2,183 research outputs found
High-Redshift Quasars Found in Sloan Digital Sky Survey Commissioning Data III: A Color Selected Sample at i^*<20 in the Fall Equatorial Stripe
This is the third paper in a series aimed at finding high-redshift quasars
from five-color (u'g'r'i'z') imaging data taken along the Celestial Equator by
the SDSS during its commissioning phase. In this paper, we first present the
observations of 14 bright high-redshift quasars (3.66<z<4.77, i^*<20)
discovered in the SDSS Fall Equatorial Stripe, and the SDSS photometry of two
previously known high-redshift quasars in the same region of the sky. Combined
with the quasars presented in previous papers, we define a color-selected
flux-limited sample of 39 quasars at 3.6 < z < 5.0 and i^*<20, covering a total
effective area of 182 deg^2. From this sample, we estimate the average spectral
power law slope in the rest-frame ultraviolet for quasars at z~4 to be -0.79
with a standard deviation of 0.34, and the average rest-frame equivalent width
of the Ly alpha+N V emission line to be 69 A with a standard deviation of 18 A.
The selection completeness of this multicolor sample is determined from the
model colors of high-redshift quasars, taking into account the distributions of
emission line strengths, intrinsic continuum slope, the line and continuum
absorption from intervening material, and the effects of photometric errors.
The average completeness of this sample is about 75%. The selection function
calculated in this paper will be used to correct the incompleteness of this
color-selected sample and to derive the high-redshift quasar luminosity
function in a subsequent paper. In the Appendix, we present the observations of
an additional 18 faint quasars (3.57<z<4.80, 20.1<i^*<20.8) discovered in the
region on the sky that has been imaged twice. Several quasars presented in this
paper exhibit interesting properties, including a radio-loud quasar at z=4.77,
and a narrow-line quasar (FWHM = 1500 km s^-1) at z=3.57.Comment: AJ accepted (Jan 2001), with minor changes; high-resolution finding
charts available at http://www.sns.ias.edu/~fan/papers/q3.p
Identification of A-colored Stars and Structure in the Halo of the Milky Way from SDSS Commissioning Data
A sample of 4208 objects with magnitude 15 < g* < 22 and colors of main
sequence A stars has been selected from 370 square degrees of Sloan Digital Sky
Survey (SDSS) commissioning observations. The data is from two long, narrow
stripes, each with an opening angle of greater than 60 deg, at Galactic
latitudes 36 < abs(b) < 63 on the celestial equator. An examination of the
sample's distribution shows that these stars trace considerable substructure in
the halo. Large overdensities of A-colored stars in the North at (l,b,R) =
(350, 50, 46 kpc) and in the South at (157, -58, 33 kpc) and extending over
tens of degrees are present in the halo of the Milky Way. Using photometry to
separate the stars by surface gravity, both structures are shown to contain a
sequence of low surface gravity stars consistent with identification as a blue
horizontal branch (BHB). Both structures also contain a population of high
surface gravity stars two magnitudes fainter than the BHB stars, consistent
with their identification as blue stragglers (BSs). From the numbers of
detected BHB stars, lower limits to the implied mass of the structures are
6x10^6 M_sun and 2x10^6 M_sun. The fact that two such large clumps have been
detected in a survey of only 1% of the sky indicates that such structures are
not uncommon in the halo. Simple spheroidal parameters are fit to a complete
sample of the remaining unclumped BHB stars and yield (at r < 40 kpc) a fit to
a halo distribution with flattening (c/a = 0.65+/-0.2) and a density falloff
exponent of alpha = -3.2+/-0.3.Comment: AASTeX v5_0, 26 pages, 1 table, 20 figures, ApJ accepte
Color-Induced Displacement double stars in SDSS
We report the first successful application of the astrometric color-induced
displacement technique (CID, the displacement of the photocenter between
different bandpasses due to a varying contribution of differently colored
components to the total light), originally proposed by Wielen (1996) for
discovering unresolved binary stars. Using the Sloan Digital Sky Survey (SDSS)
Data Release 1 with 2.5 million stars brighter than 21m in the u and g bands,
we select 419 candidate binary stars with CID greater than 0.5 arcsec. The SDSS
colors of the majority of these candidates are consistent with binary systems
including a white dwarf and any main sequence star with spectral type later
than ~K7. The astrometric CID method discussed here is complementary to the
photometric selection of binary stars in SDSS discussed by Smolcic et al.
(2004), but there is considerable overlap (15%) between the two samples of
selected candidates. This overlap testifies both to the physical soundness of
both methods, as well as to the astrometric and photometric quality of SDSS
data.Comment: submitted to A&A, 13 pages, 6 figure
The Angular Clustering of Galaxy Pairs
We identify close pairs of galaxies from 278 deg^2 of Sloan Digital Sky
Survey commissioning imaging data. The pairs are drawn from a sample of 330,041
galaxies with 18 < r^* < 20. We determine the angular correlation function of
galaxy pairs, and find it to be stronger than the correlation function of
single galaxies by a factor of 2.9 +/- 0.4. The two correlation functions have
the same logarithmic slope of 0.77. We invert Limber's equation to estimate the
three-dimensional correlation functions; we find clustering lengths of r_0= 4.2
+/- 0.4 h^{-1} Mpc for galaxies and 7.8 +/- 0.7 h^{-1} Mpc for galaxy pairs.
These results agree well with the global richness dependence of the correlation
functions of galaxy systems.Comment: 12 pages. ApJ, in pres
Towards Spectral Classification of L and T Dwarfs: Infrared and Optical Spectroscopy and Analysis
We present 0.6-2.5um, R~400 spectra of twenty-seven cool, low luminosity
stars and substellar objects. Based on these and previously published spectra
we develop a preliminary spectral classification system for L and T dwarfs. For
late L and T types the classification system is based entirely on four spectral
indices in the 1-2.5um interval. Two of these indices are derived from water
absorption bands at 1.15um and 1.4um, the latter of which shows a smooth
increase in depth through the L and T sequences and can be used to classify
both spectral types. The other two indices make use of methane absorption
features in the H and K bands, with the K band index also applicable to mid to
late L dwarfs. Continuum indices shortward of 1um used by previous authors to
classify L dwarfs are found to be useful only through mid L subclasses. We
employ the 1.5um water index and the 2.2um methane index to complete the L
classification through L9.5 and to link the new system with a modified version
of the 2MASS ``Color-d'' index. By correlating the depths of the methane and
water absorption features, we establish a T spectral sequence from types T0 to
T8, based on all four indices, which is a smooth continuation of the L
sequence. We reclassify two 2MASS L8 dwarfs as L9 and L9.5 and identify one
SDSS object as L9. In the proposed system methane absorption appears in the K
band approximately at L8, two subclasses earlier than its appearance in the H
band. The L and T spectral classes are distinguished by the absence and
presence, respectively, of H band methane absorption.Comment: 40 pages, 14 figures, to be published in Ap.J., Jan 1, 200
Reflection Symmetric Ballistic Microstructures: Quantum Transport Properties
We show that reflection symmetry has a strong influence on quantum transport
properties. Using a random S-matrix theory approach, we derive the
weak-localization correction, the magnitude of the conductance fluctuations,
and the distribution of the conductance for three classes of reflection
symmetry relevant for experimental ballistic microstructures. The S-matrix
ensembles used fall within the general classification scheme introduced by
Dyson, but because the conductance couples blocks of the S-matrix of different
parity, the resulting conductance properties are highly non-trivial.Comment: 4 pages, includes 3 postscript figs, uses revte
Calibration of White Dwarf cooling sequences: theoretical uncertainty
White Dwarf luminosities are powerful age indicators, whose calibration
should be based on reliable models. We discuss the uncertainty of some chemical
and physical parameters and their influence on the age estimated by means of
white dwarf cooling sequences. Models at the beginning of the white dwarf
sequence have been obtained on the base of progenitor evolutionary tracks
computed starting from the zero age horizontal branch and for a typical halo
chemical composition (Z=0.0001, Y=0.23). The uncertainties due to nuclear
reaction rates, convection, mass loss and initial chemical composition are
discussed. Then, various cooling sequences for a typical white dwarf mass
(M=0.6 Mo) have been calculated under different assumptions on some input
physics, namely: conductive opacity, contribution of the ion-electron
interaction to the free energy and microscopic diffusion. Finally we present
the evolution of white dwarfs having mass ranging between 0.5 and 0.9 Mo. Much
effort has been spent to extend the equation of state down to the low
temperature and high density regime. An analysis of the latest improvement in
the physics of white dwarf interiors is presented. We conclude that at the
faint end of the cooling sequence (log L/Lo=-5.5) the present overall
uncertainty on the age is of the order of 20%, which correspond to about 3 Gyr.
We suggest that this uncertainty could be substantially reduced by improving
our knowledge of the conductive opacity (especially in the partially degenerate
regime) and by fixing the internal stratification of C and O.Comment: 14 figures, accepted by Ap
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