2013ApJ...775...11P

In the absence of spectra, the technique of fitting model galaxy template spectra to observed photometric fluxes has become the workhorse method for determining the redshifts and other properties for high-z galaxy candidates. In this paper, we present an analysis of the most recent and possibly most distant galaxies (z ∼ 8-12) discovered in the Hubble Ultra Deep Field (HUDF) using a more robust method of redshift estimation based on Markov Chain Monte Carlo (MCMC) fitting, in contrast to the "best fit" models obtained using simpler χ² minimization techniques. The advantage of MCMC fitting is the ability to accurately estimate the probability density function of the redshift for each object as well as any input model parameters. This makes it possible to derive accurate, credible intervals by properly marginalizing over all other input model parameters. We apply our method to 13 recently identified sources in the HUDF and show that, despite claims based on χ² minimization, none of these sources can be securely ruled out as low redshift interlopers (z < 4) due to the low signal-to-noise of currently available observations. There is an average probability of 21% that these sources are low redshift interlopers.