Should Type Ia Supernova Distances be Corrected for their Local Environments?

TL;DR

This study investigates whether Type Ia supernova distances are more tightly linked to the local environment (within 1.5–3 kpc) of the explosion than to global host properties or random host regions. Using 273 low-redshift SNe Ia from Pantheon and Foundation, the authors measure local and global host stellar mass and rest-frame $u-g$ color via SED fitting, and quantify distance residual steps via a two-Gaussian likelihood. They find a significant local mass step ($0.056\pm0.017$ mag) after correcting for the global mass step ($0.058\pm0.018$ mag), while the local color step is comparable to the global color step; a residual local color signal is less robust. Random-aperture tests suggest locality is not fully mimicked by random regions, but the evidence for locality being intrinsically superior to random remains marginal. The estimated impact on the Hubble constant from local environment corrections is small but non-negligible ($\Delta H_0 \approx -0.14\pm0.14$ km s$^{-1}$ Mpc$^{-1}$), highlighting the need to understand survey selection effects and to assess locality in future SN cosmology analyses.

Abstract

Recent analyses suggest that distance residuals measured from Type Ia supernovae (SNe Ia) are correlated with local host galaxy properties within a few kpc of the SN explosion. However, the well-established correlation with global host galaxy properties is nearly as significant, with a shift of 0.06 mag across a low to high mass boundary (the mass step). Here, with 273 SNe Ia at $z<0.1$, we investigate whether stellar masses and rest-frame $u-g$ colors of regions within 1.5 kpc of the SN Ia explosion site are significantly better correlated with SN distance measurements than global properties or properties measured at random locations in SN hosts. At $\lesssim2σ$ significance, local properties tend to correlate with distance residuals better than properties at random locations, though despite using the largest low-$z$ sample to date we cannot definitively prove that a local correlation is more significant than a random correlation. Our data hint that SNe observed by surveys that do not target a pre-selected set of galaxies may have a larger local mass step than SNe from surveys that do, an increase of $0.071\pm0.036$ mag (2.0$σ$). We find a $3σ$ local mass step after global mass correction, evidence that SNe Ia should be corrected for their local mass, but we note that this effect is insignificant in the targeted low-$z$ sample. Only the local mass step remains significant at $>2σ$ after global mass correction, and we conservatively estimate a systematic shift in H$_0$ measurements of -0.14 $\textrm{km}\,\textrm{s}^{-1}\textrm{Mpc}^{-1}$ with an additional uncertainty of 0.14 $\textrm{km}\,\textrm{s}^{-1}\textrm{Mpc}^{-1}$, $\sim$10\% of the present uncertainty.

Figures (7)

• Figure 1: Local mass density and $u - g$ maps from four representative galaxies in our sample. The local mass and colors used in this work are measured from the 3 kpc diameter regions indicated by the small circles. For illustration, the local mass density is computed per pixel and has a median value of log(M$_{\ast}$/M$_{\odot})$ - log(Area) $\sim 8$ kpc$^{-2}$. To include regions of negative flux in the map, which have an undefined color measurement, the bottom row shows the probability that the true $u - g$ color is $< 1.6$ mag (the median observed color of this sample). The approximate $R=3$ isophotal radius of each galaxy is denoted by the ellipses. White colors in the map indicate regions on the border between locally high-mass and low-mass and blue $u - g$/red $u - g$ (and may also indicate pixels with higher than average noise). For the purposes of this plot, we use observer-frame $u - g$ colors that have not been corrected for host galaxy reddening and Equation 8 from Taylor11 to approximate the host galaxy mass using the observed $gi$ photometry.
• Figure 2: The dependence of SN luminosities on the mass and $u-g$ color within 1.5 kpc of the SN location. Colors indicate the probability that a SN is in a low-mass host galaxy (left) or a galaxy with blue rest-frame color (right). We see $\gtrsim 2 \sigma$ correlations with both quantities. The gap in rest-frame $u-g$ colors at $\sim$1.3 mag is likely due to a gap in the colors of the PEGASE.2 SED templates, the green valley between star-forming and passive host galaxies.
• Figure 3: The effect of bias corrections on the measured host galaxy steps. In particular, the size of the local color step is 27% larger if the necessary bias corrections are neglected, because SN shape and color are functions of host galaxy $u - g$.
• Figure 4: Local versus global information for each SN Ia in the sample, where dashed lines indicate the median mass and color along with the points where local measurements equal global measurements. The median Hubble residual for each quadrant is labeled, and shows significant departures from 0 only where local and global agree. Instead of the local and global masses used in this work, we show the mass density, log(M$_{\ast}$/M$_{\odot}$) - log(Area), where the units of area are kpc and the global mass density is averaged over the total area within the isophotal radius ($R = 3$) of each galaxy.
• Figure 5: The dependence of nuisance parameters $\alpha$ and $\beta$ on host mass and color. Interestingly, $\alpha$ is measured to be higher in locally/globally red or high-mass hosts. $\beta$ is higher (nearer to the Milky Way value) for SNe that occurred in redder regions of their host galaxies, likely due to dust effects.