Break in stellar masses at z~0.27

[ameliesaintonge]

I just noticed a couple of "features" in the stellar mass-redshift distribution, based on the fastspec output for Iron. There is a clear break at z~0.27, and narrow plateau at z>0.45, both of which almost certainly are not physical:

This figure is made using the Z and LOGMSTAR columns of this file: '/global/cfs/cdirs/desi/spectro/fastspecfit/iron/v1.0/catalogs/fastspec-iron.fits'

[moustakas]

This "feature" is due to an intentional fitting prior in FastSpecFit which removes SPS models which are older than the age of the universe at the redshift of each galaxy (+a pad of 500 Myr). Although physically motivated, this choice clearly introduces an unwanted redshift-dependence.

I've rerun fastphot using the https://github.com/desihub/fastspecfit/tree/no-age-prior branch where this prior is removed and things look much better:

And here's an updated comparison with an independent stellar mass catalog generated for the same set of BGS targets:

[moustakas]

Fixed in #132. Some additional analysis figures can be reviewed in this notebook--
https://github.com/moustakas/fastspecfit-projects/blob/main/mstarchallenge/mstarchallenge-bug.ipynb