294 research outputs found
The AMIGA sample of isolated galaxies: VIII. The rate of asymmetric HI profiles in spiral galaxies
(abridged) Measures of the HI properties of a galaxy are among the most
sensitive interaction diagnostic at our disposal. We report here on a study of
HI profile asymmetries (e.g., lopsidedness) in a sample of some of the most
isolated galaxies in the local Universe. This presents us with an excellent
opportunity to quantify the range of intrinsic HI asymmetries and provides us
with a zero-point calibration for evaluating these measurements in less
isolated samples. We characterize the HI profile asymmetries and search for
correlations between HI asymmetry and their environments, as well as their
optical and far infrared (FIR) properties. We use high signal-to-noise global
HI profiles for galaxies in the AMIGA project (http://amiga.iaa.csic.es). We
restrict our study to N=166 galaxies with accurate measures of the HI shape
properties. We quantify asymmetries using a flux ratio parameter. The asymmetry
parameter distribution of our isolated sample is well described by a Gaussian
model. The width of the distribution is sigma=0.13, and could be even smaller
(sigma=0.11) if instrumental errors are reduced. Only 2% of our carefully
vetted isolated galaxies sample show an asymmetry in excess of 3sigma. By using
this sample we minimize environmental effects as confirmed by the lack of
correlation between HI asymmetry and tidal force (one-on-one interactions) and
neighbor galaxy number density. On the other hand, field galaxy samples show
wider distributions and deviate from a Gaussian curve. As a result we find
higher asymmetry rates (~10-20%) in such samples. We find evidence that the
spiral arm strength is inversely correlated with the HI asymmetry. We also find
an excess of FIR luminous galaxies with larger HI asymmetries that may be
spirals associated with hidden accretion events. Our sample presents the
smallest fraction of asymmetric HI profiles compared with any other yet
studied.Comment: 18 pages, 17 figures, accepted for publication in A&
Energy versus information based estimations of dissipation using a pair of magnetic colloidal particles
Using the framework of stochastic thermodynamics, we present an experimental
study of a doublet of magnetic colloidal particles which is manipulated by a
time-dependent magnetic field. Due to hydrodynamic interactions, each bead
experiences a state-dependent friction, which we characterize using a
hydrodynamic model. In this work, we compare two estimates of the dissipation
in this system: the first one is energy based since it relies on the measured
interaction potential, while the second one is information based since it uses
only the information content of the trajectories. While the latter only offers
a lower bound of the former, we find it to be simple to implement and of
general applicability to more complex systems.Comment: Main text: 5 pages, 4 figures. Supplementary material: 5 pages, 5
figure
Star formation in isolated AMIGA galaxies: dynamical influence of bars
Star formation depends strongly both on the local environment of galaxies,
and on the internal dynamics of the interstellar medium. To disentangle the two
effects, we obtained, in the framework of the AMIGA project, Ha and Gunn r
photometric data for more than 200 spiral galaxies lying in very low-density
regions of the local Universe. We characterise the Ha emission, tracing current
star formation, of the 45 largest and less inclined galaxies observed for which
we estimate the torques between the gas and the bulk of the optical matter. We
could subsequently study the Ha morphological aspect of these isolated spiral
galaxies. Using Fourier analysis, we focus on the modes of the spiral arms and
also on the strength of the bars, computing the torques between the gas and
newly formed stars (Ha) and the bulk of the optical matter (Gunn r). We
interpret the various bar/spiral morphologies observed in terms of the secular
evolution experienced by galaxies in isolation. We also classify the different
spatial distributions of star forming regions in barred galaxies. The observed
frequency of particular patterns brings constraints on the lifetime of the
various evolution phases. We propose an evolutive sequence accounting for the
transitions between the different phases we could observe. Isolated galaxies
appear not to be preferentially barred or unbarred. Through numerical
simulations, trying to fit the Ha distributions yields constraints on the star
formation law, which is likely to differ from a genuine Schmidt law. In
particular, it is probable that the relative velocity of the gas in the bar
also needs to be taken into account.Comment: 15 pages, 9 figures (low resolution), 2 tables, accepted by A&
Search For Companions Of Nearby Isolated Galaxies
The radial velocities are measured for 45 galaxies located in the
neighborhoods of 29 likely isolated galaxies in a new catalog. We find that
about 85% of these galaxies actually are well isolated objects. 4% of nearby
galaxies with V_LG<3500 km/s are this kind of cosmic "orphan".Comment: 7 pages, 2 figure
Setting the normalcy level of HI properties in isolated galaxies
Studying the atomic gas (HI) properties of the most isolated galaxies is
essential to quantify the effect that the environment exerts on this sensitive
component of the interstellar medium. We observed and compiled HI data for a
well defined sample of ~ 800 galaxies in the Catalog of Isolated Galaxies, as
part of the AMIGA project (Analysis of the ISM in Isolated GAlaxies,
http://amiga.iaa.es), which enlarges considerably previous samples used to
quantify the HI deficiency in galaxies located in denser environments. By
studying the shape of 182 HI profiles, we revisited the usually accepted result
that, independently of the environment, more than half of the galaxies present
a perturbed HI disk. In isolated galaxies this would certainly be a striking
result if these are supposed to be the most relaxed systems, and has
implications in the relaxation time scales of HI disks and the nature of the
most frequent perturbing mechanisms in galaxies. Our sample likely exhibits the
lowest HI asymmetry level in the local Universe. We found that other field
samples present an excess of ~ 20% more asymmetric HI profiles than that in
CIG. Still a small percentage of galaxies in our sample present large
asymmetries. Follow-up high resolution VLA maps give insight into the origin of
such asymmetries.Comment: 4 pages, 2 figures, Conference 'Galaxies in Isolation: Exploring
Nature vs. Nurture', Granada, 12-15 May 2009. To be published in the ASP
Conference Serie
HI asymmetry in the isolated galaxy CIG 85 (UGC 1547)
We present the results from the Giant Metrewave Radio Telescope (GMRT)
interferometric HI and 20 cm radio continuum observations of CIG 85, an
isolated asymmetric galaxy from the AMIGA (Analysis of the Interstellar Medium
of Isolated GAlaxies) sample. Despite being an isolated galaxy, CIG 85 showed
an appreciable optical and HI spectral asymmetry and therefore was an excellent
candidate for resolved HI studies to understand the reasons giving rise to
asymmetries in isolated galaxies. The galaxy was imaged in HI and 20 cm radio
continuum using the GMRT. For a detailed discussion of the results we also made
use of multi-wavelength data from archival SDSS, GALEX and Halpha imaging. We
find the HI in CIG 85 to have a clumpy, asymmetric distribution which in the NW
part is correlated with optical tail like features, but the HI velocity field
displays a relatively regular rotation pattern. Evaluating all the
observational evidence, we come to a conclusion that CIG 85 is most likely a
case of a disturbed spiral galaxy which now appears to have the morphology of
an irregular galaxy. Although it is currently isolated from major companions,
the observational evidence is consistent with HI asymmetries, a highly
disturbed optical disk and recent increase in star formation having been caused
by a minor merger, remnants of which are now projected in front of the optical
disk. If this is correct, the companion will be fully accreted by CIG 85 in the
near future.Comment: 10 pages, 9 figures, accepted in A&
- …