226 research outputs found
Dense gas in nearby galaxies: XV. Hot ammonia in NGC253, Maffei2 and IC342
The detection of NH3 inversion lines up to the (J,K)=(6,6) level is reported
toward the central regions of the nearby galaxies NGC253, Maffei2, and IC342.
The observed lines are up to 406K (for (J,K)=(6,6)) and 848K (for the (9,9)
transition) above the ground state and reveal a warm (T_kin= 100 - 140 K)
molecular component toward all galaxies studied. The tentatively detected
(J,K)=(9,9) line is evidence for an even warmer (>400K) component toward IC342.
Toward NGC253, IC342 and Maffei2 the global beam averaged NH3 abundances are
1-2 10^-8, while the abundance relative to warm H2 is around 10^-7. The
temperatures and NH3 abundances are similar to values found for the Galactic
central region. C-shocks produced in cloud-cloud collisions can explain kinetic
temperatures and chemical abundances. In the central region of M82, however,
the NH3 emitting gas component is comparatively cool (~ 30K). It must be dense
(to provide sufficient NH3 excitation) and well shielded from dissociating
photons and comprises only a small fraction of the molecular gas mass in M82.
An important molecular component, which is warm and tenuous and characterized
by a low ammonia abundance, can be seen mainly in CO. Photon dominated regions
(PDRs) can explain both the high fraction of warm H_2 in M82 and the observed
chemical abundances.Comment: 11 pages, 3 Figures, 5 Table
Dense gas in nearby galaxies XVI. The nuclear starburst environment in NGC4945
A multi-line millimeter-wave study of the nearby starburst galaxy NGC 4945
has been carried out using the Swedish-ESO Submillimeter Telescope (SEST). The
study covers the frequency range from 82 GHz to 354 GHz and includes 80
transitions of 19 molecules. 1.3 mm continuum data of the nuclear source are
also presented. A large number of molecular species indicate the presence of a
prominent high density interstellar gas component characterized by cm. Abundances of molecular species are calculated and
compared with abundances observed toward the starburst galaxies NGC 253 and M
82 and galactic sources. Apparent is an `overabundance' of HNC in the nuclear
environment of NGC 4945. While the HNC/HCN =1--0 line intensity ratio is
0.5, the HNC/HCN abundance ratio is 1. While HCN is subthermally
excited (8 K), CN is even less excited (3--4
K), indicating that it arises from a less dense gas component and that its
=2--1 line can be optically thin even though its =1--0 emission is
moderately optically thick. Overall, fractional abundances of NGC 4945 suggest
that the starburst has reached a stage of evolution that is intermediate
between those observed in NGC 253 and M 82. Carbon, nitrogen, oxygen and sulfur
isotope ratios are also determined. Within the limits of uncertainty, carbon
and oxygen isotope ratios appear to be the same in the nuclear regions of NGC
4945 and NGC 253. High O/O, low O/O and
N/N and perhaps also low S/S ratios appear to be
characteristic properties of a starburst environment in which massive stars
have had sufficient time to affect the isotopic composition of the surrounding
interstellar medium.Comment: 26 pages, 16 figures, accepted bt A&
Dense Gas in Nearby Galaxies: XVII. The Distribution of Ammonia in NGC253, Maffei2 and IC342
The central few 100 pc of galaxies often contain large amounts of molecular
gas. The chemical and physical properties of these extragalactic star formation
regions differ from those in galactic disks, but are poorly constrained. This
study aims to develop a better knowledge of the spatial distribution and
kinetic temperature of the dense neutral gas associated with the nuclear
regions of three prototypical spiral galaxies, NGC253, IC342, and Maffei2. VLA
CnD and D configuration measurements have been made of three ammonia (NH3)
inversion transitions. The (J,K)=(1,1) and (2,2) transitions of NH3 were imaged
toward IC342 and Maffei2. The (3,3) transition was imaged toward NGC253. The
entire flux obtained from single-antenna measurements is recovered for all
three galaxies observed. Derived lower limits to the kinetic temperatures
determined for the giant molecular clouds in the centers of these galaxies are
between 25 and 50K. There is good agreement between the distributions of NH3
and other H2 tracers, such as rare CO isotopologues or HCN, suggesting that NH3
is representative of the distribution of dense gas. The "Western Peak" in IC342
is seen in the (6,6) line but not in lower transitions, suggesting maser
emission in the (6,6) transition.Comment: 13 pages, 8 figures, latex format, accepted by A&
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
Abundances and Isotope Ratios in the Magellanic Clouds: The Star Forming Environment of N113
With the goal of deriving the physical and chemical conditions of star
forming regions in the Large Magellanic Cloud (LMC), a spectral line survey of
the prominent star forming region N113 is presented. The observations cover
parts of the frequency range from 85 GHz to 357 GHz and include 63 molecular
transitions from a total of 16 species, among them spectra of rare
isotopologues. Maps of selected molecular lines as well as the 1.2 mm continuum
distribution are also presented. Molecular abundances in the core of the
complex are found to be consistent with a photon dominated region (PDR) that is
nitrogen deficient, with the potential exception of N2H+. Densities range from
5x10^3 cm-3 for CO to almost 10^6 for CS and HCN, indicating that only the
densest regions provide sufficient shielding even for some of the most common
species. An ortho- to para-H_2CO ratio of ~3 hints at H_2CO formation in a warm
(>=40 K) environment. Isotope ratios are 12C/13C ~ 49+-5, 16O/18O ~ 2000+-250,
18O/17O ~ 1.7+-0.2 and 32S/34S ~ 15. Agreement with data from other star
forming clouds shows that the gas is well mixed in the LMC . The isotope ratios
do not only differ from those seen in the Galaxy. They also do not form a
continuation of the trends observed with decreasing metallicity from the inner
to the outer Galaxy. This implies that the outer Galaxy, is not providing a
transition zone between the inner Galaxy and the metal poor environment of the
Magellanic Clouds. A part of this discrepancy is likely caused by differences
in the age of the stellar populations in the outer Galaxy and the LMC.Comment: 50 pages, 13 figures, accepted for publication in Ap
Warm-Dense Molecular Gas in the ISM of Starbursts, LIRGs and ULIRGs
The role of star formation in luminous and ultraluminous infrared galaxies is
a hotly debated issue: while it is clear that starbursts play a large role in
powering the IR luminosity in these galaxies, the relative importance of
possible enshrouded AGNs is unknown. It is therefore important to better
understand the role of star forming gas in contributing to the infrared
luminosity in IR-bright galaxies. The J=3 level of 12CO lies 33K above ground
and has a critical density of ~1.5 X 10^4 cm^-3. The 12CO(J=3-2) line serves as
an effective tracer for warm-dense molecular gas heated by active star
formation. Here we report on 12CO (J=3-2) observations of 17 starburst spirals,
LIRGs and ULIRGs which we obtained with the Heinrich Hertz Submillimeter
Telescope on Mt. Graham, Arizona.
Our main results are the following: 1. We find a nearly linear relation
between the infrared luminosity and warm-dense molecular gas such that the
infrared luminosity increases as the warm-dense molecular gas to the power
0.92; We interpret this to be roughly consistent with the recent results of Gao
& Solomon (2004a,b). 2. We find L_IR/M_H2 ratios ranging from ~10 to ~128
L_sun/M_sun using a standard CO-H2 conversion factor of 3 X 10^20 cm^-2 (K km
s^-1)^-1. If this conversion factor is ~an order of magnitude less, as
suggested in a recent statistical survey (Yao et al. 2003), then 2-3 of our
objects may have significant contributions to the L_IR by dust-enshrouded AGNs.Comment: 15 Pages, 2 figures, Accepted for Publication in Ap
A 2 mm spectral line survey of the starburst galaxy NGC 253
We present the first unbiased molecular line survey towards an extragalactic
source, namely the nuclear region of the starburst galaxy NGC 253. The scan
covers the frequency band from 129.1 to 175.2 GHz, i.e. most of the 2mm
atmospheric window. We identify 111 spectral features as transitions from 25
different molecular species. Eight of which (three tentatively) are detected
for the first time in the extragalactic interstellar medium. Among these newly
detected species, we detected the rare isotopomers 34SO and HC18O+. Tentative
detections of two deuterated species, DNC and N2D+, are reported for the first
time from a target beyond the Magellanic Clouds. Additionally, three hydrogen
recombination lines are identified, while no organic molecules larger than
methanol are detected. Column densities and rotation temperatures are
calculated for all the species, including an upper limit to the ethanol
abundance. A comparison of the chemical composition of the nuclear environment
of NGC 253 with those of selected nearby galaxies demonstrates the chemical
resemblance of IC 342 and NGC 4945 to that of NGC 253. On the other hand, the
chemistries characterizing NGC 253 and M 82 are clearly different. We also
present a comparison of the chemical composition of NGC 253 with those observed
in Galactic prototypical sources. The chemistry of NGC 253 shows a striking
similarity with the chemistry observed toward the Galactic center molecular
clouds, which are thought to be dominated by low-velocity shocks. This
resemblance strongly suggests that the heating in the nuclear environment of
NGC 253 is dominated by the same mechanism as that in the central region of the
Milky Way.Comment: Accepted for publication in ApJ
Outgassing of icy bodies in the solar system - I. The sublimation of hexagonal water ice through dust layers
Our knowledge about the physical processes determining the activity of comets
were mainly influenced by several extremely successful space missions, the
predictions of theoretical models and the results of laboratory experiments.
However, novel computer models should not be treated in isolation but should be
based on experimental results. Therefore, a new experimental setup was
constructed to investigate the temperature dependent sublimation properties of
hexagonal water ice and the gas diffusion through a dry dust layer covering the
ice surface. We show that this experimental setup is capable to reproduce known
gas production rates of pure hexagonal water ice. The reduction of the gas
production rate due to an additional dust layer on top of the ice surface was
measured and compared to the results of another experimental setup in which the
gas diffusion through dust layers at room temperature was investigated. We
found that the relative permeability of the dust layer is inversely
proportional to its thickness, which is also predicted by theoretical models.
However, the measured absolute weakening of the gas flow was smaller than
predicted by models. This lack of correspondence between model and experiment
may be caused by an ill-determination of the boundary condition in the
theoretical models, which further demonstrates the necessity of laboratory
investigations. Furthermore, the impedance of the dust layer to the ice
evaporation was found to be similar to the impedance at room temperature, which
means that the temperature profile of the dust layer is not influencing the
reduction of the gas production. Finally, we present the results of an extended
investigation of the sublimation coefficient, which is an important factor for
the description of the sublimation rate of water ice and, thus, an important
value for thermophysical modeling of icy bodies in the solar system.Comment: Submitted to Icaru
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