Dark Energy Survey Year 1 Results: Measurement of the Galaxy Angular
  Power Spectrum
  We use data from the first-year (Y1) observations of the DES collaboration to
measure the galaxy angular power spectrum (APS), and search for its BAO feature
using a template-fitting method. We test our methodology in a sample of 1800
DES Y1-like mock catalogs. The APS is measured with the pseudo-$C_\ell$ method,
using pixelized maps constructed from the mock catalogs and the DES mask. The
covariance matrix of the $C_\ell$'s in these tests are also obtained from the
mock catalogs. We use templates to model the measured spectra and estimate
template parameters firstly from the $C_\ell$'s of the mocks using two
different methods, a maximum likelihood estimator and a MCMC, finding
consistent results with a good reduced $\chi^2$. Robustness tests are performed
to estimate the impact of different choices of settings used in our analysis.
After these tests on mocks, we apply our method to a galaxy sample constructed
from DES Y1 data specifically for LSS studies. This catalog comprises galaxies
within an effective area of 1318 deg$^2$ and $0.6<z<1.0$. We fit the observed
spectra with our optimized templates, considering models with and without BAO
features. We find that the DES Y1 data favors a model with BAO at the
$2.6\,\sigma$ C.L. with a best-fit shift parameter of $\alpha=1.023\pm 0.047$.
However, the goodness-of-fit is somewhat poor, with $\chi^2/$(dof) = 1.49. We
identify a possible cause of this issue and show that using a theoretical
covariance matrix obtained from $C_\ell$'s that are better adjusted to data
results in an improved value of $\chi^2/$(dof) = 1.36 which is similar to the
value obtained with the real-space analysis. Our results correspond to a
distance measurement of $D_A(z_{\rm eff} = 0.81)/r_d = 10.65 \pm 0.49$,
consistent with the main DES BAO findings. This is a companion paper to the
main DES BAO article showing the details of the harmonic space analysis.
