Theoretically dark

How do objects which don't form stars remain so dark ? That's a question I've often asked, especially of dark galaxy candidates : objects which have gas that looks like it's rotating (implying a dark matter halo to keep it bound together) but lacking any detectable stars. The big problems with such objects is that it's very hard to know if seeing is believing, if they just look like dark galaxies or were instead formed by different processes. Maybe such objects are just bits of gas ripped off perfectly normal galaxies. I've covered this in detail umpteen times before, with the basic conclusion being "some are, some aren't".

In the 2000's there was a period when a few different groups ran numerical simulations looking at whether specific candidates could be explained in this way. This has largely died off, so I was very intrigued by this paper which examines dark galaxies in the context of the latest and greatest numerical simulations. These are completely different beasts from the n-body simulations run on the desktop machines of 20 years ago : instead of a few thousand SPH gas particles, now they have billions or more particles and included all kinds of fancy gas physics that would have had us all foaming at the mouth in an ecstasy of delirium back in the day.

It certainly starts in a promising way, reviewing the major candidate objects and studies (and yes, they cite me, so thanks for that) as well as some other more recent work I wasn't aware of. So I've got a couple of other references I should check up on, which is good. But I have to say that after that it's all rather more theoretical than what I was hoping for. Not that it's difficult or unimportant, but that it never makes any comparison between theory and observation. It deals with the dark galaxy candidates in the simulations very much on the terms of the simulation alone, making little or no comparisons with the observational candidates.

In some ways this is quite novel, at least to me. Normally I look at the missing satellite problem from the perspective of the galaxies, because those are what we actually observe. But the problem itself is all about how simulations predict too many dark matter halos that never light up, so examining those halos as interesting objects in their own right is a good idea.

What they find isn't terribly surprising though. The vast majority of the halos in the simulation do indeed remain dark, for what seems to be due to a combination of factors more than any one in particular. And they form a continuous sequence from the truly starless to the merely very dark to the brightest and most luminous objects of all; dark galaxies are not special, but normal. Indeed, perhaps we should instead be asking instead not what keeps some halos dark but the exact opposite : what allows such extreme levels of star formation in the apparently "normal" galaxies ! For comparison, in their simulation they identify 5.6 million halos, of which 5.5 million are completely starless, 47,000 are dim but not totally dark, and the rest – a mere 100,000 or so – are luminous.

If we stick with the standard question, "what keeps them dark ?", though, then the answer seems to be : isolation, spin, and mass. Isolation prevents them from experiencing as many mergers as the brighter galaxies, which compress the gas and trigger star formation. Isolated objects avoid this. Spin keeps the gas more extended and its density lower, thus reducing star formation. And mass prevents much gas from getting into the halo in the first place, again keeping density low. While some dark galaxies do form stars briefly early on and then lose their stellar population, it seems that most just never form any at all. 

There's an additional effect from mass. Being small means that objects are more vulnerable to the effects of reionisation : when the first, highly energetic stars light up, they ionise all the gas in the smallest halos* and drive it out, and being so small they don't have the gravitational strength to recapture it. 

* These population III stars are thought to have been true behemoths, much larger and more energetic than any stars around today. So these wouldn't necessarily have had to form inside the halo that would later become a dark galaxy, they just had to be in some reasonably-nearby larger galaxy.

And that's really all there is to it. They cover this in great quantitative detail, much of it having long been examined before but here all at once and in some depth. But how does one go about verifying this ? How many such dark halos should have enough gas to be observable with current HI surveys ? How do the line widths of the candidates compare with the theory – how many should we expect to see according to the model ? How does this quantitatively address the missing satellite problem ? What testable predictions does it make ?

Frustratingly, none of this is mentioned. It's great to see dark galaxies being used as a mainstream term but it feels like a cliffhanger ending, stopping at the point things get interesting. And I seem to recall other people having problems with making the reionisation ("squelching") solution fit the observational data, so more comparisons to earlier works would have been nice. 

Still, the idea of dark galaxies, being a once openly-derided solution to a major problem in cosmology, now seems to have transformed into an inescapable inevitability, not a problem but simply reality. Specific candidates, I suspect, will always remain problematic, but the notion in principle now appears to be greeted nor with mere tolerance but actually embraced : yes, these halos do exist, it's just that we can't see them directly. So the wheel turns.