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Fossati M.,Max Planck Institute for Extraterrestrial Physics | Fossati M.,University of Milan Bicocca | Gavazzi G.,University of Milan Bicocca | Savorgnan G.,Swinburne University of Technology | And 7 more authors.
Astronomy and Astrophysics | Year: 2013

Context. We present the analysis of the galaxy structural parameters from Ha3, an Ha narrow-band imaging follow-up survey of ∼800 galaxies selected from the HI Arecibo Legacy Fast ALFA Survey (ALFALFA) in the Local supercluster, including the Virgo cluster, and in the Coma supercluster. Aims.Taking advantage of Ha3, 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 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. Methods.We computed the concentration, asymmetry, and clumpiness (CAS) structural parameters for recently born and old stars. To quantify the sizes we computed half-light radii and a new parameter dubbed EW/r based on the half-light radius of the Ha equivalent width map. To highlight the environmental perturbation, we adopt an updated calibration of the HI-deficiency parameter (DefHI) that we use to divide the sample in unperturbed galaxies (DefHI = 0.3) and perturbed galaxies (DefHI > 0.3). Results.The concentration index computed in the r band depends on the stellar mass and on the Hubble type these variables are related because most massive galaxies are bulge dominated therefore highly concentrated. Going toward later spirals and irregulars the concentration index and the mass decrease along with the bulge-to-disk ratio. Blue compact dwarfs (BCDs) are an exception because they have similar mass, but they are more concentrated than dwarf irregulars. The asymmetry and the clumpiness increase along the spiral sequence up to Sc-Sd, but they decrease going in the dwarf regime, where the light distribution is smooth and more symmetric. When measured on Ha images, the CAS parameters show no obvious correlations with Hubble type. Irrespective of whether we used the ratio between effective radii or the EW/r parameter, 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. © 2013 ESO.


Gavazzi G.,University of Milan Bicocca | Savorgnan G.,University of Milan Bicocca | Savorgnan G.,Swinburne University of Technology | Fossati M.,University of Milan Bicocca | And 9 more authors.
Astronomy and Astrophysics | Year: 2013

Context. We present the analysis of Hα3, an Hα narrow-band imaging follow-up survey of galaxies selected from the HI Arecibo Legacy Fast ALFA Survey (ALFALFA) in the Coma supercluster. Aims. Taking advantage of Hα3, which provides the complete census of the recent star formation in HI-rich galaxies in the local universe, we explored the hypothesis that a morphological sequence of galaxies of progressively earlier type and lower gas-content exists in the neighborhood of a rich cluster of galaxies such as Coma, with a specific star formation activity that decreases with increasing local galaxy density and velocity dispersion. Methods. By using the Hα hydrogen recombination line as a tracer of the "instantaneous" star formation, complemented with optical colors from SDSS, we investigated the relationships between atomic neutral gas and newly formed stars in different local galaxy density intervals, for many morphological types, and over a wide range of stellar masses (109 to 1011.5 M ·). Results. In the dwarf regime (8.5 < log (M ·-/M·) < 9.5) we identify a four-step sequence of galaxies with progressively redder colors (corrected for dust extinction), i.e., of decreasing specific star formation, from (1) HI-rich late-type galaxies (LTGs) belonging to the blue cloud that exhibit extended plus nuclear star formation, (2) ∼0.1 mag redder, HI-poor LTGs with nuclear star formation only, (3) ∼0.35 mag redder, HI-poor galaxies without either extended or nuclear star formation, but with nuclear post-star-burst (PSB) signature, (4) ∼ 0.5 mag redder early-type galaxies (ETGs) that belong to the red sequence, and show 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 from increasing the local galaxy density, from cosmic filaments (1 2), to the rich clusters (2 3 4). Conclusions. In the dwarf regime we find evidence for an evolution of HI-rich LTGs into ETGs through HI-poor LTGs 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 galaxies has proceeded for the last 7.5 Gyr, building up the Coma cluster at a rate of approximately 100 galaxies with log (Mâ̂-/M·) > 9.0 per Gyr. © 2013 ESO.


Gavazzi G.,University of Milan Bicocca | Consolandi G.,University of Milan Bicocca | Dotti M.,University of Milan Bicocca | Dotti M.,National Institute of Nuclear Physics, Italy | And 11 more authors.
Astronomy and Astrophysics | Year: 2015

A growing body of evidence indicates that the star formation rate per unit stellar mass (sSFR) decreases with increasing mass in normal main-sequence star-forming galaxies. Many processes have been advocated as being responsible for this trend (also known as mass quenching), e.g., feedback from active galactic nuclei (AGNs), and the formation of classical bulges. In order to improve our insight into the mechanisms regulating the star formation in normal star-forming galaxies across cosmic epochs, we determine a refined star formation versus stellar mass relation in the local Universe. To this end we use the Hα narrow-band imaging follow-up survey (Hα3) of field galaxies selected from the HI Arecibo Legacy Fast ALFA Survey (ALFALFA) in the Coma and Local superclusters. By complementing this local determination with high-redshift measurements from the literature, we reconstruct the star formation history of main-sequence galaxies as a function of stellar mass from the present epoch up to z = 3. In agreement with previous studies, our analysis shows that quenching mechanisms occur above a threshold stellar mass Mknee that evolves with redshift as ∞ (1 + z)2. Moreover, visual morphological classification of individual objects in our local sample reveals a sharp increase in the fraction of visually classified strong bars with mass, hinting that strong bars may contribute to the observed downturn in the sSFR above Mknee. We test this hypothesis using a simple but physically motivated numerical model for bar formation, finding that strong bars can rapidly quench star formation in the central few kpc of field galaxies. We conclude that strong bars contribute significantly to the red colors observed in the inner parts of massive galaxies, although additional mechanisms are likely required to quench the star formation in the outer regions of massive spiral galaxies. Intriguingly, when we extrapolate our model to higher redshifts, we successfully recover the observed redshift evolution for Mknee. Our study highlights how the formation of strong bars in massive galaxies is an important mechanism in regulating the redshift evolution of the sSFR for field main-sequence galaxies. © ESO, 2015.


Gavazzi G.,University of Milan Bicocca | Consolandi G.,University of Milan Bicocca | Viscardi E.,University of Milan Bicocca | Fossati M.,Universitats Sternwarte Munich | And 8 more authors.
Astronomy and Astrophysics | Year: 2015

Neutral hydrogen represents the major observable baryonic constituent of galaxies that fuels the formation of stars through transformation of molecular hydrogen. The emission of the hydrogen recombination line Hα is the most direct tracer of the process that transforms gas (fuel) into stars. We continue to present Hα3 (acronym for Hα - αα), an extensive Hα+[NII] narrow-band imaging campaign of galaxies selected from the HI Arecibo Legacy Fast ALFA Survey (ALFALFA), using the instrumentation available at the San Pedro Martir (SPM) observatory (Mexico). In 48 nights since 2011 we were able to carry out the Hα imaging observations of 724 galaxies in the region of the Coma supercluster 10h < RA < 16h; 24° < Dec < 28° and 3900 < cz < 9000 km s-1. Of these, 603 are selected from the ALFALFA and constitute a 97% complete sample. They provide for the first time a complete census of the massive star formation properties of local gas-rich galaxies belonging to different environments (cluster vs. filaments), morphological types (spirals vs. dwarf Irr), over a wide range of stellar mass (∼ 108 - 1011.5 M⊙) in the Coma supercluster. The present Paper V provides Hα data and derived star formation rates for the observed galaxies. © ESO, 2015.


Gavazzi G.,University of Milan Bicocca | Fumagalli M.,University of California at Santa Cruz | Galardo V.,University of Milan Bicocca | Grossetti F.,University of Milan Bicocca | And 4 more authors.
Astronomy and Astrophysics | Year: 2012

Context. We present Ha3 (acronym for Ha- aa), an Ha 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. Aims. 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 (̃107.5-1011.5 M). Methods. We image in Ha+[NII] all the galaxies that contain more than 107 M of neutral atomic hydrogen in the sky region 11h>R A>16h;4°>Dec>16>;350>cz>2000 km s-1 using the San Pedro Martir 2 m telescope. This survey provides a complete census of the star formation in HI rich galaxies of the local universe. Results. We present the properties of the galaxy sample, together with Ha 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 Ha fluxes corrected for galactic and internal extinction and for [NII] contamination we derive the global star formation rates (SFRs). © ESO 2012.


Gavazzi G.,University of Milan Bicocca | Fumagalli M.,Carnegie Observatories | Fumagalli M.,Princeton University | Fossati M.,University of Milan Bicocca | And 6 more authors.
Astronomy and Astrophysics | Year: 2013

Context.We present the analysis of Ha3, an Ha narrow-band imaging follow-up survey of 409 galaxies selected from the HI Arecibo Legacy Fast ALFA Survey (ALFALFA) in the Local Supercluster, including the Virgo cluster, in the region 11h

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