Molecular hydrogen deficiency in HI-poor galaxies and its implications for star formation

We use a sample of 47 homogeneous and high sensitivity CO images taken from the Nobeyama and BIMA surveys to demonstrate that, contrary to common belief, a significant number (~40%) of HI-deficient nearby spiral galaxies are also depleted in molecular hydrogen. While HI-deficiency by itself is not a sufficient condition for molecular gas depletion, we find that H2 reduction is associated with the removal of HI inside the galaxy optical disk. Those HI-deficient galaxies with normal H2 content have lost HI mainly from outside their optical disks, where the H2 content is low in all galaxies. This finding is consistent with theoretical models in which the molecular fraction in a galaxy is determined primarily by its gas column density. Our result is supported by indirect evidence that molecular deficient galaxies form stars at a lower rate or have dimmer far infrared fluxes than gas rich galaxies, as expected if the star formation rate is determined by the molecular hydrogen content. Our result is consistent with a scenario in which, when the atomic gas column density is lowered inside the optical disk below the critical value required to form molecular hydrogen and stars, spirals become quiescent and passive evolving systems. We speculate that this process would act on the time-scale set by the gas depletion rate and might be a first step for the transition between the blue and red sequence observed in the color-magnitude diagram.Comment: 12 pages, 9 figures, accepted for publication in Ap

Mid-IR emission of galaxies in the Virgo cluster and in the Coma supercluster.IV. The nature of the dust heating sources

We study the relationship between the mid-IR (5-18 $\mu$m) emission of late-type galaxies and various other star formation tracers in order to investigate the nature of the dust heating sources in this spectral domain. The analysis is carried out using a sample of 123 normal, late-type, nearby galaxies with available data at several frequencies. The mid-IR luminosity (normalized to the H-band luminosity) correlates better with the far-IR luminosity than with more direct tracers of the young stellar population such as the H$\alpha$ and the UV luminosity. The comparison of resolved images reveals a remarkable similarity in the H$\alpha$ and mid-IR morphologies, with prominent HII regions at both frequencies. The mid-IR images, however, show in addition a diffuse emission not associated with HII regions nor with the diffuse H$\alpha$ emission. This evidence indicates that the stellar population responsible for the heating of dust emitting in the mid-IR is similar to that heating big grains emitting in the far-IR, including relatively evolved stars responsible for the non-ionizing radiation. The scatter in the mid-IR vs. H$\alpha$, UV and far-IR luminosity relation is mostly due to metallicity effects, with metal-poor objects having a lower mid-IR emission per unit star formation rate than metal-rich galaxies. Our analysis indicates that the mid-IR luminosity is not an optimal star formation tracer in normal, late-type galaxies.Comment: accepted for publication on A&

Mid--IR emission of galaxies in the Virgo cluster: II. Integrated properties

We analyse the integrated properties of the Mid-IR emission of a complete, optically selected sample of galaxies in the Virgo cluster observed with the ISOCAM instrument on board the ISO satellite. The analysis shows that the Mid-IR emission up to 15 mic of optically-selected, normal early-type galaxies (E, S0 and S0a) is dominated by the Rayleigh-Jeans tail of the cold stellar component. The Mid-IR emission of late-type galaxies is instead dominated by the thermal emission from dust. The small dust grains emitting in the Mid-IR have an excess of emission if compared to big grains emitting in the Far-IR. While the Far-IR emission increases with the intensity of the interstellar radiation field, their Mid-IR emission is non--linearly related to the UV radiation field. The spectral energy distributions of the target galaxies indicate that there is a linear relationship between the UV radiation field and the Mid-IR emission of galaxies for low or intermediate activities of star formation, while the emission from the hot dust seems to drop for strong UV fields. The Mid-IR colour of late-type galaxies is not related to their activity of star formation. The properties of the dust emission in the Mid-IR seem more related to the mass than to the morphological type of the target galaxy. Since the activity of star formation is anticorrelated to the mass of galaxies, this reflects a relationship between the emission of dust in the Mid-IR and the UV radiation field: galaxies with the lowest Mid-IR emission for a given UV field are low mass, dwarf galaxies. These observational evidences are easily explained if the carriers of the Unidentified Infrared Bands that dominate the 6.75 mic emission are destroyed by the intense UV radiation field of dwarf galaxies, although abundance effects can also play a role.Comment: 17 pages, Latex, 7 figures; to be published in Astronomy & Astrophysics, Main Journal; Figure legend should be corrected in: 1 - 1a; 2 - 1b; 3 - 2; 4 - 3a; 5 - 3b; 6 - 3c; 7 - 3d; 8 - 3e; 9 - 4; 10 - 5; 11 - 6; 12 -

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&

Rise-time effects in ggnMOSt under TLP stress

In this paper the main mechanisms that lead the turn on of the parasitic bipolar transistor of a grounded gate nMOS transistor (ggnMOS) under TLP stress have been analyzed in detail in the sub-nanoseconds range by means of a mixed-mode simulator. We showed that the breakdown voltage of the ggnMOS measured in static conditions would underestimate the maximum voltage across the protection structure obtained by TLP stress, depending on the rise-time of the applied puls

The Star Formation Rate in disk galaxies: thresholds and dependence on gas amount

We reassess the applicability of the Toomre criterion in galactic disks and we study the local star formation law in 16 disk galaxies for which abundance gradients are published. The data we use consists of stellar light profiles, atomic and molecular gas (deduced from CO with a metallicity-dependent conversion factor), star formation rates (from H-alpha emissivities), metallicities, dispersion velocities and rotation curves. We show that the Toomre criterion applies successfully to the case of the Milky Way disk, but it has limited success with the data of our sample; depending on whether the stellar component is included or not in the stability analysis, we find average values for the threshold ratio of the gas surface density to the critical surface density in the range 0.5 to 0.7. We also test various star formation laws proposed in the literature, i.e. either the simple Schmidt law or modifications of it, that take into account dynamical factors. We find only small differences among them as far as the overall fit to our data is concerned; in particular, we find that all three SF laws (with parameters derived from the fits to our data) match particularly well observations in the Milky Way disk. In all cases we find that the exponent n of our best fit SFR has slightly higher values than in other recent works and we suggest several reasons that may cause that discrepancy.Comment: 18 pages, 12 figures, accepted in MNRA