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

Abstract

We present the analysis of archival Very Large Telescope (VLT) Multi-Unit Spectroscopic Explorer (MUSE) observations of 179 HII 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 $\log (\frac{\mathrm{N}}{\mathrm{O}})=-1.3\pm 0.2$ and $12+\log (\frac{\mathrm{O}}{\mathrm{H}})=8.9\pm 0.2$. We also find the so-called `blue-bump', broad He II $\lambda 4686$, in the spectrum of this knot of massive star-formation; its luminosity being consistent with the presence of $\sim 430$ Wolf-Rayet (WR) stars of the Nitrogen late-type. We determined the O abundances for 137 HII regions using the strong-line method; we report a median value of $12+\log (\frac{\mathrm{O}}{\mathrm{H}})=8.5\pm 0.8$. 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 $\sim 2.5-20$ 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.

20 pages. Accepted in MNRAS