SDSS-IV MaNGA: the physical origin of off-galaxy H$α$ blobs in the local Universe

Abstract

Hα blobs are off-galaxy emission-line regions with weak or no optical counterparts. They are mostly visible in Hα 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 Hα blobs in the public data of MaNGA survey, by visually inspecting both the SDSS optical images and the spatially resolved maps of Hα line for ∼ 4600 galaxy systems. Among the 13 Hα 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 Hα blobs. We examine the physical properties of these Hα blobs and their associated galaxies, including their morphology, environments, gasphase metallicity, kinematics of ionized gas, and ionizing sources. We find that the Hα 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 Hα 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.