ALMA View of the High-velocity-dispersion Compact Cloud CO 0.02–0.02 at the Galactic Center

Figure 2 shows the velocity-integrated maps of the four observed molecular line emissions (H¹³CN, SiO, CH₃OH, and CO), the interferometric image of $900$ $\mu$m continuum emission, and schematic view of the characteristic features in the integrated maps. Note that the H¹³CO⁺ emission was not detected in the integrated map. The spatial distributions of the H¹³CN, SiO, and CH₃OH line emissions were roughly similar. We clearly see two filaments of intense H¹³CN emissions which range northeast to southwest. We hereafter refer to these as the “Filaments”. The faint SiO and CH₃OH emissions trace the Filaments, except for the CH₃OH emission spots near the northwest corner. These northwestern CH₃OH spots were also visible in the H¹³CN map. We also observed faint H¹³CN emission features in the south and west of the Filaments. Significantly faint SiO and CH₃OH emissions appear to be associated with the southern and western H¹³CN features.

The spatial distribution of the CO line emission (Figure 2d) differs completely from that of the other observed molecular lines. The wide spatial extent of the CO emissions may be due to the lower critical density for excitation ($n_{\rm cr}\!\simeq\!10^{4.5}$ cm^-3) and ubiquity of the CO molecule. We carefully checked whether the CO image was recovered by comparing the smoothed ALMA image with the JCMT image. The Filaments, as well as the western H¹³CN feature and the northwestern CH₃OH spots, seem to be buried in the spatially extended CO emission. The southern H¹³CN feature corresponds to the center of a more spatially extended feature of CO emissions, where the CO integrated intensity is the most intense.

The $900$ $\mu$m continuum image (Figure 2e) shows the characteristics that are a mixture of those of H¹³CN and CO. Continuum emission features trace the Filaments, further elongating them toward southwest. The western H¹³CN emission region is also visible in the continuum. In addition, an intense, spatially extended continuum emission feature was observed in the southern part. The Filaments and the southern emission feature trace the large-scale molecular gas distribution observed with single-dish telescopes (see Figure 1c).

Figure 3 shows velocity channel maps of the H¹³CN line. The Filaments appeared in the velocity range between $\hbox{$V_{\rm LSR}$}\!=\!60$ and $120$ km s^-1, with a velocity gradient from northeast to southwest. The southern feature ranges from $\hbox{$V_{\rm LSR}$}\!=\!60$ to $160$ km s^-1, breaking up into several clumps of H¹³CN emissions. The western feature appears at $\hbox{$V_{\rm LSR}$}\!=\!60$ to $120$ km s^-1 and breaks up into a few clumps. The northwestern CH₃OH spots appeared at $\hbox{$V_{\rm LSR}$}\!=\!-20$ to $0$ km s^-1, which suggests a possible irrelevance to CO 0.02–0.02.

Figure 4 shows the velocity channel maps of the CO emissions. At the slight west of the center in the $\hbox{$V_{\rm LSR}$}\!=\!60\mbox{--}80$ and $80\mbox{--}100$ km s^-1 channel maps, there is a hole of emission. Its direction is similar to that of the Filaments. We hereafter refer to the hole as “Slit”. The Slit also can be seen in the CO integrated intensity map of Figure 2d.

The wide spatial extent of CO emissions hinders our understanding of its spatial velocity structure, especially at velocities lower than $\hbox{$V_{\rm LSR}$}\!=\!140$ km s^-1. Upon investigating the CO velocity channel maps minutely, we noticed two distinctive filamentary features (F1 and F2) and a high-velocity component (HV). These features appearing at different line-of-sight velocity ranges are explained in detail below.

The distribution of CO emissions at velocities between $\hbox{$V_{\rm LSR}$}\!=\!0$ and $100$ km s^-1 shows a rough striped pattern in the northeast-southwest direction. To characterize such a pattern, we calculated autocorrelations of the CO channel maps and show characteristic three maps in the subpanels inserted in Figure 4. The correlated region corresponds to the spatial scale of the clouds, and elongation reflects the typical cloud shape and direction. The northeast-southwest direction pattern is manifested in the autocorrelation maps of $\hbox{$V_{\rm LSR}$}\!=\!80$ to $100$ km s^-1. Hereafter, we refer to this northeast-southwest striped emission feature as “F1”. The aforementioned two Filaments align along F1. At velocities between $\hbox{$V_{\rm LSR}$}\!=\!100$ and $140$ km s^-1, another rough striped pattern of the southeast-northwest direction appears. This is hereafter referred to as “F2”. The mesh-patterned CO emission features at the $\hbox{$V_{\rm LSR}$}\!=\!100\mbox{--}120$ velocity channel is a mixture of F1 and F2. In contrast to F1, we could not detect any H¹³CN, SiO, CH₃OH, or H¹³CO⁺ line emissions from F2. The absence of these high-density probe emissions indicates that less dense gas dominates F2.

Another distinct feature appeared at velocities higher than $\hbox{$V_{\rm LSR}$}\!=\!140$ km s^-1, as a $\simeq\!7\arcsec$ diameter CO emission, at the most intense part of the southern continuum emission feature. Hereafter, we referred to this high-velocity feature as “HV”. The HV corresponds to the positive highest-velocity gas identified in the JCMT CO J=3–2 map (the contours in Figure 1c) as well as to the positive high-velocity end of the southern emission feature in the velocity-integrated maps (see Figure 2f). F2 and HV seem to be connected in the position-velocity structure because the shape of the HV shrinks at the higher velocity range from F2. The autocorrelation map reflects the spatial compactness of the HV (Figure 4).

Figure 5 shows CO velocity channel maps between $\hbox{$V_{\rm LSR}$}\!=150$ km s^-1 and $190$ km s^-1 with a 10 km s^-1 interval. We found eight ultra-compact clumps (UCCs), which appear as isolated compact shapes at their positive high-velocity ends, and summarized them in Table 1. These UCCs have angular sizes that are approximately like those of the synthesized beam, indicating they these may be unresolved. Such ultra-compact clumps were only detected in the angular area of the HV envelope. UCC6 is visible in the CO J=3–2 position-velocity map (Figure 6) up to $V_{\rm LSR}\!\simeq\!200$ km s^-1. This is the “real” highest-velocity gas in CO 0.02–0.02.

We observe several small $900$ $\mu$m continuum cores along the Filaments and the southern emission feature (Figure 2e). The brightest continuum emission originates from $(l,b)\!=\!(0\fdg 0129,-0\fdg 0209)$, where a $\simeq\!2\arcsec$-sized core is located (Figure 2e). Figure 7 shows the molecular line spectra extracted from this brightest continuum core (BCC) region. All observed molecular lines were detected, including H¹³CO⁺ with a 4.8$\sigma$ level at the peak. All spectra, except for the CO line, had similar shapes. The CO spectrum consisted of three components. Two of these are attributed to the F1 and F2 features, and the lower velocity component at approximately $\hbox{$V_{\rm LSR}$}\!=\!50$ km s^-1 arises from M–0.02–0.07.

By fitting a single Gaussian function to the H¹³CN spectrum, we obtained the velocity dispersion of $\sigma_{V}\!=\!6.3$ km s^-1 with a center velocity of $101.9$ km s^-1. The H¹³CN/H¹³CO⁺ ratio at the H¹³CN peak velocity was 5.5, which is a normal value in the CMZ (2.6–6.6; Jones et al., 2012). The size parameter ($S\!\equiv\!D\mathrm{tan}\sqrt{\sigma_{l}\sigma_{b}}$) of the BCC was estimated to be $0.05$ pc using H¹³CN data. At $S\!=\!0.05$ pc, the velocity dispersion of $\sigma_{V}\!=\!6.3$ km s^-1 is approximately one order of magnitude higher than that estimated from the extrapolated $S$-$\sigma_{V}$ relation of the CMZ clouds (Oka et al., 2001b; Shetty et al., 2012). This behavior follows that of the high velocity-dispersion compact clouds (Tokuyama et al., 2017; Oka et al., 2022).

The originally proposed formation scenario of CO 0.02–0.02 is based on the acceleration by a series of supernova explosions (Oka et al., 1999, 2008). This idea came from the association of the “Small Shell” and “Emission Cavity” with a group of point-like infrared sources (stellar cluster candidate). This scenario was also successful in energetics, requiring a young ($\mbox{age}\!=\!10\mbox{--}30$ Myr) and very massive ($M\!\sim\!10^{6}$ $M_{\odot}$) star cluster in the immediate vicinity of CO 0.02–0.02 (Oka et al., 2008).

Figure 8a summarizes the characteristic features in CO 0.02–0.02 identified in the single-dish data (Oka et al., 1999, 2008) and the ALMA data. The Slit, BCC, and HV were spatially adjacent to the Emission Cavity, which included the stellar cluster candidate. The CO J=3–2 position velocity maps along and perpendicular to the Slit (Figure 8c) show that the Slit closed at $\hbox{$V_{\rm LSR}$}\!\simeq\!110$ km s^-1, tracing a rough half-ellipsoid in the position-velocity space. This behavior suggests the expansion of the kinematics of the Slit. The southwestern tip of the Filaments, where the BCC was located, also traced this ellipsoid (Figure 8f). The broad velocity width of the BCC may be partially owing to its interaction with the Slit.

The central velocity of the Slit ellipsoid $\hbox{$V_{\rm LSR}$}\!\simeq\!75$ km s^-1, was almost equal that of the Emission Cavity. The proximity in position and velocity suggests that their origins are closely related. The expansion time of the Slit is estimated to be $\sim\!10^{4}$ yr, a factor of three shorter than that of the Emission Cavity. This discrepancy suggests that a series of explosions, which are responsible for the formation of the Emission Cavity, still continues in CO 0.02–0.02. The discovery of the Slit may reinforce the endogenous acceleration of CO 0.02–0.02, but it cannot account for the higher-velocity features, that are F2 and HV.

The spatial structure of CO 0.02–0.02 changes at $\hbox{$V_{\rm LSR}$}\!\simeq\!120$ km s^-1 from F1 to F2. Because the HV seems to arise from F2, their origins are likely to be closely related. Taking another look at the full view of CO 0.02–0.02 (Figure 1c), we noticed a northwestern protrusion extending from its intensity peak, along the direction of F2. This protrusion appears only in the CO maps (Oka et al., 1999) at $\hbox{$V_{\rm LSR}$}\!\simeq\!120$ km s^-1(Figure 8e), having similar chemical composition of F2 (Figure 7). These similarities may suggest a physical association of protrusion with F2.

In the large-scale view (Figure 1a), the protrusion curves toward the north appear to connect to the southern end of the Polar Arc (Bally et al., 1988). Figure 9 shows that the northwestern protrusion is part of a $20\,\mbox{pc}\times\!13\,\mbox{pc}$-sized ellipse at the bottom of the Polar Arc. The ellipse appearing at $\hbox{$V_{\rm LSR}$}\!=\!100\mbox{--}140$ km s^-1 could be a manifestation of a large-scale expanding shell (hereafter referred to as “Large Shell”). The lack of an emission depression at $\hbox{$V_{\rm LSR}$}\!\leq\!80$ km s^-1 suggests that the expanding velocity and expansion time of the Large Shell are likely to be $\simeq\!40$ km s^-1 and $\simeq\!10^{5.5}$ yr, respectively.

The presence of a Large Shell may evoke an exogenous acceleration scenario for CO 0.02–0.02. A collision between the main body of CO 0.02–0.02 ($\hbox{$V_{\rm LSR}$}\!\simeq\!100$ km s^-1) and the Large Shell ($\hbox{$V_{\rm LSR}$}\!\simeq\!120$ km s^-1) may have generated the northwestern protrusion and F2, which broadened the velocity extent of the cloud to $\hbox{$V_{\rm LSR}$}\!\simeq\!140$ km s^-1. The Small Shell could be one of the bubbles that form the Large Shell, whereas the Emission Cavity is outside its impact. This scenario assumes that the Large Shell has swept a part of CO 0.02–0.02 and its bulk remains unperturbed. The Slit cannot be reproduced in this scenario because its central velocity clearly differs from that of the Large Shell.

The formation of the HV is not included in either the endogenesis or exogenous scenario. Thus, an additional acceleration process is necessary to account for the presence of the HV and UCCs. The absence of expanding features in the HV may indicate acceleration by a nonexplosive process.

Such an acceleration process has been proposed to interpret the compact high-velocity feature in the W44 molecular cloud (“Bullet”; Yamada et al., 2017), where a high-velocity plunge of a point-like massive object is assumed. A magnetohydrodynamic simulation of a plunging black hole into a molecular cloud has successfully reproduced the observed position-velocity behavior of the Bullet (Nomura et al., 2018). Another example of gravitationally-kicked gas is CO–0.40–0.22, of which a point-like mass of $10^{5}$ $M_{\sun}$ was assumed to be responsible for this origin(Oka et al., 2016). ALMA observations of HCN–0.009–0.044 and HCN–0.085–0.094 revealed that the gas kinematics in each of these clouds could be reproduced by Keplerian orbit(s) around a point mass of $\sim\!10^{4}\,M_{\sun}$ (Takekawa et al., 2019, 2020).

The presence of UCCs in the HV suggests gravitational acceleration by a group of point-like massive objects. Because we could not find any counterparts at the positions of UCCs in the mid-infrared images of Spitzer/IRAC (Stolovy et al., 2006; Churchwell et al., 2009) and the X-ray point source catalog by the Chandra X-ray Observatory (Muno et al., 2009), the accelerating source may be a cluster of less (or non-) luminous stars. We suppose that such a less luminous star cluster resides in CO 0.02–0.02, at the position of the HV. The passage of the Large Shell may increase the interaction with stars, generating a number of ultra-compact clumps of high-velocity gas. The HV may be caused by the aggregation of gravitationally kicked gas.

As described above, neither the endogenous nor exogenous scenario can reproduce all the observed features in CO 0.02–0.02. Therefore, it is reasonable to expect that both processes are occurring there. Here, we describe the most plausible “hybrid”-formation scenario for CO 0.02–0.02:

1. 1.

   The filamentary molecular cloud along the F1 direction is the progenitor of CO 0.02–0.02.

2. 2.

   A huge explosion, which may be responsible for the formation of the Polar Arc, generated the Large Shell.

3. 3.

   A microburst of star formation occurred at the off-center of the progenitor.

4. 4.

   A subsequent series of supernova explosions formed the Emission Cavity and Slit.

5. 5.

   The rim of the Large Shell reached the progenitor cloud to form F2.

6. 6.

   The passage of the Large Shell at the less luminous star cluster locally accelerated molecular gas to form the HV and UCCs.

The formation scenario described above can reproduce all the observed features in the CO 0.02–0.02 well. The expansion times of $\sim\!10^{5}$ yr and $\sim\!10^{4}$ yr for the Large Shell and Slit, and the timescale of $\sim\!10^{3}$ yr for forming a UCCs-like feature (Nomura et al., 2018) also agree with the scenario. The remaining issues are (1) the origin of the progenitor cloud and (2) the nature of the putative star cluster in the HV. Because of the intense tidal field at the location of CO 0.02–0.02, even massive star clusters would dissolve into the background less than $10^{6}$ yr (Portegies Zwart et al., 2002). We expect infrared proper motion surveys with high-resolution and high-sensitivity observations to enable us to identify fragmented star clusters, which may be the driving source of the UCCs. In addition, proper motion studies of the UCCs with ALMA will provide clues to unveil the true origin of this peculiar molecular cloud in the CMZ.