SDSS-IV MaNGA: The Nature of an Off-galaxy H$\alpha$ Blob -- A
  Multi-wavelength View of Offset Cooling in a Merging Galaxy Group
  Galaxies in dense environments, such as groups and clusters, experience
various processes by which galaxies gain and lose gas. Using data from the
SDSS-IV MaNGA survey, we previously reported the discovery of a giant (6 -- 8
kpc in diameter) H$\alpha$ blob, Totoro, about 8 kpc away from a pair of
galaxies (Satsuki and Mei) residing in a galaxy group which is experiencing a
group-group merger. Here, we combine interferometric $^{12}$CO(1--0) molecular
gas data, new wide-field H$\alpha$, $u$-band data, and published X-ray data to
determine the origin of the blob. Several scenarios are discussed to account
for its multi-wavelength properties, including (1) H$\alpha$ gas being stripped
from galaxy Satsuki by ram-pressure; (2) a separated low-surface-brightness
galaxy; (3) gas being ejected or ionized by an active galactic nucleus (AGN);
and (4) a cooling intra-group medium (IGM). Scenarios (1) and (2) are less
favored by the present data. Scenario (3) is also less likely as there is no
evidence for an active ongoing AGN in the host galaxy. We find that the CO
(cold) and H$\alpha$ (warm) gas coexist with X-ray (hot) structures; moreover,
the derived cooling time is within the regime where molecular and H$\alpha$ gas
are expected. The coexistence of gas with different temperatures also agrees
with that of cooling gas in other systems. Our multi-wavelength results
strongly suggest that the CO and H$\alpha$ gas are the product of cooling from
the IGM at its current location, i.e., cooling has occurred, and may be
ongoing, well outside the host-galaxy core.
