Characterization and history of the Helmi streams with Gaia DR2
  The halo of the Milky Way has long been hypothesized to harbour significant
amounts of merger debris. This view has been supported over more than a decade
by wide-field photometric surveys which have revealed the outer halo to be
lumpy. The recent release of Gaia DR2 is allowing us to establish that mergers
also have been important and possibly built up the majority of the inner halo.
In this work we focus on the Helmi streams, a group of streams crossing the
Solar vicinity and known for almost two decades. We characterize their
properties and relevance for the build-up of the Milky Way's halo. We identify
new members of the Helmi streams in an unprecedented dataset with full
phase-space information combining Gaia DR2, and the APOGEE DR2, RAVE DR5 and
LAMOST DR4 spectroscopic surveys. Based on the orbital properties of the stars,
we find new stream members up to a distance of 5 kpc from the Sun, which we
characterize using photometry and metallicity information. We also perform
N-body experiments to constrain the time of accretion and properties of the
progenitor of the streams. We find nearly 600 new members of the Helmi streams.
Their HR diagram reveals a broad age range, from approximately 11 to 13 Gyr,
while their metallicity distribution goes from $\sim$ 2.3 to $\sim$1.0, and
peaks at [Fe/H] $\sim$1.5. These findings confirm that the streams originate in
a dwarf galaxy. Furthermore, we find 7 globular clusters to be likely
associated, and which follow a well-defined age-metallicity sequence whose
properties suggest a relatively massive progenitor object. Our N-body
simulations favour a system with a stellar mass of $\sim
10^8\,\mathrm{M}_\odot$ accreted $5 - 8$ Gyr ago. The debris from the Helmi
streams is an important donor to the MilkyWay halo, contributing approximately
15\% of its mass in field stars and 10\% of its globular clusters.
