241 research outputs found
Chemical tracers of high-metallicity environments
We present for the first time a detailed study of the properties of molecular
gas in metal-rich environments such as early-type galaxies (ETGs). We have
explored Photon-Dominated Region (PDR) chemistry for a wide range of physical
conditions likely to be appropriate for these sources. We derive fractional
abundances of the 20 most chemically reactive species as a function of the
metallicity, as a function of the optical depth and for various volume number
gas densities, Far-Ultra Violet (FUV) radiation fields and cosmic ray
ionisation rates. We also investigate the response of the chemistry to the
changes in element enhancement as seen in ETGs. We find that the
fractional abundances of CS, HS, HCS, HO, HO,
HCO and HCN seem invariant to an increase of metallicity whereas
C, CO, CH, CN, HCN, HNC and OCS appear to be the species most
sensitive to this change. The most sensitive species to the change in the
fractional abundance of elements are C, C, CN, HCN, HNC, SO,
SO, HO and CS. Finally, we provide line brightness ratios for the
most abundant species, especially in the range observable with ALMA. Discussion
of favorable line ratios to use for the estimation of super-solar metallicities
and -elements are also provided.Comment: 15 pages, 6 figures, 4 tables, Accepted for publication into MNRA
ISM chemistry in metal rich environments: molecular tracers of metallicity
In this paper we use observations of molecular tracers in metal rich and
alpha-enhanced galaxies to study the effect of abundance changes on molecular
chemistry. We selected a sample of metal rich spiral and star bursting objects
from the literature, and present here new data for a sample of early-type
galaxies (ETGs). We conducted the first survey of CS and methanol emission in
ETGs, detecting 7 objects in CS, and 5 in methanol emission. We find evidence
to support the hypothesis that CS is a better tracer of dense star-forming gas
than HCN. We suggest that the methanol emission in these sources is driven by
dust mantle destruction due to ionisation from high mass star formation, but
cannot rule out shocks dominating in some sources. The derived source averaged
CS/methanol column densities and rotation temperatures are similar to those
found in normal spiral and starburst galaxies, suggesting dense clouds are
little affected by the differences between galaxy types. Finally we used the
total column density ratios for our galaxy samples to show for the first time
that some molecular tracers do seem to show systematic variations that appear
to correlate with metallicity, and that these variations roughly match those
predicted by chemical models. Using this fact, the chemical models of Bayet et
al. (2012b), and assumptions about the optical depth we are able to roughly
predict the metallicity of our spiral and ETG sample, with a scatter of ~0.3
dex. We provide the community with linear approximations to the relationship
between the HCN and CS column density ratio and metallicity. Further study will
clearly be required to determine if this, or any, molecular tracer can be used
to robustly determine gas-phase metallically, but that a relationship exists at
all suggests that in the future it may be possible to calibrate a metallicity
indicator for the molecular interstellar medium (abridged).Comment: 14 pages, 9 figures. MNRAS, accepte
Molecular Gas Properties of the Giant Molecular Cloud Complexes in the Arms and Inter-arms of the Spiral Galaxy NGC 6946
Combining observations of multiple CO lines with radiative transfer modeling
is a very powerful tool to investigate the physical properties of the molecular
gas in galaxies. Using new observations as well as literature data, we provide
the most complete CO ladders ever generated for eight star-forming regions in
the spiral arms and inter-arms of the spiral galaxy NGC 6946, with observations
of the CO(1-0), CO(2-1), CO(3-2), CO(4-3), CO(6-5), 13CO(1-0) and 13CO(2-1)
transitions. For each region, we use the large velocity gradient assumption to
derive beam-averaged molecular gas physical properties, namely the gas kinetic
temperature (T_K), H2 number volume density n(H2) and CO number column density
N(CO). Two complementary approaches are used to compare the observations with
the model predictions: chi-square minimisation and likelihood. The physical
conditions derived vary greatly from one region to the next: T_K=10-250 K,
n(H2)=10^2.3-10^7.0 cm^-3 and N(CO)=10^15.0-10^19.3 cm^-2. The spectral line
energy distribution (SLED) of some of these extranuclear regions indicates a
star-formation activity that is more intense than that at the centre of our own
Milky Way. The molecular gas in regions with a large SLED turnover transition
(J_max>4) is hot but tenuous with a high CO column density, while that in
regions with a low SLED turnover transition (J_max<=4) is cold but dense with a
low CO column density. We finally discuss and find some correlations between
the physical properties of the molecular gas in each region and the presence of
young stellar population indicators (supernova remnants, HII regions, HI holes,
etc.)Comment: 23 pages, 11 figures, MNRAS, Accepte
Molecular tracers of PDR-dominated galaxies
Photon-dominated regions (PDRs) are powerful molecular line emitters in
external galaxies. They are expected in galaxies with high rates of massive
star formation due to either starburst (SB) events or starburst coupled with
active galactic nuclei (AGN) events. We have explored the PDR chemistry for a
range of physical conditions representing a variety of galaxy types. Our main
result is a demonstration of the sensitivity of the chemistry to changes in the
physical conditions. We adopt crude estimates of relevant physical parameters
for several galaxy types and use our models to predict suitable molecular
tracers of those conditions. The set of recommended molecular tracers differs
from that which we recommended for use in galaxies with embedded massive stars.
Thus, molecular observations can in principle be used to distinguish between
excitation by starburst and by SB+AGN in distant galaxies. Our recommendations
are intended to be useful in preparing Herschel and ALMA proposals to identify
sources of excitation in galaxies.Comment: 18 pages, 6 figures, Accepted in Ap
The influence of cosmic rays in the circumnuclear molecular gas of NGC1068
We surveyed the circumnuclear disk of the Seyfert galaxy NGC1068 between the
frequencies 86.2 GHz and 115.6 GHz, and identified 17 different molecules.
Using a time and depth dependent chemical model we reproduced the observational
results, and show that the column densities of most of the species are better
reproduced if the molecular gas is heavily pervaded by a high cosmic ray
ionization rate of about 1000 times that of the Milky Way. We discuss how
molecules in the NGC1068 nucleus may be influenced by this external radiation,
as well as by UV radiation fields.Comment: 6 pages. Conference proceeding for the workshop on "Cosmic-ray
induced phenomenology in star-forming environments" held in Sant Cugat,
Spain, on April 16-19, 201
Chemistry in Cosmic-Ray Dominated Regions (CRDRs)
Molecular line observations may serve as diagnostics of the degree to which
the number density of cosmic ray protons, having energies of 10s to 100s of
MeVs each, is enhanced in starburst galaxies and galaxies with active nuclei.
Results, obtained with the UCL\_PDR code, for the fractional abundances of
molecules as functions of the cosmic-ray induced ionisation rate, , are
presented. The aim is not to model any particular external galaxies. Rather, it
is to identify characteristics of the dependencies of molecular abundances on
, in part to enable the development of suitable observational programmes
for cosmic ray dominated regions (CRDRs) which will then stimulate detailed
modelling. For a number density of hydrogen nuclei of of cm, and
high visual extinction, the fractional abundances of some species increase as
increases to s, but for much higher values of
the fractional abundances of all molecular species are significantly below
their peak values. We show in particular that OH, HO, H,
HO and OH attain large fractional abundances () for as large as s. HCO is a poor
tracer of CRDRs when s. Sulphur-bearing species may
be useful tracers of CRDRs gas in which s. Ammonia
has a large fractional abundance for s and
nitrogen appears in CN-bearing species at significant levels as
increases, even up to s. In this paper, we also discuss
our model predictions, comparing them to recent detections in both galactic and
extragalactic sources. We show that they agree well, to a first approximation,
with the observational constraints.Comment: 11 pages, 4 figures, 2 tables, Accepted to MNRAS publicatio
Are CO lines good indicators of the star formation rate in galaxies?
In this paper, we investigate the relevance of using the CO line
emissions as indicators of star formation rates (SFR). For the first time, we
present this study for a relatively large number of CO transitions (12)
as well as over a large interval in redshift (from z0 to z6). For
the nearby sources (D10 Mpc), we have used homogeneous sample of
CO data provided by Bayet et al. (2004, 2006), mixing observational and
modelled line intensities. For higher-z sources (z 1), we have collected
CO observations from various papers and have completed the data set of
line intensities with model predictions which we also present in this paper.
Finally, for increasing the statistics, we have included recent CO(1-0)
and CO(3-2) observations of intermediate-z sources. Linear regressions
have been calculated for identifying the tightest SFR-CO line luminosity
relationships. We show that the \emph{total} CO, the CO(5-4), the
CO(6-5) and the CO(7-6) luminosities are the best indicators of
SFR (as measured by the far-infrared luminosity). Comparisons with theoretical
approaches from Krumholz and Thompson (2007) and Narayanan et al. (2008) are
also performed in this paper. Although in general agreement, the predictions
made by these authors and the observational results we present here show small
and interesting discrepancies. In particular, the slope of the linear
regressions, for J 4 CO lines are not similar between
theoretical studies and observations. On one hand, a larger high-J CO
data set of observations might help to better agree with models, increasing the
statistics. On the other hand, theoretical studies extended to high redshift
sources might also reduce such discrepancies.Comment: 10 pages, 3 figures and 4 tables, Accepted in MNRA
Dense molecular gas toward W49A: A template for extragalactic starbursts?
The HCN, HCO+, and HNC molecules are commonly used as tracers of dense
star-forming gas in external galaxies, but such observations are spatially
unresolved. Reliably inferring the properties of galactic nuclei and disks
requires detailed studies of sources whose structure is spatially resolved. We
compare the spatial distributions and abundance ratios of HCN, HCO+, and HNC in
W49A, the most massive and luminous star-forming region in the Galactic disk,
based on maps of a 2' (6.6 pc) field at 14" (0.83 pc) resolution of the J=4-3
transitions of HCN, H13CN, HC15N, HCO+, H13CO+, HC18O+ and HNC. The kinematics
of the molecular gas in W49A appears complex, with a mixture of infall and
outflow motions. Both the line profiles and comparison of the main and rarer
species show that the main species are optically thick. Two 'clumps' of
infalling gas appear to be at ~40 K, compared to ~100 K at the source centre,
and may be ~10x denser than the rest of the outer cloud. Chemical modelling
suggests that the HCN/HNC ratio probes the current gas temperature, while the
HCN/HCO+ ratio and the deuterium fractionation were set during an earlier,
colder phase of evolution. The data suggest that W49A is an appropriate
analogue of an extragalactic star forming region. Our data show that the use of
HCN/HNC/HCO+ line ratios as proxies for the abundance ratios is incorrect for
W49A, suggesting the same for galactic nuclei. Our observed isotopic line
ratios such as H13CN/H13CO+ approach our modeled abundance ratios quite well in
W49A. The 4-3 lines of HCN and HCO+ are much better tracers of the dense
star-forming gas in W49A than the 1-0 lines. Our observed HCN/HNC and HCN/HCO+
ratios in W49A are inconsistent with homogeneous PDR or XDR models, indicating
that irradiation hardly affects the gas chemistry in W49A. Overall, the W49A
region appears to be a useful template for starburst galaxies.Comment: Accepted by A&A; 17 pages, 15 figure
Molecular Tracers of Filamentary CO Emission Regions Surrounding the Central Galaxies of Clusters
Optical emission is detected from filaments around the central galaxies of
clusters of galaxies. These filaments have lengths of tens of kiloparsecs. The
emission is possibly due to heating caused by the dissipation of mechanical
energy and by cosmic ray induced ionisation. CO millimeter and submillimeter
line emissions as well as H infrared emission originating in such
filaments surrounding NGC~1275, the central galaxy of the Perseus cluster, have
been detected. Our aim is to identify those molecular species, other than CO,
that may emit detectable millimeter and submillimeter line features arising in
these filaments, and to determine which of those species will produce emissions
that might serve as diagnostics of the dissipation and cosmic ray induced
ionisation. The time-dependent UCL photon-dominated region modelling code was
used in the construction of steady-state models of molecular filamentary
emission regions at appropriate pressures, for a range of dissipation and
cosmic ray induced ionisation rates and incident radiation fields.HCO and
CH emissions will potentially provide information about the cosmic ray
induced ionisation rates in the filaments. HCN and, in particular, CN are
species with millimeter and submillimeter lines that remain abundant in the
warmest regions containing molecules. Detections of the galaxy cluster
filaments in HCO, CH, and CN emissions and further detections of
them in HCN emissions would provide significant constraints on the dissipation
and cosmic ray induced ionisation rates.Comment: 11 pages, 3 figures, 3 tables, accepted in A&
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
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