QSO Absorbers and Their Relation to Galaxies at Low Redshift
Kitt Peak National Observatory REU, Summer 1998
M. Stark working under Dr. Buell Jannuzi

Introduction

Quasi-Stellar Objects (QSOs) are defined as point-like sources of light, that emit large amounts of electromagnetic radiation at all wavelengths (they are especially noted for an excess of ultraviolet light when compared to a black-body model), all of which is characterized by large redshifts (but most noticeable in their emission lines). In some QSO spectra multiple absorption features are seen that correspond to redshifts less than those of the emission lines. These absorption features correspond to clouds of absorbing material located between the QSO and earth so the light from the QSO must pass through these absorbers to reach earth.

The the most common absorption features seen are those of: Lyman-alpha (hydrogen), C IV, and Mg II. Naturally the rest wavelengths of these features fall in the ultraviolet (UV) region of the spectrum which cannot be observed from the ground, but in the case of very distant QSOs these lines have been shifted to the visible part of the spectrum due to their high apparent recessional velocities caused by the cosmological expansion of the universe. These high apparent recessional velocities, thus lead to high redshifts (z > ~1.63 for Ly-alpha) of the QSO and absorbers, so it is possible to observe them from the ground. The next obvious question would be "what are these absorbers, where do they come from, and are they associated with anything?" To answer this, common sense would say to look for absorbers that are "near" earth so it would be possible to image them with conventional telescopes and spectrographs --- or in other words, to look at low redshift (z < ~1). Unfortunately, at low redshift these same absorption features still lie in the UV part of the spectrum, so to observe them at low redshift requires getting above the earth's atmosphere, which the Hubble Space Telescope (HST) has recently made possible.

During the first four cycles of the HST, one of the Key Projects was a Quasar Absorption Line Survey using the Faint Object Spectrograph. One of the results of this project is a census of absorption line systems at low redshift (Jannuzi et al. 1998 Preprint). Based on the analysis of the UV spectra, a catalogue of absorption lines for 66 quasar was created which is suitable for studies of gaseous systems in the nearby universe. From this catalogue, the redshifts of the intervening absorbers for each QSO can be determined. The information from surveys such as this can now be used to study the relationship of absorbers to galaxies and test the hypothesis of Bacall and Spitzer (1969, quoted in Jannuzi 1996) that these absorbers are in fact the extended halo of normal galaxies.

Process

To determine if a relationship exists between absorbers and galaxies, and to what extent, first, images were taken of the quasar fields to identify candidate galaxies located in the direction of the QSO; then, spectra were taken of the galaxies; finally the spectra were processed and will be used to determine the redshifts of the galaxies. Once the redshifts of these nearby galaxies are determined, we can look for a correlation between them and the absorbers (ex. do any of the galaxies have the same redshift as the absorbers?). When the candidate galaxies were chosen we had to be certain to include a large enough angular extent so we could recognise if any of the galaxies were grouped in clusters, then we could take that into account when determining absorber-galaxy relationships.

If we find that there are correlations between the absorbers and galaxies for low redshifts, then by using the CCD images to measure angular separations, we can get an estimation of the extent of the absorbers. Other longer-term goals may include determining to what extent and in what ways the absorbers and galaxies are associated beyond just proximity to each other, such as: " are the absorbers a part of the extended outer halo of galaxies?," "are they part of the thick disk of the galaxies?," "are they just coincidentally associated with the galaxies?," "is the material primordial material that was not incorporated into the galaxy at its formation, or is it material that has been a part of the galaxy since it formed?," and "has it been significantly affected/enriched by the galaxies?". Furthermore, in the long-term, it may be possible to postulate that there is a relationship between galaxies and absorbers that extends to all redshifts --- from this postulate we could examine the evolution, with time, of the absorbers and galaxies to get a better understanding of cosmology and the development of the universe.

Data Collection

The data that I worked with to determine the redshift of the galaxies was taken using the HYDRA Multi-Object Spectrograph on the WIYN telescope at Kitt Peak in 1995 and 1996, and covered 24 QSO fields. These spectral images were processed in the normal manner: bias subtracted, flat fielded using quartz lamp, and trimmed. Additionally several images of the same field were combined to produce higher signal-to-noise images and to remove most radiation events. After the standard reduction steps, the IRAF task "dohydra" was used to dispersion correct the object spectra using CuAr comparison spectra, sky subtract to remove the night sky emission features, and finally to extract the reduced spectra. The redshifts will be determined by comparing the offset of the features in the extracted spectra with those of a "standard spectra" using a relation such as:
Redshift Equation
to compute the actual redshift.

Once the redshifts for the galaxies have been determined, a search for correlations between the galaxies and absorbers will begin by comparing redshifts. If a galaxy and absorber are found to have the same redshift, then further steps will be taken to determine their relation, including determining the minimum angular size the absorber would have to subtend away from the galaxy to produce the absorption (based on the angular separation of the galaxy from the QSO), and looking for other noticeable relations. Should the nature of the relationships between the absorbers and the galaxies at low redshifts be determined, it may be possible to postulate that the relationship exists between absorbers and galaxies at all redshifts. It then could become possible for further studies to examine the evolution of absorbers with time and their relation to galaxies, to achieve a better understanding of cosmology and the development of the universe.

Bibliography

E-mail to mistark_(at)_umflint.edu