15 research outputs found
Blue horizontal branch stars in the Sloan Digital Sky Survey: II. Kinematics of the Galactic halo
We carry out a maximum-likelihood kinematic analysis of a sample of 1170 blue
horizontal branch (BHB) stars from the Sloan Digital Sky Survey presented in
Sirko et al. (2003) (Paper I). Monte Carlo simulations and resampling show that
the results are robust to distance and velocity errors at least as large as the
estimated errors from Paper I. The best-fit velocities of the Sun (circular)
and halo (rotational) are 245.9 +/- 13.5 km/s and 23.8 +/- 20.1 km/s but are
strongly covariant, so that v_0 - v_halo = 222.1 +/- 7.7 km/s. If one adopts
standard values for the local standard of rest and solar motion, then the halo
scarcely rotates. The velocity ellipsoid inferred for our sample is much more
isotropic [(sigma_r,sigma_theta,sigma_phi) = (101.4 +/- 2.8, 97.7 +/- 16.4,
107.4 +/- 16.6) km/s] than that of halo stars in the solar neighborhood, in
agreement with a recent study of the distant halo by Sommer-Larsen et al.
(1997). The line-of-sight velocity distribution of the entire sample, corrected
for the Sun's motion, is accurately gaussian with a dispersion of 101.6 +/- 3.0
km/s.Comment: 23 pages including 4 figures, 1 color; submitted to A
Improving Cosmological Distance Measurements by Reconstruction of the Baryon Acoustic Peak
The baryon acoustic oscillations are a promising route to the precision
measure of the cosmological distance scale and hence the measurement of the
time evolution of dark energy. We show that the non-linear degradation of the
acoustic signature in the correlations of low-redshift galaxies is a
correctable process. By suitable reconstruction of the linear density field,
one can sharpen the acoustic peak in the correlation function or, equivalently,
restore the higher harmonics of the oscillations in the power spectrum. With
this, one can achieve better measurements of the acoustic scale for a given
survey volume. Reconstruction is particularly effective at low redshift, where
the non-linearities are worse but where the dark energy density is highest. At
z=0.3, we find that one can reduce the sample variance error bar on the
acoustic scale by at least a factor of 2 and in principle by nearly a factor of
4. We discuss the significant implications our results have for the design of
galaxy surveys aimed at measuring the distance scale through the acoustic peak.Comment: 5 pages, LaTeX. Submitted to the Astrophysical Journa
Initial conditions to cosmological N-body simulations, or how to run an ensemble of simulations
The conventional method of generating initial conditions for cosmological
N-body simulations introduces a significant error in the real-space statistical
properties of the particles. More specifically, the finite box size leads to a
significant underestimate of sigma_8, the correlation function, and nonlinear
effects. I implement a method of generating initial conditions for N-body
simulations that accurately models the real-space statistical properties, such
as the mass variance in spheres and the correlation function. The method
requires running ensembles of simulations because the power in the DC mode is
no longer assumed to be zero. For moderately sized boxes, I demonstrate that
the new method corrects the underestimate in the mass variance in spheres and
the shape of the correlation function. I also argue that subtracting Poisson
noise from the power spectrum is a dangerous practice. Code to generate initial
conditions to second order in Lagrangian perturbation theory with the new
method is available at http://www.astro.princeton.edu/~esirko/ic .Comment: 17 pages, 11 figures, 6 colo
The Second Data Release of the Sloan Digital Sky Survey
The Sloan Digital Sky Survey (SDSS) has validated and made publicly available its Second Data Release. This data release consists of 3324 deg2 of five-band (ugriz) imaging data with photometry for over 88 million unique objects, 367,360 spectra of galaxies, quasars, stars, and calibrating blank sky patches selected over 2627 deg2 of this area, and tables of measured parameters from these data. The imaging data reach a depth of r ≈ 22.2 (95% completeness limit for point sources) and are photometrically and astrometrically calibrated to 2% rms and 100 mas rms per coordinate, respectively. The imaging data have all been processed through a new version of the SDSS imaging pipeline, in which the most important improvement since the last data release is fixing an error in the model fits to each object. The result is that model magnitudes are now a good proxy for point-spread function magnitudes for point sources, and Petrosian magnitudes for extended sources. The spectroscopy extends from 3800 to 9200 Å at a resolution of 2000. The spectroscopic software now repairs a systematic error in the radial velocities of certain types of stars and has substantially improved spectrophotometry. All data included in the SDSS Early Data Release and First Data Release are reprocessed with the improved pipelines and included in the Second Data Release. Further characteristics of the data are described, as are the data products themselves and the tools for accessing them
The First Data Release of the Sloan Digital Sky Survey
The Sloan Digital Sky Survey has validated and made publicly available its
First Data Release. This consists of 2099 square degrees of five-band (u, g, r,
i, z) imaging data, 186,240 spectra of galaxies, quasars, stars and calibrating
blank sky patches selected over 1360 square degrees of this area, and tables of
measured parameters from these data. The imaging data go to a depth of r ~ 22.6
and are photometrically and astrometrically calibrated to 2% rms and 100
milli-arcsec rms per coordinate, respectively. The spectra cover the range
3800--9200 A, with a resolution of 1800--2100. Further characteristics of the
data are described, as are the data products themselves.Comment: Submitted to The Astronomical Journal. 16 pages. For associated
documentation, see http://www.sdss.org/dr