Outlying H$α$ emitters in SDSS IV MaNGA

Abstract: We have carried out a systematic search for outlying H$\alpha$ emitters in the entire data release 14 of the Sloan Digital Sky Survey (SDSS) IV Mapping Nearby Galaxies at APO (MaNGA) survey. We have discovered six outlying H$\alpha$ emitters with no bright underlying optical continuum emission in the imaging data release 5 from the Dark Energy Camera Legacy Survey (DECaLS) and data release 6 of the Mayall $z$-band Legacy Survey (MzLS) + Beijing-Arizona Sky Survey (BASS). They also show a velocity field which is different from that of the host galaxy. These outlying H$\alpha$ emitters all have extended structure in the H$\alpha$ image. Their emission line ratios show that they are photoionised due to an active galactic nucleus (AGN) or a mixture of both an AGN and star formation. Some of them are very likely to be fainter counterparts of Hanny's Voorwerp like objects.