15,913 research outputs found
Stellar Associations and their Field East of LMC 4 in the Large Magellanic Cloud
We report about the stellar content and the luminosity and mass functions of
three stellar associations and their field located on the north-east edge of
the super-bubble LMC 4 in the Large Magellanic Cloud.Comment: To be appeared in the meeting Proceedings of ``Modes of Star
Formation and the Origin of Field Populations'', Heidelberg, Germany, October
2000; to be published in the ASP Conference Series, edited by E. K. Grebel
and W. Brandne
Chandra Detection of X-ray Absorption Associated with a Damped Lyman Alpha System
We have observed three quasars, PKS 1127-145, Q 1331+171 and Q0054+144, with
the ACIS-S aboard the Chandra X-ray Observatory, in order to measure soft X-ray
absorption associated with intervening 21-cm and damped Ly absorbers.
For PKS 1127-145, we detect absorption which, if associated with an intervening
z_{abs}=0.312 absorber, implies a metallicity of 23% solar. If the absorption
is not at z_{abs}=0.312, then the metallicity is still constrained to be less
than 23% solar. The advantage of the X-ray measurement is that the derived
metallicity is insensitive to ionization, inclusion of an atom in a molecule,
or depletion onto grains. The X-ray absorption is mostly due to oxygen, and is
consistent with the oxygen abundance of 30% solar derived from optical nebular
emission lines in a foreground galaxy at the redshift of the absorber.
For Q1331+171 and Q 0054+144, only upper limits were obtained, although the
exposure times were intentionally short, since for these two objects we were
interested primarily in measuring flux levels to plan for future observations.
The imaging results are presented in a companion paper.Comment: 23 pages, 6 figures, accepted for publication in the Astrophysical
Journa
A non-renormalization theorem for chiral primary 3-point functions
In this note we prove a non-renormalization theorem for the 3-point functions
of 1/2 BPS primaries in the four-dimensional N = 4 SYM and chiral primaries in
two dimensional N =(4,4) SCFTs. Our proof is rather elementary: it is based on
Ward identities and the structure of the short multiplets of the superconformal
algebra and it does not rely on superspace techniques. We also discuss some
possible generalizations to less supersymmetric multiplets.Comment: 32 pages, added references, corrected typo
Can filamentary accretion explain the orbital poles of the Milky Way satellites?
Several scenarios have been suggested to explain the phase-space distribution
of the Milky Way (MW) satellite galaxies in a disc of satellites (DoS). To
quantitatively compare these different possibilities, a new method analysing
angular momentum directions in modelled data is presented. It determines how
likely it is to find sets of angular momenta as concentrated and as close to a
polar orientation as is observed for the MW satellite orbital poles. The method
can be easily applied to orbital pole data from different models. The observed
distribution of satellite orbital poles is compared to published angular
momentum directions of subhalos derived from six cosmological state-of-the-art
simulations in the Aquarius project. This tests the possibility that
filamentary accretion might be able to naturally explain the satellite orbits
within the DoS. For the most likely alignment of main halo and MW disc spin,
the probability to reproduce the MW satellite orbital pole properties turns out
to be less than 0.5 per cent in Aquarius models. Even an isotropic distribution
of angular momenta has a higher likelihood to produce the observed
distribution. The two Via Lactea cosmological simulations give results similar
to the Aquarius simulations. Comparing instead with numerical models of
galaxy-interactions gives a probability of up to 90 per cent for some models to
draw the observed distribution of orbital poles from the angular momenta of
tidal debris. This indicates that the formation as tidal dwarf galaxies in a
single encounter is a viable, if not the only, process to explain the
phase-space distribution of the MW satellite galaxies.Comment: 14 pages, 4 figures, 3 tables. Accepted for publication in MNRA
Monitoring the Variable Interstellar Absorption toward HD 219188 with HST/STIS
We discuss the results of continued spectroscopic monitoring of the variable
intermediate-velocity (IV) absorption at v = -38 km/s toward HD 219188. After
reaching maxima in mid-2000, the column densities of both Na I and Ca II in
that IV component declined by factors >= 2 by the end of 2006. Comparisons
between HST/STIS echelle spectra obtained in 2001, 2003, and 2004 and HST/GHRS
echelle spectra obtained in 1994--1995 indicate the following: (1) The
absorption from the dominant species S II, O I, Si II, and Fe II is roughly
constant in all four sets of spectra -- suggesting that the total N(H) and the
(mild) depletions have not changed significantly over a period of nearly ten
years. (2) The column densities of the trace species C I (both ground and
excited fine-structure states) and of the excited state C II* all increased by
factors of 2--5 between 1995 and 2001 -- implying increases in the hydrogen
density n_H (from about 20 cm^{-3} to about 45 cm^{-3}) and in the electron
density n_e (by a factor >= 3) over that 6-year period. (3) The column
densities of C I and C II* -- and the corresponding inferred n_H and n_e --
then decreased slightly between 2001 and 2004. (4) The changes in C I and C II*
are very similar to those seen for Na I and Ca II. The relatively low total
N(H) and the modest n_H suggest that the -38 km/s cloud toward HD 219188 is not
a very dense knot or filament. Partial ionization of hydrogen appears to be
responsible for the enhanced abundances of Na I, C I, Ca II, and C II*. In this
case, the variations in those species appear to reflect differences in density
and ionization [and not N(H)] over scales of tens of AU.Comment: 33 pages, 6 figures, aastex, accepted to Ap
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