Supernovae without host galaxies? : The low surface brightness host of SN 2009Z

• Context. 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. Aims. 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. Methods. We use archival ultraviolet (UV) and optical imaging, as well as an HI spectrum taken with the 100m Effelsberg Radio Telescope to measure various parameters of the host galaxy, in particular its redshift, stellar and HI mass, and metallicity. Results. From the Effelsberg spectrum, a redshift z = 0.02513 +/- 0.00001 and an HI mass of 2.96 +/- 0.12 x 10(9) M-circle dot 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-circle dot yr(-1). 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. Conclusions. 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, rather than a continuous but very low SFR. Discovering faint (LSB) galaxies via bright supernova events happening in them offers an excellent opportunity to improve our understanding of the nature of LSB galaxies.

Ämnesord

NATURVETENSKAP  -- Fysik -- Astronomi, astrofysik och kosmologi (hsv//swe)

NATURAL SCIENCES  -- Physical Sciences -- Astronomy, Astrophysics and Cosmology (hsv//eng)

Nyckelord

supernovae: individual: SN 2009Z

galaxies: evolution

galaxies: stellar content

methods: observational

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