The first detection of radio recombination lines at cosmological
  distances
  Recombination lines involving high principal quantum numbers populate the
radio spectrum in large numbers. Low-frequency (<1 GHz) observations of radio
recombination lines (RRLs) primarily from carbon and hydrogen offer a new, if
not unique, way to probe cold, largely atomic gas and warm, ionised gas in
other galaxies. Furthermore, RRLs can be used to determine the physical state
of the emitting regions, such as temperature and density. These properties make
RRLs, potentially, a powerful tool of extragalactic ISM physics. At low radio
frequencies, its conceivable to detect RRLs out to cosmological distances when
illuminated by a strong radio continuum. However, they are extremely faint (tau
~ 1e-3 -- 1e-4) and have so far eluded detection outside of the local universe.
  With LOFAR observations of the radio quasar 3C 190 (z=1.1946), we aim to
demonstrate that the ISM can be explored out to great distances through
low-frequency RRLs.
  We report the detection of RRLs in the frequency range 112--163 MHz in the
spectrum of 3C 190. Stacking 13 a-transitions with principal quantum numbers
n=266-301, a peak 6sigma feature of optical depth, tau(peak) = (1.0 +\- 0.2) x
1e-3 and FWHM = 31.2 +/- 8.3 km/s was found at z=1.124. This corresponds to a
velocity offset of -9965 km/s with respect to the systemic redshift of 3C 190.
  We consider three interpretations of the origin of the RRL emission: an
intervening dwarf-like galaxy, an AGN-driven outflow, and the inter-galactic
medium. We argue that the RRLs most likely originate in a dwarf-like galaxy (M
~ 1e9 Msun) along the line of sight, although we cannot rule out an AGN-driven
outflow. We do find the RRLs to be inconsistent with an inter-galactic medium
origin. With this detection, we have opened up a new way to study the physical
properties of cool, diffuse gas out to cosmological distances.
