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Two separate outflows in the dual supermassive black hole system NGC 6240

Nature volume 556, pages 345–348 (2018)Cite this article

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Abstract

Theoretical models and numerical simulations have established a framework of galaxy evolution in which galaxies merge and create dual supermassive black holes (with separations of one to ten kiloparsecs), which eventually sink into the centre of the merger remnant, emit gravitational waves and coalesce. The merger also triggers star formation and supermassive black hole growth, and gas outflows regulate the stellar content1,2,3. Although this theoretical picture is supported by recent observations of starburst-driven and supermassive black hole-driven outflows4,5,6, it remains unclear how these outflows interact with the interstellar medium. Furthermore, the relative contributions of star formation and black hole activity to galactic feedback remain unknown7,8,9. Here we report observations of dual outflows in the central region of the prototypical merger NGC 6240. We find a black-hole-driven outflow of [O iii] to the northeast and a starburst-driven outflow of Hα to the northwest. The orientations and positions of the outflows allow us to isolate them spatially and study their properties independently. We estimate mass outflow rates of 10 and 75 solar masses per year for the Hα bubble and the [O iii] cone, respectively. Their combined mass outflow is comparable to the star formation rate10, suggesting that negative feedback on star formation is occurring.

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Fig. 1: Three-colour composite image of Hα (red), [O iii] (blue) and V-band continuum (green) emission in NGC 6240, obtained by the HST.
Fig. 2: VLT/SINFONI maps of NGC 6240.
Fig. 3: Kinematics of [O iii].

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Acknowledgements

Some of the data presented in this paper were obtained from the Mikulski Archive for Space Telescopes (MAST). The Space Telescope Science Institute is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555. The optical spectroscopic data reported here were obtained at the Apache Point Observatory 3.5-m telescope, which is owned and operated by the Astrophysical Research Consortium. F.M.-S. acknowledges financial support from NASA HST Grant HST-AR-13260.001. G.C.P. acknowledges support from a FONDECYT Postdoctoral Fellowship (number 3150361) and the University of Florida. E.T. acknowledges support from CONICYT Anillo ACT1101, FONDECYT regular grants 1120061 and 1160999, and Basal-CATA PFB-06/2007.

Author information

Authors and Affiliations

  1. Department of Astrophysical and Planetary Sciences, University of Colorado, Boulder, CO, USA

    F. Müller-Sánchez, R. Nevin & J. M. Comerford

  2. Max-Planck-Institut für Extraterrestrische Physik, Garching, Germany

    R. I. Davies

  3. Instituto de Astrofísica and Centro de Astroingeniería, Facultad de Física, Pontificia Universidad Católica de Chile, Santiago, Chile

    G. C. Privon & E. Treister

  4. Department of Astronomy, University of Florida, Gainesville, FL, USA

    G. C. Privon

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Contributions

F.M.-S. conceived the project, analysed the data, coordinated the activities and prepared the manuscript. R.N. prepared and reduced the APO/DIS observations and created the BPT diagrams. F.M.-S. and J.C. analysed the HST images. R.D. reduced the VLT/SINFONI data. E.T. and G.C.P. contributed to the analyses and discussion. All authors discussed the results and implications and commented on the manuscript at all stages.

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Correspondence to F. Müller-Sánchez.

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Extended data figures and tables

Extended Data Fig. 1 Contour image of [O iii] emission in NGC 6240.

The blue curves show linear contours for the HST/F502N observations. The contours are set at 7.5%, 15%, 30%, 45%, 60%, 75% and 90% of the peak of emission. The extended [O iii] emission is traced by the contour representing 7.5% of the peak of emission. The other contours (15–90% of the peak of emission) are located mostly around the two nuclei. A geometric model of the [O iii] cone is shown in light blue. The model was created using the software Shape48. We constrained the model (size and opening angle) to follow the outer contours (7.5% of the peak of emission) of the wedge-shaped structure in region 1. Interestingly, a regular cone (a cone with a sharp apex) does not provide a good fit to the wedge-shaped structure. The best fit is obtained for a truncated cone. If we had used a regular cone, the apex would be located exactly at the position of the southwestern nucleus. This is consistent with our interpretation that the [O iii] cone is probably produced by the two AGNs, with a larger contribution from the southwestern nucleus. For the [O iii] cone, we obtained a size of 3.7 ± 0.2 kpc and an opening angle of 50.2 ± 3.1°. The red-shaded rectangles indicate the spatial coverage of the long slits of the DIS. PA1 = 22° is oriented along the major axis of the galaxy disk, and PA2 = 56° covers the region where the [O iii] cone is observed (region 1). Both slits were centred between the nuclei. The dashed rectangle represents the SINFONI field of view. North is up and east is to the left.

Extended Data Fig. 2 Comparison of the morphologies of [O iii] and H2.

An image of H2 emission obtained with the near-infrared camera 2 (NIRC2)42 of the Keck adaptive optics system is superimposed on the [O iii] contours from Extended Data Fig. 1. Black represents fluxes < 0.011 of the peak of emission. The absence of molecular gas at the locations of the [O iii] cone and the Hα bubble clearly indicates that these two structures are located in regions that are not greatly influenced by the merger process. By contrast, the majority of perturbations caused by the merger activity are seen in the central region between the nuclei, and as gas streamers in the regions east and southwest of the SW nucleus (regions 3 and 4 in our analysis; see also Fig. 2b).

Extended Data Fig. 3 Optical emission-line diagnostic diagrams.

The galaxy was observed at two position angles, PA1 = 22° and PA2 = 56° (see Extended Data Fig. 1). The positive values of angular distance (green to red in the colour bar) correspond to the direction north of the centre of the galaxy, at that position angle. Negative angular distance values (green to blue in the colour bar) correspond to the direction south of the centre of the galaxy, at that position angle. The BPT diagram is usually divided into three regions: AGN (or Seyfert), LINER (or LIER, low-ionization emission-line region; see also ref. 50) and H ii (or starburst region). In both panels, we plot the extreme starburst diagnostic line21 (curved dashed line) and the LIER/LINER diagnostic line49 (straight dashed line). Hβ emission was detected with a signal-to-noise ratio higher than 3 in 16 spatial elements at PA1 = 22° (from r = −6″ to r = 2″) and in 26 spatial elements at PA2 = 56° (from r = −5″ to r = 6″). There are 15 spatial elements inside the [O iii] cone (Fig. 3). The error bars correspond to the uncertainties of the flux ratios (one standard deviation) and were calculated via standard error propagation for the flux of each emission line.

Extended Data Fig. 4 Map of Brγ velocity dispersion.

The contours delineate the Brγ flux distribution and are set at 15%, 30%, 45%, 60%, 75% and 90% of the peak of emission. The dashed rectangle delimits the base of the Hα bubble. Regions in white correspond to pixels where the Brγ flux is less than 5% of the peak of emission and thus were masked out. North is up and east is to the left. The colour bar indicates the range of velocity dispersion values observed in units of kilometres per second.

Extended Data Fig. 5 Map of H2/Brγ flux ratio.

The contours delineate the Brγ flux distribution and are set at 15%, 30%, 45%, 60%, 75% and 90% of the peak of emission. The dashed rectangle delimits the base of the Hα bubble. Regions in white correspond to pixels where the Brγ flux is less than 5% of the peak of emission and thus were masked out. North is up and east is to the left. The colour bar indicates the range of ratios observed.

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Müller-Sánchez, F., Nevin, R., Comerford, J.M. et al. Two separate outflows in the dual supermassive black hole system NGC 6240. Nature 556, 345–348 (2018). https://doi.org/10.1038/s41586-018-0033-2

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