arXiv subjects

Giuseppe Gavazzi

Publications and source records attributed to Giuseppe Gavazzi.

35 records · Page 2Linked to original sources

arXiv2013

Halpha3: an Halpha imaging survey of HI selected galaxies from ALFALFA. III. Nurture shapes up the Hubble sequence in the Great Wall

We present the analysis of Halpha3, an Halpha imaging survey of galaxies selected from the HI ALFALFA Survey in the Coma Supercluster. By using the Halpha line as a tracer of the "instantaneous" star formation, complemented with optical colors from SDSS we explore the hypothesis that a morphological sequence of galaxies of progressively earlier type, lower gas-content exists in the neighborhood of the Coma cluster, with specific star formation activity decreasing with increasing local galaxy density and velocity dispersion. In the dwarf regime (8.5<\log(M*)< 9.5) we identify a 4-step sequence of galaxies with progressively redder colors, i.e. of decreasing specific star formation, from (1) HI-rich Late-Type Galaxies belonging to the blue-cloud exhibiting extended plus nuclear star formation, (2) HI-poor LTGs with nuclear star formation only, (3) HI-poor galaxies with no star formation either extended or nuclear, but with nuclear Post-Star-Burst signature,(4) Early-type Galaxies in the red-sequence, with no gas or star formation on all scales. Along this sequence the quenching of the star formation proceeds radially outside-in. The progression toward redder colors found along this "morphological" (gas content) sequence is comparable to the one obtained increasing the local galaxy density, from the cosmic filaments (1,2), to the rich clusters (2,3,4). In the dwarf regime we find evidence for evolution of HI-rich LTGs into ETGs, passing through HI-poor and PSB galaxies, driven by the environment. We identify ram-pressure as the mechanism most likely responsible for this transformation. We conclude that infall of low-mass galaxies has proceeded for the last 7.5 Gyr building up the Coma cluster at a rate of approximately 100 galaxies per Gyr.

astro-ph.CO↗

arXiv2013

Halpha3: an Halpha imaging survey of HI selected galaxies from ALFALFA. IV. Structure of galaxies in the Local and Coma Superclusters

We present the analysis of the galaxy structural parameters from Halpha3, an Halpha narrow-band imaging follow-up survey of ~800 galaxies selected from the HI ALFALFA Survey in the Local and Coma Superclusters. Taking advantage of Halpha3 which provides the complete census of the recent star-forming, HI-rich galaxies in the local universe, we aim to investigate the structural parameters of both the young (<10 Myr) and the old (>1 Gyr) stellar populations. By comparing the sizes of these stellar components we investigated the spatial scale on which galaxies are growing at the present cosmological epoch and the role of the environment in quenching the star-formation activity. We computed the concentration, asymmetry, and clumpiness structural parameters. To quantify the sizes we computed half-light radii and a new parameter dubbed EW/r. The concentration index computed in the r band depends on the stellar mass and on the Hubble type, these variables being related since most massive galaxies are bulge dominated thus most concentrated. Going toward later spirals and irregulars both the concentration index and the stellar mass decrease. Blue Compact dwarfs represent an exception since they have similar stellar mass but they are more concentrated than dwarf irregulars. The asymmetry and the clumpiness increase along the spiral sequence then they decrease going into the dwarf regime, where the light distribution is smooth and more symmetric. When measured on Halpha images, the CAS parameters do not exhibit obvious correlations with Hubble type. We found that the concentration index is the main parameter that describes the current growth of isolated galaxies but, for a fixed concentration, the stellar mass plays a second order role. At the present epoch, massive galaxies are growing inside-out, conversely the dwarfs are growing on the scale of their already assembled mass.

astro-ph.CO↗

arXiv2013

Halpha3: an Halpha imaging survey of HI selected galaxies from ALFALFA. II. The star formation properties of galaxies in the Virgo cluster and surroundings

We present the analysis of Halpha3, an Halpha imaging survey of 409 galaxies selected from the HI Arecibo ALFALFA Survey in the Local Supercluster, including the Virgo cluster. We explore the relations between the stellar mass, the HI mass and the current, massive SFR of nearby galaxies in the Virgo cluster and we compare them with those of isolated galaxies in the Local Supercluster, disentangling the role of the environment in shaping the star formation properties of galaxies at the present cosmological epoch. We investigate the relationships between atomic neutral gas and newly formed stars in different environments, across many morphological types, and over a wide range of stellar masses adopting an updated calibration of the HI deficiency parameter. Studying the mean properties of late-type galaxies in the Local Supercluster, we find that galaxies in increasing local galaxy density conditions (or decreasing projected angular separation from M87) show a significant decrease in the HI content and in the mean specific star formation rate, along with a progressive reddening of their stellar populations. The gradual quenching of the star formation occurs outside-in, consistently with the predictions of the ram pressure model. Once considered as a whole, the Virgo cluster is effective in removing neutral hydrogen from galaxies, and this perturbation is strong enough to appreciably reduce the SFR of its entire galaxy population. An estimate of the present infall rate of 300-400 galaxies per Gyr in the Virgo cluster is obtained from the number of existing HI-rich late-type systems, assuming 200-300 Myr as the time scale for HI ablation. If the infall process has been acting at constant rate this would imply that the Virgo cluster has formed approximately 2 Gyr ago, consistently with the idea that Virgo is in a young state of dynamical evolution.

astro-ph.CO↗

arXiv2013

The Herschel Virgo Cluster Survey - XIII. Dust in early-type galaxies

Aims. We study the dust content of a large optical input sample of 910 early-type galaxies (ETG) in the Virgo cluster, extending also to the dwarf ETG, and examine the results in relation with those on the other cold ISM components. Methods. We searched for far-infrared emission in all galaxies of the input sample using the 250 micron image of the Herschel Virgo Cluster Survey (HeViCS). This image covers a large fraction of the cluster. For the detected ETG we measured fluxes in 5 bands from 100 to 500 micron, and estimated the dust mass and temperature with modified black-body fits. Results. Dust is detected above the completeness limit of 25.4 mJy at 250 micron in 46 ETG, 43 of which are in the optically complete part of the input sample. In addition dust is present at fainter levels in another 6 ETG. We detect dust in the 4 ETG with synchrotron emission, including M 87. Dust appears to be much more concentrated than stars and more luminous ETG have higher dust temperatures. Dust detection rates down to the 25.4 mJy limit are 17% for ellipticals, about 40% for lenticulars (S0 + S0a) and around 3% for dwarf ETG. Dust mass does not correlate clearly with stellar mass and is often much more than that expected for a passive galaxy in a closed-box model. The dust-to-stars mass ratio anticorrelates with galaxy luminosity, and for some dwarf ETG reaches values as high as for dusty late-type galaxies. In the Virgo cluster slow rotators appear more likely to contain dust than fast ones. Comparing the dust results with those on HI from ALFALFA, there are only 8 ETG detected both in dust and in HI in the HeViCS area; 39 have dust but only an upper limit on HI, and 8 have HI but only an upper limit on dust. The locations of these galaxies in the cluster are different, with the dusty ETG concentrated in the densest regions, while the HI rich ETG are at the periphery.

astro-ph.CO↗

arXiv2012

65 kpc of ionized gas trailing behind NGC 4848 during its first crossing of the Coma cluster

In a 5 hour Halpha exposure of the N-W region of the Coma cluster with the 2.1m telescope at SPM (Mx) we discovered a 65 kpc cometary emission of ionized gas trailing behind the SBab galaxy NGC 4848. The tail points in the opposite direction of the cluster center, in the same direction where stripped HI has been detected in previous observations. The galaxy shows bright HII regions in an inner ring-like pattern, where the star formation takes place at the prodigious rate of 8.9 Msun/yr. From the morphology of the galaxy and of the trailing material, we infer that the galaxy is suffering from ram pressure due to its high velocity motion through the cluster IGM. We estimate that 4 x 10^9 Msun of gas is swept out from the galaxy forming the tail. Given the ambient conditions in the Coma cluster (rho = 6.3 x 10^-27 g/cm^3; sigma_vel = 940 km/s) simulations predict that the ram pressure mechanism is able to remove such an amount of gas in less than 200 Myr. This, combined with the geometry of the interaction, indicating radial infall into the cluster, leads to the conclusion that NGC 4848 is caught in its first passage through the dense cluster environment.

astro-ph.CO↗

arXiv2012

Halpha3: Halpha imaging survey of HI selected galaxies from ALFALFA

We present Halpha3 (acronym for Halpha-alpha-alpha), an Halpha narrow-band imaging survey of ~400 galaxies selected from the HI Arecibo Legacy Fast ALFA Survey (ALFALFA) in the Local Supercluster, including the Virgo cluster. By using hydrogen recombination lines as a tracer of recent star formation, we aim to investigate the relationships between atomic neutral gas and newly formed stars in different environments (cluster and field), morphological types (spirals and dwarfs), and over a wide range of stellar masses (~10^7.5-10^11.5 Msun). We image in Halpha+[NII] all the galaxies that contain more than 10^7 Msun of neutral atomic hydrogen in the sky region 11^h < R.A. <16^h 4^o < Dec. <16^o; 350< cz <2000 km/s using the San Pedro Martir 2m telescope. This survey provides a complete census of the star formation in HI rich galaxies of the local universe. We present the properties of the galaxy sample, together with Halpha fluxes and equivalent widths. We find an excellent agreement between the fluxes determined from our images in apertures of 3 arcsec diameter and the fluxes derived from the SDSS spectral database. From the Halpha fluxes corrected for galactic and internal extinction and for [NII] contamination we derive the global star formation rates (SFRs).

astro-ph.CO↗

arXiv2012

Stripped gas as fuel for newly formed HII regions in the encounter between VCC1249 and M49: a unified picture from NGVS and GUViCS

Context: We study the peculiar interacting galaxy system of VCC1249/M49 located in the core of the Virgo B subcluster. Owing to a recent interaction between the dwarf galaxy VCC1249 and the halo gas of the gE M49, neutral hydrogen has been displaced from the interstellar medium of this dwarf into the Virgo ICM. Observations also reveal multiple compact star-forming regions that are embedded in this HI cloud, with a projected separation up to 13 kpc from VCC1249 in the northwest direction. Aims: Motivated by recent NUV imaging from GUViCS of the VCC1249/M49 system that shows significant ongoing/recent star formation in the compact regions, we aim to constrain the origin of these outlying HII regions with a multi-wavelength approach. Methods: Using deep optical (u, g, i, z) imaging from NGVS and new Halpha imaging obtained at the San Pedro Martir observatory together with Keck long-slit spectroscopy, we characterize the SFR, ages, and metallicity of VCC1249 and its outlying compact regions. Moreover, we analyze the color and luminosity profile of the galaxy to investigate its recent interaction with M49. Results: Our new observations indicate that VCC1249 underwent a recent interaction with M49 in which both ram-pressure stripping and tidal interaction occured. The joint action of the two mechanisms led to the removal of the HI gas from the ISM of VCC1249, while the gravitational tides triggered the stellar tail and counter-tail of VCC1249. Our SED analysis reveals that the star formation in this galaxy was truncated around 200 Myr ago and that the outlying HII regions were born in situ about 10 Myr ago out of pre-enriched gas removed from the dwarf galaxy. These observations also reveal that interactions between central and satellite galaxies similar to the one between VCC1249/M49 may be an effective way of dispersing metals into the halos of massive galaxies.

astro-ph.CO↗

arXiv2011

The complete census of optically selected AGNs in the Coma Supercluster: the dependence of AGN activity on the local environment

To investigate the dependence of the occurrence of active galactic nuclei (AGNs) on local galaxy density, we study the nuclear properties of ~5000 galaxies in the Coma Supercluster whose density spans 2 orders of magnitude from the sparse filaments to the cores of the rich clusters. We obtained optical spectra of the nuclei of 177 galaxies using the 1.5m Cassini telescope, which are added to the 4785 spectra available from SDSS (DR7) to fill-in the incomplete coverage by SDSS of luminous galaxies. We perform a spectral classification of the nuclei of galaxies (with a completeness of 98% at r<17.77), classifying the nuclear spectra in six classes: three of them (SEY, sAGN, LIN) refer to AGNs and the remaining three (HII, RET, PAS) refer to different stages of starburst activity. To perform such classification, we use the WHAN diagnostic, after correcting Halpha by 1.3 A for underlying absorption. We find that 482 (10%) of 5027 galaxies host an AGN: their frequency strongly increases with increasing luminosity of the parent galaxies, such that 32% of galaxies with Log(i-Lum)<10.2 (Solar) harbor an AGN at their interior. In addition to their presence in luminous galaxies, AGNs are also found in red galaxies with < 1.15 \pm 0.15 mag. The majority of SEY and sAGN (strong AGNs) are associated with luminous late-type (or S0a) galaxies, while LIN (weak AGNs) and RET ("retired"), are mostly found among E/S0as. The number density of AGNs, HII region-like, and retired galaxies is found to anti-correlate with the local density of galaxies, such that their frequency drops by a factor of two near the cluster cores, while the frequency of galaxies containing passive nuclei increases by the same amount towards the center of rich clusters. The dependence of AGN number density on the local galaxy density is greater than the one implied by morphology segregation alone.

astro-ph.CO↗

arXiv2010

Constraining the ages of the fireballs in the wake of the dIrr galaxy VCC1217 / IC3418

A complex of Halpha emitting blobs with strong FUV excess is associated to the dIrr galaxy VCC1217 / IC3418 (Hester et al. 2010), and extends up to 17 Kpc in the South-East direction. These outstanding features can be morphologically divided into diffuse filaments and compact knots, where most of the star formation activity traced by Halpha takes place. We investigate the properties of the galaxy and the blobs using a multiwavelength approach in order to constrain their origin. We collect publicly available data in UV and Halpha and observe the scene in the optical U,g,r,i bands with LBT. The photometric data allows to evaluate the star formation rate and to perform a SED fitting separately of the galaxy and the blobs in order to constrain their stellar population age. Moreover we analyze the color and luminosity profile of the galaxy and its spectrum to investigate its recent interaction with the Virgo cluster. Our analysis confirms that the most plausible mechanism for the formation of the blobs is ram pressure stripping by the Virgo cluster IGM. The galaxy colors, luminosity profile and SED are consistent with a sudden gas depletion in the last few hundred Myr. The SED fitting of the blobs constrains their ages in < 400 Myr.

astro-ph.CO↗

arXiv2010

A snapshot on galaxy evolution occurring in the Great Wall: the role of Nurture at z=0

With the aim of quantifying the contribution of the environment on the evolution of galaxies at z=0 we have used the DR7 catalogue of the Sloan Digital Sky Survey (SDSS) to reconstruct the 3-D distribution of 4132 galaxies in 420 square degrees of the Coma supercluster, containing two rich clusters (Coma and A1367), several groups, and many filamentary structures belonging to the "Great Wall", at the approximate distance of 100 Mpc. At this distance the galaxy census is complete to Mi=-17.5 mag, i.e. approx 4 mag fainter than M*. The morphological classification of galaxies into early- (ellipticals) and late-types (spirals) was carried out by inspection of individual SDSS images and spectra. The density around each galaxies was determined in cylinders of 1 Mpc radius and 1000 km s^-1 half length. The color-luminosity relation was derived for galaxies in bins morphological type and in four thresholds of galaxy density-contrast, ranging from delta{1,1000} <= 0 (UL = the cosmic web); 0 < delta{1,1000} <= 4 (L = the loose groups); 4 < delta{1,1000} <= 20 (H = the large groups and the cluster's outskirts) and delta{1,1000} > 20 (UH = the cluster's cores). The fraction of early-type galaxies increases with the log of the over-density. A well defined "red sequence" composed of early-type galaxies exists in all environments at high luminosity, but it lacks of low luminosity (dwarf) galaxies in the lowest density environment. Conversely low luminosity isolated galaxies are predominantly of late-type. In other words the low luminosity end of the distribution is dominated by red dE galaxies in clusters and groups and by dwarf blue amorphous systems in the lowest density regions. At z=0 we find evidence for strong evolution induced by the environment (Nurture). Transformations take place mostly at low luminosity when star forming dwarf galaxies inhabiting low density environments migrate into amorphous passive dwarf ellipticals in their infall into denser regions. The mechanism involves suppression of the star formation due to gas stripping, without significant mass growth, as proposed by Boselli et al. (2008a). This process is more efficient and fast in ambients of increasing density. In the highest density environments (around clusters) the truncation of the star formation happens fast enough (few 100 Myr) to produce the signature of post-star-burst in galaxy spectra. PSB galaxies, that are in fact found significantly clustered around the largest dynamical units, represent the remnants of star forming isolated galaxies that had their star formation violently suppressed during their infall in clusters in the last 0.5-1.5 Gyrs, and the progenitors of future dEs.

astro-ph.CO↗

arXiv2009

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.

astro-ph.CO↗

arXiv2009

Downsizing among disk galaxies and the role of the environment

The study of PopI and PopII indicators in galaxies has a profound impact on our understanding of galaxy evolution. Their present (z=0) ratio suggests that the star formation history of galaxies was primarily dictated by their global mass. Since 1989 Luis Carrasco and I spent most of our sleepless nights gathering H_alpha and near infrared surface photometry of galaxies in the local Universe and focused most of our scientific career on these two indicators trying to convince the community that the mass was the key parameter to their evolution. We were unsuccessful, until in 2004 the Sloan team rediscovered this phenomenon and named it "downsizing"

astro-ph.GA↗

arXiv2008

HI content and other structural properties of galaxies in the Virgo cluster from the Arecibo Legacy Fast ALFA Survey

We report the results of an HI blind survey of 80deg^2 of the Virgo cluster, based on the 08deg < delta < 16 deg strip of ALFALFA, the Arecibo Legacy Fast ALFA Survey. 187 HI sources of high significance are found providing a complete census of HI sources in this region of the Virgo cluster (-1000 10^7.5-8 M_sun. 156/187 (83%) sources are identified with optical galaxies from the Virgo Cluster Catalogue (Binggeli et al. 1985), all but 8 with late-type galaxies. Ten sources are not associated with optical galaxies and were found to correspond to tidally-disrupted systems (see Kent et al. 2007 and Haynes, Giovanelli and Kent 2007). The remaining 21 (11%) are associated with galaxies that are not listed in the Virgo Cluster Catalogue. For all sources with an optical counterpart in the Sloan Digital Sky Survey, we analyzed i-band SDSS plates to measure optical structural parameters. We find that in the Virgo cluster: i) HI inhabits galaxies that are structurally similar to ordinary late-type galaxies; ii) their HI content can be predicted from their optical luminosity; iii) low surface brightness galaxies have low optical luminosity and contain small quantities of neutral hydrogen; iv) low surface brightness, massive Malin1 type galaxies are comfortably rare objects (less than 0.5%); v) there are no "dark-galaxies" with HI masses M_HI > 10^7.5-8 M_sun; vi) less than 1% of early-type galaxies contain neutral hydrogen with M_HI>10^7.5-8 M_sun (di Serego Alighieri et al. 2007).

astro-ph↗

arXiv2005

The trail of discrete X-ray sources in the early-type galaxy NGC4261: anisotropy in the globular cluster distribution?

Chandra images of the elliptical galaxy NGC4261 have revealed an anisotropy in the spatial distribution of the off-nuclear X-ray sources, interpreted by Zezas et al. as evidence of an association with a young stellar population. Our independent analysis of archive X-ray (Chandra) and optical (INT and HST) observations confirms the anisotropy of the X-ray sources but conducts to a different interpretation for their origin. We find that nearly half of the X-ray sources are associated to a globular cluster (optical counterpart) suggesting that they are accreting low-mass X-ray binaries. Where color index information is available, the X-ray sources are found to reside in red (metal-rich) systems. The luminosity function of the X-ray sources is also consistent with the one drawn from a population of Low Mass X-ray Binaries. We further investigate the properties of the sample of point-like sources obtained from archival optical images that we suggest are good globular cluster candidates and for which we find that the projected spatial distribution is non-homogeneous. In addition, we show that the distributions of the optical and X-ray populations are very similar, which leads us to conclude that the spatial anisotropy of the X-ray sources in NGC4261 is mostly a reflection of the anisotropy of the globular cluster population.

astro-ph↗

arXiv2004

Spectrophotometry of galaxies in the Virgo cluster.II: The data

Drift-scan mode (3600-6800 A) spectra with 500<R<1000 resolution are presented for 333 galaxies members to nearby clusters, covering the whole Hubble sequence. The majority (225) were obtained for galaxies in the Virgo cluster where a completeness of 36%, if all Hubble types are considered, and of 51%, restricting to late-types, was reached at m_p<=16. Our data can be therefore considered representative of the integrated spectral properties of giant and dwarf galaxies in this cluster. Intensities and equivalent widths (EWs) are derived for the principal lines, both in emission and in absorption. Deblending of the underlying absorption from emission was achieved in most cases.

astro-ph↗

arXiv2002

Spectro-photometry of galaxies in the Virgo cluster

As a result of an extensive observational campaign targeting the Virgo cluster, we obtained integrated (drift-scan mode) optical spectra and multiwavelength (UV,U,B,V,H) photometry for 124 and 330 galaxies, respectively, spanning the whole Hubble sequence, and with m_p<=16 (M_p<=-15). These data were combined to obtain galaxy SEDs extending from 2000 to 22000 A. By fitting these SEDs with synthetic ones derived using Bruzual & Charlot population synthesis models we try to constrain observationally the SFH of these galaxies. Assuming a Salpeter IMF and an analytical form for the SFH, the fit free parameters are: the age (T) of the star formation event, its characteristic time-scale (tau) and the initial metallicity (Z). We test the (simplistic) case where all galaxies have a common age T=13 Gyr, exploring a SFH with "delayed" exponential form, thus allowing for an increasing SFR with time. This SFH is consistent with the full range of observed SEDs, provided that the characteristic time-scale tau is let free to vary between 0.1 and 25 Gyr and Z between 1/5 and 2.5 Z solar. Elliptical galaxies (including dEs) are best fitted with short time scales (tau~3 Gyr) and Z varying between 1/5 and 1 Z solar. The model metallicity is found to increase with H band luminosity. Spiral galaxies require that both tau and Z correlate with H band luminosity: low mass Im+BCD have sub-solar Z and tau>=10 Gyr, whereas giant spirals have solar metallicities and tau~3 Gyr, consistent with ellipticals. Moreover we find that the SFH of spirals depends upon the presence at their interior of fresh gas capable of sustaining the star formation. In fact the residuals of the tau vs. L_H relation depend significantly on the HI content. HI deficient galaxies have shorter tau than spirals with normal HI content.

astro-ph↗

arXiv2000

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.

astro-ph↗