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

TL;DR

This study identifies and analyzes 13 off-galaxy Hα emission blobs in the MaNGA survey, revealing their origins are linked to galaxy interactions or isolated star-forming regions in gas-rich systems.

Contribution

First statistical analysis of Hα blobs using MaNGA data, classifying their physical origins and properties in the local universe.

Findings

Hα blobs are associated with galaxy interactions or isolated star-forming regions.

Blobs in interacting systems show shock ionization and tidal stripping.

Blobs in gas-rich systems are low-metallicity, star-forming, and detached from main galaxies.

Abstract

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.