ABSTRACT: We present the design and performance of the multi-object fiber spectrographs
for the Sloan Digital Sky Survey (SDSS) and their upgrade for the Baryon
Oscillation Spectroscopic Survey (BOSS). Originally commissioned in Fall 1999
on the 2.5-m aperture Sloan Telescope at Apache Point Observatory, the
spectrographs produced more than 1.5 million spectra for the SDSS and SDSS-II
surveys, enabling a wide variety of Galactic and extra-galactic science
including the first observation of baryon acoustic oscillations in 2005. The
spectrographs were upgraded in 2009 and are currently in use for BOSS, the
flagship survey of the third-generation SDSS-III project. BOSS will measure
redshifts of 1.35 million massive galaxies to redshift 0.7 and Lyman-$\alpha$
absorption of 160,000 high redshift quasars over 10,000 square degrees of sky,
making percent level measurements of the absolute cosmic distance scale of the
Universe and placing tight constraints on the equation of state of dark energy.
The twin multi-object fiber spectrographs utilize a simple optical layout
with reflective collimators, all-refractive cameras, gratings, and
state-of-the-art CCD detectors to produce hundreds of spectra simultaneously in
two channels over a bandpass covering the near ultraviolet to the near
infrared, with a resolving power R = \lambda/FWHM ~ 2000. Building on proven
heritage, the spectrographs were upgraded for BOSS with volume-phase
holographic gratings and modern CCD detectors, improving the peak throughput by
nearly a factor of two, extending the bandpass to cover 360 < \lambda < 1000
nm, and increasing the number of fibers from 640 to 1000 per exposure. In this
paper we describe the original SDSS spectrograph design and the upgrades
implemented for BOSS, and document the predicted and measured performances.
ABSTRACT: We measure the acoustic scale from the angular power spectra of the Sloan
Digital Sky Survey III (SDSS-III) Data Release 8 imaging catalog that includes
872,921 galaxies over ~ 10,000 deg^2 between 0.45<z<0.65. The extensive
spectroscopic training set of the Baryon Oscillation Spectroscopic Survey
(BOSS) luminous galaxies allows precise estimates of the true redshift
distributions of galaxies in our imaging catalog. Utilizing the redshift
distribution information, we build templates and fit to the power spectra of
the data, which are measured in our companion paper, Ho et al. 2011, to derive
the location of Baryon acoustic oscillations (BAO) while marginalizing over
many free parameters to exclude nearly all of the non-BAO signal. We derive the
ratio of the angular diameter distance to the sound horizon scale D_A/r_s=
9.212 + 0.416 -0.404 at z=0.54, and therefore, D_A= 1411+- 65 Mpc at z=0.54;
the result is fairly independent of assumptions on the underlying cosmology.
Our measurement of angular diameter distance D_A is 1.4 \sigma higher than what
is expected for the concordance LCDM (Komatsu et al. 2011), in accordance to
the trend of other spectroscopic BAO measurements for z >~ 0.35. We report
constraints on cosmological parameters from our measurement in combination with
the WMAP7 data and the previous spectroscopic BAO measurements of SDSS
(Percival et al. 2010) and WiggleZ (Blake et al. 2011). We refer to our
companion papers (Ho et al. 2011; de Putter et al. 2011) for investigations on
information of the full power spectrum.
ABSTRACT: Statistical weak lensing by large-scale structure -- cosmic shear -- is a
promising cosmological tool, which has motivated the design of several large
upcoming surveys. Here, we present a measurement of cosmic shear using coadded
Sloan Digital Sky Survey (SDSS) imaging in 168 square degrees of the equatorial
region, with r<23.5 and i<22.5, a source number density of 2.2 galaxies per
square arcminute and median redshift of 0.52. These coadds were generated using
a new method described in the companion Paper I that was intended to minimise
systematic errors in the lensing measurement due to coherent PSF anisotropies
that are otherwise prevalent in the SDSS imaging data. We present measurements
of cosmic shear out to angular separations of 2 degrees, along with systematics
tests that (combined with those from Paper I on the catalogue generation)
demonstrate that our results are dominated by statistical rather than
systematic errors. Assuming a cosmological model corresponding to WMAP7 and
allowing only the amplitude of matter fluctuations to vary, we find a best-fit
value of sigma_8=0.636 +0.109 -0.154 (1-sigma); without systematic errors this
would be sigma_8=0.636 +0.099 -0.137 (1-sigma). Assuming a flat LCDM model, the
combined constraints with WMAP7 are sigma_8=0.784 +0.028 -0.026 (1-sigma),
+0.055 -0.054 (2-sigma) and Omega_m h^2=0.1303 +0.0047 -0.0048 (1-sigma)+0.009
-0.009 (2-sigma); the 2-sigma error ranges are respectively 14 and 17 per cent
smaller than WMAP7 alone. Aside from the intrinsic value of such cosmological
constraints from the growth of structure, we identify some important lessons
for upcoming surveys that may face similar issues when combining multi-epoch
data to measure cosmic shear.
ABSTRACT: Weak lensing by large-scale structure is an invaluable cosmological tool
given that most of the energy density of the concordance cosmology is
invisible. Several large ground-based imaging surveys will attempt to measure
this effect over the coming decade, but reliable control of the spurious
lensing signal introduced by atmospheric turbulence and telescope optics
remains a challenging problem. We address this challenge with a demonstration
that point-spread function (PSF) effects on measured galaxy shapes in current
ground-based surveys can be corrected with existing analysis techniques. In
this work, we co-add existing Sloan Digital Sky Survey imaging on the
equatorial stripe in order to build a data set with the statistical power to
measure cosmic shear, while using a rounding kernel method to null out the
effects of the anisotropic PSF. We build a galaxy catalogue from the combined
imaging, characterise its photometric properties, and show that the spurious
shear remaining in this catalogue after the PSF correction is negligible
compared to the expected cosmic shear signal. We identify a new source of
systematic error in the shear-shear auto-correlations arising from selection
biases related to masking. Finally, we discuss the circumstances in which this
method is expected to be useful for upcoming ground-based surveys that have
lensing as one of the science goals, and identify the systematic errors that
can reduce its efficacy.
ABSTRACT: We use SDSS photometry of 73 million stars to simultaneously obtain best-fit
main-sequence stellar energy distribution (SED) and amount of dust extinction
along the line of sight towards each star. Using a subsample of 23 million
stars with 2MASS photometry, whose addition enables more robust results, we
show that SDSS photometry alone is sufficient to break degeneracies between
intrinsic stellar color and dust amount when the shape of extinction curve is
fixed. When using both SDSS and 2MASS photometry, the ratio of the total to
selective absorption, $R_V$, can be determined with an uncertainty of about 0.1
for most stars in high-extinction regions. These fits enable detailed studies
of the dust properties and its spatial distribution, and of the stellar spatial
distribution at low Galactic latitudes. Our results are in good agreement with
the extinction normalization given by the Schlegel et al. (1998, SFD) dust maps
at high northern Galactic latitudes, but indicate that the SFD extinction map
appears to be consistently overestimated by about 20% in the southern sky, in
agreement with Schlafly et al. (2010). The constraints on the shape of the dust
extinction curve across the SDSS and 2MASS bandpasses support the models by
Fitzpatrick (1999) and Cardelli et al. (1989). For the latter, we find an
$R_V=3.0\pm0.1$(random) $\pm0.1$(systematic) over most of the high-latitude
sky. At low Galactic latitudes (|b|<5), we demonstrate that the SFD map cannot
be reliably used to correct for extinction as most stars are embedded in dust,
rather than behind it. We introduce a method for efficient selection of
candidate red giant stars in the disk, dubbed "dusty parallax relation", which
utilizes a correlation between distance and the extinction along the line of
sight. We make these best-fit parameters, as well as all the input SDSS and
2MASS data, publicly available in a user-friendly format.
ABSTRACT: Over the last decade optical spectroscopic surveys have characterized the low redshift galaxy population and uncovered populations of star-forming galaxies back to z ~ 7. This work has shown that the primary epoch of galaxy building and black hole growth occurs at redshifts of 2 to 3. The establishment of the concordance LCDM cosmology shifted the focus of galaxy population studies from constraining cosmological parameters to characterizing the processes which regulate the formation and evolution of galaxies.In the next decade, high redshift observers will attempt to formulate a coherent evolutionary picture connecting galaxies in the high redshift Universe to galaxies today. In order to link galaxy populations at different redshifts, we must not only characterize their evolution in a systematic way, we must establish which physical processes are responsible for it. Considerable progress has already been made in understanding how galaxies evolved from z ~ 1 to the present day. Large spectroscopic surveys in the near infrared are required to push these studies back towards the main epoch of galaxy building. Only then will we understand the full story of the formation of L* galaxies like our own Milky Way. A large near-IR spectroscopic survey will also provide the calibration needed to avoid systematics in the large photometric programs proposed to study the nature of dark matter and dark energy. We provide an outline design for a multi-object 0.4 to 1.8 micron spectrograph, which could be placed on an existing telescope, and which would allow a full characterization of the galaxy population out to z ~ 2. We strongly recommend a serious further study to design a real instrument, which will be required for galaxy formation studies to advance to the next frontier.
ABSTRACT: BOSS, the Baryon Oscillation Spectroscopic Survey, is a 5-year program to measure the absolute cosmic distance scale and expansion rate with percent-level precision at redshifts z<0.7 and z~2.5. BOSS uses the "standard ruler" provided by baryon acoustic oscillations (BAO). BOSS will achieve a near optimal measurement of the BAO scale at z<0.7, with a redshift survey of 1.5 million luminous galaxies. It will pioneer a new method of BAO measurement at high redshift, using the LyA forest to 160,000 QSOs in the redshift range 2.1<z<3.0. The forecast measurement precision for angular diameter distance d_A is 1.0%, 1.0%, and 1.5% at z=0.35, 0.6, and 2.5, respectively, and the forecast precision for the Hubble parameter H(z) is 1.8%, 1.7%, and 1.2% at the same redshifts. These measurements will provide powerful constraints on the nature of dark energy and the curvature of space, complementing the constraints obtained from other probes. BOSS will also provide a superb data set for studying large- and small-scale clustering, the evolution of massive galaxies and the luminosity function and clustering of QSOs at 2.3 < z < 6.5. BOSS is one of four surveys that comprise SDSS-III (the Sloan Digital Sky Survey III), a 6-year program that will use highly multiplexed spectrographs on the 2.5-m Sloan Foundation Telescope to investigate cosmological parameters, the history and structure of the Milky Way galaxy, and the population of giant planet systems.
ABSTRACT: Early photometric data from the Sloan Digital Sky Survey (SDSS) contain angular positions for 1.5 million galaxies. In companion papers, the angular correlation function w(θ) and two-dimensional power spectrum Cl of these galaxies are presented. Here we invert Limber's equation to extract the three-dimensional power spectrum from the angular results. We accomplish this using an estimate of dn/dz, the redshift distribution of galaxies in four different magnitude slices in the SDSS photometric catalog. The resulting three-dimensional power spectrum estimates from w(θ) and Cl agree with each other and with previous estimates over a range in wavenumbers 0.03 < k/(h Mpc-1) < 1. The galaxies in the faintest magnitude bin (21 < r* < 22, which have median redshift zm = 0.43) are less clustered than the galaxies in the brightest magnitude bin (18 < r* < 19 with zm = 0.17), especially on scales where nonlinearities are important. The derived power spectrum agrees with that of Szalay et al., who go directly from the raw data to a parametric estimate of the power spectrum. The strongest constraints on the shape parameter Γ come from the faintest galaxies (in the magnitude bin 21 < r* < 22), from which we infer Γ = 0.14 (95% CL).
The Astrophysical Journal 12/2008; 572(1):140. · 6.02 Impact Factor
ABSTRACT: We report the discovery of a locus of binary stars in the Sloan Digital Sky Survey (SDSS) g-r versus u-g color-color diagram that connects the colors of white dwarfs and M dwarfs. While its contrast with respect to the main stellar locus is only ~1 : 2300, this previously unrecognized feature includes 863 stars from the SDSS Data Release 1 (DR1). The position and shape of the feature are in good agreement with predictions of a simple binary star model that consists of a white dwarf and an M dwarf, with the components' luminosity ratio controlling the position along this binary system locus. SDSS DR1 spectra for 47 of these objects strongly support this model. The absolute magnitude-color distribution inferred for the white dwarf component is in good agreement with the models of Bergeron et al.
The Astrophysical Journal 12/2008; 615(2):L141. · 6.02 Impact Factor
ABSTRACT: We present initial results for counts in cell statistics of the angular distribution of galaxies in early data from the Sloan Digital Sky Survey (SDSS). We analyze a rectangular stripe 25 wide, covering approximately 160 deg2, containing over 106 galaxies in the apparent magnitude range 18 < r' < 22, with areas of bad seeing, contamination from bright stars, ghosts, and high galactic extinction masked out. This survey region, which forms part of the SDSS early data release, is the same as that for which two-point angular clustering statistics have recently been computed. The third and fourth moments of the cell counts, s3 (skewness) and s4 (kurtosis), constitute the most accurate measurements to date of these quantities (for r' < 21) over angular scales 0015-03. They display the approximate hierarchical scaling expected from nonlinear structure formation models and are in reasonable agreement with the predictions of Λ-dominated cold dark matter models with galaxy biasing that suppresses higher order correlations at small scales. The results are, in general, consistent with previous measurements in the APM, EDSGC, and Deeprange surveys. These results suggest that the SDSS imaging data are free of systematics to a high degree and will therefore enable determination of the skewness and kurtosis to 1% and less then 10%, as predicted earlier.
The Astrophysical Journal 12/2008; 570(1):75. · 6.02 Impact Factor
ABSTRACT: We compute the angular power spectrum Cl from 1.5 million galaxies in early Sloan Digital Sky Survey (SDSS) data on large angular scales, l 600. The data set covers about 160 deg2, with a characteristic depth on the order of 1 h-1 Gpc in the faintest (21 < r* < 22) of our four magnitude bins. Cosmological interpretations of these results are presented in a companion paper by Dodelson and coworkers. The data in all four magnitude bins are consistent with a simple flat "concordance" model with nonlinear evolution and linear bias factors on the order of unity. Nonlinear evolution is particularly evident for the brightest galaxies. A series of tests suggests that systematic errors related to seeing, reddening, etc. are negligible, which bodes well for the 60-fold larger sample that the SDSS is currently collecting. Uncorrelated error bars and well-behaved window functions make our measurements a convenient starting point for cosmological model fitting.
The Astrophysical Journal 12/2008; 571(1):191. · 6.02 Impact Factor
ABSTRACT: The angular distribution of galaxies encodes a wealth of information about large-scale structure. Ultimately, the Sloan Digital Sky Survey (SDSS) will record the angular positions of order of 108 galaxies in five bands, adding significantly to the cosmological constraints. This is the first in a series of papers analyzing a rectangular stripe of 25 × 90° from early SDSS data. We present the angular correlation function for galaxies in four separate magnitude bins on angular scales ranging from 0003 to 15°. Much of the focus of this paper is on potential systematic effects. We show that the final galaxy catalog—with the mask accounting for regions of poor seeing, reddening, bright stars, etc.—is free from external and internal systematic effects for galaxies brighter than r* = 22. Our estimator of the angular correlation function includes the effects of the integral constraint and the mask. The full covariance matrix of errors in these estimates is derived using mock catalogs with further estimates using a number of other methods.
The Astrophysical Journal 12/2008; 579(1):48. · 6.02 Impact Factor
ABSTRACT: We present the first measurements of clustering in the Sloan Digital Sky Survey (SDSS) galaxy redshift survey. Our sample consists of 29,300 galaxies with redshifts 5700 km s-1 ≤ cz ≤ 39,000 km s-1, distributed in several long but narrow (25-5°) segments, covering 690 deg2. For the full, flux-limited sample, the redshift-space correlation length is approximately 8 h-1 Mpc. The two-dimensional correlation function ξ(rp,π) shows clear signatures of both the small-scale, "fingers-of-God" distortion caused by velocity dispersions in collapsed objects and the large-scale compression caused by coherent flows, though the latter cannot be measured with high precision in the present sample. The inferred real-space correlation function is well described by a power law, ξ(r) = (r/6.1 ± 0.2 h-1 Mpc)-1.75±0.03, for 0.1 h-1 Mpc ≤ r ≤ 16 h-1 Mpc. The galaxy pairwise velocity dispersion is σ12 ≈ 600 ± 100 km s-1 for projected separations 0.15 h-1 Mpc ≤ rp ≤ 5 h-1 Mpc. When we divide the sample by color, the red galaxies exhibit a stronger and steeper real-space correlation function and a higher pairwise velocity dispersion than do the blue galaxies. The relative behavior of subsamples defined by high/low profile concentration or high/low surface brightness is qualitatively similar to that of the red/blue subsamples. Our most striking result is a clear measurement of scale-independent luminosity bias at r 10 h-1 Mpc: subsamples with absolute magnitude ranges centered on M* - 1.5, M*, and M* + 1.5 have real-space correlation functions that are parallel power laws of slope ≈-1.8 with correlation lengths of approximately 7.4, 6.3, and 4.7 h-1 Mpc, respectively.
The Astrophysical Journal 12/2008; 571(1):172. · 6.02 Impact Factor
ABSTRACT: The Sloan Digital Sky Survey is one of the first multicolor photometric and spectroscopic surveys designed to measure the statistical properties of galaxies within the local universe. In this paper we present some of the initial results on the angular two-point correlation function measured from the early SDSS galaxy data. The form of the correlation function, over the magnitude interval 18 < r* < 22, is shown to be consistent with results from existing wide-field, photographic-based surveys and narrower CCD galaxy surveys. On scales between 1' and 1° the correlation function is well described by a power law with an exponent of ≈-0.7. The amplitude of the correlation function, within this angular interval, decreases with fainter magnitudes in good agreement with analysis from existing galaxy surveys. There is a characteristic break in the correlation function on scales of approximately 1°-2°. On small scales, θ < 1', the SDSS correlation function does not appear to be consistent with the power-law form fitted to the 1' < θ < 05 data. With a data set that is less than 2% of the full SDSS survey area, we have obtained high-precision measurements of the power-law angular correlation function on angular scales 1' < θ < 1°, which are robust to systematic uncertainties. Because of the limited area and the highly correlated nature of the error covariance matrix, these initial results do not yet provide a definitive characterization of departures from the power-law form at smaller and larger angles. In the near future, however, the area of the SDSS imaging survey will be sufficient to allow detailed analysis of the small- and large-scale regimes, measurements of higher order correlations, and studies of angular clustering as a function of redshift and galaxy type.
The Astrophysical Journal 12/2008; 579(1):42. · 6.02 Impact Factor
ABSTRACT: We present evidence for a ring of stars in the plane of the Milky Way, extending at least from l = 180° to 227° with turnoff magnitude g ~ 19.5; the ring could encircle the Galaxy. We infer that the low Galactic latitude structure is at a fairly constant distance of R = 18 ± 2 kpc from the Galactic center above the Galactic plane and has R = 20 ± 2 kpc in the region sampled below the Galactic plane. The evidence includes 500 Sloan Digital Sky Survey spectroscopic radial velocities of stars within 30° of the plane. The velocity dispersion of the stars associated with this structure is found to be 27 km s-1 at (l, b) = (198°, - 27°), 22 km s-1 at (l, b) = (225°, 28°), 30 km s-1 at (l, b) = (188°, 24°), and 30 km s-1 at (l, b) = (182°, 27°). The structure rotates in the same prograde direction as the Galactic disk stars but with a circular velocity of 110 ± 25 km s-1. The narrow measured velocity dispersion is inconsistent with power-law spheroid or thick-disk populations. We compare the velocity dispersion in this structure with the velocity dispersion of stars in the Sagittarius dwarf galaxy tidal stream, for which we measure a velocity dispersion of 20 km s-1 at (l, b) = (165°, - 55°). We estimate a preliminary metallicity from the Ca II (K) line and color of the turnoff stars of [Fe/H] = -1.6 with a dispersion of 0.3 dex and note that the turnoff color is consistent with that of the spheroid population. We interpret our measurements as evidence for a tidally disrupted satellite of 2 × 107 to 5 × 108 M☉ that rings the Galaxy.
The Astrophysical Journal 12/2008; 588(2):824. · 6.02 Impact Factor
ABSTRACT: We have produced a new conformal map of the universe illustrating recent discoveries, ranging from Kuiper Belt objects in the solar system to the galaxies and quasars from the Sloan Digital Sky Survey. This map projection, based on the logarithm map of the complex plane, preserves shapes locally and yet is able to display the entire range of astronomical scales from the Earth's neighborhood to the cosmic microwave background. The conformal nature of the projection, preserving shapes locally, may be of particular use for analyzing large-scale structure. Prominent in the map is a Sloan Great Wall of galaxies 1.37 billion light-years long, 80% longer than the Great Wall discovered by Geller and Huchra and therefore the largest observed structure in the universe.
The Astrophysical Journal 12/2008; 624(2):463. · 6.02 Impact Factor
ABSTRACT: Using effective temperature and metallicity derived from SDSS spectra for ~60,000 F and G type main sequence stars (0.2<g-r<0.6), we develop polynomial models for estimating these parameters from the SDSS u-g and g-r colors. We apply this method to SDSS photometric data for about 2 million F/G stars and measure the unbiased metallicity distribution for a complete volume-limited sample of stars at distances between 500 pc and 8 kpc. The metallicity distribution can be exquisitely modeled using two components with a spatially varying number ratio, that correspond to disk and halo. The two components also possess the kinematics expected for disk and halo stars. The metallicity of the halo component is spatially invariant, while the median disk metallicity smoothly decreases with distance from the Galactic plane from -0.6 at 500 pc to -0.8 beyond several kpc. The absence of a correlation between metallicity and kinematics for disk stars is in a conflict with the traditional decomposition in terms of thin and thick disks. We detect coherent substructures in the kinematics--metallicity space, such as the Monoceros stream, which rotates faster than the LSR, and has a median metallicity of [Fe/H]=-0.96, with an rms scatter of only ~0.15 dex. We extrapolate our results to the performance expected from the Large Synoptic Survey Telescope (LSST) and estimate that the LSST will obtain metallicity measurements accurate to 0.2 dex or better, with proper motion measurements accurate to ~0.2 mas/yr, for about 200 million F/G dwarf stars within a distance limit of ~100 kpc (g<23.5). [abridged]
ABSTRACT: 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 by Sirko and coworkers. Monte Carlo simulations and resampling show that the results are robust to distance and velocity errors at least as large as the errors they estimated. The best-fit velocities of the Sun (circular) and halo (rotational) are 245.9 ± 13.5 km s-1 and 23.8 ± 20.1 km s-1 but are strongly covariant, so that v - vhalo = 222.1 ± 7.7 km s-1. 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 [(σr,σθ,σ) = (101.4 ± 2.8, 97.7 ± 16.4, 107.4 ± 16.6) km s-1] than that of halo stars in the solar neighborhood, in agreement with a recent study of the distant halo by Sommer-Larsen and coworkers. 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-1.
The Astronomical Journal 12/2007; 127(2):914. · 4.03 Impact Factor
ABSTRACT: We discuss the optical and radio properties of ~30,000 FIRST (radio, 20 cm, sensitive to 1 mJy) sources positionally associated within 15 with a Sloan Digital Sky Survey (SDSS) (optical, sensitive to r* ~ 22.2) source in 1230 deg2 of sky. The matched sample represents ~30% of the 108,000 FIRST sources and 0.1% of the 2.5 × 107 SDSS sources in the studied region. SDSS spectra are available for 4300 galaxies and 1154 quasars from the matched sample and for a control sample of 140,000 galaxies and 20,000 quasars in 1030 deg2 of sky. Here we analyze only core sources, which dominate the sample; the fraction of SDSS-FIRST sources with complex radio morphology is determined to be less than 10%. This large and unbiased catalog of optical identifications provides much firmer statistical footing for existing results and allows several new findings. The majority (83%) of the FIRST sources identified with an SDSS source brighter than r* = 21 are optically resolved; the fraction of resolved objects among the matched sources is a function of the radio flux, increasing from ~50% at the bright end to ~90% at the FIRST faint limit. Nearly all optically unresolved radio sources have nonstellar colors indicative of quasars. We estimate an upper limit of ~5% for the fraction of quasars with broadband optical colors indistinguishable from those of stars. The distribution of quasars in the radio flux–optical flux plane suggests the existence of the "quasar radio dichotomy"; 8% ± 1% of all quasars with i* < 18.5 are radio-loud, and this fraction seems independent of redshift and optical luminosity. The radio-loud quasars have a redder median color by 0.08 ± 0.02 mag, and show a 3 times larger fraction of objects with extremely red colors. FIRST galaxies represent 5% of all SDSS galaxies with r* < 17.5, and 1% for r* < 20, and are dominated by red (u*-r* > 2.22) galaxies, especially those with r* > 17.5. Magnitude- and redshift-limited samples show that radio galaxies have a different optical luminosity distribution than nonradio galaxies selected by the same criteria; when galaxies are further separated by their colors, this result remains valid for both blue and red galaxies. For a given optical luminosity and redshift, the observed optical colors of radio galaxies are indistinguishable from those of all SDSS galaxies selected by identical criteria. The distributions of radio-to-optical flux ratio are similar for blue and red galaxies in redshift-limited samples; this similarity implies that the difference in their luminosity functions and resulting selection effects are the dominant cause for the preponderance of red radio galaxies in flux-limited samples. The fraction of radio galaxies whose emission-line ratios indicate an AGN (30%), rather than starburst, origin is 6 times larger than the corresponding fraction for all SDSS galaxies (r* < 17.5). We confirm that the AGN-to-starburst galaxy number ratio increases with radio flux and find that radio emission from AGNs is more concentrated than radio emission from starburst galaxies.
The Astronomical Journal 12/2007; 124(5):2364. · 4.03 Impact Factor
ABSTRACT: We isolate samples of 733 bright (g < 18) and 437 faint (g > 18) high Galactic latitude blue horizontal-branch stars with photometry and spectroscopy in the Sloan Digital Sky Survey (SDSS). Comparison of independent photometric and spectroscopic selection criteria indicates that contamination from F and blue straggler stars is less than 10% for bright stars (g < 18) and about 25% for faint stars (g > 18), and this is qualitatively confirmed by proper motions based on the USNO-A catalog as first epoch. Analysis of repeated observations shows that the errors in radial velocity are ≈26 km s-1. A relation between absolute magnitude and color is established using the horizontal branches of halo globular clusters observed by SDSS. Bolometric corrections and colors are synthesized in the SDSS filters from model spectra. The redder stars agree well in absolute magnitude with accepted values for RR Lyrae stars. The resulting photometric distances are accurate to about 0.2 mag, with a median of about 25 kpc. Modest clumps in phase space exist and are consistent with the previously reported tidal stream of the Sagittarius dwarf galaxy.
The Astronomical Journal 12/2007; 127(2):899. · 4.03 Impact Factor