93 research outputs found
Unproceedings of the Fourth .Astronomy Conference (.Astronomy 4), Heidelberg, Germany, July 9-11 2012
The goal of the .Astronomy conference series is to bring together
astronomers, educators, developers and others interested in using the Internet
as a medium for astronomy. Attendance at the event is limited to approximately
50 participants, and days are split into mornings of scheduled talks, followed
by 'unconference' afternoons, where sessions are defined by participants during
the course of the event. Participants in unconference sessions are discouraged
from formal presentations, with discussion, workshop-style formats or informal
practical tutorials encouraged. The conference also designates one day as a
'hack day', in which attendees collaborate in groups on day-long projects for
presentation the following morning. These hacks are often a way of
concentrating effort, learning new skills, and exploring ideas in a practical
fashion. The emphasis on informal, focused interaction makes recording
proceedings more difficult than for a normal meeting. While the first
.Astronomy conference is preserved formally in a book, more recent iterations
are not documented. We therefore, in the spirit of .Astronomy, report
'unproceedings' from .Astronomy 4, which was held in Heidelberg in July 2012.Comment: 11 pages, 1 figure, .Astronomy 4, #dotastr
Improved background subtraction for the Sloan Digital Sky Survey images
We describe a procedure for background subtracting Sloan Digital Sky Survey
(SDSS) imaging that improves the resulting detection and photometry of large
galaxies on the sky. Within each SDSS drift scan run, we mask out detected
sources and then fit a smooth function to the variation of the sky background.
This procedure has been applied to all SDSS-III Data Release 8 images, and the
results are available as part of that data set. We have tested the effect of
our background subtraction on the photometry of large galaxies by inserting
fake galaxies into the raw pixels, reanalyzing the data, and measuring them
after background subtraction. Our technique results in no size-dependent bias
in galaxy fluxes up to half-light radii of 100 arcsec; in contrast, for
galaxies of that size the standard SDSS photometric catalog underestimates
fluxes by about 1.5 mag. Our results represent a substantial improvement over
the standard SDSS catalog results and should form the basis of any analysis of
nearby galaxies using the SDSS imaging data.Comment: accepted by the Astronomical Journa
Characterizing unknown systematics in large scale structure surveys
Photometric large scale structure (LSS) surveys probe the largest volumes in
the Universe, but are inevitably limited by systematic uncertainties. Imperfect
photometric calibration leads to biases in our measurements of the density
fields of LSS tracers such as galaxies and quasars, and as a result in
cosmological parameter estimation. Earlier studies have proposed using
cross-correlations between different redshift slices or cross-correlations
between different surveys to reduce the effects of such systematics. In this
paper we develop a method to characterize unknown systematics. We demonstrate
that while we do not have sufficient information to correct for unknown
systematics in the data, we can obtain an estimate of their magnitude. We
define a parameter to estimate contamination from unknown systematics using
cross-correlations between different redshift slices and propose discarding
bins in the angular power spectrum that lie outside a certain contamination
tolerance level. We show that this method improves estimates of the bias using
simulated data and further apply it to photometric luminous red galaxies in the
Sloan Digital Sky Survey as a case study.Comment: 24 pages, 6 figures; Expanded discussion of results, added figure 2;
Version to be published in JCA
The Open Cluster Chemical Analysis and Mapping Survey: Local Galactic Metallicity Gradient with APOGEE using SDSS DR10
The Open Cluster Chemical Analysis and Mapping (OCCAM) Survey aims to produce
a comprehensive, uniform, infrared-based dataset for hundreds of open clusters,
and constrain key Galactic dynamical and chemical parameters from this sample.
This first contribution from the OCCAM survey presents analysis of 141 members
stars in 28 open clusters with high-resolution metallicities derived from a
large uniform sample collected as part of the SDSS-III/Apache Point Observatory
Galactic Evolution Experiment (APOGEE). This sample includes the first
high-resolution metallicity measurements for 22 open clusters. With this
largest ever uniformly observed sample of open cluster stars we investigate the
Galactic disk gradients of both [M/H] and [alpha/M]. We find basically no
gradient across this range in [alpha/M], but [M/H] does show a gradient for
R_{GC} < 10 kpc and a significant flattening beyond R_{GC} = 10 kpc. In
particular, whereas fitting a single linear trend yields an [M/H] gradient of
-0.09 +/- 0.03$ dex/kpc --- similar to previously measure gradients inside 13
kpc --- by independently fitting inside and outside 10 kpc separately we find a
significantly steeper gradient near the Sun (7.9 <= R_{GC} <= 10) than
previously found (-0.20 +/- 0.08 dex/kpc) and a nearly flat trend beyond 10 kpc
(-0.02 +/- 0.09 dex/kpc).Comment: 6 pages, 4 figures, ApJ letters, in pres
The Apache Point Observatory Galactic Evolution Experiment: First Detection of High Velocity Milky Way Bar Stars
Commissioning observations with the Apache Point Observatory Galactic
Evolution Experiment (APOGEE), part of the Sloan Digital Sky Survey III, have
produced radial velocities (RVs) for ~4700 K/M-giant stars in the Milky Way
bulge. These high-resolution (R \sim 22,500), high-S/N (>100 per resolution
element), near-infrared (1.51-1.70 um; NIR) spectra provide accurate RVs
(epsilon_v~0.2 km/s) for the sample of stars in 18 Galactic bulge fields
spanning -1-32 deg. This represents the largest
NIR high-resolution spectroscopic sample of giant stars ever assembled in this
region of the Galaxy. A cold (sigma_v~30 km/s), high-velocity peak (V_GSR \sim
+200 km/s) is found to comprise a significant fraction (~10%) of stars in many
of these fields. These high RVs have not been detected in previous MW surveys
and are not expected for a simple, circularly rotating disk. Preliminary
distance estimates rule out an origin from the background Sagittarius tidal
stream or a new stream in the MW disk. Comparison to various Galactic models
suggests that these high RVs are best explained by stars in orbits of the
Galactic bar potential, although some observational features remain
unexplained.Comment: 7 pages, 4 figures, accepted for publication in ApJ Letter
The clustering of galaxies in the SDSS-III Baryon Oscillation Spectroscopic Survey: measuring structure growth using passive galaxies
We explore the benefits of using a passively evolving population of galaxies
to measure the evolution of the rate of structure growth between z=0.25 and
z=0.65 by combining data from the SDSS-I/II and SDSS-III surveys. The
large-scale linear bias of a population of dynamically passive galaxies, which
we select from both surveys, is easily modeled. Knowing the bias evolution
breaks degeneracies inherent to other methodologies, and decreases the
uncertainty in measurements of the rate of structure growth and the
normalization of the galaxy power-spectrum by up to a factor of two. If we
translate our measurements into a constraint on sigma_8(z=0) assuming a
concordance cosmological model and General Relativity (GR), we find that using
a bias model improves our uncertainty by a factor of nearly 1.5. Our results
are consistent with a flat Lambda Cold Dark Matter model and with GR.Comment: Accepted for publication in MNRAS (clarifications added, results and
conclusions unchanged
The APOKASC Catalog: An Asteroseismic and Spectroscopic Joint Survey of Targets in the Kepler Fields
We present the first APOKASC catalog of spectroscopic and asteroseismic
properties of 1916 red giants observed in the Kepler fields. The spectroscopic
parameters provided from the Apache Point Observatory Galactic Evolution
Experiment project are complemented with asteroseismic surface gravities,
masses, radii, and mean densities determined by members of the Kepler
Asteroseismology Science Consortium. We assess both random and systematic
sources of error and include a discussion of sample selection for giants in the
Kepler fields. Total uncertainties in the main catalog properties are of order
80 K in Teff , 0.06 dex in [M/H], 0.014 dex in log g, and 12% and 5% in mass
and radius, respectively; these reflect a combination of systematic and random
errors. Asteroseismic surface gravities are substantially more precise and
accurate than spectroscopic ones, and we find good agreement between their mean
values and the calibrated spectroscopic surface gravities. There are, however,
systematic underlying trends with Teff and log g. Our effective temperature
scale is between 0-200 K cooler than that expected from the Infrared Flux
Method, depending on the adopted extinction map, which provides evidence for a
lower value on average than that inferred for the Kepler Input Catalog (KIC).
We find a reasonable correspondence between the photometric KIC and
spectroscopic APOKASC metallicity scales, with increased dispersion in KIC
metallicities as the absolute metal abundance decreases, and offsets in Teff
and log g consistent with those derived in the literature. We present mean
fitting relations between APOKASC and KIC observables and discuss future
prospects, strengths, and limitations of the catalog data.Comment: 49 pages. ApJSupp, in press. Full machine-readable ascii files
available under ancillary data. Categories: Kepler targets, asteroseismology,
large spectroscopic survey
- …