280 research outputs found
Spatial distribution of Far infrared emission in spiral galaxies II. Heating sources and gas-to-dust ratio
We study the radial distribution of the temperature of the warm dust and
gas-to-dust mass ratios in a sample of 22 spiral galaxies. The heating
capabilities of the diffuse interstellar radiation field (ISRF), based on
Desert et al. model, are investigated in 13 of the sample galaxies. In general,
the temperature of the warm dust decreases away from the center, reaches a
minimum value at the mid-disk and increases again in the outer parts of
galaxies. Heating a mixture of small and big grains by the ISRF is able to
explain the observed behavior qualitatively. However, ultraviolet photons from
recent star formation events are necessary for a detailed matching of the warm
dust temperature profiles. Very small grains contribute typically more than 50%
to the observed flux at 60 micron beyond half the disk radius in galaxies.
Optical depth profiles, derived from the observed 60 micron and warm dust
temperature profiles, peak at or close to the galactic center. In 13 of the
galaxies, where dust temperature profiles are modeled, we obtain gas-to-dust
mass ratio profiles, after correction for the contaminating effects of very
small grains. The gas-to-dust mass ratio decreases by a factor of 8 from the
center to the optical isophotal radius, where the value approaches the local
galactic value. We demonstrate that the observed steep gradient is a result of
the over-estimation of the molecular mass, and can be flattened out to within a
factor of 2, if the molecular hydrogen mass (H2) is recomputed assuming a
metallicity dependent conversion factor from CO intensity to H2 column density.
The flattened radial profiles indicate a global gas-to-dust ratio of around
300, which is within a factor of two of the local galactic value.Comment: Uses aas2pp4.sty and epsfig.sty, 27 pages. To appear in Astronomical
Journal, September 199
Spatial distribution of far infrared emission in spiral galaxies I. Relation with radio continuum emission
We use high resolution IRAS and 20 cm radio continuum (RC) images of a sample
of 22 spiral galaxies to study the correlation between the far infra-red (FIR)
and RC emissions within the galactic disks. A combination of exponential and
gaussian profiles rather than a single exponential profile is found to be a
better representation of the observed intensity profiles in the two bands. The
gaussian component, which we show is not due to the effects of limited
beam-resolution, contains more than 60% of the total flux in majority of the
galaxies. The dominance of the gaussian component suggests that the nuclear
star forming regions and the bulge stars are more important contributors to the
emission in the two bands, rather than the outer exponential stellar disks. The
RC profile is flatter compared to the FIR profile, resulting in a decrease of
their ratio, Q60, away from the center. However, the Q60 increases in the
extreme outer parts, where the dispersion in the FIR and RC correlation is also
higher than in the central regions. The global Q60 and its dispersion match
those in the inner parts of the galaxies. These results imply that the observed
tight correlation in the global quantities reflects processes in the inner
regions only where OB stars and the associated Type II supernovae control the
FIR and RC emission. In the outer parts heating of very small dust grains by
the old disk stars provides a secondary component in the FIR emission, without
associated RC emission. The edge-on galaxy NGC3079 shows extended FIR and RC
emissions along its minor axis, probably associated with the nuclear starburst
activity.Comment: Uses aas2pp4.sty and epsfig.sty, 21 pages. Figure 1 is spread over 22
pages and is available at http://www.inaoep.mx/~ydm/preprint.html To appear
in Astronomical Journal, September 199
EMBEDDED CLUSTERS IN GIANT EXTRAGALACTIC HII REGIONS II. EVOLUTIONARY POPULATION SYNTHESIS MODEL
A stellar population synthesis model, suitable for comparison with Giant
Extragalactic HII Regions (GEHRs), is constructed incorporating the recent
developments in modelling stellar evolution by Maeder and co-workers and
stellar atmospheres by Kurucz. A number of quantities suitable for comparison
with broad band data of GEHRs in visible and near infrared parts of the
spectrum are synthesized in addition to the hydrogen and helium ionizing photon
production rates at solar metallicities, for three scenarios of star formation
--- (i) Instantaneous burst (IB) (ii) Continuous star formation (CSF) and (iii)
Two bursts of star formation, with the older burst rich in red supergiants. For
IB case, evolution of colors shows three distinct phases --- an initial steady
blue phase, followed by a red bump (5--15~Myr) and another steady phase with
colors intermediate to the earlier two phases. CSF colors asymptotically reach
peak values at ~Myr, never reaching the reddest IB colors. Ionizing
photon production rate falls off by an order of magnitude in 6~Myr for IB,
where as it almost remains constant for CSF model. Two-burst models with burst
separations ~Myr have properties of both IB and CSF, simultaneously
producing the red IB colors and high ionizing photon rate, making such regions
easily distinguishable using optical observations. Flat IMFs result in bluest
colors when the massive stars are on the main sequence and reddest colors
during the red supergiant phase of the evolving massive stars. Errors on the
computed quantities due to the statistical uncertainties inherent in the
process of star formation become negligible for cluster masses in excess of
\,\msun.Comment: 15 pages, AASTeX, 5 tables and 11 figures available on request at
[email protected] . To appear in Astronomical Journa
Optical Color Gradients in Star-Forming Ring Galaxies
We compute radial color gradients produced by an outwardly propagating
circular wave of star formation and compare our results with color gradients
observed in the classical ring galaxy, the ``Cartwheel''. We invoke two
independent models of star formation in the ring galaxies. The first one is the
conventional density wave scenario, in which an intruder galaxy creates a
radially propagating density wave accompanied by an enhanced star formation
following the Schmidt law. The second scenario is a pure self-propagating star
formation model, in which the intruder only sets off the first burst of stars
at the point of impact. Both models give essentially the same results.
Systematic reddening of B-V, V-K colors towards the center, such as that
observed in the Cartwheel, can be obtained only if the abundance of heavy
elements in the star-forming gas is a few times below solar. The B-V and V-K
color gradients observed in the Cartwheel can be explained as a result of
mixing of stellar populations born in a star-forming wave propagating through a
low-metallicity gaseous disk, and a pre-existing stellar disk of the size of
the gaseous disk with color properties typical to those observed in nearby disk
galaxies.Comment: 16 pages, 12 figures; accepted for publication in the Astrophysical
Journa
Massive young stellar object W42-MME: The discovery of an infrared jet using VLT/NACO near-infrared images
We report on the discovery of an infrared jet from a deeply embedded infrared
counterpart of 6.7 GHz methanol maser emission (MME) in W42 (i.e. W42-MME). We
also investigate that W42-MME drives a parsec-scale H2 outflow, with detection
of bow shock feature at ~0.52 pc to the north. The inner ~0.4 pc part of the H2
outflow has a position angle of ~18 deg and the position angle of ~40 deg is
found farther away on either side of outflow from W42-MME. W42-MME is detected
at wavelengths longer than 2.2 microns and is a massive young stellar object,
with the estimated stellar mass of 19+-4 Msun. We map the inner circumstellar
environment of W42-MME using VLT/NACO adaptive optics Ks and L' observations at
resolutions ~0.2 arcsec and ~0.1 arcsec, respectively. We discover a collimated
jet in the inner 4500 AU using the L' band, which contains prominent Br alpha
line emission. The jet is located inside an envelope/cavity (extent ~10640 AU)
that is tapered at both ends and is oriented along the north-south direction.
Such observed morphology of outflow cavity around massive star is scarcely
known and is very crucial for understanding the jet-outflow formation process
in massive star formation. Along the flow axis, which is parallel to the
previously known magnetic field, two blobs are found in both the NACO images at
distances of ~11800 AU, located symmetrically from W42-MME. The observed
W42-MME jet-outflow configuration can be used to constrain the jet launching
and jet collimation models in massive star formation.Comment: 6 pages, 5 figures, Accepted for publication in The Astrophysical
Journa
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