As part of the 100IAS survey , a program aimed to obtain nebular-phase spectra for a volume-limited and homogeneous sample of Type Ia supernovae ( SNe Ia ) , we observed ASASSN-18tb ( SN 2018fhw ) at 139 days past maximum light .
ASASSN-18tb was a fast-declining , sub-luminous event that fits well within the observed photometric and spectroscopic distributions of the SN Ia population .
We detect a prominent H \alpha emission line ( L _ { { H } \alpha } = 2.2 \pm 0.2 \times 10 ^ { 38 } ergs s ^ { -1 } ) with FWHM \approx 1100 km s ^ { -1 } in the nebular-phase spectrum of this SN Ia .
High luminosity H \alpha emission ( L _ { { H } \alpha } \gtrsim 10 ^ { 40 } ergs s ^ { -1 } ) has previously been discovered in a rare class of SNe Ia-like objects showing CSM interactions ( SNe Ia-CSM ) .
They predominantly belong to over-luminous ( M _ { max } < -19 mag in optical ) 1991T-like SNe Ia and are exclusively found in star-forming galaxies .
By contrast , ASASSN-18tb is a sub-luminous SN Ia ( M _ { B, { max } } \sim - 17.7 mag ) found in an early-type galaxy dominated by old stellar populations .
We discuss possible origins for the observed hydrogen .
Out of 75 SNe Ia for which we have so far obtained nebular spectra in 100IAS , no other SN shows a \sim 1000 { km s ^ { -1 } } H \alpha emission line with comparable line luminosity as ASASSN-18tb , emphasizing the rarity of such emission in the nebular phase .
Based on preliminary results from our survey , the rate for ASASSN-18tb-like nebular H \alpha emission could be as high as \sim 10 \% level among sub-luminous SNe Ia .