from the ROSAT All Sky Survey. For each fossil and normal system we homogeneously compute Lopt in the r-band within the characteristic cluster radius, using data from the SDSS DR7. We sample the Lx-Lopt relation over two orders of magnitude in Lx. Our analysis shows that fossil systems are not statistically distinguishable from the normal systems both through the 2D KS test and the fit of the Lx-Lopt relation. The optical luminosity of the galaxy system does strongly correlate with the X-ray luminosity of the hot gas component, independently of whether the system is fossil or not. We conclude that our results are consistent with the classical "merging scenario" of the brightest galaxy formed via merger/cannibalism of other group galaxies, with conservation of the optical light. We find no evidence for a peculiar state of the hot intracluster medium.Comment: A&A, 12 pages, 4 figures, 3 tables, typos corr. and paper re-numbe