711 research outputs found
Lopsided spiral galaxies: evidence for gas accretion
We quantify the degree of lopsidedness for a sample of 149 galaxies observed
in the near-infrared from the OSUBGS sample, and try to explain the physical
origin for the observed disk lopsidedness. We confirm previous studies, but now
for a larger sample, that a large fraction of galaxies show significant
lopsidedness in their stellar disks, measured as the Fourier amplitude of the
m=1 component, normalised to the average or m=0 component, in the surface
density. Late-type galaxies are found to be more lopsided, while the presence
of m=2 spiral arms and bars is correlated. The m=1 amplitude is found to be
uncorrelated with the tidal forces acting on a galaxy via nearby companions.
Numerical simulations are carried out to study the generation of m=1 via
different processes: galaxy tidal encounters, galaxy mergers, and external gas
accretion and subsequent star formation. The simulations show that galaxy
interactions and mergers can trigger strong lopsidedness, but do not explain
several independent statistical properties of observed galaxies. To explain all
the observational results, it is required that a large fraction of lopsidedness
results from cosmological accretion of gas on galactic disks, which can create
strongly lopsided disks when this accretion is asymmetrical enough.Comment: accepted for publication in Astronomy and Astrophysics - Final
version after language editio
Gravitational torques in spiral galaxies: gas accretion as a driving mechanism of galactic evolution
The distribution of gravitational torques and bar strengths in the local
Universe is derived from a detailed study of 163 galaxies observed in the
near-infrared. The results are compared with numerical models for spiral galaxy
evolution. It is found that the observed distribution of torques can be
accounted for only with external accretion of gas onto spiral disks. Accretion
is responsible for bar renewal - after the dissolution of primordial bars - as
well as the maintenance of spiral structures. Models of isolated, non-accreting
galaxies are ruled out. Moderate accretion rates do not explain the
observational results: it is shown that galactic disks should double their mass
in less than the Hubble time. The best fit is obtained if spiral galaxies are
open systems, still forming today by continuous gas accretion, doubling their
mass every 10 billion years.Comment: 4 pages, 2 figures, Astronomy and Astrophysics Letters (accepted
Birth, life and survival of Tidal Dwarf Galaxies
Advances on the formation and survival of the so-called Tidal Dwarf Galaxies
(TDGs) are reviewed. The understanding on how objects of the mass of dwarf
galaxies may form in debris of galactic collisions has recently benefited from
the coupling of multi-wavelength observations with numerical simulations of
galaxy mergers. Nonetheless, no consensual scenario has yet emerged and as a
matter of fact the very definition of TDGs remains elusive. Their real
cosmological importance is also a matter of debate, their presence in our Local
Group of galaxies as well. Identifying old, evolved, TDGs among the population
of regular dwarf galaxies and satellites may not be straightforward. However a
number of specific properties (location, dark matter and metal content) that
objects of tidal origin should have are reminded here. Examples of newly
discovered genuine old TDGs around a nearby elliptical galaxy are finally
presented.Comment: 9 pages, 5 figures, invited talk at JENAM 2010 symposium on "Dwarf
Galaxies", v2:reference and acknowledgements update
AM 1934-563: A giant spiral polar-ring galaxy in a triplet
We have observed the emission-line kinematics and photometry of a southern
triplet of galaxies. The triplet contains a giant spiral galaxy AM 1934-563
which optical structure resembles a polar-ring galaxy: distorted spiral disk,
seen almost edge-on, and a faint large-scale (45 kpc in diameter) warped
structure, inclined by 60^o-70^o with respect to the disk major axis. The
triplet shows relatively small velocity dispersion (69 km/s) and a large
crossing time (0.17 in units of the Hubble time). The disk of AM 1934-563
demonstrates optical colors typical for an early-type spirals, strong radial
color gradient, and almost exponential surface brightness distribution with an
exponential scale-length value of 3.1 kpc (R passband). The galaxy shows a
maximum rotation velocity of about 200 km/s and it lies close to the
Tully-Fisher relation for spiral galaxies. The suspected polar ring is faint
(\mu(B) > 24) and strongly warped. Its total luminosity comprises (10-15)% of
the total luminosity of AM 1934-563. We then try to model this system using
numerical simulations, and study its possible formation mechanisms. We find
that the most robust model, that reproduces the observed characteristics of the
ring and the host galaxy, is the tidal transfer of mass from a massive gas-rich
donor galaxy to the polar ring. The physical properties of the triplet of
galaxies are in agreement with this scenario.Comment: Accepted for publication in A&
Modelling CO emission from hydrodynamic simulations of nearby spirals, starbursting mergers, and high-redshift galaxies
We model the intensity of emission lines from the CO molecule, based on
hydrodynamic simulations of spirals, mergers, and high-redshift galaxies with
very high resolutions (3pc and 10^3 Msun) and detailed models for the
phase-space structure of the interstellar gas including shock heating, stellar
feedback processes and galactic winds. The simulations are analyzed with a
Large Velocity Gradient (LVG) model to compute the local emission in various
molecular lines in each resolution element, radiation transfer and opacity
effects, and the intensity emerging from galaxies, to generate synthetic
spectra for various transitions of the CO molecule. This model reproduces the
known properties of CO spectra and CO-to-H2 conversion factors in nearby
spirals and starbursting major mergers. The high excitation of CO lines in
mergers is dominated by an excess of high-density gas, and the high turbulent
velocities and compression that create this dense gas excess result in broad
linewidths and low CO intensity-to-H2 mass ratios. When applied to
high-redshift gas-rich disks galaxies, the same model predicts that their
CO-to-H2 conversion factor is almost as high as in nearby spirals, and much
higher than in starbursting mergers. High-redshift disk galaxies contain giant
star-forming clumps that host a high-excitation component associated to gas
warmed by the spatially-concentrated stellar feedback sources, although CO(1-0)
to CO(3-2) emission is overall dominated by low-excitation gas around the
densest clumps. These results overall highlight a strong dependence of CO
excitation and the CO-to-H2 conversion factor on galaxy type, even at similar
star formation rates or densities. The underlying processes are driven by the
interstellar medium structure and turbulence and its response to stellar
feedback, which depend on global galaxy structure and in turn impact the CO
emission properties.Comment: A&A in pres
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