129 research outputs found
Submillimeter Array CO(2-1) Imaging of the NGC 6946 Giant Molecular Clouds
We present a CO(2-1) mosaic map of the spiral galaxy NGC 6946 by combining
data from the Submillimeter Array and the IRAM 30 m telescope. We identify 390
giant molecular clouds (GMCs) from the nucleus to 4.5 kpc in the disk. GMCs in
the inner 1 kpc are generally more luminous and turbulent, some of which have
luminosities >10^6 K km/s pc^2 and velocity dispersions >10 km/s. Large-scale
bar-driven dynamics likely regulate GMC properties in the nuclear region.
Similar to the Milky Way and other disk galaxies, GMC mass function of NGC 6946
has a shallower slope (index>-2) in the inner region, and a steeper slope
(index<-2) in the outer region. This difference in mass spectra may be
indicative of different cloud formation pathways: gravitational instabilities
might play a major role in the nuclear region, while cloud coalescence might be
dominant in the outer disk. Finally, the NGC 6946 clouds are similar to those
in M33 in terms of statistical properties, but they are generally less luminous
and turbulent than the M51 clouds.Comment: Published in Ap
A Luminous Infrared Merger with Two Bipolar Molecular Outflows: ALMA and SMA Observations of NGC 3256
We report ALMA and SMA observations of the luminous infrared merger NGC 3256,
the most luminous galaxy within z=0.01. Both of the two merger nuclei separated
by 5" (0.8 kpc) on the sky have a compact concentration of molecular gas, i.e.,
nuclear disks with Sigma_mol > 10^3 Msun pc^-2. The one at the northern nucleus
is face-on while the southern nuclear disk is almost edge-on. The northern
nucleus is more massive and has molecular arcs and spiral arms around. The
high-velocity molecular gas previously found in the system is resolved to two
molecular outflows associated with each of the two nuclei. The molecular
outflow from the northern nuclear disk is part of a starburst-driven superwind
seen nearly pole on. Its maximum velocity is >750 km/s and its mass outflow
rate is estimated to be > 60 Msun/yr for a conversion factor
N_{H_2}/I_{CO(1-0)}=1x10^20 cm^-2/(K km/s). The outflow from the southern
nucleus is a highly collimated bipolar molecular jet seen nearly edge-on. Its
line-of-sight velocity increases with distance out to 300 pc from the southern
nucleus. Its maximum de-projected velocity is ~2000 km/s for the estimated
inclination and should exceed 1000 km/s even allowing for its uncertainty. The
mass outflow rate is estimated to be >50 Msun/yr for this outflow. There are
possible signs that this southern outflow has been driven by a bipolar radio
jet from an AGN that became inactive very recently. The sum of these outflow
rates, although subject to the uncertainty in the molecular mass estimate,
either exceeds or compares to the total star formation rate in NGC 3256. The
feedback from nuclear activities in the form of molecular outflows is therefore
significant in the gas consumption budget, and hence evolution, of this
luminous infrared galaxy. (abridged)Comment: 36 pages, 21 figures, submitted to ApJ on Mar. 4, 201
Star-forming Cloud Complexes in the Central Molecular Zone of NGC 253
We report 350 and 230 GHz observations of molecular gas and dust in the
starburst nucleus of NGC 253 at 20-40 pc (1"-2") resolution. The data contain
CO(3-2), HCN(4-3), CO(2-1), 13CO(2-1), C18O(2-1), and continuum at 0.87 mm and
1.3 mm toward the central kiloparsec. The CO(2-1) size of the galaxy's central
molecular zone (CMZ) is measured to be about 300 pc x 100 pc at the half
maximum of intensity. Five clumps of dense and warm gas stand out in the CMZ at
arcsecond resolution, and they are associated with compact radio sources due to
recent massive star formation. They contribute one third of the CO emission in
the central 300 pc and have 12CO peak brightness temperatures around 50 K,
molecular gas column densities on the order of 10^{4} Msun pc^{-2}, gas masses
on the order of 10^{7} Msun in the size scale of 20 pc, volume-averaged gas
densities of n(H2) ~ 4000 cm^{-3}, and high HCN-to-CO ratios suggestive of
higher fractions of dense gas than in the surrounding environment. It is
suggested that these are natal molecular cloud complexes of massive star
formation. The CMZ of NGC 253 is also compared with that of our Galaxy in
CO(2-1) at the same 20 pc resolution. Their overall gas distributions are
strikingly similar. The five molecular cloud complexes appear to be akin to
such molecular complexes as Sgr A, Sgr B2, Sgr C, and the l=1.3deg cloud in the
Galactic center. On the other hand, the starburst CMZ in NGC 253 has higher
temperatures and higher surface (and presumably volume) densities than its
non-starburst cousin.Comment: ApJ in press, 18 page
High Density Molecular Gas in the IR-bright Galaxy System VV114
New high resolution CO(3-2) interferometric map of the IR-bright interacting
galaxy system VV114 observed with the Submillimeter Array (SMA) reveal a
substantial amount of warm and dense gas in the IR-bright but optically
obscured galaxy, VV114E, and the overlap region connecting the two nuclei. A
1.8 x 1.4 kpc concentration of CO(3-2) emitting gas with a total mass of 4 x
10^9 Msun coincides with the peaks of NIR, MIR, and radio continuum emission
found previously by others, identifying the dense fuel for the AGN and/or the
starburst activity there. Extensive CO(2-1) emission is also detected,
revealing detailed distribution and kinematics that are consistent with the
earlier CO(1-0) results. The widely distributed molecular gas traced in CO(2-1)
and the distributed discrete peaks of CO(3-2) emission suggest that a spatially
extended intense starbursts may contribute significantly to its large IR
luminosity. These new observations further support the notion that VV114 is
approaching its final stage of merger, when violent central inflow of gas
triggers intense starburst activity possibly boosting the IR luminosity above
the ultraluminous threshold.Comment: 11 pages, 2 figures, accepted for publication in ApJ
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