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 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

The evolution of the mass-size relation to z=3.5 for UV-bright galaxies and sub-mm galaxies in the GOODS-NORTH field

We study the evolution of the size - stellar mass relation for a large spectroscopic sample of galaxies in the GOODs North field up to $z \sim 3.5$. The sizes of the galaxies are measured from $\textit{K}_{s}$-band images (corresponding to rest-frame optical/NIR) from the Subaru 8m telescope. We reproduce earlier results based on photometric redshifts that the sizes of galaxies at a given mass evolve with redshift. Specifically, we compare sizes of UV-bright galaxies at a range of redshifts: Lyman break galaxies (LBGs) selected through the U-drop technique ($z \sim 2.5-3.5$), BM/BX galaxies at $z \sim 1.5-2.5$, and GALEX LBGs at low redshift ($z \sim 0.6-1.5$). The median sizes of these UV-bright galaxies evolve as $(1+z)^{-1.11\pm0.13}$ between $z \sim 0.5-3.5$. The UV-bright galaxies are significantly larger than quiescent galaxies at the same mass and redshift by $0.45\pm0.09$ dex. We also verify the correlation between color and stellar mass density of galaxies to high redshifts. The sizes of sub-mm galaxies in the same field are measured and compared with BM/BX galaxies. We find that median half-light radii of SMGs is $2.90 \pm 0.45$ kpc and there is little difference in their size distribution to the UV-bright star forming galaxies.Comment: 12 pages, 10 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

Discovery of a large-scale galaxy filament near a candidate intergalactic X-ray absorption system

We present an analysis of the large-scale galaxy distribution around two possible warm-hot intergalactic medium (WHIM) absorption systems reported along the Markarian 421 sightline. Using the Sloan Digital Sky Survey, we find a prominent galaxy filament at the redshift of the z=0.027 X-ray absorption line system. The filament exhibits a width of 3.2 Mpc and length of at least 20 Mpc, comparable to the size of WHIM filaments seen in cosmological simulations. No individual galaxies fall within 350 projected kpc so it is unlikely that the absorption is associated with gas in a galaxy halo or outflow. Another, lower-significance X-ray absorption system was reported in the same Chandra spectrum at z=0.011, but the large-scale structure in its vicinity is far weaker and may be a spurious alignment. By searching for similar galaxy structures in 140 random smoothed SDSS fields, we estimate a ~5-10% probability of the z=0.027 absorber-filament alignment occurring by chance. If these two systems are indeed physically associated, this would represent the first known coincidence between large-scale galaxy structure and a blind X-ray WHIM detection.Comment: Accepted to ApJ Letters. 8 pages emulateapj, 4 figures, minor changes in response to referee repor