51 research outputs found
The molecular gas content of the advanced S+E merger NGC 4441 - Evidence for an extended decoupled nuclear disc?
Mergers between a spiral and an elliptical (S+E mergers) are poorly studied
so far despite the importance for galaxy evolution. NGC4441 is a nearby
candidate for an advanced remnant of such a merger, showing typical tidal
structures like an optical tail and two shells as well as two HI tails. The
study of the molecular gas content gives clues on the impact of the recent
merger event on the star formation. Simulations of S+E mergers predict
contradictory scenarios concerning the strength and the extent of an induced
starburst. Thus, observations of the amount and the distribution of the
molecular gas, the raw material of star formation, are needed to understand the
influence of the merger on the star formation history. 12CO and 13CO (1-0) and
(2-1) observations were obtained using the Onsala Space Observatory 20m and
IRAM 30m telescope as well as the Plateau de Bure interferometer. These data
allow us to carry out a basic analysis of the molecular gas properties such as
estimates of the molecular gas mass, its temperature and density and the star
formation efficiency. The CO observations reveal an extended molecular gas
reservoir out to ~4kpc, with a total molecular gas mass of ~5x10^8 M_sun.
Furthermore, high resolution imaging shows a central molecular gas feature,
most likely a rotating disc hosting most of the molecular gas ~4x10^8 M_sun.
This nuclear disc shows a different sense of rotation than the large-scale HI
structure, indicating a kinematically decoupled core. (abbreviated)Comment: 11 pages, accepted by A&
Molecular gas in blue compact dwarf galaxies
Blue compact dwarf galaxies (BCDGs) are currently undergoing strong bursts of
star formation. Nevertheless, only a few of them have been clearly detected in
CO, which is thought to trace the "fuel" of star formation: H_2. In this paper,
we present a deep search for CO J=1-->0 and J=2-->1 emission lines in a sample
of 8 BCDGs and two companions. Only 2 of them (Haro 2 and UM 465) are detected.
For the other galaxies we have obtained more stringent upper limits on the CO
luminosity than published values. We could not confirm the previously reported
``detection'' of CO for the galaxies UM 456 and UM 462. We analyze a possible
relation between metallicity, CO luminosity, and absolute blue magnitude of the
galaxies. We use previously determined relations between X = N(H_2)/I_CO and
the metallicity to derive molecular cloud masses or upper limits for them. With
these ``global'' X_CO values we find that for those galaxies which we detect in
CO, the molecular gas mass is similar to the HI mass, whereas for the
non-detections, the upper limits on the molecular gas masses are significantly
lower than the HI mass. Using an LVG (Large Velocity Gradient) model we show
that X_CO depends not only on metallicity, but also on other physical
parameters such as volume density and kinetic temperature, which rises the
question on the validity of ``global'' X_CO factors.Comment: 9 pages, 6 figures, to be published on MNRA
Warm molecular gas, dust and ionized gas in the 500 central pc of the Galaxy
We present infrared and millimeter observations of molecular gas, dust and
ionized gas towards a sample of clouds distributed along the 500 central pc of
the Galaxy. The clouds were selected to investigate the physical state, in
particular the high gas temperatures, of the Galactic center region (GCr)
clouds located far from far-infrared of thermal radio continuum sources. We
have found that there is ionized gas associated with the molecular gas. The
ionizing radiation is hard (~35000 K) but diluted due to the inhomogeneity of
the medium. We estimate that ~30 % of the warm molecular gas observed in the
GCr clouds is heated by ultra-violet radiation in photo-dissociation regions.Comment: 5 pages, to be published in: Astron. Nachr., Vol. 324, No. S1 (2003),
Special Supplement "The central 300 parsecs of the Milky Way", Eds. A.
Cotera, H. Falcke, T. R. Geballe, S. Markof
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