Supernovae without host galaxies? The low surface brightness host of SN
  2009Z
  A remarkable fraction of supernovae (SNe) have no obvious host galaxy. Two
possible explanations are that (i) the host galaxy is simply not detected
within the sensitivity of the available data or that (ii) the progenitor is a
hypervelocity star that has escaped its parent galaxy. We use the Type IIb SN
2009Z as a prototype of case (i), an example of how a very faint (here Low
Surface Brightness; LSB) galaxy can be discovered via the observation of a
seemingly host-less SN. By identifying and studying LSB galaxies that host SNe
related to the death of massive stars, we can place constraints on the stellar
population and environment of LSB galaxies, which at present are poorly
understood. From an HI spectrum, a redshift of z = 0.02513+-0.00001 and an HI
mass of (2.96+-0.12) 10^9 M_sun are computed. This redshift is consistent with
that obtained from optical emission lines of SN 2009Z. Furthermore, a gas mass
fraction of f_g = 0.87+-0.04 is obtained, one of the highest fractions ever
measured. The host galaxy shows signs of recently enhanced star formation
activity with a far-UV derived extinction-corrected Star Formation Rate (SFR)
of 0.44+-0.34 M_sun/yr. Based on the B-band luminosity we estimate an
extinction-corrected metallicity following the calibration by Pilyugin (2001)
of 12 + log(O/H) = 8.24+-0.70. The presence of a Type IIb SN in an LSB galaxy
suggests, contrary to popular belief, that massive stars can be formed in this
type of galaxies. Furthermore, our results imply that LSB galaxies undergo
phases of small, local burst activity intermittent with longer phases of
inactivity.
