81 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
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
1.65 micron H-band Surface Photometry of Galaxies. X: Structural and Dynamical Properties of Elliptical Galaxies
We analyse the structural and dynamical properties of a sample of 324 nearby
elliptical and dwarf elliptical galaxies observed during an extensive NIR
survey in H-band (1.65 micron). The Fundamental Plane (FP) is determined and a
significant tilt is assessed. The origins of such a tilt are investigated by
means of a spherically symmetric, isotropic pressure supported dynamical model
relying on the observed surface brightness profiles. The systematic variation
of the shape coefficient converting the measured central velocity dispersion into the virial rms velocity is found to be the main
cause of the tilt, due to aperture effects. Moreover the ratio between the
dynamical mass and the total H-band luminosity turns out to be
roughly constant along the luminosity sequence of ellipticals: H-band
luminosity is therefore a reliable and cheap estimator of the dynamical mass of
the Es.Comment: 28 pages, 6 figures, accepted for publication on Ap
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.Comment: 7 pages, 2 figures, 3 tables, Accepted for publication in Astronomy
and Astrophysics (18.07.12
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