705 research outputs found
HI and OH absorption in the lensing galaxy of MG J0414+0534
We report the detection of \HI 21-cm absorption in the early-type
lensing galaxy towards MG J0414+0534 with the Green Bank Telescope. The
absorption, with total , is resolved into two strong components, probably due to the two
strongest lens components, which are separated by 0.4\arcsec. Unlike the other
three lenses which have been detected in \HI, J0414+0534 does not exhibit
strong OH absorption, giving a OH/\HI column density ratio of N_{\rm
OH}/N_{\rm HI}\lapp10^{-6} (for K, K and
). This underabundance of molecular gas may indicate
that the extreme optical--near-IR colour () along the line-of-sight
is not due to the lens. We therefore suggest that despite the strong upper
limits on molecular absorption at the quasar redshift, as traced by millimetre
lines, the extinction occurs primarily in the quasar host galaxy.Comment: Accepted by MNRAS Letters, 5 (and a bit) pages, 5 figure
Unusually Luminous Giant Molecular Clouds in the Outer Disk of M33
We use high spatial resolution (~7pc) CARMA observations to derive detailed
properties for 8 giant molecular clouds (GMCs) at a galactocentric radius
corresponding to approximately two CO scale lengths, or ~0.5 optical radii
(r25), in the Local Group spiral galaxy M33. At this radius, molecular gas
fraction, dust-to-gas ratio and metallicity are much lower than in the inner
part of M33 or in a typical spiral galaxy. This allows us to probe the impact
of environment on GMC properties by comparing our measurements to previous data
from the inner disk of M33, the Milky Way and other nearby galaxies. The outer
disk clouds roughly fall on the size-linewidth relation defined by
extragalactic GMCs, but are slightly displaced from the luminosity-virial mass
relation in the sense of having high CO luminosity compared to the inferred
virial mass. This implies a different CO-to-H2 conversion factor, which is on
average a factor of two lower than the inner disk and the extragalactic
average. We attribute this to significantly higher measured brightness
temperatures of the outer disk clouds compared to the ancillary sample of GMCs,
which is likely an effect of enhanced radiation levels due to massive star
formation in the vicinity of our target field. Apart from brightness
temperature, the properties we determine for the outer disk GMCs in M33 do not
differ significantly from those of our comparison sample. In particular, the
combined sample of inner and outer disk M33 clouds covers roughly the same
range in size, linewidth, virial mass and CO luminosity than the sample of
Milky Way GMCs. When compared to the inner disk clouds in M33, however, we find
even the brightest outer disk clouds to be smaller than most of their inner
disk counterparts. This may be due to incomplete sampling or a potentially
steeper cloud mass function at larger radii.Comment: Accepted for Publication in ApJ; 7 pages, 4 figure
The Structure of a Low-Metallicity Giant Molecular Cloud Complex
To understand the impact of low metallicities on giant molecular cloud (GMC)
structure, we compare far infrared dust emission, CO emission, and dynamics in
the star-forming complex N83 in the Wing of the Small Magellanic Cloud. Dust
emission (measured by Spitzer as part of the S3MC and SAGE-SMC surveys) probes
the total gas column independent of molecular line emission and traces
shielding from photodissociating radiation. We calibrate a method to estimate
the dust column using only the high-resolution Spitzer data and verify that
dust traces the ISM in the HI-dominated region around N83. This allows us to
resolve the relative structures of H2, dust, and CO within a giant molecular
cloud complex, one of the first times such a measurement has been made in a
low-metallicity galaxy. Our results support the hypothesis that CO is
photodissociated while H2 self-shields in the outer parts of low-metallicity
GMCs, so that dust/self shielding is the primary factor determining the
distribution of CO emission. Four pieces of evidence support this view. First,
the CO-to-H2 conversion factor averaged over the whole cloud is very high 4-11
\times 10^21 cm^-2/(K km/s), or 20-55 times the Galactic value. Second, the
CO-to-H2 conversion factor varies across the complex, with its lowest (most
nearly Galactic) values near the CO peaks. Third, bright CO emission is largely
confined to regions of relatively high line-of-sight extinction, A_V >~ 2 mag,
in agreement with PDR models and Galactic observations. Fourth, a simple model
in which CO emerges from a smaller sphere nested inside a larger cloud can
roughly relate the H2 masses measured from CO kinematics and dust.Comment: 17 pages, 10 figures (including appendix), accepted for publication
in the Astrophysical Journa
The Role of Stellar Feedback in the Dynamics of HII Regions
Stellar feedback is often cited as the biggest uncertainty in galaxy
formation models today. This uncertainty stems from a dearth of observational
constraints as well as the great dynamic range between the small scales (<1 pc)
where the feedback occurs and the large scales of galaxies (>1 kpc) that are
shaped by this feedback. To bridge this divide, in this paper we aim to assess
observationally the role of stellar feedback at the intermediate scales of HII
regions. In particular, we employ multiwavelength data to examine several
stellar feedback mechanisms in a sample of 32 HII regions in the Large and
Small Magellanic Clouds (LMC and SMC, respectively). Using optical, infrared,
radio, and X-ray images, we measure the pressures exerted on the shells from
the direct stellar radiation, the dust-processed radiation, the warm ionized
gas, and the hot X-ray emitting gas. We find that the warm ionized gas
dominates over the other terms in all of the sources, although two have
comparable dust-processed radiation pressures to their warm gas pressures. The
hot gas pressures are comparatively weak, while the direct radiation pressures
are 1-2 orders of magnitude below the other terms. We discuss the implications
of these results, particularly highlighting evidence for hot gas leakage from
the HII shells and regarding the momentum deposition from the dust-processed
radiation to the warm gas. Furthermore, we emphasize that similar observational
work should be done on very young HII regions to test whether direct radiation
pressure and hot gas can drive the dynamics at early times.Comment: 19 pages, 8 figures; accepted by Ap
Infrared Dark Clouds in the Small Magellanic Cloud?
We have applied the unsharp-masking technique to the 24 m image of the
Small Magellanic Cloud (SMC), obtained with the Spitzer Space Telescope, to
search for high-extinction regions. This technique has been used to locate very
dense and cold interstellar clouds in the Galaxy, particularly infrared dark
clouds (IRDCs). Fifty five candidate regions of high-extinction, namely
high-contrast regions (HCRs), have been identified from the generated
decremental contrast image of the SMC. Most HCRs are located in the southern
bar region and mainly distributed in the outskirts of CO clouds, but most
likely contain a significant amount of H2. HCRs have a peak-contrast at 24
m of 2 - 2.5 % and a size of 8 - 14 pc. This corresponds to the size of
typical and large Galactic IRDCs, but Galactic IRDCs are 2 - 3 times darker at
24 m than our HCRs. To constrain the physical properties of the HCRs, we
have performed NH3, N2H+, HNC, HCO+, and HCN observations toward one of the
HCRs, HCR LIRS36-EAST, using the Australia Telescope Compact Array and the
Mopra single-dish radio telescope. We did not detect any molecular line
emission, however, our upper limits to the column densities of molecular
species suggest that HCRs are most likely moderately dense with n ~ 10^{3}
cm-3. This volume density is in agreement with predictions for the cool atomic
phase in low metallicity environments. We suggest that HCRs may be tracing
clouds at the transition from atomic to molecule-dominated medium, and could be
a powerful way to study early stages of gas condensation in low metallicity
galaxies. Alternatively, if made up of dense molecular clumps < 0.5 pc in size,
HCRs could be counterparts of Galactic IRDCs, and/or regions with highly
unusual abundance of very small dust grains.Comment: accepted for publication in the Astronomical Journa
Quasar Feedback in the Ultraluminous Infrared Galaxy F11119+3257: Connecting the Accretion Disk Wind with the Large-Scale Molecular Outflow
In Tombesi et al. (2015), we reported the first direct evidence for a quasar
accretion disk wind driving a massive molecular outflow. The target was
F11119+3257, an ultraluminous infrared galaxy (ULIRG) with unambiguous type-1
quasar optical broad emission lines. The energetics of the accretion disk wind
and molecular outflow were found to be consistent with the predictions of
quasar feedback models where the molecular outflow is driven by a hot
energy-conserving bubble inflated by the inner quasar accretion disk wind.
However, this conclusion was uncertain because the energetics were estimated
from the optically thick OH 119 um transition profile observed with Herschel.
Here, we independently confirm the presence of the molecular outflow in
F11119+3257, based on the detection of broad wings in the CO(1-0) profile
derived from ALMA observations. The broad CO(1-0) line emission appears to be
spatially extended on a scale of at least ~7 kpc from the center. Mass outflow
rate, momentum flux, and mechanical power of (80-200) R_7^{-1} M_sun/yr,
(1.5-3.0) R_7^{-1} L_AGN/c, and (0.15-0.40)% R_7^{-1} L_AGN are inferred from
these data, assuming a CO-to-H_2 conversion factor appropriate for a ULIRG (R_7
is the radius of the outflow normalized to 7 kpc and L_AGN is the AGN
luminosity). These rates are time-averaged over a flow time scale of 7x10^6
yrs. They are similar to the OH-based rates time-averaged over a flow time
scale of 4x10^5 yrs, but about a factor 4 smaller than the local
("instantaneous"; <10^5 yrs) OH-based estimates cited in Tombesi et al. The
implications of these new results are discussed in the context of time-variable
quasar-mode feedback and galaxy evolution. The need for an energy-conserving
bubble to explain the molecular outflow is also re-examined.Comment: 15 pages, 6 figures, 4 tables, accepted for publication in Ap
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