The Molecular Gas in the NGC 6240 Merging Galaxy System at the Highest Spatial Resolution

Ezequiel Treister,Vivian U,George C Privon,Kevin Schawinski,Anne M Medling,Julia M Comerford,Michael Koss,Chin-Shin Chang,Kartik Sheth,Neil M Nagar ,Caitlin M Casey ,L Armus ,A S Evans ,Loreto Barcos Muñoz,C Cicone ,F Müller–Sánchez ,F E Bauer ,H Messias ,C Megan Urry,D B Sanders ,G Venturi ,N Z Scoville

doi:10.3847/1538-4357/ab6b28

Abstract

Abstract We present the highest-resolution—15 pc (0.″03)—ALMA 12CO(2–1) line emission and 1.3 mm continuum maps, tracers of the molecular gas and dust, respectively, in the nearby merging galaxy system NGC 6240, which hosts two supermassive black holes growing simultaneously. These observations provide an excellent spatial match to existing Hubble Space Telescope (HST) optical and near-infrared observations of this system. A significant molecular gas mass, ∼9 × 109 M ⊙, is located between the two nuclei, forming a clumpy stream kinematically dominated by turbulence, rather than a smooth rotating disk, as previously assumed from lower-resolution data. Evidence for rotation is seen in the gas surrounding the southern nucleus but not in the northern one. Dynamical shells can be seen, likely associated with nuclear supernova remnants. We further detect the presence of significant high-velocity outflows, some of them reaching velocities >500 km s−1, affecting a significant fraction, ∼11%, of the molecular gas in the nuclear region. Inside the spheres of influence of the northern and southern supermassive black holes, we find molecular masses of 7.4 × 108 and 3.3 × 109 M ⊙, respectively. We are thus directly imaging the reservoir of gas that can accrete onto each supermassive black hole. These new ALMA maps highlight the critical need for high-resolution observations of molecular gas in order to understand the feeding of supermassive black holes and its connection to galaxy evolution in the context of a major galaxy merger.

Highlights

IntroductionComputational simulations (e.g., Barnes & Hernquist 1991) show that the dynamical interactions between merging galaxies cause gas to lose angular momentum and fall into the center of each nucleus, triggering significant episodes of both star formation and SMBH accretion and generating a so-called luminous active galactic nucleus (AGN; Treister et al 2012; Glikman et al 2015; Kocevski et al 2015; Fan et al.2016; Trakhtenbrot et al 2017; Weston et al 2017; Donley et al 2018; Goulding et al 2018; Weigel et al 2018)
The bulk of the molecular gas emission in the nuclear region of the system appears to be directly connected to the material found between the nuclei
The unprecedented high-resolution Atacama Large Millimeter/submillimeter Array (ALMA) observations of the molecular gas in the merging system NGC 6240 allow us to pin down how SMBH growth and star formation are proceeding in this complex and chaotic environment

Summary

Introduction

Computational simulations (e.g., Barnes & Hernquist 1991) show that the dynamical interactions between merging galaxies cause gas to lose angular momentum and fall into the center of each nucleus, triggering significant episodes of both star formation and SMBH accretion and generating a so-called luminous active galactic nucleus (AGN; Treister et al 2012; Glikman et al 2015; Kocevski et al 2015; Fan et al.2016; Trakhtenbrot et al 2017; Weston et al 2017; Donley et al 2018; Goulding et al 2018; Weigel et al 2018). The estimated global star formation rate ranges from a lower value of 25 Me yr−1 (Engel et al 2010) to >100 Me yr−1 (Howell et al 2010), while specific nuclear regions have been claimed to have values up to ∼270 Me yr−1 (Pasquali et al 2004)

Results

Discussion

Conclusion