4,336 research outputs found
Galaxy Evolution in Local Group Analogs. I. A GALEX study of nearby groups
Understanding the astrophysical processes acting within galaxy groups and
their effects on the evolution of the galaxy population is one of the crucial
topic of modern cosmology, as almost 60% of galaxies in the Local Universe are
found in groups. We imaged in the far (FUV 1539 A) and near ultraviolet (NUV
2316 A) with GALEX three nearby groups, namely LGG93, LGG127 and LGG225. We
obtained the UV galaxy surface photometry and, for LGG225, the only group
covered by the SDSS, the photometry in u, g, r, i, z bands. We discuss galaxy
morphologies looking for interaction signatures and we analyze the SED of
galaxies to infer their luminosity-weighted ages. The UV and optical photometry
was also used to perform a kinematical and dynamical analysis of each group and
to evaluate the stellar mass. A few member galaxies in LGG225 show a distorted
UV morphology due to ongoing interactions. (FUV-NUV) colors suggest that
spirals in LGG93 and LGG225 host stellar populations in their outskirts younger
than that of M31 and M33 in the LG or with less extinction. The irregular
interacting galaxy NGC3447A has a significantly younger stellar population (few
Myr old) than the average of the other irregular galaxies in LGG225 suggesting
that the encounter triggered star formation. The early-type members of LGG225,
NGC3457 and NGC3522, have masses of the order of a few 10^9 Mo, comparable to
the Local Group ellipticals. For the most massive spiral in LGG225, we estimate
a stellar mass of ~4x10 Mo, comparable to M33 in the LG. Ages of stellar
populations range from a few to ~7 Gyr for the galaxies in LGG225. The
kinematical and dynamical analysis indicates that LGG127 and LGG225 are in a
pre-virial collapse phase, i.e. still undergoing dynamical relaxation, while
LGG93 is likely virialized. (Abridged)Comment: 20 pages, 13 figures, accepted for publication in Astronomy and
Astrophysic
Low redshift quasars in the SDSS Stripe 82. Host galaxy colors and close environment
We present a photometrical and morphological multicolor study of the
properties of low redshift (z<0.3) quasar hosts based on a large and
homogeneous dataset of quasars derived from the Sloan Digital Sky Survey (DR7).
We used quasars that were imaged in the SDSS Stripe82 that is up to 2 mag
deeper than standard Sloan images. This sample is part of a larger dataset of
~400 quasars at z<0.5 for which both the host galaxies and their galaxy
environments were studied (Falomo et al. 2014,Karhunen et al. 2014). For 52
quasars we undertake a study of the color of the host galaxies and of their
close environments in u,g,r,i and z bands. We are able to resolve almost all
the quasars in the sample in the filters g,r,i and z and also in for about
50% of the targets. We found that the mean colors of the QSO host galaxy
(g-i=0.82+-0.26; r-i=0.26+-0.16 and u-g=1.32+-0.25) are very similar to the
values of a sample of inactive galaxies matched in terms of redshift and galaxy
luminosity with the quasar sample. There is a suggestion that the most massive
QSO hosts have bluer colors.Both quasar hosts and the comparison sample of
inactive galaxies have candidates of close ( 50 kpc) companion galaxies for
~30% of the sources with no significant difference between active and inactive
galaxies. We do not find significant correlation between the central black hole
(BH) mass and the quasar host luminosity that appears to be extra luminous at a
given BH mass with respect to the local relation (M_BH -- M_host) for inactive
galaxies. This confirms previous suggestion that a substantial disc component,
not correlated to the BH mass, is present in the galaxies hosting low z
quasars. These results support a scenario where the activation of the nucleus
has negligible effects on the global structural and photometrical properties of
the hosting galaxies.Comment: Accepted for publication in MNRAS, 13 page
The evolution of the galactic morphological types in clusters
The morphological types of galaxies in nine clusters in the redshift range
0.1<z<0.25 are derived from very good seeing images taken at the NOT and the La
Silla Danish telescopes. With the purpose of investigating the evolution of the
fraction of different morphological types with redshift, we compare our results
with the morphological content of nine distant clusters studied by the MORPHS
group, five clusters observed with HST-WFPC2 at redshift z = 0.2-0.3, and
Dressler's (1980) large sample of nearby clusters. After having checked the
reliability of our morphological classification both in an absolute sense and
relative to the MORPHS scheme, we analyze the relative occurrence of
elliptical, S0 and spiral galaxies as a function of the cluster properties and
redshift. We find a large intrinsic scatter in the S0/E ratio, mostly related
to the cluster morphology. In particular, in our cluster sample, clusters with
a high concentration of ellipticals display a low S0/E ratio and, vice-versa,
low concentration clusters have a high S0/E. At the same time, the trend of the
morphological fractions and ratios with redshift clearly points to a
morphological evolution: as the redshift decreases, the S0 population tends to
grow at the expense of the spiral population, whereas the frequency of Es
remains almost constant. We also analyze the morphology-density (MD) relation
in our clusters and find that -similarly to higher redshift clusters- a good MD
relation exists in the high-concentration clusters, while it is absent in the
less concentrated clusters. Finally, the comparison of the MD relation in our
clusters with that of the D97 sample suggests that the transformation of
spirals into S0 galaxies becomes more efficient with decreasing local density.Comment: 24 pages including 11 figures and 4 tables, accepted for publication
in Ap
Are truncated stellar disks linked to the molecular gas density?
We know that the slope of the radial, stellar light distribution in galaxies
is well described by an exponential decline and this distribution is often
truncated at a break radius (). We don't have a clear understanding for
the origin of these outer truncations and several hypotheses have been proposed
to explain them. We want to test the various theories with direct observations
of the cold molecular gas for a few truncated galaxies in comparison with the
non-truncated ones. The answer to the existence of a possible link between
truncated stellar disks and the molecular gas density cannot be obtained from
CO maps in the literature, because so far there are no galaxies with a clear
truncation observed in CO at high resolution.Comment: 3 pages, 6 figures, to appear in Astrophysics and Space Science
(Apss), special issue of "Science with ALMA: a new era for Astrophysics"
conference, ed. Dr. Bachille
Rotation curves and metallicity gradients from HII regions in spiral galaxies
In this paper we study long slit spectra in the region of H emission
line of a sample of 111 spiral galaxies with recognizable and well defined
spiral morphology and with a well determined environmental status, ranging from
isolation to non-disruptive interaction with satellites or companions. The form
and properties of the rotation curves are considered as a function of the
isolation degree, morphological type and luminosity. The line ratios are used
to estimate the metallicity of all the detected HII regions, thus producing a
composite metallicity profile for different types of spirals. We have found
that isolated galaxies tend to be of later types and lower luminosity than the
interacting galaxies. The outer parts of the rotation curves of isolated
galaxies tend to be flatter than in interacting galaxies, but they show similar
relations between global parameters. The scatter of the Tully-Fisher relation
defined by isolated galaxies is significantly lower than that of interacting
galaxies. The [NII]/H ratios, used as metallicity indicator, show a
clear trend between Z and morphological type, t, with earlier spirals showing
larger ratios; this trend is tighter when instead of t the gradient of the
inner rotation curve, G, is used; no trend is found with the interaction
status. The Z-gradient of the disks depends on the type, being almost flat for
early spirals, and increasing for later types. The [NII]/H ratios
measured for disk HII regions of interacting galaxies are higher than for
normal/isolated objects, even if all the galaxy families present similar
distributions of H Equivalent Width.Comment: accepted for publication in A&A (tables for HII region parameters
incomplete, contact [email protected] for the whole set of tables
The UV window on counter rotating ETGs: insight from SPH simulations with chemo-photometric implementation
The Galaxy Evolution Explorer (GALEX) detected ultraviolet emission in about
50% of multi-spin early-type galaxies (ETGs), suggesting the occurrence of a
recent rejuvenation episode connected to the formation of these kinematical
features. With the aim at investigating the complex evolutionary scenario
leading to the formation of counter rotating ETGs (CR-ETGs) we use our Smooth
Particle Hydrodynamic (SPH) code with chemo-photometric implementation. We
discuss here the UV evolutionary path of two CR-ETGs, NGC 3593 and NGC 5173,
concurrently best fitting their global observed properties, i.e., morphology,
dynamics, as well as their total B-band absolute magnitude and spectral energy
distribution (SED) extended over three orders of magnitude in wavelength. These
simulations correspond to our predictions about the target evolution which we
follow in the color-magnitude diagram (CMD), near-UV (NUV) versus r-band
absolute magnitude, as a powerful diagnostic tool to emphasize rejuvenation
episodes.Comment: 7 pages, 3 figures, accepted for publication in ApS
Passive spiral formation from halo gas starvation: Gradual transformation into S0s
Recent spectroscopic and high resolution -imaging observations have
revealed significant numbers of ``passive'' spiral galaxies in distant
clusters, with all the morphological hallmarks of a spiral galaxy (in
particular, spiral arm structure), but with weak or absent star formation.
Exactly how such spiral galaxies formed and whether they are the progenitors of
present-day S0 galaxies is unclear. Based on analytic arguments and numerical
simulations of the hydrodynamical evolution of a spiral galaxy's halo gas
(which is a likely candidate for the source of gas replenishment for star
formation in spirals), we show that the origin of passive spirals may well be
associated with halo gas stripping. Such stripping results mainly from the
hydrodynamical interaction between the halo gas and the hot intracluster gas.
Our numerical simulations demonstrate that even if a spiral orbits a cluster
with a pericenter distance 3 times larger than the cluster core radius,
80 % of the halo gas is stripped within a few Gyr and, accordingly,
cannot be accreted by the spiral. Furthermore, our study demonstrates that this
dramatic decline in the gaseous infall rate leads to a steady increase in the
parameter for the disk, with the spiral arm structure, although persisting,
becoming less pronounced as the star formation rate gradually decreases. These
results suggest that passive spirals formed in this way, gradually evolve into
red cluster S0s.Comment: 13 pages 4 figures (fig.1 = jpg format), accepted by Ap
Evolution of Galaxy morphologies in Clusters
We have studied the evolution of galaxian morphologies from ground-based,
good-seeing images of 9 clusters at z=0.09-0.25. The comparison of our data
with those relative to higher redshift clusters (Dressler et al. 1997) allowed
us to trace for the first time the evolution of the morphological mix at a
look-back time of 2-4 Gyr, finding a dependence of the observed evolutionary
trends on the cluster properties.Comment: 4 pages with 2 figures in Latex-Kluwer style. To be published in the
proceedings of the Granada Euroconference "The Evolution of
Galaxies.I-Observational Clues
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