V. K. Khersonsky and D. A. Turnshek
Department of Physics and Astronomy, University of Pittsburgh,
Pittsburgh, PA 15260 USA
F. H. Briggs
Kapteyn Astronomical Institute, Groningen, The Netherlands
0.69.
Keywords: Intergalactic medium: - Gunn-Peterson test; Young galaxies: - Interstellar medium; Primordial abundances: - deuterium.
One of the potentially most effective methods of studying the intergalactic
medium (IGM) at different cosmological epochs (up to the redshifts of the most
distant QSOs) is the so-called Gunn-Peterson (GP) test. For neutral
hydrogen, this involves measurement of absorption from the Ly
resonance line in a QSO's ultraviolet continuum by a small fraction of
cosmologically distributed diffuse neutral hydrogen in the IGM. If diffuse
neutral hydrogen were dense
enough throughout the Universe, the GP effect should result in the appearance
of an absorption trough shortward of the Ly
emission line in all QSO
spectra. However, the shape and depth of the GP absorption trough will, in
fact, depend on the conditions and structure in the IGM at various epochs.
Under the right conditions, similar troughs might be observed shortward of the
resonance HeI 584Å and HeII 304Å lines, and Jakobsen et al. (1994) and
Davidsen et al. (1995) recently announced a clear detection of HeII 304Å at
high redshift which could be due to diffuse GP absorption (but we will not
consider this here). Constraints on GP absorption provide extremely important
results which are needed to understand the physical processes in the IGM, the
equation of state of the intergalactic gas, problems involving the
intergalactic UV background radiation, and mechanisms for the evolution of
different structures in the Universe.
There have been many attempts to observe the neutral hydrogen GP effect in QSO
spectra at high redshifts (Steidel & Sargent 1987, Jenkins & Ostriker 1991,
Giallongo et al. 1992, Webb et al. 1992, Fang & Crotts 1995). However, in
many cases the interpretation of the data is not very clear due
to various factors, the most important of which are: (1) uncertainties in the
extrapolation of continuum spectra of QSOs from longward of the Ly
emission line to shortward of this line and (2) the huge number of high
redshift Ly
forest lines shortward of the Ly
emission line
which result in uncertainties in the determination of the local continuum at
wavelengths where the GP trough is expected. The influence of this second
factor can be reduced by studying GP absorption at lower redshifts with
HST. In this case the number of Ly
forest lines decreases drastically.
However, without the appropriate data, the uncertainties related to the
extrapolation of a QSO's continuum spectrum from longward of the Ly
emission line to shortward of this line is still very significant (Bahcall et
al. 1991). Thus, no reliable detection of neutral hydrogen GP absorption
has been made.
Recently, during an archival investigation of HST-FOS QSO spectra aimed at
placing limits on GP absorption, we studied a sample of moderate redshift QSO
spectra. While many objects exhibited no evidence for neutral hydrogen GP
absorption, we wish to report on one object (Q2145+067 with 
)
which did exhibit some evidence (Khersonsky, Turnshek & Sherer 1995,
Khersonsky et al. 1996). The spectrum of this object is shown in Figure 1. We
used a carefully developed conservative technique to estimate the
continuum level and the minimum amount of GP absorption apparently
present. In the original spectrum, we determined that the apparent depression
in the continuum was extremely significant, 
30% centered at
2150--2300Å. This corresponds to the redshift interval 
0.75--0.90. However, this wavelength region coincides with potential
absorption from interstellar dust. An estimate of the interstellar extinction
toward Q2145+067 is 
0.06 mag as determined by the galaxy counts
method of Burstein & Heiles (1982). Based on the observed Galactic neutral
hydrogen and an assumed dust-to-gas ratio, the study of Lockman & Savage
(1995) gives a more conservative value, 
0.09 mag. We have
extinction-corrected the spectrum using this more conservative estimate and
still find a depression of 
13--15% (see Figure 1). Note that we
believe that the spectrum has been over extinction-corrected, because OVI
emission is much stronger than normal; any additional extinction correction
would make OVI emission much too strong. Therefore, it would seem that the
depression in this wavelength region can not be adequately explained by the
effects of Galactic dust and so this should be considered as an intriguing
possible case of GP absorption in the IGM near redshift 
0.8. [The
alternative interpretation would apparently require a substantial complex of
blended emission filling in the region shortward of the SiIV emission line, and
this is unexpected.]
Figure: The extinction-corrected spectrum of PKS 2145+067. The dashed line
shows the continuum of the QSO as derive by fitting the data at the locations
marked by the arrows. The 1-sigma flux errors are given by dashed-dotted line.
If real, the spectral width of the observed absorption feature is at least
250Å. Given the absence of GP absorption along other sight-lines we have
studied, this suggests that the IGM exhibits an over-density of neutral hydrogen
in the direction toward Q2145+067 extending over > 500
Mpc.
The result represents an over-density of neutral hydrogen by a factor of 3 to 7
more compared to values that would typically cause non-detections to be
reported. If confirmed, this would be the first unambiguous observation
of HI GP absorption. If the typical linear scale of any inhomogeneity in the
IGM is about the same as large scale structure (
50--100 Mpc) in
directions transverse to the sight-line toward Q2145+067, then the GP
absorption should vary on angular scales of several degrees (for 
0.8). Therefore, if the IGM is inhomogeneous on these scales, GP absorption
should be found along the lines-of-sight toward some other QSOs in the close
vicinity of 2145+067, but not found in other directions far apart from the
position of the QSO 2145+067 as analysis of other archival spectra indicate. A
search for such absorption would be highly desirable in order to understand the
real sizes of gas inhomogeneities in the IGM and to compare these sizes with
theoretical expectations.
It has been recognized by many investigators that Damped Ly
systems
(DLS) seen in QSO spectra are best tracers of galaxy evolution at large
redshifts (Wolfe 1993 and references therein). One possible interpretation of
this population of absorbers is that they are formed in giant disks of spiral
galaxies or progenitors of present day spiral galaxies (see, e.g.,
Lanzetta, Wolfe & Turnshek 1995, hereafter LWT, and references therein).
However, this interpretation is not the only possible one. We have considered
the possibility that these systems result from absorption of QSO radiation in
giant hydrogen clouds (GHCs) in different types of young galaxies, independent
of whether they are galaxies that have already formed or perhaps they are the
progenitors of ellipticals, spiral or even dwarf galaxies. The available data
on the properties of the DLSs, including data obtained with ground-based
telescopes and recent HST data, show that there are no significant differences
in the physical characteristics (except for metallicity) of the absorbers
associated with DLSs and GHCs in our and nearby spiral and dwarf galaxies (see,
e.g., Kulkarni & Heiles 1988 and Blitz 1991 for a review of the
properties of GHCs). One of the most prominent features which was found in a
statistical study of the distribution of the number of DLSs in column density
was that 
evolves with redshift. At high redshifts
the number of high column density systems is significantly higher than at low
redshifts. This evolution is clearly seen in Figure 2a. Crosses show the
result of observations of different authors (Lanzetta et al. 1991, Rao &
Briggs 1993, RB, LWT, Rao, Turnshek & Briggs 1995, RTB). Solid lines show
approximations of the data. For simplicity, we approximated the distribution of
DLSs in column density in each redshift interval with a power law. One can see
that the slope of the power law changes systematically with redshift. We
interpret this evolution as an indication of the evolution of the mass spectrum
of GHCs.
We have quantified this interpretation through analysis of the two-dimensional
distribution of DLSs in redshift and column density, 
. By making some simple assumptions about the
properties of GHCs (they are assumed to be homogeneous spheres of the same
density but with different mass) the two-dimensional distribution of DLSs can
be related to the mass spectrum of GHCs. This relation is described by the
integral equation
In this equation, 
is the mass spectrum or number density of GHCs
per co-moving volume, averaged over the surface of sky at redshift z, and 
where 
is the
mean atomic weight. The left side of this equation, i.e., the function
, was obtained from the observational data and presented in the
form of a two-dimensional approximation, 
, where 
and 
can be represented by
polynomials in the redshift interval z = 0--3.5 and 
10
cm
. Figure 2b shows the derived relation for 
. The solid
lines in Figure 2a are derived from this approximation.
Figure: (a) Distribution of DLSs in column density at
different redshifts. Crosses represent binned data. Solid lines show
our approximations. (b) The parameters of 
which approximate the
data. Solid lines connect the coefficients derived from Fig.1 data.
Dashed lines show the polynomial approximations.
By differentiating both parts of the integral equation with respect to
, one can obtain the mass spectrum of GHCs in the form of a power-law
relation,
In this result 
and 
are
dimensionless parameters where 
and 
. For a mean number density of 
4 cm
, the derived mass spectrum of GHCs associated with DLSs is shown in
Figure 3a.
In Figure 3b we also show the evolution of the cosmological mass density
parameter for neutral hydrogen, 
, where 
is critical density
and 
is assumed to be equal to 2
10
, consistent
with the derived mass spectrum. A comparison of the derived curve with the
observational data clearly shows that equating DLSs to GHCs is consistent with
the main observational features of the cosmological evolution of neutral
hydrogen gas. Thus, this result can be used to study different aspects of the
evolution of the interstellar medium in galaxies.
Figure: (a) Evolution of the mass spectrum of GHCs with cosmic time.
(b) Derived evolution of 
with redshift.
The most prominent feature in the evolution of the derived mass spectrum is that slope of this spectrum systematically increases when redshift decreases. This feature can be considered a manifestation of fundamental processes in young galaxies related to large scale (galactic scale) star formation. The most massive stars are forming in the most massive clouds. On the other hand, they effectively destroy these clouds and form a lot of smaller GHCs from the bigger ones (i.e., a transfer of mass from the high mass GHC range to the low mass GHC range). Both sinks of GHC mass (star formation and cloud destruction) reduce the number of high mass GHCs, significantly increasing the slope of the mass spectrum (see Khersonsky & Turnshek 1996 for details).
Measurements of the cosmic deuterium abundance provide a critical test for
physical conditions in the early universe. Recent attempts to detect
primordial deuterium often rely on observations of absorption lines in QSO
spectra. Songalia et al. (1994) reported the detection of a feature at the
expected position of deuterium in the wing of a strong Ly
absorption
line in a Lyman limit system at redshift z=3.32015 in the spectrum of
Q0014+813. By interpreting this feature as a deuterium absorption line, the
authors derived 
. However, Tytler et al. (1995)
obtained 
based on a ground-based study of the
Ly
and Ly
lines in the 
3.57 Lyman limit system in
Q1937-1009.
A detailed examination of the detectability of the Lyman series of deuterium in
QSO absorption-line systems shows that observations of the high-order Lyman
lines in clouds of very high column density, low velocity dispersion (such as
``damped Ly
systems''), and low heavy element abundance might produce
efficient and precise measures of the deuterium to hydrogen ratio (Khersonsky,
Briggs & Turnshek 1995, KBT).
Using HST-GHRS we have made a recent attempt to search for primordial deuterium
using the criteria developed by KBT (F. H. Briggs is the PI of this program). A
long GHRS integration was taken on the quasar 3C286 in the spectral region near
the Lyman continuum cut-off at 
0.69. The continuum strength in
this region is consistent with extrapolations of values in published FOS
spectra in the regions around the Ly
and Ly
absorption lines
from a system that was originally selected on the basis of a strong, very
narrow 21 centimeter absorption line. Our new spectrum includes 
9
identifiable Lyman series absorption lines from the gas causing the 21 cm
absorption line. A preliminary analysis of the spectrum shows the presence of
the expected deuterium Lyman lines, as well as some additional low-column
density, turbulent gas associated with the high column density cloud seen in
the 21 cm data. Since the observation is pushing the GHRS to its sensitivity
limit, the interpretation of the observation hinges on the wavelength stability
and spectral resolution over the many orbits of the exposures. The detailed
analysis and interpretation of the result is in progress (Briggs, Khersonsky &
Turnshek 1996, in preparation), but it is already clear that the added
sensitivity and low dark count of STIS will make a big improvement in future
observations of this object and to these types of observations in general.
This research was supported by a grants from the Space Telescope Science Institute, which is operated by AURA, Inc., under NASA contract NAS5-26555.
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V. K. Khersonsky, D. A. Turnshek, and F. H. BriggsKheronsky, Turnshek, and BriggsSome Recent Results on QSO Absorption Line Studies with HST