Millimetre-wave emission from an intermediate-mass black hole candidate in the Milky Way

Abstract

It is widely accepted that black holes (BHs) with masses greater than a million solar masses (M ⊙ ) lurk at the centres of massive galaxies. The origins of such 'supermassive' black holes (SMBHs) remain unknown 1 , while those of stellar-mass BHs are well-understood. One possible scenario is that intermediate-mass black holes (IMBHs), which are formed by the runaway coalescence of stars in young compact star clusters 2 , merge at the centre of a galaxy to form an SMBH 3 . Although many candidates for IMBHs have been proposed to date, none of them are accepted as definitive. Recently we discovered a peculiar molecular cloud, CO-0.40-0.22, with an extremely broad velocity width near the centre of our Milky Way galaxy.Based on the careful analysis of gas kinematics, we concluded that a compact object with a mass of ∼ 10 5 M ⊙ is lurking in this cloud 4 . Here we report the detection of a point-like continuum source as well as a compact gas clump near the center of CO-0.40-0.22. This point-like continuum source (CO-0.40-0.22 * ) has a wide-band spectrum consistent with 1/500 of the Galactic SMBH (Sgr A * ) in luminosity. Numerical simulations around a point-like massive object reproduce the kinematics of dense molecular gas well, which suggests that CO-0.40-0.22 * is the most promising candidate for an intermediate-mass black hole.CO-0.40-0.22 is a compact cloud (∼ 5 pc) with an extremely broad velocity width (∼ 100 km s −1 ) and very high CO J=3-2/J=1-0 intensity ratio (≥ 1.5) detected at a projected distance of ∼ 60 pc away from the galactic nucleus 5 . It belongs to a peculiar category of molecular clouds called high-velocity compact clouds (HVCCs) that were originally identified in the CO J=1-0 survey data 6-8 . CO-0.40-0.22 is only a dense cloud with a negative velocity detected in HCN J=4-3 line within the 0.3 • ×0.3 • area including it 4 . It has a continuous and roughly straight entity in the position-velocity maps, seeming not to be an aggregate of unrelated clouds with less broad velocity widths. The kinematical structure of CO-0.40-0.22 can be explained as being due to a gravitational kick experienced by the molecular cloud caused by an invisible compact object with a mass of ∼ 10 5 M ⊙ . The compactness and absence of a counterpart at other wavelengths suggest that this massive object is an inactive IMBH, which is not currently accreting matter. This is the second-largest black hole candidate in the Milky Way galaxy affter Sgr A * , as well as the second IMBH candidate in the Galaxy after that in the nuclear subcluster IRS13E (M BH ≃ 1300 M ⊙ ) 9,10 .ALMA band 6 observations towards CO-0.40-0.22 have provided high-resolution HCN J=3-2 (265.9 GHz) and CO J=2-1 (230.5 GHz) images. Dense molecular gas traced by HCN J=3-2 emission seems to concentrate near the centre of CO-0.40-0.22 as previously determined by the coarse-resolution ASTE HCN J=4-3 map (Fig. 1a). The displacement of 0.2 pc from the centre is within the ASTE beamwidth (22 ′′ = 0.9 pc). This dense gas clump is very compact (∼ 0.3

Weighing the IMBH candidate CO-0.40-0.22* in the Galactic Centre

“…This value is much smaller than that of their simulated cloud, with Msim = 10 3 M . In particular, in Oka et al (2017) the authors distribute their particles with a Gaussian radial mass distribution with dispersion of σr = 0.2 pc and internal velocity dispersion of σv = 1.43km s −1 , claiming that this leads to an initially virialized cloud. However, in this case, GMsim/σr ≈ 10σ 2 v , hence this setup is actually strongly subvirial.…”

“…3 A virial equilibrium would require a σv ≈ 5km s −1 . If the molecular gas has a temperature of 60 K, as estimated by Oka et al (2017), this means that the turbulence has a Mach number M ≈ 10, which is probably too high for clouds with that size (e.g., Larson 1981).…”

“…Nonetheless, the cloud size and its distance from the putative IMBH are of the same order. In this case, from a simple "Roche limit" argument, the self-gravity of the 1 22 arcsec = 0.9 pc (Oka et al 2017).…”

“…Detections of the upper end of the IMBH mass distribution have also been claimed for dwarf galaxies (see Kormendy & Ho 2013;Reines & Volonteri 2015;Mezcua 2017, and references therein). Tanaka et al (2014), Oka et al (2016) and Oka et al (2017) have reported the detection, by means of molecular emission lines, of a high velocity compact cloud, CO-0.40-0.22, with internal velocity gradient of several tens of km s −1 . In particular, through high resolution ALMA observations, Oka et al (2017) set its size, 1 pc, and its internal line of sight velocity, possibly ranging from -80 to -40 km s −1 with respect to us.…”

A Field Concept of the Black Hole

“…On the one hand we have an observational BH process that ionizes neutral matter into charged particles resulting in transformation from visibility to invisibility and on the other we have an unknown invisible process that emits radio waves and neutral atomic and molecular matter from invisible to visible space, Donley et al (2002), Yale (2006), Tombesi et al (2015), Leroy et al (2015), Pan et al (2015), Das et al (2015), Dasyra et al (2015), Whitney (2015), Gallimore et al (2016), Sanders et al (2016), Oka et al (2017). Usually, the emission is through galactic nuclei, Richings and Faucher-Giguere (2018), however, off-nucleus evidences exist, Pinto et al (2016).…”