A systematic DECam search for RR Lyrae in the outer halo of the Milky Way

TL;DR

This paper reports a systematic search for distant RR Lyrae stars in the Milky Way's outer halo using DECam, identifying over 650 candidates up to 250 kpc, which aids in understanding the galaxy's mass and accretion history.

Contribution

It presents a new, extensive catalog of distant RR Lyrae stars in the Milky Way's outer halo from DECam observations, expanding previous surveys.

Findings

Detected over 650 candidate RR Lyrae stars beyond 100 kpc.

Catalog reaches distances up to approximately 250 kpc.

Results are consistent with recent studies of the outer halo.

Abstract

The discovery of very distant stars in the halo of the Milky Way provides valuable tracers on the Milky Way mass and its formation. Beyond 100 kpc from the Galactic center, most of the stars are likely to be in faint dwarf galaxies or tidal debris from recently accreted dwarfs, making the outer reaches of the Galaxy important for understanding the Milky Way's accretion history. However, distant stars in the halo are scarce. In that context, RR Lyrae are ideal probes of the distant halo as they are intrinsically bright and thus can be seen at large distances, follow well-known period-luminosity relations that enable precise distance measurements, and are easily identifiable in time-series data. Therefore, a detailed study of RR Lyrae will help us understand the accreted outskirts of the Milky Way. In this contribution, we present the current state of our systematic search for distant RR Lyrae stars in the halo using the DECam imager at the 4m telescope on Cerro Tololo (Chile). The total surveyed area consists of more than 110 DECam fields (~ 350 sq. deg) and includes two recent independent campaigns carried out in 2017 and 2018 with which we have detected > 650 candidate RR Lyrae stars. Here we describe the methodology followed to analyze the two latest campaigns. Our catalog contains a considerable number of candidate RR Lyrae beyond 100 kpc, and reaches out up to ~ 250 kpc. The number of distant RR Lyrae found is consistent with recent studies of the outer halo. These stars provide a set of important probes of the mass of the Milky Way, the nature of the halo, and the accretion history of the Galactic outskirts.