We report the discovery of a mysterious giant H \alpha blob that is \sim 8 kpc away from the main MaNGA target 1-24145 , one component of a dry galaxy merger , identified in the first-year SDSS-IV MaNGA data .
The size of the H \alpha blob is \sim 3-4 kpc in radius , and the H \alpha distribution is centrally concentrated .
However , there is no optical continuum counterpart in deep broadband images reaching \sim 26.9 mag arcsec ^ { -2 } in surface brightness .
We estimate that the masses of ionized and cold gases are 3.3 \times 10 ^ { 5 } M _ { \odot } and < 1.3 \times 10 ^ { 9 } M _ { \odot } , respectively .
The emission-line ratios indicate that the H \alpha blob is photoionized by a combination of massive young stars and AGN .
Furthermore , the ionization line ratio decreases from MaNGA 1-24145 to the H \alpha blob , suggesting that the primary ionizing source may come from MaNGA 1-24145 , likely a low-activity AGN .
Possible explanations of this H \alpha blob include AGN outflow , the gas remnant being tidally or ram-pressure stripped from MaNGA 1-24145 , or an extremely low surface brightness ( LSB ) galaxy .
However , the stripping scenario is less favoured according to galaxy merger simulations and the morphology of the H \alpha blob .
With the current data , we can not distinguish whether this H \alpha blob is ejected gas due to a past AGN outburst , or a special category of ‘ ultra-diffuse galaxy ’ ( UDG ) interacting with MaNGA 1-24145 that further induces the gas inflow to fuel the AGN in MaNGA 1-24145 .