95 research outputs found
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
Luminous Infrared Galaxies With the Submillimeter Array. III. The Dense Kiloparsec Molecular Concentrations of Arp 299
We have used high resolution (~2.3") observations of the local (D = 46 Mpc)
luminous infrared galaxy Arp 299 to map out the physical properties of the
molecular gas which provides the fuel for its extreme star formation activity.
The 12CO J=3-2, 12CO J=2-1 and 13CO J=2-1 lines were observed with the
Submillimeter Array and the short spacings of the 12CO J=2-1 and J=3-2
observations have been recovered using James Clerk Maxwell Telescope single
dish observations. We use the radiative transfer code RADEX to estimate the
physical properties (density, column density and temperature) of the different
regions in this system. The RADEX solutions of the two galaxy nuclei, IC 694
and NGC 3690, are consistent with a wide range of gas components, from warm
moderately dense gas with T_{kin} > 30 K and n(H_{2}) ~ 0.3 - 3 x 10^{3}
cm^{-3} to cold dense gas with T_{kin} ~ 10-30 K and n(H_{2}) > 3 x 10^{3}
cm^{-3}. The overlap region is shown to have a better constrained solution with
T_{\rm{kin}}$ ~ 10-50 K and n(H_{2}) ~ 1-30 x 10^{3} cm^{-3}. We estimate the
gas masses and star formation rates of each region in order to derive molecular
gas depletion times. The depletion times of all regions (20-60 Myr) are found
to be about 2 orders of magnitude lower than those of normal spiral galaxies.
This rapid depletion time can probably be explained by a high fraction of dense
gas on kiloparsec scales in Arp 299. We estimate the CO-to-H_{2} factor,
\alpha_{co} to be 0.4 \pm 0.3 (3 x 10^{-4}/ x_{CO}) M_{sol} (K km s^{-1}
pc^{2})^{-1} for the overlap region. This value agrees well with values
determined previously for more advanced merger systems.Comment: 24 pages, 4 figures, ApJ accepte
Highly turbulent gas on GMC-scales in NGC 3256, the nearest luminous infrared galaxy
We present the highest resolution CO (2-1) observations obtained to date
(0.25") of NGC 3256 and use them to determine the detailed properties of the
molecular interstellar medium in the central 6 kpc of this merger.
Distributions of physical quantities are reported from pixel-by-pixel
measurements at 55 and 120 pc scales and compared to disc galaxies observed by
PHANGS-ALMA. Mass surface densities range from 8 to 5500 M pc
and velocity dispersions from 10 to 200 km s. Peak brightness
temperatures as large as 37 K are measured, indicating the gas in NGC 3256 may
be hotter than all regions in nearby disc galaxies measured by PHANGS-ALMA.
Brightness temperatures even surpass those in the overlap region of NGC 4038/9
at the same scales. The majority of the gas appears unbound with median virial
parameters of 7 to 19, although external pressure may bind some of the gas.
High internal turbulent pressures of 10 to 10 K cm are
found. Given the lack of significant trends in surface density, brightness
temperature, and velocity dispersion with physical scale we argue the molecular
gas is made up of a smooth medium down to 55 pc scales, unlike the more
structured medium found in the PHANGS-ALMA disc galaxies.Comment: accepted to MNRAS, 21 pages, 8 figure
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