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High resolution mapping of molecular tori with ALMA

F. Combes, A. Audibert, S. Garcia-Burillo, L. Hunt, S. Aalto, V. Casasola, K. Dasyra, A. Eckart, M. Krips, S. Martin, S. Muller, K. Sakamoto, P. van der Werf, S. Viti

TL;DR

This work presents 1–2 pc-scale ALMA maps of the circumnuclear molecular tori in two nearby Seyfert galaxies, revealing geometrically thin, clumpy, and warped structures that depart from the classical thick-torus paradigm. By combining CO(3-2) and HCO+(4-3) data at unprecedented resolution, the authors measure distinct torus morphologies in NGC 613 (a small ring with a central NW hole) and NGC 1672 (an edge-on, warped torus embedded in a decoupled disk), derive black hole masses ($M_{\rm BH}\approx3.4\times10^7$ and $4.5\times10^7\,M_\odot$), and assess the presence of molecular outflows (not detected at pc scales). The results imply short-lived, transient tori shaped by misalignment and warp evolution, challenging the notion of a universal thick dusty torus and highlighting gravity torques as a key driver of SMBH fueling. Overall, the study demonstrates ALMA’s power to dissect AGN feeding and feedback at the crucial 1–10 pc scale, with implications for torus lifetimes, obscuration, and the variability of AGN.

Abstract

Recent high resolution mapping of the circum-nuclear regions of Active Galactic Nuclei (AGN) has revealed the existence of geometrically thin nuclear disks, in general randomly oriented with respect to their galaxy hosts. These molecular tori have typical radii of 10~pc, and contain a few 10$^7$ M$_\odot$ of H$_2$, with H$_2$ column densities between 10$^{23}$ and 10$^{25}$ cm$^{-2}$. We mapped two of the most massive of these molecular tori with higher resolution, in order to unveil their morphology and kinematics, their possible warp and clumpiness, and derive their stability and life-time. We used the highest resolution possible with ALMA (16~km baseline) in Band 7, taking into account for mapping CO(3-2) and HCO$^+$(4-3) the compromise between sensitivity and resolution. New features are discovered at the high resolution, obtained with a beam of 0.015\arcsec, equivalent to $\sim$ 1~pc scale, at their $\sim$ 15~Mpc distance. The molecular torus in NGC~613 appears like a ring, depleted in molecular gas near the center. The depletion region is displaced by 3~pc toward the NW from the AGN position, meaning some $m=1$ asymmetry in the torus. The molecular torus in NGC~1672 has a different position angle from previous lower-resolution observations, and is edge-on, revealing a geometrically very thin torus (axis ratio 6.5 to 10), with a clear warp. This confirms that the classical model of a simple geometrically thick dusty torus is challenged by high resolution observations. The nuclear disks appear clumpy, and slightly lopsided. The molecular outflow in NGC~613 is now resolved out. Well inside the sphere of influence of the black holes (BH), we are now able to determine more accurately their mass, for those Seyfert spiral galaxies, in a region of the M-sigma relation where the scatter is maximum.

High resolution mapping of molecular tori with ALMA

TL;DR

This work presents 1–2 pc-scale ALMA maps of the circumnuclear molecular tori in two nearby Seyfert galaxies, revealing geometrically thin, clumpy, and warped structures that depart from the classical thick-torus paradigm. By combining CO(3-2) and HCO+(4-3) data at unprecedented resolution, the authors measure distinct torus morphologies in NGC 613 (a small ring with a central NW hole) and NGC 1672 (an edge-on, warped torus embedded in a decoupled disk), derive black hole masses ( and ), and assess the presence of molecular outflows (not detected at pc scales). The results imply short-lived, transient tori shaped by misalignment and warp evolution, challenging the notion of a universal thick dusty torus and highlighting gravity torques as a key driver of SMBH fueling. Overall, the study demonstrates ALMA’s power to dissect AGN feeding and feedback at the crucial 1–10 pc scale, with implications for torus lifetimes, obscuration, and the variability of AGN.

Abstract

Recent high resolution mapping of the circum-nuclear regions of Active Galactic Nuclei (AGN) has revealed the existence of geometrically thin nuclear disks, in general randomly oriented with respect to their galaxy hosts. These molecular tori have typical radii of 10~pc, and contain a few 10 M of H, with H column densities between 10 and 10 cm. We mapped two of the most massive of these molecular tori with higher resolution, in order to unveil their morphology and kinematics, their possible warp and clumpiness, and derive their stability and life-time. We used the highest resolution possible with ALMA (16~km baseline) in Band 7, taking into account for mapping CO(3-2) and HCO(4-3) the compromise between sensitivity and resolution. New features are discovered at the high resolution, obtained with a beam of 0.015\arcsec, equivalent to 1~pc scale, at their 15~Mpc distance. The molecular torus in NGC~613 appears like a ring, depleted in molecular gas near the center. The depletion region is displaced by 3~pc toward the NW from the AGN position, meaning some asymmetry in the torus. The molecular torus in NGC~1672 has a different position angle from previous lower-resolution observations, and is edge-on, revealing a geometrically very thin torus (axis ratio 6.5 to 10), with a clear warp. This confirms that the classical model of a simple geometrically thick dusty torus is challenged by high resolution observations. The nuclear disks appear clumpy, and slightly lopsided. The molecular outflow in NGC~613 is now resolved out. Well inside the sphere of influence of the black holes (BH), we are now able to determine more accurately their mass, for those Seyfert spiral galaxies, in a region of the M-sigma relation where the scatter is maximum.

Paper Structure

This paper contains 17 sections, 3 equations, 18 figures, 4 tables.

Table of Contents

  1. Introduction
  2. The sample
  3. Observations and data analysis
  4. Results
  5. Continuum maps
  6. CO(3-2) line maps: morphology
  7. CO(3-2) line: Kinematics
  8. Molecular gas outflow
  9. Black hole mass
  10. HCO$^+$(4-3) line
  11. Discussion
  12. NGC 613 model in view of the new morphology
  13. NGC 1672 model in view of the new morphology
  14. Warping and misalignment
  15. Conclusions
  16. ...and 2 more sections

Figures (18)

  • Figure 1: Continuum map at 0.86mm of the NGC 613 (left) and NGC 1672 (right) nucleus. The spatial scale is RA-DEC offset in arcsec from the central position of Table \ref{['tab:samp']}. The color scale is in mJy beam$^{-1}$. The first contour is at 2$\sigma$, and the next contours spaced by 1$\sigma$, see Sec. \ref{['sec:cont']} for the value of $\sigma$. The AGN center is indicated by a black cross, and the bar at bottom left is 1 pc long. A red arrow indicates the jet direction in NGC 613.
  • Figure 2: Low resolution (but without the most compact configurations) at $\sim$ 0.1" (left), zoomed in the middle and high resolution at $\sim$ 0.015" (right) of the CO(3-2) surface density toward NGC 613. The left and middle moment-0 maps have been integrated over a velocity range -175 to 175 km s$^{-1}$, and with a threshold of 2.3 mJy beam$^{-1}$. In the right panel, the CO(3-2) contours of the low resolution map are overlaid. While the left panel is plotted in absolute coordinates, the spatial scales of the two others are RA-DEC offsets in arc-seconds, from the center of Table \ref{['tab:samp']}. The color-scale is in Jy beam$^{-1}$ km s$^{-1}$.
  • Figure 3: Same as Fig \ref{['fig:n613-lo-hires']}, for NGC 1672, with a different color bar. The left and middle moment-0 maps have been integrated over a velocity range -200 to 200 km s$^{-1}$, and with a threshold of 4 mJy beam$^{-1}$. The color-scale is in Jy beam$^{-1}$ km s$^{-1}$.
  • Figure 4: Comparison of the total spectrum within a circle of 0.2 in diameter between the high resolution CO(3-2) observation of NGC 613 (left, bottom) and low resolution (left, top). Same for NGC 1672 at right. The vertical scale is Flux in Jy. The involved aperture is much larger than the torus here.
  • Figure 5: Cartoons to identify the principal components in NGC 613 (left) and NGC 1672 (right). For NGC 613, there is an H$_2$O maser detected in the (0,0) nucleus position Kondratko2006Castangia2008. The scales are RA-DEC offsets in arc-seconds. The AGN centers are indicated by faint crosshairs.
  • ...and 13 more figures