Supernovae without host galaxies?

The low surface brightness host of SN 2009Z

¹ Astronomical Institute, Ruhr-University Bochum, Universitätsstraße 150, 44801 Bochum, Germany
e-mail: zinn@astro.rub.de
² CSIRO Astronomy & Space Science, PO Box 76, Epping, NSW, 1710, Australia
³ The Oskar Klein Centre, Department of Astronomy, Stockholm University, AlbaNova, 10691 Stockholm, Sweden
⁴ Dark Cosmology Centre, Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30, 2100 Copenhagen Ø, Denmark
⁵ Argelander Institut für Astronomie, Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
⁶ Las Campanas Observatory, Carnegie Observatories, Casilla 601, La Serena, Chile
⁷ Max-Planck-Institute for Radio Astronomy, Auf dem Hügel 69, 53121 Bonn, Germany

Received: 4 January 2011
Accepted: 7 November 2011

Abstract

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 H I spectrum taken with the 100 m Effelsberg Radio Telescope to measure various parameters of the host galaxy, in particular its redshift, stellar and H I mass, and metallicity.

Results. From the Effelsberg spectrum, a redshift z = 0.02513 ± 0.00001 and an H I mass of 2.96 ± 0.12 × 10⁹ M_⊙ 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_⊙ 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.