11,631 research outputs found
Rings Over Which Cyclics are Direct Sums of Projective and CS or Noetherian
R is called a right WV -ring if each simple right R-module is injective
relative to proper cyclics. If R is a right WV -ring, then R is right uniform
or a right V -ring. It is shown that for a right WV-ring R, R is right
noetherian if and only if each right cyclic module is a direct sum of a
projective module and a CS or noetherian module. For a finitely generated
module M with projective socle over a V -ring R such that every subfactor of M
is a direct sum of a projective module and a CS or noetherian module, we show M
= X \oplus T, where X is semisimple and T is noetherian with zero socle. In the
case that M = R, we get R = S \oplus T, where S is a semisimple artinian ring,
and T is a direct sum of right noetherian simple rings with zero socle. In
addition, if R is a von Neumann regular ring, then it is semisimple artinian.Comment: A Para\^itre Glasgow Mathematical Journa
The Structure of a Low-Metallicity Giant Molecular Cloud Complex
To understand the impact of low metallicities on giant molecular cloud (GMC)
structure, we compare far infrared dust emission, CO emission, and dynamics in
the star-forming complex N83 in the Wing of the Small Magellanic Cloud. Dust
emission (measured by Spitzer as part of the S3MC and SAGE-SMC surveys) probes
the total gas column independent of molecular line emission and traces
shielding from photodissociating radiation. We calibrate a method to estimate
the dust column using only the high-resolution Spitzer data and verify that
dust traces the ISM in the HI-dominated region around N83. This allows us to
resolve the relative structures of H2, dust, and CO within a giant molecular
cloud complex, one of the first times such a measurement has been made in a
low-metallicity galaxy. Our results support the hypothesis that CO is
photodissociated while H2 self-shields in the outer parts of low-metallicity
GMCs, so that dust/self shielding is the primary factor determining the
distribution of CO emission. Four pieces of evidence support this view. First,
the CO-to-H2 conversion factor averaged over the whole cloud is very high 4-11
\times 10^21 cm^-2/(K km/s), or 20-55 times the Galactic value. Second, the
CO-to-H2 conversion factor varies across the complex, with its lowest (most
nearly Galactic) values near the CO peaks. Third, bright CO emission is largely
confined to regions of relatively high line-of-sight extinction, A_V >~ 2 mag,
in agreement with PDR models and Galactic observations. Fourth, a simple model
in which CO emerges from a smaller sphere nested inside a larger cloud can
roughly relate the H2 masses measured from CO kinematics and dust.Comment: 17 pages, 10 figures (including appendix), accepted for publication
in the Astrophysical Journa
Unusually Luminous Giant Molecular Clouds in the Outer Disk of M33
We use high spatial resolution (~7pc) CARMA observations to derive detailed
properties for 8 giant molecular clouds (GMCs) at a galactocentric radius
corresponding to approximately two CO scale lengths, or ~0.5 optical radii
(r25), in the Local Group spiral galaxy M33. At this radius, molecular gas
fraction, dust-to-gas ratio and metallicity are much lower than in the inner
part of M33 or in a typical spiral galaxy. This allows us to probe the impact
of environment on GMC properties by comparing our measurements to previous data
from the inner disk of M33, the Milky Way and other nearby galaxies. The outer
disk clouds roughly fall on the size-linewidth relation defined by
extragalactic GMCs, but are slightly displaced from the luminosity-virial mass
relation in the sense of having high CO luminosity compared to the inferred
virial mass. This implies a different CO-to-H2 conversion factor, which is on
average a factor of two lower than the inner disk and the extragalactic
average. We attribute this to significantly higher measured brightness
temperatures of the outer disk clouds compared to the ancillary sample of GMCs,
which is likely an effect of enhanced radiation levels due to massive star
formation in the vicinity of our target field. Apart from brightness
temperature, the properties we determine for the outer disk GMCs in M33 do not
differ significantly from those of our comparison sample. In particular, the
combined sample of inner and outer disk M33 clouds covers roughly the same
range in size, linewidth, virial mass and CO luminosity than the sample of
Milky Way GMCs. When compared to the inner disk clouds in M33, however, we find
even the brightest outer disk clouds to be smaller than most of their inner
disk counterparts. This may be due to incomplete sampling or a potentially
steeper cloud mass function at larger radii.Comment: Accepted for Publication in ApJ; 7 pages, 4 figure
Arm & Interarm Star Formation in Spiral Galaxies
We investigate the relationship between spiral arms and star formation in the
grand-design spirals NGC 5194 and NGC 628 and in the flocculent spiral NGC
6946. Filtered maps of near-IR (3.6 micron) emission allow us to identify "arm
regions" that should correspond to regions of stellar mass density
enhancements. The two grand-design spirals show a clear two-armed structure,
while NGC 6946 is more complex. We examine these arm and interarm regions,
looking at maps that trace recent star formation - far-ultraviolet (GALEX NGS)
and 24 micron emission (Spitzer, SINGS) - and cold gas - CO (Heracles) and HI
(Things). We find the star formation tracers and CO more concentrated in the
spiral arms than the stellar 3.6 micron flux. If we define the spiral arms as
the 25% highest pixels in the filtered 3.6 micron images, we find that the
majority (60%) of star formation tracers occurs in the interarm regions; this
result persists qualitatively even when considering the potential impact of
finite data resolution and diffuse interarm 24 micron emission. Even with a
generous definition of the arms (45% highest pixels), interarm regions still
contribute at least 30% to the integrated star formation rate tracers. We look
for evidence that spiral arms trigger star or cloud formation using the ratios
of star formation rate (SFR, traced by a combination of FUV and 24 micron
emission) to H_2 (traced by CO) and H_2 to HI. Any enhancement of SFR / M(H_2)
in the arm region is very small (less than 10%) and the grand design spirals
show no enhancement compared to the flocculent target. Arm regions do show a
weak enhancement in H_2/HI compared to the interarm regions, but at a fixed gas
surface density there is little clear enhancement in the H_2/HI ratio in the
arm regions. Thus, it seems that spiral arms may only act to concentrate the
gas to higher densities in the arms.Comment: 11 pages, 9 Figures, accepted by Ap
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