{Chemo-spectrophotometric evolution of spiral galaxies: IV. Star formation efficiency and effective ages of spirals

We study the star formation history of normal spirals by using a large and homogeneous data sample of local galaxies. For our analysis we utilise detailed models of chemical and spectrophotometric galactic evolution, calibrated on the Milky Way disc. We find that star formation efficiency is independent of galactic mass, while massive discs have, on average, lower gas fractions and are redder than their low mass counterparts; put together, these findings convincingly suggest that massive spirals are older than low mass ones. We evaluate the effective ages of the galaxies of our sample and we find that massive spirals must be several Gyr older than low mass ones. We also show that these galaxies (having rotational velocities in the 80-400 km/s range) cannot have suffered extensive mass losses, i.e. they cannot have lost during their lifetime an amount of mass much larger than their current content of gas+stars.Comment: 11 pages, 8 figures, MNRAS in pres

Magnetic ordering in GaAlAs:Mn double well structure

The magnetic order in the diluted magnetic semiconductor barrier of double AlAs/GaAs: Mn quantum well structures is investigated by Monte Carlo simulations. A confinement adapted RKKY mechanism is implemented for indirect exchange between Mn ions mediated by holes. It is shown that, depending on the barrier width and the hole concentration a ferromagnetic or a spin-glass order can be established.Comment: 3 figure

Completing HI observations of galaxies II. The Coma Supercluster

High sensitivity 21-cm HI line observations, with an rms noise level of \sim 0.5 mJy, were made of 35 spiral galaxies in the Coma Supercluster, using the refurbished Arecibo telescope, which resulted in the detection of 25 objects. These data, combined with the measurements available from the literature, provide the set of HI data for 94% of all late-type galaxies in the Coma Supercluster with an apparent photographic magnitude m_p <15.7 mag. We confirm that the typical scale of HI deficiency around the Coma cluster is 2 Mpc, i.e. one virial radius. Comparing the HI mass function (HIMF) of cluster with non-cluster members of the Coma Supercluster we detect a shortage of high HI mass galaxies among cluster members that can be ascribed to the pattern of HI deficiency found in rich clusters.Comment: 16 pages, 8 figures, 4 tables. Accepted for publication on A&

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&

1.65mic (H-band) surface photometry of galaxies. VI: The history of star formation in normal late-type galaxies

We have collected a large body of NIR (H band), UV (2000 A) and Halpha measurements of late-type galaxies. These are used, jointly with spectral evolutionary synthesis models, to study the initial mass function (IMF) in the mass range m > 2 Mo. For spirals (Sa-Sd), Magellanic irregulars (Im) and blue compact dwarfs (BCD), our determination is consistent with a Salpeter IMF with an upper mass cutoff M_up = 80 Mo. The history of star formation and the amount of total gas (per unit mass) of galaxies are found to depend primarily on their total masses (as traced by the H band luminosities) and only secondarily on morphological type. The present star formation activity of massive spirals is up to 100 times smaller than that average over their lifetime, while in low mass galaxies it is comparable to or higher than that at earlier epochs. Dwarf galaxies have presently larger gas reservoirs per unit mass than massive spirals. The efficiency in transforming gas into stars and the time scale for gas depletion (10 Gyrs) are independent of the luminosity and/or of the morphological type. These evidences are consistent with the idea that galaxies are coeval systems,that they evolved as closed-boxes forming stars following a simple, universal star formation law whose characteristic time scale is small (1 Gyr) in massive spirals and large (10 Gyr) in low mass galaxies. A similar conclusion was drawn by Gavazzi and Scodeggio (1996) to explain the colour-magnitude relation of late-type galaxies. The consequences of this interpretation on the evolution of the star formation rate and of the gas density per comoving volume of the Universe with look-back time are discussed.Comment: LaTex, 24 pages, 12 figures, accepted for publication on Astronomical Journa

The tilt of the Fundamental Plane of Early-type galaxies: wavelength dependence

The photometric parameters R_e and mu_e of 74 early-type (E+S0+S0a) galaxies in the Coma cluster are derived for the first time in the near IR H band. These are used, coupled with measurements of the central velocity dispersion found in the literature, to determine the H band Fundamental Plane (FP) relation of this cluster. The same procedure is applied to previously available photometric data in the B, V, r, I, and K bands, to perform a multi-wavelength study of the FP. Because systematic uncertainties in the value of the FP parameters are introduced both by the choice of the fitting algorithm, and by the presence of statistical biases connected with the sample selection procedure, we emphasize the importance of deriving the FP parameters in the six photometric bands using an identical fitting algorithm, and appropriate corrections to eliminate the effects of sample incompleteness. We find that the FP mu_e coefficient is stable with wavelength, while the sigma coefficient increases significantly with increasing wavelength, in agreement with an earlier result presented by Pahre & Djorgovski. The slope of the FP relation, although changing with wavelength, never approaches the virial theorem expectation. We also find that the magnitude of the slope change can be entirely explained by the presence of the well known relation between color and magnitude among early-type galaxies. We conclude that the tilt of the Fundamental Plane is significant, and must be due to some form of broken homology among early-type galaxies, while its wavelength dependence derives from whatever mechanism (currently the preferred one is the existence of a mass-metal content sequence) produces the color-magnitude relation in those galaxies.Comment: 8 pages, 7 figures, 6 tables; table 3 should be printed in landscape mode, and inserted into the text. Accepted for publication in MNRA

Halpha surface photometry of galaxies in the Virgo cluster. IV: the current star formation in nearby clusters of galaxies

Halpha+[NII] imaging observations of 369 late-type galaxies in the Virgo cluster and in the Coma/A1367 supercluster are analyzed. They constitute an optically selected sample (m_p<16.0) observed with 60% c.a. completeness.These observations provide us with the current (T<10^7 yrs) star formation properties of galaxies. The expected decrease of the star formation rate (SFR),as traced by the Halpha E.W., with decreasing clustercentric projected distance is found only when galaxies brighter than M_p=-19.5 are considered. We also include in our analysis Near Infrared data, providing us with informations on the old (T>10^9yrs) stars. Put together, the young and the old stellar indicators give the ratio of currently formed stars over the stars formed in the past, or "birthrate" parameter b. We also determine the "global gas content" combining HI with CO observations. We define the "gas deficiency" parameter as the logarithmic difference between the gas content of isolated galaxies of a given Hubble type and the measured gas content.For the isolated objects we find that b decreases with increasing NIR luminosity. The gas-deficient objects, primarily members to the Virgo cluster, have their birthrate significantly lower than the isolated objects with normal gas content and of similar NIR luminosity. This indicates that the current star formation is regulated by the gaseous content of spirals.Whatever mechanism (most plausibly ram-pressure stripping) is responsible for the pattern of gas deficiency observed in spiral galaxies members to rich clusters, it also produces the observed quenching of the current star formation.Comment: 22 pages,14 figures,3 figures available in jpeg format.To be published in A&

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