357 research outputs found
Methanol detection in M82
We present a multilevel study of the emission of methanol, detected for the
first time in this galaxy, and discuss the origin of its emission. The high
observed methanol abundance of a few 10^-9 can only be explained if injection
of methanol from dust grains is taken into account. While the overall
[CH3OH]/[NH3] ratio is much larger than observed towards other starbursts, the
dense high excitation component shows a similar value to that found in NGC 253
and Maffei 2. Our observations suggest the molecular material in M 82 to be
formed by dense warm cores, shielded from the UV radiation and similar to the
molecular clouds in other starbursts, surrounded by a less dense
photodissociated halo. The dense warm cores are likely the location of recent
and future star formation within M 82.Comment: Accepted for publication in A&A Letter
Spatially Resolved Chemistry in Nearby Galaxies I. The Center of IC 342
We have imaged emission from the millimeter lines of eight molecules--C2H,
C34S, N2H+, CH3OH, HNCO, HNC, HC3N, and SO--in the central half kpc of the
nearby spiral galaxy IC 342. The 5" (~50 pc) resolution images were made with
OVRO. Using these maps we obtain a picture of the chemistry within the nuclear
region on the sizescales of individual GMCs. Bright emission is detected from
all but SO. There are marked differences in morphology for the different
molecules. A principal component analysis is performed to quantify similarities
and differences among the images. This analysis reveals that while all
molecules are to zeroth order correlated, that is, they are all found in dense
molecular clouds, there are three distinct groups of molecules distinguished by
the location of their emission within the nuclear region. N2H+, C18O, HNC and
HCN are widespread and bright, good overall tracers of dense molecular gas. C2H
and C34S, tracers of PDR chemistry, originate exclusively from the central
50-100 pc region, where radiation fields are high. The third group of
molecules, CH3OH and HNCO, correlates well with the expected locations of
bar-induced orbital shocks. The good correlation of HNCO with the established
shock tracer molecule CH3OH is evidence that this molecule, whose chemistry has
been uncertain, is indeed produced by processing of grains. HC3N is observed to
correlate tightly with 3mm continuum emission, demonstrating that the young
starbursts are the sites of the warmest and densest molecular gas. We compare
our HNC images with the HCN images of Downes et al. (1992) to produce the first
high resolution, extragalactic HCN/HNC map: the HNC/HCN ratio is near unity
across the nucleus and the correlation of both of these gas tracers with the
star formation is excellent. (Abridged).Comment: 54 pages including 10 figures and 8 tables. Accepted for publication
in Ap
Tracing shocks and photodissociation in the Galactic center region
We present a systematic study of the HNCO, C18O, 13CS, and C34S emission
towards 13 selected molecular clouds in the Galactic center region. The
molecular emission in these positions are used as templates of the different
physical and chemical processes claimed to be dominant in the circumnuclear
molecular gas of galaxies. The relative abundance of HNCO shows a variation of
more than a factor of 20 amo ng the observed sources. The HNCO/13CS abundance
ratio is highly contrasted (up to a factor of 30) between the shielded
molecular clouds mostly affected by shocks, where HNCO is released to gas-phase
from grain mantles, and those pervaded by an intense UV radiation field, where
HNCO is photo-dissociated and CS production favored via ion reactions. We
propose the relative HNCO to CS abundance ratio as a highly contrasted
diagnostic tool to distinguish between the influence of shocks and/or the
radiation field in the nuclear regions of galaxies and their relation to the
evolutionary state of their nuclear star formation bursts.Comment: 25 pages, 5 figures, Accepted for publication in Ap
Photodissociation chemistry footprints in the Starburst galaxy NGC 253
We report the first detection of PDR molecular tracers, namely HOC+, and CO+,
and confirm the detection of the also PDR tracer HCO towards the starburst
galaxy NGC 253, claimed to be mainly dominated by shock heating and in an
earlier stage of evolution than M 82, the prototypical extragalactic PDR. Our
CO+ detection suffers from significant blending to a group of transitions of
13CH3OH, tentatively detected for the first time in the extragalactic
interstellar medium. These species are efficiently formed in the highly UV
irradiated outer layers of molecular clouds, as observed in the late stage
nuclear starburst in M 82. The molecular abundance ratios we derive for these
molecules are very similar to those found in M 82. This strongly supports the
idea that these molecules are tracing the PDR component associated with the
starburst in the nuclear region of NGC 253. A comparison with the predictions
of chemical models for PDRs shows that the observed molecular ratios are
tracing the outer layers of UV illuminated clouds up to two magnitudes of
visual extinction. Chemical models, which include grain formation and
photodissociation of HNCO, support the scenario of a photo-dominated chemistry
as an explanation to the abundances of the observed species. From this
comparison we conclude that the molecular clouds in NGC 253 are more massive
and with larger column densities than those in M 82, as expected from the
evolutionary stage of the starbursts in both galaxies.Comment: 32 pages, 4 figures, Published in Ap
- âŠ