1,058 research outputs found
Discovery of Water Vapor in the High-redshift Quasar APM 08279+5255 at z = 3.91
We report a detection of the excited 2_(20)-2_(11) rotational transition of para-H_2O in APM 08279+5255 using the IRAM Plateau de Bure Interferometer. At z = 3.91, this is the highest-redshift detection of interstellar water to date. From large velocity gradient modeling, we conclude that this transition is predominantly radiatively pumped and on its own does not provide a good estimate of the water abundance. However, additional water transitions are predicted to be detectable in this source, which would lead to an improved excitation model. We also present a sensitive upper limit for the hydrogen fluoride (HF) J = 1-0 absorption toward APM 08279+5255. While the face-on geometry of this source is not favorable for absorption studies, the lack of HF absorption is still puzzling and may be indicative of a lower fluorine abundance at z = 3.91 compared with the Galactic interstellar medium
CH2D+, the Search for the Holy Grail
CH2D+, the singly deuterated counterpart of CH3+, offers an alternative way
to mediate formation of deuterated species at temperatures of several tens of
K, as compared to the release of deuterated species from grains. We report a
longstanding observational search for this molecular ion, whose rotational
spectroscopy is not yet completely secure. We summarize the main spectroscopic
properties of this molecule and discuss the chemical network leading to the
formation of CH2D+, with explicit account of the ortho/para forms of H2, H3+
and CH3+. Astrochemical models support the presence of this molecular ion in
moderately warm environments at a marginal level.Comment: 25 pages, 6 Figures Accepted in Journal of Physical Chemistry A. "Oka
Festschrift: Celebrating 45 years of Astrochemistry
Interstellar CH absorption in the diffuse interstellar medium along the sight-lines to G10.6–0.4 (W31C), W49N, and W51
We report the detection of the ground state N, J = 1, 3/2 → 1, 1/2 doublet of the methylidyne radical CH at ~532 GHz and ~536 GHz with
the Herschel/HIFI instrument along the sight-line to the massive star-forming regions G10.6–0.4 (W31C), W49N, and W51. While the molecular
cores associated with these massive star-forming regions show emission lines, clouds in the diffuse interstellar medium are detected in absorption
against the strong submillimeter background. The combination of hyperfine structure with emission and absorption results in complex profiles,
with overlap of the different hyperfine components. The opacities of most of the CH absorption features are linearly correlated with those of CCH,
CN, and HCO^+ in the same velocity intervals. In specific narrow velocity intervals, the opacities of CN and HCO^+ deviate from the mean trends,
giving rise to more opaque absorption features. We propose that CCH can be used as another tracer of the molecular gas in the absence of better
tracers, with [CCH]/[H_2] ~3.2 ± 1.1 × 10^(−8). The observed [CN]/[CH], [CCH]/[CH] abundance ratios suggest that the bulk of the diffuse matter
along the lines of sight has gas densities n_H = n(H) + 2n(H_2) ranging between 100 and 1000 cm^(−3)
Atomic carbon at redshift ~2.5
Using the IRAM 30m telescope we detected the lower fine structure line of
neutral carbon towards three high--redshift sources: IRAS FSC10214 (z=2.3),
SMMJ14011+0252 (z=2.5) and H1413+117 (Cloverleaf quasar, z=2.5). SMMJ14011+0252
is the first high--redshift, non--AGN source in which CI has been detected. The
CI(1-0) line from FSC10214 is almost an order of magnitude weaker than
previously claimed, while our detection in the Cloverleaf is in good agreement
with earlier observations. The CI(1-0) linewidths are similar to the CO widths,
indicating that both lines trace similar regions of molecular gas on galactic
scales. Derived CI masses for all three objects are of order few 10^7 solar
masses and the implied CI(1-0)/CO(3-2) line luminosity ratio is about 0.2. This
number is similar to values found in local galaxies. We derive a CI abundance
of 5x10^{-5} which implies significant metal enrichment of the cold molecular
gas at redshifts 2.5 (age of the universe 2.7 Gyr). We conclude that the
physical properties of systems at large lookback times are similar to today's
starburst/AGN environments.Comment: 4 pages, 2 figures; accepted by A&
Molecular Gas in Candidate Double-Barred Galaxies II. Cooler, Less Dense Gas Associated with Stronger Central Concentrations
We have performed a multi-transition CO study of the centers of seven
double-barred galaxies that exhibit a variety of molecular gas morphologies to
determine if the molecular gas properties are correlated with the nuclear
morphology and star forming activity. Near infrared galaxy surveys have
revealed the existence of nuclear stellar bars in a large number of barred or
lenticular galaxies. High resolution CO maps of these galaxies exhibit a wide
range of morphologies. Recent simulations of double-barred galaxies suggest
that variations in the gas properties may allow it to respond differently to
similar gravitational potentials. We find that the 12CO J=3-2/J=2-1 line ratio
is lower in galaxies with centrally concentrated gas distributions and higher
in galaxies with CO emission dispersed around the galactic center in rings and
peaks. The 13CO/12CO J=2-1 line ratios are similar for all galaxies, which
indicates that the J=3-2/J=2-1 line ratio is tracing variations in gas
temperature and density, rather than variations in optical depth. There is
evidence that the galaxies which contain more centralized CO distributions are
comprised of molecular gas that is cooler and less dense. Observations suggest
that the star formation rates are higher in the galaxies containing the warmer,
denser, less centrally concentrated gas. It is possible that either the bar
dynamics are responsible for the variety of gas distributions and densities
(and hence the star formation rates) or that the star formation alone is
responsible for modifying the gas properties.Comment: 27 pages + 6 figures; to appear in the April 20, 2003 issue of Ap
A Constant Bar Fraction out to Redshift z~1 in the Advanced Camera for Surveys Field of the Tadpole Galaxy
Bar-like structures were investigated in a sample of 186 disk galaxies larger
than 0.5 arcsec that are in the I-band image of the Tadpole galaxy taken with
the HST ACS. We found 22 clear cases of barred galaxies, 21 galaxies with small
bars that appear primarily as isophotal twists in a contour plot, and 11 cases
of peculiar bars in clump-cluster galaxies, which are face-on versions of chain
galaxies. The latter bars are probably young, as the galaxies contain only weak
interclump emission. Four of the clearly barred galaxies at z~0.8-1.2 have
grand design spirals. The bar fraction was determined as a function of galaxy
inclination and compared with the analogous distribution in the local Universe.
The bar fraction was also determined as a function of galaxy angular size.
These distributions suggest that inclination and resolution effects obscure
nearly half of the bars in our sample. The bar fraction was also determined as
a function of redshift. We found a nearly constant bar fraction of 0.23+-0.03
from z~0 to z=1.1. When corrected for inclination and size effects, this
fraction is comparable to the bar fraction in the local Universe, ~0.4, as
tabulated for all bar and Hubble types in the Third Reference Catalogue of
Galaxies. The average major axis of a barred galaxy in our sample is ~10 kpc
after correcting for redshift with a LambdaCDM cosmology. Galaxy bars were
present in normal abundance at least ~8 Gy ago (z~1); bar dissolution cannot be
common during a Hubble time unless the bar formation rate is comparable to the
dissolution rate.Comment: to appear in ApJ, Sept 1, 2004, Vol 612, 18 pg, 12 figure
Nitrogen isotopic ratios in Barnard 1: a consistent study of the N2H+, NH3, CN, HCN and HNC isotopologues
The 15N isotopologue abundance ratio measured today in different bodies of
the solar system is thought to be connected to 15N-fractionation effects that
would have occured in the protosolar nebula. The present study aims at putting
constraints on the degree of 15N-fractionation that occurs during the
prestellar phase, through observations of D, 13C and 15N-substituted
isotopologues towards B1b. Both molecules from the nitrogen hydride family,
i.e. N2H+ and NH3, and from the nitrile family, i.e. HCN, HNC and CN, are
considered in the analysis. As a first step, we model the continuum emission in
order to derive the physical structure of the cloud, i.e. gas temperature and
H2 density. These parameters are subsequently used as an input in a non-local
radiative transfer model to infer the radial abundances profiles of the various
molecules. Our modeling shows that all the molecules are affected by depletion
onto dust grains, in the region that encompasses the B1-bS and B1-bN cores.
While high levels of deuterium fractionation are derived, we conclude that no
fractionation occurs in the case of the nitrogen chemistry. Independently of
the chemical family, the molecular abundances are consistent with 14N/15N~300,
a value representative of the elemental atomic abundances of the parental gas.
The inefficiency of the 15N-fractionation effects in the B1b region can be
linked to the relatively high gas temperature ~17K which is representative of
the innermost part of the cloud. Since this region shows signs of depletion
onto dust grains, we can not exclude the possibility that the molecules were
previously enriched in 15N, earlier in the B1b history, and that such an
enrichment could have been incorporated into the ice mantles. It is thus
necessary to repeat this kind of study in colder sources to test such a
possibility.Comment: accepted in A&
Detection of a dense clump in a filament interacting with W51e2
In the framework of the Herschel/PRISMAS Guaranteed Time Key Program, the
line of sight to the distant ultracompact HII region W51e2 has been observed
using several selected molecular species. Most of the detected absorption
features are not associated with the background high-mass star-forming region
and probe the diffuse matter along the line of sight. We present here the
detection of an additional narrow absorption feature at ~70 km/s in the
observed spectra of HDO, NH3 and C3. The 70 km/s feature is not uniquely
identifiable with the dynamic components (the main cloud and the large-scale
foreground filament) so-far identified toward this region. The narrow
absorption feature is similar to the one found toward low-mass protostars,
which is characteristic of the presence of a cold external envelope. The
far-infrared spectroscopic data were combined with existing ground-based
observations of 12CO, 13CO, CCH, CN, and C3H2 to characterize the 70 km/s
component. Using a non-LTE analysis of multiple transitions of NH3 and CN, we
estimated the density (n(H2) (1-5)x10^5 cm^-3) and temperature (10-30 K) for
this narrow feature. We used a gas-grain warm-up based chemical model with
physical parameters derived from the NH3 data to explain the observed
abundances of the different chemical species. We propose that the 70 km/s
narrow feature arises in a dense and cold clump that probably is undergoing
collapse to form a low-mass protostar, formed on the trailing side of the
high-velocity filament, which is thought to be interacting with the W51 main
cloud. While the fortuitous coincidence of the dense clump along the line of
sight with the continuum-bright W51e2 compact HII region has contributed to its
non-detection in the continuum images, this same attribute makes it an
appropriate source for absorption studies and in particular for ice studies of
star-forming regions.Comment: Accepted for publication in A&
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