  We tested the performance of photometric redshifts for galaxies in the Hubble
Ultra Deep field down to 30th magnitude. We compared photometric redshift
estimates from three spectral fitting codes from the literature (EAZY, BPZ and
BEAGLE) to high quality redshifts for 1227 galaxies from the MUSE integral
field spectrograph. All these codes can return photometric redshifts with bias
|Dzn|=|z-z_phot|/(1+z)<0.05 down to F775W=30 and spectroscopic incompleteness
is unlikely to strongly modify this statement. We have, however, identified
clear systematic biases in the determination of photometric redshifts: in the
0.4<z<1.5 range, photometric redshifts are systematically biased low by as much
as Dzn=-0.04 in the median, and at z>3 they are systematically biased high by
up to Dzn = 0.05, an offset that can in part be explained by adjusting the
amount of intergalactic absorption applied. In agreement with previous studies
we find little difference in the performance of the different codes, but in
contrast to those we find that adding extensive ground-based and IRAC
photometry actually can worsen photo-z performance for faint galaxies. We find
an outlier fraction, defined through |Dzn|>0.15, of 8% for BPZ and 10% for EAZY
and BEAGLE, and show explicitly that this is a strong function of magnitude.
While this outlier fraction is high relative to numbers presented in the
literature for brighter galaxies, they are very comparable to literature
results when the depth of the data is taken into account. Finally, we
demonstrate that while a redshift might be of high confidence, the association
of a spectrum to the photometric object can be very uncertain and lead to a
contamination of a few percent in spectroscopic training samples that do not
show up as catastrophic outliers, a problem that must be tackled in order to
have sufficiently accurate photometric redshifts for future cosmological
surveys.
