1,333 research outputs found
The Intrinsic Shapes of Stellar Systems
I compute the estimated distribution function f(q) for the apparent axis
ratio q of various types of stellar systems, using a nonparametric kernel
method. I then invert f(q) to find the distribution of intrinsic axis ratios,
using two different hypotheses: first, that the stellar systems are all oblate,
and second, that they are all prolate. The shapes of globular clusters in our
own galaxy are consistent, at the 99% confidence level, with both the oblate
and prolate hypothesis. The shapes of dwarf galaxies in the Virgo cluster are
consistent, at the 99% confidence level, with the prolate hypothesis, but
inconsistent with the oblate hypothesis. The shapes of star clusters in the
Large Magellanic Cloud, of ordinary elliptical galaxies, of brightest cluster
ellipticals, and of galaxy clusters are all inconsistent, at the 99% confidence
level, with both the oblate and prolate hypotheses. The globular clusters in
our galaxy are older than their half-mass relaxation time, and are most likely
rotationally flattened oblate spheroids. The other stellar systems considered
are generally younger than their half-mass relaxation time, and thus are
triaxial bodies flattened by anisotropy of their velocity dispersion.Comment: 23 pages, uuencoded compressed PostScript, includes 6 figures,
accepted for publication in the Ap
Dependence of Galaxy Shape on Environment in the Sloan Digital Sky Survey
Using a sample of galaxies from the Sloan Digital Sky Survey (SDSS) Data
Release 4, we study the trends relating surface brightness profile type and
apparent axis ratio to the local galaxy environment. We use the SDSS parameter
`fracDeV' to quantify the profile type. We find that galaxies with M_r > -18
are mostly described by exponential profiles in all environments. Galaxies with
-21 < M_r < -18 mainly have exponential profiles in low density environments
and de Vaucouleurs profiles in high density environments. The most luminous
galaxies, with M_r < -21, are mostly described by de Vaucouleurs profiles in
all environments. For galaxies with M_r < -19, the fraction of de Vaucouleurs
galaxies is a monotonically increasing function of local density, while the
fraction of exponential galaxies is monotonically decreasing. For a fixed
surface brightness profile type, apparent axis ratio is frequently correlated
with environment. As the local density of galaxies increases, we find that for
-20 < M_r < -18, galaxies of all profile types become slightly rounder, on
average; for -22 < M_r < -20, galaxies with exponential profiles tend to become
flatter, while galaxies with de Vaucouleurs profiles become rounder; for M_r <
-22, galaxies with exponential profiles become flatter, while the de
Vaucouleurs galaxies become rounder in their inner regions, yet exhibit no
change in their outer regions. We comment on how the observed trends relate to
the merger history of galaxies.Comment: 23 pages, 7 figures, accepted by Ap
Inclination-Dependent Extinction Effects in Disk Galaxies in the Sloan Digital Sky Survey
We analyze the r-band absolute magnitude and u − r color of low-redshift (z 0.9) to ~0.26 for nearly edge-on galaxies (q < 0.3). When the dimming law ΔM_r ⍺ (log q)^2 is used to create an inclination-corrected sample of bright exponential galaxies, their apparent shapes are consistent with a distribution of mildly noncircular disks, with median short-to-long axis ratio γ approx 0.22 and median disk ellipticity ε approx 0.08
Galaxy Alignments with Surrounding Structure in the Sloan Digital Sky Survey
Using data from the Sloan Digital Sky Survey (SDSS) Legacy Survey, we study
the alignment of relatively luminous galaxies with spectroscopic data with the
surrounding larger-scale structure as defined by galaxies with only photometric
data. We find that galaxies from the red sequence have a statistically
significant tendency for their images to align parallel to the projected
surrounding structure. Red galaxies brighter than the median of our sample
() have a mean alignment angle ,
indicating preferred parallel alignment, at a significance level on projected scales . Fainter
red galaxies have only at scales . Galaxies from the blue
sequence show no statistically significant () tendency for their
images to align with larger-scale structure. No dependence of alignment angle
is seen as a function of local overdensity or of offset from the local
distribution of surrounding galaxies.Comment: 9 pages, 5 figures, 2 tables, submitted to Ap
Formation of Massive Counterrotating Disks in Spiral Galaxies
We present results of numerical simulations of the formation of a massive
counterrotating gas disk in a spiral galaxy. Using a hierarchical tree gravity
solver combined with a sticky-particle gas dissipation scheme for our
simulations, we have investigated three mechanisms: episodic and continuous gas
infall, and a merger with a gas-rich dwarf galaxy. We find that both episodic
and continuous gas infall work reasonably well and are able to produce a
substantial gas counterrotating disk without upsetting the stability of the
existing disk drastically, but it is very important for the gas to be
well-dispersed in phase-space and not form concentrated clumps prior to its
absorption by the disk galaxy. The initial angular momentum of the gas also
plays a crucial role in determining the scale length of the counterrotating
disk formed and the time it takes to form. The rate of infall, i.e. the mass of
gas falling in per unit time, has to be small enough to preclude excessive
heating of the preexisting disk. It is much easier in general to produce a
smaller counterrotating disk than it is to produce an extensive disk whose
scale length is similar to that of the original prograde disk.
A gas-rich dwarf merger does not appear to be a viable mechanism to produce a
massive counterrotating disk, because only a very small dwarf galaxy can
produce a counterrotating disk without increasing the thickness of the existing
disk by an order of magnitude, and the time-scale for this process is
prohibitively long because it makes it very unlikely that several such mergers
can accumulate a massive counterrotating disk over a Hubble time.Comment: Accepted by ApJ, 22 pages, uuencoded compressed Postscript. 18
Figures (compressed Postscript) available from anonymous ftp at
ftp://bessel.mps.ohio-state.edu/pub/thakar/cr1/figs.ps.Z A complete
(text+figs) compressed PostScript preprint is also available at
ftp://bessel.mps.ohio-state.edu/pub/thakar/cr1/pp.ps.g
The Shapes of Dense Cores and Bok Globules
The shapes of isolated Bok globules and embedded dense cores of molecular
clouds are analyzed using a nonparametric method, under the alternate
hypotheses that they are randomly oriented prolate objects or that they are
randomly oriented oblate objects. In all cases, the prolate hypothesis gives a
better fit to the data. If Bok globules are oblate, they must be very flat; the
average axis ratio is b/a = 0.3, and few or no globules can have b/a > 0.7. If
Bok globules are prolate, then the mean axis ratio is b/a = 0.5. For most data
samples of dense cores, the randomly-oriented oblate hypothesis can be rejected
at the 99% confidence level. If the dense cores are prolate, their mean axis
ratio is approximately 0.4 to 0.5. Dense cores are significantly different in
shape from the clouds in which they are embedded; clouds have flatter apparent
shapes, and are inconsistent with a population of randomly oriented
axisymmetric objects.Comment: 26 pages (LaTeX) including 8 postscript figures; to appear in Ap
A Small Ion Propelled Spacecraft for Near Earth Experiments
It has become increasingly apparent that designers of today\u27s large and complex spacecraft are reluctant to adopt many new and innovative technologies due to the lack of in-orbit operational experience with these devices, a particular example being ion propulsion. The reasons for this are understandable but as a consequence many new ideas are caught in a \u27chicken and egg\u27 situation where they will not be flown until they have been and shown to work reliably. A study has been carried out in an attempt to provide a simple, low cost spacecraft that can be launched from almost any vehicle into any orbits that could subsequently be used as a testbed for small innovative ideas
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