142 research outputs found
First Detection of CO in a Low Surface Brightness Galaxy
We report on the first attempts at searching for CO in red low surface
brightness galaxies, and the first detection of molecular gas in a low surface
brightness (mu_B(0)_{obs} > 23 mag arcsec^{-2}) galaxy. Using the IRAM 30m
telescope, CO(1-0) and CO(2-1) lines were searched for in four galaxies --
P06-1, P05-5, C05-3, & C04-2. In three of the galaxies no CO was detected, to
T_{MB} ~ 1.8mK (at the 3 sigma level). In the fourth galaxy, P06-1, both lines
were detected. Comparing our findings with previous studies shows P06-1 to have
a molecular-to-atomic mass ratio considerably lower than is predicted using
theoretical models based on high surface brightness galaxy studies. This
indicates the N(H_2)/(int{T(CO)dv}) conversion factor for low surface
brightness galaxies may currently be consistently underestimated by a factor of
3 - 20.Comment: 8 pages, 2 figures, accepted by the ApJ
A Search for Formaldehyde 6 cm Emission toward Young Stellar Objects. II. H2CO and H110α Observations
We report the results of our second survey for Galactic H2CO maser emission toward young stellar objects. Using the GBT and the VLA in the A configuration we observed 58 star-forming regions and discovered the fifth H2CO 6 cm maser region in the Galaxy (G23.71-0.20). We have discussed the detection toward G23.71-0.20 in a previous paper. Here we present all the other results from our survey, including detection of H2CO absorption features toward 48 sources, detection of the H110α recombination line toward 29 sources, detection (including tentative detections) of the carbon recombination line C110α toward 14 sources, subarcsecond angular resolution images of 6 cm continuum emission toward five sources, and observations of the H2CO masers in IRAS 18566+0408 and NGC 7538. In the case of NGC 7538, we detected the two main H2CO maser components, and our observations confirm variability of the blueshifted component recently reported by Hoffman et al. The variability of both maser components in NGC 7538 could be caused by a shock wave that reached the redshifted component approximately 14 yr before reaching the blueshifted component. If that were the case, we would expect to detect an increase in the flux density rate of change of the blueshifted component sometime after the year 2009. Our data also support the use of H2CO/H110α observations as a tool to resolve the kinematic distance ambiguity of massive star-forming regions in the Galaxy
Possible magnetic field variability during the 6.7 GHz methanol maser flares of G09.62+0.20
(Abridged) Recently, the magnetic field induced Zeeman splitting was measured
for the strongest known 6.7 GHz methanol maser, which arises in the massive
star forming region G09.62+0.20. This maser is one of a handful of periodically
flaring methanol masers. The 100-m Effelsberg telescope was used to monitor the
6.7 GHz methanol masers of G09.62+0.20. With the exception of a two week period
during the peak of the maser flare, we measure a constant magnetic field of
B_||~11+-2 mG in the two strongest maser components of G09.62+0.20 that are
separated by over 200 AU. In the two week period that coincides exactly with
the peak of the maser flare of the strongest maser feature, we measure a sharp
decrease and possible reversal of the Zeeman splitting. The exact cause of both
maser and polarization variability is still unclear, but it could be related to
either background amplification of polarized emission or the presence of a
massive protostar with a close-by companion. Alternatively, the polarization
variability could be caused by non-Zeeman effects related to the radiative
transfer of polarized maser emission.Comment: 4 pages, 3 figures, accepted for publication Astronomy and
Astrophysic
A New Galactic 6cm Formaldehyde Maser
We report the detection of a new H2CO maser in the massive star forming
region G23.71-0.20 (IRAS 18324-0820), i.e., the fifth region in the Galaxy
where H2CO maser emission has been found. The new H2CO maser is located toward
a compact HII region, and is coincident in velocity and position with 6.7 GHz
methanol masers and with an IR source as revealed by Spitzer/IRAC GLIMPSE data.
The coincidence with an IR source and 6.7 GHz methanol masers suggests that the
maser is in close proximity to an embedded massive protostar. Thus, the
detection of H2CO maser emission toward G23.71-0.20 supports the trend that
H2CO 6cm masers trace molecular material very near young massive stellar
objects.Comment: Accepted for publication in The Astrophysical Journal Letter
A Review of H2CO 6cm Masers in the Galaxy
We present a review of the field of formaldehyde (H2CO) 6cm masers in the
Galaxy. Previous to our ongoing work, H2CO 6cm masers had been detected in the
Galaxy only toward three regions: NGC7538 IRS1, Sgr B2, and G29.96-0.02.
Current efforts by our group using the Very Large Array, Arecibo, and the Green
Bank Telescope have resulted in the detection of four new H2CO 6cm maser
regions. We discuss the characteristics of the known H2CO masers and the
association of H2CO 6cm masers with very young regions of massive star
formation. We also review the current ideas on the pumping mechanism for H2CO
6cm masers.Comment: 10 pages, 5 figures, IAU Symposium 242: Astrophysical Masers and
their Environment
An H2CO 6cm Maser Pinpointing a Possible Circumstellar Torus in IRAS18566+0408
We report observations of 6cm, 3.6cm, 1.3cm, and 7mm radio continuum,
conducted with the Very Large Array towards IRAS18566+0408, one of the few
sources known to harbor H2CO 6cm maser emission. Our observations reveal that
the emission is dominated by an ionized jet at cm wavelengths. Spitzer/IRAC
images from GLIMPSE support this interpretation, given the presence of 4.5um
excess emission at approximately the same orientation as the cm continuum. The
7mm emission is dominated by thermal dust from a flattened structure almost
perpendicular to the ionized jet, thus, the 7mm emission appears to trace a
torus associated with a young massive stellar object. The H2CO 6cm maser is
coincident with the center of the torus-like structure. Our observations rule
out radiative pumping via radio continuum as the excitation mechanism for the
H2CO 6cm maser in IRAS18566+0408.Comment: 20 pages, 4 figures, ApJ (in press
Further Discoveries of 12CO in Low Surface Brightness Galaxies
Using the IRAM 30m telescope we have obtained seven new, deep CO J(1-0) and
J(2-1) observations of low surface brightness (LSB) galaxies. Five of the
galaxies have no CO detected to extremely low limits (0.1-0.4 K km/s at
J(1-0)), while two of the galaxies, UGC 01922 and UGC 12289, have clear
detections in both line transitions. When these observations are combined with
all previous CO observations taken of LSB systems, we compile a total of 34
observations, in which only 3 galaxies have had detections of their molecular
gas. Comparing the LSB galaxies with and without CO detections to a sample of
high surface brightness (HSB) galaxies with CO observations indicates that it
is primarily the low density of baryonic matter within LSB galaxies which is
causing their low CO fluxes. Finally, we note that one of the massive LSB
galaxies studied in this project, UGC 06968 (a Malin-1 `cousin'), has upper
limits placed on both M_H2 and M_H2/M_HI which are 10-20 times lower than the
lowest values found for any galaxy (LSB or HSB) with similar global properties.
This may be due to an extremely low temperature and metallicity within UGC
06968, or simply due to the CO distribution within the galaxy being too diffuse
to be detected by the IRAM beam.Comment: 18 pages, 8 figures, 4 tables. Accepted by Ap
Excited Hydroxyl Outflow in the High-Mass Star-Forming Region G34.26+0.15
G34.26+0.15 is a region of high-mass star formation that contains a broad
range of young stellar objects in different stages of evolution, including a
hot molecular core, hyper-compact HII regions and a prototypical cometary
ultra-compact HII region. Previous high-sensitivity single dish observations by
our group resulted in the detection of broad 6035 MHz OH absorption in this
region; the line showed a significant blue-shifted asymmetry indicative of
molecular gas expansion. We present high-sensitivity Karl G. Jansky Very Large
Array (VLA) observations of the 6035 MHz OH line conducted to image the
absorption and investigate its origin with respect to the different star
formation sites in the region. In addition, we report detection of 6030 MHz OH
absorption with the VLA and further observations of 4.7 GHz and 6.0 GHz OH
lines obtained with the Arecibo Telescope. The 6030 MHz OH line shows a very
similar absorption profile as the 6035 MHz OH line. We found that the 6035 MHz
OH line absorption region is spatially unresolved at " scales, and it
is coincident with one of the bright ionized cores of the cometary HII region
that shows broad radio recombination line emission. We discuss a scenario where
the OH absorption is tracing the remnants of a pole-on molecular outflow that
is being ionized inside-out by the ultra-compact HII region.Comment: 19 pages, 6 figures. Accepted for publication in The Astrophysical
Journa
Modeling spatiotemporal abundance of mobile wildlife in highly variable environments using boosted GAMLSS hurdle models
Modeling organism distributions from survey data involves numerous statistical challenges, including accounting for zeroâinflation, overdispersion, and selection and incorporation of environmental covariates. In environments with high spatial and temporal variability, addressing these challenges often requires numerous assumptions regarding organism distributions and their relationships to biophysical features. These assumptions may limit the resolution or accuracy of predictions resulting from surveyâbased distribution models. We propose an iterative modeling approach that incorporates a negative binomial hurdle, followed by modeling of the relationship of organism distribution and abundance to environmental covariates using generalized additive models (GAM) and generalized additive models for location, scale, and shape (GAMLSS). Our approach accounts for key features of survey data by separating binary (presenceâabsence) from count (abundance) data, separately modeling the mean and dispersion of count data, and incorporating selection of appropriate covariates and response functions from a suite of potential covariates while avoiding overfitting. We apply our modeling approach to surveys of sea duck abundance and distribution in Nantucket Sound (Massachusetts, USA), which has been proposed as a location for offshore wind energy development. Our model results highlight the importance of spatiotemporal variation in this system, as well as identifying key habitat features including distance to shore, sediment grain size, and seafloor topographic variation. Our work provides a powerful, flexible, and highly repeatable modeling framework with minimal assumptions that can be broadly applied to the modeling of survey data with high spatiotemporal variability. Applying GAMLSS models to the count portion of survey data allows us to incorporate potential overdispersion, which can dramatically affect model results in highly dynamic systems. Our approach is particularly relevant to systems in which little a priori knowledge is available regarding relationships between organism distributions and biophysical features, since it incorporates simultaneous selection of covariates and their functional relationships with organism responses
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