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Ionization and the Detection of Low HI Mass Galaxies

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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.

HIDEEP found no low HI column density galaxies.
One possible explanation is that the disks are "frozen". Simulations show that below column densities of a few times 10¹⁹ cm^-2, it is easy to ionize neutral gas by the UV background. A much smaller neutral component of N_H ~ 10¹⁷ cm^-2 remains.
- Average N(M_H) will therefore be much higher.

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:

Expontential disk
Exponentials flattened in center, so assume various actual central N_H values
Assume various disk scale lengths
TF relation, optical properties

Then we calculate the average N_H 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 N_H inside 10¹⁸ cm^-2. We find a gap in the average column densities around N_H 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 N_H 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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