4 research outputs found
A search for the most massive galaxies: Double Trouble?
We describe the results of a search for galaxies with large (> 350 km/s)
velocity dispersions. The largest systems we have found appear to be the
extremes of the early-type galaxy population: compared to other galaxies with
similar luminosities, they have the largest velocity dispersions and the
smallest sizes. However, they are not distant outliers from the Fundamental
Plane and mass-to-light scaling relations defined by the bulk of the early-type
galaxy population. They may host the most massive black holes in the Universe,
and their abundance and properties can be used to constrain galaxy formation
models. Clear outliers from the scaling relations tend to be objects in
superposition (angular separations smaller than 1 arcsec), evidence for which
comes sometimes from the spectra, sometimes from the images, and sometimes from
both. The statistical properties of the superposed pairs, e.g., the
distribution of pair separations and velocity dispersions, can be used to
provide useful information about the expected distribution of image
multiplicities, separations and flux ratios due to gravitational lensing by
multiple lenses, and may also constrain models of their interaction rates.Comment: 20 pages, 8 figures. Accepted by AJ. The full set of figures in
Appendix B is available at
http://www.physics.upenn.edu/~bernardm/PAPERS/BIGEtypes/bernardi.FIG-B.ps.gz
Figure 8 did not show the set of galaxies described in the text of the
appendix. This has now been correcte
Candidate Isolated Neutron Stars and Other Optically Blank X-ray Fields Identified from the ROSAT All-Sky and Sloan Digital Sky Surveys
Only seven radio-quiet isolated neutron stars (INSs) emitting thermal X rays
are known, a sample that has yet to definitively address such fundamental
issues as the equation of state of degenerate neutron matter. We describe a
selection algorithm based on a cross-correlation of the ROSAT All-Sky Survey
(RASS) and the Sloan Digital Sky Survey (SDSS) that identifies X-ray error
circles devoid of plausible optical counterparts to the SDSS g~22 magnitudes
limit. We quantitatively characterize these error circles as optically blank;
they may host INSs or other similarly exotic X-ray sources such as radio-quiet
BL Lacs, obscured AGN, etc. Our search is an order of magnitude more selective
than previous searches for optically blank RASS error circles, and excludes the
99.9% of error circles that contain more common X-ray-emitting subclasses. We
find 11 candidates, nine of which are new. While our search is designed to find
the best INS candidates and not to produce a complete list of INSs in the RASS,
it is reassuring that our number of candidates is consistent with predictions
from INS population models. Further X-ray observations will obtain pinpoint
positions and determine whether these sources are entirely optically blank at
g~22, supporting the presence of likely isolated neutron stars and perhaps
enabling detailed follow-up studies of neutron star physics.Comment: Accepted for publication in the AJ; higher resolution figures
available at http://www.astro.washington.edu/agueros/pub
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Sdssj103913.70+533029.7: a super star cluster in the outskirts of a galaxy merger
We describe the serendipitous discovery in the spectroscopic data of the Sloan Digital Sky Survey of a star-like object, SDSSJ103913.70+533029.7, at a heliocentric radial velocity of +1012 km s{sup -1}. Its proximity in position and velocity to the spiral galaxy NGC 3310 suggests an association with the galaxy. At this distance, SDSSJ103913.70+533029.7 has the luminosity of a super star cluster and a projected distance of 17 kpc from NGC 3310. Its spectroscopic and photometric properties imply a mass of > 10{sup 6} M{sub {circle_dot}} and an age close to that of the tidal shells seen around NGC 3310, suggesting that it formed in the event which formed the shells
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The milky way tomography with sdss
Using the photometric parallax method, we estimate the distances to {approx}48 million stars detected by the Sloan Digital Sky Survey (SDSS), and map their three-dimensional number density distribution in the Galaxy. The currently available data sample the distance range from 100 pc to 15 kpc and cover 6,500 deg{sup 2} of sky, mostly at high galactic latitudes (|b| > 25). These stellar number density maps allow an investigation of the Galactic structure without any a priori assumptions about its components. The data show strong evidence for a Galaxy consisting of an oblate halo, disk components, and a number of localized overdensities. The number density distribution of stars in the Solar neighborhood (D < 1.5kpc) favors a model having a ''thin'' and a ''thick'' exponential disk, with scale heights and lengths of H{sub 1} {approx} 280 pc and L{sub 1} {approx} 2400pc, and H{sub 2} {approx} 1200pc and L{sub 2} {approx} 3500pc, respectively, and local thick-to-thin disk normalization {rho}{sub thick} (R{sub {circle_dot}})/{rho}{sub thin}(R{sub {circle_dot}}) = 4%. Fits applied to the entire dataset are significantly more uncertain due to the presence of clumps and overdensities. The halo power law index is very poorly constrained, but we find an oblate halo with c/a {approx} 0.5 to be strongly preferred. While roughly consistent with this simple model, the measured density distribution shows a number of statistically significant deviations from the model predictions. In addition to known features, such as the Monoceros stream, a remarkable density enhancement covering over a thousand square degrees of sky is detected towards the constellation of Virgo, at distances of {approx} 5-15 kpc. Compared to counts in a region symmetric with respect to the l = 0 line and with the same Galactic latitude, it is responsible for a factor of 2 number density excess, and may be a nearby tidal stream or a low-surface brightness dwarf galaxy merging with the Milky Way. The u - g color distribution of these stars implies metallicities lower than those of the thick disk, and consistent with the halo metallicity distribution