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Ionization and the Detection of Low HI Mass Galaxies next up previous
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Minutes of the talk on :
      Ionization and the Detection of Low HI Mass Galaxies

Suzanne Linder (Cardiff)


In this talk, I will discuss a study of the effect of ionization on the low end of HI mass function.

Why study low mass/dwarf galaxies?

The existence of a large population of low mass galaxies follows from CDM theory. Studying the properties of such galaxies is important both for understanding QSO absorption lines and for constraining galaxy formation theories.

Damped ${\rm Ly}\alpha$ absorbers often arise in dwarfs, while Lyman limit systems are still thought to arise in luminous galaxies.

Does ionization explain the lack of low HI column density galaxies?

We have investigated the role of ionization, modeling disk galaxies using pressure and gravity confinement. We assume that galaxies are exposed to an ionizing background radiation, from a uniform quasar distribution. We explore a range of possible HI properties. The model parameters include: Then we calculate the average NH in the model.

The "Ionization Gap"

One of the tricks is that you have to decide the radius at which you measure, for example, the mean NH inside 1018 cm-2. We find a gap in the average column densities around NH which drops to 1017 cm-2. This seems to be an "ionization" gap, seen in the simulations.

The implication of this is that there may be some galaxies yet missed.


Questions/Discussion:

Wolfram Can you explain the gap?
Suzanne At some point, the ionizing flux does penetrate disk if NH gets too low.
Bob But why is there a gap, not a continuity?
Suzanne I'm not really sure yet.
Ed It might also depend on whether you are looking at a cold phase. We have to know what is the shape of the outer HI disk.


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