Impact of photometric redshifts on the galaxy power spectrum and BAO
  scale in the LSST survey
  Imaging billions of galaxies every few nights during ten years, LSST should
be a major contributor to precision cosmology in the 2020 decade. High
precision photometric data will be available in six bands, from near-infrared
to near-ultraviolet. The computation of precise, unbiased, photometric
redshifts up to z = 2, at least, is one of the main LSST challenges and its
performance will have major impact on all extragalactic LSST sciences. We
evaluate the efficiency of our photometric redshift reconstruction on mock
galaxy catalogs up to z=2.45 and estimate the impact of realistic photometric
redshift (hereafter photo-z) reconstruction on the large-scale structures (LSS)
power spectrum and the baryonic acoustic oscillation (BAO) scale determination
for a LSST-like photometric survey. We study the effectiveness of the BAO scale
as a cosmological probe in the LSST survey. We have performed a detailed
modelling of the photo-z distribution as a function of galaxy type, redshift
and absolute magnitude using our photo-z reconstruction code with a quality
selection cut based on a Boosted decision tree (BDT). We have computed the
fractional error on the recovered power spectrum which is dominated by the
shot-noise at z>1 for scales k>0.1, due to the photo-z damping. The BAO scale
can be recovered with a percent or better accuracy level from z = 0.5 to z =
1.5 using realistic photo-z reconstruction. Outliers can represent a
significant fraction of galaxies at z>2, causing bias and errors on LSS power
spectrum measurement. Although the BAO scale is not the most powerful
cosmological probe in LSST, it can be used to check the consistency of the LSS
measurement. Moreover we show that the impact of photo-z smearing on the
recovered isotropic BAO scale in LSST should stay limited up to z=1.5, so as
long as the galaxy number density balances the photo-z smoothing.
