1,268 research outputs found
Does CO trace H2 at high galactic latitude
A CO survey of 342 Infrared Excess Clouds (IRECs) distributed uniformly across the sky is presented. Following comparison of the integrated CO brightness with the 100 micron infrared brightness B(sub 4) obtained from the IRAS data, evidence was found for a threshold in B(sub 4) of 4-5 MJy sr(exp -1) below which CO does not form. Evidence is also presented that the threshold effect can be seen within an individual cloud, providing evidence for a phase transition between atomic and molecular gas. While the main thrust was to examine the CO content of the IRECs, it was also attempted to detect CO toward a number of UV stars so that CO brightness could be correlated with direct measurements of H2 column density and E(B-V). Of the 26 observed stars CO was detected toward 6. It is consistent with the results obtained using infrared data
High resolution CO images of Seyfert Galaxies
The CO (J = 1-0) emission of three Seyfert galaxies, NGC 3227, NGC 7469, and NGC 5033 was imaged. The CO emission in NGC 3227 and NGC 7469 appears as compact structures centered on the active nuclei, containing substantial fractions of the single-dish flux. In NGC 3227, 10 percent of the CO flux detected by the interferometer is contained within the ionized narrow-line region. The unresolved molecular gas concentrations in the nucleus of NGC 3227 imply a CO mass of 65 million solar masses concentrated within a diameter less than 50 pc. The CO emission in NGC 5033 is not detected at this resolution, implying a CO structure size of 20 to 60 arcsec. Continuum emission at 2.7 mm is not detected in any of the three galaxies. In the center of NGC 7469, the H2 mass is comparable to the dynamical mass. Kinematic studies of the detected gas reveal a rotational motion of the gas in NGC 3227 and NGC 7469, allowing identification of the gas in NGC 7469 with a nuclear starburst. These data are consistent with the idea that interactions between galaxies cause gas to concentrate in their nuclei thereby feeding starburst and Seyfert activity
A Universal Neutral Gas Profile for Nearby Disk Galaxies
Based on sensitive CO measurements from HERACLES and HI data from THINGS, we
show that the azimuthally averaged radial distribution of the neutral gas
surface density (Sigma_HI + Sigma_H2) in 33 nearby spiral galaxies exhibits a
well-constrained universal exponential distribution beyond 0.2*r25 (inside of
which the scatter is large) with less than a factor of two scatter out to two
optical radii r25. Scaling the radius to r25 and the total gas surface density
to the surface density at the transition radius, i.e., where Sigma_HI and
Sigma_H2 are equal, as well as removing galaxies that are interacting with
their environment, yields a tightly constrained exponential fit with average
scale length 0.61+-0.06 r25. In this case, the scatter reduces to less than 40%
across the optical disks (and remains below a factor of two at larger radii).
We show that the tight exponential distribution of neutral gas implies that the
total neutral gas mass of nearby disk galaxies depends primarily on the size of
the stellar disk (influenced to some degree by the great variability of
Sigma_H2 inside 0.2*r25). The derived prescription predicts the total gas mass
in our sub-sample of 17 non-interacting disk galaxies to within a factor of
two. Given the short timescale over which star formation depletes the H2
content of these galaxies and the large range of r25 in our sample, there
appears to be some mechanism leading to these largely self-similar radial gas
distributions in nearby disk galaxies.Comment: 7 pages, 4 figures, accepted for publication in the Astrophysical
Journa
Tightly Correlated HI and FUV Emission in the Outskirts of M83
We compare sensitive HI data from The HI Nearby Galaxy Survey (THINGS) and
deep far UV (FUV) data from GALEX in the outer disk of M83. The FUV and HI maps
show a stunning spatial correlation out to almost 4 optical radii (r25),
roughly the extent of our maps. This underscores that HI traces the gas
reservoir for outer disk star formation and it implies that massive (at least
low level) star formation proceeds almost everywhere HI is observed. Whereas
the average FUV intensity decreases steadily with increasing radius before
leveling off at ~1.7 r25, the decline in HI surface density is more subtle. Low
HI columns (<2 M_solar/pc^2) contribute most of the mass in the outer disk,
which is not the case within r25. The time for star formation to consume the
available HI, inferred from the ratio of HI to FUV intensity, rises with
increasing radius before leveling off at ~100 Gyr, i.e., many Hubble times,
near ~1.7 r25. Assuming the relatively short H2 depletion times observed in the
inner parts of galaxies hold in outer disks, the conversion of HI into bound,
molecular clouds seems to limit star formation in outer galaxy disks. The long
consumption times suggest that most of the extended HI observed in M83 will not
be consumed by in situ star formation. However, even these low star formation
rates are enough to expect moderate chemical enrichment in a closed outer disk.Comment: Accepted for Publication in ApJ
The Distances of SNR W41 and overlapping HII regions
New HI images from the VLA Galactic Plane Survey show prominent absorption
features associated with the supernovae remnant G23.3-0.3 (SNR W41). We
highlight the HI absorption spectra and the CO emission spectra of eight
small regions on the face of W41, including four HII regions, three non-thermal
emission regions and one unclassified region. The maximum velocity of
absorption for W41 is 782 km/s and the CO cloud at radial velocity
955 km/s is behind W41. Because an extended TeV source, a diffuse X-ray
enhancement and a large molecular cloud at radial velocity 775 km/s are
also projected at the center of W41, these yield the kinematic distance of 3.9
to 4.5 kpc for W41. For HII regions, our analyses reveal that both G23.42-0.21
and G23.07+0.25 are at the far kinematic distances (9.9 kpc and
10.6 kpc respectively) of their recombination-line velocities (1030.5 km/s
and 89.62.1 km/s respectively), G23.07-0.37 is at the near kinematic
distance (4.40.3 kpc) of its recombination-line velocity (82.72.0
km/s), and G23.27-0.27 is probably at the near kinematic distance (4.10.3
kpc) of its recombination-line velocity (76.10.6 km/s).Comment: 11 pages, 3 figs., 2 tables, accepted by A
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
A Magellanic Origin for the Warp of the Galaxy
We show that a Magellanic Cloud origin for the warp of the Milky Way can
explain most quantitative features of the outer HI layer recently identified by
Levine, Blitz & Heiles (2005). We construct a model similar to that of Weinberg
(1998) that produces distortions in the dark matter halo, and we calculate the
combined effect of these dark-halo distortions and the direct tidal forcing by
the Magellanic Clouds on the disk warp in the linear regime. The interaction of
the dark matter halo with the disk and resonances between the orbit of the
Clouds and the disk account for the large amplitudes observed for the vertical
m=0,1,2 harmonics. The observations lead to six constraints on warp forcing
mechanisms and our model reasonably approximates all six. The disk is shown to
be very dynamic, constantly changing its shape as the Clouds proceed along
their orbit. We discuss the challenges to MOND placed by the observations.Comment: 4 pages, 3 figures, submitted to ApJ Letters. Additional graphics, 3d
visualizations and movies available at
http://www.astro.umass.edu/~weinberg/lm
Kinetics of CH₂OO reactions with SO₂, NO₂, NO, H₂O and CH₃CHO as a function of pressure
Kinetics of CH₂OO Criegee intermediate reactions with SO₂, NO₂, NO, H₂O and CH₃CHO and CH₂I radical reactions with NO₂ are reported as a function of pressure at 295 K. Measurements were made under pseudo-first-order conditions using flash photolysis of CH₂I₂–O₂–N₂ gas mixtures in the presence of excess co-reagent combined with monitoring of HCHO reaction products by laser-induced fluorescence (LIF) spectroscopy and, for the reaction with SO₂, direct detection of CH₂OO by photoionisation mass spectrometry (PIMS). Rate coefficients for CH₂OO + SO₂ and CH₂OO + NO₂ are independent of pressure in the ranges studied and are (3.42 ± 0.42) × 10‾¹¹ cm³ s‾¹ (measured between 1.5 and 450 Torr) and (1.5 ± 0.5) × 10‾¹² cm³ s‾¹ (measured between 25 and 300 Torr), respectively. The rate coefficient for CH₂OO + CH₃CHO is pressure dependent, with the yield of HCHO decreasing with increasing pressure. Upper limits of 2 × 10−13 cm³ s‾¹ and 9 × 10−17 cm³ s‾¹ are placed on the rate coefficients for CH₂OO + NO and CH₂OO + H₂O, respectively. The upper limit for the rate coefficient for CH₂OO + H₂O is significantly lower than has been reported previously, with consequences for modelling of atmospheric impacts of CH₂OO chemistry
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