376 research outputs found
Insights into the Carbon chemistry of Mon R2
Aiming to learn about the chemistry of the dense PDR around the ultracompact
(UC) HII region in Mon R2, we have observed a series of mm-wavelength
transitions of C3H2 and C2H. In addition, we have traced the distribution of
other molecules, such as H13CO+, SiO, HCO, and HC3N. These data, together with
the reactive ions recently detected, have been considered to determine the
physical conditions and to model the PDR chemistry. We then identified two kind
of molecules. The first group, formed by the reactive ions (CO+, HOC+) and
small hydrocarbons (C2H, C3H2), traces the surface layers of the PDR and is
presumably exposed to a high UV field (hence we called it as "high UV", or
HUV). HUV species is expected to dominate for visual absorptions 2 < Av < 5
mag. A second group (less exposed to the UV field, and hence called "low UV",
or LUV) includes HCO and SiO, and is mainly present at the edges of the PDR (Av
> 5 mag). While the abundances of the HUV molecules can be explained by gas
phase models, this is not the case for the studied LUV ones. Although some
efficient gas-phase reactions might be lacking, grain chemistry sounds like a
probable mechanism able to explain the observed enhancement of HCO and SiO.
Within this scenario, the interaction of UV photons with grains produces an
important effect on the molecular gas chemistry and constitutes the first
evidence of an ionization front created by the UC HII region carving its host
molecular cloud. The physical conditions and kinematics of the gas layer which
surrounds the UC HII region were derived from the HUV molecules. Molecular
hydrogen densities > 4 10^6 cm^(-3) are required to reproduce the observations.
Such high densities suggest that the HII region could be pressure-confined by
the surrounding high density molecular gas.Comment: 32 pages, 8 figures. Accepted by Astrophysical Journa
Tracing high density gas in M 82 and NGC 4038
We present the first detection of CS in the Antennae galaxies towards the NGC
4038 nucleus, as well as the first detections of two high-J (5-4 and 7-6) CS
lines in the center of M 82. The CS(7-6) line in M 82 shows a profile that is
surprisingly different to those of other low-J CS transitions we observed. This
implies the presence of a separate, denser and warmer molecular gas component.
The derived physical properties and the likely location of the CS(7-6) emission
suggests an association with the supershell in the centre of M 82.Comment: 10 pages, 3 figures, ApJ Letter - ACCEPTE
The cold gaseous halo of NGC 891
We present HI observations of the edge-on galaxy NGC 891. These are among the
deepest ever performed on an external galaxy. They reveal a huge gaseous halo,
much more extended than seen previously and containing almost 30 % of the HI.
This HI halo shows structures on various scales. On one side, there is a
filament extending (in projection) up to 22 kpc vertically from the disk. Small
(M_HI ~ 10^6 Msol) halo clouds, some with forbidden (apparently
counter-rotating) velocities, are also detected. The overall kinematics of the
halo gas is characterized by differential rotation lagging with respect to that
of the disk. The lag, more pronounced at small radii, increases with height
from the plane. There is evidence that a significant fraction of the halo is
due to a galactic fountain. Accretion from intergalactic space may also play a
role in building up the halo and providing low angular momentum material needed
to account for the observed rotation lag. The long HI filament and the
counter-rotating clouds may be direct evidence of such accretion.Comment: Accepted for publication in The Astronomical Journal. High-resolution
version available at http://www.astron.nl/~oosterlo/n891Pape
Atomic Hydrogen Properties of AGN Host Galaxies: HI in 16 NUclei of GAlaxies (NUGA) Sources
We present a comprehensive spectroscopic imaging survey of the distribution
and kinematics of atomic hydrogen (HI) in 16 nearby spiral galaxies hosting low
luminosity AGN, observed with high spectral and spatial resolution (resolution:
~20 arcsec, 5 km/s) using the NRAO Very Large Array (VLA). The sample contains
a range of nuclear types, ranging from Seyfert to star-forming nuclei and was
originally selected for the NUclei of GAlaxies project (NUGA) - a spectrally
and spatially resolved interferometric survey of gas dynamics in nearby
galaxies designed to identify the fueling mechanisms of AGN and the relation to
host galaxy evolution. Here we investigate the relationship between the HI
properties of these galaxies, their environment, their stellar distribution and
their AGN type. The large-scale HI morphology of each galaxy is classified as
ringed, spiral, or centrally concentrated; comparison of the resulting
morphological classification with AGN type reveals that ring structures are
significantly more common in LINER than in Seyfert host galaxies, suggesting a
time evolution of the AGN activity together with the redistribution of the
neutral gas. Dynamically disturbed HI disks are also more prevalent in LINER
host galaxies than in Seyfert host galaxies. While several galaxies are
surrounded by companions (some with associated HI emission), there is no
correlation between the presence of companions and the AGN type
(Seyfert/LINER).Comment: 54 pages, 7 figures, accepted for publication in AJ. The
full-resolution version is available at
http://www.mpia.de/homes/haan/research.htm
Diagnóstico por imagen de un caso de invaginación intestinal
Aprovechando como base un caso clínico de intususcepción intestinal, se discuten las ventajas e inconvenientes de los dos métodos de diagnóstico por imagen más usuales en medicina veterinaria, como son la radiología y la ecografía.A clinical case of intestinal intussusception in a dog is described, emphatazing the imaging diagnosis and discussing about the advantages and disadvantages of the two most used methods, as radiology and ultrasonography
Sulfur chemistry and isotopic ratios in the starburst galaxy NGC 253
Based on observations of the most abundant sulfur-bearing molecules (H2S, CS,
NS, SO, H2CS, OCS, and SO2) carried out with the IRAM 30m telescope and SEST,
we present the first analysis of the sulfur chemistry of an extragalactic
source, the nuclear region of the starburst galaxy NGC 253. This is the first
time that H2S and, tentatively, H2CS are detected towards the nucleus of a
starburst galaxy. Source averaged fractional abundances of these molecules are
a few 10^-9, except for CS and OCS which are more abundant (10^-8). Sulfur
isotopic ratios, 32S/34S~8+-2 and 34S/33S>9, are measured through observations
of 13CS, C34S, and C33S. A comparison with the observed relative abundances
towards different prototypical Galactic sources suggests that the chemical
composition of NGC 253 is similar to that found towards the molecular clouds
complexes like Sgr B2 in the nuclear region of the Milky Way. The large
overabundance of OCS compared to the predictions of time-dependent sulfur
chemistry models supports the idea that OCS is likely injected into the gas
phase from the grain mantles by low velocity shocks.Comment: Accepted for publication in ApJ. 19 pages, 3 figure
Spatial distribution of small hydrocarbons in the neighborhood of the Ultra Compact HII region Monoceros R2
We study the chemistry of small hydrocarbons in the photon-dominated regions
(PDRs) associated with the ultra-compact HII region Mon R2. Our goal is to
determine the variations of the abundance of small hydrocarbons in a high-UV
irradiated PDR and investigate their chemistry. We present an observational
study of CH, CCH and c-CH in Mon R2 combining data obtained with the
IRAM 30m telescope and Herschel. We determine the column densities of these
species, and compare their spatial distributions with that of polycyclic
aromatic hydrocarbon (PAH). We compare the observational results with different
chemical models to explore the relative importance of gas-phase, grain-surface
and time-dependent chemistry in these environments. The emission of the small
hydrocarbons show different patterns. The CCH emission is extended while CH and
c-CH are concentrated towards the more illuminated layers of the PDR.
The ratio of the column densities of c-CH and CCH shows spatial
variations up to a factor of a few, increasing from
_3_2 in the envelope to a maximum of
towards the 8m emission peak. Comparing these results
with other galactic PDRs, we find that the abundance of CCH is quite constant
over a wide range of G, whereas the abundance of c-CH is higher in
low-UV PDRs. In Mon R2, the gas-phase steady-state chemistry can account
relatively well for the abundances of CH and CCH in the most exposed layers of
the PDR, but falls short by a factor of 10 to reproduce c-CH.
In the molecular envelope, time-dependent effects and grain surface chemistry
play a dominant role in determining the hydrocarbons abundances. Our study
shows that CCH and c-CH present a complex chemistry in which UV
photons, grain-surface chemistry and time dependent effects contribute to
determine their abundances.Comment: 18 pages, 11 figures, 7 tables. Proposed for acceptance in A&A.
Abstract abridge
Evidence for a chemically differentiated outflow in Mrk 231
Aims: Our goal is to study the chemical composition of the outflows of active
galactic nuclei and starburst galaxies.
Methods: We obtained high-resolution interferometric observations of HCN and
HCO and of the ultraluminous infrared
galaxy Mrk~231 with the IRAM Plateau de Bure Interferometer. We also use
previously published observations of HCN and HCO and
, and HNC in the same source.
Results: In the line wings of the HCN, HCO, and HNC emission, we find
that these three molecular species exhibit features at distinct velocities
which differ between the species. The features are not consistent with emission
lines of other molecular species. Through radiative transfer modelling of the
HCN and HCO outflow emission we find an average abundance ratio
. Assuming a clumpy outflow,
modelling of the HCN and HCO emission produces strongly inconsistent
outflow masses.
Conclusions: Both the anti-correlated outflow features of HCN and HCO and
the different outflow masses calculated from the radiative transfer models of
the HCN and HCO emission suggest that the outflow is chemically
differentiated. The separation between HCN and HCO could be an indicator of
shock fronts present in the outflow, since the HCN/HCO ratio is expected to
be elevated in shocked regions. Our result shows that studies of the chemistry
in large-scale galactic outflows can be used to better understand the physical
properties of these outflows and their effects on the interstellar medium (ISM)
in the galaxy.Comment: 12 pages, 8 figures, accepted for publication in A&
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
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