Updated: 6 March 2011

UC, Santa Cruz Department of Astronomy and Astrophysics

Master Project 2004−2005

Characterizing the Low-redshift Intergalactic Medium towards PKS1302−102

Advisor: J. Xavier Prochaska
UCO/Lick Observatory; UCSC

Characterizing the Low-redshift Intergalactic Medium towards PKS1302−102

Cooksey, Prochaska, Chen, Mulchaey, and Weiner 2007 (PDF; 2008, Astrophysical Journal, 676, 262)


We present a detailed analysis of the intergalactic metal-line absorption systems in the archival HST/STIS and FUSE ultraviolet spectra of the low-redshift quasar PKS1302−102 (zQSO = 0.2784). We supplement the archive data with CLOUDY ionization models and a survey of galaxies in the quasar field. There are 14 strong Lyα absorbers with column densities log NH I > 14. Of these, six are associated with at least C III λ977 absorption (log N(C++) > 13); this implies a redshift density dNC III/dz =38+14-10 (68% confidence limits) for the five detections with rest equivalent width Wr > 50 mÅ. Two systems show O VI λλ1031,1037 absorption in addition to C III (log N(O+5) > 14). One is a partial Lyman limit system (log NH I = 17) with associated C III, O VI, and Si III λ1206 absorption. There are two tentative O VI systems that do not have C III detected. For one O VI doublet with both lines detected at 3σ with Wr > 50 mÅ, dNO VI/dz = 7+9-4. We also search for O VI doublets without Lyα absorption but identify none. From CLOUDY modeling, these metal-line systems have metallicities spanning the range -3 ≤ [M/H] ≤-0.5. The two O VI systems with associated C III absorption cannot be single-phase, collisionally-ionized media based on the relative abundances of the metals and kinematic arguments. From the galaxy survey, we discover that the absorption systems are in a diverse set of galactic environments. Each metal-line system has at least one galaxy within 500 km s-1 and 600 h75-1 kpc with L > 0.1L.

Searching for O VI Absorption Systems toward PKS1302−102

Poster (PDF) presented at the 205th Meeting of the American Astronomical Society, San Diego, CA, 2005.


The baryonic content of the Universe is well known from Big Bang nucleosynthesis models, deuterium abundances, and the high-redshift Lyman-α forest. However, surveys of the nearby Universe reveal a dearth of baryons in stars, galaxies, and clusters. Recent cosmological simulations have placed the most likely reservoir of baryons at low redshift—upward of 40%—in moderately overdense (δ ~ 10−30), collisionally ionized gas, called the warm-hot intergalactic medium. With temperatures in the range 105−7 K, the WHIM proves tricky to observe. The most reasonable tracer is the O VI doublet λλ1031.93, 1037.62 Å, which dominates at ~3x105 K.

Presented here are preliminary results from analysis of far-ultraviolet FUSE and HST/STIS spectra of the quasar PKS1301−102 (zem = 0.286). Column densities and Doppler parameters are measured for metal-line systems, and the redshift density of O VI absorbers is determined. In addition, the FUV spectra are supported by a galaxy survey of the field surrounding PKS1302−102, made at Las Campanas Observatory. These observations will be used to determine whether the O VI absorbers are correlated with galaxies. This work was funded by FUSE grant NAG5-12496.