Indication of Another Intermediate-mass Black Hole in the Galactic Center

Our high-resolution observations have unveiled the entity HCN–0.009–0.044. Figure 1(a) shows the integrated intensity map of the HCN J = 4–3 line. HCN–0.009–0.044 is resolved into several components. The main body appears as a balloon-like structure (hereafter Balloon). A stream-like structure (hereafter Stream) lies on the southeast of the Balloon. An ultra-compact clump (UCC) is associated with the tip of the Stream. Figure 1(b) shows the averaged line-of-sight velocity (moment 1) map of the HCN line. The Balloon and Stream clearly exhibit velocity gradients. The gas of the Balloon is gradually accelerated clockwise from the northeastern part at the line-of-sight velocity of V_LSR ∼ −70 km s⁻¹, indicating a rotational motion (see also Figure 2). The Stream is accelerated from south to north (V_LSR ∼ −60 km s⁻¹ to −40 km s⁻¹) as if it is orbiting around the center of the Balloon.

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HCN–0.009–0.044 is likely to be at least 5 kpc apart from the Sun (R > 5 kpc) because CO J = 3–2 emission from HCN–0.009–0.044 (Parsons et al. 2018) suffers from absorption by molecular gas in the 3-kpc arm, which is the Galactic spiral arm at ∼5 kpc (Sofue 2006). The components traced by the HCN J = 4–3 emission also appear in the ALMA images of the HCO⁺ J = 4–3 and CS J = 7–6 lines (Figures 1(c) and (d)). HCN–0.009–0.044 consists of highly dense and hot molecular gases as the critical densities for these transitions are ∼10⁷ cm⁻³, and the J = 7 level of CS is 65.8 K above the ground state. Such dense/hot molecular gases are abundant in the central 200 pc of our Galaxy but rare in the Galactic disk region. Hence, HCN–0.009–0.044 is probably in the Galactic center (R = 8 kpc). The molecular gas masses of the Balloon, Stream, and UCC were estimated to be M_LTE ∼ 6 M_⊙, 1 M_⊙ and 0.1 M_⊙, respectively, assuming the local thermodynamic equilibrium (LTE) with an excitation temperature of 22 K in the optically thin limit (Takekawa et al. 2017) and a fractional abundance of [HCN]/[H₂] = 4.8 × 10⁻⁸ (Tanaka et al. 2009; Oka et al. 2011).

Figure 1(e) shows the continuum image at 354.5 GHz. A faint filamentary structure appears in the eastern side of the Balloon. Although this filament also appears in the 353.6 and 342.9 GHz bands, we could not derive the reliable spectral index because of the insufficient sensitivities. M–0.02–0.07 (also referred to as the +50 km s⁻¹ cloud) and the Northern Ridge at V_LSR ≃ 0 km s⁻¹ (McGary et al. 2001) overlap with the filament in the line of sight. The filament may reflect dust emission from them. The physical relation between the continuum filament and the HVCC is currently ambiguous.

Interactions with supernova remnants (Oka et al. 2008; Tanaka et al. 2009; Yalinewich & Beniamini 2018) and cloud–cloud collisions (Tanaka et al. 2015, 2018; Ravi et al. 2018) have been considered as the origins of HVCCs. However, the position–velocity structure of HCN–0.009–0.044 exhibits neither the expanding motion indicative of a supernova–cloud interaction (e.g., see Figure 12 in Fukui et al. 2012) nor V-shaped pattern characteristic of a cloud–cloud collision (e.g., see Figure 10(c) in Torii et al. 2017). Natural explanations for the velocity gradients along the Balloon and Stream (Figure 1(b)) are orbital motions caused by gravity. The observed kinematics implies that a massive gravitational source lurks in the Balloon. The detection of the CS J = 7–6 line (Figure 1(d)), which is a high-density and possibly shock probe (e.g., Tanaka et al. 2018), may support the interpretation that the molecular gas has experienced tidal compression by strong gravity. The enclosed mass (M) can be roughly estimated by M ∼ ΔV² RΔθ/2G, where ΔV is the velocity width, R is the distance to the cloud, Δθ is the angular diameter, and G is the gravitational constant. The observed velocity width and diameter of the Balloon are ΔV ∼ 20 km s⁻¹ and RΔθ ∼ 0.4 pc, respectively. Thus, a massive object with ∼10⁴ M_⊙ may be hidden in the Balloon.

In order to confirm whether or not Keplerian motions explain the kinematical structures of the Balloon and Stream, we performed orbital fittings to the observed data with the similar procedure as done for the Sgr A West by Zhao et al. (2009). Assuming the dynamical center at (l, b, R) = (−00096, −00451, 8 kpc), we determined three-dimensional orbital parameters (a, e, Ω, ω, i) for the orbital geometries of the Balloon and Stream by least-square fitting to the loci we chose (red and blue points in Figure 3(b)). We carefully chose these loci (including the dynamical center) so that they represent the kinematics of the observed features. The orbital parameters are the semimajor axis (a), eccentricity (e), longitude of ascending node (Ω), argument of pericenter (ω), and inclination angle (i). We set the X-, Y-, and Z-axes parallel to the Galactic longitude, Galactic latitude, and line-of-sight direction, respectively. The best-fit orbital parameters are listed in Table 1 and the resultant orbits are projected in Figures 3(a)–(c). Note that there remains a double degeneracy in the inclination angle. Orbits with i and (180° − i) produce the same projected orbits and line-of-sight velocities.

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The line-of-sight velocity (V_Z) at each (X, Y) of the orbits depend on the mass (M_dyn) and line-of-sight velocity (V_dyn) of the dynamical center. After determining the orbital geometries, we fitted the modeled velocities V_Z to the observed velocities on the orbits of the Balloon and Stream with free parameters of M_dyn and V_dyn through a chi-square (χ²) minimization approach. We used the moment-1 values of the HCN image smoothed with a Gaussian kernel of 3'' FWHM (Figure 3(a)) for the model fit to reduce the noise effect. As a result, we derived the best-fit values of M_dyn = (3.2 ± 0.6) × 10⁴ M_⊙ and ${V}_{\mathrm{dyn}}=-{49.5}_{-0.7}^{+1.0}$ km s⁻¹. Figure 3(c) shows the velocity field of the model with the best-fit parameters. Figure 3(d) shows the confidence level contours as a function of M_dyn and V_dyn. These suggest that the observed kinematics of the Balloon and Stream can be well explained by two Keplerian orbits around a single gravitational source with M_dyn ≃ 3 × 10⁴ M_⊙.

Figure 4 shows the three-dimensional schematic view of HCN–0.009–0.044 based on the model with the best-fit parameters. The cloud lying on the south of the Balloon with V_LSR ∼ −60 km s⁻¹ may share the same orbit as the Stream. The dynamical timescales of the Balloon and Stream are estimated to be 4 × 10⁴ and 6 × 10⁴ yr, respectively. The similarity of the dynamical timescales may indicate that the Balloon and Stream are derived from a common parent molecular cloud. These results are consistent with the notion that the Balloon and Stream have been captured by the gravitational potential well of a massive compact object in the relatively recent past.

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It should be noted that the UCC shows an abnormally broad velocity width that cannot be explained by the Keplerian model. The UCC is located at the northern tip of the Stream, and it has velocities from V_LSR ≃ −50 km s⁻¹ to 0 km s⁻¹ without velocity gradient (Figure 2). The positive-velocity end is contaminated with emission from the Northern Ridge (McGary et al. 2001). The virial mass is roughly estimated to be ∼10⁴ M_⊙, which is far greater than the LTE mass (∼0.1 M_⊙). The UCC may contain a stellar object as a core, and the broad velocity width could be attributed to an outflow from it. Further observations with higher spatial resolution are necessary to reveal the origin of the UCC.

According to the model, a huge mass of (3.2 ± 0.6) × 10⁴ M_⊙ must be concentrated within a radius significantly smaller than 0.07 pc (the pericenter distance for the Balloon's orbit). The averaged mass density is much larger than that of the core of M15 (ρ ∼ 2 × 10⁷ M_⊙ pc⁻³), which is one of the most densely packed globular clusters (Djorgovski & King 1984). However, considering the star cluster as the gravitational source is implausible because of the absence of luminous stellar counterparts. Therefore, the most promising candidate for the gravitational source is a massive IMBH. Although accreting black holes can be traced through X-ray emission from their accretion disks (e.g., Done 2010), no X-ray point source has been detected in the extent of the Balloon (Muno et al. 2009). This is possibly because the IMBH is isolated and inactive. HCN–0.009–0.044 is the third case of the possible IMBH holder in the Galactic center after IRS13E (Schödel et al. 2005; Tsuboi et al. 2017) and CO–0.40–0.22 (Oka et al. 2016, 2017).

Many black holes follow the Fundamental Plane of black hole activity, which is an empirical relation between radio luminosity, X-ray luminosity, and black hole mass (e.g., Merloni et al. 2003; Falcke et al. 2004). Diffuse continuum emission at 5.5 GHz (Zhao et al. 2013, 2016) overlaps with HCN–0.009–0.044 (see Figure 2(b) in Takekawa et al. 2017). This sensitive image provides an upper limit radio luminosity of ∼2 × 10²⁸ erg s⁻¹ for the putative IMBH. If the IMBH is placed on the Fundamental Plane derived by Plotkin et al. (2012), then the X-ray luminosity can be inferred to be ≲10³¹ erg s⁻¹ from the radio luminosity of ≲2 × 10²⁸ erg s⁻¹ and the mass of 3.2 × 10⁴ M_⊙. On the other hand, the X-ray image at 0.5–8 keV (Muno et al. 2009) provides an upper-limit X-ray luminosity of ∼7 × 10³⁰ erg s⁻¹ for the IMBH. This is consistent with the Fundamental Plane for low-accretion-rate black holes (Plotkin et al. 2012), thereby providing another support for the IMBH interpretation.

Although there is no point source that is suggestive of the IMBH, we found an emission counterpart for the Balloon in the Pα recombination line at 1.87 μm obtained through the Hubble Space Telescope (Dong et al. 2011, Figures 2 and 3(a)). The Pα-emitting region seems to be surrounded by the orbiting molecular gas. This probably suggests that the inner part of the Balloon's orbit has been ionized. The ionization may be attributed to the dissociative shock caused by the sudden acceleration of molecular gas by the strong gravitational force. Hence, the detection of the Pα emission may also support the presence of the IMBH, although a stellar emission as the origin of the Pα cannot be ruled out.

Several observational studies have reported interactions of outflows and/or intense radiation from non-nuclear black holes with their ambient gas (e.g., Soria et al. 2014; Tetarenko et al. 2018). For instance, a persistent jet from Cygnus X-1 has been suggested to create a shell-like structure of ionized gas (Gallo et al. 2005; Russell et al. 2007). In addition, the elongated ionized gas feature associated with M51 ULX-1 has been considered as evidence for an interaction with a black hole jet (Urquhart et al. 2018). The Pα-emission feature that we noticed could indicate a past activity of the IMBH although its spatial distribution shows neither shell-like nor elongated morphology. Further investigations of the kinematics and physical conditions of the ionized gas would provide more convincing evidence for the presence of an IMBH.

Several globular clusters and dwarf galaxies have been suggested to harbor massive black holes with masses lesser than 10⁶ M_⊙ as their nuclei (Reines et al. 2013; Baldassare et al. 2015; Kızıltan et al. 2017). Recently, it has been indicated that there is likely to be a 4 × 10⁴ M_⊙ IMBH in the center of ω Centauri, which is the most luminous globular cluster in our Galaxy (Baumgardt & Hilker 2018). The IMBH in HCN–0.009–0.044 could be a remnant of such a globular cluster. The parent cluster have possibly already been dissolved before falling into the Galactic center.

The detected rotational motion of HCN–0.009–0.044 strongly indicates the presence of a dark massive object, which is probably an IMBH. An emission counterpart of the IMBH could be identified through future multi-wavelength observations. Similar to the simulations conducted for HVCC CO–0.40–0.22 (Ballone et al. 2018), the hydrodynamical simulations of the molecular clouds would more accurately restrict the orbital geometries and IMBH mass. Although only ∼60 black holes have been identified in our Galaxy to date (Corral-Santana et al. 2016), the total number of black holes in our Galaxy is theoretically estimated as ∼10⁸–10⁹ (Agol & Kamionkowski 2002; Caputo et al. 2017). Additionally, at least 10⁴ black hole X-ray binaries are observationally inferred to lurk in our Galaxy (Tetarenko et al. 2016). These suggest that almost all black holes are inactive with low accretion rates. Regardless of the activity status of a black hole, its strong gravity can disturb the ambient gas. As shown, high-resolution observations of compact high-velocity gas features have the potential to increase the number of candidates for non-luminous black holes, providing a new perspective to search for the missing black holes.