Metal abundances of the absorber systems toward HS1700+6416

1. The basic concepts

This section is intended to provide an accessible introduction to the basic ideas underlying quasar absorption line astrophysics. You may wish to skip it if you are familiar with the subject.
Four steps are necessary for this introduction. First of all something has to be said about absorption lines. In the second step, we have to deal with cosmological redshift and finally the question "What is a quasar?" is adressed briefly.

1. Absorption lines: Atoms (for convenience the term "Atom" is always meant to include also "Molecules" and "Ions") have the property to absorb electromagnetic radiation. But they cannot absorb electromagnetic radiation of arbitrary wavelength (i.e. arbitrary colour). Instead the specific types (colours) of light that is absorbed by a specific Atom uniquely characterizes this atom. Each atom thus has a unique fingerprint. For example neutral hydrogene (abbrev. H I) absorbs light at the wavelengths 1215.67 Å, 1025.72 Å and many more. No other atom absorbs at precisely these wavelengths. If we receive light with precisely these wavelengths (colours) missing then we can deduce, that somewhere on the sightline to the source there must be neutral hydrogen. It is very important to note, that not only the wavelengths of these hydrogen absorption lines are unique, but also their ratio. Thus no other atom has absorption lines with a wavelength ratio of 1215.67/1025.72 = 1.185.

2. Cosmological Redshift: When astronomers started to analyse the light from other galaxies they discovered, that in almost all cases this light was shifted to the red. That means that, for instance, the characteristic neutral hydrogen lines were detected, but at different wavelengths. Namely both wavelengths were multiplied by a constant that we will denote (1+z). Thus, of course, the ratio of the wavelengths retained the chracteristic value 1.185 making possible the identification as neutral hydrogen. Quantitative analysis of distant galaxies revealed, that the redshift increases linearly with the distance of the galaxy (the famous Hubble relation). A shift of wavelength was known to result from the source moving away from the observer or towards the observer (the Doppler Effect). It is, therefore, a convenient (though not correct) way to visualize the cosmologic redshift as the galaxies moving away from us, the more quickly the further they are away from us. It is worth mentioning, that this point of view does not distinguish our position in the universe from any other position. Any observer, wherever in the universe, would see all galaxies moving away from him according to the same Hubble relation.

3. Quasars: Quasars are the most distant objects observed by astrophysicists. According to our understanding they are galaxies with extremely active nuclei. In fact all we see is the nucleus. The rest of the galaxy is too faint to be seen at such large distances (billions of lightyears). In contrast to galaxies, quasars are pointlike sources, which gave them their name: QUAsi StellAr Radiosource.

4. Quasar absorption lines Now we regard the quasar simply as a very distant source of light which emits energy in a wide spectrum of electromagnetic radiation. In a simple picture, this light contains all possible "colours". On the sightline from the earth to the quasar, there are numerous gasclouds containing different sorts of atoms and ions. These absorb at certain chracteristic wavelengths (colours) determined by the type of ion and the position on the sightline (redshift). The redshift labels the cloud and the wavelength divided by the redshift the ion. examining the different lines of all ions, we are able to derive the chemical structure of the intervening clouds.


This picture illustrates the absorption processes. The colors absorbed in an intervening gascloud depend on the redshift and the ions present. We can derive a lot of information about both from examining the light that reaches the earth.

2. Introduction to HS1700+6416

Absorption lines in the spectra of quasars originate either in intervening interstellar matter in our miky way or, more interestingly, in extragalactic gas clouds on the sightline to the distant QSO. With the detection of these absorption lines a unique oportunity has arisen to study the physical and chemical properties of regions of the universe otherwise not accessible for astrophysical observations. QSO absorption lines thus provide an important gateway to infer observational constraints on galaxy formation and evolution and to probe conditions in the early universe.
Different types of absorbers give rise to these absorption lines. Whereas the majority of Ly alpha and higher series lines are produced in Ly alpha clouds until recently believed to be generally metal deficient, metal line systems (hereafter MLS) also reveal distinct absorption caused by heavier elements. Yet, after the detection of metal absorption lines associated with approximately 75 % of the Ly alpha absorbers of HI column-densities as low as 3·10^14; cm² (Songaila and Cowie 1996), the distinction between these absorber types has become somewhat unclear.
Due to sufficiently high neutral hydrogen column densities, so called Lyman limit systems (LLS) are optically thick for Lyman continuum photons (lambda < 912 Å in the absorber restframe) and produce sharp edges in the quasar spectra, sometimes even completely absorbing the quasar flux below (1+z)·912 Å, where z is the redshift of the absorber (one of these edges can be seen at 1670 Å (<=>  z = 0.722) in the FOS spectrum of HS1700 presented below).
Since the majority of strong heavy element lines falls into the ultraviolet frequency range, even if originating at redshifts about 2, the successfull implementation of the Hubble Space Telescope (HST) has provided a substantial breakthrough in the studies of QSO absorption lines. Yet, due to the LLS mentioned above only a few quasars are known to exhibit detectable flux over a wide range of the UV part of the electromagnetic spectrum. As shown by IUE observations between 1200 Å and 3200 Å, the bright (V = 16.1) high-redshift (z = 2.72) quasar HS1700+6416 discovered in the course of the Hamburg Quasar Survey (Reimers et al. [3]), belongs to this class.
The graphic depicts UV spectra (with 1 sigma error) taken with the Faint Object Spectrograph (FOS) and the Goddard High Resolution Spectrograph (GHRS) onboard the Hubble Space Telescope in May 1996. The red line marks the estimated continuum which shows likely emission lines of NeIII and NeV and possibly NeIV and OIV. The GHRS spectrum is contaminated by geocoronal HI emission at 1215 Å and airglow (OI) at 1310 Å.


Analysis of these data in combination with results determined from high quality optical data (Tripp et al. [4]) and earlier UV data (Vogel and Reimers [5], Köhler, Reimers and Wamsteker [2]) provides more accurate metal abundances of the absorber systems with particular regard to Neon and Sulphur and systems at low redshift as well as further information about the intergalactic ionizing radiation field.

3. Results

1. Absorption Systems: A new MLS at z=0.09 has been found on the basis of strong but blended absorption for Ly alpha and the CIV doublet at 1548/1550 Å. This redshift coincides well with z=0.086 of a galaxy detected at 11'' (i.e. appr. 30 kpc for H0 = 50 km/sMpc) from HS1700 by Reimers et al. [3]).

2. Photoionization Models: Assuming photoionization, cloud models under the influence of a metagalactic radiation field as computed by Haardt & Madau [1] for different redshifts have been constructed with Ferland's ionization code CLOUDY. The cloud models consist of two regions of different hydrogen density : low- (LIP) and high-ionization phase (HIP).
In some cases, these models fail to produce sufficient amounts of higly ionized (OVI, NeVII, NeVIII). If the corresponding measured column densities are real, a harder radiation field obtained by decreasing the HeII-edge by a factor of 3 leads to adequate amounts of highly ionized elements. This modification, though, has no significant effect on the metal abundances, apart from neon, for which a slightly larger value has been found.

3. The Absorber Complex at z = 2.315: Tripp et al. [4] derive [M/H] = -0.45 for the strongest component of this system from a one-phase cloud model. Adopting their model, we were not able to reproduce the wide variety of different ionization stages observed in the new UV data, which suggests the introduction of an additional HIP. Information from UV data of higher resolution are required to derive precise metal abundances, but prominent absorption by various ions including SIII and NeIV and correspondingly high observed metal column densities indicate the possibility of a high metallicity of this system.

4. Evolution of Metal Abundances with Redshift: In general, metal abundances increase with decreasing redshift with [M/H] appr. -2 at z=2, [M/H} appr. -1 at z=1 and values slightly below solar at z=0.5 (see Table 1). The investigation of the system at z = 2.315, however, indicates that exception from this trend do occur. If the measured column densities of high-ionization species are real, neon abundances would be greater by appr. 0.3 dex (higher [Ne/H] values are given in parentheses).
Except for the absorber at z = 0.8643, no significant overabundance of O and N relative to C as compared with solar values has been found. According to the models, though, Si, Mg, S and in particular Ne are enriched relative to C. However, unresolved saturation of the strong C, N and O lines and/or unrecognized blending of Si, S and Ne lines might spuriously mimic enrichment of the latter elements.

Table 1: Metal abundances derived from two component cloud models
z2.1681.84511.15740.8643 0.7220.5524
[O/H]-2.15-1.7-1.2-0.4 -0.75-0.55
[C/H]-2.15-1.6-1.3-1.2 -0.95-0.55
[N/H]-2.05-1.8-1.5-0.9 -0.65-0.2
[S/H]-1.7-1.5-0.9-0.25 -0.5 -
[Si/H] - - - -0.7 -0.85-0.25
[Ne/H]-1.3 (-1.0)-1.2 (-0.9)-0.9 (-0.6) - - -


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