Delayed Stellar Mass Assembly in the Low Surface Brightness Dwarf Galaxy KDG 215

Abstract

We present H i spectral line and optical broadband images of the nearby low surface brightness dwarf galaxy KDG 215. The H $I$ images, acquired with the Karl G. Jansky Very Large Array, reveal a dispersion-dominated interstellar medium with only weak signatures of coherent rotation. The H $I$ gas reaches a peak mass surface density of 6 M ⊙ pc$^{−2}$ at the location of the peak surface brightness in the optical and the ultraviolet. Although KDG 215 is gas-rich, the Hα non-detection implies a very low current massive star formation rate. In order to investigate the recent evolution of this system, we have derived the recent and lifetime star formation histories from archival Hubble Space Telescope images. The recent star formation history shows a peak star formation rate ~1 Gyr ago, followed by a decreasing star formation rate to the present day quiescent state. The cumulative star formation history indicates that a significant fraction of the stellar mass assembly in KDG 215 has occurred within the last 1.25 Gyr. KDG 215 is one of only a few known galaxies that demonstrates such a delayed star formation history. While the ancient stellar population (predominantly red giants) is prominent, the look-back time by which 50% of the mass of all stars ever formed had been created is among the youngest of any known galaxy.