Tracing the Intrinsic Shapes of Dwarf Galaxies out to Four Effective Radii: Clues to Low-Mass Stellar Halo Formation

TL;DR

This study measures the shapes of dwarf galaxies out to four effective radii, revealing a trend towards spheroidal forms at larger radii, supporting in-situ formation models driven by star formation feedback.

Contribution

It provides the first detailed shape analysis of dwarf galaxies out to four effective radii, demonstrating a universal trend towards spheroidal outskirts regardless of galaxy type or environment.

Findings

Dwarfs are slightly triaxial with a >~ 0.75.

Galaxies become more spheroidal at larger radii.

Both blue and red dwarfs show increasing spheroidality with radius.

Abstract

Though smooth, extended spheroidal stellar outskirts have long been observed around nearby dwarf galaxies, it is unclear whether dwarfs generically host an extended stellar halo. We use imaging from the Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP) to measure the shapes of dwarf galaxies out to four effective radii for a sample of dwarfs at 0.005<z<0.2 and 10^7<M_star/M_sun<10^9.6. We find that dwarfs are slightly triaxial, with a <B/A> >~ 0.75 (where the ellipsoid is characterized by three principle semi-axes constrained by C<=B<=A). At M_star>10^8.5 M_sun, the galaxies grow from thick disk-like near their centers towards the spheroidal extreme at four effective radii. We also see that although blue dwarfs are, on average, characterized by thinner discs than red dwarfs, both blue and red dwarfs grow more spheroidal as a function of radius. This relation also holds true for a comparison between field and satellite dwarfs. This uniform trend towards relatively spheroidal shapes as a function of radius is consistent with an in-situ formation mechanism for stellar outskirts around low-mass galaxies, in agreement with proposed models where star formation feedback produces round stellar outskirts around dwarfs.