New techniques to investigate the AGN-SF connection with integral field spectroscopy

TL;DR

This study introduces a robust, non-parametric basis-spectrum method to decompose spatially resolved emission into star-formation and AGN components using IFU data, enabling accurate, spaxel-level SFRs and AGN luminosities. By combining this with pPXF-based star-formation-history fitting (including an AGN continuum) and thorough recovery tests, the work derives LW stellar ages and an Eddington-ratio proxy $L_{ m [OIII]}/\sigma_*^4$ for 54 S7 galaxies. The results show a moderately strong link between AGN accretion and SFR across apertures, and a tendency for stronger correlations with very young nuclear stellar populations ($\tau_{\rm LW}<100$ Myr), though overall correlations are weak due to observational uncertainties and sample biases. The authors highlight the method’s applicability to large IFU surveys (e.g., MaNGA, CALIFA, LVM, SAMI, TYPHOON) and future work with radio-quiet AGN to better constrain the physical mechanisms and timescales of the AGN–SF connection.

Abstract

Understanding the connection between active galactic nuclei and star-formation (the AGN-SF connection) is one of the longest standing problems in modern astrophysics. In the age of large Integral Field Unit (IFU) surveys, studies of the AGN-SF connection greatly benefit from spatially resolving AGN and SF contributions to study the two processes independently. Using IFU data for 54 local active galaxies from the S7 sample, we present a new method to separate emission from AGN activity and SF using mixing sequences observed in the [NII]$λ6584$/H$α$ vs. [OIII]$λ5007$/H$β$ Baldwin-Phillips-Terlevich (BPT) diagram. We use the new decomposition method to calculate the H$α$ star-formation rate and AGN [OIII] luminosity for the galaxies. Our new method is robust to outliers in the line-ratio distribution and can be applied to large galaxy samples with little manual intervention. We infer star-formation histories (SFHs) using pPXF, conducting detailed recovery tests to determine the quantities that can be considered robust. We test the correlation between the AGN Eddington ratio, using the proxy L[OIII]/$σ_*^4$, and star-formation properties. We find a moderately strong correlation between the Eddington ratio and the star-formation rate (SFR). We also observe marginally significant correlations between the AGN Eddington ratio and the light-weighted stellar age under 100 Myr. Our results point to higher AGN accretion being associated with young nuclear star formation under 100 Myr, consistent with timelines presented in previous studies. The correlations found in this paper are relatively weak; extending our methods to larger samples, including radio-quiet galaxies, will help better constrain the physical mechanisms and timescales of the AGN-SF connection.

Figures (23)

• Figure 1: BPT diagram (a), spatial map (inset in a), and [Sii] diagram (b) of a representative galaxy from the clean sample. There is a tight and complete mixing sequence extending from the Hii region and AGN NLR region. The [Sii] diagram shows data extending to the Seyfert region. The 2D spatial distributions show high line ratios extending radially outwards from the central nucleus, consistent with ionization dominated by the central AGN. Please note that axis limits are chosen individually for each galaxy to avoid compressing the spaxel distributions within a fixed plotting range.
• Figure 2: Same plots as \ref{['fig:clean']} for two representative galaxies from the ambiguous sample. ESO420-G13 is likely edge-on with the ionization cone pointing to the right. ESO420-G13 may be shock-heated as evidenced by the uneven and dispersed ionization seen in the 2D spatial map and the cluster of spaxels extending off the bottom of the mixing sequence lying in the LINER region. NGC5128 shows a well-defined mixing sequence. However, it contains a high number of spaxels in the LINER region and the same spaxels are widespread throughout the galaxy, consistent with shock excitation.
• Figure 3: Same plots as \ref{['fig:clean']} for a representative galaxy from the AGN-dominated sample. The majority of spaxels lie in the Seyfert region in the [Sii]/${\rm H}\upalpha$ vs. [Oiii]$\uplambda$5007$\angstrom$/${\rm H}\upbeta$ diagram.
• Figure 4: Same plots as \ref{['fig:clean']} for two representative galaxies from the unsuitable sample. NGC3831 does not display a complete mixing sequence and is shock-dominated. NGC4507 shows high scatter above the Ke01 line in the BPT diagram, indicating significant shock contamination as the SF+AGN and SF+shock mixing sequences overlap.
• Figure 5: A comparison of basis spectra points from the Davies2016 fitting method (a) and the Mahalanobis method (b) in each plot pair. The Mahalanobis method is more robust to issues in the linear fit, including outliers (NGC6860), high scatter (ESO362-G18), a strong SF-metallicity sequence (IC1657), and dense clustering in the AGN region (NGC424). Please note that axis limits are chosen individually for each galaxy to avoid compressing the spaxel distributions within a fixed plotting range.