Publishing a paper that is not related your field
My honest advise is to try to publish it.
Your work speaks for yourself. 
It means you were dedicated, motivated and got some results. 
If it is not some kind of 2 years writing thing, I would do it.
All your work represents you and your education and motivation, and I would always chose to work with someone who showed passion for some area, even if it is not his major.

P.S. In my country, mathematics and astrophysics are ojd the same faculty. They are connected.