37 research outputs found
Molecular Gas in Candidate Double-Barred Galaxies II. Cooler, Less Dense Gas Associated with Stronger Central Concentrations
We have performed a multi-transition CO study of the centers of seven
double-barred galaxies that exhibit a variety of molecular gas morphologies to
determine if the molecular gas properties are correlated with the nuclear
morphology and star forming activity. Near infrared galaxy surveys have
revealed the existence of nuclear stellar bars in a large number of barred or
lenticular galaxies. High resolution CO maps of these galaxies exhibit a wide
range of morphologies. Recent simulations of double-barred galaxies suggest
that variations in the gas properties may allow it to respond differently to
similar gravitational potentials. We find that the 12CO J=3-2/J=2-1 line ratio
is lower in galaxies with centrally concentrated gas distributions and higher
in galaxies with CO emission dispersed around the galactic center in rings and
peaks. The 13CO/12CO J=2-1 line ratios are similar for all galaxies, which
indicates that the J=3-2/J=2-1 line ratio is tracing variations in gas
temperature and density, rather than variations in optical depth. There is
evidence that the galaxies which contain more centralized CO distributions are
comprised of molecular gas that is cooler and less dense. Observations suggest
that the star formation rates are higher in the galaxies containing the warmer,
denser, less centrally concentrated gas. It is possible that either the bar
dynamics are responsible for the variety of gas distributions and densities
(and hence the star formation rates) or that the star formation alone is
responsible for modifying the gas properties.Comment: 27 pages + 6 figures; to appear in the April 20, 2003 issue of Ap
Radio Continuum and Star Formation in CO-rich Early Type Galaxies
In this paper we present new high resolution VLA 1.4 GHz radio continuum
observations of five FIR bright CO-rich early-type galaxies and two dwarf
early-type galaxies. The position on the radio-FIR correlation combined with
striking agreements in morphology between high resolution CO and radio maps
show that the radio continuum is associated with star formation in at least
four of the eight galaxies. The average star formation rate for the sample
galaxies detected in radio is approximately 2 solar masses per year. There is
no evidence of a luminous AGN in any of our sample galaxies. We estimate Toomre
Q values and find that the gas disks may well be gravitationally unstable,
consistent with the above evidence for star formation activity. The radio
continuum emission thus corroborates other recent suggestions that star
formation in early type galaxies may not be uncommon.Comment: 21 pages, 7 figures, to be published in the Astronomical Journa
Evidence for chemical evolution in the spectra of high redshift galaxies
Using a sample of 57 VLT FORS spectra in the redshift range 1.37<z<3.40
(selected mainly from the FORS Deep Field survey) and a comparison sample with
36 IUE spectra of local (z ~ 0) starburst galaxies we derive CIV and SiIV
equivalent width values and estimate metallicities of starburst galaxies as a
function of redshift. Assuming that a calibration of the CIV equivalent widths
in terms of the metallicity based on the local sample of starburst galaxies is
applicable to high-z objects, we find a significant increase of the average
metallicities from about 0.16 Z_sun at the cosmic epoch corresponding to z ~
3.2 to about 0.42 Z_sun at z ~ 2.3. A significant further increase in
metallicity during later epochs cannot be detected in our data. Compared to the
local starburst galaxies our high-redshift objects tend to be overluminous for
a fixed metallicity. Our observational results are in good agreement with
published observational data by other authors and with theoretical predictions
of the cosmic chemical evolution.Comment: accepted to be published in A&A, 12 pages, 11 figures Latex2e using
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Near-Infrared photometry in the J, H and Kn bands for Polar Ring Galaxies: I. Data, structural parameters
We present new Near-Infrared (NIR) observations, in the J, H and Kn bands,
for a sample of Polar Ring Galaxies (PRGs), selected from the Polar Ring
Catalogue (Whitmore et al. 1990). Data were acquired with the CASPIR near-IR
camera at the 2.3 m telescope of Mount Stromlo and Siding Spring Observatory.
We report here on the detail morphological study for the central host galaxy
and the polar structure in all PRGs of our sample. Total magnitudes,
bulge-to-disk decomposition and structural parameters are computed for all
objects. These data are crucial for an accurate modeling of the stellar
population and the estimate of the star formation rates in the two components.Comment: 14 pages, 8 postscript figures, accepted for publication in A&A. For
high resolution Figure 1 and Figure 4, see http://www.na.astro.it/~enric
Deep Near Infrared Mapping of Young and Old Stars in Blue Compact Dwarf Galaxies
We analyze J, H and Ks near-infrared data for 9 Blue Compact Dwarf (BCD)
galaxies, selected from a larger sample that we have already studied in the
optical. We present contour maps, surface brightness and color profiles, as
well as color maps of the sample galaxies. The morphology of the BCDs in the
NIR has been found to be basically the same as in the optical. The inner
regions of these systems are dominated by the starburst component. At low
surface brightness levels the emission is due to the underlying host galaxy;
the latter is characterized by red, radially constant colors and isophotes well
fit by ellipses. We derive accurate optical near--infrared host galaxy colors
for eight of the sample galaxies; these colors are typical of an evolved
stellar population. Interestingly, optical near--infrared color maps reveal the
presence of a complex, large-scale absorption pattern in three of the sample
galaxies. We study the applicability of the Sersic law to describe the surface
brightness profiles of the underlying host galaxy, and find that, because of
the limited surface brightness interval over which the fit can be made, the
derived Sersic parameters are very sensitive to the selected radial interval
and to errors in the sky subtraction. Fitting an exponential model gives
generally more stable results, and can provide a useful tool to quantify the
structural properties of the host galaxy and compare them with those of other
dwarf classes as well as with those of star-forming dwarfs at higher redshifts.Comment: 49 pages, 9 figures, 10 tables, accepted for publication in the
Astrophysical Journa