The metal-poor dwarf irregular galaxy candidate next to Mrk 1172

ABSTRACT In this work, we characterize the properties of the object SDSS J020536.84−081424.7, an extended nebular region with projected extension of 14 × 14 kpc2 in the line of sight of the ETG Mrk 1172, using unprecedented spectroscopic data from MUSE. We perform a spatially resolved stellar population synthesis and estimate the stellar mass for both Mrk 1172 (1 × 1011 M⊙) and our object of study (3 × 109 M⊙). While the stellar content of Mrk 1172 is dominated by an old (∼10 Gyr) stellar population, the extended nebular emission has its light dominated by young to intermediate age populations (from ∼100 Myr to ∼1 Gyr) and presents strong emission lines such as H β; [O iii] λλ4959, 5007 Å; H α; [N ii] λλ6549, 6585 Å; and [S ii] λλ6717, 6732 Å. Using these emission lines, we find that it is metal poor (with Z ∼ 1/3 Z⊙, comparable to the LMC) and is actively forming stars (0.70 M⊙ yr−1), especially in a few bright clumpy knots that are readily visible in H α. The object has an ionized gas mass ≥3.8 × 105 M⊙. Moreover, the motion of the gas is well described by a gas in circular orbit in the plane of a disc and is being affected by interaction wtih Mrk 1172. We conclude that SDSS J020536.84−081424.7 is most likely a dwarf irregular galaxy (the dIGal).