Millimeter-wave CO and SiO Observations toward the Broad-velocity-width Molecular Feature CO 16.134–0.553: A Smith Cloud Scenario?

Figures 1(a), (c) show the velocity-integrated and longitude–velocity (l–V) maps of CO J = 1–0 line emission. CO 16.134–0.553 comprises of a 5 pc length ridge elongated from Galactic southwest to northeast (hereafter referred to as "ridge") and a 2 pc size clump in the north of the ridge ("northern clump"). The ridge turns southward at (l, b) ≃ (1611, − 058). A faint, filamentary structure ("northwestern filament") elongates toward the northwest from the center of the ridge. In the l–V plane, the ridge spreads in a fan shape toward higher velocities and connects two spatially extended emission components at V_LSR ≃ 40 km s⁻¹ and 65 km s⁻¹. The 40 km s⁻¹ component corresponds to molecular gas in the Norma Arm, while the 65 km s⁻¹ component is not associated with the known Galactic spiral arm. Faint high-velocity features appear to penetrate the 65 km s⁻¹ component, reaching V_LSR ≃ 80 km s⁻¹.

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Prominent SiO J = 2–1 line emission was detected from the ridge and northern clump of CO 16.134–0.553 (Figures 1(b), (d)). The SiO emission is less spatially extended than the CO emission. The southern half of the ridge is faint and clumpy. In the l–V plane, the ridge bifurcates at V_LSR ≃ 55 km s⁻¹, forming a V shape, which traces the edge of the fan shape in the CO l–V map. The spatially extended 40 km s⁻¹ component is barely visible in the SiO l–V map, while the 65 km s⁻¹ component appears to be absent. The positive high-velocity end of SiO emission also reaches to V_LSR ≃ 80 km s⁻¹.

Figures 2 and 3 show the velocity-channel maps of the CO J = 1–0 and SiO J = 2–1 line emissions. In the CO velocity-channel maps, the ridge of CO 16.134–0.553 comprises several clumps of 1–2 pc in size. The northern clump appears in the velocity range of V_LSR = 44–64 km s⁻¹ and merges with the ridge at the higher velocities. The highest CO intensity was observed in the northern clump at (l, b, V_LSR) = (1613, − 054, 51 km s⁻¹). The 40 km s⁻¹ component and faint 65 km s⁻¹ component spread over the frames at V_LSR = 40–44 km s⁻¹ and V_LSR = 64–68 km s⁻¹ channels, respectively. The northwestern filament appears at V_LSR = 64–72 km s⁻¹. A pair of faint, compact high-velocity features originate from the northwestern filament at (l, b) ∼ (1610, − 055) on both velocity sides. The velocity extent of this pair of compact high-velocity features is V_LSR ≃ 48–80 km s⁻¹.

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The SiO velocity-channel maps (Figure 3) delineate the less-extended spatial distribution of SiO emission. The northern clump and the northern two-thirds of the ridge are prominent in the SiO maps. The highest SiO intensity was observed at the center of the northern clump, with almost the same l–b–V position as that of the CO emission peak, (l, b, V_LSR) = (1613, − 054, 50 km s⁻¹). The 40 km s⁻¹ component in the SiO map is less spatially extended and distributed around the northern part of the ridge. The 65 km s⁻¹ component, the northwestern filament, and the southern one-third of the ridge are absent from the SiO maps.

Using the newly obtained CO J = 1–0 data in the range of 1610 ≤ l ≤ 1615, −060 ≤ b ≤ − 052, and 42 km s⁻¹ ≤ V_LSR ≤ 82 km s⁻¹, we recalculated the physical parameters of CO 16.134–0.553. The size parameter and velocity dispersion (definitions in Solomon et al. 1987) were calculated as S = 1.2 pc and σ_V = 8.5 km s⁻¹, respectively. These yield a dynamical time as t_dyn = 1.4 × 10⁵ yr. Assuming the local thermodynamic equilibrium (LTE), the molecular gas mass (M_LTE) of CO 16.134–0.553 was obtained as 5.1 × 10³ M_☉. Here, we employed an optical depth of τ = 3.6 and excitation temperature of T_ex = 18 K, which were derived from the FUGIN CO and ¹³CO J = 1–0 data using Equations (1)–(2) in Oka et al. (1998) with [CO]/[¹³CO] = 60. The kinetic energy was calculated using the definition, ${E}_{\mathrm{kin}}=(3/2){M}_{\mathrm{LTE}}{\sigma }_{V}^{2}$, and obtained as 1.3 × 10⁴⁹ erg. The kinetic energy and the dynamical time calculated above correspond to the kinetic power (P_kin ≡ E_kin/t_dyn) as high as 7.8 × 10² L_⊙ (=3.0 × 10³⁶ erg). This is at least 1 order of magnitude higher than that of bipolar outflows from massive protostars observed to date (e.g., Maud et al. 2015). Moreover, the absence of infrared counterpart resulted in a large departure of CO 16.134–0.553 from the infrared luminosity to kinetic power (L_IR–P_kin) trend of protostellar outflows (Yokozuka et al. 2021).

Gas-phase SiO is a well-established probe for strong shock in interstellar space (e.g., Ziurys et al. 1982), and the critical density of SiO J = 2–1 line is as high as ∼10⁵ cm⁻³ (e.g., Huang et al. 2022). Thus, the detection of widespread, broad-velocity-width SiO J = 2–1 emission from CO 16.134–0.553 indicates the presence of dense molecular gas that is affected by strong interstellar shock. Here we referred to the SiO J = 2–1/CO J = 1–0 intensity ratio (hereafter R_SiO/CO) to assess the abundance of dense/shocked gas (Table 1). Ratios were calculated from velocity-integrated line intensities in units of kelvin kilometers per second. Apparently, R_SiO/CO is determined with high significance across the whole of CO 16.134–0.553, being comparable to or even higher than those found at protostellar outflows (e.g., IRAS 00338+6312, IRAS 00494+5617). The ratio is the highest in the northern clump and is slightly lower in the northwestern filament. The 40 km s⁻¹ component exhibits a very low R_SiO/CO, while the 65 km s⁻¹ component exhibits a value similar to the typical value in the CMZ of our Galaxy (S. Takekawa et al. 2024, in preparation). These results indicate that CO 16.134–0.553, and possibly the 65 km s⁻¹ component, have recently experienced the passage of a strong interstellar shock.

The presence of the spatially extended 65 km s⁻¹ component and evidence for a widespread, strong shock indicate that a large-scale process is responsible for the acceleration of CO 16.134–0.553. Again, we referred to CO J = 1–0 data from the FUGIN survey (Umemoto et al. 2017) to reexamine the large-scale distribution and kinematics of molecular gas around CO 16.134–0.553. In the velocity-integrated CO map (Figure 4(a)), we noticed a shell-like structure with a diameter of ∼15 pc. Here, CO 16.134–0.553 appears to define the eastern edge of this 15 pc diameter CO shell (hereafter, "CO shell"). The 65 km s⁻¹ component exhibits a spatial extent as large as that of the CO shell, while the 40 km s⁻¹ component spreads over the frame (Figures 4(a), (b)). Several BVFs, including CO 16.134–0.553, connect the 40 km s⁻¹ and 65 km s⁻¹ components (Figures 4(b), (c)). The total molecular gas mass of the CO shell is ∼2 × 10⁴ M_☉ (assuming ${X}_{\mathrm{CO}}=2\times {10}^{20}\,{\mathrm{cm}}^{-2}{({{\rm{K}}\,{\rm{km}}\,{\rm{s}}}^{-1})}^{-1}$), and the kinetic energy amounts to ∼10⁵¹ erg. The situation of CO 16.134–0.553/CO shell is very similar to that of Smith Cloud (Smith 1963), which is a high-velocity cloud plunging into the H i disk of our Galaxy (Lockman 1984; Lockman et al. 2008); however, the scale (size, mass, and kinetic energy) is far smaller (details in Section 4.4).

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We also referred to H i 21 cm line data obtained from the H i 4π survey (HI4PI Collaboration et al. 2016). Figure 5 shows two large-scale maps of the H i 21 cm line emission with different area sizes and velocity ranges for integration. We noticed a local decrease of emission at velocities from V_LSR = 60 km s⁻¹ to 100 km s⁻¹ in the intense H i disk of the Galaxy (Figure 5(a)). This "H i void" is located at ∼05 above the CO shell, having a longitudinal size of ∼1° (=70 pc at a distance of 4 kpc). Far below the H i void/CO shell, we found a large filament with faint H i emission at velocities from V_LSR = 40 km s⁻¹ to 120 km s⁻¹ (Figure 5(b)). This "H i filament" has the apparent length and width of ∼4° and ∼1°, respectively, corresponding to 280 pc and 70 pc at 4 kpc, respectively. This extends from the Galactic H i disk, from the position of the H i void/CO shell, toward negative Galactic latitudes. The total atomic gas mass of the H i filament is ∼10⁵ M_☉ (using the method in Dickey 1990). This situation may indicate that the high-velocity plunging of a massive object from the Galactic halo was responsible for the formation of the H i void/CO shell (CO 16.134–0.553)/H i filament.

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Based on the observational facts presented above, we propose the following scenario (Figure 6):

• 1.  
  A small dark matter subhalo (DMSH) accompanied by a baryonic matter clump (BM) plunged into the Galactic disk.
• 2.  
  The BM was stopped by the CO disk, forming the CO shell and CO 16.134–0.553, while the DMSH penetrated the Galactic disk.
• 3.  
  The DMSH formed the H i filament by gravitational attraction, leaving the H i void in the Galactic disk.

This scenario was invoked by an analogy with the Smith Cloud (Lockman et al. 2008) and theoretical works that simulate a DMSH with BM impacting the Galactic disk (e.g., Bekki & Chiba 2006; Tepper-Garcia & Bland-Hawthorn 2018; Shah et al. 2019). The plunging scenario naturally explains all the observed kinematical features as well as the signatures of strong shock, whereas it assumed the presence of invisible objects (DMSH and BM).

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According to this scenario, the clump of BM, which is responsible for the formation of the CO shell, should measure ∼15 pc and predominantly consist of atomic hydrogen with an internal pressure comparable to that of molecular clouds, p/k ≳ 10³ cm⁻³ K. Limited angular resolution of H i surveys ($16\buildrel{\,\prime}\over{.} 2$ for HI4PI) and widespread H i emission in the Galactic plane prevents the direct detection of the BM in the CO shell. We assume that the velocity width of the CO shell (V_CO ≃ 30 km s⁻¹) roughly represents the line-of-sight (LOS) component of the plunging velocity and that its longitudinal component is similar to the LOS component (∣V_l ∣ ≃ ∣V_LOS∣). Using the longitudinal and latitudinal extents of the H i filament (Δl: Δb = 1: 4), we roughly estimated the latitudinal component of the plunging velocity as ∣V_b ∣ ≃ 4 × ∣V_l ∣ = 120 km s⁻¹. Thus, the total plunging velocity of the BM/DMSH relative to the Galactic disk can be estimated as ${V}_{\mathrm{BM}}\,=\sqrt{{V}_{l}^{2}+{V}_{b}^{2}+{V}_{\mathrm{LOS}}^{2}}\simeq 130$ km s⁻¹. Considering the Galactic rotational velocity at the CO shell (≃220 km s⁻¹), the total plunging velocity with respect to the Galactic center is ≃250 km s⁻¹, which is a reasonable value for Galactic halo objects (e.g., Lockman 2002). Since the BM is left far behind the DMSH, it may have almost been stopped by the CO shell. The conservation of momentum of the BM+CO shell system, M_BM V_BM ≲ (M_BM + M_CO)V_CO, provides a mass estimate of the BM, approximately M_BM ≲ 6 × 10³ M_☉. The estimated parameters of the BM and Smith Cloud are summarized in Table 2, which demonstrates a sharp discrepancy in scale between these two objects.

The size of the DMSH should fall in the range from 15 to 70 pc, corresponding to the diameter of the CO shell and the thickness of the H i filament (see Figure 6). Equating the size of the H i void with twice the Hoyle–Littleton radius (Equation (6) in Edgar 2004 with v_∞ = 130 km s⁻¹), we can roughly estimate the mass of the DMSH as ∼6 × 10⁷ M_☉. This value lies at the middle of the theoretically hypothesized "small" DMSH mass range of 10^6–9 M_☉ (e.g., Bovy et al. 2017; Garrison-Kimmel et al. 2017). The detection and quantification of such small DMSHs is one of the most important goals of the ΛCDM cosmology (Bullock & Boylan-Kolchin 2017). It is expected that the high-velocity plunge of such an object, which makes a H i void and filament, would have left certain signatures in the Galactic disk stellar population as well. The detection of a "vertical-velocity-field anomaly" toward the root of the H i filament in the Gaia astrometric data will be presented in a separate paper (K. Udagawa et al. 2024, in preparation).