PHAT: PHoto-z accuracy testing
Absztrakt:
Context. Photometric redshifts (photo-z's) have become an
essential tool in extragalactic astronomy. Many current and
upcoming observing programmes require great accuracy of photo-
z's to reach their scientific goals. Aims. Here we introduce
PHAT, the PHoto-z Accuracy Testing programme, an international
initiative to test and compare different methods of photo-z
estimation. Methods. Two different test environments are set up,
one (PHAT0) based on simulations to test the basic functionality
of the different photo-z codes, and another one (PHAT1) based on
data from the GOODS survey including 18-band photometry and
∼2000 spectroscopic redshifts. Results. The accuracy of the
different methods is expressed and ranked by the global photo-z
bias, scatter, and outlier rates. While most methods agree very
well on PHAT0 there are differences in the handling of the
Lyman-α forest for higher redshifts. Furthermore, different
methods produce photo-z scatters that can differ by up to a
factor of two even in this idealised case. A larger spread in
accuracy is found for PHAT1. Few methods benefit from the
addition of mid-IR photometry. The accuracy of the other methods
is unaffected or suffers when IRAC data are included. Remaining
biases and systematic effects can be explained by shortcomings
in the different template sets (especially in the mid-IR) and
the use of priors on the one hand and an insufficient training
set on the other hand. Some strategies to overcome these
problems are identified by comparing the methods in detail.
Scatters of 4-8% in Δz/(1+z) were obtained, consistent with
other studies. However, somewhat larger outlier rates (>7.5%
with Δz/(1+z)>0.15; >4.5% after cleaning) are found for all
codes that can only partly be explained by AGN or issues in the
photometry or the spec-z catalogue. Some outliers were probably
missed in comparisons of photo-z's to other, less complete
spectroscopic surveys in the past. There is a general trend that
empirical codes produce smaller biases than template-based
codes. Conclusions. The systematic, quantitative comparison of
different photo-z codes presented here is a snapshot of the
current state-of-the-art of photo-z estimation and sets a
standard for the assessment of photo-z accuracy in the future.
The rather large outlier rates reported here for PHAT1 on real
data should be investigated further since they are most probably
also present (and possibly hidden) in many other studies. The
test data sets are publicly available and can be used to compare
new, upcoming methods to established ones and help in guiding
future photo-z method development. © 2010 ESO.