Kavli Affiliate: Richard Kessler
| First 5 Authors: Ayan Mitra, Richard Kessler, Surhud More, Renee Hlozek, The LSST Dark Energy Science Collaboration
| Summary:
We perform a rigorous cosmology analysis on simulated type Ia supernovae
(SN~Ia) and evaluate the improvement from including photometric host-galaxy
redshifts compared to using only the "zspec" subset with spectroscopic
redshifts from the host or SN. We use the Deep Drilling Fields (~50 deg^2) from
the Photometric LSST Astronomical Time-Series Classification Challenge
(PLaSTiCC), in combination with a low-z sample based on Data Challenge2 (DC2).
The analysis includes light curve fitting to standardize the SN brightness, a
high-statistics simulation to obtain a bias-corrected Hubble diagram, a
statistical+systematics covariance matrix including calibration and photo-z
uncertainties, and cosmology fitting with a prior from the cosmic microwave
background. Compared to using the zspec subset, including events with SN+host
photo-z results in i) more precise distances for z>0.5, ii) a Hubble diagram
that extends 0.3 further in redshift, and iii) a 50 % increase in the Dark
Energy Task Force figure of merit (FoM) based on the w0-wa CDM model. Analyzing
25 simulated data samples, the average bias on w0 and wa is consistent with
zero. The host photo-z systematic of 0.01 reduces FoM by only 2 % because i)
most z<0.5 events are in the zspec subset, ii) the combined SN+host photo-z has
X 2 smaller bias, and iii) the anti-correlation between fitted redshift and
color self corrects distance errors. To prepare for analysing real data, the
next SNIa-cosmology analysis with photo-z’s should include non SN-Ia
contamination and host galaxy mis-associations.
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