Discovery of the Tadpole Molecular Cloud near the Galactic Nucleus

The ¹²CO J = 3–2 line (345.795990 GHz) observations of the CMZ were performed using the JCMT from 2013 July to 2014 July (Parsons et al. 2018). The Heterodyne Array Receiver Program (Buckle et al. 2009) and autocorrelation spectral imaging system (ACSIS) were used during these observations. The half-power beam width (HPBW) of the telescope was approximately 14'' at 345 GHz. The ACSIS was operated in the 1 GHz bandwidth (976.56 kHz resolution) mode. During these observations, the system noise temperature (T_sys) ranged between 100 and 200 K. The rms noise level of the image cubes was between 0.4 and 0.84 K. The details of the CO J = 3–2 data and JCMT observations are presented in Parsons et al. (2018). We use the data after resampling onto a 75 × 75 × 1 km s⁻¹ grid.

The CO J = 1–0 observations of the CMZ were performed using the NRO 45 m telescope (Tokuyama et al. 2019). The ¹²CO J = 1–0 (115.27120 GHz) line data were obtained from 2011 January 19 to 29 using the 25 beam array receiver system (BEARS; Sunada et al. 2000). As the receiver backend, the AC45 spectrometer system (Sorai et al. 2000) was employed in the 500 MHz bandwidth (0.5 MHz resolution) mode. The ¹³CO J = 1–0 (110.20135 GHz) line data were obtained from 2016 February to March using the four-beam receiver system on the 45 m telescope (FOREST; Holland et al. 2016). The spectral analysis machine on the 45 m telescope (SAM45; Kamazaki et al. 2012) was operated in the 1 GHz (244.14 kHz resolution) mode. The HPBW of the telescope was approximately 15'' at 115 and 110 GHz. The typical T_sys was ∼800 and 150–300 K during the ¹²CO and ¹³CO observations, respectively. The data were resampled onto a 75 × 75 × 2 km s⁻¹ grid. The rms noise levels of the resultant ¹²CO and ¹³CO data cubes were 1.0 and 0.2 K in main-beam temperature (T_MB), respectively.

The CS J = 2–1 (97.98096 GHz) line observations of the CMZ were performed during the NRO 45 m Telescope Large Program through 2019 January–May and 2020 January–April. The mapping area was set to −15 ≤ l ≤+15 and −025 ≤ b ≤+025. The FOREST receiver and SAM45 spectrometer were used. The SAM45 was operated in the 1 GHz bandwidth (244.14 kHz resolution) mode. The HPBW of the telescope was ≃19'' at 86 GHz. The T_sys ranged from 150–300 K during the CS J = 2–1 line observations. The data were resampled onto a 75 × 75 × 2 km s⁻¹ grid. The rms noise level of the resultant CS J = 2–1 data cube was 0.14 K in T_MB. The details of the CS data and NRO 45 m observations will be presented in the forthcoming paper (S. Takekawa et al. 2022, in preparation).

Figure 2 shows the spatial and velocity structure of the Tadpole in various molecular lines. The Tadpole appears as a compact, well-defined clump in velocity-integrated maps (Figures 2(a), (c)–(e)). In the CO J = 3–2 integrated map (Figure 2(a)), the FWHM angular diameter of the Tadpole is 28''. This angular diameter is 2 times the angular resolution (14''), corresponding to a distance of 1.1 pc from the Galactic center. In the ¹³CO J = 1–0 and CS J = 2–1 maps, the Tadpole has angular sizes similar to that depicted in the CO J = 3–2 map, while the ¹²CO J = 1–0 map depicts an appearance that is larger by a factor of 2 compared with those depicted in the other maps.

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The velocity extent of the Tadpole is from V_LSR ≃ −135 to −90 km s⁻¹ (Figure 2(b)), indicating that it may be in the CMZ. The longitude–velocity behavior of the Tadpole is characterized by a "head-tail" structure, from which its name is derived. It depicts a steep velocity gradient of $\left|{\rm{\Delta }}V/{\rm{\Delta }}l\right|\sim 16$ km s⁻¹ pc⁻¹. The tail is obscure in the ¹³CO J = 1–0 and CS J = 2–1 data sets. We show the spectra of CO and CS lines in Figure 3.

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We learned that CO J = 3–2 line can trace the kinematics of the Tadpole best because of its high intensity and well-defined appearance in the l–b–V space. Figure 4 shows the velocity channel maps of CO J = 3–2 emission. The Tadpole appears with an elliptical shape in each velocity channel, changing its angular size with respect to velocity. It has the largest angular size of 36'' × 24'' in the V_LSR = −130 to −125 km s⁻¹ channel, while the smallest size of ∼20'' is detected at both velocity ends. These angular sizes, which are comparable to the telescope's HPBWs, indicate that the spatial structure of the Tadpole is not well resolved using these single-dish observations; thus, the actual angular size must be far smaller than those observed.

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The CO J = 3–2 line emission shows a maximum intensity of 64 K km s⁻¹ in the −130 to −125 km s⁻¹ channel. We identified that the intensity maximum pixel in the Tadpole at each velocity channel continuously changes with respect to velocity. Note that these intensity maximum pixels trace an arc in the plane of the sky (Figure 4, bottom right panel). This behavior will be analyzed in more detail in the discussion section (Section 4.1).

The physical size of the Tadpole was evaluated as the size parameter; defined by $S\equiv D\tan \left(\sqrt{{\sigma }_{l}{\sigma }_{b}}\right)$ to be 2.2 pc. The velocity dispersion was calculated to be σ_V = 22 km s⁻¹. These give a size–line width coefficient (σ_V /S^0.5) of 15 km s⁻¹ pc^−0.5 and virial theorem mass (M_VT ≡ 8.7Sσ_V ²/G) of 4.7 × 10⁵ M_⊙. The molecular gas mass (M_gas) was estimated by summing the CO J = 3–2 line integrated intensity and using the CO(3–2)-to-H₂ conversion factor (X_{CO 3–2} = 1.4 × 10²⁰ [cm⁻² (K km s⁻¹)⁻¹]; Oka et al. 2022 ) to be 6.6 × 10² M_⊙. The X_{CO 3–2} we employed is close to 1 × 10²⁰ that estimated from an large velocity gradient (LVG) calculation for n(H₂) = 1 × 10⁴ cm⁻³, T_k = 60 K, and N(CO)/dV = 1 × 10¹⁷ cm⁻² ( km s⁻¹)⁻¹. Using the physical parameters described above, we derived the dynamical timescale (t_dyn ≡ S/σ_V ), kinetic energy (E_kin ≡ 1.5M_gas σ_V ²), and kinetic power (P_kin ≡ E_kin/t_dyn) of the Tadpole as 2.2 × 10⁴ yr, 9.4 × 10⁴⁸ erg, and 1.4 × 10³⁸ erg s⁻¹, respectively. The kinetic power of the Tadpole is equal to 3.7 × 10⁴ L_⊙, which is far greater than those of molecular outflows from massive young stellar objects  (0.01–100 L_⊙; Maud et al. 2015).

The virial theorem mass and molecular gas mass of the Tadpole yield a significantly high virial parameter, M_VT/M_gas ∼ 700. This indicates that the Tadpole is not in gravitational equilibrium. Following the method described in Stark et al. (1989), we estimated the self-gravity of the Tadpole and tidal force by the Galactic potential. The self-gravity of the cloud estimated here (∼10⁻¹⁰ m s⁻²) is 2.5 orders of magnitude weaker than the tidal force (∼4 × 10⁻⁸ m s⁻²). These assessments clearly demonstrate that the Tadpole cannot be bound by its self-gravity. This strongly indicates to the presence of an object with a mass comparable to the virial mass (4.7 × 10⁵ M_⊙) inside the Tadpole.

To search for the driving source behind the Tadpole, we referred to the 1.284 GHz data obtained using MeerKAT (Heywood et al. 2022), mid-infrared data obtained using the Spitzer Space Telescope (Ramirez et al. 2008; Churchwell et al. 2009), and X-ray data obtained at the Chandra X-ray Observatory (Muno et al. 2009).

No radio source brighter than 0.4 mJy beam⁻¹ at 1.284 GHz was detected near the angular extent of the Tadpole, while a faint filament was seen in the Galactic south (Figure 5(a)). In the mid-infrared image (Figure 5(b)), we identified a faint triangular rim which may be the irradiated surface of the Tadpole. Considering the angular resolution of the Spitzer (∼2''), the mid-infrared rim defines the angular extent of the Tadpole better than the CO J = 3–2 appearance. We also checked the ATLASGAL (870 μm) and Hi-GAL (70, 160, 250, 350, and 500 μm) images (Contreras et al. 2013; Molinari et al. 2016), and found no separate feature toward the Tadpole above the sea of intense dust emission. A bright point-like source near the Galactic southwestern edge of the Tadpole is the long-period variable star V4872 Sgr (Matsunaga et al. 2009). In addition, we also identify dozens of point-like, mid-infrared sources toward the Tadpole. The X-ray image also shows numerous faint point-like sources toward the Tadpole (Figure 5(c)). Although the nature of these mid-infrared/X-ray point-like sources is unclear, we will refer to them in the discussion section (Section 4.5). In short, the multiwavelength view confirms the absence of any energetic objects that can drive the Tadpole.

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The continuous change in the CO J = 3–2 intensity maximum pixel along the arc with velocity (Section 3.1) suggests that the bulk of warm molecular gas belongs to a certain trajectory, such as a closed orbit. To determine the orbit trajectory accurately, we constructed a CO J = 3–2 data cube with a 2'' × 2'' × 1 km s⁻¹ grid. Then, we calculated the accurate intensity peak position as the CO emission center of gravity using 5 × 5 pixels around the intensity maximum pixel in the Tadpole for each 1 km s⁻¹ width velocity channel.

Figure 6 shows the l–b–V and l–b distributions of the intensity peak position in the Tadpole for each velocity channel. The arc shape apparent in the l–b distribution became clearer than that depicted in Figure 4. The l–b–V distribution demonstrates the striking continuity of peak positions in the velocity direction, indicating that the bulk of warm molecular gas in the Tadpole may predominantly follow a certain closed orbit.

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The arc-shaped spatial structure and continuous velocity change along the arc suggest that a rotational motion exists. Thus, we expect that the l–b–V behavior of warm molecular gas in the Tadpole can be reproduced by a Keplerian orbit around a point mass. ⁶ We performed the fitting of a Keplerian orbit to the intensity peak positions (Figure 6) following the method described in Zhao et al. (2009). We determined nine parameters. Five of these parameters are three-dimensional orbital parameters; namely, the semimajor axis (a), eccentricity (e), longitude of ascending node (Ω), argument of pericenter (ω), and inclination angle (i). The remaining parameters are the mass (M_dyn), line-of-sight velocity (V_dyn), and l–b position (l₀, b₀) of the dynamical center. The distance to the Tadpole is assumed to be same as that to the Galactic center. The χ² minimization approach was utilized in the l–b–V space. The χ² consists of two terms, namely, the spatial (χ² _s) and velocity terms (χ² _v). The spatial term is defined by the sum of the orthogonal distances between the peak position and the modeled orbit divided by the square of the positional uncertainty, which was set to 0.15 pc (36). The velocity term is, similarly, the sum of the velocity deviations between those of the peak positions and nearest positions in the modeled orbit divided by a square of the velocity uncertainty, which was set to 1 km s⁻¹.

The best-fit parameters with 1σ uncertainties are listed in Table 1. The l–b–V locus of the orbit is presented in Figure 6. Note that the best-fit solution is bivalent because the orbits with i → (180° − i) and Ω → (Ω + 180°) yield the same l–b–V locus. The fitting result indicates the presence of a point mass of 1.0 × 10⁵ M_⊙ in the northwestern periphery of the Tadpole (Figure 6).

We examined the tidal stability of the head, assuming the Newtonian potential of a point mass, following the method described in Stark et al. (1989). The tidal force caused by the point mass and self-gravity doing work on the head were calculated as functions of the head diameter (Figure 7). A central mass of 1 × 10⁵ M_⊙ and head mass of 660 M_⊙ were assumed. The curves of the two forces at the pericenter of the best-fit orbit (0.5 pc) intersect at d = 0.14 pc, while those at the apocenter intersect at d = 0.35 pc. Because the observed head diameter (∼1 pc) is larger than the critical value (0.14 pc) by a factor of 7, the Tadpole head is tidally unstable at the pericenter. If the head includes a self-gravitating core smaller than 0.14 pc, it may be able to survive several turns. It is possibly that the Tadpole has been trapped by the gravitational potential of the 10⁵ M_⊙ point mass, being stretched by the strong tidal force to form the characteristic head-tail structure. This situation is similar to the simulation of a cloud tidally disrupted by an SMBH (Saitoh et al. 2014). Note that the Tadpole must have been lost its angular momentum to be captured during the encounter with the point mass.

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Here, we refer to the CO J = 1–0, CO J = 3–2, and CS J = 2–1 line intensities, as well as CO J = 3–2/CO J = 1–0 (R_3–2/1–0) and CS J = 2–1/CO J = 1–0 (R_CS/CO) line intensity ratios to examine the Keplerian orbit scenario. The critical densities of the CO J = 1–0, CO J = 3–2, and CS J = 2–1 transitions are approximately 10^2.5, 10⁴, and 10⁵ cm⁻³, respectively, while their upper state energies are 5.6, 33, and 7.1 K, respectively. Thus, R_CS/CO is sensitive to the variations in density while R_3–2/1–0 is affected by both the temperature and density. Figure 8 shows plots of line intensities and intensity ratios along the best-fit orbit. The motion along the orbit increases the orbital phase (ϕ) at all times, where the pericenter of the orbit corresponds to ϕ = 0°.

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Both ratios exhibit higher values at the head, indicating that both the density and temperature are enhanced there. At the beginning of the tail, ϕ ∼ 170°, R_3–2/1–0 demonstrates a prominent peak while R_CS/CO does not. This suggest that the temperature is enhanced at the beginning of the tail. Then, both ratios decrease toward the tip of the tail. The higher temperature in the head may have been caused by the shock occurring at the first encounter with the point mass. This shock may be caused by collisions of gas clumps at the pericenter, where adjacent orbits are very close. In a fluid mechanical treatment, the same process is described as the "tidal compression" (Saitoh et al. 2014). The gas accumulation near the apocenter and/or the presence on a self-gravitating core can cause the higher density in the head. The tail may be really the low-density, tidally stretched tail of the head. The temperature increase at the beginning of the tail could be caused by another shock at the congestion near the apocenter. Thus, the behaviors of intensity ratios are consistent with the Keplerian orbit scenario for the Tadpole.

The Keplerian orbit model requires the presence of a huge point mass at the dynamical center. What is this point-like massive object? The mass of 10⁵ M_⊙ is larger than the Arches or Quintuplet clusters (∼10⁴ M_⊙ Figer et al. 1999a, 1999b). According to the best-fit model, a mass of 10⁵ M_⊙ must be concentrated within a radius significantly smaller than 0.5 pc (the pericenter distance), resulting in an average mass density higher than that of the Arches cluster (ρ ∼ 2 × 10⁵ M_⊙ pc⁻³ Espinoza et al. 2009). Anyway, the absence of a bright infrared counterpart toward the Tadpole (Ramirez et al. 2008; Churchwell et al. 2009; Molinari et al. 2011) clearly rules out a stellar cluster from being a candidate for the point mass.

The most promising candidate for the point-like massive object in the Tadpole may be an IMBH. We searched for a counterpart for this point mass referring to X-ray images (Section 3.3). Figure 9 shows the positions of point-like sources in the X-ray images superimposed on the composite mid-infrared images around (l, b) = (l₀, b₀). The small statistical uncertainties in the l–b position of the dynamical center (Table 1) should be considered with some caution because the entire gas mass in the Tadpole is not confined within the single closed orbit. Notice that three point-like sources in the X-ray and mid-infrared reside within a 5'' (∼10 times of the positional ambiguity of the dynamical center) angular distance from (l₀, b₀) (Table 2). One source, SSTGC 489898, is apparent in both X-ray and mid-infrared images. The X-ray detected sources, SSTGC 489898 and CXOGC 174526.9–290124, exhibit hard spectra, indicating that they may be at the same distance to the Galactic center. No time variation has been detected from these sources (Zhu et al. 2018). If we assume the standard accretion disk model (L = $\dot{M}$ c²/12), their X-ray luminosities correspond to $\dot{M}$ ∼ 1 × 10⁻¹⁴ M_⊙ yr⁻¹. This very low mass accretion rate could be a challenge to our interpretation. Anyway, we suppose these three point-like sources are candidates for the luminous counterpart of the point mass. Although we currently have very little knowledge on these point-like sources, they should be considered as candidates for an IMBH in future studies.

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