Dark Energy Survey Year 3 results: Cosmological constraints from galaxy
  clustering and galaxy-galaxy lensing using the MagLim lens sample
  Two of the most sensitive probes of the large scale structure of the universe
are the clustering of galaxies and the tangential shear of background galaxy
shapes produced by those foreground galaxies, so-called galaxy-galaxy lensing.
Combining the measurements of these two two-point functions leads to
cosmological constraints that are independent of the galaxy bias factor. The
optimal choice of foreground, or lens, galaxies is governed by the joint, but
conflicting requirements to obtain accurate redshift information and large
statistics. We present cosmological results from the full 5000 sq. deg. of the
Dark Energy Survey first three years of observations (Y3) combining those
two-point functions, using for the first time a magnitude-limited lens sample
(MagLim) of 11 million galaxies especially selected to optimize such
combination, and 100 million background shapes. We consider two cosmological
models, flat $\Lambda$CDM and $w$CDM. In $\Lambda$CDM we obtain for the matter
density $\Omega_m = 0.320^{+0.041}_{-0.034}$ and for the clustering amplitude
$S_8 = 0.778^{+0.037}_{-0.031}$, at 68\% C.L. The latter is only 1$\sigma$
smaller than the prediction in this model informed by measurements of the
cosmic microwave background by the Planck satellite. In $w$CDM we find
$\Omega_m = 0.32^{+0.044}_{-0.046}$, $S_8=0.777^{+0.049}_{-0.051}$, and dark
energy equation of state $w=-1.031^{+0.218}_{-0.379}$. We find that including
smaller scales while marginalizing over non-linear galaxy bias improves the
constraining power in the $\Omega_m-S_8$ plane by $31\%$ and in the
$\Omega_m-w$ plane by $41\%$ while yielding consistent cosmological parameters
from those in the linear bias case. These results are combined with those from
cosmic shear in a companion paper to present full DES-Y3 constraints from the
three two-point functions (3x2pt).
