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&

Effects of dust scattering albedo and 2175 A bump on ultraviolet colours of normal disc galaxies

We discuss dust properties in the interstellar medium (ISM) of nearby normal galaxies, by comparing observations in the ultraviolet (UV) with simulations by a radiative transfer model. The observed UV colours of nearby galaxies show a reddening relative to their expected intrinsic colours. Some authors argued that the Milky Way dust cannot reproduce the reddening because of the prominent 2175 \AA absorption bump. Other authors proposed a reduction mechanism of the bump strength in an {\it attenuation law} derived from the ratio of the observed intensity to the intrinsic one through an age-selective attenuation (i.e., young stars are more attenuated selectively). We newly find that the wavelength dependence of the scattering albedo also has a strong effect on the UV colour; an albedo decreasing toward shorter wavelengths (except for the absorption bump range) produces a significant UV reddening. After comparing the observed UV colours of nearby normal galaxies with those expected from radiative transfer simulations assumed several dust models, we find two sorts of dust suitable for these galaxies: (1) dust with a bump and a smaller albedo for a shorter wavelength (except for the bump range), and (2) dust without any bump but with an almost constant albedo. If very small carbonaceous grains responsible for the common unidentified infrared emission band are also the bump carrier, the former dust is favorable. Finally, we derive mean attenuation laws of various dust models as a function of the UV attenuation, and derive some relations between the UV attenuation and observable/theoretical quantities.Comment: MNRAS in pres

The X-ray luminosity function of galaxies in the Coma cluster

The XMM-Newton survey of the Coma cluster of galaxies covers an area of 1.86 square degrees with a mosaic of 16 pointings and has a total useful integration time of 400 ksec. Detected X-ray sources with extent less than 10" were correlated with cataloged galaxies in the Coma cluster region. The redshift information, which is abundant in this region of the sky, allowed us to separate cluster members from background and foreground galaxies. For the background sources, we recover a typical LogN-LogS in the flux range 1.e-15 - 1.e-13 ergs/s/cm^2 in the 0.5-2.0 keV band. The X-ray emission from the cluster galaxies exhibits X-ray colors typical of thermal emission. The luminosities of Coma galaxies lie in the 1.e39-1.e41 ergs/s interval in the 0.5-2.0 keV band. The luminosity function of Coma galaxies reveals that their X-ray activity is suppressed with respect to the field by a factor of 5.6, indicating a lower level of X-ray emission for a given stellar mass.Comment: 16 pages, 2004 A&A in pres

Aperture-free star formation rate of SDSS star-forming galaxies

Large area surveys with a high number of galaxies observed have undoubtedly marked a milestone in the understanding of several properties of galaxies, such as star-formation history, morphology, and metallicity. However, in many cases, these surveys provide fluxes from fixed small apertures (e.g. fibre), which cover a scant fraction of the galaxy, compelling us to use aperture corrections to study the global properties of galaxies. In this work, we derive the current total star formation rate (SFR) of Sloan Digital Sky Survey (SDSS) star-forming galaxies, using an empirically based aperture correction of the measured $\rm H\alpha$ flux for the first time, thus minimising the uncertainties associated with reduced apertures. All the $\rm H\alpha$ fluxes have been extinction-corrected using the $\rm H\alpha/H\beta$ ratio free from aperture effects. The total SFR for $\sim$210,000 SDSS star-forming galaxies has been derived applying pure empirical $\rm H\alpha$ and $\rm H\alpha/H\beta$ aperture corrections based on the Calar Alto Legacy Integral Field Area (CALIFA) survey. We find that, on average, the aperture-corrected SFR is $\sim$0.65dex higher than the SDSS fibre-based SFR. The relation between the SFR and stellar mass for SDSS star-forming galaxies (SFR--$\rm M_\star$) has been obtained, together with its dependence on extinction and $\rm H\alpha$ equivalent width. We compare our results with those obtained in previous works and examine the behaviour of the derived SFR in six redshift bins, over the redshift range $\rm 0.005 \leq z\leq 0.22$. The SFR--$\rm M_\star$ sequence derived here is in agreement with selected observational studies based on integral field spectroscopy of individual galaxies as well as with the predictions of recent theoretical models of disc galaxies

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