Supernovae without host galaxy? - Hypervelocity stars in foreign galaxies

Abstract

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.

11 pages, 7 figures, accepted for publication by A&A, abstract abridged due to arXiv requirements, rev. 2 after language corrections

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