Chemical abundances and ionizing mechanisms in the star-forming double-ring of AM 0644-741 using MUSE data

Abstract

ABSTRACT We present the analysis of archival Very Large Telescope Multi-Unit Spectroscopic Explorer observations of 179 H ii regions in the star-forming double-ring collisional galaxy AM 0644-741 at 98.6 Mpc. We determined ionic abundances of He, N, O, and Fe using the direct method for the brightest H ii region (ID 39); we report $\log \rm {(\frac{N}{O})}=-1.3\pm 0.2$ and $12+\log \rm {(\frac{O}{H})}=8.9\pm 0.2$. We also find the so-called ‘blue-bump’, broad He ii λ4686, in the spectrum of this knot of massive star formation; its luminosity being consistent with the presence of ∼430 Wolf–Rayet (WR) stars of the nitrogen late-type. We determined the O abundances for 137 H ii regions using the strong-line method; we report a median value of $12+\log \rm {(\frac{O}{H})}=8.5\pm 0.8$. The location of three objects, including the WR complex, coincide with that of an Ultra Luminous X-ray source. Nebular He ii is not detected in any H ii region. We investigate the physical mechanisms responsible for the observed spectral lines using appropriate diagnostic diagrams and ionization models. We find that the H ii regions are being photoionized by star clusters with ages ∼2.5–20 Myr and ionization potential −3.5 <log 〈U〉<−3.0. In these diagrams, a binary population is needed to reproduce the observables considered in this work.