Updated: 6 March 2011

Collaborator: J. Xavier Prochaska (UCSC), Hsiao-Wen Chen (U Chicago)

Conference presentation (pdf) from the first Space Astronomy: The UV Window to the Universe, El Escorial, Spain, May 2007

1. Metal-line System Survey: Characterizing the Low-z IGM

   Thank you for the opportunity to talk a little about metal-line absorbers in the low-z IGM. This is work I do in large part with Xavier Prochaska at UCSC, and, more recently, Hsiao-Wen Chen at U Chicago has proved useful in understanding the relation between absorbers and galaxies.

2. Outline

   In this brief talk I'm going to start broad by mentioning a couple interesting science questions that can be addressed with QAL spectroscopy. Then I'll discuss work that's wrapping up on one sightline where we've found evidence for a multi-phase IGM and a qualitative association between metal absorbers and galaxy groups. I'll wrap up with a peek at work that's just beginning and is basically repeating "this" analysis on a larger sample, hoping to dig out more statistics.

3. Quasar Absorption Line Spectroscopy

   Just a quick illustration of QAL spectroscopy technique. You observe some background quasar, and in its spectra will be (usually) many absorption features that probe structure at various redshifts along the pencil beam. Once you've sorted the features into Galactic and intergalactic, you can start characterizing e.g., chemical composition and evolution, structure.

4. Chemical Evolution of IGM

   One interesting question that can be addressed at least in part by QAL spectroscopy is the enrichment history of the IGM. Pettini et al (2003) and others have found the IGM is enriched early with C IV and the mass density is roughly constant from that early time on (z ~ 1.5 to 5). So the interesting question is what happens in the lowest redshift bins, which probe the last 9 Gyr of the Universe and span the time after the peak in the star formation rate so you would expect some evolution. I will return to this question later (and I don't have an answer now.)

5. "Missing" Baryons

   The next interesting topic I wanted to discuss briefly is the "missing" baryons problem. From Big Bang nucleosynthesis models and measure of the baryons at z ~ 3 in the Lyα forest, we know how many baryons there should be around at z ~ 1. However, surveys of the baryon content in various populations show a lack (Fukugita and Peebles). From cosmological simulations, we see that there may be a large reservoir of baryons in what is called the WHIM. The simulations predict that the WHIM is collisionally-ionized, shock-heated, moderately overdense medium that may have up to 40% of the baryons at z ~ 1. Conveniently, this is difficult to observe but the current best probe is the O VI doublet, which traces the lower WHIM temperature range. (Could mention Mg X, O VIII,...) So how are observations doing?

6. PKS1302-102: Search for O VI

   This question started this project of studying one sightline. After identifying all the lines, we walk away with a couple dozen Lyα, half dozen C III absorbers, and 2 O VI detected with confidence and 1 tentative one. There's overlap between C III and O VI systems. Here are examples from the spectra which is not the best S/N. I'm going to focus on this strong OVI absorber and this strong Lyα absorber.

7. OVI at z_abs = 0.04231

   Shown here is the system stacked in velocity space centered at the center of Lyα. The H I features show a Voigt profile from log N_H I and b from curve of growth analysis (example later). The Lyα feature is multi-component, and it appears that C III traces the stronger H I component and O VI traces a redward, weaker component. So we have a kinematic argument for a two-phase medium, and we next can examine the possible ionization mechanism from CLOUDY models.

8. Photoionization

   For this photoionization model, the relative abundances of the various C ions (one detection and two limits) constrain the ionization parameter, which is the number of H I ionizing photons to the number of H atoms. The abundance-dependent ratio N(O VI)/N(C IV) indicates that O VI is from a different photoionized phase.

9. Collisional Ionization

   In the collisional ionization model, the idea that there are two phases is stronger. As expected, O VI traces a hotter component than C III, and in CIE, you absolutely can't have C III and O VI.

10. Two-phase Medium

    So we conclude that there are two phases (and this is supported by kinematic arguments). Then you can decide whether it's two photoionized media or a photoionized medium with collisionally-ionized medium.

11. z_abs = 0.09487 Partial Lyman Limit System

    For an example of another interesting system in the PKS1302 sightline, I turn to the partial Lyman limit system. It has properties similar to the z ~ 0.042 system previously discussed. there's a kinematic difference between elements (show on the next slide). And modeling supports two phases. It's a partial system because it's at the edge of having enough column density. The COG analysis was for a single component and that works well event though the profile is multi-component.

12. Line Profiles

    Lyα, C III, Si III have multi-components where as O VI is broad. Note the Voigt profiles look pretty good (values from single-component COG).

13. Lyman Limit Optical Depth

    As an extra treat, we can try to measure the total log N_H I independent of the Doppler parameter by measuring the flux decrement at the Lyman limit. The constraint isn't as tight as the COG but is consistent. We were unlucky that the Lyman limit falls at the edge of LiF 1A, and we're fighting noise.

14. Galaxy Survey

    Last, I'd like to do a splash from our galaxy survey, in a qualitative sense. Here's the quasar, 20' on a side, 95% complete within 5' to R ~ 19.5 mag. Galaxies marked are within 1000 km/s of an absorber. Strong O VI system has two galaxies within 300 kpc, and there is no galaxy brighter than 0.01 L_* within 200 kpc. pLLS system, which also has strong C III and Lyα system within 250 km/s, has 10 galaxies within 1000 km/s.

15. HST/STIS Archive Survey

    Moving on to a slightly broader topic. Here's the next project(s) for me. basically repeat the analysis I just presented on all metal-line systems in the archives. Hearkening back to the Pettini et al (2003) work, we'll be interested in C IV mass density at these low redshifts.

16. How many absorbers will we find?

    Taking the redshift pathlength from those 12 QSO's highlighted on the previous slide, we can get a a rough sense of how many metals we'll find. Prospects for C IV and Si IV will improve when I include z > 0.6 QSOs.

17. Summary

    To conclude, I hope I showed you a little of the power in QAL spectroscopy. For the specific example of PKS1302, we come to the same idea of a multi-phase IGM as others have seen. We also think galaxy groups and metal absorbers are related. And I've mentioned the next phase and what we can expect to find in the archives. Thanks.