M31 halo paper - question on metallicity

[jim mcnamara]

http://iopscience.iop.org/0004-637X/804/2/79/pdf/0004-637X_804_2_79.pdf
The report finds evidence for a halo of ionized gas and small amounts of other heavier elements, in a very large halo around the Andromeda galaxy. The report estimates the mass of the CGM halo's metal component at 2 million solar masses.

Ok.

So here are some ways to explain how those "extra" heavier elements got out there:
created by supernovae in situ
expulsion from supernovae closer to home
hypervelocity stars(?)
remnants of galactic collisions

I am aware of one report (cannot get the citation) on the presence of rogue hypervelocity stars far from the center of mass of our own Milky Way. These could be larger staller mass objects, I do not know.

Most references - like Wikipedia - cite models with Population I stars making up most of the sparse stellar population out there. So, what hypothesis/explanation/data accounts for those heavy elements?

Heavy elements are derived from supernovae, which AFAIK does not appear to be happening out there right now.

Thanks for any corrections.

 

[Greg Bernhardt]

Which explanation do you think is most likely?