28,539 research outputs found
External Shear in Quadruply Imaged Lens Systems
We use publicly available N-body simulations and semi-analytic models of
galaxy formation to estimate the levels of external shear due to structure near
the lens in gravitational lens systems. We also describe two selection effects,
specific to four-image systems, that enhance the probability of observing
systems to have higher external shear. Ignoring additional contributions from
"cosmic shear" and assuming that lens galaxies are not significantly flattened,
we find that the mean shear at the position of a quadruple lens galaxy is 0.11,
the rms shear is roughly 0.15, and there is roughly a 45% likelihood of
external shear greater than 0.1. This is much larger than previous estimates
and in good agreement with typical measured external shear. The higher shear
primarily stems from the tendency of early-type galaxies, which are the
majority of lenses, to reside in overdense regions.Comment: 5 pages, 2 figures, ApJ in press, minor revision
The bottom of the white dwarf cooling sequence in the old open cluster NGC 2158
We use 10 orbits of Advanced Camera for Surveys observations to reach the end
of the white dwarf cooling sequence in the solar-metallicity open cluster NGC
2158. Our photometry and completeness tests show that the end falls at
magnitude m_F606W = 27.5 +/- 0.15, which implies an age between ~1.8 and ~2.0
Gyr, consistent with the age of 1.9 +/- 0.2 Gyr obtained from fits to the
main-sequence turn-off. The faintest white dwarfs show a clear turn toward
bluer colors, as predicted by theoretical isochrones.Comment: 12 pages, 4 figures (2 in low resolution, and 1 bonus for
astro-ph-only). ApJ Letter accepted on December 1, 200
Two tails in NGC 3656, and the major merger origin of shell and minor axis dust lane ellipticals
I report on the discovery of two faint (~ 26.8 Rmag/arcsec^2) tidal tails
around the shell elliptical NGC 3656 (Arp 155). This galaxy had previously been
interpreted as a case of accretion, or minor merger. The two tidal tails are
inconsistent with a minor merger, and point instead to a disk-disk major merger
origin. NGC 3656 extends Toomre's merger sequence toward normal elliptical
galaxies, and hints at a major merger origin for shells and minor-axis dust
lanes. A dwarf galaxy lies at the tip of one of the tidal tails. A prominent
shell, which shows sharp azymuthal color discontinuities, belongs to a rotating
dynamical component of young stars which includes the inner dust lane.Comment: 9 pages, 2 plates, 1 figure, uses aaspp.sty, accepted for publication
in The Astrophysical Journal Letters. Files also available by anonymous ftp
at ftp.iac.es, directory ./pub/balcell
Resolving the Controversy Over the Core Radius of 47 Tucanae (NGC 104)
This paper investigates the discrepancy between recent measurements of the
density profile of the globular cluster 47 Tuc that have used HST data sets.
Guhathakurta et al. (1992) used pre-refurbishment WFPC1 V-band images to derive
r_c = 23" +/- 2". Calzetti et al. (1993) suggested that the density profile is
a superposition of two King profiles (r_c = 8" and r_c = 25") based on U-band
FOC images. De Marchi et al. (1996) used deep WFPC1 U-band images to derive r_c
= 12" +/- 2". Differences in the adopted cluster centers are not the cause of
the discrepancy. Our independent analysis of the data used by De Marchi et al.
reaches the following conclusions: (1) De Marchi et al.'s r_c ~ 12" value is
spuriously low, a result of radially-varying bias in the star counts in a
magnitude limited sample -- photometric errors and a steeply rising stellar
luminosity function cause more stars to scatter across the limiting magnitude
into the sample than out of it, especially near the cluster center where
crowding effects are most severe. (2) Changing the limiting magnitude to the
main sequence turnoff, away from the steep part of the luminosity function,
partially alleviates the problem and results in r_c = 18". (3) Combining such a
limiting magnitude with accurate photometry derived from PSF fitting, instead
of the less accurate aperture photometry employed by De Marchi et al., results
in a reliable measurement of the density profile which is well fit by r_c = 22"
+/- 2". Archival WFPC2 data are used to derive a star list with a higher degree
of completeness, greater photometric accuracy, and wider areal coverage than
the WFPC1 and FOC data sets; the WFPC2-based density profile supports the above
conclusions, yielding r_c = 24" +/- 1.9".Comment: 22 pages, 5 figures, 1 table; accepted for publication in PASP; see
http://www.ucolick.org/~raja/hgg.tar.gz for full-resolution figure
Absolute motions of globular clusters. II. [HST astrometry and VLT radial velocities in NGC6397]
In this paper we present a new, accurate determination of the three
components of the absolute space velocity of the Galactic globular cluster
NGC6397 (l 338d, b -12d). We used three HST/WFPC2 fields with multi-epoch
observations to obtain astrometric measurements of objects in three different
fields in this cluster. The identification of 33 background galaxies with sharp
nuclei allowed us to determine an absolute reference point and measure the
absolute proper motion of the cluster. The third component has been obtained
from radial velocities measured on spectra from the multi-fiber spectrograph
FLAMES at UT2-VLT. We find [mu_alpha cos(delta), mu_delta](J2000.0) = [+3.39
+/- 0.15, -17.55 +/- 0.15] mas/yr, and V_rad = +18.36 +/- 0.09 (+/-0.10) km/s.
Assuming a Galactic potential, we calculate the cluster orbit for various
assumed distances, and briefly discuss the implications.Comment: 7 pages, 5 figures, 4 tables. Accepted for publication in A&A, on
April 27 200
Development of a Microsatellite Library in \u3cem\u3eLolium Perenne\u3c/em\u3e
Lolium perenne, as one of the most important forage grasses of temperate regions, combines a number of very useful characteristics, e.g., good seedling establishment, with a low resistance to drought and limited winter hardiness. Trait selection and introgression can be greatly enhanced by the use of molecular markers in a genetic linkage map. The aim of this project was the generation of a genomic microsatellite library which when combined with microsatellites developed from a Genethresher database would give good genome coverage coupled to high levels of marker polymorphism
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