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We report the discovery of a mysterious giant $H_{\alpha}$ blob that is $\sim8$ kpc away from the main MaNGA target 1-24145, one component of a dry galaxymerger, 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 continuumcounterpart in deep broadband images reaching $\sim$26.9 mag arcsec$^{-2}$ insurface brightness. We estimate that the masses of ionized and cold gases are$3.3 \times 10^{5}$ $\rm M_{\odot}$ and $< 1.3 \times 10^{9}$ $\rm M_{\odot}$,respectively. The emission-line ratios indicate that the $H_{\alpha}$ blob isphotoionized by a combination of massive young stars and AGN. Furthermore, theionization line ratio decreases from MaNGA 1-24145 to the $H_{\alpha}$ blob,suggesting that the primary ionizing source may come from MaNGA 1-24145, likelya low-activity AGN. Possible explanations of this $H_{\alpha}$ blob include AGNoutflow, the gas remnant being tidally or ram-pressure stripped from MaNGA1-24145, or an extremely low surface brightness (LSB) galaxy. However, thestripping scenario is less favoured according to galaxy merger simulations andthe morphology of the $H_{\alpha}$ blob. With the current data, we can notdistinguish whether this $H_{\alpha}$ blob is ejected gas due to a past AGNoutburst, or a special category of `ultra-diffuse galaxy' (UDG) interactingwith MaNGA 1-24145 that further induces the gas inflow to fuel the AGN in MaNGA1-24145.

SDSS IV MaNGA: Discovery of an $H_{\alpha}$ blob associated with a dry galaxy pair -- ejected gas or a `dark' galaxy candidate?