First Cosmology Results using Type Ia Supernovae from the Dark Energy Survey: The Effect of Host Galaxy Properties on Supernova Luminosity

Author(s): Smith, M; Sullivan, M; Wiseman, P; Kessler, R; Scolnic, D; Brout, D; D'Andrea, CB; Davis, TM; Foley, RJ; Frohmaier, C; Galbany, L; Gupta, RR; Gutierrez, CP; Hinton, SR; Kelsey, L; Lidman, C; Macaulay, E; Moller, A; Nichol, RC; Nugent, P; Palmese, A; Pursiainen, M; Sako, M; Thomas, RC; Tucker, BE; Carollo, D; Lewis, GF; Sommer, NE; Abbott, TMC; Aguena, M; Allam, S; Avila, S; Bertin, E; Bhargava, S; Brooks, D; Buckley-Geer, E; Burke, DL; Rosell, A Carnero; Kind, M Carrasco; Costanzi, M; Costa, LN da; Vicente, J De; Desai, S; Diehl, HT; Doel, P; Eifler, TF; Everett, S; Flaugher, B; Fosalba, P; Frieman, J; Garcia-Bellido, J; Gaztanaga, E; Glazebrook, K; Gruen, D; Gruendl, RA; Gschwend, J; Gutierrez, G; Hartley, WG; Hollowood, DL; Honscheid, K; James, DJ; Krause, E; Kuehn, K; Kuropatkin, N; Lima, M; MacCrann, N; Maia, MAG; Marshall, JL; Martini, P; Melchior, P; Menanteau, F; Miquel, R; Paz-Chinchon, F; Plazas, AA; Romer, AK; Roodman, A; Rykoff, ES; Sanchez, E; Scarpine, V; Schubnell, M; Serrano, S; Sevilla-Noarbe, I; Suchyta, E; Swanson, MEC; Tarle, G | Abstract: We present improved photometric measurements for the host galaxies of 206 spectroscopically confirmed type Ia supernovae discovered by the Dark Energy Survey Supernova Program (DES-SN) and used in the first DES-SN cosmological analysis. Fitting spectral energy distributions to the $griz$ photometric measurements of the DES-SN host galaxies, we derive stellar masses and star-formation rates. For the DES-SN sample, when considering a 5D ($z$, $x_1$, $c$, $\alpha$, $\beta$) bias correction, we find evidence of a Hubble residual `mass step', where SNe Ia in high mass galaxies ($g10^{10} \textrm{M}_{\odot}$) are intrinsically more luminous (after correction) than their low mass counterparts by $\gamma=0.040\pm0.019$mag. This value is larger by $0.031$mag than the value found in the first DES-SN cosmological analysis. This difference is due to a combination of updated photometric measurements and improved star formation histories and is not from host-galaxy misidentification. When using a 1D (redshift-only) bias correction the inferred mass step is larger, with $\gamma=0.066\pm0.020$mag. The 1D-5D $\gamma$ difference for DES-SN is $0.026\pm0.009$mag. We show that this difference is due to a strong correlation between host galaxy stellar mass and the $x_1$ component of the 5D distance-bias correction. To better understand this effect, we include an intrinsic correlation between light-curve width and stellar mass in simulated SN Ia samples. We show that a 5D fit recovers $\gamma$ with $-9$mmag bias compared to a $+2$mmag bias for a 1D fit. This difference can explain part of the discrepancy seen in the data. Improvements in modeling correlations between galaxy properties and SN is necessary to determine the implications for $\gamma$ and ensure unbiased precision estimates of the dark energy equation-of-state as we enter the era of LSST.