# SDSS IV MaNGA: Discovery of an $H_{\alpha}$ blob associated with a dry galaxy pair -- ejected gas or a `dark' galaxy candidate?

## TL;DR

This paper reports the discovery of a large, optically faint H-alpha emitting blob near a dry galaxy merger, exploring its possible origins including outflows, stripped gas, or a dark galaxy candidate.

## Contribution

First identification of a giant H-alpha blob associated with a dry galaxy merger, analyzing its properties and potential origins using SDSS-IV MaNGA data.

## Key findings

- H-alpha blob is 8 kpc from the galaxy with no optical continuum counterpart.
- Ionized and cold gas masses are estimated at 3.3×10^5 M_sun and <1.3×10^9 M_sun.
- Ionization likely driven by a low-activity AGN in the galaxy.

## Abstract

We report the discovery of a mysterious giant $H_{\alpha}$ blob that is $\sim 8$ kpc away from the main MaNGA target 1-24145, one component of a dry galaxy merger, identified in the first-year SDSS-IV MaNGA data. The size of the $H_{\alpha}$ blob is $\sim$ 3-4 kpc in radius, and the $H_{\alpha}$ distribution is centrally concentrated. However, there is no optical continuum counterpart in deep broadband images reaching $\sim$26.9 mag arcsec$^{-2}$ in surface brightness. We estimate that the masses of ionized and cold gases are $3.3 \times 10^{5}$ $\rm M_{\odot}$ and $< 1.3 \times 10^{9}$ $\rm M_{\odot}$, respectively. The emission-line ratios indicate that the $H_{\alpha}$ blob is photoionized by a combination of massive young stars and AGN. Furthermore, the ionization line ratio decreases from MaNGA 1-24145 to the $H_{\alpha}$ blob, suggesting that the primary ionizing source may come from MaNGA 1-24145, likely a low-activity AGN. Possible explanations of this $H_{\alpha}$ blob include AGN outflow, the gas remnant being tidally or ram-pressure stripped from MaNGA 1-24145, or an extremely low surface brightness (LSB) galaxy. However, the stripping scenario is less favoured according to galaxy merger simulations and the morphology of the $H_{\alpha}$ blob. With the current data, we can not distinguish whether this $H_{\alpha}$ blob is ejected gas due to a past AGN outburst, or a special category of `ultra-diffuse galaxy' (UDG) interacting with MaNGA 1-24145 that further induces the gas inflow to fuel the AGN in MaNGA 1-24145.