  We present the discovery and interpretation of ionized nebulae around two
ultraluminous X-ray sources in M 51; both sources share the rare property of
showing X-ray eclipses by their companion stars, and are therefore prime
targets for follow-up studies. Using archival Hubble Space Telescope images, we
found an elongated, 100-pc-long emission-line structure associated with one
X-ray source (CXOM51 J132940.0+471237; ULX-1 for simplicity), and a more
circular, ionized nebula at the location of the second source (CXOM51
J132939.5+471244; ULX-2 for simplicity). We observed both nebulae with the
Large Binocular Telescope's Multi-Object Double Spectrograph. From our analysis
of the optical spectra, we argue that the gas in the ULX-1 bubble is
shock-ionized, consistent with the effect of a jet with a kinetic power of
$\approx$2 $\times 10^{39}$ erg s$^{-1}$. Additional X-ray photo-ionization may
also be present, to explain the strength of high-ionization lines such as He II
$\lambda 4686$ and [Ne V] $\lambda 3426$. On the other hand, the emission lines
from the ULX-2 bubble are typical for photoionization by normal O stars
suggesting that the nebula is actually an H II region not physically related to
the ULX but is simply a chance alignment. From archival Very Large Array data,
we also detect spatially extended, steep-spectrum radio emission at the
location of the ULX-1 bubble (consistent with its jet origin), but no radio
counterpart for ULX-2 (consistent with the lack of shock-ionized gas around
that source).
