247 research outputs found
A Water Maser and Ammonia Survey of GLIMPSE Extended Green Objects (EGOs)
We present the results of a Nobeyama 45-m water maser and ammonia survey of
all 94 northern GLIMPSE Extended Green Objects (EGOs), a sample of massive
young stellar objects (MYSOs) identified based on their extended 4.5 micron
emission. We observed the ammonia (1,1), (2,2), and (3,3) inversion lines, and
detect emission towards 97%, 63%, and 46% of our sample, respectively (median
rms ~50 mK). The water maser detection rate is 68% (median rms ~0.11 Jy). The
derived water maser and clump-scale gas properties are consistent with the
identification of EGOs as young MYSOs. To explore the degree of variation among
EGOs, we analyze subsamples defined based on MIR properties or maser
associations. Water masers and warm dense gas, as indicated by emission in the
higher-excitation ammonia transitions, are most frequently detected towards
EGOs also associated with both Class I and II methanol masers. 95% (81%) of
such EGOs are detected in water (ammonia(3,3)), compared to only 33% (7%) of
EGOs without either methanol maser type. As populations, EGOs associated with
Class I and/or II methanol masers have significantly higher ammonia linewidths,
column densities, and kinetic temperatures than EGOs undetected in methanol
maser surveys. However, we find no evidence for statistically significant
differences in water maser properties (such as maser luminosity) among any EGO
subsamples. Combining our data with the 1.1 mm continuum Bolocam Galactic Plane
Survey, we find no correlation between isotropic water maser luminosity and
clump number density. Water maser luminosity is weakly correlated with clump
(gas) temperature and clump mass.Comment: Astrophysical Journal, accepted. Emulateapj, 24 pages including 24
figures, plus 9 tables (including full content of online-only tables
Patterns of silver eel (Anguilla anguilla L.) sex ratio in a catchment
Changes in the numbers and size-class structure of European silver eels, Anguilla anguilla, in the River Fre´mur (France) were examined over a 9-year period after installation of downstream eel passes. The number of silver eels migrating downstream peaked in 1999, then decreased strongly and steadily after 2000, reaching relatively low levels. At the same time, a gradual shift in the silver eel sex ratio from a dominance of males (size from 270 to 442 mm, age from 3 to 6 years) to females (size from 366 to 1112 mm, age from 4 to 9 years) was recorded. Possible explanations for the escapement patterns observed are environmental sex determination and the installation of eel passes on the main hydraulic engineering structures in 1992 and 1996
The Kinematically Measured Pattern Speeds of NGC 2523 and NGC 4245
We have applied the Tremaine-Weinberg continuity equation method to derive
the bar pattern speed in the SB(r)b galaxy NGC 2523 and the SB(r)0/a galaxy NGC
4245 using the Calcium Triplet absorption lines. These galaxies were selected
because they have strong inner rings which can be used as independent tracers
of the pattern speed. The pattern speed of NGC 2523 is 26.4 6.1 km
s kpc, assuming an inclination of 49.7 and a distance
of 51.0 Mpc. The pattern speed of NGC 4245 is 75.5 31.3 km s
kpc, assuming an inclination of 35.4 and a distance of 12.6
Mpc. The ratio of the corotation radius to the bar radius of NGC 2523 and NGC
4245 is 1.4 0.3 and 1.1 0.5, respectively. These values place the
bright inner rings near and slightly inside the corotation radius, as predicted
by barred galaxy theory. Within the uncertainties, both galaxies are found to
have fast bars that likely indicate dark halos of low central concentration.
The photometric properties, bar strengths, and disk stabilities of both
galaxies are also discussed.Comment: Accepted for publication in The Astronomical Journal, 11 figures, 2
table
Dynamical Simulations of NGC 2523 and NGC 4245
We present dynamical simulations of NGC 2523 and NGC 4245, two barred
galaxies (types SB(r)b and SB(r)0/a, respectively) with prominent inner rings.
Our goal is to estimate the bar pattern speeds in these galaxies by matching a
sticky-particle simulation to the -band morphology, using near-infrared
-band images to define the gravitational potentials. We compare the
pattern speeds derived by this method with those derived in our previous paper
using the well-known Tremaine-Weinberg continuity equation method. The inner
rings in these galaxies, which are likely to be resonance features, help to
constrain the dynamical models. We find that both methods give the same pattern
speeds within the errors.Comment: 29 pages, 3 tables, 13 figures. Accepted for publication in The
Astronomical Journa
How do different spiral arm models impact the ISM and GMC population?
The nature of galactic spiral arms in disc galaxies remains elusive.
Regardless of the spiral model, arms are expected to play a role in sculpting
the star-forming interstellar medium. As such, different arm models may result
in differences in the structure of the interstellar medium and molecular cloud
properties. In this study we present simulations of galactic discs subject to
spiral arm perturbations of different natures. We find very little difference
in how the cloud population or gas kinematics vary between the different
grand-design spirals, indicting that the interstellar medium on cloud scales
cares little about where spiral arms come from. We do, however, see a
difference in the interarm/arm mass spectra, {and minor differences in tails of
the distributions of cloud properties} (as well as radial variations in the
stellar/gaseous velocity dispersions). These features can be attributed to
differences in the radial dependence of the pattern speeds between the
different spiral models, and could act as a metric of the nature of spiral
structure in observational studies.Comment: 18 pages, 17 figures. Accepted for publication in MNRA
A Water Maser and NH_3 Survey of GLIMPSE Extended Green Objects
We present the results of a Nobeyama 45 m H_(2)O maser and NH_3 survey of all 94 northern GLIMPSE extended green objects (EGOs), a sample of massive young stellar objects (MYSOs) identified based on their extended 4.5 μm emission. We observed the NH3(1,1), (2,2), and (3,3) inversion lines, and detected emission toward 97%, 63%, and 46% of our sample, respectively (median rms ~ 50 mK). The H_(2)O maser detection rate is 68% (median rms ~ 0.11 Jy). The derived H_(2)O maser and clump-scale gas properties are consistent with the identification of EGOs as young MYSOs. To explore the degree of variation among EGOs, we analyze subsamples defined based on mid-infrared (MIR) properties or maser associations. H_(2)O masers and warm dense gas, as indicated by emission in the higher-excitation NH_3 transitions, are most frequently detected toward EGOs also associated with both Class I and II CH_(3)OH masers. Ninety-five percent (81%) of such EGOs are detected in H_(2)O (NH_(3)(3,3)), compared to only 33% (7%) of EGOs without either CH_(3)OH maser type. As populations, EGOs associated with Class I and/or II CH3OH masers have significantly higher NH_3 line widths, column densities, and kinetic temperatures than EGOs undetected in CH_(3)OH maser surveys. However, we find no evidence for statistically significant differences in H_(2)O maser properties (such as maser luminosity) among any EGO subsamples. Combining our data with the 1.1 mm continuum Bolocam Galactic Plane Survey, we find no correlation between isotropic H_(2)O maser luminosity and clump number density. H_(2)O maser luminosity is weakly correlated with clump (gas) temperature and clump mass
The Potential-Density Phase Shift Method for Determining the Corotation Radii in Spiral and Barred Galaxies
We have developed a new method for determining the corotation radii of
density waves in disk galaxies, which makes use of the radial distribution of
an azimuthal phase shift between the potential and density wave patterns. The
approach originated from improved theoretical understandings of the relation
between the morphology and kinematics of galaxies, and on the dynamical
interaction between density waves and the basic-state disk stars which results
in the secular evolution of disk galaxies. In this paper, we present the
rationales behind the method, and the first application of it to several
representative barred and grand-design spiral galaxies, using near-infrared
images to trace the mass distributions, as well as to calculate the potential
distributions used in the phase shift calculations. We compare our results with
those from other existing methods for locating the corotations, and show that
the new method both confirms the previously-established trends of bar-length
dependence on galaxy morphological types, as well as provides new insights into
the possible extent of bars in disk galaxies. Application of the method to a
larger sample and the preliminary analysis of which show that the phase shift
method is likely to be a generally-applicable, accurate, and essentially
model-independent method for determining the pattern speeds and corotation
radii of single or nested density wave patterns in galaxies. Other implications
of this work are: most of the nearby bright disk galaxies appear to possess
quasi-stationary spiral modes; that these density wave modes and the associated
basic state of the galactic disk slowly transform over time; and that
self-consistent N-particle systems contain physics not revealed by the passive
orbit analysis approaches.Comment: 48 pages, 16 figures. Accepted for publication in the Astronomical
Journa
The Schmidt-Kennicutt Law of Matched-Age Star Forming Regions; Pa-alpha Observations of the Early-Phase Interacting Galaxy Taffy I
In order to test a recent hypothesis that the dispersion in the
Schmidt-Kennicutt law arises from variations in the evolutionary stage of star
forming molecular clouds, we compared molecular gas and recent star formation
in an early-phase merger galaxy pair, Taffy I (UGC\ 12915/UGC\ 12914, VV\ 254)
which went through a direct collision 20 Myr ago and whose star forming regions
are expected to have similar ages. Narrow-band Pa-alpha image is obtained using
the ANIR near-infrared camera on the mini-TAO 1m telescope. The image enables
us to derive accurate star formation rates within the galaxy directly. The
total star formation rate, 22.2 M_sun/yr, was found to be much higher than
previous estimates. Ages of individual star forming blobs estimated from
equivalent widths indicate that most star forming regions are ~7 Myr old,
except for a giant HII region at the bridge which is much younger. Comparison
between star formation rates and molecular gas masses for the regions with the
same age exhibits a surprisingly tight correlation, a slope of unity, and star
formation efficiencies comparable to those of starburst galaxies. These results
suggest that Taffy I has just evolved into a starburst system after the
collision, and the star forming sites are at a similar stage in their evolution
from natal molecular clouds except for the bridge region. The tight
Schmidt-Kennicutt law supports the scenario that dispersion in the star
formation law is in large part due to differences in evolutionary stage of star
forming regions.Comment: Accepted for publication in Astrophysical Journa
Simulations of the grand design galaxy M51: a case study for analysing tidally induced spiral structure
We present hydrodynamical models of the grand design spiral M51 (NGC 5194),
and its interaction with its companion NGC 5195. Despite the simplicity of our
models, our simulations capture the present day spiral structure of M51
remarkably well, and even reproduce details such as a kink along one spiral
arm, and spiral arm bifurcations. We investigate the offset between the stellar
and gaseous spiral arms, and find at most times (including the present day)
there is no offset between the stars and gas to within our error bars. We also
compare our simulations with recent observational analysis of M51. We compute
the pattern speed versus radius, and like the observations, find no single
global pattern speed. We also show that the spiral arms cannot be fitted well
by logarithmic spirals. We interpret these findings as evidence that M51 does
not exhibit a quasi-steady density wave, as would be predicted by density wave
theory. The internal structure of M51 derives from the complicated and
dynamical interaction with its companion, resulting in spiral arms showing
considerable structure in the form of short-lived kinks and bifurcations.
Rather than trying to model such galaxies in terms of global spiral modes with
fixed pattern speeds, it is more realistic to start from a picture in which the
spiral arms, while not being simple material arms, are the result of tidally
induced kinematic density `waves' or density patterns, which wind up slowly
over time.Comment: 23 pages, 20 figures, accepted for publication in MNRA
Temperature Variations of the Cold Dust in the Triangulum Galaxy M33
We present wide-field 1.1 mm continuum imaging of the nearby spiral galaxy M
33, conducted with the AzTEC bolometer camera on ASTE. We show that the 1.1 mm
flux traces the distribution of dust with T ~20 K. Combined with far-infrared
imaging at 160um, we derive the dust temperature distribution out to a galactic
radius of ~7 kpc with a spatial resolution of ~100 parsecs. Although the 1.1 mm
flux is observed predominantly near star forming regions, we find a smooth
radial temperature gradient declining from ~20 K to ~13 K, consistent with
recent results from the Herschel satellite. Further comparison of individual
regions show a strong correlation between the cold dust temperature and the Ks
band brightness, but not with the ionizing flux. The observed results imply
that the dominant heating source of cold dust at few hundred parsec scales are
due to the non-OB stars, even when associated with star forming regions.Comment: Accepted for publication in PAS
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