Galaxy Infall Kinematics as a Test of Modified Gravity
Zu, Y., Weinberg, D. H., Jennings, E., Li, B., & Wyman, M. 2013, arXiv:1310.6768
Infrared modifications of General Relativity (GR) can be revealed
by comparing the mass of galaxy clusters estimated from weak
lensing to that from infall kinematics. We measure the 2D
galaxy velocity distribution in the cluster infall region by
applying the galaxy infall kinematics (GIK) model developed by Zu and Weinberg
(2013) to two suites of f(R) and Galileon modified gravity simulations.
Despite having distinct screening mechanisms, the f(R) and Galileon
clusters exhibit very similar deviations in their GIK profiles from GR
(Figure on the left). In combination with the stacked weak lensing
measurements, this will provide powerful diagnostics of modified gravity
theories and the origin of cosmic acceleration.
[on astro-ph][on ADS]
 Redshift-Space Cluster-Galaxy Cross-Correlation: I. Modeling Galaxy
Infall onto Millennium Simulation Clusters and SDSS Groups
Zu, Y., & Weinberg D. H., 2013, MNRAS, 431, 3319
We develop and test a method to recover galaxy infall kinematics (GIK)
from measurements of the redshift-space cluster-galaxy cross-correlation
function xi_cg, by calibrating an analytic model of the galaxy kinematic
profiles comprised of a virialized component and an infall component.
We show that convolving the real-space cross-correlation function with
this velocity distribution accurately predicts the redshift-space xi_cg,
and that measurements of xi_cg can be inverted to provide
diagnostics of cluster mass profiles.
As a proof of concept we measure xi_cg for rich galaxy groups in the
Sloan Digital Sky Survey and recover GIK profiles for groups in bins of
central galaxy stellar mass. The figure on the left compares the best-fit
(dashed contours) to the measurement of xi_cg (color contours) for one of the bins.
 Cosmological Constraints from the Large Scale Weak Lensing of SDSS
Zu, Y., Weinberg D. H., Rozo, Eduardo, Sheldon, E. S., Tinker, J. L., Becker, M. R. arXiv:1207.3794
We derive constraints on the matter density Omega_m and the amplitude of
matter clustering sigma_8 from measurements of large scale weak lensing
by clusters in the SDSS MaxBCG catalog. The weak lensing signal is
proportional to the product of Omega_m and the cluster-mass correlation
function xi_cm, breaking the degeneracies between cosmological and
We find sigma_8(Omega_m/0.325)^0.501=0.828 +/- 0.049 (beige contours)
and the constraint is consistent with and orthogonal to the one inferred
from WMAP CMB data (red contours), reflecting agreement with the structure
growth predicted by GR for an LCDM cosmological model. A joint constraint
assuming LCDM yields Omega_m=0.298 +/- 0.020 and sigma_8=0.831 +/- 0.020
 Intergalactic Dust Extinction in Hydrodynamic Cosmological
Zu, Y., Weinberg, D. H., Davé, R., Fardal, M., Katz, N., Kereš, D., Oppenheimer, B. D. 2011, MNRAS, 412, 1059
We study how the intergalactic dust should be distributed based on the
observation from Menard et al (2010) and cosmological hydrodynamic
simulations. We found that it is very hard to explain the observed
intergalactic dust distribution and abundance using the simulations
without winds, and it requires a dust-to-metal ratio of 0.24 with the
wind simulations to match to the data.
Figure on the left shows the cosmic metalicity distribution in our
wind(left) and no-wind(right) simulations. The bottom panels are the zoom-in
versions of the clusters on the top. Circles indicate the positions of the
galaxies, with the radii proportional to the stellar masses.
 Reverberation Mapping With Photometry
Zu, Y., Kochanek, C. S., Kozlowski, Szymon & Peterson, B. M.
Using both simulated and real quasar light curves we explore the
feasibility of one and two--band photometric reverberation mapping
(RM) and compare to the results for spectroscopic RM. We find that
two-band photometric RM can be competitive with spectroscopic RM for
strong lines like H\alpha and H\beta, and that the one-band method is
feasible, but requires very small photometric uncertainties.
Our approach is directly applicable to the time-domain programs within
ongoing and future wide-field imaging surveys. Figure on the left shows
the lag detection significance (three colors representing 1, 2, and 3 sigma
leves) as a function of line strength for simulated LSST quasar light curves
with different variability parameters (marked on top right of each panel).
 Is Quasar Optical Variability a Damped Random Walk?
Zu, Y., Kochanek, C. S., Kozlowski, S., Udalski A. 2013, ApJ, 765, 106
Using a sample of OGLE quasar light
curves, we consider four modifications to the DRW model by introducing
additional parameters into the covariance function to search for deviations
from the DRW model on both short and long timescales. We find good agreement
with the DRW model on timescales that are well sampled by the data (from a
month to a few years), possibly with some intrinsic scatter in the
additional parameters, but this conclusion depends on the statistical test
employed and is sensitive to whether the estimates of the photometric errors
are correct to within ~10%.
Figure on the left illustrates the four different covariance functions we
employed in the paper.
 An Alternative Approach to Measuring Reverberation Lags in Active Galactic Nuclei
Zu, Y., Kochanek, C. S., & Peterson, B. M. 2010, ApJ, 735, 80
We developed a new reverberation mapping technique to infer lags in
the AGN emission lines. Assuming the emission light curves are lagged,
smoothed, and scaled versions of the continuum light curve, and the
quasar variability can be well described by a damped random walk, we can
statistical interpolate the observed light curves and align each other
to find the best-fit parameters for the transfer functions of emissoin
Figure on the left compares the new lag estimates from our
method JAVELIN (formerly known
as SPEAR) and from correlation
based methods. We recovered lags from all the measured light
curves and removed some old outliers from the lag-luminosity
is an open source software hosted on bitbucket. Please feel free
or simply check out the documents.
 Environmental Effects on Real-Space and Redshift-Space Galaxy
Zu, Y., Zheng, Z., Zhu, G. T., & Jing, Y. P. 2008, ApJ, 686, 41
We study the environmental effect on the clustering of galaxies in both
real and redshift spaces using semi-analytical galaxy formation models,
by shuffling the galaxy content between halos of similar masses, both
with and without retaining the 1-halo term. We found around 5 percent
impact from the environment on the two point correlations of galaxies
through this shuffling technique.
Figure on the left compares the two point correlation functions of the
original and shuffled galaxy samples, defined by different redshifts and
magnitude limits. Each correlation is decomposed into five components,
including 1-halo central-satellite, 1-halo satellite-satellite, 2-halo
central-central, 2-halo central-satellite, and 2-halo satellite-satellite