5,608 research outputs found
Seeing Galaxies Through Thick & Thin. III. HST Imaging of the Dust in Backlit Spiral Galaxies
We present analysis of WFPC2 imaging of two spiral galaxies partially backlit
by E/S0 systems in the pairs AM1316-241 and AM0500-620, and the spiral
foreground system in NGC 1275. Images in B and I are used to determine the
reddening curve of in these systems. The spiral component of AM1316-241 shows
dust strongly concentrated in discrete arms, with a reddening law very close to
the Milky Way mean. The dust distribution is scale-free between about 100 pc
and the arm scale. The spiral in AM0500-620 shows dust concentrated in arms and
interarm spurs, with measurable interarm extinction as well. Although its dust
properties are less well-determined, we find evidence for a steeper extinction
law here. The shape of the reddening law suggests that, at least in AM1316-241,
we have resolved most of the dust structure. In AM0500-620, the slope of the
fractal perimeter-scale relation steepens systematically from low to high
extinction. In AM1316-241, we cannot determine a unique fractal dimension from
the defining area-perimeter relation, so the projected dust distribution is
best defined as fractal-like. In neither galaxy do we see regions even on
single-pixel scales in spiral arms with AB > 2.5. The measurements in NGC 1275
are compromised by our lack of independent knowledge of the foreground system's
light distribution, but masked sampling of the absorption suggests an effective
reddening curve much flatter than the Milky Way mean (perhaps indicating that
the foreground system has been affected by immersion in the hot intracluster
gas).Comment: Astronomical Journal, in press. 13 figures. Full-size PostScript
figures available at http://www.astr.ua.edu/preprints/kee
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Congestion management in electricity networks: Nodal, zonal and discriminatory pricing
Wholesale electricity markets use different market designs to handle congestion in the transmission network. We compare nodal, zonal and discriminatory pricing in general networks with transmission constraints and loop flows. We conclude that in large games with many producers who are allowed to participate in the real-time market the three market designs result in the same efficient dispatch. However, zonal pricing with counter-trading results in additional payments to producers in exportconstrained nodes.discriminatory pricin
Price Instability in Multi-Unit Auctions
We consider a procurement auction, where each supplier has private costs and submits a stepped supply function. We solve for a Bayesian Nash equilibrium and show that the equilibrium has a price instability in the sense that a minor change in a supplier's cost sometimes result in a major change in the market price. In wholesale electricity markets, we predict that the bid price of the most expensive production unit can change by 1-10% due to price instability. The price instability is reduced when suppliers have more steps in their supply functions for a given production technology. In the limit, as the number of steps increases and the cost uncertainty decreases, the Bayesian equilibrium converges to a pure-strategy NE without price instability, the Supply Function Equilibrium (SFE).convergence of Nash equilibri
The Properties of Satellite Galaxies in External Systems. I. Morphology and Structural Parameters
We present the first results of an ongoing project to study the
morphological, kinematical, dynamical, and chemical properties of satellite
galaxies of external giant spiral galaxies. The sample of objects has been
selected from the catalogue by Zaritsky et al. (1997). The paper analyzes the
morphology and structural parameters of a subsample of 60 such objects. The
satellites span a great variety of morphologies and surface brightness
profiles. About two thirds of the sample are spirals and irregulars, the
remaining third being early-types. Some cases showing interaction between pairs
of satellites are presented and briefly discussed.Comment: Accepted for publication in Astrophys. Journal Supp. Se
Anisotropic Distribution of SDSS Satellite Galaxies: Planar (not Polar) Alignment
The distribution of satellite galaxies relative to isolated host galaxies in
the Sloan Digital Sky Survey (SDSS) is investigated. Host-satellite systems are
selected using three different methods, yielding samples of ~3300, ~1600, and
\~950 satellites. In the plane of the sky, the distributions of all three
samples show highly significant deviations from circular symmetry (> 99.99%, >
99.99%, and 99.79% confidence levels, respectively), and the degree of
anisotropy is a strong function of the projected radius, r_p, at which the
satellites are found. For r_p < 100 kpc, the SDSS satellites are aligned
preferentially with the major axes of the hosts. This is in stark contrast to
the Holmberg effect, in which satellites are aligned with the minor axes of
host galaxies. The degree of anisotropy in the distribution of the SDSS
satellites decreases with r_p and is consistent with an isotropic distribution
at of order the 1-sigma level for 250 kpc < r_p < 500 kpc.Comment: ApJ Letters (in press); Discussion section substantially revised,
SDSS DR3 included in the analysis, no significant changes to the result
The identification of physical close galaxy pairs
A classification scheme for close pairs of galaxies is proposed. The scheme
is motivated by the fact that the majority of apparent close pairs are in fact
wide pairs in three-dimensional space. This is demonstrated by means of
numerical simulations of random samples of binary galaxies and the scrutiny of
the resulting projected and spatial separation distributions.
Observational strategies for classifying close pairs according to the scheme
are suggested. As a result, physical (i.e., bound and spatially) close pairs
are identified.Comment: 16 pages, 5 figures, accepted for publication in The Astronomical
Journal, added text corrections on proof
Substructure around M31 : Evolution and Effects
We investigate the evolution of a population of 100 dark matter satellites
orbiting in the gravitational potential of a realistic model of M31. We find
that after 10 Gyr, seven subhalos are completely disrupted by the tidal field
of the host galaxy. The remaining satellites suffer heavy mass loss and
overall, 75% of the mass initially in the subhalo system is tidally stripped.
Not surprisingly, satellites with pericentric radius less than 30 kpc suffer
the greatest stripping and leave a complex structure of tails and streams of
debris around the host galaxy. Assuming that the most bound particles in each
subhalo are kinematic tracers of stars, we find that the halo stellar
population resulting from the tidal debris follows an r^{-3.5} density profile
at large radii. We construct B-band photometric maps of stars coming from
disrupted satellites and find conspicuous features similar both in morphology
and brightness to the observed Giant Stream around Andromeda. An assumed star
formation efficiency of 5-10% in the simulated satellite galaxies results in
good agreement with the number of M31 satellites, the V-band surface brightness
distribution, and the brightness of the Giant Stream. During the first 5 Gyr,
the bombardment of the satellites heats and thickens the disk by a small
amount. At about 5 Gyr, satellite interations induce the formation of a strong
bar which, in turn, leads to a significant increase in the velocity dispersion
of the disk.Comment: 45 pages, 18 figures. To be submitted to the Astrophysical Journal,
version 2.0 : scale height value corrected, references added, and some
figures have been modifie
The Structure of the Outer Halo of the Galaxy and its Relationship to Nearby Large-Scale Structure
We present evidence to support an earlier indication that the Galaxy is
embedded in an extended, highly inclined, triaxial halo outlined by the spatial
distribution of companion galaxies to the Milky Way. Signatures of this spatial
distribution are seen in 1) the angular variation of the radial-velocity
dispersion of the companion galaxies, 2) the spatial distribution of the M~31
sub-group of galaxies, 3) the spatial distribution of the isolated, mainly
dwarf irregular, galaxies of the Local Group, 4) the velocity anisotropy
quadrupole of a sub-group of high-velocity clouds, and 5) the spatial
distribution of galaxies in the Coma-Sculptor cloud. Tidal effects of M~31 and
surrounding galaxies on the Galaxy are not strong enough to have affected the
observed structure. We conclude that this distribution is a reflection of
initial conditions. A simple galaxy formation scenario is proposed which ties
together the results found here with those of Holmberg (1969) and Zaritsky et
al. (1997) on the peculiar distribution of satellites around a large sample of
spiral galaxies.Comment: Accepted for publication in the Astron J., March 2000, 12 pages with
1 figur
The Relationship Between Baryons and Dark Matter in Extended Galaxy Halos
The relationship between gas-rich galaxies and Ly-alpha absorbers is
addressed in this paper in the context of the baryonic content of galaxy halos.
Deep Arecibo HI observations are presented of two gas-rich spiral galaxies
within 125 kpc projected distance of a Ly-alpha absorber at a similar velocity.
The galaxies investigated are close to edge-on and the absorbers lie almost
along their major axes, allowing for a comparison of the Ly-alpha absorber
velocities with galactic rotation. This comparison is used to examine whether
the absorbers are diffuse gas rotating with the galaxies' halos, outflow
material from the galaxies, or intergalactic gas in the low redshift cosmic
web. The results indicate that if the gas resides in the galaxies' halos it is
not rotating with the system and possibly counter-rotating. In addition, simple
geometry indicates the gas was not ejected from the galaxies and there are no
gas-rich satellites detected down to 3.6 - 7.5 x 10^6 Msun, or remnants of
satellites to 5-6 x 10^{18} cm^{-2}. The gas could potentially be infalling
from large radii, but the velocities and distances are rather high compared to
the high velocity clouds around the Milky Way. The most likely explanation is
the galaxies and absorbers are not directly associated, despite the vicinity of
the spiral galaxies to the absorbers (58-77 kpc from the HI edge). The spiral
galaxies reside in a filament of intergalactic gas, and the gas detected by the
absorber has not yet come into equilibrium with the galaxy. These results also
indicate that the massive, extended dark matter halos of spiral galaxies do not
commonly have an associated diffuse baryonic component at large radii.Comment: Accepted by AJ, 33 pages preprint format, see
http://www.astro.lsa.umich.edu/~mputman/putman1.pdf for a higher resolution
versio
A Hubble Space Telescope Snapshot Survey of Dynamically Close Galaxy Pairs in the CNOC2 Redshift Survey
We compare the structural properties of two classes of galaxies at
intermediate redshift: those in dynamically close galaxy pairs, and those which
are isolated. Both samples are selected from the CNOC2 Redshift Survey, and
have redshifts in the range 0.1 < z <0.6. Hubble Space Telescope WFPC2 images
were acquired as part of a snapshot survey, and were used to measure bulge
fraction and asymmetry for these galaxies. We find that paired and isolated
galaxies have identical distributions of bulge fractions. Conversely, we find
that paired galaxies are much more likely to be asymmetric (R_T+R_A >= 0.13)
than isolated galaxies. Assuming that half of these pairs are unlikely to be
close enough to merge, we estimate that 40% +/- 11% of merging galaxies are
asymmetric, compared with 9% +/- 3% of isolated galaxies. The difference is
even more striking for strongly asymmetric (R_T+R_A >= 0.16) galaxies: 25% +/-
8% for merging galaxies versus 1% +/- 1% for isolated galaxies. We find that
strongly asymmetric paired galaxies are very blue, with rest-frame B-R colors
close to 0.80, compared with a mean (B-R)_0 of 1.24 for all paired galaxies. In
addition, asymmetric galaxies in pairs have strong [OII]3727 emission lines. We
conclude that close to half of the galaxy pairs in our sample are in the
process of merging, and that most of these mergers are accompanied by triggered
star formation.Comment: Accepted for publication in the Astronomical Journal. 40 pages,
including 15 figures. For full resolution version, please see
http://www.trentu.ca/physics/dpatton/hstpairs
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