Talks & Events

Ph.D. Thesis Defenses: 2010
Luminous halos surrounding edgeon disk galaxies in the Sloan Digital Sky Survey We present the first measurement of both majoraxis and minoraxis average surface brightness profiles of lowredshift (0.009 ≤ z ≤ 0.1), edgeon ScSd galaxies. The 906 objects in our sample have 90% Petrosian radii between 5 kpc and 15 kpc and were selected from the Fourth Data Release of the Sloan Digital Sky Survey. The minoraxis profile extends to 20 kpc above the disk and a limiting surface brightness of 30 mag/arcsec 2. We demonstrate that this surface brightness profile is best fit by an exponential+powerlaw with slope 2.3 1 0.3 in agreement with simulations and star counts around individual galaxies. We attribute the powerlaw component to a diffuse stellar halo and present the first measurement of such around isolated, latetype galaxies. We also divide our sample according to physical size and show that the minoraxis powerlaw slopes of the two size categories do not differ by more than 2sigma, also in agreement with simulations. We find that the average majoraxis scale length is reap = 4.37 10.012 kpc, somewhat larger than the typical value for faceon galaxies (see, for example, Kent (1985)). We also present the first measurement of the ratio of the disk luminosity to that of the halo, and provide a context for this ratio. Average luminosity distance in inhomogeneous universes Using numerical ray tracing, the paper studies how the average distance modulus in an inhomogeneous universe differs from its homogeneous counterpart. The averaging is over all directions from a fixed observer not over all possible observers (cosmic), thus it is more directly applicable to our observations. Unlike previous studies, the averaging is exact, nonperturbative, and includes all possible nonlinear effects. The inhomogeneous universes are represented by Sweesecheese models containing random and simple cubic lattices of masscompensated voids. The Earth observer is in the homogeneous cheese which has an Einstein  de Sitter metric. For the first time, the averaging is widened to include the supernovas inside the voids by assuming the probability for supernova emission from any comoving volume is proportional to the rest mass in it. Despite the well known argument for photon flux conservation, the average distance modulus correction at low redshifts is not zero due to the peculiar velocities. A formula for the maximum possible average correction as a function of redshift is derived and shown to be in excellent agreement with the numerical results. The actual average correction calculated in random and simple cubic void lattices is severely damped below the predicted maximal average. That is traced to cancelations between the corrections coming from the fronts and backs of different voids at the same redshift from the observer. The calculated correction at low redshifts allows one to readily predict the redshift at which the averaged fluctuation in the Hubble diagram is below a required precision and suggests a method to extract the background Hubble constant from low redshift data without the need to correct for peculiar velocities. The largescale threepoint correlation function of SDSS luminous red galaxies We present new measurements of the redshiftspace threepoint correlation function (3PCF) of Luminous Red Galaxies (LRGs) from the Sloan Digital Sky Survey (SDSS). Using the largest dataset to date, the Data Release 7 (DR7) LRGs, and an improved binning scheme compared to previous measurements, we measure the LRG 3PCF on large scales up to ~ 90 h1 Mpc, from the mildly nonlinear to quasilinear regimes. Comparing the LRG correlations to the dark matter two and threepoint correlation functions, obtained from Nbody simulations we infer linear and nonlinear bias parameters. As expected, LRGs are highly biased tracers of large scale structure, with a linear bias b1 ~ 2; the LRGs also have a large positive nonlinear bias parameter, in agreement with predictions of galaxy population models. The use of the 3PCF to estimate biasing helps to also make estimates of the cosmological parameter sigma8, as well as to infer bestfit parameters of the Halo Occupation Distribution parameters for LRGs. We also use a large suite of public mock catalogs to characterize the error covariance matrix for the 3PCF and compare the variance among simulation results with jackknife error estimates. 