Talks & Events
Ph.D. Thesis Defenses: 2010
Luminous halos surrounding edge-on disk galaxies in the Sloan Digital Sky Survey
We present the first measurement of both major-axis and minor-axis average surface brightness profiles of low-redshift (0.009 ≤ z ≤ 0.1), edge-on Sc-Sd 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 minor-axis 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+power-law with slope 2.3 1 0.3 in agreement with simulations and star counts around individual galaxies. We attribute the power-law component to a diffuse stellar halo and present the first measurement of such around isolated, late-type galaxies. We also divide our sample according to physical size and show that the minor-axis power-law slopes of the two size categories do not differ by more than 2sigma, also in agreement with simulations. We find that the average major-axis scale length is reap = 4.37 10.012 kpc, somewhat larger than the typical value for face-on 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, non-perturbative, and includes all possible non-linear effects. The inhomogeneous universes are represented by Sweese-cheese models containing random and simple cubic lattices of mass-compensated 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 large-scale three-point correlation function of SDSS luminous red galaxies
We present new measurements of the redshift-space three-point 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 h-1 Mpc, from the mildly non-linear to quasi-linear regimes. Comparing the LRG correlations to the dark matter two- and three-point correlation functions, obtained from N-body 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 non-linear 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 best-fit 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.
Characterizing galaxy clusters with gravitational potential
Global self-similar protostellar disk/wind models
The magnetocentrifugal disk wind mechanism is the leading candidate for producing the large-scale, bipolar jets commonly seen in protostellar systems. I present a detailed formulation of a global, radially self-similar model for a nonideal disk that launches a magnetocentrifugal wind. This formulation generalizes the conductivity tensor formalism previously used in radially localized disk models. The model involves matching a solution of the equations of non-ideal MHD describing matter in the disk to a solution of the equations of ideal MHD describing a "cold" wind. The disk solution must pass smoothly through the sonic point, the wind solution must pass smoothly through the Alfv'en point, and the two solutions must match at the disk/wind interface. This model includes for the first time a self-consistent treatment of the evolution of magnetic flux threading the disk, which can change on the disk accretion timescale. The formulation presented here also allows a realistic conductivity profile for the disk to be used in a global disk/wind model for the first time. The physical constraints on the model solutions fix the distribution of the magnetic field threading the disk, the midplane accretion speed, and the midplane migration speed of flux surfaces. I present a representative solution that corresponds to a disk in the ambipolar conductivity regime with a nominal neutral-matter-magnetic-field coupling parameter that is constant along field lines. I conclude with a brief discussion of the importance of self-similar disk/wind models in studying global processes such as dust evolution in protostellar systems.