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

The origin of the mu_e - M_B and Kormendy relations in dwarf elliptical galaxies

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 exercise 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 ellipticalsComment: Accepted for publication on A&

The GALEX Ultraviolet Virgo Cluster Survey (GUViCS). II. Constraints on star formation in ram-pressure stripped gas

Context: Several galaxies in the Virgo cluster are known to have large HI gas tails related to a recent ram-pressure stripping event. The Virgo cluster has been extensively observed at 1539 A in the far-ultraviolet for the GALEX Ultraviolet Virgo Cluster Survey (GUViCS), and in the optical for the Next Generation Virgo Survey (NGVS), allowing a study of the stellar emission potentially associated with the gas tails of 8 cluster members. On the theoretical side, models of ram-pressure stripping events have started to include the physics of star formation. Aim: We aim to provide quantitative constraints on the amount of star formation taking place in the ram-pressure stripped gas, mainly on the basis of the far-UV emission found in the GUViCS images in relation with the gas content of the tails. Methods: We have performed three comparisons of the young stars emission with the gas column density: visual, pixel-by-pixel and global. We have compared our results to other observational and theoretical studies. Results: We find that the level of star formation taking place in the gas stripped from galaxies by ram-pressure is low with respect to the available amount of gas. Star formation is lower by at least a factor 10 compared to the predictions of the Schmidt Law as determined in regular spiral galaxy disks. It is also lower than measured in dwarfs galaxies and the outer regions of spirals, and than predicted by some numerical simulations. We provide constraints on the star formation efficiency in the ram-pressure stripped gas tails, and compare these with current models.Comment: Accepted in A&A, 17 pages (including the appendix and "on-line" figures of the paper

The effect of the environment on the HI scaling relations

We use a volume-, magnitude-limited sample of nearby galaxies to investigate the effect of the environment on the HI scaling relations. We confirm that the HI-to-stellar mass ratio anti correlates with stellar mass, stellar mass surface density and NUV-r colour across the whole range of parameters covered by our sample (10^9 <M*<10^11 Msol, 7.5 <mu*<9.5 Msol kpc^-2, 2<NUV-r<6 mag). These scaling relations are also followed by galaxies in the Virgo cluster, although they are significantly offset towards lower gas content. Interestingly, the difference between field and cluster galaxies gradually decreases moving towards massive, bulge-dominated systems. By comparing our data with the predictions of chemo-spectrophotometric models of galaxy evolution, we show that starvation alone cannot explain the low gas content of Virgo spirals and that only ram-pressure stripping is able to reproduce our findings. Finally, motivated by previous studies, we investigate the use of a plane obtained from the relations between the HI-to-stellar mass ratio, stellar mass surface density and NUV-r colour as a proxy for the HI deficiency parameter. We show that the distance from the `HI gas fraction plane' can be used as an alternative estimate for the HI deficiency, but only if carefully calibrated on pre-defined samples of `unperturbed' systems.Comment: Accepted for publication on MNRAS main journal. 11 pages, 6 figures, 1 tabl

High mass star formation in normal late-type galaxies: observational constraints to the IMF

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, namely 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.Comment: Accepted for publication on Ap

The IRX-beta relation on sub-galactic scales in star-forming galaxies of the Herschel Reference Survey

UV and optical surveys are essential to gain insight into the processes driving galaxy formation and evolution. The rest-frame UV emission is key to measure the cosmic SFR. However, UV light is strongly reddened by dust. In starburst galaxies, the UV colour and the attenuation are linked, allowing to correct for dust extinction. Unfortunately, evidence has been accumulating that the relation between UV colour and attenuation is different for normal star-forming galaxies when compared to starburst galaxies. It is still not understood why star-forming galaxies deviate from the UV colour-attenuation relation of starburst galaxies. Previous work and models hint that the role of the shape of the attenuation curve and the age of stellar populations have an important role. In this paper we aim at understanding the fundamental reasons to explain this deviation. We have used the CIGALE SED fitting code to model the far UV to the far IR emission of a set of 7 reasonably face-on spiral galaxies from the HRS. We have explored the influence of a wide range of physical parameters to quantify their influence and impact on the accurate determination of the attenuation from the UV colour, and why normal galaxies do not follow the same relation as starburst galaxies. We have found that the deviation can be best explained by intrinsic UV colour differences between different regions in galaxies. Variations in the shape of the attenuation curve can also play a secondary role. Standard age estimators of the stellar populations prove to be poor predictors of the intrinsic UV colour. These results are also retrieved on a sample of 58 galaxies when considering their integrated fluxes. When correcting the emission of normal star-forming galaxies for the attenuation, it is crucial to take into account possible variations in the intrinsic UV colour as well as variations of the shape of the attenuation curve.Comment: Accepted for publication in A&A, 18 pages, 14 figures. The paper with high resolution figures can be downloaded at http://www.oamp.fr/people/mboquien/HRS/boquien_IRX_beta.pd

Non-linear Dependence of L(B) on L(FIR) and M(H2) among Spiral Galaxies and Effects of Tidal Interaction

Through the study of a carefully selected sample of isolated spiral galaxies, we have established that two important global physical quantities for tracing star forming activities, L(FIR) and M(H2), have non-linear dependence on another commonly cited global quantity L(B). Furthermore we show that simple power law relations can effectively describe these non-linear relations for spiral galaxies spanning four orders of magnitude in FIR and M(H2) and nearly three orders of magnitude in L(B). While the existence of non-linear dependence of M(H2) (assuming a constant CO-to-H2 conversion) and L(FIR) on optical luminosity L(B) has been previously noted in the literature, an improper normalization of simple scaling by L(B) has been commonly used in many previous studies to claim enhanced molecular gas content and induced activities among tidally interacting and other types of galaxies. We remove these non-linear effects using the template relations derived from the isolated galaxy sample and conclude that strongly interacting galaxies do not have enhanced molecular gas content, contrary to previous claims. With these non-linear relations among L(B), L(FIR) and M(H2) properly taken into account, we confirm again that the FIR emission and the star formation efficiency L(FIR)/M(H2) are indeed enhanced by tidal interactions. Virgo galaxies show the same level of M(H2) and L(FIR) as isolated galaxies. We do not find any evidence for enhanced star forming activity among barred galaxies.Comment: 19 pages and 5 figures, requires AAS style files, ApJ, accepte

1.65 micrometers (H-band) surface photometry of galaxies. III: observations of 558 galaxies with the TIRGO 1.5m telescope

We present near-infrared H-band (1.65 micron) surface photometry of 558 galaxies in the Coma Supercluster and in the Virgo cluster. This data set, obtained with the Arcetri NICMOS3 camera ARNICA mounted on the Gornergrat Infrared Telescope, is aimed at complementing, with observations of mostly early-type objects, our NIR survey of spiral galaxies in these regions, presented in previous papers of this series. Magnitudes at the optical radius, total magnitudes, isophotal radii and light concentration indices are derived. We confirm the existence of a positive correlation between the near-infrared concentration index and the galaxy H-band luminosity. (Tables 1 and 2 are only available in electronic form upon request to [email protected])Comment: 12 pages, 6 figures. Accepted for publication in A&A