Peter Jakobsen
Astrophysics Division, Space Science Department of ESA,
ESTEC, 2200 AG Noordwijk, The Netherlands
absorption from singly
ionized intergalactic helium has so far been detected in the far-UV
spectra of three quasars, twice with HST at 
and once with
HUT/Astro-2 at 
. Weak HeI 
lines of neutral
helium have also been detected in several metal line systems with
HST. The helium detections confirm that intergalactic gas is highly
ionized and provide an important, but qualitative, consistency check
of Big Bang nucleosynthesis theory. However, since the existing HeII
observations cannot distinguish between absorption due to unresolved
lines in the HeII forest and true Gunn-Peterson absorption, it is not
clear at present whether a diffuse IGM is detected in the data.
Nearly all that is known about the content and physical conditions in
intergalactic space at high redshift stems from the study of
absorption lines in quasar spectra. A long-standing goal of
ultraviolet space astronomy---and HST in particular---has been to
extend such studies to cover the redshifted extreme ultraviolet
resonance lines of neutral helium at 
Å and singly
ionized helium at 
Å. The element helium not only plays
a central role in modern cosmology, but the singly ionized HeII ion
is also an important diagnostic of ionized gas.
The Lyman forest clouds are of particular interest in the context of
helium observations. Although it is now known that at least some
forest systems contain heavy elements, (Cowie et al. 1995, Tytler et
al. 1995), the forest clouds are still the closest approximation to
primordial gas currently known. From the so-called proximity effect
(Carswell et al. 1982, Murdoch et al. 1986, Bajtlik, Duncan &
Ostriker 1988, Lu, Wolfe & Turnshek 1991, Bechtold 1994), the Lyman
forest clouds are inferred to be highly photoionized (to of order one
part on 
in HI) by a diffuse meta-galactic background flux
believed to be due to the integrated light of quasars and/or young
galaxies (Bechtold et al. 1987, Miralda-Escudé & Ostriker
1990,1992, Madau 1992). The helium present in the forest clouds is
therefore also expected to be highly ionized, and detectable mainly in
the form of singly ionized HeII rather than neutral HeI. In
particular, one clear prediction is that the forest should be very
weak in HeI
584, but extremely strong in HeII
304,
with the density of residual HeII in the forest clouds exceeding that
of HI by one order of magnitude or more, depending on the spectral
shape of the ionizing background (Sargent et al. 1980,
Miralda-Escudé 1993, Jakobsen et al. 1993).
A second motivation for pursuing observations of helium absorption is
the hope of detecting a smoothly distributed intergalactic medium
(IGM) through its redshift-smeared line absorption (Gunn & Peterson
1995). Although Gunn-Peterson absorption has not been reliably
detected in HI Ly
(Steidel & Sargent 1987, Schneider, Schmidt
& Gunn 1991, Webb et al. 1992, Giallongo, Cristiani & Trevese 1992)
or HeI
584 (Beaver et al. 1991, Reimers et al. 1989, Tripp,
Bechtold & Green 1994), a highly photoionized IGM could still reveal
itself through detectable Gunn-Peterson absorption in the
HeII 
304 line.
A considerable hurdle standing in the way of helium absorption
measurements, however, is presented by the fact that the HeI and HeII
resonance lines fall in the extreme ultraviolet. The
HeII 
304 transition is only accessible above the MgF
short wavelength cutoff of the HST optics in the case of the most
remote---and therefore faint---quasars at redshifts z>3.
Moreover, since the transition lies below the Lyman limit at
Å, the vast majority of z>3 quasars are predicted
to be completely unobservable at redshifted HeII 
304 due to
strong photoelectric absorption in the numerous Lyman forest and Lyman
limit systems intercepted by the line of sight out to such large
redshift (Møller & Jakobsen 1990, Picard & Jakobsen 1993, Zuo &
Phinney 1993).
Since the launch of HST, there have been two systematic searches for
rare cases of z>3 quasars whose brightness and exceptionally clear
line of sight combine to make detection of redshifted HeII absorption
feasible: the FOC objective prism survey (Jakobsen et al. 1993) and
the FOC ``snapshot'' imaging survey of David Tytler and coworkers.
Between them, these two surveys have so far sampled the far-UV fluxes
more than 110 bright z>3 quasars selected from the literature. Both
surveys have now met with success and each have uncovered an object
against which the first detections of strong intergalactic HeII
absorption at 
have been made (Jakobsen et al. 1994, Tytler
et al. 1995).
Other successes concern the remarkable UV bright 
quasar
HS1700+6416 discovered by Reimers et al. (1989). Although of too
low redshift to enable HeII
304 to be reached with HST, FOS
observations of HS1700+6416 have led to the first detections of HeI
in quasar absorption systems (Reimers et al. 1992, Reimers & Vogel
1993). More recently, shorter wavelength spectroscopic observations of
HS1700+6416 obtained with the Hopkins Ultraviolet Telescope flown
on the Astro-2 mission have detected HeII absorption at redshifts
z<3 which are inaccessible to HST (Davidsen, Kriss & Zheng 1996)
The HeI and HeII detections confirm that HeI absorption is universally very weak, while HeII absorption is strong compared to HI. This provides further confirmation that quasar absorption systems---and the Lyman forest clouds in particular---are indeed highly ionized, and that the observable ions HI, HeI, and possibly HeII, therefore, represent only minority species. One consequence of this high degree ionization is that it precludes being able to derive helium abundances from the measurements at any level of accuracy of interest to Big Bang nucleosynthesis theory because of the large and uncertain ionization corrections involved. Nonetheless, the mere fact that intergalactic gas at these large redshifts has been shown to contain helium still provides an important, albeit qualitative, confirmation of a key prediction of Big Bang cosmology
The UV continuum of HS1700+6416 was first detected by Reimers et al.
(1989) using IUE. The high intrinsic brightness of HS1700+6416
(
) and its exceptionally clear line of sight conspire to
make this object the most UV bright high redshift quasar presently
known (see, however, Reimers et al. 1994). The incredibly rich extreme
ultraviolet absorption spectrum of HS1700+6416 has been extensively
studied with HST by Reimers et al. (1992) and Vogel & Reimers (1995)
using the FOS (and more recently the GHRS). These observations confirm
the prediction that HeI 
lines corresponding to the
Ly
forest are too weak to be detectable, but weak HeI lines
are detected in the case of four denser metal line systems in the
spectrum at 
--2.43.
A detailed analysis of the HeI absorption in HS1700+6416 is given
by Reimers & Vogel (1993), who show that the strengths of the HeI
lines appear to be a factor of 
higher than predicted for the
single component photoionization models that best fit the ionization
of the heavy elements, assuming a cosmological helium abundance. As
also discussed by Reimers & Vogel (1993), however, this discrepancy
probably does not imply a high helium abundance, but rather reflects
inadequacies in accurately modeling highly ionized quasar absorption
systems.
Singly ionized HeII absorption has now been detected toward three
quasars, twice with HST and once with HUT/Astro-2. It is important
to appreciate that because of the exceeding faint UV fluxes involved,
all these detections push the sensitivity limits of present day
instrumentation. Neither the existing HST nor the HUT observations
have the spectral resolution nor signal to noise ratio to resolve
individual spectral lines. What is detected is the redshift-smeared
absorption trough shortward of the HeII line in the quasar rest frame
due to the combined HeII opacity of both the HeII forest and the
diffuse IGM, and the quantitative strength of this ``Gunn-Peterson''
trough as measured by its wavelength-averaged effective optical
depth, 
.
Figure: Far-ultraviolet spectrum of Q0302-003 obtained with the
FOC in objective prism mode. The thin solid line gives the
1
uncertainty per 
Å wavelength bin.
The position of the HeII 
line in the quasar rest frame
is also marked.
The quasar Q0302-003 (
, 
) was first detected in
the far-UV in the FOC prism survey (Jakobsen et al. 1993), and then
re-observed with the COSTAR-corrected FOC (Jakobsen et al. 1994). The
low resolution ultraviolet spectrum of this quasar obtained with the
FOC far-UV objective prism is shown in Figure 1. The spectrum of
Q0302-003 is seemingly completely absorbed on the blue side of the
redshifted HeII line, with no flux detected below the edge at the
sensitivity limit of the 3 hour FOC integration. This places a lower
limit on the total effective HeII optical depth at
of 
(90%
confidence).
Figure: FOC far-UV prism spectrum of PKS 1935-692 with the position
of the HeII 
line in the quasar rest frame marked. The
thin solid line gives the 1
uncertainty per
Å wavelength bin. The depressions in the
spectrum near 
1580Å and 
1330Å are due to an
intervening damped Ly
system at a redshift low enough not to
obscure the HeII absorption.
This finding has recently been confirmed by a second HST detection
toward the quasar PKS 1935-692 (
, 
) . This
object was first detected in the UV during FOC snapshot survey of
David Tytler and coworkers, and was subsequently re-observed
spectroscopically using the FOS (Tytler et al. 1995). The FOS
spectrum of PKS 1935-692 initially suggested less intense HeII
absorption (
) than seen toward Q0302-003 at
slightly higher redshift. However, this has not been confirmed by very
recent follow-up observations of PKS 1935-692 taken with the FOC
objective prism (Jakobsen & Tytler, Director's Discretionary Program
6156). The FOC prism spectrum of PKS 1935-692 (Figure 2) instead
shows no detectable flux below the redshifted HeII line, yielding the
limit 
(90% confidence). Given that the FOC
spectrum (which is otherwise in good agreement with the FOS spectrum
at wavelengths above the HeII edge) is technically speaking the
cleaner of the two in terms of measuring 
on account
of the accurate background subtraction afforded by the two-dimensional
FOC detector, it is believed that the discrepancy with the initial
suggestion of 
can be traced to residual
background/stray light in the FOS at the painfully faint signal levels
involved. The two existing HeII detections thus appear to be
mutually consistent and can be combined statistically to yield the
measure: 
(90% confidence) at
.
The HST HeII measurement at high redshift can now be compared to the
recent successful detection of HeII absorption at lower redshifts
toward HS1700+6416 by Davidsen, Kriss, & Zheng (1996) with
HUT/Astro-2. The impressive HUT spectrum of HS1700+6416 shows a
clear and finite flux in the HeII trough, which implies a lower HeII
opacity over the 
redshift range sampled by the HUT
data. The inferred value of 
at
quoted by Davidsen, Kriss, & Zheng (1996) is
plotted together with the combined HST measure in Figure 3.
Figure: Comparison of the HST and HUT measurements of 
and the redshift ranges probed by the data. The two continuous
curves show the expected evolution in redshift for a non-evolving
ionizing background in the two extreme cases where the absorption is
completely dominated by unresolved line absorption in the HeII forest
and true Gunn-Peterson absorption in a diffuse IGM.
It is presently an open question whether the strong HeII absorption detected is dominated by unresolved line absorption or by true Gunn-Peterson absorption from a diffuse IGM. Since it is only the sum of the two contributions that is observed, and none of the existing HeII spectra is capable of resolving the HeII forest, this question can only be addressed indirectly. As a consequence, the parameter space of the problem is still very large, permitting both forest- and IGM-dominated interpretations.
The optical depth of the Gunn-Peterson HeII 
304 absorption
trough produced by a smooth IGM having HeII volume density 
in an 
universe is given by
where 
km s
Mpc
is the Hubble constant,
cm
the integrated cross section
of the HeII 304Å transition. The corresponding
wavelength-averaged contribution to 
from HeII line
blanketing in the Lyman forest systems can be written
where 
is the number of forest lines per unit redshift and
is the mean HeII 304Å
equivalent width of the forest systems averaged over their
distribution in column density.
Both contributions to 
are expected to evolve rapidly
with redshift. In the simplest case of an IGM held photoionized by a
background flux of constant intensity and spectral shape, it is easy
to show that Eqn.(1) predicts that 
should
evolve as 
. Similarly,
since the Lyman forest line density evolves as 
where 
(Sargent et al. 1980, Murdoch
et al. 1986), the same assumptions lead to the prediction 
. From Figure 3 it is seen that---at
least to first order---the observed relative increase in
seen between the HUT and HST measurements is
consistent with any combination of these two naive evolution models.
The absolute value of the forest contribution to 
can be also estimated from Eqn.(2) based on the known characteristics
of the forest in HI Ly
as measured from the ground. However,
the calculated value of 
depends
critically on both the assumed ionization level of the forest gas and
its detailed distribution in column density. Of particular importance
to this problem is the recent Keck discovery that the column density
spectrum of Lyman forest clouds extends uninterrupted down to 
cm
and possibly lower (Songaila, Hu, &
Cowie 1995, Tytler et al. 1995). This population of very low column
forest clouds is very important to the HeII opacity problem since it
is conceivable that the ionization conditions in the forest are such
that the residual density of HeII in the forest clouds is much
larger than the HI density, in which case the low column population
dominates the HeII absorption to the point of being able to explain
all the absorption observed without any need for a diffuse IGM
(Miralda-Escudé 1993, Jakobsen et al. 1994, Madau & Meiksin 1994,
Songaila, Hu, & Cowie 1995, Giroux, Fardal & Shull 1995, Davidsen,
Kriss & Zheng 1996).
This basic uncertainty concerning the exact geometry and ionization conditions in the plasma giving rise to the detected HeII absorption also affects the quantitative estimates of the total amount of intergalactic baryonic matter detected. The HeII measurements can only provide a lower limit on the baryonic mass detected since there is no way of directly measuring the amount of unseen mass present in the form of invisible HeIII. Moreover, since it is the net opacity in HeII that is measured, the assumed geometry of the absorbing gas is crucial in determining the amount of absorbing gas detected, since it takes many more HeII ions to produce a given amount of absorption if the gas is clumped in self-shielding clouds rather than uniformly distributed. This last issue is also important for the question of whether a proximity effect should be detectable in HeII (Zheng & Davidsen 1994, Giroux, Fardal, & Shull 1995).
In one extreme, if it is assumed that all the HeII detected is in a
diffuse IGM, applying equation (1) to the HST measurement of
at 
leads to the
following lower limit on the cosmological density of gas detected:
where 
is the fraction of helium in singly ionized
form, and a cosmological helium to hydrogen abundance of 
by number has been assumed. The matching calculation in
the opposite extreme case of Eqn.(2) where pure forest line absorption
is assumed leads to
i.e., a density some three orders of magnitude higher. Very similar estimates result from the equivalent analysis of the HUT results.
In the first case, only an extremely low density IGM having 
is required to explain the observations. In
contrast, the required baryonic density in the second case is
comparable to the canonical Big Bang nucleosynthesis density of
(Walker et al.
1991). In other words, for the detected HeII absorption to be
dominated by unresolved forest absorption, it is necessary that the
forest clouds at high redshift contain the bulk of the baryonic mass
of the universe. Interestingly, entirely different considerations
prompted by other HST observations of the forest clouds suggest that
the latter extreme may be closest to the truth (Rauch & Haehnelt
1995).
Bajtlik, S., Duncan, R. C., & Ostriker, J. P. 1988, ApJ, 327, 570
Beaver, E. A, Burbidge, E. M., Cohen, R. D., Junkkarinen, V. T., Lyons, R. W., Rosenblatt, E. I., Hartig, G. F., Margon, B., & Davidsen, A. F. 1991, ApJ, 377, L9
Bechtold, J. 1994, ApJS, 91, 1
Bechtold, J., Czerny, B., Elvis, M., Fabbiano, G., & Green, R. F. 1987, ApJ, 314, 699
Carswell, R. F., Whelan, J. A. J., Smith, M. G., Boksenberg, A., & Tytler, D. 1982, MNRAS, 198,91
Cowie, L. L., Songaila, A., Kim, T. S., Hu, E. M. 1995, AJ, 109, 1522
Davidsen, A. F., Kriss, G. A., & Zheng, W. 1996, Nature (in press)
Giallongo, E., Cristiani, S., & Trevese, D. 1992,ApJ, 398, L9
Giroux, M. L., Fardal, M. A., & Shull, M. J. 1995 ApJ, 451, 477
Gunn, J. E., & Peterson, B. A. 1965, ApJ, 142, 1633
Hu, E. M., Kim, T. S. Cowie, L. L., Songaila, A., & Rauch, M. 1995, AJ, 110, 1526
Jakobsen, P. et al. 1993, ApJ, 417, 528
Jakobsen, P., Boksenberg, A., Deharveng, J. M., Greenfield, P., Jedrzejewski, R., & Paresce, F. 1994, Nature 370,35
Lu, L., Wolfe, A. M., & Turnshek, D. A. 1991, ApJ, 367, 19
Madau, P. & Meiksin, A. 1994, ApJ, 433, L53
Madua, P. 1992, ApJ, 389, L1
Miralda-Escudé, J. 1993, MNRAS, 262, 273
Miralda-Escudé, J. & Ostriker, J. P. 1990, ApJ, 350, 1
Miralda-Escudé, J. & Ostriker, J. P. 1992, ApJ, 392, 15
Murdoch, H. S., Hunstead, R. W., Pettini, M., & Blades, J. C. 1986, ApJ, 309
Møller, P. & Jakobsen, P. 1990, A&A, 228, 299
Picard. A. & Jakobsen, P. 1993 A&A, 276, 331
Rauch, M. & Haehnelt, M. G. 1995, MNRAS, 275, L76
Reimers, D. & Vogel, S. 1993, A&A, 276, L13
Reimers, D., Vogel, S., Hagen, H. J., Engels, D., Groote, D., Wamsteker, W., Clavel, J., & Rosa, M. R. 1992, Nature 360, 561
Reimers, D., Clavel, J., Groote, D., Engels, D., Hagen, H. J., Naylor, T., Wamsteker, W., & Hopp, U. 1989, A&A, 218, 71
Reimers, D., Rodriguez-Pascual, P., Hagen, H. J., Wisotski, L. 1994, A&A, 293, L21
Sargent, W. L. W., Steidel, C. C., & Boksenberg, A. 1989, ApJS, 69, 703
Sargent, W. L. W., Young, P. J., Boksenberg, A., & Tytler, D. 1980, ApJS, 42, 41
Schneider, D. P., Schmidt, M., & Gunn, J. E. 1991, AJ, 101, 2004
Songaila, A., Hu, E. M., & Cowie, L. L. 1995, Nature, 375, 124
Steidel, C. C. & Sargent, W. L. W. 1987, ApJ, 318, L11
Tripp, T. M., Bechtold, J., & Green, R. F. 1994, ApJ, 433, 533
Tytler, D., Fan, X. M., Burles, S., Cottrell, L., Davis, C., Kirkman, D., & Zuo, L. 1995, in QSO Absorption Lines, ed. G. Meylan (Berlin: Springer), p.289
Vogel, S. & Reimers, D. 1995, A&A, 294, 377
Walker, T. P., Steigman, G. Schramm, D. N., Olive, K. A., & Kang, H.-S. 1991, ApJ, 376, 51
Webb, J. K., Barcons, X., Carswell, R. F., & Parnell, H. C. 1992, MNRAS, 255, 319
Zheng, W. & Davidsen, A. 1994, ApJ, 440, L53
Zuo, L. & Phinney, E. S. 1993, ApJ, 418, 28