A New View of the Size-Mass Distribution of Galaxies: Using r 20 and r 80 
Instead of r50

Miller, Tim B.; van Dokkum, Pieter; Mowla, Lamiya; van der Wel, Arjen

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A New View of the Size-Mass Distribution of Galaxies: Using r ₂₀ and r ₈₀ Instead of r ₅₀

MPS-Authors

Miller, Tim B.
Max Planck Institute for Astronomy, Max Planck Society and Cooperation Partners;

van Dokkum, Pieter
Max Planck Institute for Astronomy, Max Planck Society and Cooperation Partners;

Mowla, Lamiya
Max Planck Institute for Astronomy, Max Planck Society and Cooperation Partners;

van der Wel, Arjen
Max Planck Institute for Astronomy, Max Planck Society and Cooperation Partners;

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https://ui.adsabs.harvard.edu/abs/2019ApJ...872L..14M
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Citation

Miller, T. B., van Dokkum, P., Mowla, L., & van der Wel, A. (2019). A New View of the Size-Mass Distribution of Galaxies: Using r ₂₀ and r ₈₀ Instead of r ₅₀. The Astrophysical Journal, 872.

Cite as: https://hdl.handle.net/21.11116/0000-0005-D060-3

Abstract

When investigating the sizes of galaxies it is standard practice to use the half-light radius, r ₅₀. Here we explore the effects of the size definition on the distribution of galaxies in the size-stellar mass plane. Specifically, we consider r ₂₀ and r ₈₀, the radii that contain 20% and 80% of a galaxy’s total luminosity, as determined from a Sérsic profile fit, for galaxies in the 3D-HST/CANDELS and COSMOS-DASH surveys. These radii are calculated from size catalogs based on a simple calculation assuming a Sérsic profile. We find that the size-mass distributions for r ₂₀ and r ₈₀ are markedly different from each other and also from the canonical r ₅₀ distribution. The most striking difference is in the relative sizes of star-forming and quiescent galaxies at fixed stellar mass. Whereas quiescent galaxies are smaller than star-forming galaxies in r ₅₀, this difference nearly vanishes for r ₈₀. By contrast, the distance between the two populations increases for r ₂₀. Considering all galaxies in a given stellar mass and redshift bin we detect a significant bimodality in the distribution of r ₂₀, with one peak corresponding to star- forming galaxies and the other to quiescent galaxies. We suggest that different measures of the size are tracing different physical processes within galaxies; r ₂₀ is closely related to processes controlling the star formation rate of galaxies and r ₈₀ may be sensitive to accretion processes and the relation of galaxies with their halos.