Automated measurement of redshift from mid-infrared low resolution
  spectroscopy
  We present a new SED-fitting based routine for redshift determination that is
optimised for mid-infrared (MIR) low-resolution spectroscopy. Its flexible
template scaling increases the sensitivity to slope changes and small scale
features in the spectrum, while a new selection algorithm called Maximum
Combined Pseudo-Likelihood (MCPL) provides increased accuracy and a lower
number of outliers compared to the standard maximum-likelihood (ML) approach.
Unlike ML, MCPL searches for local (instead of absolute) maxima of a
'pseudo-likelihood' (PL) function, and combines results obtained for all the
templates in the library to weed out spurious redshift solutions. The
capabilities of MCPL are demonstrated by comparing its results to those of
regular ML and to the optical spectroscopic redshifts of a sample of 491
Spitzer/IRS spectra from sources at 0<z<3.7. MCPL achieves a redshift accuracy
dz/(1+z)<0.005 for 78% of the galaxies in the sample compared to 68% for ML.
The rate of outliers (dz/(1+z)>0.02) is 14% for MCPL and 22% for ML. chi^2
values for ML solutions are found to correlate with the SNR of the spectra, but
not with redshift accuracy. By contrast, the peak value of the normalised
combined PL (gamma) is found to provide a good indication on the reliability of
the MCPL solution for individual sources. The accuracy and reliability of the
redshifts depends strongly on the MIR SED. Sources with significant polycyclic
aromatic hydrocarbon emission obtain much better results compared to sources
dominated by AGN continuum. Nevertheless, for a given gamma the frequency of
accurate solutions and outliers is largely independent on their SED type. This
reliability indicator for MCPL solutions allows to select subsamples with
highly reliable redshifts. In particular, a gamma>0.15 threshold retains 79% of
the sources with dz/(1+z)<0.005 while reducing the outlier rate to 3.8%
(abridged).
