SkyMapper SEDs of nearby galaxies: quenching and bursting probed by a change index for star formation

TL;DR

This paper introduces a new photometric diagnostic called the 'change index' to detect and analyze recent quenching or bursting of star formation in nearby galaxies using SkyMapper data, effective within 150 Mpc.

Contribution

The paper presents a novel, dust-insensitive photometric change index for identifying star formation rate changes, applicable to large galaxy samples with SkyMapper data.

Findings

The change index responds to star formation rate changes on 20-500 Myr timescales.

It is effective for galaxies up to 150 Mpc away.

The index provides qualitative insights into star formation activity.

Abstract

The wish list of astronomers includes a tool that reveals quenching of star formation in galaxies directly as it proceeds. Here, we present a proof-of-concept for a new quenching-and-bursting diagnostic, a "change index" for star formation, that requires only photometric data, provided they include filters such as the violet $uv$ bands used by SkyMapper. The index responds mostly to changes in star-formation rate on a timescale of 20 to 500 Myr and is nearly insensitive to dust extinction. It works effectively to distances of 100 to 150 Mpc. We explore its application to eight example galaxies in SkyMapper DR2, including known E+A and Seyfert-1 galaxies. Owing to the degeneracies inherent in broad-band photometry, the change index can only be a qualitative indicator of changes in star-formation rate. But once the SkyMapper Southern Survey is complete, the change index will be available for every spatial resolution element of every galaxy in the Southern sky within its working distance range.