3.1. Past and present UV instruments
The first and second generations of space based UV spectrographs such
as Copernicus and the International Ultraviolet Explorer
(IUE) did not have sufficient sensitivity
to systematically study intervening absorption in the intergalactic medium
along a large number of sightlines.
The early low- and intermediate resolution spectrographs installed
on the Hubble Space Telescope (HST), namely the Faint Object
Spectrograph (FOS) and the Goddard High Resolution Spectrograph
(GHRS), were used to study the properties of the local
Ly forest
and intervening metal-line systems (e.g.,
Stocke et
al. 1995;
Shull et
al. 1998).
While intervening O VI absorption has been detected with
these instruments (e.g.,
Tripp et
al. 1998),
the concept of a warm-hot intergalactic gas phase was not really
established at that time. With the implementation of the high-resolution
capabilities of the Space Telescope
Imaging Spectrograph (STIS) installed on HST
the first systematic analyses of WHIM O VI absorbers as
significant low-redshift baryon reservoirs came out in 2000 (see
Tripp et
al. 2000),
thus relatively soon after the importance
of a shock-heated intergalactic gas phase was realised in cosmological
simulations for the first time (e.g.,
Cen & Ostriker
1999;
Davé et
al. 2001).
The STIS echelle spectrograph together with the E140M
grating provides a high spectral-resolution of R
45000,
corresponding to a velocity resolution of ~ 7 km s-1
in the STIS E140M wavelength band between 1150 and 1730 Å (e.g.,
Kimble et
al. 1998;
Woodgate 1998).
An example for a STIS quasar spectrum with intervening hydrogen and
metal-line absorption is shown in Fig. 4.
Note that at the spectral resolution of the STIS E140M grating
all intergalactic absorption lines (i.e., hydrogen and metal lines) are
fully resolved. In 1999, the Far Ultraviolet Spectroscopic
Explorer (FUSE) became available,
covering the wavelength range between 912 and 1187 Å.
Equipped with a Rowland-type spectrograph providing a medium spectral
resolution of R
20000 (FWHM ~ 20 km s-1) FUSE is able to observe
extragalactic UV background sources brighter than V = 16.5 mag
with acceptable integration time and signal-to-noise (S/N) ratios
(for a description of FUSE see
Moos et al. 2000;
Sahnow et
al. 2000).
With this resolution, FUSE is able to resolve the broader intergalactic
absorption from the H I Lyman series, while most of the narrow metal-line
absorbers remain just unresolved. This is not a problem for O VI WHIM
studies with FUSE, however, since the spectral resolution is very close
to the actual line widths and the O VI absorption usually is not
saturated. FUSE complements the STIS instruments at lower wavelengths
down to the Lyman limit and consequently combined FUSE and STIS spectra
of ~ 15 low redshift QSOs and AGN have been used to study the
low-redshift WHIM via intervening O VI and BLA absorption (see
Tripp et al. 2007
and references therein).
Unfortunately, since 2006/2007 both STIS
and FUSE are out of commission due to technical problems.
![]() |
Figure 4. STIS spectrum of the quasar
PG 1259+593 in the wavelength range between 1300
and 1400 Å. Next to absorption from the local
Ly |
Fresh spectroscopic UV data from
WHIM absorption line studies will become available once the
Cosmic Origins Spectrograph (COS) will be installed on HST during
the next HST service mission (SM-4), which currently is scheduled for late
2008. COS will observe in the UV wavelength band between 1150 and
3000 Å at medium resolution (R
20000). COS has been
designed with maximum effective area as the primary constraint: it
provides more than an order of magnitude gain in sensitivity over previous
HST instruments. Due to its very high sensitivity, COS thus will be able
to observe hundreds of low- and intermediate redshift QSOs and
AGN and thus will deliver an enormous data archive to study the
properties of WHIM UV absorption lines systems in great detail (see also
Paerels et
al. 2008
- Chapter 19, this volume).
3.2. Intervening WHIM absorbers at low redshift
We start with the O VI absorbers which are believed
to trace the low-temperature tail of the
WHIM at T < 5 × 105 K.
Up to now, more than 50 detections
of intervening O VI absorbers at z < 0.5
have been reported in the literature (e.g.,
Tripp et al. 2000;
Oegerle et
al. 2000;
Chen &
Prochaska 2000;
Savage et
al. 2002;
Richter et
al. 2004;
Sembach et
al. 2004;
Savage et
al. 2005;
Danforth &
Shull 2005;
Tripp et
al. 2007).
All of these detections are based on FUSE and STIS data.
Fig. 5 shows two examples for intervening
O VI absorption at z = 0.23351 and z = 0.26656 in the
direction of PG 0953+415 and H 1821+643, as observed with STIS.
The most recent compilation of low-redshift intervening O VI absorbers
is that of
Tripp et
al. (2007),
who have analysed 16 sightlines toward low-redshift QSOs
observed with STIS and FUSE along a total redshift path
of z
3.
They find a total of 53 intervening O VI absorbers
(i.e., they are not within 5000 km s-1 of zQSO)
comprised of 78 individual absorption components
1.
The measurements imply a number density of O VI
absorbing systems per unit redshift of dNOVI /
dz
18 ± 3 for equivalent widths
W
30 mÅ. The
corresponding number density of O VI absorption components is
dNO VI / dz
25 ± 3. These
values are slightly higher than what is found by
earlier analyses of smaller O VI samples
(Danforth &
Shull 2005),
but lie within the cited
2
error ranges.
The discrepancy between the measured O VI number densities
probably is due to the different approaches of estimating the redshift
path
z along
which the O VI absorption takes place.
If one assumes that the gas is in a collisional ionisation equilibrium,
i.e., that ~ 20 percent of the oxygen is present
in the form of O VI (fO VI
0.2),
and further assumes that the mean oxygen abundance is 0.1 Solar,
the measured number density of O VI absorbers
corresponds to a cosmological mass density
of
b(O VI)
0.0020-0.0030
h70-1.
These values imply that intervening O VI absorbers trace
~ 5-7 percent of the total baryon mass in the local Universe.
For the interpretation of
b(O VI)
it has to be noted that O VI absorption traces
collisionally ionised gas at
temperatures around 3 × 105 K (and also
low-density, photoionised gas at lower temperatures), but not the
million-degree gas phase which probably contains
the majority of the baryons in the WHIM.
![]() |
Figure 5. Examples for H I and O VI absorption in two absorption systems at z = 0.23351 and z = 0.6656 towards PG 0953+415 and H 1821+643, respectively, plotted on a rest frame velocity scale (observed with STIS). Adapted from Tripp et al. (2007). |
The recent analysis of
Tripp et
al. (2007)
indicates, however,
that this rather simple conversion from measured O VI column densities to
b(O VI)
may not be justified in general, as the CIE assumption possibly breaks
down for a considerable fraction of the O VI systems.
From the measured line widths of the H I
Ly
absorption that is associated with the O VI Tripp et al. conclude
that ~ 40 percent of their O VI systems belong to
cooler, photoionised gas with T < 105 K, possibly
not at all
associated with shock-heated warm-hot gas. In addition, about half of the
intervening O VI absorbers arise in rather complex, multi-phase
systems that can accommodate hot gas at relatively low metallicity.
It thus appears that - without having additional information about the
physical conditions in each O VI absorber - the estimate of the
baryon budget in intervening O VI systems is afflicted
with rather large systematic uncertainties.
In high-column density O VI systems at redshifts z > 0.18,
such desired additional information may be
provided by the presence or absence of Ne VIII (see
Sect. 2.2.1),
which in CIE traces gas at T ~ 7 × 105 K.
Toward the quasar PG 1259+593
Richter et
al. (2004)
have reported a tentative detection of Ne VIII absorption
at ~ 2 significance in
an O VI absorber
at z
0.25. The
first secure detection of intervening Ne VIII absorption
(at ~ 4
significance)
was presented by
Savage et
al. (2005)
in a multi-phase O VI absorption system
at z
0.21 in
the direction of the quasar HE 0226-4110.
The latter authors show that in this particular absorber the high-ion ratio
Ne VIII / O VI = 0.33 is in agreement with gas in CIE
at temperature of T ~ 5 × 105 K.
With future high S/N absorption line data of low-redshift QSOs
(as will be provided by COS) it is expected
that the number of detections of WHIM Ne VIII absorbers
will increase substantially, so that an important new diagnostic
will become available for the analysis of high-ion absorbers.
One other key aspect in understanding the distribution and nature
of intervening O VI systems concerns their relation to the
large-scale distribution of galaxies.
Combining FUSE data of 37 O VI absorbers with a database
of more than a million galaxy positions and redshifts,
Stocke et
al. (2006)
find that all of these O VI systems
lie within 800 h70-1 kpc of the nearest galaxy.
These results suggest that O VI systems preferentially
arise in the immediate circumgalactic environment and extended halos
of galaxies, where the metallicity of the gas is expected to be
relatively high compared to regions far away from galactic structures.
Some very local analogs of intervening O VI systems thus may be
the O VI high-velocity clouds in the Local Group
that are discussed in the next subsection.
Due to apparent strong connection between intervening
O VI systems and galactic structures
and a resulting galaxy/metallicity bias problem it is
of great interest to consider other tracers
of warm-hot gas, which are independent of the metallicity of the gas.
The broad hydrogen Ly
absorbers - as will be discussed in the following - therefore represent
an important alternative for studying the WHIM at low redshift.
As described in Sect. 2.2.1, BLAs
represent H I Ly
absorbers with large Doppler parameters b > 40 km s-1.
If thermal line broadening dominates the width of the
absorption, these systems trace the WHIM at temperatures
between 105 and 106 K, typically (note that for
most systems with T > 106 K BLAs are both too broad
and too shallow to be unambiguously identified with the limitations of
current UV spectrographs). The existence of H I
Ly
absorbers with
relatively large line widths has been occasionally reported in
earlier absorption-line studies of the local intergalactic medium (e.g.,
Tripp et al. 2001;
Bowen et
al. 2002).
Motivated by the rather frequent occurrence of broad absorbers
along QSO sightlines with relatively large redshift paths,
the first systematic analyses of BLAs in STIS low-z
data were carried out by
Richter et
al. (2004) and
Sembach et
al. (2004).
Richter et
al. (2006a)
have inspected four sightlines observed with STIS
towards the quasars PG 1259+593
(zem = 0.478), PG 1116+215 (zem =
0.176), H 1821+643 (zem = 0.297), and PG 0953+415
(zem = 0.239) for the presence of
BLAs and they identified a number of good candidates. Their study
implies a BLA number density per unit redshift
of dNBLA / dz
22-53 for Doppler
parameters b
40 km
s-1 and above a sensitivity limit of
log (N(cm-2) / b(km s-1))
11.3.
The large range for dNBLA / dz
partly is due to the uncertainty about defining reliable selection
criteria for separating spurious cases from good broad
Ly
candidates
(see discussions in
Richter et
al. 2004,
2006a)
and
Sembach et
al. 2004).
Transforming the number density dNBLA /
dz into a cosmological baryonic mass density,
Richter et
al. (2006a)
obtains
b(BLA)
0.0027
h70-1.
This limit is about 6 percent of the total baryonic mass density
in the Universe expected from the current cosmological models (see above),
and is comparable with the value derived for the intervening O VI absorbers
(see above). Examples for several BLAs in the STIS
spectrum of the quasar H 1821+643 are shown in
Fig. 6.
![]() |
Figure 6. Broad Lyman
|
More recently,
Lehner et
al. (2007)
have analysed BLAs in low-redshift
STIS spectra along seven sightlines. They find a BLA number density
of dNBLA / dz = 30 ± 4 for
b = 40-150 km s-1 and
log N(H I) > 13.2 for the redshift range z = 0-0.4. They
conclude that BLAs host at least 20 percent of the baryons in the
local Universe, while the photoionised
Ly forest, which
produces a large number of narrow
Ly
absorbers (NLAs),
contributes with ~ 30 percent to the total baryon budget.
In addition,
Prause et
al. (2007)
have investigated
the properties of BLAs at intermediate redshifts (z = 0.9 - 1.9)
along five other quasars using STIS high- and intermediate-resolution
data. They find a number density of reliably detected BLA candidates
of dNBLA / dz
14
and obtain a lower limit of the contribution of BLAs to the total
baryon budget of ~ 2 percent in this redshift range. The frequency
and baryon content of BLAs at intermediate redshifts obviously
is lower than at z = 0, indicating that at intermediate redshifts
shock-heating of the intergalactic gas from the infall
in large-scale filaments is not yet very efficient.
This is in line with the predictions from
cosmological simulations.
3.3. The Milky Way halo and Local Group gas
One primary goal of the FUSE mission was to constrain the
distribution and kinematics of hot gas in the thick
disk and lower halo of the Milky Way by studying the properties
of Galactic O VI absorption systems at radial
velocities |vLSR|
100 km s-1
(Savage et
al. 2000;
Savage et
al. 2003;
Wakker et
al. 2003).
However, as the FUSE data unveil,
O VI absorption associated with Milky Way gas
is observed not only at low velocities
but also at |vLSR| > 100 km s-1
(Sembach et
al. 2003).
The topic of cool and hot gas in the halo of the Milky Way
recently has been reviewed by
Richter et
al. (2006c).
These detections imply that next to the Milky Way's hot "atmosphere"
(i.e., the Galactic Corona) individual pockets of hot gas exist that move
with high velocities through the circumgalactic environment of the Milky
Way. Such high-velocity O VI absorbers may contain
a substantial fraction of the baryonic matter in the
Local Group in the form of warm-hot gas and thus
- as discussed in the previous subsection -
possibly represent the local counterparts of some of the
intervening O VI absorbers observed towards
low-redshift QSOs.
From their FUSE survey of high-velocity O VI absorption Sembach et al. (2003) find that probably more than 60 percent of the sky at high velocities is covered by ionised hydrogen (associated with the O VI absorbing gas) above a column density level of log N(H II) = 18, assuming a metallicity of the gas of 0.2 Solar. Some of the high-velocity O VI detected with FUSE appears to be associated with known high-velocity H I 21 cm structures (e.g., the high-velocity clouds complex A, complex C, the Magellanic Stream, and the Outer Arm). Other high-velocity O VI features, however, have no counterparts in H I 21 cm emission. The high radial velocities for most of these O VI absorbers are incompatible with those expected for the hot coronal gas (even if the coronal gas motion is decoupled from the underlying rotating disk). A transformation from the Local Standard of Rest to the Galactic Standard of Rest and the Local Group Standard of Rest velocity reference frames reduces the dispersion around the mean of the high-velocity O VI centroids (Sembach et al. 2003; Nicastro et al. 2003). This can be interpreted as evidence that some of the O VI high-velocity absorbers are intergalactic clouds in the Local Group rather than clouds directly associated with the Milky Way. However, it is extremely difficult to discriminate between a Local Group explanation and a distant Galactic explanation for these absorbers. The presence of intergalactic O VI absorbing gas in the Local Group is in line with theoretical models that predict that there should be a large reservoir of hot gas left over from the formation of the Local Group (see, e.g., Cen & Ostriker 1999).
It is unlikely that the high-velocity O VI is produced by photoionisation. Probably, the gas is collisionally ionised at temperatures of several 105 K. The O VI then may be produced in the turbulent interface regions between very hot (T > 106 K) gas in an extended Galactic Corona and the cooler gas clouds that are moving through this hot medium (see Sembach et al. 2003). Evidence for the existence of such interfaces also comes from the comparison of absorption lines from neutral and weakly ionised species with absorption from high ions like O VI (Fox et al. 2004).