93 research outputs found
The star formation properties of disk galaxies: Halpha imaging of galaxies in the Coma supercluster
We present integrated H alpha measurements obtained from imaging observations
of 98 late-type galaxies, primarily selected in the Coma supercluster. These
data, combined with H alpha photometry from the literature, include a magnitude
selected sample of spiral (Sa to Irr) galaxies belonging to the "Great Wall"
complete up to mp=15.4, thus composed of galaxies brighter than Mp=-18.8
(H0=100 km Mpc^-1 s^-1). The frequency distribution of the H alpha E.W.,
determined for the first time from an optically complete sample, is
approximately gaussian peaking at E.W. ~25 A. We find that, at the present
limiting luminosity, the star formation properties of spiral+Irr galaxies
members of the Coma and A1367 clusters do not differ significantly from those
of the isolated ones belonging to the Great Wall. The present analysis confirms
the well known increase of the current massive star formation rate (SFR) with
Hubble type. Moreover perhaps a more fundamental anticorrelation exists between
the SFR and the mass of disk galaxies: low-mass spirals and dwarf systems have
present SFRs ~50 times higher than giant spirals. This result is consistent
with the idea that disk galaxies are coeval, evolve as "closed systems" with
exponentially declining SFR and that the mass of their progenitor protogalaxies
is the principal parameter governing their evolution. Massive systems having
high initial efficiency of collapse, or a short collapse time-scale, have
retained little gas to feed the present epoch of star formation. These findings
support the conclusions of Gavazzi & Scodeggio (1996) who studyed the
color-mass relation of a local galaxy sample and agree with the analysis by
Cowie et al. (1996) who traced the star formation history of galaxies up to
z>1.Comment: 13 pages (LateX) + 24 figures + 4 tables. To appear in Astronomical
Journal, April 1998 issu
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&
Red-channel (6000-8000 {\AA}) nuclear spectra of 376 local galaxies
We obtained long-slit optical spectra of the nuclear regions of 376 galaxies
in the local Universe using the 1.5m Cassini telescope of Bologna Observatory.
Of these spectra, 164 were either never taken before by the Sloan Digital Sky
Survey (SDSS), or given by the Nasa Extragalactic Database (NED). With these
new spectra, we contribute investigating the occurrence of active galactic
nuclei (AGNs). Nevertheless, we stress that the present sample is by no means
complete, thus, it cannot be used to perform any demographic study. Following
the method presented in Gavazzi et al. (2011), we classify the nuclear spectra
using a six bin scheme: SEY (Seyfert), sAGN (strong AGN), and wAGN (weak AGN)
represent active galactic nuclei of different levels of activity; HII accounts
for star-forming nuclei; RET (retired) and PAS (passive) refer to nuclei with
poor or no star-formation activity. The spectral classification is performed
using the ratio of 6584 {\lambda} [NII] to H{\alpha} lines and the equivalent
width (EW) of H{\alpha} versus [NII]/H{\alpha} (WHAN diagnostic introduced by
Cid Fernandes and collaborators) after correcting H{\alpha} for underlying
absorption. The obtained spectra are made available in machine readable format
via the Strasbourg Astronomical Data Center (CDS) and NED.Comment: 8 pages, 6 Figures, 4 Tables, accepted for publication in Astronomy &
Astrophysic
A complete spectroscopic catalogue of local galaxies in the Northern spring sky -- Gas properties and nuclear activity in different environments
With the aim of providing the complete demography of galaxies in the local
Universe, including their nuclear properties, we present SPRING, a complete
census of local galaxies limited to the spring quarter of the Northern sky
(10h< RA <16h; 0< Dec <65). The SPRING catalogue is a flux- and volume-limited
sample (r < 17.7 mag, cz < 10000 km/s) of 30597 galaxies, including the Virgo,
Coma and A1367 clusters. To inspect possible secular and environmental
dependencies of the various nuclear excitation properties (SF vs. AGN), we
perform a multidimensional analysis by dividing the sample according to (i)
their position in the (NUV-i) vs. M* diagram,(ii) local galaxy density, (iii)
stellar-mass, (iv) halo-mass of the group to which galaxies belong, and (v)
neutral Hydrogen content. We present a new calibration of the optical
diameter-based HI-deficiency parameter employing a reference sample of isolated
galaxies. At intermediate distances between Virgo and Coma, we identify a
ring-like structure of galaxies constituted by three large filaments. The
fraction of HI-deficient galaxies within the filament suggests that filaments
are a transitioning environment between field and cluster in terms of HI
content. We classify the nuclear spectra according to the four-line BPT and the
two-line WHAN diagrams, and investigate the variation in the fraction of AGN
with stellar-mass, as well as their colours and environments. In general, we
observe that the mass-dependency of the fraction of Seyfert nuclei is little
sensitive to the environment, whereas the fraction of star-forming nuclei is a
steeper function of M* in lower-density environments and in blue-cloud
galaxies. We find that the fraction of LINERs depends on galaxy colour and, for
logM* > 9.5-10, increases in galaxies belonging to the green valley.Comment: 26 pages, 22 figures. Accepted for publication in Astronomy &
Astrophysic
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