Recent observations have shown that the characteristic luminosity of the rest-frame ultraviolet ( UV ) luminosity function does not significantly evolve at 4 < z < 7 and is approximately M ^ { \ast } _ { \mathrm { UV } } \sim - 21 .
We investigate this apparent non-evolution by examining a sample of 178 bright , M _ { \mathrm { UV } } < - 21 galaxies at z = 4 to 7 , analyzing their stellar populations and host halo masses .
Including deep Spitzer /IRAC imaging to constrain the rest-frame optical light , we find that M ^ { \ast } _ { \mathrm { UV } } galaxies at z = 4–7 have similar stellar masses of \log ( M / M _ { \odot } ) = 9.6–9.9 and are thus relatively massive for these high redshifts .
However , bright galaxies at z = 4–7 are less massive and have younger inferred ages than similarly bright galaxies at z = 2–3 , even though the two populations have similar star formation rates and levels of dust attenuation for a fixed dust-attenuation curve .
Matching the abundances of these bright z = 4–7 galaxies to halo mass functions from the Bolshoi \Lambda CDM simulation implies that the typical halo masses in \sim M ^ { \ast } _ { UV } galaxies decrease from \log ( M _ { h } / M _ { \odot } ) = 11.9 at z = 4 to \log ( M _ { h } / M _ { \odot } ) = 11.4 at z = 7 .
Thus , although we are studying galaxies at a similar mass across multiple redshifts , these galaxies live in lower mass halos at higher redshift .
The stellar baryon fraction in units of the cosmic mean \Omega _ { b } / \Omega _ { m } rises from 5.1 % at z = 4 to 11.7 % at z = 7 ; this evolution is significant at the \sim 3 \sigma level .
This rise does not agree with simple expectations of how galaxies grow , and implies that some effect , perhaps a diminishing efficiency of feedback , is allowing a higher fraction of available baryons to be converted into stars at high redshifts .