521 research outputs found
On the Spiral Structure of the Milky Way Galaxy
We consider the possible pattern of the overall spiral structure of the
Galaxy, using data on the distribution of neutral (atomic), molecular, and
ionized hydrogen, on the base of the hypothesis of the spiral structure being
symmetric, i.e. the assumption that spiral arms are translated into each other
for a rotation around the galactic center by 180{\deg} (a two-arm pattern) or
by 90{\deg} (a four-arm pattern). We demonstrate that, for the inner region,
the observations are best represented with a four-arm scheme of the spiral
pattern, associated with all-Galaxy spiral density waves. The basic position is
that of the Carina arm, reliably determined from distances to HII regions and
from HI and H2 radial velocities. This pattern is continued in the quadrants
III and IV with weak outer HI arms; from their morphology, the Galaxy should be
considered an asymmetric multi-arm spiral. The kneed shape of the outer arms
that consist of straight segments can indicate that these arms are transient
formations that appeared due to a gravitational instability in the gas disk.
The distances between HI superclouds in the two arms that are the brightest in
neutral hydrogen, the Carina arm and the Cygnus (Outer) arm, concentrate to two
values, permitting to assume the presence of a regular magnetic field in these
arms.Comment: 21 pages, 14 fugures; accepted for publication in Astronomichesky
Journal (Astron. Rep.
On the chains of star complexes and superclouds in spiral arms
The relation is studied between occurrence of a regular chain of star
complexes and superclouds in a spiral arm, and other properties of the latter.
A regular string of star complexes is located in the north-western arm of M31;
they have about the same size 0.6 kpc with spacing of 1.1 kpc. Within the same
arm segment the regular magnetic field with the wavelength of 2.3 kpc was found
by Beck et al. (1989). We noted that this wavelength is twice as large as the
spacing between complexes and suggested that they were formed in result of
magneto-gravitational instability developed along the arm. In this NW arm, star
complexes are located inside the gas-dust lane, whilst in the south-western arm
of M31 the gas-dust lane is upstream of the bright and uniform stellar arm.
Earlier, evidence for the age gradient has been found in the SW arm. All these
are signatures of a spiral shock, which may be associated with unusually large
(for M31) pitch-angle of this SW arm segment. Such a shock may prevent the
formation of the regular magnetic field, which might explain the absence of
star complexes there. Anti-correlation between shock wave signatures and
presence of star complexes is observed in spiral arms of a few other galaxies.
Regular chains of star complexes and superclouds in spiral arms are rare, which
may imply that a rather specific mechanism is involved in their formation, and
the most probable one is the Parker-Jeans instability. The spiral pattern of
our Galaxy is briefly discussed; it may be of M101 type in the outer parts. The
regular bi-modal spacing of HI superclouds is found in Carina and Cygnus
(Outer) arms, which may be an indirect evidence for the regular magnetic field
along these arms.Comment: 20 pages, 12 figures, accepted for publication in MNRA
Evolution of twist-3 multi-parton correlation functions relevant to single transverse-spin asymmetry
We constructed two sets of twist-3 correlation functions that are responsible
for generating the novel single transverse-spin asymmetry in the QCD collinear
factorization approach. We derive evolution equations for these universal
three-parton correlation functions. We calculate evolution kernels relevant to
the gluonic pole contributions to the asymmetry at the order of . We
find that all evolution kernels are infrared safe as they should be and have a
lot in common to the DGLAP evolution kernels of unpolarized parton
distributions. By solving the evolution equations, we explicitly demonstrate
the factorization scale dependence of these twist-3 correlation functions.Comment: 28 pages, 17 figures, new materials and references adde
Hierarchical Star-Formation in M33: Fundamental properties of the star-forming regions
Star-formation within galaxies appears on multiple scales, from spiral
structure, to OB associations, to individual star clusters, and often
sub-structure within these clusters. This multitude of scales calls for
objective methods to find and classify star-forming regions, regardless of
spatial size. To this end, we present an analysis of star-forming groups in the
local group spiral galaxy M33, based on a new implementation of the Minimum
Spanning Tree (MST) method. Unlike previous studies which limited themselves to
a single spatial scale, we study star-forming structures from the effective
resolution limit (~20pc) to kpc scales. We find evidence for a continuum of
star-forming group sizes, from pc to kpc scales. We do not find a
characteristic scale for OB associations, unlike that found in previous
studies, and we suggest that the appearance of such a scale was caused by
spatial resolution and selection effects. The luminosity function of the groups
is found to be well represented by a power-law with an index, -2, similar to
that found for clusters and GMCs. Additionally, the groups follow a similar
mass-radius relation as GMCs. The size distribution of the groups is best
described by a log-normal distribution and we show that within a hierarchical
distribution, if a scale is selected to find structure, the resulting size
distribution will have a log-normal distribution. We find an abrupt drop of the
number of groups outside a galactic radius of ~4kpc, suggesting a change in the
structure of the star-forming ISM, possibly reflected in the lack of GMCs
beyond this radius. (abridged)Comment: 12 pages, 16 figures, accepted MNRA
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