SDSS IV MaNGA: Discovery of an Hα Blob Associated with a Dry Galaxy Pair—Ejected Gas or a “Dark” Galaxy Candidate?

Abstract

We report the discovery of a mysterious giant Hα blob that is ∼ 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 Hα blob is ∼ 3-4 kpc in radius, and the Hα distribution is centrally concentrated. However, there is no optical continuum counterpart in deep broadband images reaching ∼26.9 mag arcsec −2 in surface brightness. We estimate that the masses of ionized and cold gases are 3.3 × 10 5 M ⊙ and < 1.3 × 10 9 M ⊙ , respectively. The emission-line ratios indicate that the Hα blob is photoionized by a combination of massive young stars and AGN. Furthermore, the ionization line ratio decreases from MaNGA 1-24145 to the Hα blob, suggesting that the primary ionizing source may come from MaNGA 1-24145, likely a low-activity AGN. Possible explanations of this Hα 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 Hα blob. With the current data, we can not distinguish whether this Hα 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.

Outlying Hα emitters in SDSS IV MaNGA

“…Our final sample of outlying Hα emitters is summarised in Table 1. We have also independently rediscovered the outlying Hα emitter from Lin et al (2017), but do not discuss it here as it has been extensively studied in their paper.…”

“…The BPT diagram is widely used to classify galaxies into star forming (SF), Seyferts, and low-ionisation emission line regions (LINERs) (e.g., Kewley et al 2006;Kauffmann et al 2003). The BPT classification scheme is also used in resolved IFU studies (e.g., Belfiore et al 2016;Lin et al 2017 [NII]([SII])/Hα diagnostic diagram for our sample of outlying Hα emitters. The corresponding line ratios for the host galaxies are determined using SDSS DR14 spectroscopy.…”

“…Using a detailed analysis of the gas kinematics, gas phase metallicity, and emission-line ratios the authors argue that the scenario is most consistent with an gas accretion event. Recently, Lin et al (2017) have discovered a large outlying Hα blob (∼ 3-4 kpc in radius) around 8 kpc away from the host galaxy which is undergoing a dry galaxy merger. Here the authors argue that this could either be an AGN outburst or a possible "ultra-diffuse galaxy".…”

The Fourteenth Data Release of the Sloan Digital Sky Survey: First Spectroscopic Data from the Extended Baryon Oscillation Spectroscopic Survey and from the Second Phase of the Apache Point Observatory Galactic Evolution Experiment

“…PIPE3D is an IFU-focused analysis pipeline that calculates intermediate data products and is able to obtain both the stellar population and the ionized gas properties extracted from the data cubes in an automatic way. This pipeline is based on FIT3D, the details of which are presented in Sánchez et al (2016aSánchez et al ( , 2016b, which show some examples based on CALIFA (Cano-Díaz et al 2016;Sánchez-Menguiano et al 2016;Sánchez et al 2017b) and MaNGA/P-MaNGA (BarreraBallesteros et al 2016;Ibarra-Medel et al 2016;BarreraBallesteros et al 2017;Lin et al 2017) data sets. The MaNGA data products provided by Pipe3D are presented in Sánchez et al (2017a).…”

“…For example, published results based on the MaNGA DR14 data include Barrera-Ballesteros et al (2017), who discuss the integrated stellar mass-metallicity relation for more than 1700 galaxies; Zhu et al (2017), who revisit the relation between the stellar surface density, the gas surface density, and the gasphase metallicity of typical disk galaxies in the local universe; Belfiore et al (2017), who study the gas-phase metallicity and nitrogen abundance gradients traced by star-forming regions in a representative sample of 550 nearby galaxies; and Lin et al (2017), who report the discovery of a mysterious giant Hα blob that is ∼8 kpc away from a component of a dry galaxy merger. Bizyaev et al (2017) presented a study of the kinematics of the extraplanar ionized gas around several dozen galaxies, while Jones et al (2017) conducted a detailed study of extraplanar diffuse ionized gas stacking spectra from 49 edge-on, late-type galaxies as a function of distance from the midplane of the galaxy.…”