We report the discovery of a 20-kpc-sized \ha emission in SDSS J083803.68 + 540642.0 , a ringed dwarf galaxy ( M _ { V } = -17.89 mag ) hosting an accreting intermediate-mass black hole at z = 0.02957 .
Analysis of the HST images indicates that it is an early-type galaxy with a featureless low-surface brightness disk ( \mu _ { 0 } = 20.39 mag arcsec ^ { -2 } in the V band ) and a prominent , relatively red bulge ( V - I = 2.03 , R _ { e } = 0.28 kpc or 0 \farcs 48 ) that accounts for \approx 81 % of the total light in the I band .
A circumgalactic ring of a diameter 16 kpc is also detected , with a disperse shape on its south side .
The optical emission lines reveal the nucleus to be a broad-line LINER .
Our MMT longslit observation indicates that the kinematics of the extended \ha emission is consistent with a rotational gaseous disk , with a mean blueshifted velocity of 162 \mathrm { km~ { } s ^ { -1 } } and mean redshifted velocity of 86 \mathrm { km~ { } s ^ { -1 } } .
According to our photoionization calculations , the large-scale \ha emission is unlikely to be powered by the central nucleus or by hot evolved ( post-AGB ) stars interspersed in the old stellar populations , but by in situ star formation ; this is vindicated by the line-ratio diagnostic of the extended emission .
We propose that both the ring and large-scale \ha -emitting gas are created by the tidal accretion in a collision—and then merger—with a gas-rich galaxy of a comparable mass .