On How to Extend the NIR Tully-Fisher Relation to be Truly All-Sky

Abstract

Dust extinction and stellar confusion by the Milky Way reduce the efficiency of detecting galaxies at low Galactic latitudes, creating the so-called Zone of Avoidance. This stands as a stumbling block in charting the distribution of galaxies and cosmic flow fields, and therewith our understanding of the local dynamics in the Universe (CMB dipole, convergence radius of bulk flows). For instance, ZoA galaxies are generally excluded from the whole-sky Tully-Fisher Surveys ($|b| \leq 5^\circ$) even if catalogued. We show here that by fine-tuning the near-infrared TF relation, there is no reason not to extend peculiar velocity surveys deeper into the ZoA. Accurate axial ratios ($b/a$) are crucial to both the TF sample selection and the resulting TF distances. We simulate the effect of dust extinction on the geometrical properties of galaxies. As expected, galaxies appear rounder with increasing obscuration level, even affecting existing TF samples. We derive correction models and demonstrate that we can reliably reproduce the intrinsic axial ratio from the observed value up to extinction level of about $A_J\simeq3$ mag ($A_V\sim11$ mag), we also recover a fair fraction of galaxies that otherwise would fall out of an uncorrected inclination limited galaxy sample. We present a re-calibration of the 2MTF relation in the NIR $J$, $H$, and $K_s$-bands for isophotal rather than total magnitudes, using their same calibration sample. Both TF relations exhibit similar scatter at high Galactic latitudes. However, the isophotal TF relation results in a significant improvement in the scatter for galaxies in the ZoA, and low surface brightness galaxies in general, because isophotal apertures are more robust in the face of significant stellar confusion.