The effect of foreground galaxies on the estimation of the Hubble constant from type Ia supernovae

Context. Type Ia supernovae are the established standard candle in the construction of the Hubble diagram out to high-luminosity distances. The Hubble constant that fits observations of these supernovae best often turns out to be higher than fits to other data, which are therefore currently investigated for possible systematic effects. Many studies focused on the calibration of supernova measurements either through the distance ladder in the local Universe or through large-scale anchors such as baryon acoustic oscillations or distant clusters. These different calibration methods yield different values for the Hubble constant, which means that calibration is a crucial step in the analysis of supernova measurements. Aims. We present a simulation-based assessment of another type of systematic effect that is related to the possibility that the line of sight to a distant supernova passes close to a foreground galaxy. Methods. We considered two cases separately: First, the foreground galaxy may block the line of sight so that the supernova is not observed. Since foreground galaxies are correlated with overdensities that typically magnify the flux of background sources, this effect systematically removes lensed supernovae from the sample and biases the high-redshift Hubble diagram towards demagnified (fainter) supernovae. Second, if the supernova can be observed, its proximity to the foreground galaxy can lead to an incorrect host assignment, especially when the true host has a low surface brightness. Since foreground galaxies are typically found at lower redshifts, this effect introduces another systematic bias. The probability of line-of-sight alignments with foreground galaxies increases with redshift and therefore affects distant supernovae more strongly. Results. We find that these two effects are both weak, but the effect of host misidentification should be included in the systematic error budget at the current levels of measurement precision.