Dr. Alaina Henry

This question was posed to me by the citizen scientist, Richard Nowell, who played a major role in the discovery of Green Peas in the Galaxy Zoo.

I do not wish to intrude upon any research that might be happening to the GPs, but I am wondering about what the role, if any, is of Dark Matter in the GPs. No-one seems to have mentioned it in any papers, yet if they are being investigated for leaking LyA or LyC, then DM would effect that? Are they without DM?

Great question, Richard.  Here is my answer:

Thanks for your interest!  All galaxies have dark matter; the “normal” matter resides in halos of dark matter.  You can think of dark matter halos as seeds, and their gravity attracts gas that can form stars. Without a seed a galaxy cannot form. This is non-controversial.

There are a couple why reasons no one talks about it in the Green Pea papers:

1. because all galaxies have dark matter and this is not controversial, then investigating whether the GPs have dark matter is not that interesting.  (You may be shocked!)   The answer is yes and we should try to answer questions that we don’t know… that’s just how science works.

2. only in rare cases is it possible to directly measure the mass of the dark matter halo that hosts a galaxy.  And the Green Peas do not fall under this category of object.  We have other means that will tell us the dark matter mass for an average Green Pea should be, but we can never really figure this out for individual ones.

Finally, dark matter affects the properties of all galaxies. Because it is the largest source of mass in a galaxy, it is the main source of gravity.   For the Green Peas, one thing we want to think about is whether they are actually spewing gas away from their stars (from exploding supernovae), out into intergalactic space.  Whether they can do this or not will depend on gravity, because if it is strong then the gas will not be able to escape the regions nearest the galaxy. Ultimately, these “galactic outflows”  may relate to LyA and LyC, because they are changing the spatial distribution of gas in the galaxies. And LyA and LyC should be able to get out more easily if there are holes in the distribution of gas, and the Lya might also get out more easily if the gas is moving.   But it is an active area of research and there are no easily distilled answers.  The dark matter is always implicit, but we try to understand the physics by using stellar mass as a proxy for dark matter mass, because it is all we can do. (Stellar mass and dark matter mass do correlate, so this approach is not unreasonable.)

If you are interested in further reading, NASA’s “Ask an Astrophysicist” forum has a page on Dark Matter:

http://imagine.gsfc.nasa.gov/ask_astro/dark_matter.html

Also, the Hubble and Chandra images of M82 are really nice examples of outflows:

https://en.wikipedia.org/wiki/Messier_82