198 research outputs found
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
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
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
Diffuse and Gravitationally Stable Molecular Gas in the Post-Starburst Galaxy NGC 5195
The Nobeyama Millimeter Array (NMA) has been used to make aperture synthesis
CO(1-0) observations of the post-starburst galaxy NGC 5195. CO(1-0) and
HCN(1-0) observations of NGC 5195 using the Nobeyama 45 m telescope are also
presented. High-resolution (1".9 x 1".8 or 86 pc x 81 pc at D = 9.3 Mpc) NMA
maps show a strong concentration of CO emission toward the central a few 100 pc
region of NGC 5195, despite the fact that the current massive star formation is
suppressed there. The HCN-to-CO integrated intensity ratio on the brightness
temperature scale, R_{HCN/CO}, is about 0.02 within the central r < 400 pc
region. This R_{HCN/CO} is smaller than those in starburst regions by a factor
of 5 - 15. These molecular gas properties would explain why NGC 5195 is in a
post-starburst phase; most of the dense molecular cores (i.e., the very sites
of massive star formation) have been consumed away by a past starburst event,
and therefore a burst of massive star formation can no longer last, although a
large amount of low density gas still exists. We propose that dense molecular
gas can not be formed from remaining diffuse molecular gas because the
molecular gas in the center of NGC 5195 is too stable to form dense cores via
gravitational instabilities of diffuse molecular gas.Comment: 26 pages, 10 figures, PASJ, vol. 54, in press. For the preprint with
high resolution figures, see
http://www.nro.nao.ac.jp/library/report/list.html or
http://www.ioa.s.u-tokyo.ac.jp/~kkohno/n5195/all.ps.g
The Structure, Kinematics and Physical Properties of the Molecular Gas in the Starburst Nucleus of NGC 253
We present 5.2" x 2.6" resolution interferometry of CO J=1-0 emission from
the starburst galaxy NGC 253. The high spatial resolution of these new data, in
combination with recent high resolution maps of 13CO, HCN and near-infrared
emission, allow us for the first time to link unambiguously the gas properties
in the central starburst of NGC 253 with its bar dynamics. We confirm that the
star formation results from bar-driven gas flows as seen in "twin peaks"
galaxies. Two distinct kinematic features are evident from the CO map and
position-velocity diagram: a group of clouds rotating as a solid body about the
kinematic center of the galaxy, and a more extended gas component associated
with the near-infrared bar. We model the line intensities of CO, HCN and 13CO
to infer the physical conditions of the gas in the nucleus of NGC 253. The
results indicate increased volume densities around the radio nucleus in a
twin-peaks morphology. Compared with the CO kinematics, the gas densities
appear highest near the radius of a likely inner Linblad resonance, and
slightly lead the bar minor axis. This result is similar to observations of the
face-on, twin-peaks galaxy NGC 6951, and is consistent with models of starburst
generation due to gas inflow along a bar.Comment: To appear in the ApJ, 28 pages, 12 figure file
Tracing star formation in galaxies with molecular line and continuum observations
We report our recent progress on extragalactic spectroscopic and continuum
observations, including HCN(J=1-0), HCO(J=1-0), and CN(N=1-0) imaging
surveys of local Seyfert and starburst galaxies using the Nobeyama Millimeter
Array, high-J CO observations (J=3-2 observations using the Atacama
Submillimeter Telescope Experiment (ASTE) and J=2-1 observations with the
Submillimeter Array) of galaxies, and 1.1 mm continuum observations
of high-z violent starburst galaxies using the bolometer camera AzTEC mounted
on ASTE.Comment: 6 pages, 5 figures, To appear in proceedings of "Far-Infrared and
Submillimeter Emission of the Interstellar Medium", EAS Publication Series,
Bad Honnef, November 2007, Eds. C. Kramer, S. Aalto, R. Simon. See
http://www.nro.nao.ac.jp/~f0212kk/FIR07/kk-ver20.pdf for a version with high
resolution figure
AzTEC/ASTE 1.1-mm Survey of the AKARI Deep Field South: source catalogue and number counts
We present results of a 1.1 mm deep survey of the AKARI Deep Field South
(ADF-S) with AzTEC mounted on the Atacama Submillimetre Telescope Experiment
(ASTE). We obtained a map of 0.25 sq. deg area with an rms noise level of
0.32-0.71 mJy. This is one of the deepest and widest maps thus far at
millimetre and submillimetre wavelengths. We uncovered 198 sources with a
significance of 3.5-15.6 sigma, providing the largest catalog of 1.1 mm sources
in a contiguous region. Most of the sources are not detected in the
far-infrared bands of the AKARI satellite, suggesting that they are mostly at z
~ 1.5 given the detection limits. We constructed differential and cumulative
number counts in the ADF-S, the Subaru/XMM Newton Deep Field (SXDF), and the
SSA 22 field surveyed by AzTEC/ASTE, which provide currently the tightest
constraints on the faint end. The integration of the best-fit number counts in
the ADF-S find that the contribution of 1.1 mm sources with fluxes >=1 mJy to
the cosmic infrared background (CIB) at 1.1 mm is 12-16%, suggesting that the
large fraction of the CIB originates from faint sources of which the number
counts are not yet constrained. We estimate the cosmic star-formation rate
density contributed by 1.1 mm sources with >=1 mJy using the best-fit number
counts in the ADF-S and find that it is lower by about a factor of 5-10
compared to those derived from UV/optically-selected galaxies at z ~ 2-3. The
fraction of stellar mass of the present-day universe produced by 1.1 mm sources
with >=1 mJy at z >= 1 is ~20%, calculated by the time integration of the
star-formation rate density. If we consider the recycled fraction of >0.4,
which is the fraction of materials forming stars returned to the interstellar
medium, the fraction of stellar mass produced by 1.1 mm sources decrease to
<~10%.Comment: 15 pages, 12 figure, accepted for publication in MNRA
Multi-Transition Study of M51's Molecular Gas Spiral Arms
Two selected regions in the molecular gas spiral arms in M51 were mapped with
the Owens Valley Radio Observatory (OVRO) mm-interferometer in the 12CO(2-1),
13CO(1-0), C18O(1-0), HCN(1-0) and HCO+(1-0) emission lines. The CO data have
been combined with the 12CO(1-0) data from Aalto et al. (1999) covering the
central 3.5kpc to study the physical properties of the molecular gas. All CO
data cubes were short spacing corrected using IRAM 30m (12CO(1-0): NRO 45m)
single dish data. A large velocity gradient (LVG) analysis finds that the giant
molecular clouds (GMCs) are similar to Galactic GMCs when studied at 180pc
(120pc) resolution with an average kinetic temperature of T_kin = 20(16)K and
H_2 density of n(H_2) = 120(240)cm^(-3) when assuming virialized clouds (a
constant velocity gradient dv/dr. The associated conversion factor between H_2
mass and CO luminosity is close to the Galactic value for most regions
analyzed. Our findings suggest that the GMC population in the spiral arms of
M51 is similar to those of the Milky Way and therefore the strong star
formation occurring in the spiral arms has no strong impact on the molecular
gas in the spiral arms. Extinction inferred from the derived H_2 column density
is very high (A_V about 15 - 30 mag), about a factor of 5-10 higher than the
average value derived toward HII regions. Thus a significant fraction of the
ongoing star formation could be hidden inside the dust lanes of the spiral
arms. A comparison of MIPS 24um and H_alpha data, however, suggests that this
is not the case and most of the GMCs studied here are not (yet) forming stars.
We also present low (4.5") resolution OVRO maps of the HCN(1-0) and HCO+(1-0)
emission at the location of the brightest 12CO(1-0) peak.Comment: 41 pages, 12 figures, 7 tables; accepted for publication by Ap
Spatial correlation between submillimetre and Lyman-alpha galaxies in the SSA 22 protocluster
Lyman-alpha emitters are thought to be young, low-mass galaxies with ages of
~10^8 yr. An overdensity of them in one region of the sky (the SSA 22 field)
traces out a filamentary structure in the early Universe at a redshift of z =
3.1 (equivalent to 15 per cent of the age of the Universe) and is believed to
mark a forming protocluster. Galaxies that are bright at (sub)millimetre
wavelengths are undergoing violent episodes of star formation, and there is
evidence that they are preferentially associated with high-redshift radio
galaxies, so the question of whether they are also associated with the most
significant large-scale structure growing at high redshift (as outlined by
Lyman-alpha emitters) naturally arises. Here we report an imaging survey of
1,100-um emission in the SSA 22 region. We find an enhancement of submillimetre
galaxies near the core of the protocluster, and a large-scale correlation
between the submillimetre galaxies and the low-mass Lyman-alpha emitters,
suggesting synchronous formation of the two very different types of
star-forming galaxy within the same structure at high redshift. These results
are in general agreement with our understanding of the formation of cosmic
structure.Comment: Published in Nature (7th May 2009 issue). The astro-ph paper includes
the main text (10 pages, 2 figures, 1 table) and supplementary material (6
pages, 4 figures, 1 table
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