[Paper Review] Supernovae without host galaxy? - Hypervelocity stars in foreign galaxies

Peter-Christian Zinn, P. Grunden+1 more|arXiv (Cornell University)|Sep 22, 2011

Gamma-ray bursts and supernovae64 references3 citations

TL;DR

This study investigates supernovae (SNe) appearing to lack a host galaxy by distinguishing between faint, undetected low-surface-brightness (LSB) galaxies and hypervelocity star (HVS) progenitors. Using deep imaging and velocity analysis, the authors identify SN 2006bx in UGC 5434 as the first confirmed hypervelocity supernova in a galaxy other than the Milky Way, with a projected velocity of ~800 km s⁻¹, indicating ejection via a supermassive black hole slingshot mechanism.

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.

Motivation & Objective

• To determine whether supernovae without visible host galaxies are due to undetected low-surface-brightness (LSB) galaxies or hypervelocity star (HVS) progenitors.
• To test the hypothesis that some SNe are produced by HVSs ejected from their host galaxies at high velocities.
• To identify and characterize the first HVS progenitor in a galaxy other than the Milky Way using deep imaging and kinematic analysis.
• To assess the limitations of current surveys in detecting faint host galaxies and the implications for SN host association.

Proposed method

• Used deep imaging with a detection limit of ~27 mag arcsec⁻² to search for faint host galaxies around host-less SNe.
• Applied a surface brightness threshold based on the faintest known LSB galaxy (μ_B > 23 mag arcsec⁻²) to assess detectability.
• Calculated projected space velocities of progenitor stars using angular separation and redshift data to infer ejection mechanisms.
• Analyzed archival Hubble Space Telescope (HST) ACS data with a sensitivity limit of 27.4 mag for host galaxy detection.
• Evaluated SN 2005nc using HST data, placing an upper limit of 1.2×10⁸ L☉ on its host luminosity.
• Used kinematic and morphological criteria to rule out in situ star formation and interaction with the host galaxy.

Experimental results

Research questions

• RQ1 Can the absence of a visible host galaxy for certain SNe be attributed to the host being too faint to detect, or is it due to the progenitor being a hypervelocity star?
• RQ2 What is the minimum detectable surface brightness of a host galaxy in current optical surveys, and how does this affect SN host identification?
• RQ3 Can the kinematic properties of a SN's progenitor, such as projected velocity, confirm its origin as a hypervelocity star?
• RQ4 Is it possible to identify hypervelocity supernovae in galaxies other than the Milky Way using existing imaging and catalog data?
• RQ5 How do extreme environments like low-luminosity or low-surface-brightness galaxies influence the occurrence and detectability of supernovae?

Key findings

• SN 2006bx in UGC 5434 is the first identified hypervelocity supernova in a galaxy other than the Milky Way, with a projected velocity of ~800 km s⁻¹.
• The progenitor of SN 2006bx likely achieved its high velocity via a gravitational slingshot around a supermassive black hole in its host galaxy.
• No detectable host galaxy was found within a 27 mag arcsec⁻² sensitivity limit, ruling out a faint LSB galaxy as the host of SN 2006bx.
• SN 1969L in NGC 1058 is best explained by a very extended star-forming disk visible only in the far-UV, not optical bands.
• For SN 2005nc (associated with GRB 050525), no unambiguous host galaxy was detected even with HST ACS data, with an upper luminosity limit of 1.2×10⁸ L☉.
• The sample selection criterion (projected separation >10 kpc) effectively biases toward SNe with HVS progenitors, enabling the discovery of extragalactic HVSs.