  Current exoplanet surveys using the radial velocity (RV) technique are
targeting M dwarfs because any habitable zone terrestrial-mass planets will
induce a high RV and orbit on shorter periods than for more massive stars. One
of the main caveats is that M dwarfs show a wide range of activity levels from
inactive to very active, which can induce an asymmetry in the line profiles
and, consequently, a spurious RV measurement. We aim to benchmark the impact of
stellar activity on high-precision RV measurements using regular-cadence
CARMENES visible and near-infrared observations of the active M3.5 dwarf EV
Lac. We used the newly developed technique of low-resolution Doppler imaging to
determine the centre-of-light, or spot-induced RV component, for eight
observational epochs. We confirm a statistically significant and strong
correlation between the independently measured centre-of-light and the
chromatic index, which is a measure of the amplitude variation with wavelength
of the RVs. We also find circular closed-loop relations of several activity
indices with RV for a subset of data that covers only several rotation periods.
We also investigate the implications of large phase gaps in the periodograms of
activity indicators. Finally, by removing the spot-induced RV component we
improve the planet-mass sensitivity by a factor of at least three. We conclude
that for active M stars, a regular-cadence observing strategy is the most
efficient way to identify and eliminate sources of correlated noise.
