88 research outputs found
Detection of Ne VIII in the Low-Redshift Warm-Hot IGM
High resolution FUSE and STIS observations of the bright QSO HE 0226-4110
(zem = 0.495) reveal the presence of a multi-phase absorption line system at
zabs(O VI) = 0.20701 containing absorption from H I (Ly alpha to Ly theta), C
III, O III, O IV, O VI, N III, Ne VIII, Si III, S VI and possibly S V. Single
component fits to the Ne VIII and O VI absorption doublets yield logN(Ne VIII)
= 13.89+/-0.11 and logN(O VI) = 14.37+/-0.03. The Ne VIII and O VI doublets are
detected at 3.9 sigma and 16 sigma significance levels, respectively. This
represents the first detection of intergalactic Ne VIII, a diagnostic of gas
with temperature in the range from 5x10(5) to 1x10(6) K. The O VI and Ne VIII
are not likely created in a low density medium photoionized solely by the
extragalactic background at z = 0.2 since the required path length of ~11 Mpc
implies the Hubble flow absorption line broadening would be ~10 times greater
than the observed line widths. A collisional ionization origin is therefore
more likely. Assuming [Ne/H] and [O/H] = -0.5, the value N(Ne VIII)/N(O VI) =
0.33+/-0.10 is consistent with gas in collisional ionization equilibrium near
T=5.4x10(5) K with logN(H)= 19.9 and N(H)/N(H I) = 1.7x10(6). The observations
support the basic idea that a substantial fraction of the baryonic matter at
low redshift exists in hot very highly ionized gaseous structures.Comment: 32 pages text and 9 pages of figures. Accepted by the Astrophysical
Journa
A Survey of O VI, C III, and H I in Highly Ionized High-Velocity Clouds
(ABRIDGED) We present a Far-Ultraviolet Spectroscopic Explorer survey of
highly ionized high-velocity clouds (HVCs) in 66 extragalactic sight lines. We
find a total of 63 high-velocity O VI absorbers, 16 with 21 cm-emitting H I
counterparts and 47 ``highly ionized'' absorbers without 21 cm emission. 11 of
these high-velocity O VI absorbers are positive-velocity wings (broad O VI
features extending asymmetrically to velocities of up to 300 km/s). The highly
ionized HVC population is characterized by =38+/-10 km/s and <log
N_a(O VI)>=13.83+/-0.36. We find that 81% (30/37) of high-velocity O VI
absorbers have clear accompanying C III absorption, and 76% (29/38) have
accompanying H I absorption in the Lyman series. The lower average width of the
high-velocity H I absorbers implies the H I lines arise in a separate, lower
temperature phase than the O VI. We find that the shape of the wing profiles is
well reproduced by a radiatively cooling, vertical outflow. However, the
outflow has to be patchy and out of ionization equilibrium. An alternative
model, consistent with the observations, is one where the highly ionized HVCs
represent the low N(H I) tail of the HVC population, with the O VI formed at
the interfaces around the embedded H I cores. Though we cannot rule out a Local
Group explanation, we favor a Galactic origin. This is based on the recent
evidence that both H I HVCs and the million-degree gas detected in X-ray
absorption are Galactic phenomena. Since the highly ionized HVCs appear to
trace the interface between these two Galactic phases, it follows that highly
ionized HVCs are Galactic themselves. However, the non-detection of
high-velocity O VI in halo star spectra implies that any Galactic high-velocity
O VI exists at z-distances beyond a few kpc.Comment: 36 pages, 14 figures (3 in color), accepted to ApJS. Some figures
downgraded to limit file siz
Molecular Hydrogen in High-Velocity Clouds
We present Far Ultraviolet Spectroscopic Explorer (FUSE) observations of
interstellar molecular hydrogen (H_2) in two Galactic high-velocity clouds
(HVCs). Molecular hydrogen absorption is detected in the Magellanic Stream
(abundance ~0.3 solar) toward the Seyfert galaxy Fairall 9 in the lowest three
rotational states (J=0-2) at v(LSR)=+190 km/s, yielding a total H_2 column
density of log N(H_2)=16.40(+0.26)(-0.53). In contrast, no H_2 absorption is
seen in the high-velocity cloud Complex C (abundance ~0.1 solar) toward the
quasar PG 1259+593 (log N(H_2)<13.96 at v(LSR)=-130 km/s) although both HVCs
have similar HI column densities on the order of log N(HI)~20. Weak H_2
absorption is detected in the Intermediate-Velocity Arch (IV Arch; abundance
\~1.0 solar) toward PG 1259+593 (log N(H_2)=14.10(+0.21)(-0.44) at v(LSR)=-55
km/s and log N(HI)=19.5). It thus appears that metal- and dust-poor halo clouds
like Complex C are not able to form and maintain widely distributed H_2,
whereas metal and dust-rich halo clouds like the IV Arch can maintain H_2 even
at low HI column densities.Comment: 12 pages, 3 figures; Accepted for publication in ApJ Letter
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