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Low-surface-brightness galaxies (LSBs) contribute to a significant fraction of all the galaxies in the Universe. Ultra-diffuse galaxies (UDGs) form a subclass of LSBs that has attracted a lot of attention in recent years (although its definition may vary between studies). Although UDGs are found in large numbers in galaxy clusters, groups, and in the field, their formation and evolution are still very much debated. Using a comprehensive set of multiwavelength data from the NGVS (optical), VESTIGE (H$\alpha$ narrowband), and GUViCS (UV) surveys, we studied a sample of 64 diffuse galaxies and UDGs in the Virgo cluster to investigate their formation history. We analyzed the photometric colors and surface-brightness profiles of these galaxies and then compared them to models of galaxy evolution, including ram-pressure stripping (RPS) events to infer any possible strong interactions with the hot cluster gas in the past. While our sample consists mainly of red LSBs, which is typical in cluster environments, we found evidence of a color variation with the cluster-centric distance. Blue, HI-bearing, star-forming diffuse galaxies are found at larger distances from the cluster center than the rest of the sample. The comparison of our models with multifrequency observations suggests that most of the galaxies of the sample might have undergone a strong RPS event in their lifetime, on average 1.6 Gyr ago (with a large dispersion, and RPS still ongoing for some of them). This process resulted in the transformation of initially gas-rich diffuse blue galaxies into gas-poor and red ones that form the dominant population now, the more extreme UDGs having undergone the process in a more distant past on average. The RPS in dense environments could be one of the major mechanisms for the formation of the large number of quiescent UDGs we observe in galaxy clusters., Comment: 57 pages, 14 Figures, accepted for publication in A&A

We study the chemical evolution of the disks of the Milky Way (MW) and of Andromeda (M31), in order to reveal common points and differences between the two major galaxies of the Local group. We use a large set of observational data for M31, including recent observations of the Star Formation Rate (SFR) and gas profiles, as well as stellar metallicity distributions along its disk. We show that, when expressed in terms of the corresponding disk scale lengths, the observed radial profiles of MW and M31 exhibit interesting similarities, suggesting the possibility of a description within a common framework. We find that the profiles of stars, gas fraction and metallicity of the two galaxies, as well as most of their global properties, are well described by our model, provided the star formation efficiency in M31 disk is twice as large as in the MW. We show that the star formation rate profile of M31 cannot be fitted with any form of the Kennicutt-Schmidt law (KS Law) for star formation. We attribute those discrepancies to the fact that M31 has undergone a more active star formation history, even in the recent past, as suggested by observations of a "head-on" collision with the neighboring M32 galaxy about 200 Myr ago. The MW has most probably undergone a quiescent secular evolution, making possible a fairly successful description with a simple model. If M31 is more typical of spiral galaxies, as recently suggested by Hammer et al. (2007), more complex models, involving galaxy interactions, will be required for the description of spirals., 13 Pages, 7 figures, 2 tables, accepted for publication in Astronomy and Astrophysics

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 ellipticals

Dust depletion plays a key role in understanding the nature of Damped Lyman or systems (DLAs). In this paper we point out a previously unnoticed anticorrelation between the observed abundance ratio [X/Zn] (where Zn is assumed to be undepleted and X stands for the refractories Fe, Cr and Ni) and metal column density ([Zn/H] + log(N-HI)) in DLAs. We suggest that this trend is an unambiguous sign of dust depletion, since metal column density is a measure of the amount of dust along the line of sight. Assuming that DLAs are (proto-) galactic disks and using detailed chemical evolution models with metallicity dependent yields we study chemical evolution and dust depletion patterns for ct and iron-peak elements in DLAs. When observational constraints on the metal column density of DLAs are taken into account las suggested in Boisse et al. 1998) we find that our models reproduce fairly well the observed mild redshift evolution of the abundances of 8 elements (Al, Si, S, Cr, Mn, Fe, Zn and Ni) as well as the observed scatter at a given redshift. By considering the aforementioned dependence of abundance ratios on metal column density, we further explore the general dust depletion pattern in DLAs, comparing to our model results and to a solar reference pattern. We find that for low metal column densities (no depletion), our models compare fairly well to the data, while a solar pattern has difficulties with Mn. At high metal column densities (amount of depletion similar to0.5 dex), the solar pattern describes the data quite well, while our models have difficulties with S. We suggest that further measurements of those key elements, i.e. Zn, S and Mn, will help us to gain more insight into the nature of DLAs. The presently uncertain nucleosynthesis of Zn in massive stars Ion which a large part of these conclusions is based) should be carefully scrutinised.

We obtained using MegaCam at the CFHT a deep narrow band Halpha+[NII] wide field image of NGC 4569, the brightest late-type galaxy in the Virgo cluster. The image reveals the presence of long tails of diffuse ionised gas without any associated stellar component extending from the disc of the galaxy up to ~ 80 kpc (projected distance) with a typical surface brightness of a few 10^-18 erg s-1 cm-2 arcsec-2. These features provide direct evidence that NGC 4569 is undergoing a ram presure stripping event. The image also shows a prominent 8 kpc spur of ionised gas associated to the nucleus that spectroscopic data identify as an outflow. With some assumptions on the 3D distribution of the gas, we use the Halpha surface brightness of these extended low surface brightness features to derive the density and the mass of the gas stripped during the interaction of the galaxy with the ICM. The comparison with ad-hoc chemo-spectrophotometric models of galaxy evolution indicates that the mass of the Halpha emitting gas in the tail is comparable to that of the cold phase stripped from the disc, suggesting that the gas is ionised within the tail during the stripping process. The lack of star forming regions suggests that mechanisms other than photoionisation are responsible for the excitation of the gas (shocks, heat conduction, magneto hydrodynamic waves). This analysis indicates that ram pressure stripping is efficient in massive (M_star ~ 10^10.5 Mo) galaxies located in intermediate mass (~ 10^14 Mo) clusters under formation. It also shows that the mass of gas expelled by the nuclear outflow is ~ 1 % than that removed during the ram pressure stripping event. All together these results indicate that ram pressure stripping, rather than starvation through nuclear feedback, can be the dominant mechanism responsible for the quenching of the star formation activity of galaxies in high density environments., Comment: Accepted for publication on Astronomy & Astrophysics