Stellar velocity profiles and line strengths out to four effective radii
  in the early-type galaxies NGC 3379 and NGC 821
  We use the integral-field spectrograph SAURON to measure the stellar
line-of-sight velocity distribution and absorption line strengths out to four
effective radii (Re) in the early-type galaxies NGC 3379 and NGC 821. With our
newly developed observing technique we can now probe these faint regions in
galaxies that were previously not accessible with traditional long-slit
spectroscopy. We make optimal use of the large field-of-view and high
throughput of the spectrograph: by adding the signal of all ~1400 lenslets into
one spectrum, we obtain sufficient signal-to-noise in a few hours of observing
time to reliably measure the absorption line kinematics and line strengths out
to large radius.
  We find that the line strength gradients previously observed within 1 Re
remain constant out to at least 4 Re, which puts constraints on the merger
histories of these galaxies. The stellar halo populations are old and
metal-poor. By constructing orbit-based Schwarzschild dynamical models we find
that dark matter is necessary to explain the observed kinematics in NGC 3379
and NGC 821, with 30 - 50 per cent of the total matter being dark within 4 Re.
The radial anisotropy in our best-fit halo models is less than in our models
without halo, due to differences in orbital structure. The halo also has an
effect on the Mgb - Vesc relation: its slope is steeper when a dark matter halo
is added to the model.
