Are passive red spirals truly passive? - The current star formation activity of optically-red disc galaxies

TL;DR

This study reveals that optically-red spiral galaxies, previously thought to be passive, are actively forming stars at rates comparable to normal star-forming galaxies, challenging the use of optical color alone to identify passive systems.

Contribution

It demonstrates that optical red colors do not reliably indicate quiescence, showing that high-mass red spirals can be actively star-forming based on UV and infrared data.

Findings

Red spirals have significant star formation rates (>= 1 M_sun/yr).

Red spirals follow the same star formation relations as normal star-forming galaxies.

Optical colors alone cannot distinguish passive from active star-forming galaxies at high stellar masses.

Abstract

We use GALEX ultraviolet and WISE 22 micron observations to investigate the current star formation activity of the optically-red spirals recently identified as part of the Galaxy Zoo project. These galaxies were accurately selected from the Sloan Digital Sky Survey in order to be pure discs with low or no current star formation activity, representing one of the best optically-selected samples of candidate passive spirals. However, we show that these galaxies are not only still forming stars at a significant rate >= 1 M_sun/yr but, more importantly, their star formation activity is not different from that of normal star-forming discs of the same stellar mass (M* >= 10^10.2 M_sun). Indeed, these systems lie on the UV-optical blue sequence, even without any corrections for internal dust attenuation, and they follow the same specific star formation rate vs. stellar mass relation of star-forming galaxies. Our findings clearly show that, at high stellar masses, optical colours do not allow to discriminate between actively star-forming and truly quiescent systems.