89 research outputs found
A Dimensional study of Disk Galaxies
We present a highly simplified model of the dynamical structure of a disk
galaxy where only two parameters fully determine the solution, mass and angular
momentum. We show through simple physical scalings that once the mass has been
fixed, the angular momentum parameter is expected to regulate such
critical galactic disk properties as colour, thickness of the disk and disk to
bulge ratio. It is hence expected to be the determinant physical ingredient
resulting in a given Hubble type. A simple analytic estimate of for
an observed system is provided. An explicit comparison of the distribution of
several galactic parameters against both Hubble type and is performed
using observed galaxies. Both such distributions exhibit highly similar
characteristics for all galactic properties studied, suggesting as a
physically motivated classification parameter for disk galaxies.Comment: 10 pages including 11 figures, Final version, MNRAS in pres
Empirical distributions of galactic spin parameters from the SDSS
Using simple dimensional arguments for both spiral and elliptical galaxies,
we present formulas to derive an estimate of the halo spin parameter
for any real galaxy, in terms of common observational parameters. This allows a
rough estimate of , which we apply to a large volume limited sample of
galaxies taken from the SDSS data base. The large numbers involved (11,597)
allow the derivation of reliable distributions, as signal adds up
significantly in spite of the errors in the inferences for particular galaxies.
We find that if the observed distribution of is modeled with a
log-normal function, as often done for this distribution in dark matter halos
that appear in cosmological simulations, we obtain parameters and , interestingly consistent with
values derived from simulations. For spirals, we find a good correlation
between empirical values of and visually assigned Hubble types,
highlighting the potential of this physical parameter as an objective
classification tool.Comment: 8 pages, 6 figures, expanded final version, MNRAS (in press
Quantifying galactic morphological transformations in the cluster environment
We study the effects of the cluster environment on galactic morphology by
defining a dimensionless angular momentum parameter , to obtain a
quantitative and objective measure of galaxy type. The use of this physical
parameter allows us to take the study of morphological transformations in
clusters beyond the measurements of merely qualitative parameters, e.g. S/E
ratios, to a more physical footing. To this end, we employ an extensive Sloan
Digital Sky Survey sample (Data Release 7), with galaxies associated with Abell
galaxy clusters. The sample contains 121 relaxed Abell clusters and over 51,000
individual galaxies, which guarantees a thorough statistical coverage over a
wide range of physical parameters. We find that the median value
tends to decrease as we approach the cluster center, with different dependences
according to the mass of the galaxies and the hosting cluster; low and
intermediate mass galaxies showing a strong dependence, while massive galaxies
seems to show, at all radii, low values. By analysing trends in
as functions of the nearest neighbour environment, clustercentric
radius and velocity dispersion of clusters, we can identify clearly the leading
physical processes at work. We find that in massive clusters (
km/s), the interaction with the cluster central region dominates, whilst in
smaller clusters galaxy-galaxy interactions are chiefly responsible for driving
galactic morphological transformations.Comment: 10 pages, 6 figures. Accepted for publication in MNRA
Clues on the origin of galactic angular momentum from looking at galaxy pairs
We search for correlations between the spin in pairs of spiral galaxies, to
study if the angular momentum gain for each galaxy was the result of tidal
torques imprint by the same tidal field. To perform our study we made use of a
sample of galaxy pairs identified using the Sloan Digital Sky Survey. We find a
weak, but statistically significant correlation between the spin magnitude of
neighbouring galaxies, but no clear alignment between their orientation. We
show that events such as interactions with close neighbours play an important
role in the value of the spin for the final configuration, as we find these
interactions tend to reduce the value of the spin parameter of
late-type galaxies considerably, with dependence on the morphology of the
neighbour. This implies that the original tidal field for each pair could have
been similar, but the redistribution of angular momentum at later stages of
evolution is important.Comment: 10 pages, 4 figures, 1 table. Replaced to match the version accepted
for publication in MNRA
On the nature of Delta Scuti star HD 115520
Observing Delta Scuti stars is most important as their multi-frequency
spectrum of radial pulsations provide strong constraints on the physics of the
stars interior; so any new detection and observation of these stars is a
valuable contribution to asteroseismology. While performing uvby-beta
photoelectric photometry of some RR Lyrae stars acquired in 2005 at the
Observatorio Astronomico Nacional, Mexico, we also observed several standard
stars, HD115520 among them. After the reduction this star showed indications of
variability. In view of this, a new observing run was carried out in 2006
during which we were able to demonstrate its variability and its nature as a
Delta Scuti star. New observations in 2007 permitted us to determine its
periodic content with more accuracy. This, along with the uvby-beta
photoelectric photometry allowed us to deduce its physical characteristics and
pulsational modes.Comment: 6 pages. To be published in Journal of Physics: Conference Series.
Proc. of the workshop Helioseismology, Asteroseismology and MHD Connections
held in Gottingen, Germany, 20-24 August 200
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