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

Abstract

We report the discovery of a mysterious giant $H_{\hat{I}}\pm$ 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 $H_{\hat{I}}\pm$ blob is $\sim$ 3-4 kpc in radius, and the $H_{\hat{I}}\pm$ 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 $3.3\times 10^5$ ${\mathrm{M}}_{\odot }$ and $<1.3\times 10^9$ ${\mathrm{M}}_{\odot }$, respectively. The emission-line ratios indicate that the $H_{\hat{I}}\pm$ blob is photoionized by a combination of massive young stars and AGN. Furthermore, the ionization line ratio decreases from MaNGA 1-24145 to the $H_{\hat{I}}\pm$ blob, suggesting that the primary ionizing source may come from MaNGA 1-24145, likely a low-activity AGN. Possible explanations of this $H_{\hat{I}}\pm$ 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 $H_{\hat{I}}\pm$ blob. With the current data, we can not distinguish whether this $H_{\hat{I}}\pm$ 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.

13 pages, 11 figures, ApJ accepted