  Recent controversy regarding the existence of massive (log(M/Msun)>11)
galaxies at z>6 poses a challenge for galaxy formation theories. Hence, it is
of critical importance to understand the effects of SED fitting methods on
stellar mass estimates of Epoch of Reionization galaxies. In this work, we
perform a case study on the AGN host galaxy candidate COS-87259 with
spectroscopic redshift z=6.853, that is claimed to have an extremely high
stellar mass of log(M/Msun)~11.2. We test a suite of different SED fitting
algorithms and stellar population models on our independently measured
photometry in 17 broad bands for this source. Between five different code
setups, the stellar mass estimates for COS-87259 span log(M/Msun)=10.24-11.00,
whilst the reduced chi-squared values of the fits are all close to unity within
dchi2=1.2, so that the quality of the SED fits is basically indistinguishable.
Only when we adopt a nonparametric star formation history model within
Prospector do we retrieve a stellar mass exceeding log(M/Msun)=11. Although the
derived stellar masses change when using previously reported photometry for
this source, the nonparametric SED-fitting method always yields the highest
values. As these models are becoming increasingly popular for James Webb Space
Telescope high-redshift science, we stress the absolute importance of testing
various SED fitting routines particularly on apparently very massive galaxies
at such high redshifts.
