On The Robustness of z=0-1 Galaxy Size Measurements Through Model and Non-Parametric Fits

We present the size-stellar mass relations of nearby (z=0.01-0.02) Sloan Digital Sky Survey galaxies, for samples selected by color, morphology, Sersic index n, and specific star formation rate. Several commonly employed size measurement techniques are used, including single Sersic fits, two-component Sersic models, and a non-parametric method. Through simple simulations, we show that the non-parametric and two-component Sersic methods provide the most robust effective radius measurements, while those based on single Sersic profiles are often overestimates, especially for massive red/early-type galaxies. Using our robust sizes, we show for all sub-samples that the mass-size relations are shallow at low stellar masses and steepen above ~ 3-4 x 10^{10}\msun. The mass-size relations for galaxies classified as late-type, low-n, and star-forming are consistent with each other, while blue galaxies follow a somewhat steeper relation. The mass-size relations of early-type, high-n, red, and quiescent galaxies all agree with each other but are somewhat steeper at the high-mass end than previous results. To test potential systematics at high redshift, we artificially redshifted our sample (including surface brightness dimming and degraded resolution) to z=1 and re-fit the galaxies using single Serisc profiles. The sizes of these galaxies before and after redshifting are consistent and we conclude that systematic effects in sizes and the size-mass relation at z ~ 1 are negligible. Interestingly, since the poorer physical resolution at high redshift washes out bright galaxy substructures, single-Sersic fitting appears to provide more reliable and unbiased effective radius measurements at high z than for nearby, well-resolved galaxies.Comment: 18 pages, 15 figures, Accepted for publication in Ap

Chandra discovery of the intracluster medium around UM425 at redshift 1.47

We report on a discovery of a candidate cluster of galaxies at redshift z=1.47 based on Chandra observations in the field of quasars UM425 A & B. We detect with high significance diffuse emission due the intracluster hot gas around the quasar pair. This is the second highest redshift cluster candidate after 3C294 at z=1.786. The diffuse emission is elliptical in shape with about 17" extent. If indeed at z=1.47, this corresponds to a physical size of 140 h_{70}^{-1} Kpc and 2--10 keV luminosity of about 3 times 10^{43} erg/s. The cluster is unlikely to be the long sought gravitational lens invoked to explain unusual brightness of UM425 A and the close quasar pair. Coexistence of the quasars with the cluster suggests a link of activity to cluster environment. The unusual brightness of UM425 A may then be due to a higher accretion rate. We also comment briefly on the X-ray spectra of UM 425 A & B which also happen to be broad absorption line quasars. We argue that present evidence suggests that the quasars are just a pair and not lensed images of the same quasar.Comment: Submitted to ApJ Letter

Chandra and Far Ultraviolet Spectroscopic Explorer Observations of z~0 Warm-Hot Gas Toward PKS 2155-304

The X-ray bright z=0.116 quasar PKS 2155-304 is frequently observed as a Chandra calibration source, with a total of 483 ksec of Low Energy Transmission Grating (LETG) exposure time accumulated through May 2006. Highly-ionized metal absorption lines, including numerous lines at z=0 and a putative OVIII K-alpha line at z=0.055, have been reported in past Chandra studies of this source. Using all available Chandra LETG spectra and analysis techniques developed for such z=0 X-ray absorption along other sightlines, we revisit these previous detections. We detect 4 absorption lines at >3\sigma significance (OVII K-alpha/beta, OVIII K-alpha, and NeIX K-alpha), with OVII K-alpha being a 7.3\sigma detection. The 1\sigma ranges of z=0 OVII column density and Doppler parameter are consistent with those derived for Mrk 421 and within 2\sigma of the Mrk 279 absorption. Temperatures and densities inferred from the relative OVII and other ionic column densities are found to be consistent with either the local warm-hot intergalactic medium or a Galactic corona. Unlike the local X-ray absorbers seen in other sightlines, a link with the low- or high-velocity far-ultraviolet OVI absorption lines cannot be ruled out. The z=0.055 OVIII absorption reported by Fang et al. is seen with 3.5\sigma confidence in the ACIS/LETG spectrum, but no other absorption lines are found at the same redshift.Comment: 11 pages, 9 figures; minor changes, accepted to Ap

Chandra Detection of Local Warm-Hot Gas Toward Markarian 279

We report the Chandra detection of OVII K\alpha absorption at z=0 in the direction of the z=0.03 Seyfert 1 galaxy Mkn 279. The high velocity cloud Complex C lies along this line of sight, with HI 21-cm emission and OVI 1032\AA absorption both observed at velocities of ~ -150 km/s relative to the local standard of rest. We present an improved method for placing limits on the Doppler parameter and column density of a medium when only one unresolved line can be measured; this method is applied to the OVII absorption seen here, indicating that the OVII Doppler parameter is inconsistent with that of any low-velocity (Galactic thick disk) or high-velocity OVI (OVI_HV) component. Direct association of the OVII with the OVI_HV is further ruled out by the high temperatures required to produce the observed OVI_HV/OVII ratio and the significant velocity difference between the OVII and OVI_HV lines. If the OVII absorption is associated with a very broad, undetected OVI component, then the absorption must be broadened by primarily nonthermal processes. The large velocity dispersion and possible slight redshift of the OVII absorption (as well as limits on the absorber's temperature and density) may be indicative of a local intergalactic medium origin, though absorption from a hot, low-density Galactic corona cannot be ruled out.Comment: 20 pages including 5 figures, 1 table; minor changes to text/figures, accepted to Ap