Far‐Ultraviolet Spectroscopy of the Intergalactic and Interstellar Absorption toward 3C 273

Abstract

We present Far Ultraviolet Spectroscopic Explorer observations of the molecular, neutral atomic, weakly ionized, and highly ionized components of the interstellar and intergalactic material toward the quasar 3C 273. We identify Lyβ absorption in 8 of the known intergalactic Lyα absorbers along the sight line with rest-frame equivalent widths W r (Lyα) 50 mÅ. Refined estimates of the H I column densities and Doppler parameters (b) of the clouds are presented. We find a range of b ≈ 16 − 46 km s −1 . We detect multiple H I lines (Lyβ -Lyθ) in the 1590 km s −1 Virgo absorber and estimate log N (H I) = 15.85± 0.10 0.08 , ten times more H I than all of the other absorbers along the sight line combined. The Doppler width of this absorber, b ≈ 16 km s −1 , implies T 15, 000 K. We detect O VI absorption at 1015 km s −1 at the 2 − 3σ level that may be associated with hot, X-ray emitting gas in the Virgo Cluster. We detect weak C III and O VI absorption in the IGM at z = 0.12007; this absorber is predominantly ionized and has N(H + )/N(H I) ≥ 4000 Z −1 , where Z is the metallicity.Strong Galactic interstellar O VI is present between −100 and +100 km s −1 , with an additional high-velocity wing containing about 13% of the total O VI between ∼ +100 and ∼ +240 km s −1 . The Galactic O VI, N V, and C IV lines have similar shapes, with roughly constant ratios across the -100 to +100 km s −1 velocity range. The high velocity O VI wing is not detected in other species. Much of the interstellar high ion absorption probably occurs within a highly fragmented medium within the Loop IV remnant or in the outer cavity walls of the remnant. Multiple hot gas production mechanisms are required. The broad O VI absorption wing likely traces the expulsion of hot gas out of the Galactic disk into the halo. A flux limit of 5.4×10 −16 erg cm −2 s −1 on the amount of diffuse O VI emission present ∼ 3.5 ′ off the 3C 273 sight line combined with the observed -2 -O VI column density toward 3C 273, log N(O VI) = 14.73± 0.04, implies n e 0.02 cm −3 and P/k 11, 500 cm −3 K for an assumed temperature of 3 × 10 5 K. The elemental abundances in the neutral and weakly-ionized interstellar clouds are similar to those found for other halo clouds. The warm neutral and warm ionized clouds along the sight line have similar dust-phase abundances, implying that the properties of the dust grains in the two types of clouds are similar. Interstellar H 2 absorption is present at positive velocities at a level of log N (H 2 ) ∼ 15.71, but is very weak at the velocities of the main column density concentration along the sight line observed in H I 21 cm emission.

The Local Lyα Forest. III. Relationship between Lyα Absorbers and Galaxies, Voids, and Superclusters

“…Absorbers in that redshift range are located in the southern portion of the Virgo Cluster region. Updated column densities for the two strongest Virgo cluster absorbers along the 3C 273 sightline have been reported by Sembach et al (2001) on the basis of FUSE spectra.…”

Hot gas in the Local Group and low-redshift intergalactic medium

“…Many occur at the redshifts of groups or clusters of galaxies along the same sight lines. Several sight lines have been examined in detail for O vi absorbers: H 1821+643 (Tripp, Savage, & Jenkins 2000;Oegerle et al 2000), PG 0953+415 , 3C 273 (Sembach et al 2001), PKS 2155-304 (Shull, Tumlinson, & Giroux 2003, PG 1259+593 ), PG 1116+215 , and PG 1211+143 ). An important piece of information that is lacking for many of the well-observed O vi absorbers is the amount of C iv associated with the O vi.…”

First Results from the FUSE Mission

“…The HVC has a metallicity of 0.2-0.4 solar, similar to that of the Small Magellanic Cloud (Lu et al 1998;Sembach et al 2001). The relative abundance pattern for the elements studied by FUSE (Si, P, Fe) also resembles that of warm gas in the SMC.…”

“…Only an unbiased sample will reveal whether the SMC actually has a higher H2 to extinction ratio than the LMC. In addition to the Magellanic Clouds, there have been detections of H2 in high-velocity clouds, such as the Magellanic Stream (Sembach et al 2001).…”

“…While valuable for assessing whether the clouds contain large quantities of molecules, these searches are relatively insensitive to diffuse gas with Af(H 2 ) < 10 18 c m -2 . Recent FUSE observations of the high velocity cloud HVC 287.5+22.5+240 demonstrate that it is possible for molecular hydrogen to exist in HVCs (Sembach et al 2001). The amount of molecular hydrogen is small compared to atomic hydrogen (AT(H 2 ) ~6 x 10 16 cm" 2 , / H 2 = 2JV(H 2 )/[2iV(H2)+Ar(Hi)] ~ 1.6 x 10" 3 ), but it is easily detected in more than 30 lines arising in rotational levels J = 0 to J = 3.…”