2,904 research outputs found
Molecular content of a type-Ia SN host galaxy at z=0.6
We study the properties and the molecular content of the host of a type-Ia
supernova (SN1997ey). This z=0.575 host is the brightest submillimetre source
of the sample of type-Ia supernova hosts observed at 450um and 850um by Farrah
et al.. Observations were performed at IRAM-30m to search for CO(2-1) and
CO(3-2) lines in good weather conditions but no signal was detected. The star
formation rate cannot exceed 50 M_sol/yr. These negative results are confronted
with an optical analysis of a Keck spectrum and other data archives. We reach
the conclusion that this galaxy is a late-type system (0.7 L^B_*), with a small
residual star-formation activity (0.2 M_sol/yr) detected in the optical. No
source of heating (AGN or starburst) is found to explain the
submillimetre-continuum flux and the non-CO detection excludes the presence of
a large amount of cold gas. We thus suggest that either the star formation
activity is hidden in the nucleus (with A_V ~ 4) or this galaxy is passive or
anemic and this flux might be associated with a background galaxy.Comment: 8 pages, 7 figures, accepted for publication in MNRA
CO investigation of z=0.4-1.5 galaxies
We report on the results of an IRAM-30m search for CO emission lines in three
galaxies at intermediate redshifts. The idea was to investigate the molecular
content of galaxies bright in the infrared at z=0.4-1.5, a redshift desert for
molecular line studies, poorly investigated as of yet. We integrated 8-10h per
source and did not succeed in detecting any of the sources. From our upper
limits, we are able to constrain the molecular gas content in these systems to
less than 4 to 8 x 10^9 Mo, assuming a CO-to-H_2 conversion factor (\alpha=0.8
Mo/(K km s^-1 pc^2)). We stress the current difficulty of selecting sources
with a detectable molecular content, a problem that will be faced by the ALMA
First Science projects.Comment: 6 pages, 5 figures. Accepted for publication in Astronomy and
Astrophysic
Molecular Gas and Star Formation in the SAURON Early-type Galaxies
We present the results of a survey of CO emission in 43 of the 48
representative E/S0 galaxies observed in the optical with the SAURON
integral-field spectrograph. The CO detection rate is 12/43 or 28%. This is
lower than previous studies of early-types but can probably be attributed to
different sample selection criteria. As expected, earlier type, more luminous
and massive galaxies have a relatively lower molecular gas content. We find
that CO-rich galaxies tend to have higher H\beta but lower Fe5015 and Mgb
absorption indices than CO-poor galaxies. Those trends appear primarily driven
by the age of the stars, an hypothesis supported by the fact that the galaxies
with the strongest evidence of star formation are also the most CO-rich. In
fact, the early-type galaxies from the current sample appear to extend the
well-known correlations between FIR luminosity, dust mass and molecular mass of
other galaxy types. The star formation interpretation is also consistent with
the SAURON galaxies' radio continuum and FIR flux ratios, and their inferred
star formation efficiencies are similar to those in spiral galaxies. It thus
appears that we have identified the material fueling (residual) star formation
in early-type galaxies, and have demonstrated that it is actively being
transformed. Nevertheless, the lack of strong correlations between the CO
content and most stellar parameters is compatible with the idea that, in a
significant number of sample galaxies, the molecular gas has been accreted from
the outside and has properties rather independent from the old, pre-existing
stellar component.Comment: 14 pages, 9 figures, accepted in MNRA
Conductivity and the current-current correlation measure
We review various formulations of conductivity for one-particle Hamiltonians
and relate them to the current-current correlation measure. We prove that the
current-current correlation measure for random Schr\"odinger operators has a
density at coincident energies provided the energy lies in a localization
regime. The density vanishes at such energies and an upper bound on the rate of
vanishing is computed. We also relate the current-current correlation measure
to the localization length
Star formation efficiency in galaxy interactions and mergers: a statistical study
We investigate the enhancement of star formation efficiency in galaxy
interactions and mergers, by numerical simulations of several hundred galaxy
collisions. All morphological types along the Hubble sequence are considered in
the initial conditions of the two colliding galaxies, with varying
bulge-to-disk ratios and gas mass fractions. Different types of orbits are
simulated, direct and retrograde, according to the initial relative energy and
impact parameter, and the resulting star formation history is compared to that
occuring in the two galaxies when they are isolated. Our principal results are:
(1) retrograde encounters have a larger star formation efficiency (SFE) than
direct encounters; (2) the amount of gas available in the galaxy is not the
main parameter governing the SFE in the burst phase; (3) there is an
anticorrelation between the amplitude of the star forming burst and the tidal
forces exerted per unit of time, which is due to the large amount of gas
dragged outside the galaxy by tidal tails in strong interactions; (4) globally,
the Kennicutt-Schmidt law is retrieved statistically for isolated galaxies,
interacting pairs and mergers; (5) the enhanced star formation is essentially
occurring in nuclear starbursts, triggered by inward gas flows driven by
non-axisymmetries in the galaxy disks. Direct encounters develop more
pronounced asymmetries than retrograde ones. Based on these statistical
results, we derive general laws for the enhancement of star formation in galaxy
interactions and mergers, as a function of the main parameters of the
encounter.Comment: 22 pages, 37 figures, 4 tables. Accepted on Astronomy & Astrophysic
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
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