153 research outputs found
Optimal Galaxy Shape Measurements for Weak Lensing Applications Using the Hubble Space Telescope Advanced Camera for Surveys
We present three-epoch multiband (, , )
measurements of galaxy shapes using the ``polar shapelet'' or
Laguerre-expansions method with the Hubble Space Telescope () Advanced
Camera for Surveys (ACS) data, obtained as part of the {\it Great Observatories
Origin Deep Survey} (GOODS). We take advantage of the unique features of the
GOODS/ACS Fields to test the reliability of this relatively new method of
galaxy shape measurement for weak lensing analysis and to quantify the impact
of the ACS Point Spread Function (PSF) on /ACS data. We estimate the bias
introduced by the sharp PSF of the ACS on shape measurement. We show that the
bias in the tangential shear due to galaxy-galaxy lensing can be safely
neglected provided only well-resolved galaxies are used, while it would be
comparable to the signal level (1--3%) for cosmic shear measurements. These
results should of be general utility in planning and analyzing weak lensing
measurements with /ACS data.Comment: 12 pages, 3 figures. Accepted for ApJ
A test of Gaia Data Release 1 parallaxes: implications for the local distance scale
We present a comparison of Gaia Data Release 1 (DR1) parallaxes with
photometric parallaxes for a sample of 212 Galactic Cepheids at a median
distance of 2~kpc, and explore their implications on the distance scale and the
local value of the Hubble constant H_0. The Cepheid distances are estimated
from a recent calibration of the near-infrared Period-Luminosity P-L relation.
The comparison is carried out in parallax space, where the DR1 parallax errors,
with a median value of half the median parallax, are expected to be
well-behaved. With the exception of one outlier, the DR1 parallaxes are in
remarkably good global agreement with the predictions, and the published errors
may be conservatively overestimated by about 20%. The parallaxes of 9 Cepheids
brighter than G = 6 may be systematically underestimated, trigonometric
parallaxes measured with the HST FGS for three of these objects confirm this
trend. If interpreted as an independent calibration of the Cepheid luminosities
and assumed to be otherwise free of systematic uncertainties, DR1 parallaxes
would imply a decrease of 0.3% in the current estimate of the local Hubble
constant, well within their statistical uncertainty, and corresponding to a
value 2.5 sigma (3.5 sigma if the errors are scaled) higher than the value
inferred from Planck CMB data used in conjunction with Lambda-CDM. We also test
for a zeropoint error in Gaia parallaxes and find none to a precision of ~20
muas. We caution however that with this early release, the complete systematic
properties of the measurements may not be fully understood at the statistical
level of the Cepheid sample mean, a level an order of magnitude below the
individual uncertainties. The early results from DR1 demonstrate again the
enormous impact that the full mission will likely have on fundamental questions
in astrophysics and cosmology.Comment: A&A, submitted, 6 pages, 3 figure
A Gaia Data Release 3 View on the Tip of the Red Giant Branch Luminosity
The tip of the red giant branch (TRGB) is a standard candle that can be used
to help refine the determination of the Hubble constant. Data Release 3
(DR3) provides synthetic photometry constructed from low-resolution BP/RP
spectra for Milky Way field stars that can be used to directly calibrate the
luminosity of the TRGB in the Johnson-Cousins I band, where the TRGB is least
sensitive to metallicity. We calibrate the TRGB luminosity using a
two-dimensional maximum likelihood algorithm with field stars and
synthetic photometry and parallaxes. For a high-contrast and low-contrast break
(characterized by the values of the contrast parameter or the magnitude of
the break ), we find = and mag
respectively, or a midpoint of (sys)
(stat) mag. This measurement improves upon the TRGB measurement from Li
et al. (2022), as the higher precision photometry based on DR3 allows
us to constrain two additional free parameters of the luminosity function. We
also investigate the possibility of using DR3 synthetic photometry to
calibrate the TRGB luminosity with Centauri, but find evidence of
blending within the inner region for cluster member photometry that precludes
accurate calibration with DR3 photometry. We instead provide an updated
TRGB measurement of = mag in Centauri
using ground-based photometry from the most recent version of the database
described in Stetson et al. (2019), which gives =
(stat) 0.10 (sys) mag when tied to the EDR3 parallax
distance from the consensus of Vasiliev & Baumgardt (2021), Soltis et al.
(2021), and Ma\'{i}z Apell\'{a}niz et al. (2022a).Comment: 17 pages, 10 figures, 3 tables. Accepted by Ap
The WFPC2 Archival Parallels Project
We describe the methods and procedures developed to obtain a near-automatic
combination of WFPC2 images obtained as part of the WFPC2 Archival Pure
Parallels program. Several techniques have been developed or refined to ensure
proper alignment, registration, and combination of overlapping images that can
be obtained at different times and with different orientations. We quantify the
success rate and the accuracy of the registration of images of different types,
and we develop techniques suitable to equalize the sky background without
unduly affecting extended emission. About 600 combined images of the 1,500
eventually planned have already been publicly released through the STScI
Archive. The images released to date are especially suited to study star
formation in the Magellanic Clouds, the stellar population in the halo of
nearby galaxies, and the properties of star-forming galaxies at .Comment: 12 pages, 7 figures, to appear in the PAS
The Luminosity Function of QSO Host Galaxies
We present some results from our HST archival image study of 71 QSO host
galaxies. The objects are selected to have z < 0.46 and total absolute
magnitude M_V < -23 (H_0=50 km/s/Mpc). This initial study investigates host
morphology and radio loudness, and it derives the QSO host galaxy luminosity
function. We have analyzed available WFPC2 broad band images, using a uniform
set of procedures. The host galaxies span a narrow range of luminosities and
are exceptionally bright, much more so than normal galaxies, usually L > L*_V.
The QSOs are almost equally divided among radio-loud QSOs with elliptical
hosts, radio-quiet QSOs with elliptical hosts, and radio-quiet QSOs with spiral
hosts. Using a weighting procedure, we derive the combined luminosity function
of QSO host galaxies. We find that the luminosity function of QSO hosts differs
in shape from that of normal galaxies but that they coincide at the highest
luminosities. The ratio of the number of quasar hosts to the number of normal
galaxies at a luminosity L_V is R = [L_V/(11.48 L*_V)]^2.46, where L*_V
corresponds to M*_V = -22.35, and a QSO is defined to be an object with total
nuclear plus host light M_V < -23. This ratio can be interpreted as the
probability that a galaxy with luminosity L_V will host a QSO at redshift z ~
0.26. (Abridged)Comment: 36 pages, 11 figures, LaTeX. Expanded analysis and additional
figures. To be published in the Astrophysical Journal, v576, September 1,
200
Cosmic Gravitational Shear from the HST Medium Deep Survey
We present a measurement of cosmic shear on scales ranging from 10\arcsec
to 2\arcmin in 347 WFPC2 images of random fields. Our result is based on
shapes measured via image fitting and on a simple statistical technique;
careful calibration of each step allows us to quantify our systematic
uncertainties and to measure the cosmic shear down to very small angular
scales. The WFPC2 images provide a robust measurement of the cosmic shear
signal decreasing from at 10\arcsec to at 130\arcsec .Comment: 4 pages 2 Postscript figures, uses emulateapj.cls Astrophysical
Journal Letters, December 1, 200
Statistical analysis of trigonometric parallaxes
The present project was initiated with two specific medium-term goals: first, to develop a novel approach, based on global modeling and maximum likelihood, to the study of databases of stellar data, with specific attention to the results to be obtained by the Hipparcos mission; and second, to apply and test the methodology developed to existing ground-based data. The long-term goal was of course to integrate the methodology and the ground-based data into a global study of the results of the Hipparcos mission, which were expected to be made available in 1995 or 1996. The closing of the project is due to the recent change of home institution of the PI, who has moved from the University of Illinois to Johns Hopkins University. As a consequence of this move, the three scientists involved in this project will be in close proximity, thereby hopefully improving their ability to collaborate and increasing the productivity of the project. Continued funding for the project has been requested from Johns Hopkins University. This report describes briefly the results obtained so far both on the technical aspect of software development and on the scientific side of applications to existing ground-based data. Significant progress has been made on both counts, with several papers published in (or submitted to) refereed journals and in conference proceedings. Because it is hoped that the project can be successfully continued with NASA support, the report of the progress in each area includes also an assessment of how the current results fit in the expected continuation of the project. Our results to date include: code development (essentially completed); a detailed study of the kinematics and dynamics of stars escaping from the Hyades cluster, relevant to the question of membership in the Hyades; a study of the kinematics and luminosity calibration of nearby dwarfs; an assessment of the quality of the photometry included in the Hipparcos Input Catalog; and two studies of properties of nearby clusters, including a moving-cluster determination of the distance to Praesepe. The bibliography includes three papers submitted to refereed journals, two of which have already been published, and four contributions to conference proceedings. Finally, the work so far has also provided a very good introduction to stellar dynamics and astrometry for an undergraduate student, with educational benefits that had not been foreseen in the original proposal
A 2.4% DETERMINATION of the LOCAL VALUE of the HUBBLE CONSTANT
We use the Wide Field Camera 3 (WFC3) on the Hubble Space Telescope (HST) to reduce the uncertainty in the local value of the Hubble constant from 3.3% to 2.4%. The bulk of this improvement comes from new near-infrared (NIR) observations of Cepheid variables in 11 host galaxies of recent type Ia supernovae (SNe Ia), more than doubling the sample of reliable SNe Ia having a Cepheid-calibrated distance to a total of 19; these in turn leverage the magnitude-redshift relation based on 300 SNe Ia at z < 0.15. All 19 hosts as well as the megamaser system NGC 4258 have been observed with WFC3 in the optical and NIR, thus nullifying cross-instrument zeropoint errors in the relative distance estimates from Cepheids. Other noteworthy improvements include a 33% reduction in the systematic uncertainty in the maser distance to NGC 4258, a larger sample of Cepheids in the Large Magellanic Cloud (LMC), a more robust distance to the LMC based on late-type detached eclipsing binaries (DEBs), HST observations of Cepheids in M31, and new HST-based trigonometric parallaxes for Milky Way (MW) Cepheids. We consider four geometric distance calibrations of Cepheids: (i) megamasers in NGC 4258, (ii) 8 DEBs in the LMC, (iii) 15 MW Cepheids with parallaxes measured with HST/FGS, HST/WFC3 spatial scanning and/or Hipparcos, and (iv) 2 DEBs in M31. The Hubble constant from each is 72.25, 2.51, 72.04,2.67, 76.18,2.37, and 74.50,3.27 km s-1 Mpc-1, respectively. Our best estimate of H 0 = 73.24, 1.74 km s-1 Mpc-1 combines the anchors NGC 4258, MW, and LMC, yielding a 2.4% determination (all quoted uncertainties include fully propagated statistical and systematic components). This value is 3.4σ higher than 66.93, 0.62 km s-1 Mpc-1 predicted by ΛCDM with 3 neutrino flavors having a mass of 0.06 eV and the new Planck data, but the discrepancy reduces to 2.1σ relative to the prediction of 69.3, 0.7 km s-1 Mpc-1 based on the comparably precise combination of WMAP+ACT+SPT+BAO observations, suggesting that systematic uncertainties in CMB radiation measurements may play a role in the tension. If we take the conflict between Planck high-redshift measurements and our local determination of H 0 at face value, one plausible explanation could involve an additional source of dark radiation in the early universe in the range of ΔN eff ≈ 0.4-1. We anticipate further significant improvements in H 0 from upcoming parallax measurements of long-period MW Cepheid
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