750 research outputs found
The Distances of SNR W41 and overlapping HII regions
New HI images from the VLA Galactic Plane Survey show prominent absorption
features associated with the supernovae remnant G23.3-0.3 (SNR W41). We
highlight the HI absorption spectra and the CO emission spectra of eight
small regions on the face of W41, including four HII regions, three non-thermal
emission regions and one unclassified region. The maximum velocity of
absorption for W41 is 782 km/s and the CO cloud at radial velocity
955 km/s is behind W41. Because an extended TeV source, a diffuse X-ray
enhancement and a large molecular cloud at radial velocity 775 km/s are
also projected at the center of W41, these yield the kinematic distance of 3.9
to 4.5 kpc for W41. For HII regions, our analyses reveal that both G23.42-0.21
and G23.07+0.25 are at the far kinematic distances (9.9 kpc and
10.6 kpc respectively) of their recombination-line velocities (1030.5 km/s
and 89.62.1 km/s respectively), G23.07-0.37 is at the near kinematic
distance (4.40.3 kpc) of its recombination-line velocity (82.72.0
km/s), and G23.27-0.27 is probably at the near kinematic distance (4.10.3
kpc) of its recombination-line velocity (76.10.6 km/s).Comment: 11 pages, 3 figs., 2 tables, accepted by A
The EGNoG Survey: Gas Excitation in Normal Galaxies at z~0.3
As observations of molecular gas in galaxies are pushed to lower star
formation rate galaxies at higher redshifts, it is becoming increasingly
important to understand the conditions of the gas in these systems to properly
infer their molecular gas content. The rotational transitions of the carbon
monoxide (CO) molecule provide an excellent probe of the gas excitation
conditions in these galaxies. In this paper we present the results from the gas
excitation sample of the Evolution of molecular Gas in Normal Galaxies (EGNoG)
survey at the Combined Array for Research in Millimeter-wave Astronomy (CARMA).
This subset of the full EGNoG sample consists of four galaxies at z~0.3 with
star formation rates of 40-65 M_Sun yr^-1 and stellar masses of ~2x10^11 M_Sun.
Using the 3 mm and 1 mm bands at CARMA, we observe both the CO(1-0) and CO(3-2)
transitions in these four galaxies in order to probe the excitation of the
molecular gas. We report robust detections of both lines in three galaxies (and
an upper limit on the fourth), with an average line ratio, r_31 = L'_CO(3-2) /
L'_CO(1-0), of 0.46 \pm 0.07 (with systematic errors \lesssim 40%), which
implies sub-thermal excitation of the CO(3-2) line. We conclude that the
excitation of the gas in these massive, highly star-forming galaxies is
consistent with normal star-forming galaxies such as local spirals, not
starbursting systems like local ultra-luminous infrared galaxies. Since the
EGNoG gas excitation sample galaxies are selected from the main sequence of
star-forming galaxies, we suggest that this result is applicable to studies of
main sequence galaxies at intermediate and high redshifts, supporting the
assumptions made in studies that find molecular gas fractions in star forming
galaxies at z~1-2 to be an order of magnitude larger than what is observed
locally.Comment: Accepted for publication in the Astrophysical Journal, to appear
January 2013; 18 pages, 10 figures, 6 table
Use of External Cephalic Version and Amnioreduction in the Delivery of a Fetal Demise with Macrocephaly Secondary to Massive Intracranial Teratoma
Introduction Congenital intracranial tumors are rare and often incidentally diagnosed on routine ultrasound. We report a case of a fetal demise with a massive intracranial teratoma at 25 weeks of gestation and the management of her delivery in the setting of macrocephaly, breech presentation, and polyhydramnios. Case A 31-year-old G3P1011 woman at 25 weeks\u27 gestation presented with a recent fetal demise and a fetal intracranial tumor first identified at 16 weeks\u27 gestational age. The patient had declined termination of pregnancy. Biometry was consistent with 24 weeks\u27 gestation, except for a head circumference of 394.4 mm consistent with 39 weeks\u27 gestation. The fetus was in a breech presentation. An external cephalic version (ECV) was successfully performed under epidural anesthesia and an amnioreduction was then performed to stabilize the fetal position. Immediate induction of labor and vaginal delivery followed. Discussion ECV and amnioreduction may help facilitate delivery in cases of fetal demise complicated by macrocephaly, malpresentation, and polyhydramnios
High-Resolution Measurements of the Dark Matter Halo of NGC 2976: Evidence for a Shallow Density Profile
We have obtained two-dimensional velocity fields of the dwarf spiral galaxy
NGC 2976 in Halpha and CO. The high spatial (~75 pc) and spectral (13 km/s and
2 km/s, respectively) resolution of these observations, along with our
multicolor optical and near-infrared imaging, allow us to measure the shape of
the density profile of the dark matter halo with good precision. We find that
the total (baryonic plus dark matter) mass distribution of NGC 2976 follows a
rho_tot ~ r^(-0.27 +/- 0.09) power law out to a radius of 1.8 kpc, assuming
that the observed radial motions provide no support. The density profile
attributed to the dark halo is even shallower, consistent with a nearly
constant density of dark matter over the entire observed region. A maximal disk
fit yields an upper limit to the K-band stellar mass-to-light ratio (M*/L_K) of
0.09^{+0.15}_{-0.08} M_sun/L_sun,K (including systematic uncertainties), with
the caveat that for M*/L_K > 0.19 M_sun/L_sun,K the dark matter density
increases with radius, which is unphysical. Assuming 0.10 M_sun/L_sun,K <
M*/L_K < 0.19 M_sun/L_sun,K, the dark matter density profile lies between
rho_dm ~ r^-0.17 and rho_dm ~ r^-0.01. Therefore, independent of any
assumptions about the stellar disk or the functional form of the density
profile, NGC 2976 does not contain a cuspy dark matter halo. We also
investigate some of the systematic effects that can hamper rotation curve
studies, and show that 1) longslit rotation curves are far more vulnerable to
systematic errors than two-dimensional velocity fields, 2) NGC 2976 contains
large radial motions at small radii, and 3) the Halpha and CO velocity fields
of NGC 2976 agree within their uncertainties. [slightly abridged]Comment: 30 pages, 4 tables, 13 figures (7 in color; Figures 1 and 3 are
low-resolution to save space). Accepted for publication in ApJ. Version with
full-resolution figures available at
http://astro.berkeley.edu/~bolatto/ngc2976rotation.ps (46 MB
Evidence for a Weak Galactic Center Magnetic Field from Diffuse Low Frequency Nonthermal Radio Emission
New low-frequency 74 and 330 MHz observations of the Galactic center (GC)
region reveal the presence of a large-scale (6\arcdeg\times 2\arcdeg) diffuse
source of nonthermal synchrotron emission. A minimum energy analysis of this
emission yields a total energy of ergs
and a magnetic field strength of \muG (where is
the proton to electron energy ratio and is the filling factor of the
synchrotron emitting gas). The equipartition particle energy density is
\evcm, a value consistent with cosmic-ray data. However,
the derived magnetic field is several orders of magnitude below the 1 mG field
commonly invoked for the GC. With this field the source can be maintained with
the SN rate inferred from the GC star formation. Furthermore, a strong magnetic
field implies an abnormally low GC cosmic-ray energy density. We conclude that
the mean magnetic field in the GC region must be weak, of order 10 \muG (at
least on size scales \ga 125\arcsec).Comment: 12 pages, 1 JPEG figure, uses aastex.sty; Accepted for publication,
ApJL (2005, published
Giant Molecular Clouds in M33 - I. BIMA All Disk Survey
We present the first interferometric CO(J=1->0) map of the entire H-alpha
disk of M33. The 13" diameter synthesized beam corresponds to a linear
resolution of 50 pc, sufficient to distinguish individual giant molecular
clouds (GMCs). From these data we generated a catalog of 148 GMCs with an
expectation that no more than 15 of the sources are spurious. The catalog is
complete down to GMC masses of 1.5 X 10^5 M_sun and contains a total mass of
2.3 X 10^7 M_sun. Single dish observations of CO in selected fields imply that
our survey detects ~50% of the CO flux, hence that the total molecular mass of
M33 is 4.5 X 10^7 M_sun, approximately 2% of the HI mass. The GMCs in our
catalog are confined largely to the central region (R < 4 kpc). They show a
remarkable spatial and kinematic correlation with overdense HI filaments; the
geometry suggests that the formation of GMCs follows that of the filaments. The
GMCs exhibit a mass spectrum dN/dM ~ M^(-2.6 +/- 0.3), considerably steeper
than that found in the Milky Way and in the LMC. Combined with the total mass,
this steep function implies that the GMCs in M33 form with a characteristic
mass of 7 X 10^4 M_sun. More than 2/3 of the GMCs have associated HII regions,
implying that the GMCs have a short quiescent period. Our results suggest the
rapid assembly of molecular clouds from atomic gas, with prompt onset of
massive star formation.Comment: 19 pages, Accepted for Publication in the Astrophysical Journal
Supplemen
Constraints on the galactic distribution of cosmic rays from the COS-B gamma-ray data
The diffuse component of the galactic high energy gamma rays results mainly from the interaction of CR nuclei and electrons with the nuclei of the interstellar gas. An additional contribution is obtained from the interaction of CR electrons with the interstellar photons through the inverse-Compton (IC) process. Gamma ray astronomy therefore offers an excellent means to study the distribution of CR particles throughout the Galaxy, but it is essential to know the distribution of the target interstellar gas particles, the major constituents being atomic and molecular hydrogen. Large scale millimeter wave surveys of the CO molecule covering more than half of the Milky Way, obtained with the Columbia 1.2 m telescopes, are currently available and are used to trace the H2; the COS-B observations have sufficient resolution and sensitivity to constrain the relation between the integrated CO line intensity and the molecular hydrogen column density
Giant Molecular Clouds in the Early-type Galaxy NGC 4526
D. Utomo, et al., âGiant Molecular Clouds in the Early-Type Galaxy NGC 4526â, The Astrophysical Journal, Vol. 803(1), April 2015. © 2015. The American Astronomical Society. All rights reserved.We present a high spatial resolution (â20 pc) of 12CO(2 â1) observations of the lenticular galaxy NGC 4526. We identify 103 resolved giant molecular clouds (GMCs) and measure their properties: size R, velocity dispersion Ïv, and luminosity L. This is the first GMC catalog of an early-type galaxy. We find that the GMC population in NGC 4526 is gravitationally bound, with a virial parameter α ⌠1. The mass distribution, dN/dM â Mâ2.39 ± 0.03, is steeper than that for GMCs in the inner Milky Way, but comparable to that found in some late-type galaxies. We find no sizeâline width correlation for the NGC 4526 clouds, in contradiction to the expectation from Larsonâs relation. In general, the GMCs in NGC 4526 are more luminous, denser, and have a higher velocity dispersion than equal-size GMCs in the Milky Way and other galaxies in the Local Group. These may be due to higher interstellar radiation field than in the Milky Way disk and weaker external pressure than in the Galactic center. In addition, a kinematic measurement of cloud rotation shows that the rotation is driven by the galactic shear. For the vast majority of the clouds, the rotational energy is less than the turbulent and gravitational energy, while the four innermost clouds are unbound and will likely be torn apart by the strong shear at the galactic center. We combine our data with the archival data of other galaxies to show that the surface density ÎŁ of GMCs is not approximately constant, as previously believed, but varies by âŒ3 orders of magnitude. We also show that the size and velocity dispersion of the GMC population across galaxies are related to the surface density, as expected from the gravitational and pressure equilibrium, i.e., Ïv Râ1/2 â ÎŁ1/2.Peer reviewe
The Role of Pressure in GMC Formation II: The H_2 - Pressure Relation
We show that the ratio of molecular to atomic gas in galaxies is determined
by hydrostatic pressure and that the relation between the two is nearly linear.
The pressure relation is shown to be good over three orders of magnitude for 14
galaxies including dwarfs, HI-rich, and H_2-rich galaxies as well as the Milky
Way. The sample spans a factor of five in mean metallicity. The rms scatter of
individual points of the relation is only about a factor of two for all the
galaxies, though some show much more scatter than others. Using these results,
we propose a modified star formation prescription based on pressure determining
the degree to which the ISM is molecular. The formulation is different in high
and low pressure regimes defined by whether the gas is primarily atomic or
primarily molecular. This formulation can be implemented in simulations and
provides a more appropriate treatment of the outer regions of spiral galaxies
and molecule-poor systems such as dwarf irregulars and damped Lyman-alpha
systems.Comment: 14 pages, 7 figures, Accepted to the Astrophysical Journa
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