SDSS-IV MaNGA: the physical origin of off-galaxy H$\alpha$ blobs in the
  local Universe
  H$\alpha$ blobs are off-galaxy emission-line regions with weak or no optical
counterparts. They are mostly visible in H$\alpha$ line, appearing as
concentrated blobs. Such unusual objects have been rarely observed and studied,
and their physical origin is still unclear. We have identified 13 H$\alpha$
blobs in the public data of MaNGA survey, by visually inspecting both the
optical images and the spatially resolved maps of H$\alpha$ line for $\sim
4600$ galaxy systems. Among the 13 H$\alpha$ blobs, 2 were reported in
previously MaNGA-based studies and 11 are newly discovered. This sample, though
still small in size, is by far the largest sample with both deep imaging and
integral field spectroscopy. Therefore, for the first time we are able to
perform statistical studies to investigate the physical origin of H$\alpha$
blobs. We examine the physical properties of these H$\alpha$ blobs and their
associated galaxies, including their morphology, environments, gas-phase
metallicity, kinematics of ionized gas, and ionizing sources. We find that the
H$\alpha$ blobs in our sample can be broadly divided into two groups. One is
associated with interacting/merging galaxy systems, of which the ionization is
dominated by shocks or diffuse ionized gas. It is likely that these H$\alpha$
blobs used to be part of their nearby galaxies, but were stripped away at some
point due to tidal interactions. The other group is found in gas-rich systems,
appearing as low-metallicity star-forming regions that are visually detached
from the main galaxy. These H$\alpha$ blobs could be associated with faint
disks, spiral arms, or dwarf galaxies.
