Clustered Cepheid Variables 90 kiloparsec from the Galactic Center
  Distant regions close to the plane of our Galaxy are largely unexplored by
optical surveys as they are hidden by dust. We have used near-infrared data
(that minimizes dust obscuration) from the ESO Public survey VISTA Variables of
the Via Lactea (VVV) (Minniti et al. 2011; Saito et al. 2012; henceforth S12)
to search for distant stars at low latitudes. We have discovered four Cepheid
variables within an angular extent of one degree centered at Galactic longitude
of $l = -27.4^\circ$ and Galactic latitude of $b = -1.08 ^\circ$. We use the
tightly constrained period-luminosity relationship that these pulsating stars
obey (Persson et al. 2004; Matsunaga et al. 2011) to derive distances. We infer
an average distance to these Cepheid variables of 90 kpc. The Cepheid variables
are highly clustered in angle (within one degree) and in distance (the standard
deviation of the distances is 12 kpc). They are at an average distance of $\sim
2~\rm kpc$ from the plane and their maximum projected separation is $\sim 1~
\rm kpc$. These young ($\sim$ 100 Myr old), pulsating stars (Bono et al. 2005)
are unexpected at such large distances from the Galactic disk, which terminates
at $\sim$ 15 kpc (Minniti et al. 2011). The highly clustered nature in distance
and angle of the Cepheid variables suggests that the stars may be associated
with a dwarf galaxy, one that was earlier predicted by a dynamical analysis
(Chakrabarti \& Blitz 2009).
