Discovery of disc truncations above the galaxies' mid-plane in Milky
  Way-like galaxies
  Disc truncations are the closest feature to an edge that galaxies have, but
the nature of this phenomena is not yet understood. In this paper, we explore
the truncations in two nearby (D ~15 Mpc) Milky Way-like galaxies: NGC 4565 and
NGC 5907. We cover a wide wavelength range from the NUV and optical, to 3.6
{\mu}m. We find that the radius of the truncation (26+/-0.5 kpc) is independent
of wavelength. Surprisingly, we identify (at all wavelengths) the truncation at
altitudes as high as 3 kpc above the mid-plane, which implies that the thin
disc in those outer regions has a width of at least this value. We find the
characteristic U-shape radial colour profile associated with a star formation
threshold at the location of the truncation. Further supporting such an origin,
the stellar mass density at the position of the truncation is ~1-2 M_sun pc^-2,
in good agreement with the critical gas density for transforming gas into
stars. Beyond the truncation, the stellar mass in the mid-plane of the disc
drops to just 0.1-0.2% of the total stellar mass of the galaxies. The detection
of the truncation at high altitude in combination with the U shape of the
radial colour profile allows us to establish, for the first time, an upper
limit to the present-day growth rate of galactic discs. We find that, if the
discs of the galaxies are growing inside-out, their growth rate is less than
0.6-0.9 kpc Gyr^-1.
