APMUKS(BJ) B215839.70-615403.9: The massive host galaxy candidate of ASASSN-15lh

ATel #7776; J. L. Prieto (U. Diego Portales; MAS), B. J. Shappee (Hubble Fellow, Carnegie Observatories), Subo Dong (KIAA-PKU), S. W. Jha (Rutgers), K. Z. Stanek, T. W.-S. Holoien, C. S. Kochanek, T. A. Thompson (Ohio State), N. Morell, I. B. Thompson (Carnegie Observatories), F. Olivares, G. Pignata (Andres Bello; MAS)
on 8 Jul 2015; 21:13 UT
Credential Certification: Jose L. Prieto (jose.prietok@mail.udp.cl)

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We searched public data archives and surveys in order to characterize the candidate host galaxy of the most luminous SLSN ASASSN-15lh, APMUKS(BJ) B215839.70-615403.9 at z=0.2326 (ATel #7774). The candidate host is detected in the CRTS survey (Drake et al. 2009, ApJ, 696, 870) data at V=18.5 (absolute magnitude M_V ~ -22) and the longterm V-band light curve does not show any strong evidence for variability. We found excellent near-IR H and Ks-band images of the field in the ESO archive from the VISTA Hemisphere Survey (VHS; McMahon et al. 2013, The Messenger, 154, 35). After calibrating the images using 2MASS stars in the field, we measure total aperture magnitudes of the host of J= 16.04 +/- 0.03 and Ks= 14.82 +/- 0.04. Using Sextractor, we measure a half light radius in the Ks-band of 1.0" (~4 kpc). The Ks absolute magnitude of M_Ks = -25.1 implies a high stellar mass of Mstar ~ 2x10^11 Msun (e.g., Mannucci et al. 2005, A&A, 433, 807). This is very different from the host galaxies of previously discovered SLSN-I, which have significantly lower stellar masses / absolute magnitudes (e.g., Neill et al. 2011, ApJ, 727, 15; Stoll et al. 2011, ApJ, 730, 34; Lunnan et al. 2014, ApJ, 787, 138; Lunnan et al. 2015, ApJ, 804, 90). This high stellar mass also implies a black-hole mass > 10^8 Msun, too large to make a tidal disruption event physically likely (e.g., Rees 1988, Nature, 333, 523). Furthermore, limits on its star-formation rate from the non-detection of [O II] 3727 lines in the spectra reported in ATel #7774 (following the procedure described in Leonard & Filippenko 2001, PASP, 113, 920) suggest current SFR <~ 0.4 Msun/year (Savaglio et al. 2009, ApJ, 691, 182). The high stellar mass and relatively low current SFR (specific SFR of log(sSFR) < -11.7) make the candidate host of ASASSN-15lh quite unique among SLSN-I host galaxies (see this figure, Figure 4 in Lunnan et al. 2015 adding the candidate host galaxy of ASASSN-15lh in green). It has, however, striking similarities compared to the S0/a galaxy NGC1260, the host galaxy of the SLSN-II SN 2006gy (e.g., Kelly et al. 2014, ApJ, 789, 23).

Based in part on data products from observations made with ESO Telescopes at the Paranal Observatories under ESO programme ID 179.A-2010.