372 research outputs found
Characterizing the line emission from molecular clouds. II. A comparative study of California, Perseus, and Orion A
We characterize the molecular-line emission of three clouds whose
star-formation rates span one order of magnitude: California, Perseus, and
Orion A.
We use stratified random sampling to select positions representing
the different column density regimes of each cloud and observe them with the
IRAM-30m telescope. We cover the 3 mm wavelength band and focus our analysis on
CO, HCN, CS, HCO+, HNC, and N2H+.
We find that the line intensities depend most strongly on the H2
column density. A secondary effect, especially visible in Orion A, is a
dependence of the line intensities on the gas temperature. We explored a method
that corrects for temperature variations and show that, when it is applied, the
emission from the three clouds behaves very similarly. CO intensities vary
weakly with column density, while the intensity of traditional dense-gas
tracers such as HCN, CS, and HCO+ varies almost linearly with column density.
N2H+ differs from all other species in that it traces only cold dense gas. The
intensity of the rare HCN and CS isotopologs reveals additional
temperature-dependent abundance variations. Overall, the clouds have similar
chemical compositions that, as the depth increases, are sequentially dominated
by photodissociation, gas-phase reactions, molecular freeze-out, and stellar
feedback in the densest parts of Orion A. Our observations also allowed us to
calculate line luminosities for each cloud, and a comparison with literature
values shows good agreement. We used our HCN data to explore the behavior of
the HCN conversion factor, finding that it is dominated by the emission from
the outermost cloud layers. It also depends strongly on the gas kinetic
temperature. Finally, we show that the HCN/CO ratio provides a gas volume
density estimate, and that its correlation with the column density resembles
that found in extragalactic observations.Comment: 36 pages, 19 figures, accepted for publication in A&
Molecular line probes of activity in galaxies
The use of specific tracers of the dense molecular gas phase can help to
explore the feedback of activity on the interstellar medium (ISM) in galaxies.
This information is a key to any quantitative assessment of the efficiency of
the star formation process in galaxies. We present the results of a survey
devoted to probe the feedback of activity through the study of the excitation
and chemistry of the dense molecular gas in a sample of local universe
starbursts and active galactic nuclei (AGNs). Our sample includes also 17
luminous and ultraluminous infrared galaxies (LIRGs and ULIRGs). From the
analysis of the LIRGs/ULIRGs subsample, published in Gracia-Carpio et al.(2007)
we find the first clear observational evidence that the star formation
efficiency of the dense gas, measured by the L_FIR/L_HCN ratio, is
significantly higher in LIRGs and ULIRGs than in normal galaxies. Mounting
evidence of overabundant HCN in active environments would even reinforce the
reported trend, pointing to a significant turn upward in the Kennicutt-Schmidt
law around L_FIR=10^11 L_sun. This result has major implications for the use of
HCN as a tracer of the dense gas in local and high-redshift luminous infrared
galaxies.Comment: 4 pages, 2 figures, contributed paper to Far-Infrared Workshop 07
(FIR 2007
Characterizing the line emission from molecular clouds. Stratified random sampling of the Perseus cloud
The traditional approach to characterize the structure of
molecular clouds is to map their line emission.
We aim to test and apply a stratified random sampling technique that
can characterize the line emission from molecular clouds more efficiently than
mapping.
We sampled the molecular emission from the Perseus cloud using the
H2 column density as a proxy. We divided the cloud into ten logarithmically
spaced column density bins, and we randomly selected ten positions from each
bin. The resulting 100 cloud positions were observed with the IRAM 30m
telescope, covering the 3mm-wavelength band and parts of the 2 and 1mm bands.
We focus our analysis on 11 molecular species detected toward most
column density bins. In all cases, the line intensity is tightly correlated
with the H2 column density. For the CO isotopologs, the trend is relatively
flat, while for high-dipole moment species such as HCN, CS, and HCO+ the trend
is approximately linear. We reproduce this behavior with a cloud model in which
the gas density increases with column density, and where most species have
abundance profiles characterized by an outer photodissociation edge and an
inner freeze-out drop. The intensity behavior of the high-dipole moment species
arises from a combination of excitation effects and molecular freeze out, with
some modulation from optical depth. This quasi-linear dependence with the H2
column density makes the gas at low column densities dominate the
cloud-integrated emission. It also makes the emission from most high-dipole
moment species proportional to the cloud mass inside the photodissociation
edge.
Stratified random sampling is an efficient technique for
characterizing the emission from whole molecular clouds. It shows that despite
the complex appearance of Perseus, its molecular emission follows a relatively
simple pattern.Comment: 27 pages, 19 figures, accepted for publication in A&
Modelling the Molecular Gas in NGC 6240
We present the first observations of HCN, HCO
and SiO in NGC\,6240, obtained with the IRAM PdBI. Combining a Markov
Chain Monte Carlo (MCMC) code with Large Velocity Gradient (LVG) modelling, and
with additional data from the literature, we simultaneously fit three gas
phases and six molecular species to constrain the physical condition of the
molecular gas, including massluminosity conversion factors. We find
of dense molecular gas in cold, dense clouds (\,K, \,cm) with a volume filling factor
, embedded in a shock heated molecular medium (\,K,
\,cm), both surrounded by an extended diffuse
phase (\,K, \,cm). We
derive a global with gas masses
, dominated by the
dense gas. We also find , which traces the
cold, dense gas. The [C]/[C] ratio is only slightly elevated
(), contrary to the very high [CO]/[CO] ratio (300-500)
reported in the literature. However, we find very high [HCN]/[HCN] and
[HCO]/[HCO] abundance ratios which we
attribute to isotope fractionation in the cold, dense clouds.Comment: 27 pages, 17 figures, 9 tables. Accepted in Ap
Detection of CO+ in the nucleus of M82
We present the detection of the reactive ion CO+ towards the prototypical
starburst galaxy M82. This is the first secure detection of this short-lived
ion in an external galaxy. Values of [CO+]/[HCO+]>0.04 are measured across the
inner 650pc of the nuclear disk of M82. Such high values of the [CO+]/[HCO+]
ratio had only been previously measured towards the atomic peak in the
reflection nebula NGC7023. This detection corroborates that the molecular gas
reservoir in the M82 disk is heavily affected by the UV radiation from the
recently formed stars. Comparing the column densities measured in M82 with
those found in prototypical Galactic photon-dominated regions (PDRs), we need
\~20 clouds along the line of sight to explain our observations. We have
completed our model of the molecular gas chemistry in the M82 nucleus. Our PDR
chemical model successfully explains the [CO+]/[HCO+] ratios measured in the
M~82 nucleus but fails by one order of magnitude to explain the large measured
CO+ column densities (~1--4x10^{13} cm^{-2}). We explore possible routes to
reconcile the chemical model and the observations.Comment: 12 pages, 2 figure
Chemically Distinct Nuclei and Outflowing Shocked Molecular Gas in Arp 220
We present the results of interferometric spectral line observations of Arp
220 at 3.5mm and 1.2mm from the Plateau de Bure Interferometer (PdBI), imaging
the two nuclear disks in HCN and , HCO and , and HNC as well as SiO and , HCN, and SO. The gas traced by SiO
has a complex and extended kinematic signature including a prominent P Cygni
profile, almost identical to previous observations of HCO. Spatial
offsets north and south of the continuum centre in the emission and
absorption of the SiO P Cygni profile in the western nucleus (WN)
imply a bipolar outflow, delineating the northern and southern edges of its
disk and suggesting a disk radius of pc, consistent with that found by
ALMA observations of Arp 220. We address the blending of SiO and
HCO by considering two limiting cases with regards to the
HCO emission throughout our analysis. Large velocity gradient (LVG)
modelling is used to constrain the physical conditions of the gas and to infer
abundance ratios in the two nuclei. Our most conservative lower limit on the
[HCN]/[HCO] abundance ratio is 11 in the WN, cf. 0.10 in the
eastern nucleus (EN). Comparing these ratios to the literature we argue on
chemical grounds for an energetically significant AGN in the WN driving either
X-ray or shock chemistry, and a dominant starburst in the EN.Comment: 28 pages, 17 figures, accepted to Ap
Widespread HCO emission in the M82's nuclear starburst
We present a high-resolution (~ 5'') image of the nucleus of M82 showing the
presence of widespread emission of the formyl radical (HCO). The HCO map, the
first obtained in an external galaxy, reveals the existence of a structured
disk of ~ 650 pc full diameter. The HCO distribution in the plane mimics the
ring morphology displayed by other molecular/ionized gas tracers in M82. More
precisely, rings traced by HCO, CO and HII regions are nested, with the HCO
ring lying in the outer edge of the molecular torus. Observations of HCO in
galactic clouds indicate that the abundance of HCO is strongly enhanced in the
interfaces between the ionized and molecular gas. The surprisingly high overall
abundance of HCO measured in M82 (X(HCO) ~ 4x10^{-10}) indicates that its
nuclear disk can be viewed as a giant Photon Dominated Region (PDR) of ~ 650 pc
size. The existence of various nested gas rings, with the highest HCO abundance
occurring at the outer ring (X(HCO) ~ 0.8x10^{-9}), suggests that PDR chemistry
is propagating in the disk. We discuss the inferred large abundances of HCO in
M82 in the context of a starburst evolutionary scenario, picturing the M82
nucleus as an evolved starburst.Comment: 13 pages, 3 figures, to appear in ApJ Letters; corrected list of
author
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