How to accrete neutral gas after the reionization of the universe?

[philherna]

Hi,

Once the Universe has completed reionization (at redshift >~6 ), the intergalactic medium (IGM) is completely ionized (even if some neutral clumps can persist). On the other hand, it seems that galaxies were accreting a lot of gas from the IGM after reionization so that they could form stars. It appears that this gas accretion can follow 2 modes, either (1) "cold gas" from IGM directly feed galaxies, or (2) IGM gas is heated when falling in the potential well of dark matter halos and then cools onto galaxies.

My question is the following: this IGM gas which is accreted by galaxies/halos should be ionized after reionization. Then, since the fuel for star formation is neutral gas, how the ionized gas can recombine in each of these 2 regimes of accretion?

Regards.

 

[Chronos]

Ionized after reionizatiton?? please clarify

 

[philherna]

yes, all I am talking about is related to the post-reionization epoch, when the IGM is ionized by definition.

 

[zhermes]

Hi philherna, welcome to PhysicsForums!

My question is the following: this IGM gas which is accreted by galaxies/halos should be ionized after reionization. Then, since the fuel for star formation is neutral gas, how the ionized gas can recombine in each of these 2 regimes of accretion?

Excellent question. General gravitational collapse is usually described using the http://www.astro.rug.nl/~onderwys/ACTUEELONDERZOEK/JAAR2003/college2/jeans_def.html. The idea is that for gas to collapse, its self-gravity must overcome its internal pressure. The internal pressure depends strongly on the temperature, and thus either a given clump of gas needs to be cold enough---to lose pressure support---or massive enough to overcome it.

Its actually a lot more complicated than that suggests... for example: its not just the total mass versus the temperature, but also the particular density is important. None-the-less, the overarching idea is the same---the gas has to cool before it can collapse, and during cooling it recombines (de-ionizes).

Lots of methods allow the gas to cool, which are important depends on what the density and temperature is... thermal emission, line-cooling, etc all play a roll.

Ionized after reionizatiton?? please clarify

Ionized can be used as an adjective.

 

[philherna]

Thanks for your reply and your explanation. So, I agree, in general you need gas to cool (and potentially recombine) in order to collapse.
However, as you may know, galaxies seem to be able to accrete IGM gas which is already cool enough to infall without being heated by shocks when penetrating the environment of galaxies (i.e the halo). This scenario is detailed in this article : http://arxiv.org/pdf/0808.0553v3.pdf

What i don't really understand in this picture is how or when the cold accreted gas (which comes from the ionized IGM in the post-reinozation epoch) recombine when falling onto the galaxy.

Does it become neutral when settling in the galactic disk? or during its free fall in the halo? or what is already neutral before entering the halo?

 

[zhermes]

Those "cold" streams are only relatively cold; about 0.1(-0.01) times the virial temperature, which will be at least KeV---and thus still ionized. Most gas would recombine in more dense regions, i.e. disks---or clumps in more elliptical galaxies. Its a stochastic process occurring anywhere the gas can cool.

 

[philherna]

OK, thanks for the explanation! That helps.
Cheers.