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