arXiv subjects

Wentao Luo

Publications and source records attributed to Wentao Luo.

81 records · Page 5Linked to original sources

arXiv2017

Approaching the Cramer-Rao Bound in Weak Lensing with PDF Symmetrization

Weak lensing statistics is typically measured as weighted sum of shear estimators or their products (shear-shear correlation). The weighting schemes are designed in the hope of minimizing the statistical error without introducing systematic errors. It would be ideal to approach the Cramer-Rao bound (the lower bound of the statistical uncertainty) in shear statistics, though it is generally difficult to do so in practice. The reasons may include: difficulties in galaxy shape measurement, inaccurate knowledge of the probability-distribution-function (PDF) of the shear estimator, misidentification of point sources as galaxies, etc.. Using the shear estimators defined in Zhang et al. (2015), we show that one can overcome all these problems, and allow shear measurement accuracy to approach the Cramer-Rao bound. This can be achieved by symmetrizing the PDF of the shear estimator, or the joint PDF of shear estimator pairs (for shear-shear correlation), without any prior knowledge of the PDF. Using simulated galaxy images, we demonstrate that under general observing conditions, this idea works as expected: it minimizes the statistical uncertainty without introducing systematic error.

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arXiv2016

Galaxy-Galaxy Weak Lensing Measurements from SDSS: I. Image Processing and Lensing signals

As the first paper in a series on the study of the galaxy-galaxy lensing from Sloan Digital Sky Survey Data Release 7 (SDSS DR7), we present our image processing pipeline that corrects the systematics primarily introduced by the Point Spread Function (PSF). Using this pipeline, we processed SDSS DR7 imaging data in $r$ band and generated a background galaxy catalog containing the shape information of each galaxy. Based on our own shape measurements of the galaxy images from SDSS DR7, we extract the galaxy-galaxy (GG) lensing signals around foreground spectroscopic galaxies binned in different luminosity and stellar mass. The overall signals are in good agreement with those obtained by \citet{Mandelbaum2005, Mandelbaum2006} from the SDSS DR4. The results in this paper with higher signal to noise ratio is due to the larger survey area than SDSS DR4, confirm that more luminous/massive galaxies bear stronger GG lensing signal. We also divide the foreground galaxies into red/blue and star forming/quenched subsamples and measured their GG lensing signals, respectively. We find that, at a specific stellar mass/luminosity, the red/quenched galaxies have relatively stronger GG lensing signals than their counterparts especially at large radii. These GG lensing signals can be used to probe the galaxy-halo mass relations and their environmental dependences in the halo occupation or conditional luminosity function framework.

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arXiv2016

An empirical model to form and evolve galaxies in dark matter halos

Based on the star formation histories (SFH) of galaxies in halos of different masses, we develop an empirical model to grow galaxies in dark mattet halos. This model has very few ingredients, any of which can be associated to observational data and thus be efficiently assessed. By applying this model to a very high resolution cosmological $N$-body simulation, we predict a number of galaxy properties that are a very good match to relevant observational data. Namely, for both centrals and satellites, the galaxy stellar mass function (SMF) up to redshift $z\simeq4$ and the conditional stellar mass functions (CSMF) in the local universe are in good agreement with observations. In addition, the 2-point correlation is well predicted in the different stellar mass ranges explored by our model. Furthermore, after applying stellar population synthesis models to our stellar composition as a function of redshift, we find that the luminosity functions in $^{0.1}u$, $^{0.1}g$, $^{0.1}r$, $^{0.1}i$ and $^{0.1}z$ bands agree quite well with the SDSS observational results down to an absolute magnitude at about -17.0. The SDSS conditional luminosity functions (CLF) itself is predicted well. Finally, the cold gas is derived from the star formation rate (SFR) to predict the HI gas mass within each mock galaxy. We find a remarkably good match to observed HI-to-stellar mass ratios. These features ensure that such galaxy/gas catalogs can be used to generate reliable mock redshift surveys.

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arXiv2015

Accurate Shear Measurement with Faint Sources

For cosmic shear to become an accurate cosmological probe, systematic errors in the shear measurement method must be unambiguously identified and corrected for. Previous work of this series has demonstrated that cosmic shears can be measured accurately in Fourier space in the presence of background noise and finite pixel size, without assumptions on the morphologies of galaxy and PSF. The remaining major source of error is source Poisson noise, due to the finiteness of source photon number. This problem is particularly important for faint galaxies in space-based weak lensing measurements, and for ground-based images of short exposure times. In this work, we propose a simple and rigorous way of removing the shear bias from the source Poisson noise. Our noise treatment can be generalized for images made of multiple exposures through MultiDrizzle. This is demonstrated with the SDSS and COSMOS/ACS data. With a large ensemble of mock galaxy images of unrestricted morphologies, we show that our shear measurement method can achieve sub-percent level accuracy even for images of signal-to-noise ratio less than 5 in general, making it the most promising technique for cosmic shear measurement in the ongoing and upcoming large scale galaxy surveys.

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arXiv2014

Spin alignments of spiral galaxies within the large-scale structure from SDSS DR7

Using a sample of spiral galaxies selected from the Sloan Digital Sky Survey Data Release 7 (SDSS DR7) and Galaxy Zoo 2 (GZ2), we investigate the alignment of spin axes of spiral galaxies with their surrounding large scale structure, which is characterized by the large-scale tidal field reconstructed from the data using galaxy groups above a certain mass threshold. We find that the spin axes of only have weak tendency to be aligned with (or perpendicular to) the intermediate (or minor) axis of the local tidal tensor. The signal is the strongest in a \cluster environment where all the three eigenvalues of the local tidal tensor are positive. Compared to the alignments between halo spins and local tidal field obtained in N-body simulations, the above observational results are in best agreement with those for the spins of inner regions of halos, suggesting that the disk material traces the angular momentum of dark matter halos in the inner regions.

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arXiv2014

Connections between galaxy mergers and Starburst: evidence from local Universe

Major mergers and interactions between gas-rich galaxies with comparable masses are thought to be the main triggers of starburst. In this work, we study, for a large stellar mass range, the interaction rate of the starburst galaxies in the local universe. We focus independently on central and satellite star forming galaxies extracted from the Sloan Digital Sky Survey. Here the starburst galaxies are selected in the star formation rate (SFR) stellar mass plane with SFR five times larger than the median value found for "star forming" galaxies of the same stellar mass. Through visual inspection of their images together with close companions determined using spectroscopic redshifts, we find that ~50% of the "starburst" populations show evident merger features, i.e., tidal tails, bridges between galaxies, double cores and close companions. In contrast, in the control sample we selected from the normal star forming galaxies, only ~19% of galaxies are associated with evident mergers. The interaction rates may increase by ~5% for the starburst sample and 2% for the control sample if close companions determined using photometric redshifts are considered. The contrast of the merger rate between the two samples strengthens the hypothesis that mergers and interactions are indeed the main causes of starburst.

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arXiv2013

Measuring the X-ray luminosities of SDSS DR7 clusters from RASS

We use ROSAT All Sky Survey (RASS) broadband X-ray images and the optical clusters identified from SDSS DR7 to estimate the X-ray luminosities around $\sim 65,000$ candidate clusters with masses $\ga 10^{13}\msunh$ based on an Optical to X-ray (OTX) code we develop. We obtain a catalogue with X-ray luminosity for each cluster. This catalog contains 817 clusters (473 at redshift $z\le 0.12$) with $S/N> 3$ in X-ray detection. We find about $65\%$ of these X-ray clusters have their most massive member located near the X-ray flux peak; for the rest $35\%$, the most massive galaxy is separated from the X-ray peak, with the separation following a distribution expected from a NFW profile. We investigate a number of correlations between the optical and X-ray properties of these X-ray clusters, and find that: the cluster X-ray luminosity is correlated with the stellar mass (luminosity) of the clusters, as well as with the stellar mass (luminosity) of the central galaxy and the mass of the halo, but the scatter in these correlations is large. Comparing the properties of X-ray clusters of similar halo masses but having different X-ray luminosities, we find that massive halos with masses $\ga 10^{14}\msunh$ contain a larger fraction of red satellite galaxies when they are brighter in X-ray. ... A cluster catalog containing the optical properties of member galaxies and the X-ray luminosity is available at {\it http://gax.shao.ac.cn/data/Group.html}.

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arXiv2013

First Galaxy-Galaxy Lensing Measurement of Satellite Halo Mass in the CFHT Stripe-82 Survey

We select satellite galaxies from the galaxy group catalog constructed with the SDSS spectroscopic galaxies and measure the tangential shear around these galaxies with source catalog extracted from CFHT/MegaCam Stripe-82 Survey to constrain the mass of subhalos associated with them. The lensing signal is measured around satellites in groups with masses in the range [10^{13}, 5x10^{14}]h^{-1}M_{sun}, and is found to agree well with theoretical expectation. Fitting the data with a truncated NFW profile, we obtain an average subhalo mass of log M_{sub}= 11.68 \pm 0.67 for satellites whose projected distances to central galaxies are in the range [0.1, 0.3] h^{-1}Mpc, and log M_{sub}= 11.68 \pm 0.76 for satellites with projected halo-centric distance in [0.3, 0.5] h^{-1}Mpc. The best-fit subhalo masses are comparable to the truncated subhalo masses assigned to satellite galaxies using abundance matching and about 5 to 10 times higher than the average stellar mass of the lensing satellite galaxies.

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arXiv2012

Cross identification between X-ray and Optical Clusters of Galaxies in the SDSS DR7 Field

We use the ROSAT all sky survey X-ray cluster catalogs and the optical SDSS DR7 galaxy and group catalogs to cross-identify X-ray clusters with their optical counterparts, resulting in a sample of 201 X-ray clusters in the sky coverage of SDSS DR7. We investigate various correlations between the optical and X-ray properties of these X-ray clusters, and find that the following optical properties are correlated with the X-ray luminosity: the central galaxy luminosity, the central galaxy mass, the characteristic group luminosity ($\propto \Lx^{0.43}$), the group stellar mass ($\propto \Lx^{0.46}$), with typical 1-$σ$ scatter of $\sim 0.67$ in $\log \Lx$. Using the observed number distribution of X-ray clusters, we obtain an unbiased scaling relation between the X-ray luminosity, the central galaxy stellar mass and the characteristic satellite stellar mass as ${\log L_X} = -0.26 + 2.90 [\log (M_{\ast, c} + 0.26 M_{\rm sat}) -12.0]$ (and in terms of luminosities, as ${\log L_X} = -0.15 + 2.38 [\log (L_{c} + 0.72 L_{\rm sat}) -12.0]$). We find that the systematic difference between different halo mass estimations, e.g., using the ranking of characteristic group stellar mass or using the X-ray luminosity scaling relation can be used to constrain cosmology. Comparing the properties of groups of similar stellar mass (or optical luminosities) and redshift that are X-ray luminous or under-luminous, we find that X-ray luminous groups have more faint satellite galaxies and higher red fraction in their satellites. The cross-identified X-ray clusters together with their optical properties are provided in Appendix B.

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