Galaxy Activity, Torus and Outflow Survey (GATOS) X: Molecular gas clumpiness under the influence of AGN

TL;DR

This study uses high-resolution ALMA CO($3-2$) observations of 16 nearby Seyfert galaxies to quantify the small-scale clumpiness of molecular gas in nuclear (50 pc) and circumnuclear (200 pc) regions. By applying three distinct clumpiness metrics across a range of smoothing scales and employing robust statistical tests, the authors demonstrate a consistent anti-correlation between gas clumpiness and AGN power, with a notable turnover near $L_{ ext{X}} oughly 10^{42}$ erg s$^{-1}$. The results imply that AGN feedback smooths dense molecular structures in the central regions, potentially suppressing star formation, while larger-scale gas shows a more gradual response. This work underscores the scale- and method-dependent impact of AGN on the ISM and provides observational constraints for models of AGN-galaxy co-evolution.

Abstract

The distribution of molecular gas on small scales regulates star formation and the growth of supermassive black holes in galaxy centers, yet the role of active galactic nuclei (AGN) feedback in shaping this distribution remains poorly constrained. We investigate how AGN influence the small-scale structure of molecular gas in galaxy centers, by measuring the clumpiness of CO(3 - 2) emission observed with the Atacama Large Millimeter/submillimeter Array (ALMA) in the nuclear regions (50 - 200 pc from the AGN) of 16 nearby Seyfert galaxies from the Galaxy Activity, Torus, and Outflow Survey (GATOS). To quantify clumpiness, we apply three different methods: (1) the median of the pixel-by-pixel contrast between the original and smoothed maps; (2) the ratio of the total excess flux to the total flux, after substracting the background smoothed emission; and (3) the fraction of total flux coming from clumpy regions, interpreted as the mass fraction in clumps. We find a negative correlation between molecular gas clumpiness and AGN X-ray luminosity (L_X), suggesting that higher AGN activity is associated with smoother gas distributions. All methods reveal a turnover in this relation around L_X = 10^{42} erg/s, possibly indicating a threshold above which AGN feedback becomes efficient at dispersing dense molecular structures and suppressing future star formation. Our findings provide new observational evidence that AGN feedback can smooth out dense gas structures in galaxy centers.

Figures (13)

• Figure 1: ALMA observations of the CO($3-2$) emission in the target galaxy NGC 3227 and the clumpiness computation applied within a 400 pc $\times$ 400 pc box around the AGN. First panel: CO($3-2$) emission at native resolution ($10.5 \times 9.4$ pc), with contours at $(3, 5, 10, 20, 30, 50) \times \sigma$. Second panel: CO($3-2$) emission smoothed to a common physical resolution of 16 pc, using contours at the same $\sigma$ levels. Third panel: CO($3-2$) emission further smoothed with a Gaussian kernel of FWHM 50 pc, with the same contour levels. Fourth panel: clumpiness map computed with Method #1 (see Section \ref{['sec:methods_description']}), with contours every 0.1 from 0 to 1. In all panels, the two ellipses correspond to circles of 50 and 200 pc radius in the galaxy plane, projected onto the sky. The red ellipse within a square in the bottom right of each panel shows the native ALMA beam ($10.5 \times 9.4$ pc in this case).
• Figure 2: Clumpiness as a function of AGN intrinsic X-ray $2-10$ keV luminosity. Clumpiness is measured within the nuclear 50-pc-radius aperture with the three different methods described in Section \ref{['sec:methods_description']} (panels left to right). Data points (square markers; nine per galaxy) are coloured according to the nine smoothing FWHM sizes adopted (see colour bar in the central panel). Coloured lines show the pwlf fits to the data points, with the same colour as the fitted dataset; lines are solid where a linear fit is statistically accepted, and dashed where it is rejected (see Section \ref{['sec:stat_methods']}).
• Figure 3: Clumpiness as a function of AGN X-ray luminosity. Clumpiness is measured within the nuclear 50-pc-radius aperture with the three different methods described in Section \ref{['sec:methods_description']} (panels left to right). Data points are shown as blue square markers with error bars, computed for a smoothing kernel FWHM of 90 pc. The broken orange lines indicate the pwlf fits to the data, and the shaded regions show the corresponding 95% confidence intervals.
• Figure 4: Clumpiness measured within the larger 200-pc-radius aperture, plotted as a function of AGN X-ray luminosity, Panels, lines, and markers are as in Figure \ref{['fig:smooths_comparison']}.
• Figure 5: Clumpiness measured within the 50-pc-radius aperture, plotted as a function of cold molecular gas concentration index (CCI $\equiv \log (\Sigma_{50} / \Sigma_{200})$), where $\Sigma_r$ is the surface density within an aperture of radius $r$ parsec. Panels, lines, and markers are as in Figure \ref{['fig:smooths_comparison']}.