787 research outputs found
CO (J=1-0) Observation of the cD Galaxy of AWM7: Constraints on the Evaporation of Molecular Gas
We have searched for molecular gas in the cD galaxy of a poor cluster of
galaxies AWM7 using Nobeyama 45 m telescope. We do not detect CO emission in
the galaxy. Our limit of molecular gas in the inner 7.5 kpc is M_H_2< 4 x 10^8
M_sun. We estimate the total mass of molecular gas left in the cD galaxy when
the gas deposited by a cooling flow once becomes molecular gas and the
molecular gas is continuously evaporated by the ambient hot gas. The
observational limit of molecular gas requires f >~ 10^{-3}, where f is the
ratio of the heat conduction rate to that of Spitzer. However, this contradicts
recent X-ray observations showing f<10^{-5}. Thus, the non-detection of CO
cannot be explained by the evaporation, and most of the cooled gas predicted by
a cooling flow model may not change into molecular gas in the cD galaxy.
Moreover, we estimate the evaporation time of molecular clouds brought to a cD
galaxy through the capture of gas-rich galaxies and find that these clouds
should not be evaporated if f <~ 10^{-3}-10^{-4}. Therefore, the non-detection
of CO in a cD galaxy could constrain the total mass of the molecular clouds
brought into it.Comment: 5 pages, 1 figure, to be published in PAS
Detection of Molecular Clouds in the Interarm of the Flocculent Galaxy NGC 5055
We present high-resolution (~ 4") 12CO (J = 1 - 0) mapping observations with
high - velocity resolution (~ 2.6 km s^{-1}) toward the disk of flocculent
galaxy NGC 5055, using the Nobeyama Millimeter Array in order to study the
physical properties of the molecular clouds in the arm and the interarm. The
obtained map shows clumpy structures. Although these are mainly distributed
along a spiral arm seen in near-infrared observations, some clouds are located
far from the arm, namely in the interarm. These clouds in both the arm and the
interarm have a typical size and mass of a few 100 pc and a few 10^6 Mo,
respectively. These correspond to the largest Giant Molecular Cloud (GMC) in
our Galaxy, and are slightly smaller than Giant Molecular Associations (GMAs)
in the grand design spiral M 51. Their CO flux-based masses show good agreement
with their virial masses. A size - velocity dispersion relation is also plotted
on an extension of the relation for the Galactic GMCs. These facts suggest that
the properties of these clouds are similar to that of the Galactic GMCs. We
also found no clear systematic offset between the molecular gas and HII regions
unlike M 51. This fact and no existense of GMAs suggest the view that, in NGC
5055, cloud formation and following star formation in both the arm and the
interarm are due to enhancement of gas by local fluctuation. On the other hand,
in grand design spiral galaxies, such as M 51, GMA formations may occur only in
the arm due to a strong density wave also enhanced star formation in GMA
formation may also occur. These may control the optical morphology of spiral
arms in spiral galaxies.Comment: 16 pages, 8 figure
ASTE CO(3-2) Mapping toward the Whole Optical Disk of M 83: Properties of Inter-arm GMAs
We present a new on-the-fly (OTF) mapping of CO(J=3-2) line emission with the
Atacama Submillimeter Telescope Experiment (ASTE) toward the 8' x 8' (or 10.5 x
10.5 kpc at the distance of 4.5 Mpc) region of the nearby barred spiral galaxy
M 83 at an effective resolution of 25''. Due to its very high sensitivity, our
CO(J=3-2) map can depict not only spiral arm structures but also spur-like
substructures extended in inter-arm regions. This spur-like substructures in
CO(J=3-2) emission are well coincident with the distribution of massive star
forming regions traced by Halpha luminosity and Spitzer/IRAC 8 um emission. We
have identified 54 CO(J=3-2) clumps as Giant Molecular-cloud Associations
(GMAs) employing the CLUMPFIND algorithm, and have obtained their sizes,
velocity dispersions, virial masses, and CO luminosity masses. We found that
the virial parameter alpha, which is defined as the ratio of the virial mass to
the CO luminosity mass, is almost unity for GMAs in spiral arms, whereas there
exist some GMAs whose alpha are 3 -- 10 in the inter-arm region. We found that
GMAs with higher tend not to be associated with massive star forming
regions, while other virialized GMAs are. Since alpha mainly depends on
velocity dispersion of the GMA, we suppose the onset of star formation in these
unvirialized GMAs with higher alpha are suppressed by an increase in internal
velocity dispersions of Giant Molecular Clouds within these GMAs due to shear
motion.Comment: 42 pages, 16 figures, ApJ in press, version with high resolution
figures is available via
http://www.nro.nao.ac.jp/~kmuraoka/m83paper/m83aste-otf.pd
Variation of Molecular Cloud Properties across the Spiral Arm in M 51
We present the results of high-resolution 13CO(1-0) mapping observations with
the NRO 45m telescope of the area toward the southern bright arm region of M51,
including the galactic center. The obtained map shows the central depression of
the the circumnuclear ring and the spiral arm structure.The arm-to-interarm
ratio of the 13CO(1-0) integrated intensity is 2-4. We also have found a
feature different from that found in the 12CO results. The 12CO/13CO ratio
spatially varies, and shows high values (~20) for the interarm and the central
region, but low values(~10) for the arm. These indicate that there is a denser
gas in the spiral arm than in the interarm. The distribution of the 13CO shows
a better correspondence with that of the H\alpha emission than with the 12CO in
the disk region, except for the central region. We found that the 13CO emission
is located on the downstream side of the 12CO arm, namely there is an offset
between the 12CO and the 13CO as well as the H\alpha emission. This suggests
that there is a time delay between the accumulation of gas caused by the
density wave and dense gas formation, accordingly star formation. This time
delay is estimated to be ~10^7 yr based on the assumption of galactic rotation
derived by the rotation curve and the pattern speed of M51. It is similar to
the growth timescale of a gravitational instability in the spiral arm of M51,
suggesting that the gravitational instability plays an important role for dense
gas formation.Comment: 23 pages, 10 figures, PASJ Vol.54, No.2 (2002), in pres
Dense and Warm Molecular Gas and Warm Dust in Nearby Galaxies
We performed 12CO(1-0), 13CO(1-0), and HCN(1-0) single-dish observations
(beam size ~14"-18") toward nearby starburst and non-starburst galaxies using
the Nobeyama 45 m telescope. The 13CO(1-0) and HCN(1-0) emissions were detected
from all the seven starburst galaxies, with the intensities of both lines being
similar (i.e., the ratios are around unity). On the other hand, for case of the
non-starburst galaxies, the 13CO(1-0) emission was detected from all three
galaxies, while the HCN(1-0) emission was weakly or not detected in past
observations. This result indicates that the HCN/13CO intensity ratios are
significantly larger (~1.15+-0.32) in the starburst galaxy samples than the
non-starburst galaxy samples (<0.31+-0.14). The large-velocity-gradient model
suggests that the molecular gas in the starburst galaxies have warmer and
denser conditions than that in the non-starburst galaxies, and the
photon-dominated-region model suggests that the denser molecular gas is
irradiated by stronger interstellar radiation field in the starburst galaxies
than that in the non-starburst galaxies. In addition, HCN/13CO in our sample
galaxies exhibit strong correlations with the IRAS 25 micron flux ratios. It is
a well established fact that there exists a strong correlation between dense
molecular gas and star formation activities, but our results suggest that
molecular gas temperature is also an important parameter.Comment: 14 pages, 6 figures. Accepted for publication in PAS
ASTE CO(3--2) Survey of Elliptical Galaxies
We report CO(3--2) observations of 15 nearby elliptical galaxies
carried out with the ASTE telescope. Thirteen were selected without regard to
the presence of other tracers of cold interstellar matter. CO emission was
detected from three of the galaxies, two of which are undetected by IRAS at 100
microns, suggesting that cold ISM may be present in more ellipticals than
previously thought. The molecular gas masses range from to
. The ratio of the CO(3--2) and (1--0) lines,
, has a lower value for elliptical galaxies than for spiral galaxies
except for NGC 855, for which the value is close to the mean for spirals. The
molecular gas in NGC 855 has a mean density in the range 300 -- 1000 cm
adopting a temperature range of 15 -- 100 K.Comment: 6 pages, 3 figures, Accepted for publication in PAS
Arc-like distribution of high CO(J=3-2)/CO(J=1-0) ratio gas surrounding the central star cluster of the supergiant HII region NGC 604
We report the discovery of a high CO(J=3-2)/CO(J=1-0) ratio gas with an
arc-like distribution (``high-ratio gas arc'') surrounding the central star
cluster of the supergiant HII region NGC 604 in the nearby spiral galaxy M 33,
based on multi-J CO observations of a 5' 5' region of NGC 604
conducted using the ASTE 10-m and NRO 45-m telescopes. The discovered
``high-ratio gas arc'' extends to the south-east to north-west direction with a
size of 200 pc. The western part of the high-ratio gas arc closely
coincides well with the shells of the HII regions traced by H and radio
continuum peaks. The CO(J=3-2)/CO(J=1-0) ratio, R_{3-2/1-0}, ranges between 0.3
and 1.2 in the observed region, and the R_{3-2/1-0} values of the high-ratio
gas arc are around or higher than unity, indicating very warm (T_kin > 60 K)
and dense (n(H_2) > 10^{3-4} cm^{-3}) conditions of the high-ratio gas arc. We
suggest that the dense gas formation and second-generation star formation occur
in the surrounding gas compressed by the stellar wind and/or supernova of the
first-generation stars of NGC 604, i.e., the central star cluster of NGC 604.Comment: 4 pages, 4 figures. The Astrophysical Journal Letters, in pres
High Resolution Molecular Gas Maps of M33
New observations of CO (J=1->0) line emission from M33, using the 25 element
BEARS focal plane array at the Nobeyama Radio Observatory 45-m telescope, in
conjunction with existing maps from the BIMA interferometer and the FCRAO 14-m
telescope, give the highest resolution (13'') and most sensitive (RMS ~ 60 mK)
maps to date of the distribution of molecular gas in the central 5.5 kpc of the
galaxy. A new catalog of giant molecular clouds (GMCs) has a completeness limit
of 1.3 X 10^5 M_sun. The fraction of molecular gas found in GMCs is a strong
function of radius in the galaxy, declining from 60% in the center to 20% at
galactocentric radius R_gal ~ 4 kpc. Beyond that radius, GMCs are nearly
absent, although molecular gas exists. Most (90%) of the emission from low mass
clouds is found within 100 pc projected separation of a GMC. In an annulus 2.1<
R_gal <4.1 kpc, GMC masses follow a power law distribution with index -2.1.
Inside that radius, the mass distribution is truncated, and clouds more massive
than 8 X 10^5 M_sun are absent. The cloud mass distribution shows no
significant difference in the grand design spiral arms versus the interarm
region. The CO surface brightness ratio for the arm to interarm regions is 1.5,
typical of other flocculent galaxies.Comment: 14 pages, 14 figures, accepted in ApJ. Some tables poorly typeset in
emulateapj; see source files for raw dat
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