How robustly can we constrain the low-mass end of the z ∼ 6-7 stellar mass function? The limits of lensing models and stellar population assumptions in the Hubble Frontier Fields

We present new measurements of the very low mass end of the galaxy stellar mass function (GSMF) at z similar to 6-7 computed from a rest-frame ultraviolet selected sample of dropout galaxies. These galaxies lie behind the six Hubble Frontier Field clusters and are all gravitationally magnified. Using deep Spitzer/IRAC and Hubble Space Telescope imaging, we derive stellar masses by fitting galaxy spectral energy distributions and explore the impact of different model assumptions and parameter degeneracies on the resulting GSMF. Our sample probes stellar masses down to M-star > 10(6)M(circle dot) and we find the z similar to 6-7 GSMF to be best parametrized by a modified Schechter function that allows for a turnover at very low masses. Using a Monte Carlo Markov chain analysis of the GSMF, including accurate treatment of lensing uncertainties, we obtain a relatively steep low-mass end slope alpha similar or equal to -1.96(-0.08)(+0.09) and a turnover at log(M-T/M-circle dot) similar or equal to 7.10(-0.56)(+0.17) with a curvature of beta similar or equal to 1.00(-0.73)(+0.87) for our minimum assumption model with constant star formation history (SFH) and low dust attenuation, A(V) <= 0.2. We find that the z similar to 6-7 GSMF, in particular its very low mass end, is significantly affected by the assumed functional form of the star formation history and the degeneracy between stellar mass and dust attenuation. For example, the low-mass end slope ranges from alpha similar to -1.82(-0.07)(+0.08) for an exponentially rising SFH to alpha similar to -2.34(-0.10)(+0.11) when allowing A(V) of up to 3.25. Future observations at longer wavelengths and higher angular resolution with the James Webb Space Telescope are required to break these degeneracies and to robustly constrain the stellar mass of galaxies on the extreme low-mass end of the GSMF.

Automated summary

New measurements of the very low mass end of the galaxy stellar mass function (GSMF) at z ~ 6-7 were presented, using a rest-frame ultraviolet selected sample of dropout galaxies. The study found that the z ~ 6-7 GSMF was best described by a modified Schechter function with a turnover at very low masses, and that the assumed functional form of the star formation history and the degeneracy between stellar mass and dust attenuation significantly impacted the results. Further observations with the James Webb Space Telescope are needed to break these degeneracies and better constrain the stellar mass of galaxies on the extreme low-mass end of the GSMF.