A historical perspective on the concept of galaxy size

TL;DR

This paper reviews the historical development of galaxy size measures and proposes a new physically motivated size definition based on star formation thresholds, which reduces scatter in size-mass relations.

Contribution

It introduces a new galaxy size measure grounded in astrophysical theory, improving the precision of size-stellar mass relations.

Findings

New size measure reduces scatter in size-mass relation to 0.06 dex

Historical measures were based solely on observational premises

Proposed measure is suitable for future deep imaging surveys

Abstract

A brief narrative on how the effective radius and isophotal diameters were accepted as galaxy size measures is presented. Evidence suggests that these parameters were defined only based on observational premises, independent of any astrophysical theories. An alternative, new physically motivated size definition based on the expected gas density threshold required for star formation in galaxies is discussed. The intrinsic scatter of the size-stellar mass relation using the new size measure is 0.06 dex, three times smaller than that of the relation with the effective radius as size. The new physically motivated size measure can be adopted in upcoming deep, wide imaging surveys.