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Searching the Environment around LyAlpha Absorbers

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Minutes of the talk on:
Searching the Environment around Ly $\alpha$ Absorbers

Jessica Rosenberg (U. Colorado)

A more detailed version of this talk by the speaker is available in HTML or PDF format.

This talk focuses on exploring the connection between Ly $\alpha$ absorbers and galaxies.

The theorist view of gas in the low z universe is based on the filamentary structures that are evident in simulations based on the CDM paradigm. But we can ask: what is gas in local universe doing?

The observer's view of gas in the low-z universe is based on the Ly $\alpha$ absorbers which have column densities as low as 10 $^{12}$ cm $^{-2}$ , while galaxies are at column densities of 10 $^{20}$ cm $^{-2}$ . We ask: What is the connection between the two?

There are separate categories of absorbers:

Damped absorbers are associated with galaxies (2 $\times 10^{20}$ cm $^{-2}$ ).
Ly limit systems are associated with halos (10 $^{17}$ cm $^{-2}$ to 2 $\times 10^{20}$ cm $^{-2}$ ).
High column density forest absorbers (W>0.3A, N $>10^{13.5}$ cm $^{-2}$ ) may also be associated with halos.

Lower column density absorbers are very interesting. In particular, their relationship to galaxies is less well known and they are useful test of models. For example, we can ask: How large are halos? How do metals get out into IGM from the disks where they are manufactured by stars?

On particular example is that of the VLA and WSRT search for HI emission conducted by van Gorkom et al. (1996, Astron. J. 112, 1397). They found absorption clearly identified in a group of galaxies. This absorber is also seen in HI with =17000 km s $^{-1}$ . The absorber has N $_{\rm H} \sim 10^{14}$ cm $^{-2}$ .

Right now there are only about 100 quasars bright enough to study with HST.

Another example is the low z absorption seen towards 3C273, near the edge of the Virgo cluster.McLin et al. have detected 10 $^{16,1}$ cm $^{-2}$ metal lines. There is a dI of dwarf post starburst at a projected distance of 71 kpc with cz=1556 km/s, M $_{\rm B} \sim$ -14.5.

Arecibo study

We have used the Arecibo telescope to map a region 0.5 x 0.5 degrees around low z absorbers in 4 AGN sight lines. Area surveyed is 140 kpc to 1 Mpc around these nearby absorbers.
Mass sensitivities to 2 $\times 10^6$ to 5 $\times 10^8$ solar masses.
HIPASS data cubes also looked at.

What will ALFA contribute?

Can examine LSS defined by gas-rich galaxies around absorbers.
Deep large area surveys will provide better understanding of the LS environment.
Search for gas-rich LSBs near sightlines.

COS: The Cosmic Origins Spectrograph

COS, the next UV spectrograph on HST is scheduled to be launched in 2004. This will have 10-30 times the throughput of present instruments so a much large sample of low-z absorbers will be available for statistical study. There will be many opportunities also to use AGN as probes of galaxies halos.

We must tie our understanding of low column density gas in the local universe to the distribution of higher column density gas in galaxies

Discussion/Questions

Desh	You mentioned "large scale" structure; How large is large?
Jessica	It depends on the particular group/region. 3C273 is near Virgo so you'd like to go over 500 kpc/ 1Mpc scales. You want to ask: are you in a filament or void?
Jon	Saying you want to map out LSS in gas and in galaxies? HIPASS would say that distributions are similar. So what is new?
Jessica	Yes, but you find new galaxies not previously known/catalogued, especially very low surface brightness ones. They do tend to follow the same structures outlined by higher surface brightness galaxies but their correlation amplitude is lower.
Lister	The correlation function of HI selected galaxies is same as that of late types. The question might be: is it different in clusters/groups from very low density regions?