26th Australian Institute of Physics Congress and the Australian and New Zealand Conference on Optics and Photonics

From OzDES to TiDES: host-dependent redshift recovery in supernova cosmology

Not scheduled

20m

Belinda Hutchinson Building (The University of Sydney )

Contributed Oral AIP | Astronomy (ASTRO)

Speaker

Mr Alan Chan (Mount Stromlo Observatory, The Australian National University)

Description

Type Ia supernovae remain one of the key observational probes of dark energy, but their cosmological constraining power depends on constructing Hubble diagrams with well-understood observational biases. In future high-statistics time-domain surveys, including TiDES and Rubin/LSST-era supernova programmes, host-galaxy redshifts will often need to be obtained under finite spectroscopic resources. It is therefore important to understand whether shallow host-galaxy follow-up recovers all galaxy populations equally, and whether missed hosts could affect supernova cosmology analyses.

Using OzDES DR2 spectra together with DES-SN5YR host-galaxy data, we construct shallow three-exposure spectra for 1,280 supernova host galaxies and test redshift recovery using MARZ. We find a strong host-dependent selection effect: star-forming hosts are recovered with 35.9% completeness, while passive hosts are recovered with only 16.4% completeness. This difference arises because narrow nebular emission lines remain identifiable at low signal-to-noise, whereas passive galaxies rely primarily on weaker continuum and absorption features.

These results show that shallow spectroscopic follow-up does not simply reduce the number of recovered redshifts; it systematically changes the recovered host-galaxy population. We are extending this analysis using PIPPIN and SNANA by simulating DES-like surveys and comparing bias corrections derived from an average selection function against those derived from host-type-dependent selection functions. This connects the observed redshift-recovery efficiencies directly to the bias-correction stage used in supernova cosmology, making the work both an OzDES measurement and a practical test case for future TiDES-style follow-up strategies. This will allow us to test whether host-dependent redshift incompleteness changes the recovered distance-redshift relation for DES-like samples. More broadly, this work provides a data-driven framework for identifying and modelling spectroscopic selection effects in next-generation supernova dark-energy measurements.