193 research outputs found
First detection of ammonia in M82
We report the detection of the (J,K) = (1,1), (2,2), and (3,3) inversion
lines of ammonia (NH3) towards the south--western molecular lobe in M82. The
relative intensities of the ammonia lines are characterized by a rotational
temperature of T_rot=29+/-5 K which implies an average kinetic temperature of
T_kin~60 K. A Gaussian decomposition of the observed spectra indicates
increasing kinetic temperatures towards the nucleus of M82, consistent with
recent findings based on CO observations. The observations imply a very low NH3
abundance relative to H2, X(NH3)~5x10^(-10). We present evidence for a
decreasing NH3 abundance towards the central active regions in M82 and
interpret this abundance gradient in terms of photodissociation of NH3 in PDRs.
The low temperature derived here from NH3 also explains the apparent
underabundance of complex molecules like CH_3OH and HNCO, which has previously
been reported.Comment: 4 pages, 4 figures, accepted by ApJ
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
Results of the REFLEX (Return Flux Experiment) Flight Mission
The numerous problems occurring in this first flight of the REFLEX experiment, both in the spacecraft and with the instrument package, seriously constrained the acquisition and analysis of data and severely limited the interpretation of the data that were obtained. Of these, the ambient helium measurements appear to be the most promising. They are summarized and discussed in Appendix A. Further analyses could be attempted to establish the correct values for the energy centers as they varied during the mission. In addition, an extensive laboratory recalibration on a high-speed beam system could in principle provide corrections to be used in analyzing and interpreting the returned data set. The unknown malfunction which generated the energy drift needs to be understood and corrected before the REFLEX experiment is reflown; some hardware modification, or at least retuning, is likely to be required
HCN Survey of Normal Spiral, IR-luminous and Ultraluminous Galaxies
We report systematic HCN J=1-0 (and CO) observations of a sample of 53
infrared (IR) and/or CO-bright and/or luminous galaxies, including seven
ultraluminous infrared galaxies, nearly 20 luminous infrared galaxies, and more
than a dozen of the nearest normal spiral galaxies. This is the largest and
most sensitive HCN survey of galaxies to date. All galaxies observed so far
follow the tight correlation between the IR luminosity and the HCN
luminosity initially proposed by Solomon, Downes, & Radford,
which is detailed in a companion paper. We also address here the issue of HCN
excitation. There is no particularly strong correlation between
and the 12m luminosity; in fact, of all the four \IRAS bands, the 12m
luminosity has the weakest correlation with the HCN luminosity. There is also
no evidence of stronger HCN emission or a higher ratio of HCN and CO
luminosities for galaxies with excess 12m
emission. This result implies that mid-IR radiative pumping, or populating, of
the J=1 level of HCN by a mid-IR vibrational transition is not important
compared with the collisional excitation by dense molecular hydrogen.
Furthermore, large velocity gradient calculations justify the use of HCN J=1-0
emission as a tracer of high-density molecular gas (\approxgt 3\times
10^4/\tau cm) and give an estimate of the mass of dense molecular gas
from HCN observations. Therefore, may be used as a measure of the
total mass of dense molecular gas, and the luminosity ratio may indicate the fraction of molecular gas that is dense.Comment: ApJS, May issue (final version, 12 pages + 5 figures + 4 tables).
Fig. 2a,b,c and Fig. 3a,b are in GIF format due to the space limitation of
astro-ph. Added an error-bar in both Fig. 4 and Fig. 5a. Please see the
companion paper by Gao & Solomon (Paper II, astro-ph/0310339) for the
detailed analysis and implications of this HCN surve
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
Molecular Gas in Candidate Double Barred Galaxies III. A Lack of Molecular Gas?
Most models of double-barred galaxies suggest that a molecular gas component
is crucial for maintaining long-lived nuclear bars. We have undertaken a CO
survey in an attempt to determine the gas content of these systems and to
locate double barred galaxies with strong CO emission that could be candidates
for high resolution mapping. We observed 10 galaxies in CO J=2-1 and J=3-2 and
did not detect any galaxies that had not already been detected in previous CO
surveys. We preferentially detect emission from galaxies containing some form
of nuclear activity. Simulations of these galaxies require that they contain 2%
to 10% gas by mass in order to maintain long-lived nuclear bars. The fluxes for
the galaxies for which we have detections suggest that the gas mass fraction is
in agreement with these models requirements. The lack of emission in the other
galaxies suggests that they contain as little as 7 x 10^6 solar masses of
molecular material which corresponds to < 0.1% gas by mass. This result
combined with the wide variety of CO distributions observed in double barred
galaxies suggests the need for models of double-barred galaxies that do not
require a large, well ordered molecular gas component.Comment: 17 pages (3 figures embedded on pg 17). To appear in the March 10
issue of the Astrophysical Journa
[12CII] and [13CII] 158 mum emission from NGC 2024: Large column densities of ionized carbon
Context: We analyze the NGC 2024 HII region and molecular cloud interface
using [12CII] and [13CII] observations. Aims: We attempt to gain insight into
the physical structure of the interface layer between the molecular cloud and
the HII region. Methods. Observations of [12CII] and [13CII] emission at 158
{\mu}m with high spatial and spectral resolution allow us to study the detailed
structure of the ionization front and estimate the column densities and
temperatures of the ionized carbon layer in the PDR. Results: The [12CII]
emission closely follows the distribution of the 8 mum continuum. Across most
of the source, the spectral lines have two velocity peaks similar to lines of
rare CO isotopes. The [13CII] emission is detected near the edge-on ionization
front. It has only a single velocity component, which implies that the [12CII]
line shape is caused by self-absorption. An anomalous hyperfine line-intensity
ratio observed in [13CII] cannot yet be explained. Conclusions: Our analysis of
the two isotopes results in a total column density of N(H)~1.6\times10^23 cm^-2
in the gas emitting the [CII] line. A large fraction of this gas has to be at a
temperature of several hundred K. The self-absorption is caused by a cooler
(T<=100 K) foreground component containing a column density of N(H)~10^22
cm^-2
Molecular Gas in the Lensed Lyman Break Galaxy cB58
We have used the IRAM Plateau de Bure Interferometer to map CO(3-2) emission
from the gravitationally lensed Lyman break galaxy MS1512-cB58. This is the
first detection of a molecular emission line in any Lyman break system; its
integrated intensity implies a total molecular gas mass of 6.6e9 Msun, while
its width implies a dynamical mass of 1.0e10 csc^2i Msun (for a flat Lambda=0.7
cosmology). These estimates are in excellent concordance with nearly all
parameters of the system measured at other wavelengths, and yield a consistent
picture of past and future star formation with no obvious discrepancies
requiring explanation by differential lensing. In particular, we find that the
age and remaining lifetime of the current episode of star formation are likely
to be similar; the surface densities of star formation and molecular gas mass
are related by a Schmidt law; and the fraction of baryonic mass already
converted into stars is sufficient to account for the observed enrichment of
the interstellar medium to 0.4 Zsun. Barring substantial gas inflow or a major
merger, the stars forming in the current episode will have mass and coevality
at z=0 similar to those of a spiral bulge. Assuming cB58 is a typical Lyman
break galaxy apart from its magnification, its global parameters suggest that
the prescriptions for star formation used in some semi-analytic models of
galaxy evolution require moderate revision, although the general prediction
that gas mass fraction should increase with redshift is validated. [abridged]Comment: 41 pages, 6 figures, accepted by Ap
Interactions of ultrahigh-energy cosmic rays with photons in the galactic center
Ultrahigh-energy cosmic rays passing through the central region of the Galaxy
interact with starlight and the infrared photons. Both nuclei and protons
generate secondary fluxes of photons and neutrinos on their passage through the
central region. We compute the fluxes of these secondary particles, the
observations of which can be used to improve one's understanding of origin and
composition of ultrahigh-energy comic rays, especially if the violation of the
Greisen--Zatespin--Kuzmin cutoff is confirmed by the future data.Comment: 8 pages, 2 figure
Dense Gas and Star Formation: Characteristics of Cloud Cores Associated with Water Masers
We have observed 150 regions of massive star formation, selected originally
by the presence of a water maser, in the J = 5-4, 3-2, and 2-1 transitions of
CS, and 49 regions in the same transitions of CS. Over 90% of the 150
regions were detected in the J = 2-1 and 3-2 transitions of CS and 75% were
detected in the J=5-4 transition. We have combined the data with the J = 7-6
data from our original survey (Plume et al. 1992) to determine the density by
analyzing the excitation of the rotational levels. Using Large Velocity
Gradient (LVG) models, we have determined densities and column densities for 71
of these regions. The gas densities are very high (the mean log of the density
is 5.9), but much less than the critical density of the J=7-6 line. Small maps
of 25 of the sources in the J = 5-4 line yield a mean diameter of 1.0 pc. The
mean virial mass is 3800 solar masses. The mean ratio of bolometric luminosity
to virial mass (L/M) is 190, about 50 times higher than estimates using CO
emission, suggesting that star formation is much more efficient in the dense
gas probed in this study. The gas depletion time for the dense gas is roughly
1.3 x 10^7 yr. We find no statistically significant linewidth--size or
density--size relationships in our data. Instead, both linewidth and density
are larger for a given size than would be predicted by the usual relationships.
We find that the linewidth increases with density, the opposite of what would
be predicted by the usual arguments. We estimate that the luminosity of our
Galaxy (excluding the inner 400 pc) in the CS J = 5-4 transition is 15 to 23
L_sun, considerably less than the luminosity in this line within the central
100 pc of NGC 253 and M82. In addition, the ratio of far-infrared luminosity to
CS luminosity is higher in M82 than in any cloud in our sample.Comment: 26 pages, 6 postscript figures, 3 postscript tables. Uses AAS Latex
macros, accepted for Astrophysical Journa
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