North+Lone Star Supernova Host Survey I: Local Host-Galaxy H${\alpha}$ Surface Brightness and the Hubble Residuals of Type Ia Supernovae

Abstract

We present optical integral-field unit (IFU) spectroscopy acquired with the George and Cynthia Mitchell Spectrograph on the Harlan J. Smith telescope at McDonald Observatory of 94 galaxies (0.01 < z < 0.058) that have hosted Type Ia supernovae (SNe Ia). We selected host galaxies with star-forming morphology, consistent with the criteria used by Riess et al. (2022). We measured the H${\alpha}$ surface brightness of each host galaxy within 1 kpc of the location of the supernova. Using distances from the Pantheon+ sample, we find a step in Hubble residuals compared to local H${\alpha}$ surface brightness of -0.097 $\pm$ 0.051 mag at 1.9${\sigma}$ significance in a sample of 73 host galaxies, where SNe in environments with smaller H${\alpha}$ surface brightness are, on average, less luminous after correction for light-curve shape and color. Almost all of the SNe in our sample were discovered by targeted surveys. Using an independent sample primarily from the untargeted Nearby Supernova Factory survey, Rigault et al. (2020) found a step of 0.045 $\pm$ 0.029 mag where SNe in passive environments are instead brighter, which is in 2.4${\sigma}$ tension with our measurement. Rigault et al. (2013) designated SNe Ia comparatively small HRs (< -0.1) and faint local H${\alpha}$ surface brightness (SB) (<log10(H${\alpha}$ SB/(erg$^{-1}$s$^{-1}$kpc$^2$))=38.32 as the M$_2$ population. SNe that would be classified as M$_2$ are less highly represented in our sample (7% versus 21%). When we include an additional twelve early-type galaxies, the number of M$_2$ SNe is almost doubled, although the tension with the HR step measured by Rigault et al. (2020) persists at 1.7${\sigma}$.