Photometric vs dynamical stellar masses and their impact on scaling relations in nearby disc galaxies

Abstract: The study of scaling relations of disc galaxies and their evolution across cosmic time requires accurate estimates of galaxy stellar masses $M_\star$ over broad redshift ranges. While photometric $M_\star$ estimates ($M_{\rm phot}$) based on spectral energy distribution (SED) modelling methods are employed routinely at high-$z$, it is unclear to what extent these are compatible with dynamical $M_\star$ estimates ($M_{\rm dyn}$), available for nearby galaxies. Here we compare newly determined, SED-model based $M_{\rm phot}$ with previously obtained $M_{\rm dyn}$ inferred via rotation curve decomposition techniques in a sample of $\sim100$ nearby galaxies from the SPARC database. We find that the two mass estimates show a systematic agreement at the $\sim12\%$ ($0.05$ dex) level and a $\sim55\%$ ($0.22$ dex) scatter across almost $5$ dex in $M_\star$. Our $M_{\rm phot}$ estimates correspond to mass-to-light ratios in the $3.6\mu$m band that increase gradually with $3.6\mu$m luminosity, as a consequence of the earlier (later) assembly history of high-mass (low-mass) disc galaxies. The choice of using either $M_{\rm dyn}$ or $M_{\rm phot}$ has only a marginal impact on the slope and zero-point of the Tully-Fisher and Fall relations: the observed orthogonal scatter in both relations is virtually the same for the two methods, and indistinguishable from that derived using a constant mass-to-light ratio in the $3.6\mu$m band. $M_\star$ estimates based on the assumption that discs are marginally stable lead to the largest scatter in the scaling relations.