15 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
A 60 pc counter-rotating core in NGC 4621
We present adaptive optics assisted OASIS integral field spectrography of the
S0 galaxy NGC 4621. Two-dimensional stellar kinematical maps (mean velocity and
dispersion) reveal the presence of a 60 pc diameter counter-rotating core
(CRC), the smallest observed to date. The OASIS data also suggests that the
kinematic center of the CRC is slightly offset from the center of the outer
isophotes. This seems to be confirmed by archival HST/STIS data. We also
present the HST/WFPC2 V-I colour map, which exhibits a central elongated red
structure, also slightly off-centered in the same direction as the kinematic
centre. We then construct an axisymmetric model of NGC 4621: the two-integral
distribution function is derived using the Multi-Gaussian Expansion and the
Hunter & Qian (1993) formalisms. Although the stellar velocities are reasonably
fitted, including the region of the counter-rotating core, significant
discrepancies between the model and the observations demonstrate the need for a
more general model (e.g. a three-integral model).Comment: 9 pages, 8 figure
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
Minor-axis velocity gradients in disk galaxies
We present the ionized-gas kinematics and photometry of a sample of 4 spiral
galaxies which are characterized by a zero-velocity plateau along the major
axis and a velocity gradient along the minor axis, respectively. By combining
these new kinematical data with those available in the literature for the
ionized-gas component of the S0s and spirals listed in the Revised Shapley-Ames
Catalog of Bright Galaxies we realized that about 50% of unbarred galaxies show
a remarkable gas velocity gradient along the optical minor axis. This fraction
rises to about 60% if we include unbarred galaxies with an irregular velocity
profile along the minor axis. This phenomenon is observed all along the Hubble
sequence of disk galaxies, and it is particularly frequent in early-type
spirals. Since minor-axis velocity gradients are unexpected if the gas is
moving onto circular orbits in a disk coplanar to the stellar one, we conclude
that non-circular and off-plane gas motions are not rare in the inner regions
of disk galaxies.Comment: 12 pages, 4 postscript figures. Accepted for publication in A&