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Outflowing shocked gas dominates the NIR H$_2$ emission from the dual AGN NGC6240

J. Carlsen, C. Cicone, B. Hagedorn, K. Rubinur, P. Andreani, K. Dasyra, P. Severgnini, C. Vignali, R. Morganti, T. Oosterloo, A. Lasrado, E. Lopez-Rodriguez, S. Shen

Abstract

[Abridged] We present a multi-line study of the kinematics of the molecular and ionised gas phases in the central 2 kpc of NGC6240, based on JWST/NIRSpec and ALMA observations. We devised a new spectral-line fitting approach to de-blend rotating and non-rotating gas components, which is better tailored to the extreme feedback mechanisms at work in NGC6240. We find that ~65% of the Pa$α$, H$_2$, and [FeII] line fluxes within the NIRSpec field of view arise from gas components that are kinematically decoupled from the stars. The NIR H$_2$ lines show the most deviation from the stars, with peak emission between the two rotating stellar structures. The PAH 3.3$μ$m feature does not follow the NIR H$_2$ morphology, indicating that the latter does not trace PDRs. In the non-rotating gas components, we identify a biconical wind launched from the northern AGN, expanding along the minor axis of stellar rotation. This wind is dominated by ionised gas and, although it entrains some H$_2$, it does not show a H$_2$/PAH enhancement, suggesting either high UV irradiation or expansion along a relatively gas-free path. Furthermore, we find bright non-rotating gas emission between the two AGN and around the southern AGN, which we interpret as due to an outflow launched from the southern nucleus, coinciding with the molecular outflow previously studied in cold (sub-)millimeter tracers. The strong H$_2$/PAH enhancement measured in this region, coextensive with high velocity redshifted gas ($v\sim900$ km s$^{-1}$), suggests that the shocks responsible for the high H$_2$/PAH ratios are due to the outflow rather than to the collision of media during the merger. Our results show that the bulk of the NIR line emission in NGC6240 is decoupled from the stars, and that most of the warm H$_2$ is shock-excited and embedded in a powerful outflow, where it coexists with colder molecular gas.

Outflowing shocked gas dominates the NIR H$_2$ emission from the dual AGN NGC6240

Abstract

[Abridged] We present a multi-line study of the kinematics of the molecular and ionised gas phases in the central 2 kpc of NGC6240, based on JWST/NIRSpec and ALMA observations. We devised a new spectral-line fitting approach to de-blend rotating and non-rotating gas components, which is better tailored to the extreme feedback mechanisms at work in NGC6240. We find that ~65% of the Pa, H, and [FeII] line fluxes within the NIRSpec field of view arise from gas components that are kinematically decoupled from the stars. The NIR H lines show the most deviation from the stars, with peak emission between the two rotating stellar structures. The PAH 3.3m feature does not follow the NIR H morphology, indicating that the latter does not trace PDRs. In the non-rotating gas components, we identify a biconical wind launched from the northern AGN, expanding along the minor axis of stellar rotation. This wind is dominated by ionised gas and, although it entrains some H, it does not show a H/PAH enhancement, suggesting either high UV irradiation or expansion along a relatively gas-free path. Furthermore, we find bright non-rotating gas emission between the two AGN and around the southern AGN, which we interpret as due to an outflow launched from the southern nucleus, coinciding with the molecular outflow previously studied in cold (sub-)millimeter tracers. The strong H/PAH enhancement measured in this region, coextensive with high velocity redshifted gas ( km s), suggests that the shocks responsible for the high H/PAH ratios are due to the outflow rather than to the collision of media during the merger. Our results show that the bulk of the NIR line emission in NGC6240 is decoupled from the stars, and that most of the warm H is shock-excited and embedded in a powerful outflow, where it coexists with colder molecular gas.

Paper Structure

This paper contains 22 sections, 17 figures, 5 tables.

Table of Contents

  1. Introduction
  2. Data
  3. Archival JWST NIRSpec observations
  4. Archival ALMA observations
  5. Band 7 (870$\mu$m) continuum data
  6. Band 8 [CI]$^3P_1-^3P_0$ line data
  7. Band 3 CO(1-0) line data
  8. Results
  9. Spectral properties
  10. Stellar kinematics
  11. Spectral line fitting
  12. The PAH 3.3 $\mu$m feature
  13. Total moment maps of gas tracers
  14. Moment maps of non-rotating gas components
  15. H$_2$/PAH ratio map
  16. ...and 7 more sections

Figures (17)

  • Figure 1: RGB composite map of NGC 6240 obtained with JWST NIRSpec, tracing the emission at $\sim1.6~\mu$m (blue), 2.9 $\mu$m (green), and 4.6 $\mu$m (red). Four representative apertures with $0.3"$ radius are shown on the map: the VLBI positions Hagiwara+11 of the two known AGN (N and S) at $\rm{R.A.=16^h52^m58^s.9241}$, $\rm{Dec.=02^\circ24'04".776}$, and $\rm{R.A.=16^h52^m58^s.8902}$, $\rm{Dec.=02^\circ24'03".350}$ respectively; a central position between the nuclei (C), at R.A.=16$^h$52$^m$58$^s$.907, Dec.=02$^{\circ}$24$'$4$"$.063, and an offset point to the east (OE), at R.A.=16$^h$52$^m$58$^s$.978, Dec.=02$^{\circ}$24$'$3$"$.859. The physical scale of the map (500 pc) is indicated by the white line.
  • Figure 2: Relative intensity of spectra extracted using an aperture of 3 spaxels from the northern (N) and southern (S) nuclei, a centre (C) point between the AGNs, an offset (OE) point (see \ref{['fig:rgb']}), and the full FoV (Total). The spectra are joined from the three JWST/NIRSpec grating/filter compositions to cover all the wavelengths. CO bands are marked as shaded regions. Atomic lines are annotated in black, ro-vibrational H$_2$ transitions in red, and pure rotational H$_2$ transitions in blue.
  • Figure 3: Analysis of the stellar dynamics, showing the presence of two rotating structures. The left and middle panels show respectively the stellar velocity and velocity dispersion maps obtained from the 1.0 $\mu$m and 1.7 $\mu$m bands using pPXF (we note that the colour bars have different limits). The points N, S, C, and OE indicated on the maps are the same as Fig. \ref{['fig:rgb']}. The other two positions are centred at R.A.=16$^h$52$^m$58$^s$.829, Dec.=02$^{\circ}$24$'$4$"$.369 (ONW) and R.A.=16$^h$52$^m$58$^s$.863, Dec.=02$^{\circ}$24$'$2$"$.331 (OSW). Examples of pPXF spectral fits are presented in \ref{['fig:ppxf-results']}. The right panel shows the major axis PA (gray dashed lines) for the two stellar structures derived using the kinemetry tool krajnovic_kinemetry_2006. The two AGN, marked with crosses, are used as kinematic centres. The northern PA is $26.5\pm3.2^\circ$, and the southern PA is $153.0\pm0.5^\circ$. Angles are measured counter-clockwise from the north.
  • Figure 4: Left panel: Intensity map of the PAH 3.3 $\mu$m band. The AGN positions are marked with crosses. Right panel: High resolution ALMA map of the 870$\mu$m dust continuum, with overlaid PAH 3.3$\mu$m contours.
  • Figure 5: Moment maps for six selected emission lines; two hydrogen recombination lines Pa$\alpha$ and Pa$\beta$, two strong molecular hydrogen lines H$_2$ 1-0 S(1) and H$_2$ 0-0 S(8), and two ionized iron lines [Fe II]$\lambda1.644\mu$m and [Fe II]$\lambda1.257\mu$m. The zeroth moment is shown in the upper panel, the first in the middle, and the second in the lower panel. AGN positions are marked with crosses.
  • ...and 12 more figures