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
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 10cm. 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 210, 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