The Lack of RR Lyrae Variables in Ultrafaint Dwarf Galaxy Sextans II

TL;DR

This study searches for RR Lyrae variables in the ultrafaint dwarf galaxy Sextans II using archival imaging data but finds none, suggesting some faint UFDs may lack these standard candles, impacting distance measurements.

Contribution

First search for RR Lyrae in Sextans II using multiband time-series data, providing evidence for their absence in some ultrafaint dwarf galaxies.

Findings

No RR Lyrae detected in Sextans II.

Recovered a known foreground RR Lyrae, BB Sex.

Suggests some faint UFDs may lack RR Lyrae stars.

Abstract

RR Lyrae in ultrafaint dwarf (UFD) galaxies, if found, would be valuable to constrain the distances of these UFD. In this work, we report our search of RR Lyrae in a recently discovered UFD -- Sextans II. Based on multiband ($griz$) time-series archival DECam imaging data, we do not find any RR Lyrae in Sextans II. On the contrary, the sparse multiband DECam light curves allowed recovery of a known foreground RR Lyrae, BB Sex, which happened to fall within the DECam images, and its pulsation period was correctly identified. Therefore, it is possible that Sextans II lacks RR Lyrae, similar to several other fainter UFDs (with $M_V \gtrsim -5.0$~mag) that do not have RR Lyrae.