The extended Baryon Oscillation Spectroscopic Survey: measuring the
  cross-correlation between the MgII flux transmission field and quasars and
  galaxies at $z=0.59$
  We present the first attempt at measuring the baryonic acoustic oscillations
(BAO) in the large scale cross-correlation between the magnesium-II doublet
(MgII) flux transmission field and the position of quasar and galaxy tracers.
The MgII flux transmission continuous field at $0.3 < z < 1.6$ is measured from
500,589 quasar spectra obtained in the Baryonic Oscillation Spectroscopic
Survey (BOSS) and the extended BOSS (eBOSS). The position of 246,697 quasar
tracers and 1,346,776 galaxy tracers are extracted from the Sloan Digital Sky
Survey (SDSS) I, II, BOSS and eBOSS catalogs. In addition to measuring the
cosmological BAO scale and the biased matter density correlation, this study
allows tests and improvements to cosmological Lyman-$\alpha$ analyses. A
feature consistent with that of the BAO is detected at a significance of
$\Delta \chi^{2} = 7.25$. The measured MgII linear transmission bias parameters
are $b_{\mathrm{MgII(2796)}} (z = 0.59) = (-6.82 \pm 0.54) \, \times 10^{-4}$
and $b_{\mathrm{MgII(2804)}} (z = 0.59) = (-5.55 \pm 0.46) \, \times 10^{-4}$,
and the MgI bias is $b_{\mathrm{MgI(2853)}} (z = 0.59) = (-1.48 \pm 0.24) \,
\times 10^{-4}$. Their redshift evolution is characterized by the power-law
index: $\gamma_{\mathrm{Mg}} = 3.36 \pm 0.46$. These measurements open a new
window towards using BAO from flux transmission at $z < 1$ in the final eBOSS
sample and in the upcoming sample from the Dark Energy Spectroscopic
Instrument.
