Speaker
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 biases. In future high-statistics time-domain surveys, such as TiDES and LSST-era spectroscopic follow-up 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.
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 merely reduce the total number of recovered redshifts; it systematically skews the recovered host-galaxy population towards star-forming systems. We discuss the implications for future surveys using supernovae to constrain dark energy.