The host of the Type I SLSN 2017egm

Abstract

Context. Type I superluminous supernova (SLSN) host galaxies are predominantly low-metallicity, highly star-forming (SF) dwarfs. One of the current key questions is whether Type I SLSNe can only occur in such environments and hosts. Aims. Here we present an integral-field study of the massive, high-metallicity spiral NGC 3191, the host of SN 2017egm, the closest Type I SLSN known to date. We use data from PMAS/CAHA and the public MaNGA survey to shed light on the properties of the SLSN site and the origin of star formation in this non-starburst spiral galaxy. Methods. We map the physical properties of different H ii regions throughout the galaxy and characterise their stellar populations using the STARLIGHT fitting code. Kinematical information allows us to study a possible interaction with its neighbouring galaxy as the origin of recent star formation activity which could have caused the SLSN. Results. NGC 3191 shows intense star formation in the western part with three large SF regions of low metallicity. Taking only the properties of emitting gas, the central regions of the host have a higher metallicity, a lower specific star formation rate, and lower ionisation. Modelling the stellar populations gives a different picture: the SLSN region has two dominant stellar populations with different ages, the younger one with an age of 2–10 Myr and lower metallicity, likely the population from which the SN progenitor originated. Emission line kinematics of NGC 3191 show indications of interaction with its neighbour MCG+08-19-017 at ~45 kpc, which might be responsible for the recent starburst. In fact, this galaxy pair has hosted a total of four SNe, 1988B (Type Ia), SN 2003ds (Type Ic in MCG+08-19-017), PTF10bgl (Type II), and 2017egm, underlying the enhanced SF in both galaxies due to interaction. Conclusions. Our study shows that care should be taken when interpreting global host and even gas properties without looking at the stellar population history of the region. The SLSNe seem to be consistent with massive stars (>20 M⊙) requiring low metallicity (<0.6 Z⊙), environments that can also occur in massive late-type galaxies, but not necessarily with starbursts.

Far-UV HST Spectroscopy of an Unusual Hydrogen-poor Superluminous Supernova: SN2017egm

“…This is consistent with nebular gasphase metallicities measured from optical spectra at the SN location (IFU data) and the companion galaxy (SDSS) (Bose et al 2018;Chen et al 2017a;Izzo et al 2018;Nicholl et al 2017a). Combining the low H I column density result with the high-metallicity measurement, we can rule out the scenario where SN2017egm is located in a dwarf companion galaxy.…”

“…This relation is based on Galactic dust-to-gas and dust-to-metal ratios and should work reasonably well for galaxies with high metallicities. The E(B−V )=0.3 (Chen et al 2017a;Izzo et al 2018), derived from nebular emission lines at the SN location, represents the dust extinction through the full disk of NGC 3191. The low host extinction correction is further supported by the fact that the blackbody temperature derived from the uncorrected SN2017egm spectra appears to be consistent with that of other SLSNe-I in dwarf galaxies.…”

“…The projected position of SN2017egm is very close to the host nucleus, and archival integral field unit (IFU) spectra of NGC 3191 make it possible to zoom in onto the spiral arm region where the supernova exploded, and measure the nebular gas-phase metallicities with a resolution of ∼1 kpc. Two independent groups have found conflicting results, one with solar to super-solar metallicity (Chen et al 2017a), and the other with sub-solar metallicity (Izzo et al 2018).…”

A nearby super-luminous supernova with a long pre-maximum & “plateau” and strong C II features

“…To create these comparison samples we searched the literature for all SLSNe-I at a redshift lower than 0.3 with published host masses, SFRs and emission line fluxes (with the latter being taken from the exact SN sites if available and global host values if not). Emissionline fluxes were taken from Leloudas et al (2015); Perley et al (2016); Chen et al (2017a); Inserra et al (2017);and Izzo et al (2018) and oxygen abundances were calculated on the Marino et al (2013) O3N2 scale. Host masses and SFRs were estimated using the samples published in Leloudas et al (2015); Perley et al (2016); Schulze et al (2018), however the PTF hosts presented in Perley et al (2016) were remodelled using Le Phare.Within these distributions we also plot the position of each parameter for SN 2018bsz and SN 2017egm (Taggart et al in preparation).…”

Late-phase Spectropolarimetric Observations of Superluminous Supernova SN 2017egm to Probe the Geometry of the Inner Ejecta

“…We here report our late-phase spectropolarimetric observations of SN 2017egm, which is an SLSN-I (Nicholl et al 2017). It was discovered by the Gaia Satellite on 2017 May 23 (MJD=57896; Delgado et al 2017) in a massive metal-rich spiral galaxy NGC 3191 (Chen et al 2017b;Bose et al 2018;Izzo et al 2018) at redshift z=0.0307 (Dong et al 2017). By assuming Hubble flow, the luminosity distance is d L =136.6±1.0 Mpc adopting the Hubble constant H 0 =67.4±0.5 km s −1 Mpc −1 (Planck Collaboration et al 2018).…”