Ionization processes in a local analogue of distant clumpy galaxies: VLT MUSE IFU spectroscopy and FORS deep images of the TDG NGC 5291N

TL;DR

This study uses VLT MUSE IFU spectroscopy and deep FORS imaging to analyze ionization processes and physical conditions in a young dwarf galaxy formed within a collisional HI ring, revealing homogeneous star-forming regions and complex ionization features.

Contribution

It provides high-resolution spatial analysis of ionization mechanisms and physical conditions in a TDG, highlighting the homogeneity of star-forming regions and unusual ionization signatures.

Findings

Homogeneous physical conditions with half-solar oxygen abundance.

Presence of narrow dust lanes and atypical [OI] emission regions.

Diffuse blue continuum without associated star formation or old stars.

Abstract

We present IFU observations with MUSE@VLT and deep imaging with FORS@VLT of a dwarf galaxy recently formed within the giant collisional HI ring surrounding NGC 5291. This TDG-like object has the characteristics of typical z=1-2 gas-rich spiral galaxies: a high gas fraction, a rather turbulent clumpy ISM, the absence of an old stellar population, a moderate metallicity and star formation efficiency. The MUSE spectra allow us to determine the physical conditions within the various complex substructures revealed by the deep optical images, and to scrutinize at unprecedented spatial resolution the ionization processes at play in this specific medium. Starburst age, extinction and metallicity maps of the TDG and surrounding regions were determined using the strong emission lines Hbeta, [OIII], [OI], [NII], Halpha and [SII] combined with empirical diagnostics. Discrimination between different ionization mechanisms was made using BPT--like diagrams and shock plus photoionization models. Globally, the physical conditions within the star--forming regions are homogeneous, with in particular an uniform half-solar oxygen abundance. At small scales, the derived extinction map shows narrow dust lanes. Regions with atypically strong [OI] emission line immediately surround the TDG. The [OI] / Halpha ratio cannot be easily accounted for by photoionization by young stars or shock models. At larger distances from the main star--forming clumps, a faint diffuse blue continuum emission is observed, both with the deep FORS images and MUSE data. It does not have a clear counterpart in the UV regime probed by GALEX. A stacked spectrum towards this region does not exhibit any emission line, excluding faint levels of star formation, nor stellar absorption lines that might have revealed the presence of old stars. Several hypotheses are discussed for the origin of these intriguing features.