Physical properties of circumnuclear ionising clusters. IV. NGC 1097

TL;DR

This study uses MUSE integral-field spectroscopy to map the circumnuclear ring of NGC 1097, identifying 24 CNSFRs between $\sim$385 pc and $\sim$1.3 kpc and measuring auroral $[\mathrm{SIII}]\,6312$ to derive direct sulphur abundances. The CNSFRs are predominantly photoionised and exhibit supersolar abundances, with the highest region reaching $12+\log(S/H)=7.88\pm0.35$ and $T_e$([SIII]) $\approx$ $3912\pm567$ K, while CLOUDY models with young $\sim$4 Myr clusters and $\log u\sim-3$ reproduce observed line ratios. A break in the $\log u$–$[\mathrm{SII}]/[\mathrm{SIII}]$ relation at high metallicity is attributed to changes in the ionisation structure rather than photon leakage, and the data indicate a propagating starburst from the nucleus along the bar into the ring. The ionising clusters collectively account for about 8% of the ring gas, with $\log Q(H_0)$ between $50.66$ and $52.43$ and a ring-ionising mass $M_{ion}$ of $7.74\times10^7$ M$_{\odot}$, suggesting gas supply regulates star formation. Optical jets aligned with AGN activity are also detected, implying feedback operates on sub-kpc scales. These results highlight extremely elevated metal content in CNSFRs and challenge standard abundance calibrations at high metallicity, while supporting a cohesive picture of bar-driven inflows fueling circumnuclear star formation alongside AGN feedback.

Abstract

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.

Figures (10)

• Figure 1: From left to right and top to bottom: maps of the observed H$\alpha$ and [OIII]$\lambda$5007 Å emission line fluxes (in units of 10$^{-20}$ erg/s/cm$^2$ and logarithmic scale); A$_V$ extinction (in magnitudes); observed continuum fluxes in the blue and red parts of the spectrum (5400 Å and 8150Å respectively), in units of 10$^{-17}$ erg/s/cm$^2$ and logarithmic scale); and EW($H\alpha$) in Å. Orientation is North up and East to the left.
• Figure 2: HII regions selected using our segregation program on the H$\alpha$ observed emission line map. The aperture used to define the ring for the segregation of the regions is overplotted in blue. Logarithmic color scale. Orientation is north up, east to the left. The physical scale is represented at the bottom left corner of the map.
• Figure 3: Extracted spectrum of region R2.
• Figure 4: Upper left panel: map of the observed [OIII]$\lambda$ 5007 Å /[NII]$\lambda$ 6584 Å ratio. Upper right panel: Emission line spectra of regions contaminated by the AGN emission (a, b, c) and the LINER nucleus of the galaxy (*). Lower left panel: the [OIII]/H$\beta$ vs [NII]/H$\alpha$ diagnostic diagram. Over-plotted, derived separations between LINER/Seyfert 2007MNRAS.382.1415S and HII regions 2001ApJ...556..121KKauffmann2003 Lower right panel: The [SII]/H$\alpha$ - [SIII]/H$\alpha$ diagnostic diagram. Over-plotted, dust-free AGN photoionisation models 2004ApJS..153....9G and star forming models by ngc7742paper.
• Figure 5: The S$_{23}$ abundance calibration from 2022MNRAS.511.4377D. Red contours correspond to disc HII regions while blue contours correspond to HII galaxies. Purple dots represent the CNFSRs analysed in this work. The Cloudy models described in the text appear superimposed.