444 research outputs found
Bar pattern speed evolution over the last 7 Gyr
The tumbling pattern of a bar is the main parameter characterising its
dynamics. From numerical simulations, its evolution since bar formation is
tightly linked to the dark halo in which the bar is formed through dynamical
friction and angular momentum exchange. Observational measurements of the bar
pattern speed with redshift can restrict models of galaxy formation and bar
evolution. We aim to determine, for the first time, the bar pattern speed
evolution with redshift based on morphological measurements. We have selected a
sample of 44 low inclination ringed galaxies from the SDSS and COSMOS surveys
covering the redshift range 0 <z< 0.8 to investigate the evolution of the bar
pattern speed. We have derived morphological ratios between the deprojected
outer ring radius (R_{ring}) and the bar size (R_{bar}). This quantity is
related to the parameter {\cal R}=R_{CR}/R_{bar} used for classifiying bars in
slow and fast rotators, and allow us to investigate possible differences with
redshift. We obtain a similar distribution of at all redshifts. We do not
find any systematic effect that could be forcing this result. The results
obtained here are compatible with both, the bulk of the bar population (~70%)
being fast-rotators and no evolution of the pattern speed with redshift. We
argue that if bars are long-lasting structures, the results presented here
imply that there has not been a substantial angular momentum exchange between
the bar and halo, as predicted by numerical simulations. In consequence, this
might imply that the discs of these high surface-brightness galaxies are
maximal.Comment: Accepted for publication in A&
Properties of bars in the local universe
We studied the fraction and properties of bars in a sample of about 3000
galaxies extracted from SDSS-DR5. This represents a volume limited sample with
galaxies located between redshift 0.01-20, and
inclination i < 60. Interacting galaxies were excluded from the sample. The
fraction of barred galaxies in our sample is 45%. We found that 32% of S0s, 55%
of early-type spirals, and 52% of late-type spirals are barred galaxies. The
bars in S0s galaxies are weaker than those in later-type galaxies. The bar
length and galaxy size are correlated, being larger bars located in larger
galaxies. Neither the bar strength nor bar length correlate with the local
galaxy density. On the contrary, the bar properties correlate with the
properties of their host galaxies. Galaxies with higher central light
concentration host less and weaker bars.Comment: 2 pages, 1 figure to appear in the proceedings of "Formation and
Evolution of Galaxy Disks", Rome, October 2007, Eds. J. Funes and E. M.
Corsin
Deep spectroscopy in nearby galaxy clusters: III Orbital structure of galaxies in Abell 85
Galaxies in clusters are strongly affected by their environment. They evolve
according to several physical mechanisms that are active in clusters. Their
efficiency can strongly depend on the orbital configuration of the galaxies.
Our aim is to analyse the orbits of the galaxies in the cluster Abell 85, based
on the study of the galaxy velocity anisotropy parameter. We have solved the
Jeans equation under the assumption that the galaxies in A85 are collisionless
objects, within the spherically symmetric gravitational potential of the
virialized cluster. The mass of the cluster was estimated with X-ray and
caustic analyses. We find that the anisotropy profile of the full galaxy
population in A85 is an increasing monotonic function of the distance from the
cluster centre: on average, galaxies in the central region (r/r200 < 0.3) are
on isotropic orbits, while galaxies in the outer regions are on radial orbits.
We also find that the orbital properties of the galaxies strongly depend on
their stellar colour. In particular, blue galaxies are on less radial orbits
than red galaxies. The different families of cluster galaxies considered here
have the pseudo phase-space density profiles Q(r) and Qr(r) consistent with the
profiles expected in virialized dark matter halos in -body simulations. This
result suggests that the galaxies in A85 have reached dynamical equilibrium
within the cluster potential. Our results indicate that the origin of the blue
and red colour of the different galaxy populations is the different orbital
shape rather than the accretion time.Comment: 15 pages, 15 figures. Accepted for publication at MNRA
Structural properties of disk galaxies I. The intrinsic ellipticity of bulges
(Abridged) A variety of formation scenarios was proposed to explain the
diversity of properties observed in bulges. Studying their intrinsic shape can
help in constraining the dominant mechanism at the epochs of their assembly.
The structural parameters of a magnitude-limited sample of 148 unbarred S0--Sb
galaxies were derived in order to study the correlations between bulges and
disks as well as the probability distribution function (PDF) of the intrinsic
equatorial ellipticity of bulges. It is presented a new fitting algorithm
(GASP2D) to perform the two-dimensional photometric decomposition of galaxy
surface-brightness distribution. This was assumed to be the sum of the
contribution of a bulge and disk component characterized by elliptical and
concentric isophotes with constant (but possibly different) ellipticity and
position angles. Bulge and disk parameters of the sample galaxies were derived
from the J-band images which were available in the Two Micron All Sky Survey.
The PDF of the equatorial ellipticity of the bulges was derived from the
distribution of the observed ellipticities of bulges and misalignments between
bulges and disks. Strong correlations between the bulge and disk parameters
were found. About 80% of bulges in unbarred lenticular and
early-to-intermediate spiral galaxies are not oblate but triaxial ellipsoids.
Their mean axial ratio in the equatorial plane is = 0.85. There is not
significant dependence of their PDF on morphology, light concentration, and
luminosity. The interplay between bulge and disk parameters favors scenarios in
which bulges assembled from mergers and/or grew over long times through disk
secular evolution. But all these mechanisms have to be tested against the
derived distribution of bulge intrinsic ellipticities.Comment: 24 pages, 13 figures, accepted for publication in A&A, corrected
proof
Optical Surface Photometry of a Sample of Disk Galaxies. II Structural Components
This work presents the structural decomposition of a sample of 11 disk
galaxies, which span a range of different morphological types. The U, B, V, R,
and I photometric information given in Paper I (color and color-index images
and luminosity, ellipticity, and position-angle profiles) has been used to
decide what types of components form the galaxies before carrying out the
decomposition. We find and model such components as bulges, disks, bars, lenses
and rings.Comment: 14 figures. Accepted for publication in A&
Molecular gas in low-metallicity starburst galaxies: Scaling relations and the CO-to-H conversion factor
We study the molecular content and the star formation efficiency of 21 Blue
Compact Dwarfs (BCDs). We present CO(1-0) and (2-1) observations, further
supplemented with additional CO measurements and multiwavelength ancillary data
from the literature. We find the CO luminosity to be correlated with the
stellar and HI masses, SFR tracers, the size of the starburst and its
metallicity. BCDs appear offset from the Schmidt-Kennicutt (SK) law, showing
extremely low (0.1 Gyr) H2 and H2+HI depletion timescales. The
departure from the SK law is smaller when considering H2+HI rather than H2
only, and is larger for BCDs with lower metallicity and higher specific SFR.
Thus, the molecular fraction and H2 depletion timescale of BCDs is found to be
strongly correlated with metallicity. Using this and assuming that the
empirical correlation found between the specific SFR and galaxy-averaged H2
depletion timescale of more metal-rich galaxies extends to lower masses, we
derive a metallicity-dependent CO-to-H2 conversion factor , with in qualitative agreement
with previous determinations, dust-based measurements, and recent model
predictions. Our results suggest that in vigorously star-forming dwarfs the
fraction of H2 traced by CO decreases by a factor of about 40 from to , leading to a strong underestimation of
the H2 mass in metal-poor systems when a Galactic is
considered. Adopting we find that departures from the SK law
are partially resolved. Our results suggest that starbursting dwarfs have
shorter depletion gas timescales and lower molecular fractions compared to
normal late-type disc galaxies even accounting for the molecular gas not traced
by CO emission in metal-poor environments, raising additional constraints to
model predictions (Abridged).Comment: 18 pages, 14 Figures, 4 Tables: Accepted for publication in A&
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