We study the role of the environment on galaxy evolution using a sample of 868 galaxies in the Virgo cluster and in its surrounding regions selected from the GUViCS Survey with the purpose of understanding the origin of the red sequence in dense envi
ronments. We collected multifrequency data covering the whole electromagnetic spectrum for most of the galaxies. We identify the different dynamical substructures composing the Virgo cluster and we calculate the local density of galaxies using different methods. We then study the distribution of galaxies belonging to the red sequence, the green valley, and the blue cloud within the different cluster substructures. Our analysis indicates that all the most massive galaxies are slow rotators and are the dominant galaxies of the different cluster substructures generally associated with a diffuse X-ray emission. They are probably the result of major merging events that occurred at early epochs. Slow rotators of lower stellar mass are also preferentially located within the different high-density substructures of the cluster. They are virialised within the cluster, thus Virgo members since its formation. They have been shaped by gravitational perturbations occurring within the infalling groups that later formed the cluster. On the contrary, low-mass star-forming systems are extremely rare in the inner regions of the Virgo cluster A, where the density of the intergalactic medium is at its maximum. Our ram pressure stripping models consistently indicate that these star-forming systems can be rapidly deprived of their interstellar medium during their interaction with the intergalactic medium. The lack of gas quenches their star formation activity transforming them into quiescent dwarf ellipticals. This mild transformation does not perturb the kinematic properties of these galaxies which still have rotation curves typical of star-forming systems.
The HRS is a complete volume-limited sample of nearby objects including Virgo cluster and isolated objects. Using a recent compilation of HI and CO data we study the effects of the cluster on the molecular gas content of spiral galaxies. We first ide
ntify M* as the scaling variable that traces the total H2 mass of galaxies better. We show that, on average, HI-deficient galaxies are significantly offset from the M(H2) vs. M* relation for HI-normal galaxies. We use the M(H2) vs. M* scaling relation to define the H2-deficiency parameter. This parameter shows a weak and scattered relation with the HI-def, here taken as a proxy for galaxy interactions with the cluster environment. We also show that, as for the HI, the extent of the H2 disc decreases with increasing HI-deficiency. These results show that cluster galaxies have, on average, a lower H2 content than similar objects in the field. The slope of the H2-def vs. HI-def relation is less than 1, while the D(HI)/D(i) vs. HI-def relation is steeper than the D(CO)/D(i) vs. HI-def relation, thereby indicating that the H2 gas is removed less efficiently than the HI. This result can be understood if the HI is distributed on a flat disc more extended than the stellar disc, thus less anchored to the gravitational potential well of the galaxy than the H2. There is a clear trend between the NUV-i colour and H2-def, which suggests that H2 removal quenches the activity of star formation. This causes galaxies migrate from the blue cloud to the green valley and, eventually, to the red sequence. The total gas-consumption timescale of gas deficient cluster galaxies is comparable to that of isolated systems, and is significantly larger than the typical timescale for total gas removal in a ram pressure stripping process, thus suggesting that ram pressure, rather than starvation, is the dominant process driving the evolution of these cluster galaxies.
We study the properties of the cold gas component of the interstellar medium of the Herschel Reference Survey, a complete volume-limited (15<D<25 Mpc), K-band-selected sample of galaxies spanning a wide range in morphological type (from E to Im) and
stellar mass (10^9<M*<10^11 Mo). The multifrequency data in our hands are used to trace the molecular gas mass distribution and the main scaling relations of the sample, which put strong constraints on galaxy formation simulations. We extend the main scaling relations concerning the total and the molecular gas component determined for massive galaxies (M* > 10^10 Mo) from the COLD GASS survey down to stellar masses M* ~ 10^9 Mo. As scaling variables we use M*, the stellar surface density mu*, the specific star formation rate SSFR, and the metallicity of the target galaxies. By comparing molecular gas masses determined using a constant or a luminosity dependent conversion factor, we estimate the robustness of these scaling relations on the very uncertain assumptions used to transform CO line intensities into molecular gas masses. The molecular gas distribution of a K-band-selected sample is different from that of a far-infrared-selected sample since it includes a significantly smaller number of objects with M(H2) < 6 10^9 Mo. In spiral galaxies the molecular gas phase is only 25-30% of the atomic gas. The analysis also indicates that the slope of the main scaling relations depends on the adopted conversion factor. Among the sampled relations, all those concerning M(gas)/M* are statistically significant and show little variation with X_CO. We observe a significant correlation between M(H2)/M* and SSFR, M(H2)/M(HI) and mu*, M(H2)/M(HI), and 12+log(O/H) regardless of the adopted X_CO. The total and molecular gas consumption timescales are anticorrelated with the SSFR.
We present long-slit integrated spectroscopy of 238 late-type galaxies belonging to the Herschel Reference Survey, a volume limited sample representative of the nearby universe. This sample has a unique legacy value since ideally defined for any stat
istical study of the multifrequency properties of galaxies spanning a large range in morphological type and luminosity. The spectroscopic observations cover the spectral range 3600-6900 A at a resolution R ~ 1000 and are thus suitable for separating the underlying absorption from the emission of the Hbeta line as well as the two [NII] lines from the Halpha emission. We measure the fluxes and the equivalent widths of the strongest emission lines ([OII], Hbeta, [OIII], [NII], Halpha, and [SII]). The data are used to study the distribution of the equivalent width of all the emission lines, of the Balmer decrement C(Hbeta) and of the observed underlying Balmer absorption under Hbeta in this sample. Combining these new spectroscopic data with those available at other frequencies, we also study the dependence of C(Hbeta) and E.W.Hbeta_{abs} on morphological type, stellar mass and stellar surface density, star formation rate, birthrate parameter and metallicity in galaxies belonging to different environments (fields vs. Virgo). The distribution of the equivalent width of all the emission lines, of C(Hbeta) and E.W.Hbeta_{abs} are systematically different in cluster and field galaxies. The Balmer decrement increases with stellar mass, stellar surface density, metallicity and star formation rate of the observed galaxies, while it is unexpectedly almost independent from the column density of the atomic and molecular gas. The dependence of C(Hbeta) on stellar mass is steeper than that previously found in other works. The underlying Balmer absorption does not significantly change with any of these physical parameters.
We study the far infrared (60-500 $mu$m) colours of late-type galaxies in the $Herschel$ Reference Survey, a K-band selected, volume limited sample of nearby galaxies. The far infrared colours are correlated with each other, with tighter correlations
for the indices that are closer in wavelength. We also compare the different colour indices to various tracers of the physical properties of the target galaxies, such as the surface brightness of the ionising and non-ionising stellar radiation, the dust attenuation and the metallicity. The emission properties of the cold dust dominating the far infrared spectral domain are regulated by the properties of the interstellar radiation field. Consistent with that observed in nearby, resolved galaxies, our analysis shows that the ionising and the non-ionising stellar radiation, including that emitted by the most evolved, cold stars, both contribute to the heating of the cold dust component. This work also shows that metallicity is another key parameter characterising the cold dust emission of normal, late-type galaxies. A single modified black body with a grain emissivity index $beta$=1.5 better fits the observed SPIRE flux density ratios $S250/S350$ vs. $S350/S500$ than $beta$=2, although values of $beta$ $simeq$ 2 are possible in metal rich, high surface brightness galaxies. Values of $beta$ $lesssim$ 1.5 better represent metal poor, low surface brightness objects. This observational evidence provides strong constraints for dust emission models of normal, late type galaxies.
We present infrared colours (in the 25-500 mic spectral range) and UV to radio continuum spectral energy distributions of a sample of 51 nearby galaxies observed with SPIRE on Herschel. The observed sample includes all morphological classes, from qui
escent ellipticals to active starbursts. Active galaxies have warmer colour temperatures than normal spirals. In ellipticals hosting a radio galaxy, the far-infrared (FIR) emission is dominated bynthe synchrotron nuclear emission. The colour temperature of the cold dust is higher in quiescent E-S0a than in star-forming systems probably because of the different nature of their dust heating sources (evolved stellar populations, X-ray, fast electrons) and dust grain properties. In contrast to the colour temperature of the warm dust, the f350/f500 index sensitive to the cold dust decreases with star formation and increases with metallicity, suggesting an overabundance of cold dust or an emissivity parameter beta<2 in low metallicity, active systems.
The Herschel Reference Survey is a guaranteed time Herschel key project and will be a benchmark study of dust in the nearby universe. The survey will complement a number of other Herschel key projects including large cosmological surveys that trace d
ust in the distant universe. We will use Herschel to produce images of a statistically-complete sample of 323 galaxies at 250, 350 and 500 micron. The sample is volume-limited, containing sources with distances between 15 and 25 Mpc and flux limits in the K-band to minimize the selection effects associated with dust and with young high-mass stars and to introduce a selection in stellar mass. The sample spans the whole range of morphological types (ellipticals to late-type spirals) and environments (from the field to the centre of the Virgo Cluster) and as such will be useful for other purposes than our own. We plan to use the survey to investigate (i) the dust content of galaxies as a function of Hubble type, stellar mass and environment, (ii) the connection between the dust content and composition and the other phases of the interstellar medium and (iii) the origin and evolution of dust in galaxies. In this paper, we describe the goals of the survey, the details of the sample and some of the auxiliary observing programs that we have started to collect complementary data. We also use the available multi-frequency data to carry out an analysis of the statistical properties of the sample.
We use Halpha and FUV GALEX data for a large sample of nearby objects to study the high mass star formation activity of normal late-type galaxies. The data are corrected for dust attenuation using the most accurate techniques at present available, na
mely the Balmer decrement and the total far-infrared to FUV flux ratio. The sample shows a highly dispersed distribution in the Halpha to FUV flux ratio indicating that two of the most commonly used star formation tracers give star formation rates with uncertainties up to a factor of 2-3. The high dispersion is due to the presence of AGN, where the UV and the Halpha emission can be contaminated by nuclear activity, highly inclined galaxies, for which the applied extinction corrections are probably inaccurate, or starburst galaxies, where the stationarity in the star formation history required for transforming Halpha and UV luminosities into star formation rates is not satisfied. Excluding these objects we reach an uncertainty of ~50% on the SFR. The Halpha to FUV flux ratio increases with their total stellar mass. If limited to normal star forming galaxies, however, this relationship reduces to a weak trend that might be totally removed using different extinction correction recipes. In these objects the Halpha to FUV flux ratio seems also barely related with the FUV-H colour, the H band effective surface brightness, the total star formation activity and the gas fraction. The data are consistent with a Kroupa and Salpeter initial mass function in the high mass stellar range and imply, for a Salpeter IMF, that the variations of the slope cannot exceed 0.25, from g=2.35 for massive galaxies to g=2.60 in low luminosity systems. We show however that these observed trends, if real, can be due to the different micro history of star formation in massive galaxies with respect to dwarf.
Pre-processing within small groups has been proposed to explain several of the properties of galaxies inhabiting rich clusters. The aim of the present work is to see whether pre-processing is acting in the nearby universe, where the structures that a
re merging to form rich clusters are rather large and massive. We study the HI gas properties of a large sample of late-type galaxies belonging to the Coma I cloud, an association of objects close to the Virgo cluster. Contrary to what previously claimed, late-type galaxies in the Coma I cloud are not deficient in HI gas (HI-def=0.06+-0.44). If the Coma I cloud is representative of infalling groups in nearby clusters, this result suggests that, in the local universe, the evolution of late-type galaxies belonging to loose structures with high velocity dispersions (>= 300 km/s)associated to rich clusters such as Virgo is not significantly perturbed by pre-processing.
The present work is aimed at studying the distribution of galaxies of different types and luminosities along different structural scaling relations to see whether massive and dwarf ellipticals have been shaped by the same formation process. This exer
cise is here done by comparing the distribution of Virgo cluster massive and dwarf ellipticals and star forming galaxies along the B band effective surface brightness and effective radius vs. absolute magnitude relations and the Kormendy relation to the predictions of models tracing the effects of ram-pressure stripping on disc galaxies entering the cluster environment and galaxy harassment. Dwarf ellipticals might have been formed from low luminosity, late-type spirals that recently entered into the cluster and lost their gas because of a ram-pressure stripping event, stopping their activity of star formation. The perturbations induced by the abrupt decrease of the star formation activity are sufficient to modify the structural properties of disc galaxies into those of dwarf ellipticals. Galaxy harassment induce a truncation of the disc and generally an increase of the effective surface brightness of the perturbed galaxies. The lack of dynamical simulations of perturbed galaxies spanning a wide range in luminosity prevents us to drive any firm conclusion on a possible harassment-induced origin of the low surface brightness dwarf elliptical galaxy population inhabiting the Virgo cluster. Although the observed scaling relations are consistent with the idea that the distribution of elliptical galaxies along the mentioned scaling relation is just due to a gradual variation with luminosity of the Sersic index n, the comparison with models indicates that dwarf ellipticals might have been formed by a totally different process than giant ellipticals
