Physical properties of circumnuclear ionising clusters. IV. NGC 1097

The circumnuclear star-forming ring of the barred spiral galaxy NGC 1097 provides a unique laboratory to study star formation under extreme conditions. This work aims to derive the physical properties of the circumnuclear star-forming regions (CNSFRs) using MUSE integral field spectroscopy observations. A total of 24 individual ionised HII are identified and analysed within its ring, which spans from $\sim$385 pc to $\sim$1.3 kpc. Despite the complex nuclear activity, all HII regions are found to be purely photoionised. Directly derived abundances reveal supersolar metallicities, with the highest one exceeding five times the solar value (12+log(S/H) = 7.875 $\pm$ 0.353, T$_e$([SIII]) = 3912 $\pm$ 567 K), and representing the highest abundance reported to date. In this high-metallicity regime, we find a break in the ionisation parameter-[SII]/[SIII] relation, which can be explained by changes in the ionisation structure and line emissivities, as confirmed by photoionisation models that successfully reproduce the observed emission-line ratios. Our results also indicate that the local gas supply regulates the star formation activity within the ring, with the young stars ionising 8 % of the total gas in the ring. Furthermore, our findings support a propagating starburst scenario, originating in the galaxy nucleus and extending towards the ends of the bar and into the circumnuclear ring through bar-driven shocks, this being consistent with the results of previous multi-wavelength studies. Finally, we likely detect optical signatures associated with one of the two known jets in this galaxy. This finding, together with the radio core emission previously found at sub-parsec scales, reflects the presence of feedback processes operating even on small galactic disc scales.

💡 Research Summary

This paper presents a comprehensive analysis of the circumnuclear star‑forming ring in the barred spiral galaxy NGC 1097 using publicly available MUSE integral‑field spectroscopy data. The authors first identified the spatial extent of the ring (≈ 385 pc – 1.3 kpc) from the H α surface‑brightness profile and then applied an automated segmentation algorithm to the H α map, extracting 24 discrete H II regions within the ring (plus five additional regions outside the ring). Quality cuts (EW(H α) > 6 Å and 2.7 < H α/H β < 6.0) ensured that all selected regions are dominated by photo‑ionisation rather than shocks or AGN excitation.

For each region the authors measured the fluxes of the strong nebular lines (