Supernovae without host galaxy? - Hypervelocity stars in foreign
  galaxies
  Harvesting the SAI supernova catalog, we search for SNe that apparently do
not occur within a distinct host galaxy but lie a great distance apart from
their assigned host galaxy. Assuming two possible explanations for this
host-lessness of a fraction of reported SNe, namely (i) a host galaxy which is
too faint to be detected within the limits of currently available surveys or
(ii) a hypervelocity star (HVS) as progenitor of the SN,we want to distinguish
between these two cases. To do so, we use deep imaging to test explanation (i).
If within our detection limit of 27 mag/arcsec^2, the central surface
brightness of the faintest known LSB galaxy so far, no galaxy could be
identified, we discard this explanation and regard the SN, after several other
checks, to have had a hypervelocity star progenitor. Analyzing a selected
subsample of five host-less SNe we find one, SN 2006bx in UGC5434, to be put in
the hypervelocity progenitor category with a high probability, exhibiting a
projected velocity of > 800 km/s. SN 1969L in NGC1058 is most likely an example
for a very extended star-forming disk visible only in the far-UV, not in the
optical wavebands. Therefore this SN is clearly due to in situ star formation.
This mechanism may also apply for two other SNe we investigated (SN 1970L and
SN 1997C), but this cannot be determined with final certainty. Another one, SN
2005nc associated with a gamma-ray burst (GRB 050525), is a special case not
covered by our initial assumptions. Even with deep Hubble data, a host galaxy
could not be unambiguously identified.
