An early-type galaxy with an inner star-forming disk

TL;DR

This paper reports the discovery of an early-type galaxy with an inner star-forming disk, providing direct evidence of gas-rich merging as a formation process for low-mass ETGs, which is rare and transient.

Contribution

It presents the first direct observational evidence linking gas-rich merging to the formation of low-mass early-type galaxies.

Findings

Inner disk shows ongoing star formation.

Outer and inner regions have distinct kinematics.

Presence of tidal features indicating recent merging.

Abstract

Early-type galaxies (ETGs) are composed of two distinct populations: high-mass and low-mass, which are likely to be built via gas-poor merging and gas-rich merging/accretion, respectively. However, it is difficult to directly associate low-mass ETGs with gas-rich processes, because currently they are gas poor with no signs of ongoing star formation. We report a discovery of an ETG (SDSS J142055.01+400715.7) with Mstellar=10^10 Msun that offers direct evidence for gas-rich merging as the origin of low-mass ETGs. The integrated properties of the galaxy are consistent with a typical low-mass ETG, but the outer and inner regions show distinct dispersion- and rotation-dominated kinematics, respectively. There are some tidal features surrounding the galaxy. These two facts suggest very recent galaxy merging. Furthermore, the inner disk harbors on-going star formation, indicating the merging to be gas rich. This type of galaxy is rare but it may be a demonstration of the role the transient phase of gas-rich merging plays in making a low-mass ETG.