104 research outputs found
NRO M33 All-Disk Survey of Giant Molecular Clouds (NRO MAGiC): II. Dense Gas Formation within Giant Molecular Clouds in M33
We report the results of our observations of the 12CO (J=1-0) and 12CO
(J=3-2) line emission of 74 major giant molecular clouds (GMCs) within the
galactocentric distance of 5.1 kpc in the Local Group galaxy M33. The
observations have been conducted as part of the Nobeyama Radio Observatory M33
All-disk survey of Giant Molecular Clouds project (NRO MAGiC). The spatial
resolutions are 80 pc for 12CO (J=1-0) and 100 pc for 12CO (J=3-2). We detect
12CO (J=3-2) emission of 65 GMCs successfully. Furthermore, we find that the
correlation between the surface density of the star formation rate, which is
derived from a linear combination of Halpha and 24um emissions, and the 12CO
(J=3-2) integrated intensity still holds at this scale. This result show that
the star-forming activity is closely associated with warm and dense gases that
are traced with the 12CO (J=3-2) line, even in the scale of GMCs. We also find
that the GMCs with a high star-forming activity tend to show a high integrated
intensity ratio (R3-2/1-0). Moreover, we also observe a mass-dependent trend of
R3-2/1-0 for the GMCs with a low star-forming activity. From these results, we
speculate that the R3-2/1-0 values of the GMCs with a low star-forming activity
mainly depend on the dense gas fraction and not on the temperature, and
therefore, the dense gas fraction increases with the mass of GMCs, at least in
the GMCs with a low star-forming activity.Comment: 17 pages, 5 figures, Accepted for publication in PASJ, 2012, Vol. 64,
No.
ALMA Observations toward the starburst dwarf galaxy NGC 5253: I. Molecular cloud properties and scaling relations
We present high-spatial-resolution (\sim 0\farcs2, or 3\,pc) CO(2--1)
observations of the nearest young starburst dwarf galaxy, NGC\,5253, taken with
the Atacama Large Millimeter/submillimeter Array. We have identified 118
molecular clouds with average values of 4.3\,pc in radius and 2.2\,\kms\, in
velocity dispersion, which comprise the molecular cloud complexes observed
previously with 100\,pc resolution. We derive for the first time in this
galaxy the --(H) conversion factor, =
\,cm(K\,\kms), based on the
virial method. The line-width and mass-to-size relations of the resolved
molecular clouds present an offset on average toward higher line-widths and
masses with respect to quiescent regions in other nearby spiral galaxies and
our Galaxy. The offset in the scaling relation reaches its maximum in regions
close to the central starburst, where velocity dispersions are 0.5 dex
higher and gas mass surface densities are as high as =
10\,\Msol\,pc . These central clouds are gravitationally bound
despite the high internal pressure. A spatial comparison with star clusters
found in the literature enables us to identify six clouds that are associated
with young star clusters. Furthermore, the star formation efficiencies (SFEs)
of some of these clouds exceed those found in star-cluster-forming clouds
within our Galaxy. We conclude that once a super star cluster is formed, the
parent molecular clouds are rapidly dispersed by the destructive stellar
feedback, which results in such a high SFE in the central starburst of
NGC\,5253.Comment: 18 pages, 11 figures, accepted to Ap
NRO M33 All Disk Survey of Giant Molecular Clouds (NRO MAGiC): I. HI to H_2 Transition
We present the results of the Nobeyama Radio Observatory (NRO) M33 All Disk
(30'x30' or 7.3 kpc x 7.3 kpc) Survey of Giant Molecular Clouds (NRO MAGiC)
based on 12CO (1-0) observations using the NRO 45-m telescope. The spatial
resolution of the resultant map is 19".3, corresponding to 81 pc, which is
sufficient to identify each Giant Molecular Cloud (GMC) in the disk. We found
clumpy structures with a typical spatial scale of ~100 pc, corresponding to
GMCs, and no diffuse, smoothly distributed component of molecular gas at this
sensitivity. Closer inspection of the CO and HI maps suggests that not every CO
emission is associated with local HI peaks, particularly in the inner portion
of the disk (r < 2 kpc), although most of CO emission is located at the local
HI peaks in the outer radii. We found that most uncovered GMCs are accompanied
by massive star-forming regions, although the star formation rates (SFRs) vary
widely from cloud to cloud. The azimuthally averaged H{\sc i} gas surface
density exhibits a flat radial distribution. However, the CO radial
distribution shows a significant enhancement within the central 1-2 kpc region,
which is very similar to that of the SFR. We obtained a map of the molecular
fraction, f_mol = Sigma_H_2/(Sigma_HI+Sigma_H_2, at a 100-pc resolution. This
is the first f_mol map covering an entire galaxy with a GMC-scale resolution.
We find that f_mol tends to be high near the center. The correlation between
f_mol and gas surface density shows two distinct sequences. The presence of two
correlation sequences can be explained by differences in metallicity, i.e.,
higher (~ 2-fold) metallicity in the central region (r< 1.5 kpc) than in the
outer parts. Alternatively, differences in scale height can also account for
the two sequences, i.e., increased scale height toward the outer disk.Comment: Accepted for publication in PASJ, See
http://www.juen.ac.jp/lab/tosaki/paper/astro-ph/2011/tosaki2011.pdf for a
version with full resolution figure
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