SDSS-IV MaNGA: The Nature of an Off-galaxy H$\alpha$ Blob -- A Multi-wavelength View of Offset Cooling in a Merging Galaxy Group

TL;DR

This study investigates an off-galaxy Hα blob in a merging galaxy group, using multi-wavelength data to determine its origin, and finds evidence supporting cooling of intra-group medium as the source of the gas.

Contribution

The paper provides a multi-wavelength analysis of an off-galaxy Hα blob, demonstrating cooling from intra-group medium as the likely origin, which is a novel insight into gas dynamics in merging galaxy groups.

Findings

The Hα and CO gases coexist with X-ray structures, indicating multi-phase gas.

Cooling times are consistent with ongoing cooling of intra-group medium.

Scenarios involving gas stripping, low-surface-brightness galaxy, or AGN activity are less supported.

Abstract

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.