Supernovae without host galaxies?

Abstract

Context. Harvesting the SAI supernova catalog, the most complete list of supernovae (SNe) currently available, we search for SNe that apparently do not occur within a distinct host galaxy but lie a great distance (several arcmin) apart from the host galaxy given in the catalog or even show no sign of an identifiable galaxy in their direct vicinity. Aims. We attempt to distinguish between two possible explanations of this host-lessness of a fraction of reported SNe, namely (i) that a host galaxy is too faint (of too low surface brightness) to be detected within the limits of currently available surveys (presumably a low surface brightness galaxy) or (ii) the progenitor of the SN is a hypervelocity star (HVS) that exploded kiloparsecs away from its host galaxy. Methods. We use deep imaging to test the first explanation. If no galaxy is identified within our detection limit of ∼27 mag arcsec −2 , which is the central surface brightness of the faintest known LSB galaxy so far, we discard this explanation and propose that the SN, after several other checks, had a hypervelocity star progenitor. We focus on observations for which this is the case and give lower limits to the actual space velocities of the progenitors, making them the first hypervelocity stars known in galaxies other than our own Milky Way. Results. Analyzing a selected subsample of five host-less SNe, we find one, SN 2006bx in UGC 5434, is a possible hypervelocity progenitor category with a high probability, exhibiting a projected velocity of ∼800 km s −1 . SN 1969L in NGC 1058 is most likely an example of a very extended star-forming disk visible only in the far-UV, but not in the optical wavebands. Therefore, this SN is clearly due to in situ star formation. This mechanism may also apply to two other SNe that we investigated (SN 1970L and SN 1997C), but this cannot be determined with certainty. Another SN, SN 2005 nc which is associated with a gamma-ray burst (GRB 050525), is a special case that is not covered by our initial assumptions. Even with deep Hubble Space Telescope data, a host galaxy cannot be unambiguously identified.

Possible discovery of Calvera's supernova remnant

“…Getting the (massive) progenitor star to such a large height above the Galactic plane could require a high velocity runaway scenario: a star with a vertical velocity of 100 km s −1 would take ∼ 30 Myrs to reach it, which points to a star in the low-mass end of core-collapse explosions (8−10 M ), or alternatively a star in some kind of binary interaction that extends its lifetime (e.g., as discussed in Zapartas et al 2021). Hypervelocity runaway stars are rare (Renzo et al 2019) but certainly can be responsible for core-collapse supernovae in the outskirts of galaxies (e.g., Zinn et al 2011). Alternatively, the neutron star need not be formed by a massive star at all, if its origin is the accretion-induced or merger-induced collapse of a white dwarf (Grindlay & Bailyn 1988;Saio & Nomoto 1985).…”

The Fastest Unbound Stars in the Universe

“…Given that LSST is expected to detect a few 10 5 supernovae per year (Abell et al 2009), and presuming the above rates, we predict that LSST should find a few isolated supernovae (with d 100 kpc from their origin galaxy) that originate from the SHS/SHB population per year. Zinn et al (2011) found several candidate supernovae at distances of kpc from their source galaxies that likely originated from stars with very short progenitor lifetimes. They inferred that the progenitor stars must have traveled with speeds in excess of several hundred km s −1 to get so far away from their source galaxies.…”

Exploring the nature of the unidentified very-high-energy gamma-ray source HESS J1507–622

“…This limitation on a PWN origin can in principle be overcome by a scenario where the pulsar is born offset from the Galactic disk in the core-collapse of a massive star that was ejected from the Galaxy (see Zinn et al, 2011, for core-collapse supernovae connected to hypervelocity stars). Since the presupernova lifetime of massive stars is of the order of 10 7 years, significantly longer than the lifetime of young energetic pulsars, such a star can reach larger offsets from the Galactic plane.…”

“…It has further to be noted that supernovae of hypervelocity stars appear to be rare events (Zinn et al, 2011). Therefore only a very limited number of PWNe resulting from such supernovae are expected in the Milky Way.…”