Integral field spectroscopy and numerical simulations of the NGC 2207/IC 2163 system

TL;DR

This study combines SITELLE integral field spectroscopy with chemodynamical SPH simulations (GCD+) to dissect the impact of the NGC 2207/IC 2163 interaction on ionized-gas physics. By mapping over 1100 HII-region complexes, it derives oxygen abundance gradients using multiple strong-line calibrations, finds shallower gradients ($$\sim -0.015$$ dex kpc$^{-1}$) than in isolated spirals, and detects negligible azimuthal variation, indicating efficient mixing. The Hα luminosity functions reveal arm-dominated regions are typically brighter and, in IC 2163, show a potential difference in the cloud-mass spectrum associated with eyelid shocks. Simulations reproduce the observed morphology, SF bursts after pericentre passages, and metallicity evolution, underscoring gas inflows and interaction-driven star formation as key drivers of chemical enrichment; two field dwarfs with similar systemic velocities are identified as potential minor participants in the system.

Abstract

We present integral field spectroscopy of the interacting galaxy system NGC 2207/IC 2163 obtained with the imaging Fourier Transform Spectrometer SITELLE. Approximately 1000 HII region complexes are detected in both galaxies and analyzed using their strong optical emission lines. Their properties were studied via BPT diagrams and their luminosity function. We conducted a detailed study of the distribution of oxygen abundance across the system using a series of strong-line O/H indicators and calibrations. Both galaxies exhibit negative galactocentric abundance gradients with a slope -0.015 dex kpc$^{-1}$. There are marginal signs of discontinuities in the O/H gradients with some indicators while no significant azimuthal variations are seen. A shallower slope in the HII region luminosity function between the arm and inter-arm regions in IC 2163 is observed, supporting previous conclusion that the star formation process in this galaxy eyelids has been altered during the interaction. The kinematics of the ionised gas reveal disturbed velocity fields, AGN-like features in the nucleus of NGC 2207, and elevated velocity dispersion in turbulent or feedback-driven regions. To interpret these findings, we modeled the collision using the numerical algorithm GCD+. The simulation reproduces key features of the system and demonstrates how close passages drive enhanced star formation and localized chemical enrichment. Finally, two dwarf galaxies in the field are found to have very similar systemic velocities as their larger counterparts, and could well play a minor role in the global interaction based on their morphology and position.

Figures (23)

• Figure 1: SITELLE image of the NGC 2207 (right) and IC 2163 (left) field, built from the deep SN2 and SN3 images combined with the H$\alpha$ map. The field of view is $8.7\arcmin \times 8.7\arcmin$. Arrows point to particular structures in the system and the two dwarf galaxies discussed in section \ref{['section:quadruple']}.
• Figure 2: (a) Emission regions initially detected (red crosses) on the H$\alpha$ amplitude map. (b) Overview of the detection methodology for Hii region complexes. It includes illustrations of the detection process, zone of influence and the final delineation of emission regions. (c) Examples of detected emission regions in the tidal tail of IC 2163 (left) and a spiral arm of NGC 2207 (right) from the final catalog. Each example includes a broad view encompassing the zones of influence of neighboring regions on the H$\alpha$ amplitude map and a close-up view of an Hii region complex displaying H$\alpha$ amplitude along with SN3, SN2, and SN1 deep frames. Different display levels are used across the map and deep frames to better highlight the emission distribution in each. (d) Sky-subtracted spectra of these complexes are shown, highlighting the main fitted emission lines, from left to right, [Sii]$\lambda$6731, [Sii]$\lambda$6716, [Nii]$\lambda$6583, H$\alpha$, [Nii]$\lambda$6548, [Oiii]$\lambda$5007, [Oiii]$\lambda$4959, H$\beta$, and [Oii]$\lambda$3727. Note that the oscillations near each line are not noise, but rather due to the natural instrument line shape, a sinc function.
• Figure 3: The $A_v$ extinction maps for integrated fluxes from the final emission domains, derived from the H$\alpha$ to H$\beta$ ratio.
• Figure 4: BPT diagrams for Hii region complexes displaying the [Oiii]/H$\beta$ ratios as a function of (a) [Nii]/H$\alpha$, (b) [Sii]/H$\alpha$, and (c) [Oiii]/[Oii]. The color scheme in the top panel is determined by the logarithm of the H$\alpha$ flux, while in the bottom panel, the colors are based on point density. The boundaries are delineated by the equations of kauffmann2003host, kewley2001optical, schawinski2007observational, and law2021sdss.
• Figure 5: Results of the separation of Hii region complexes between NGC 2207 and IC 2163. (a) Examples of three criteria used for the separation, with colors representing, top to bottom, the ratios log(([Oiii]$\lambda$5007/H$\beta$) / ([Nii]$\lambda$6583/H$\alpha$)), log([Nii]$\lambda$6583/H$\alpha$) and log([Oiii]$\lambda$5007/H$\beta$) from the integrated fluxes of the final emission domains. The colormap boundaries have been adjusted to distinguish different values in the overlap region. (b) Contours delineating the domain of Hii region complexes with the overlap region identified in darker shade. (c) Zoomed-in view of the results in the overlap region depicted by the colored regions. The orange complexes are classified in IC 2163 by both methods, while the blue ones are classified in NGC 2207. The hatched regions represent complexes with ambiguous separations : the color corresponds to manual separation and the hatching indicates that the unsupervised machine learning classifies them in the other galaxy.