1,645 research outputs found
A Modified Magnitude System that Produces Well-Behaved Magnitudes, Colors, and Errors Even for Low Signal-to-Noise Ratio Measurements
We describe a modification of the usual definition of astronomical
magnitudes, replacing the usual logarithm with an inverse hyperbolic sine
function; we call these modified magnitudes `asinh magnitudes'. For objects
detected at signal-to-noise ratios of greater than about five, our modified
definition is essentially identical to the traditional one; for fainter objects
(including those with a formally negative flux) our definition is well behaved,
tending to a definite value with finite errors as the flux goes to zero.
This new definition is especially useful when considering the colors of faint
objects, as the difference of two `asinh' magnitudes measures the usual flux
ratio for bright objects, while avoiding the problems caused by dividing two
very uncertain values for faint objects.
The Sloan Digital Sky Survey (SDSS) data products will use this scheme to
express all magnitudes in their catalogs.Comment: 11 pages, including 3 postscript figures. Submitted to A
Probabilistic Cross-Identification of Astronomical Sources
We present a general probabilistic formalism for cross-identifying
astronomical point sources in multiple observations. Our Bayesian approach,
symmetric in all observations, is the foundation of a unified framework for
object matching, where not only spatial information, but physical properties,
such as colors, redshift and luminosity, can also be considered in a natural
way. We provide a practical recipe to implement an efficient recursive
algorithm to evaluate the Bayes factor over a set of catalogs with known
circular errors in positions. This new methodology is crucial for studies
leveraging the synergy of today's multi-wavelength observations and to enter
the time-domain science of the upcoming survey telescopes.Comment: Accepted for publication in the Astrophysical Journal, 8 pages, 1
figure, emulateapj w/ apjfont
Data Mining the SDSS SkyServer Database
An earlier paper (Szalay et. al. "Designing and Mining MultiTerabyte
Astronomy Archives: The Sloan Digital Sky Survey," ACM SIGMOD 2000) described
the Sloan Digital Sky Survey's (SDSS) data management needs by defining twenty
database queries and twelve data visualization tasks that a good data
management system should support. We built a database and interfaces to support
both the query load and also a website for ad-hoc access. This paper reports on
the database design, describes the data loading pipeline, and reports on the
query implementation and performance. The queries typically translated to a
single SQL statement. Most queries run in less than 20 seconds, allowing
scientists to interactively explore the database. This paper is an in-depth
tour of those queries. Readers should first have studied the companion overview
paper Szalay et. al. "The SDSS SkyServer, Public Access to the Sloan Digital
Sky Server Data" ACM SIGMOND 2002.Comment: 40 pages, Original source is at
http://research.microsoft.com/~gray/Papers/MSR_TR_O2_01_20_queries.do
A Robust Classification of Galaxy Spectra: Dealing with Noisy and Incomplete Data
Over the next few years new spectroscopic surveys (from the optical surveys
of the Sloan Digital Sky Survey and the 2 degree Field survey through to
space-based ultraviolet satellites such as GALEX) will provide the opportunity
and challenge of understanding how galaxies of different spectral type evolve
with redshift. Techniques have been developed to classify galaxies based on
their continuum and line spectra. Some of the most promising of these have used
the Karhunen and Loeve transform (or Principal Component Analysis) to separate
galaxies into distinct classes. Their limitation has been that they assume that
the spectral coverage and quality of the spectra are constant for all galaxies
within a given sample. In this paper we develop a general formalism that
accounts for the missing data within the observed spectra (such as the removal
of sky lines or the effect of sampling different intrinsic rest wavelength
ranges due to the redshift of a galaxy). We demonstrate that by correcting for
these gaps we can recover an almost redshift independent classification scheme.
From this classification we can derive an optimal interpolation that
reconstructs the underlying galaxy spectral energy distributions in the regions
of missing data. This provides a simple and effective mechanism for building
galaxy spectral energy distributions directly from data that may be noisy,
incomplete or drawn from a number of different sources.Comment: 20 pages, 8 figures. Accepted for publication in A
Can Baryonic Features Produce the Observed 100 Mpc Clustering?
We assess the possibility that baryonic acoustic oscillations in adiabatic
models may explain the observations of excess power in large-scale structure on
100h^-1 Mpc scales. The observed location restricts models to two extreme areas
of parameter space. In either case, the baryon fraction must be large
(Omega_b/Omega_0 > 0.3) to yield significant features. The first region
requires Omega_0 < 0.2h to match the location, implying large blue tilts
(n>1.4) to satisfy cluster abundance constraints. The power spectrum also
continues to rise toward larger scales in these models. The second region
requires Omega_0 near 1, implying Omega_b well out of the range of big bang
nucleosynthesis constraints; moreover, the peak is noticeably wider than the
observations suggest. Testable features of both solutions are that they require
moderate reionization and thereby generate potentially observable (about 1 uK)
large-angle polarization, as well as sub-arc-minute temperature fluctuations.
In short, baryonic features in adiabatic models may explain the observed excess
only if currently favored determinations of cosmological parameters are in
substantial error or if present surveys do not represent a fair sample of
100h^-1 Mpc structures.Comment: LaTeX, 7 pages, 5 Postscript figures, submitted to ApJ Letter
The Evolution of the Global Star Formation History as Measured from the Hubble Deep Field
The Hubble Deep Field (HDF) is the deepest set of multicolor optical
photometric observations ever undertaken, and offers a valuable data set with
which to study galaxy evolution. Combining the optical WFPC2 data with
ground-based near-infrared photometry, we derive photometrically estimated
redshifts for HDF galaxies with J<23.5. We demonstrate that incorporating the
near-infrared data reduces the uncertainty in the estimated redshifts by
approximately 40% and is required to remove systematic uncertainties within the
redshift range 1<z<2. Utilizing these photometric redshifts, we determine the
evolution of the comoving ultraviolet (2800 A) luminosity density (presumed to
be proportional to the global star formation rate) from a redshift of z=0.5 to
z=2. We find that the global star formation rate increases rapidly with
redshift, rising by a factor of 12 from a redshift of zero to a peak at z~1.5.
For redshifts beyond 1.5, it decreases monotonically. Our measures of the star
formation rate are consistent with those found by Lilly et al. (1996) from the
CFRS at z 2, and
bridge the redshift gap between those two samples. The overall star formation
or metal enrichment rate history is consistent with the predictions of Pei and
Fall (1995) based on the evolving HI content of Lyman-alpha QSO absorption line
systems.Comment: Latex format, 10 pages, 3 postscript figures. Accepted for
publication in Ap J Letter
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