251 research outputs found
Precision Weak Gravitational Lensing Using Velocity Fields: Fisher Matrix Analysis
Weak gravitational lensing measurements based on photometry are limited by
shape noise, the variance in the unknown unlensed orientations of the source
galaxies. If the source is a disk galaxy with a well-ordered velocity field,
however, velocity field data can support simultaneous inference of the shear,
inclination, and position angle, virtually eliminating shape noise. We use the
Fisher Information Matrix formalism to forecast the precision of this method in
the idealized case of a perfectly ordered velocity field defined on an
infinitesimally thin disk. For nearly face-on targets one shear component,
, can be constrained to where is the S/N of the central intensity pixel and
is the number of pixels across a diameter enclosing 80\% of the light. This
precision degrades with inclination angle, by a factor of three by
. Uncertainty on the other shear component, , is about
1.5 (7) times larger than the uncertainty for targets at
(). For arbitrary galaxy position angle on the sky,
these forecasts apply not to and as defined on the
sky, but to two eigenvectors in space where
is the magnification. We also forecast the potential of less expensive
partial observations of the velocity field such as slit spectroscopy. We
conclude by outlining some ways in which real galaxies depart from our
idealized model and thus create random or systematic uncertainties not captured
here. In particular, our forecast precision is currently
limited only by the data quality rather than scatter in galaxy properties
because the relevant type of scatter has yet to be measured.Comment: Accepted to ApJ, 17 pages, 14 figures. Diff from v1: added Sec 3.1 on
degeneracies and Appendix with simulations confirming Fisher result
Shaping Attitudes Toward Science in an Introductory Astronomy Class
At many universities, astronomy is a popular way for non-science majors to
fulfill a general education requirement. Because general-education astronomy
may be the only college-level science course taken by these students, it is the
last chance to shape the science attitudes of these future journalists,
teachers, politicians, and voters. I report on an attempt to measure and induce
changes in science attitudes in my general-education astronomy course. I
describe construction of the attitude survey, classroom activities designed to
influence attitudes, and give numerical results indicating a significant
improvement. In contrast, the literature on attitudes in introductory physics
courses generally reports stagnation or decline. I briefly comment on some
plausible explanations for this difference.Comment: v2 includes a copy of the surve
Optical Galaxy Clusters in the Deep Lens Survey
We present the first sample of 882 optically selected galaxy clusters in the
Deep Lens Survey (DLS), selected with the Bayesian Cluster Finder. We create
mock DLS data to assess completeness and purity rates, and find that both are
at least within 0.1 1.2 for clusters with . We verified the integrity of the sample by
performing several comparisons with other optical, weak lensing, X-ray and
spectroscopic surveys which overlap the DLS footprint: the estimated redshifts
are consistent with the spectroscopic redshifts of known clusters (for
where saturation in the DLS is not an issue); our richness estimates in
combination with a previously calibrated richness-mass relation yields
individual cluster mass estimates consistent with available SHeLS dynamical
mass estimates; synthetic mass maps made from the optical mass estimates are
correlated ( significance) with the weak lensing mass maps; and the
mass function thus derived is consistent with theoretical predictions for the
CDM scenario. With the verified sample we investigated correlations between the
brightest cluster galaxies (BCG) properties and the host cluster properties
within a broader range in redshift (0.25 0.8) and mass
() than in previous work. We find that the slope
of the BCG magnitude-redshift relation throughout this redshift range is
consistent with that found at lower redshifts. This result supports an
extrapolation to higher redshift of passive evolution of the BCG within the
hierarchical scenario.Comment: Paper accepted for publication in MNRAS, Table 1 will be available
online or under reques
Brightest Cluster Galaxy Alignments in Merging Clusters
The orientations of brightest cluster galaxies (BCGs) and their host clusters
tend to be aligned, but the mechanism driving this is not clear. To probe the
role of cluster mergers in this process, we quantify alignments of 38 BCGs in
22 clusters undergoing major mergers (up to Gyr after first
pericenter). We find alignments entirely consistent with those of clusters in
general. This suggests that alignments are robust against major cluster
mergers. If, conversely, major cluster mergers actually help orient the BCG,
such a process is acting quickly because the orientation is in place within
Gyr after first pericenter.Comment: accepted to Ap
Overconfidence in Photometric Redshift Estimation
We describe a new test of photometric redshift performance given a
spectroscopic redshift sample. This test complements the traditional comparison
of redshift {\it differences} by testing whether the probability density
functions have the correct {\it width}. We test two photometric redshift
codes, BPZ and EAZY, on each of two data sets and find that BPZ is consistently
overconfident (the are too narrow) while EAZY produces approximately the
correct level of confidence. We show that this is because EAZY models the
uncertainty in its spectral energy distribution templates, and that post-hoc
smoothing of the BPZ provides a reasonable substitute for detailed
modeling of template uncertainties. Either remedy still leaves a small surplus
of galaxies with spectroscopic redshift very far from the peaks. Thus, better
modeling of low-probability tails will be needed for high-precision work such
as dark energy constraints with the Large Synoptic Survey Telescope and other
large surveys.Comment: accepted to MNRA
MC: Multi-wavelength and dynamical analysis of the merging galaxy cluster ZwCl 0008.8+5215: An older and less massive Bullet Cluster
We analyze a rich dataset including Subaru/SuprimeCam, HST/ACS and WFC3,
Keck/DEIMOS, Chandra/ACIS-I, and JVLA/C and D array for the merging galaxy
cluster ZwCl 0008.8+5215. With a joint Subaru/HST weak gravitational lensing
analysis, we identify two dominant subclusters and estimate the masses to be
M
and 1.2 M. We estimate the
projected separation between the two subclusters to be
924 kpc. We perform a clustering analysis on
confirmed cluster member galaxies and estimate the line of sight velocity
difference between the two subclusters to be 92164 km s. We
further motivate, discuss, and analyze the merger scenario through an analysis
of the 42 ks of Chandra/ACIS-I and JVLA/C and D polarization data. The X-ray
surface brightness profile reveals a remnant core reminiscent of the Bullet
Cluster. The X-ray luminosity in the 0.5-7.0 keV band is
1.70.110 erg s and the X-ray
temperature is 4.900.13 keV. The radio relics are polarized up to 40.
We implement a Monte Carlo dynamical analysis and estimate the merger velocity
at pericenter to be 1800 km s. ZwCl
0008.8+5215 is a low-mass version of the Bullet Cluster and therefore may prove
useful in testing alternative models of dark matter. We do not find significant
offsets between dark matter and galaxies, as the uncertainties are large with
the current lensing data. Furthermore, in the east, the BCG is offset from
other luminous cluster galaxies, which poses a puzzle for defining dark matter
-- galaxy offsets.Comment: 22 pages, 19 figures, accepted for publication in the Astrophysical
Journal on March 13, 201
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