ALMA Observations of the Physical and Chemical Conditions in Centaurus A

Observations were taken with ALMA during Cycle 0 early science observing under the project code 2011.0.00010.S (PI:Ott). We observed 20 spectral windows, four in the 1 mm band (ALMA Band 6), and 16 in the 3 mm band (ALMA Band 3). The observation and image parameters are outlined in Table 1.

Table 1.  Observational Parameters

Center Freq.	Bandwidth	Δ v	Date	Beam	Channel rms	Cont. Flux Density	Spectral	Rest Freq.
(GHz)	(MHz) [km s−1]	(km s−1)		("× ,"o )	(mJy beam−1)	(Jy)	Line ID	(GHz)
(1)	(2)	(3)	(4)	(5)	(6)	(7)	(8)	(9)
Band 3 (3 mm)
85.338706	234.4 [806]	0.420	2012 Apr 07	2.07 × 1.69, −76.2	4.1	7.5
86.846824	234.4 [822]	0.428	2012 Apr 07	2.19 × 1.67, −83.7	4.8	7.4
87.169498	234.4 [806]	0.420	2012 May 08	2.19 × 1.61, 72.2	4.8	7.8	CCH(N = 1–0;J $=\,\tfrac{3}{2}-\tfrac{1}{2}$)	87.316925
87.765057	234.4 [799]	0.416	2012 May 08	2.17 × 1.60, 73.8	4.2	7.9	HNCO(404 − 303)	87.925238
88.470137	234.4 [791]	0.412	2012 May 08	2.13 × 1.59, 74.6	4.0	7.4	HCN(1 − 0)	88.631847
89.026047	234.4 [787]	0.410	2012 May 08	2.12 × 1.57, 74.8	4.2	7.9	HCO+(1 − 0)	89.188526
90.498407	234.4 [776]	0.404	2012 May 08	1.66 × 1.61, −59.0	4.4	8.1	HNC(1 − 0)	90.663564
90.813277	234.4 [772]	0.402	2012 May 08	1.66 × 1.61, −46.1	3.9	8.1	HC3N(10 − 9)	90.978989
91.803617	234.4 [764]	0.398	2012 May 08	1.66 × 1.58, −56.6	4.0	8.2	CH3CN(53 − 43)	91.971160
93.003656	234.4 [753]	0.392	2012 May 08	1.63 × 1.55, −73.0	3.8	8.2	N2H+(1 − 0)	93.173777
97.979577	234.4 [714]	0.372	2012 Apr 07	1.68 × 1.45, −59.4	4.6	7.6
100.076592	234.4 [703]	0.366	2012 Apr 07	1.88 × 1.50, −70.9	5.1	7.6
109.590043	234.4 [641]	0.334	2012 May 07	1.38 × 1.35, −73.7	5.3	9.0	C18O(1 − 0)	109.782176
110.001195	234.4 [637]	0.332	2012 May 07	1.41 × 1.32, −72.9	4.6	9.0	13CO(1 − 0)	110.201354
112.154164	234.4 [626]	0.326	2012 May 07	1.38 × 1.30, −54.1	5.4	9.1	C17O(1 − 0)	112.358988
113.230037	234.4 [618]	0.322	2012 May 07	1.38 × 1.27, −69.7	6.1	9.2	CN(N = 1 − 0;J $\,=\,\tfrac{3}{2}-\tfrac{1}{2}$)	113.490982
Band 6 (1 mm)
218.077608	937.5 [1286]	0.670	2012 Jan 24	2.22 × 1.43, −44.4	4.0	6.5	H2CO(303 − 202)	218.222195
219.152815	937.5 [1279]	0.666	2012 Jan 24	2.22 × 1.43, −44.4	4.3	6.5	C18O(2 − 1)	219.560358
219.397838	937.5 [1279]	0.666	2012 Jan 24	2.23 × 1.45, −46.0	4.7	6.5	HNCO(10010 − 909)	219.798282
220.007315	937.5 [1275]	0.664	2012 Jan 24	2.23 × 1.46, −44.6	3.9	6.8	13CO(2 − 1)	220.398684

Note. (1) The center frequencies of the bands. (2) The bandwidths. (3) The velocity width of the channels. (4) The date of the observations. (5) The synthesized beams from imaging with natural weighting. (6) RMS of emission and absorption free channels of the image cubes. (7) The flux densities of the central continuum source. Overall flux density uncertainties are taken to be 5% in Band 3 and 10% in Band 6. (8) The molecular line targeted in emission in the frequency ranges. (9) The rest frequencies of the targeted molecular transitions from Lovas & Dragoset (2004). Transitions detected in emission are in bold. For transitions that have resolved hyperfine structure, the strongest of the hyperfine is reported.

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The 1 mm observations were carried out in a single run with 17 antennas in a compact configuration on 2012 January 24, with baselines of 18 m to 125 m. The maximum recoverable scale is 10'' and the primary beam is 27''. Titan is the flux and bandpass calibrator, and complex gain (phase/gain) calibration was done by observing PKS B1424−418 (J1427−4206) every 10 minutes. There were no significant spectral lines in the atmosphere of Titan that may contaminate the Cen A spectra. The total on-source integration time is ∼33 minutes, with each spectral window separated into 3840 channels with spacings of 244.14 kHz per channel corresponding to a velocity resolution of 0.66 km s⁻¹ after Hanning smoothing and a bandwidth of 937.5 MHz (1260 km s⁻¹).

The 3 mm band observations were taken in multiple runs spanning the time between 2012 April 7 and 2012 May 9 in a more extended configuration with baselines of 36 m to 400 m. The maximum recoverable scale was 20'', and the primary beam was 62''. The 3 mm observations were performed with 17 antennas for the first two observing runs and 16 antennas for the final two observations (See Table 1 for details). As with the Band 6 observations, the flux and complex gain calibrators are Titan and PKS B1424−418 (J1427-4206), respectively. The phase calibrator was observed every 16 minutes. 3C279 (J1256−0547) was used as the bandpass calibrator. The total on-source integration time is ∼66 minutes with each spectral window, again divided into 3840 channels but with spacings of 61.04 kHz per channel corresponding to a velocity resolution between 0.32 and 0.44 km s⁻¹ with a bandwidth of 234.4 MHz (approximately 730 km s⁻¹).

The configurations were selected such that the maximum recoverable scales are approximately the same between the 3 mm and 1 mm bands, so comparisons between the fluxes in each band should, in principle, be robust on the sampled scales.

Initial calibration and editing of the data were performed by the ALMA team using the CASA (Common Astronomy Software Applications) data reduction package (McMullin et al. 2007). After delivery, self-calibration and imaging were performed by us with CASA version 4.1. We complemented our data with the Science Verification ¹²CO(2 − 1) observations of Cen A, giving us a total of nine molecular transitions with emission detected in the nuclear region.

We performed a self-calibration on the bright radio continuum core of Cen A using the CASA task gaincal to generate the calibration tables and applycal to apply the corrections to the data. The calibration tables were made by carefully avoiding channels with line emission. As the first step, we self-calibrated the phases only; then we did an iteration of simultaneous amplitude and phase self-calibration. We then used the CASA task uvcontsub2 to fit a first order polynomial continuum model to channels that were free of both line emission and absorption. This model was then subtracted from the (u, v) data to create a continuum-free spectral line data set for each line.

With the continuum emission removed, we then proceeded to image each of the spectral windows. Imaging was performed using the CASA task clean with natural weighting and an image size of 320 × 320 pixels at 0.2 arcseconds per pixel. This corresponds to an image size of 64'', which encompasses the primary beams of both the Band 3 (62'' primary beam) and the Band 6 (27'' primary beam) observations. The synthesized beams for the images are listed in Table 1. Continuum images were made using the line-free channels of the two representative frequency ranges, centered on 220.007 GHz and 90.498 GHz, with synthesized beams listed in Table 1. An overall flux accuracy of 5% was assumed for the Band 3 observations, and a flux accuracy of 10% was assumed for the Band 6 observations.¹⁵

Each frequency range was imaged at the full spectral resolution (3840 channels) to obtain the highest spectral resolution absorption spectra toward the continuum source of Cen A achievable with this data. The image cubes used to make the absorption spectra were not cleaned because of an artifact created during the execution of the clean algorithm. Near the location of the absorption, clean created a positive spike in the image cube as the map transitioned from noise to a deep absorption feature. The clean artifacts are not fully understood, but they appear to be related to the high dynamic range change between adjacent absorption/non-absorption channels. The artifacts are not present in the dirty cube that we used for the analysis of the absorption spectra.

Broader spectral resolution (averaged over 10 channels) image cubes were created and cleaned to map the emission. The emission cubes were then inspected for artifacts created by clean. The artifacts in the emission were all located in the center of the field of view, co-spatial with the absorption, which were already masked due to the absorption hole (discussed later) and so do not affect the displayed emission maps. Therefore, in summary, the artifact does not impact any map/result presented in this paper. The emission cubes were all then convolved to a common beam of 225 × 225, which is slightly larger than the original synthesized beam sizes given in Table 1. The lone exception is the ¹²CO(2 − 1) image cube, which due to u–v coverage was unable to be convolved to the same beam. In order to compare the ¹²CO(2 − 1) and the other isotopologues, the ¹²CO(2 − 1) was convolved to a 30 × 30 beam and compared to separate ¹³CO(2 − 1), ¹³CO(1 − 0), and C¹⁸O(2 − 1) images, also convolved to 30 × 30. Integrated intensity, intensity-weighted velocity, and intensity-weighted velocity dispersion maps were made for each transition with observed emission. A mask was created for each molecular transition by convolving the image cube with a 45 × 45 beam (twice the size of the synthesized beam of the image cube) and then only including emission in the original image cube where the smoothed cube contained emission stronger than ∼3 times the rms. This clipping was necessary to minimize the effect of the absorption near the radio core on the rest of the CND emission. This mask was applied to the image cube during the map making process. In addition, a mask of ∼5'' × 5'' was used to mask the absorption at the center for each of the moment maps.

Additional ALMA data obtained during science verification (2011.0.00008.SV) were used to provide the ¹²CO(2 − 1) 1.3 mm transition for comparison. The data were edited, calibrated, and imaged in a consistent manner as the rest of our data for direct comparison. However, the spectral resolution is coarser than our data, at 10 km s⁻¹ per channel, while our data were imaged with velocity resolutions ranging from 1.6 to 3.3 km s⁻¹ per channel.

The AGN is detected as an unresolved millimeter continuum source at a position of R.A.(J2000): 13^h25^m27616 Decl. (J2000): −43°01'08813. The continuum fluxes for the bright central source were determined from apertures in cleaned continuum images and are listed in column (7) of Table 1. Figure 1 shows the 3 mm band (at 90.5 GHz) and 1 mm band (at 220.0 GHz) continuum images. The dashed line in Figure 1 (lower panel) shows the half power of the 1.3 mm primary beam, while the corresponding half power for 3 mm is outside the displayed field of view. The radio jet is also detected in both bands, but it is stronger in the 3 mm continuum image. Two knots of the northern jet are detected, but no portion of the southern jet is seen. This is consistent with previous observations. The knots correspond to A1 (inner) and A2 (outer) of Clarke et al. (1992), which were studied in more detail at radio wavelengths and X-ray energies by Hardcastle et al. (2003). Self-calibration was required to achieve the dynamic range necessary to detect the jet in the presence of the strong nuclear source. We achieved rms values for the 3 mm and 1 mm images of 1.3 mJy beam⁻¹ and 2.1 mJy beam⁻¹, respectively. The jet integrated flux is around 30 ± 5 mJy in band 3 and 20 ± 5 mJy in band 6, compared to the core flux of 8.1 ± 0.4 Jy in band 3 and 6.7 ± 0.7 Jy in band 6. Both knots of the jet are well detected in both images, with the outer knot slightly stronger than the inner knot in both bands. The spectral index between the two bands is −0.2 ± 0.1 for the bright radio core continuum, and −0.4 ± 0.2 for the jet continuum, with the spectral index α defined as S ∝ ν^α. Israel et al. (2008) determine the core continuum spectral index to vary over the range −0.2 to −0.6, consistent with our value. However, the 3 mm and 1 mm continuum were not observed simultaneously. This adds uncertainty to all flux comparisons due to the known ∼30% variability of Centaurus A (See Israel et al. 2008, their Figure 5). This time variability in the flux could potentially explain the flux differences listed in Table 1. However, the fluxes determined for different frequencies in the same observing band give a positive spectral index, so there are likely systemic errors as well.

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We detected eight molecular transitions in emission toward the nuclear region of Cen A. The detected transitions are the J = 2 − 1 rotational transitions of ¹³CO and C¹⁸O [Band 6]; the J = 1 − 0 rotational transitions of ¹³CO, HCO⁺, HCN, HNC; and the N = 1 − 0; $J=\tfrac{3}{2}-\tfrac{1}{2}$ transitions of CN and CCH (hereafter CN(1 − 0) and CCH(1 − 0)) [Band 3]. In Band 6, HNCO(10_0,10 − 9_0,9) and H₂CO(3_0,3 − 2_0,2) were targeted but not detected in emission. HC₃N(10 − 9), CH₃CN(5₃ − 4₃), HNCO(4_0,4 − 3_0,3), and J = 1 − 0 of C¹⁸O, C¹⁷O, and N₂H⁺ were targeted in band 3 (3 mm), but not detected in emission.

Figure 2 shows the primary beam corrected integrated intensity (moment 0) maps of the detected CO isotopologues and HCO⁺ transitions toward the nuclear region of Cen A, as well as a comparable ¹²CO(2 − 1) moment 0 map from ALMA science verification data. Figure 3 shows the other detected dense gas tracers, HCN(1 − 0), CN(1 − 0), HNC(1 − 0), and CCH(1 − 0) toward the same region of Cen A as in Figure 2. There are two main structures in the maps visible on our observed scales: the two linear molecular arms, and the dense circumnuclear disk (CND).

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In each of the detected CO isotopologue maps, there are two clumpy linear features, referred to as molecular arms, extending roughly east-west, that coincide with the parallelogram-shaped dust lanes. The two linear features are separated by about 15'' (275 pc), each at a projected offset from the SMBH of about 75. Both molecular arms are roughly parallel and at a position angle of ∼113°. The southern arm is brighter than the northern arm in all detected molecular lines, with the brightest clumps of the southern arm ∼50% brighter than the brightest clumps of the northern arm. The molecular arms extend beyond the primary beams of both the 3 mm band and 1 mm band observations while maintaining the 15'' separation, which is consistent with observations by Espada et al. (2009).

Figure 4 shows the ¹³CO(2 − 1) integrated intensity map in grayscale with HCO⁺(1 − 0) integrated intensity contours overlaid. Eight regions are noted in the figure and are used for analysis of the molecular arms and CND. Region A is part of the northern arm, while regions F, G, and H are located along the southern molecular arm. Regions C, D, and E are in the CND. Region B represents a location of overlap between the molecular arms and the CND. The CND is measured to be ∼410 pc (22'') × 215 pc (107), and the centroid of the CND appears slightly off-center relative to the AGN (∼50 pc). Garcia-Burillo et al. (2014) have observed the molecular CND in another nearby AGN, NGC 1068, to be off-centered from its AGN as well.

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HCN(1 − 0), HCO⁺(1 − 0), HNC(1 − 0), CN(1 − 0), and CCH(1 − 0) emission are all weak or absent in the molecular arms, but present in the CND. HCO⁺ traces the majority of the CND, while the others are only detected in specific regions of the CND. If considered as a thin, flat disk, its axial ratio of ∼2 corresponds to an inclination angle of ∼60 degrees and a position angle of about 150°, nearly perpendicular to the radio jet axis. Thus the minor axis of the CND is at a position angle of 60°. The FWHM (full width at half maximum) line width of the CND is around 40 km s⁻¹ for an inner diameter of 140 pc corresponding to C and D (Figure 4) and 70 km s⁻¹ between diameters of 275 and 325 pc corresponding to B and E (Figure 4) measured by HCO⁺(1 − 0). The outer portions of the CND for these transitions are coincident with the edges of the CND that are seen in ¹³CO(2 − 1). Regions C and D are most likely composed of a combination of nuclear filaments and the nuclear ring structure (Espada et al. 2017).

In the ¹²CO(2 − 1) and ¹³CO(2 − 1) images, the outer edge of the CND is also detected; however, no part of the CND is detected in either ¹³CO(1 − 0) or C¹⁸O(2 − 1). The clumps in the molecular arms have spectral line widths (FWHM) of ∼15 km s⁻¹, while the detected portions of the CND have FWHM of ∼40 km s⁻¹ as measured by ¹³CO(2 − 1) (determined in Section 3.4.1), which are consistent with the measured line widths from the dense gas tracers.

The CND is considerably brighter along its major axis than its minor axis. The minor axis at a position angle of approximately 60° separates regions B and C on the northern side from D and E on the southern side. From the HCO⁺(1 − 0) emission, the southern side of the minor axis is almost not present, and the northern side of the minor axis is weaker than any of the emission along the CND's major axis. The underluminous minor axis may be due to deviations from a uniform disk, and the underluminous minor axis was also observed in ¹²CO(2 − 1), with the SMA as reported by Espada et al. (2009).

The intensity-weighted mean velocity (moment 1) and intensity-weighted velocity dispersion (moment 2) maps are shown in Figure 5, for the HCO⁺(1 − 0), ¹³CO(1 − 0), and ¹³CO(2 − 1) transitions. The HCO⁺(1 − 0) emission traces the CND very well in both velocity and intensity, but there is no emission from the arms. The ¹³CO(1 − 0) emission, on the other hand, traces the arms but not the CND. ¹³CO(2 − 1) exhibits both the molecular arms and the CND components and resembles a combination of the HCO⁺(1 − 0) and ¹³CO(1 − 0) velocity maps. The ¹³CO(1 − 0) transition covers velocities of ∼500–600 km s⁻¹, and shows a velocity gradient of ∼0.2 km s⁻¹ pc⁻¹ increasing from east to west, with a velocity dispersion of <10 km s⁻¹. HCO⁺(1 − 0) covers the velocity range from about 330 to 770 km s⁻¹, and reveals a much steeper velocity gradient along the same axis of 1.2 km s⁻¹ pc⁻¹, with a dispersion of ∼50 km s⁻¹. Using the central velocity of the CND as observed by HCO⁺(1 − 0), we calculate that the systemic velocity in the LSRK (local standard of rest, kinematic definition) velocity frame for Cen A is approximately 550 km s⁻¹, which is consistent with the values in the literature, ranging from about 530 to 560 km s⁻¹ (Quillen et al. 1992; Marconi et al. 2001; Häring-Neumayer et al. 2006).

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Figure 5 also shows the full field ¹³CO(1 − 0) intensity-weighted mean velocity and velocity dispersion maps. These maps show that outside the field limited by the ¹³CO(2 − 1) primary beam, the ¹³CO(1 − 0) continues to follow a similar trend of lower dispersion and a smaller velocity gradient than HCO⁺(1 − 0).

The molecular arm component in the ¹³CO(2 − 1) moment 1 map ranges from 450 km s⁻¹ up to 600 km s⁻¹ (similar to the range of ¹³CO(1 − 0)), while the circumnuclear component of the ¹³CO(2 − 1) map covers 375 km s⁻¹ to 675 km s⁻¹ (similar to the range of HCO⁺(1 − 0)). There are two locations in the map, near B and E, where the dispersion jumps to >60 km s⁻¹ but the dispersion remains around ∼10 km s⁻¹ across the remaining map. This jump in the dispersion is not due to a single component with a dispersion of 60 km s⁻¹, but the positional overlap of the narrow dust lane feature with the broad circumnuclear feature. It is worth noting that the velocity field of the inner region of the CND seems to twist in the HCO⁺(1 − 0) moment 1 map (Figure 5). This warping appears to decrease the position angle of the inner CND, consistent with the H₂ velocity map of Neumayer et al. (2007) and CO maps of Espada et al. (2017). However, in the H₂ velocity map, there appears to be another warp further in, not sampled by our data (due to the absorption).

Position-velocity (PV) diagrams for ¹³CO(2 − 1), ¹³CO(1 − 0), and HCO⁺(1 − 0) are shown in Figure 6. The top left panel of the figure shows the locations of the slices through the cube. Five slices were taken: the first through the major axis of the CND, the second through the inner peaks of the CND, the third and fourth along the northern and southern molecular arms, and the last along the axis of the radio jet. The slices were all averaged over a width of 11 pixels, approximately 40 pc. These pixels are not independent and were chosen such that the width of the slice was approximately the same width as the beam. The broad components in all of the PV diagrams come from the CND, while the narrow components come from the molecular arms. The center of the slice (the 0 offset location) is noted by a small cross on the slice line. Slices 1, 2, and 5 all share the same center.

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Slice 1 extends along the elongated axis of the CND while also crossing the molecular arms. Slice 1 shows the lack of ¹³CO(1 − 0) along with the prominence of HCO⁺ in the CND. ¹³CO(2 − 1) is present in both the molecular arm components as well as the CND components, although not as prominently as HCO⁺. The CND has components spanning a velocity range from 350 to 800 km s⁻¹. The molecular arms span a much narrower velocity range, about 520–580 km s⁻¹ within the band 6 primary beam in Slice 1.

Slice 2 intersects the inner portion of the nuclear disk and avoids the molecular arms until large offsets. Again, there is a lack of ¹³CO(1 − 0) in the broad (CND) components. ¹³CO(2 − 1) is present in both the molecular arms as well as the CND, although more prominent in the molecular arms. This slice shows that the inner portion of the CND has a velocity range of 375–700 km s⁻¹ while both molecular arms span velocities ∼475–575 km s⁻¹.

Slices 3 and 4 primarily cover the northern and southern molecular arms, crossing only the very edges of the CND. Both slices show the narrow velocity range of the molecular arms, but slice 4 (southern arm) is simply a linear feature in the PV diagram. Slice 3 has a more complicated structure. Slice 3 (northern arm) contains a linear feature similar to slice 4 with the addition of ¹³CO(1 − 0) and ¹³CO(2 − 1) detected in the same position as the linear feature, but offset by ∼30 km s⁻¹ showing a double structure. This double structure might be a result of overlapping tilted ring along the line of sight, but its origin is not entirely evident. The absence of emission from ¹³CO(2–1) at an offset of −14'' to −20'' results from being outside the 20% level of the primary beam.

Slice 5 traces approximately the AGN jet axis. However, there is no indication of significant structure that would come from an interaction of the molecular gas with the jet.

3.4.1. Emission Spectra

Figures 7–14 show the spectra from each of the detected transitions as well as ¹²CO(2 − 1) from the science verification data and the non-detection of C¹⁸O(1 − 0) toward the eight marked locations in Figure 4. Spectra were taken using a 225 apertures corresponding to the circles in Figure 4. We identify 22 velocity components throughout all of the observed molecular transitions and spatial locations. Each of these components are labeled by its spatial location and a numeric identifier that increases with velocity, irrespective of the transition. These components are identified in the top left panel in Figures 7 through 14 (the ¹²CO(2 − 1) spectra from each of the regions). Each figure shows the spectra from each of the detected transitions, as well as ¹²CO(2 − 1) from the science verification data and the non-detection of C¹⁸O(1 − 0). The properties of these spectral features—that is, for each velocity component (e.g., A-1 , A-2, A-3)—are given in Table 2, and corresponding line ratios are given in Table 3. For undetected features, a 1 sigma upper limit for the peak temperature is provided. Comparisons between detections and non-detections in Table 3 are presented as upper or lower limits of that line ratio. Comparisons between double non-detections are not presented in Table 3.

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Table 2.  Properties of Emission Components

Line Identifier	Transition	Peak	Line Center	FWHM	Integral
		(K)	(km s−1)	(km s−1)	(K km s−1)
A-1	12CO(2–1)	0.92 ± 0.12	510.0 ± 5.0	20.0 ± 5.0	19.4 ± 5.6
(13:25:27.983)	13CO(2–1)	0.61 ± 0.03	508.5 ± 0.3	10.4 ± 0.6	6.7 ± 0.5
(−43.01.01.519)	13CO(1–0)	0.42 ± 0.03	513.1 ± 0.3	7.8 ± 0.8	3.5 ± 0.4
	C18O(2–1)	0.14 ± 0.02	509.6 ± 0.6	8.5 ± 1.4	1.3 ± 0.3
	C18O(1–0)	<0.03	⋯	⋯	⋯
	HCO+(1–0)	<0.06	⋯	⋯	⋯
	HCN(1–0)	<0.06	⋯	⋯	⋯
	CN(1–0)	<0.05	⋯	⋯	⋯
	HNC(1–0)	<0.06	⋯	⋯	⋯
	CCH(1–0)	<0.06	⋯	⋯	⋯
A-2	12CO(2–1)	0.31 ± 0.05	550.0 ± 5.0	15.0 ± 5.0	4.9 ± 3.6
	13CO(2–1)	0.26 ± 0.07	553.7 ± 1.3	10.8 ± 3.2	3.0 ± 1.2
	13CO(1–0)	0.18 ± 0.01	560.1 ± 0.4	4.7 ± 0.9	0.9 ± 0.2
	C18O(2–1)	<0.02	⋯	⋯	⋯
	C18O(1–0)	<0.03	⋯	⋯	⋯
	HCO+(1–0)	<0.06	⋯	⋯	⋯
	HCN(1–0)	<0.06	⋯	⋯	⋯
	CN(1–0)	<0.05	⋯	⋯	⋯
	HNC(1–0)	<0.06	⋯	⋯	⋯
	CCH(1–0)	<0.06	⋯	⋯	⋯
A-3	12CO(2–1)	0.15 ± 0.03	614.1 ± 6.3	61.0 ± 15.0	10.0 ± 3.3
	13CO(2–1)	<0.02	⋯	⋯	⋯
	13CO(1–0)	0.02 ± 0.01	631.8 ± 12.0	40.0 ± 31.0	0.9 ± 0.8
	C18O(2–1)	<0.02	⋯	⋯	⋯
	C18O(1–0)	<0.03	⋯	⋯	⋯
	HCO+(1–0)	0.07 ± 0.03	627.1 ± 3.8	19.6 ± 9.0	1.5 ± 0.9
	HCN(1–0)	<0.06	⋯	⋯	⋯
	CN(1–0)	<0.05	⋯	⋯	⋯
	HNC(1–0)	<0.06	⋯	⋯	⋯
	CCH(1–0)	<0.06	⋯	⋯	⋯
B-1	12CO(2–1)	2.17 ± 0.05	525.9 ± 0.2	22.8 ± 0.6	52.5 ± 1.7
(13:25:27.490)	13CO(2–1)	0.42 ± 0.02	533.1 ± 0.4	16.0 ± 0.9	7.2 ± 0.5
(−43.01.00.370)	13CO(1–0)	0.29 ± 0.04	537.6 ± 0.9	13.8 ± 2.0	4.3 ± 0.8
	C18O(2–1)	0.02 ± 0.01	535.5 ± 2.2	13.5 ± 5.1	0.5 ± 0.2
	C18O(1–0)	<0.04	⋯	⋯	⋯
	HCO+(1–0)	0.14 ± 0.03	534.4 ± 1.3	11.1 ± 3.1	1.7 ± 0.6
	HCN(1–0)	0.08 ± 0.03	532.7 ± 2.3	12.9 ± 5.5	1.1 ± 0.6
	CN(1–0)	<0.05	⋯	⋯	⋯
	HNC(1–0)	<0.06	⋯	⋯	⋯
	CCH(1–0)	<0.06	⋯	⋯	⋯
B-2	12CO(2–1)	0.26 ± 0.05	570.0 ± 5.0	15.0 ± 5.0	4.1 ± 3.6
	13CO(2–1)	0.12 ± 0.03	573.2 ± 1.0	9.5 ± 2.4	1.2 ± 0.4
	13CO(1–0)	<0.04	⋯	⋯	⋯
	C18O(2–1)	<0.01	⋯	⋯	⋯
	C18O(1–0)	<0.04	⋯	⋯	⋯
	HCO+(1–0)	<0.06	⋯	⋯	⋯
	HCN(1–0)	<0.12	⋯	⋯	⋯
	CN(1–0)	<0.05	⋯	⋯	⋯
	HNC(1–0)	<0.06	⋯	⋯	⋯
	CCH(1–0)	<0.06	⋯	⋯	⋯
B-3	12CO(2–1)	1.47 ± 0.05	711.7 ± 1.2	64.4 ± 2.9	100.6 ± 5.9
	13CO(2–1)	0.07 ± 0.01	715.4 ± 2.4	68.1 ± 5.6	5.3 ± 0.6
	13CO(1–0)	<0.04	⋯	⋯	⋯
	C18O(2–1)	<0.01	⋯	⋯	⋯
	C18O(1–0)	<0.04	⋯	⋯	⋯
	HCO+(1–0)	0.40 ± 0.02	712.7 ± 1.5	65.5 ± 3.5	28.3 ± 2.0
	HCN(1–0)	0.19 ± 0.02	701.9 ± 2.8	53.8 ± 6.9	11.7 ± 1.8
	CN(1–0)	0.04 ± 0.01	697.6 ± 6.5	48.0 ± 15.0	2.2 ± 0.9
	HNC(1–0)	0.11 ± 0.02	720.0 ± 5.2	63.0 ± 12.0	7.3 ± 1.9
	CCH(1–0)	0.07 ± 0.02	726.9 ± 5.7	36.0 ± 13.0	2.7 ± 1.3
C-1	12CO(2–1)	<0.12	⋯	⋯	⋯
(13:25:27.379)	13CO(2–1)	0.03 ± 0.02	523.6 ± 3.5	10.8 ± 8.2	0.3 ± 0.3
(−43.01.05.728)	13CO(1–0)	<0.04	⋯	⋯	⋯
	C18O(2–1)	<0.01	⋯	⋯	⋯
	C18O(1–0)	<0.04	⋯	⋯	⋯
	HCO+(1–0)	<0.06	⋯	⋯	⋯
	HCN(1–0)	<0.06	⋯	⋯	⋯
	CN(1–0)	<0.04	⋯	⋯	⋯
	HNC(1–0)	<0.06	⋯	⋯	⋯
	CCH(1–0)	<0.06	⋯	⋯	⋯
C-2	12CO(2–1)	0.19 ± 0.05	560.0 ± 5.0	15.0 ± 5.0	3.1 ± 3.6
	13CO(2–1)	0.18 ± 0.05	557.5 ± 5.0	5.0 ± 3.0	0.9 ± 0.8
	13CO(1–0)	<0.04	⋯	⋯	⋯
	C18O(2–1)	<0.01	⋯	⋯	⋯
	C18O(1–0)	<0.04	⋯	⋯	⋯
	HCO+(1–0)	<0.06	⋯	⋯	⋯
	HCN(1–0)	<0.06	⋯	⋯	⋯
	CN(1–0)	<0.04	⋯	⋯	⋯
	HNC(1–0)	<0.06	⋯	⋯	⋯
	CCH(1–0)	<0.06	⋯	⋯	⋯
C-3	12CO(2–1)	1.04 ± 0.09	649.1 ± 1.9	46.4 ± 4.5	51.2 ± 6.6
	13CO(2–1)	0.04 ± 0.01	646.7 ± 2.4	46.4 ± 5.6	2.1 ± 0.3
	13CO(1–0)	<0.04	⋯	⋯	⋯
	C18O(2–1)	<0.01	⋯	⋯	⋯
	C18O(1–0)	<0.04	⋯	⋯	⋯
	HCO+(1–0)	0.64 ± 0.02	651.7 ± 1.0	56.3 ± 2.2	38.4 ± 2.0
	HCN(1–0)	0.37 ± 0.02	648.4 ± 1.4	50.7 ± 3.4	20.0 ± 1.8
	CN(1–0)	0.21 ± 0.01	649.2 ± 1.4	42.3 ± 3.4	9.6 ± 1.0
	HNC(1–0)	0.19 ± 0.02	650.2 ± 2.4	38.0 ± 5.8	7.6 ± 1.5
	CCH(1–0)	0.10 ± 0.02	664.8 ± 5.7	64.0 ± 14.0	6.6 ± 1.8
D-1	12CO(2–1)	0.86 ± 0.05	468.4 ± 1.2	42.0 ± 2.9	38.4 ± 3.6
(13:25:27.918)	13CO(2–1)	0.04 ± 0.01	459.1 ± 1.8	35.4 ± 4.3	1.4 ± 0.2
(−43.01.09.025)	13CO(1–0)	<0.03	⋯	⋯	⋯
	C18O(2–1)	<0.01	⋯	⋯	⋯
	C18O(1–0)	<0.04	⋯	⋯	⋯
	HCO+(1–0)	0.83 ± 0.02	466.9 ± 0.7	46.3 ± 1.6	40.8 ± 1.8
	HCN(1–0)	0.48 ± 0.02	465.2 ± 1.2	49.7 ± 2.9	25.5 ± 2.0
	CN(1–0)	0.20 ± 0.02	465.9 ± 1.7	27.7 ± 4.1	5.8 ± 1.2
	HNC(1–0)	0.16 ± 0.02	458.8 ± 1.8	39.6 ± 4.3	6.9 ± 1.0
	CCH(1–0)	<0.07	⋯	⋯	⋯
D-2	12CO(2–1)	0.36 ± 0.05	530.0 ± 5.0	20.0 ± 5.0	7.7 ± 4.6
	13CO(2–1)	<0.01	⋯	⋯	⋯
	13CO(1–0)	<0.03	⋯	⋯	⋯
	C18O(2–1)	<0.01	⋯	⋯	⋯
	C18O(1–0)	<0.04	⋯	⋯	⋯
	HCO+(1–0)	<0.07	⋯	⋯	⋯
	HCN(1–0)	<0.06	⋯	⋯	⋯
	CN(1–0)	<0.06	⋯	⋯	⋯
	HNC(1–0)	<0.06	⋯	⋯	⋯
	CCH(1–0)	<0.07	⋯	⋯	⋯
D-3	12CO(2–1)	<0.23	⋯	⋯	⋯
	13CO(2–1)	0.02 ± 0.01	609.9 ± 3.4	17.0 ± 8.1	0.3 ± 0.2
	13CO(1–0)	<0.03	⋯	⋯	⋯
	C18O(2–1)	<0.01	⋯	⋯	⋯
	C18O(1–0)	<0.04	⋯	⋯	⋯
	HCO+(1–0)	<0.07	⋯	⋯	⋯
	HCN(1–0)	<0.06	⋯	⋯	⋯
	CN(1–0)	<0.06	⋯	⋯	⋯
	HNC(1–0)	<0.06	⋯	⋯	⋯
	CCH(1–0)	<0.07	⋯	⋯	⋯
D-4	12CO(2–1)	0.13 ± 0.05	704.8 ± 7.9	44.0 ± 18.0	6.1 ± 3.3
	13CO(2–1)	<0.01	⋯	⋯	⋯
	13CO(1–0)	<0.03	⋯	⋯	⋯
	C18O(2–1)	<0.01	⋯	⋯	⋯
	C18O(1–0)	<0.04	⋯	⋯	⋯
	HCO+(1–0)	<0.07	⋯	⋯	⋯
	HCN(1–0)	<0.06	⋯	⋯	⋯
	CN(1–0)	<0.06	⋯	⋯	⋯
	HNC(1–0)	<0.06	⋯	⋯	⋯
	CCH(1–0)	<0.07	⋯	⋯	⋯
E-1	12CO(2–1)	1.05 ± 0.03	374.5 ± 1.0	61.2 ± 2.3	68.3 ± 3.3
(13:25:27.918)	13CO(2–1)	0.07 ± 0.01	376.1 ± 2.0	49.3 ± 4.7	3.5 ± 0.4
(−43.01.15.132)	13CO(1–0)	<0.04	⋯	⋯	⋯
	C18O(2–1)	<0.01	⋯	⋯	⋯
	C18O(1–0)	<0.04	⋯	⋯	⋯
	HCO+(1–0)	0.42 ± 0.02	374.2 ± 1.4	52.8 ± 3.4	23.6 ± 2.0
	HCN(1–0)	0.19 ± 0.02	372.8 ± 1.2	46.2 ± 6.3	9.5 ± 1.7
	CN(1–0)	0.09 ± 0.02	371.2 ± 1.8	19.3 ± 4.4	1.8 ± 0.5
	HNC(1–0)	0.05 ± 0.01	359.0 ± 11.0	105.0 ± 31.0	6.2 ± 2.2
	CCH(1–0)	0.07 ± 0.02	399.1 ± 5.3	44.0 ± 15.0	3.3 ± 1.4
E-2	12CO(2–1)	<0.23	⋯	⋯	⋯
	13CO(2–1)	0.11 ± 0.04	543.6 ± 1.6	9.1 ± 3.8	1.0 ± 0.6
	13CO(1–0)	<0.04	⋯	⋯	⋯
	C18O(2–1)	<0.01	⋯	⋯	⋯
	C18O(1–0)	<0.04	⋯	⋯	⋯
	HCO+(1–0)	0.09 ± 0.03	544.6 ± 2.1	13.6 ± 4.9	1.3 ± 0.6
	HCN(1–0)	<0.06	⋯	⋯	⋯
	CN(1–0)	<0.04	⋯	⋯	⋯
	HNC(1–0)	<0.06	⋯	⋯	⋯
	CCH(1–0)	<0.07	⋯	⋯	⋯
E-3	12CO(2–1)	0.37 ± 0.07	590.0 ± 3.7	39.8 ± 8.7	15.9 ± 4.6
	13CO(2–1)	0.03 ± 0.01	584.0 ± 1.2	8.2 ± 2.9	0.3 ± 0.1
	13CO(1–0)	<0.04	⋯	⋯	⋯
	C18O(2–1)	<0.01	⋯	⋯	⋯
	C18O(1–0)	<0.04	⋯	⋯	⋯
	HCO+(1–0)	<0.06	⋯	⋯	⋯
	HCN(1–0)	<0.06	⋯	⋯	⋯
	CN(1–0)	<0.04	⋯	⋯	⋯
	HNC(1–0)	<0.06	⋯	⋯	⋯
	CCH(1–0)	<0.07	⋯	⋯	⋯
F-1	12CO(2–1)	<0.15	⋯	⋯	⋯
(13:25:26.819)	13CO(2–1)	0.19 ± 0.02	545.6 ± 0.3	5.3 ± 0.9	1.1 ± 0.2
(−43.01.13.585)	13CO(1–0)	0.16 ± 0.03	549.2 ± 0.3	2.3 ± 0.5	0.4 ± 0.1
	C18O(2–1)	<0.01	⋯	⋯	⋯
	C18O(1–0)	<0.04	⋯	⋯	⋯
	HCO+(1–0)	<0.06	⋯	⋯	⋯
	HCN(1–0)	<0.06	⋯	⋯	⋯
	CN(1–0)	<0.04	⋯	⋯	⋯
	HNC(1–0)	<0.06	⋯	⋯	⋯
	CCH(1–0)	<0.07	⋯	⋯	⋯
F-2	12CO(2–1)	1.79 ± 0.11	581.0 ± 0.6	18.4 ± 1.3	35.1 ± 3.3
	13CO(2–1)	0.51 ± 0.03	581.2 ± 0.4	16.6 ± 1.0	8.9 ± 0.7
	13CO(1–0)	0.47 ± 0.03	584.9 ± 0.4	11.5 ± 0.9	5.7 ± 0.6
	C18O(2–1)	0.06 ± 0.01	582.4 ± 1.4	13.3 ± 3.3	0.8 ± 0.3
	C18O(1–0)	<0.04	⋯	⋯	⋯
	HCO+(1–0)	<0.06	⋯	⋯	⋯
	HCN(1–0)	<0.06	⋯	⋯	⋯
	CN(1–0)	<0.04	⋯	⋯	⋯
	HNC(1–0)	<0.06	⋯	⋯	⋯
	CCH(1–0)	<0.07	⋯	⋯	⋯
G-1	12CO(2–1)	0.25 ± 0.12	442.4 ± 5.4	23.0 ± 13.0	6.1 ± 4.6
(13:25:28.556)	13CO(2–1)	0.05 ± 0.01	469.3 ± 8.2	46.0 ± 20.0	2.2 ± 1.1
(−43.01.21.565)	13CO(1–0)	<0.04	⋯	⋯	⋯
	C18O(2–1)	<0.01	⋯	⋯	⋯
	C18O(1–0)	<0.04	⋯	⋯	⋯
	HCO+(1–0)	<0.06	⋯	⋯	⋯
	HCN(1–0)	<0.06	⋯	⋯	⋯
	CN(1–0)	<0.04	⋯	⋯	⋯
	HNC(1–0)	<0.06	⋯	⋯	⋯
	CCH(1–0)	<0.07	⋯	⋯	⋯
G-2	12CO(2–1)	2.16 ± 0.12	507.6 ± 0.6	23.6 ± 1.5	54.5 ± 4.6
	13CO(2–1)	1.22 ± 0.04	509.8 ± 0.3	18.9 ± 0.7	24.6 ± 1.3
	13CO(1–0)	0.52 ± 0.04	511.0 ± 0.6	18.2 ± 1.4	10.1 ± 1.0
	C18O(2–1)	0.09 ± 0.02	512.5 ± 1.8	19.2 ± 4.3	1.9 ± 0.6
	C18O(1–0)	<0.04	⋯	⋯	⋯
	HCO+(1–0)	0.28 ± 0.05	503.9 ± 0.9	10.4 ± 2.1	3.1 ± 0.8
	HCN(1–0)	0.14 ± 0.03	510.7 ± 1.7	17.6 ± 4.1	2.6 ± 0.8
	CN(1–0)	<0.04	⋯	⋯	⋯
	HNC(1–0)	<0.06	⋯	⋯	⋯
	CCH(1–0)	<0.07	⋯	⋯	⋯
H-1	12CO(2–1)	2.00 ± 0.10	524.2 ± 0.6	22.3 ± 1.3	47.6 ± 3.6
(13:25:28.556)	13CO(2–1)	1.23 ± 0.09	521.7 ± 0.6	14.9 ± 1.4	19.6 ± 2.3
(−43.01.20.343)	13CO(1–0)	1.02 ± 0.05	523.8 ± 0.3	11.3 ± 0.6	12.2 ± 0.9
	C18O(2–1)	0.35 ± 0.03	518.4 ± 0.3	6.3 ± 0.5	2.3 ± 0.3
	C18O(1–0)	<0.04	⋯	⋯	⋯
	HCO+(1–0)	0.17 ± 0.03	520.7 ± 1.4	15.3 ± 3.3	2.8 ± 0.8
	HCN(1–0)	0.12 ± 0.04	521.1 ± 2.7	18.4 ± 6.5	2.4 ± 1.1
	CN(1–0)	<0.04	⋯	⋯	⋯
	HNC(1–0)	<0.06	⋯	⋯	⋯
	CCH(1–0)	<0.07	⋯	⋯	⋯
H-2	12CO(2–1)	<0.08	⋯	⋯	⋯
	13CO(2–1)	<0.01	⋯	⋯	⋯
	13CO(1–0)	0.09 ± 0.03	559.9 ± 0.9	6.8 ± 2.1	0.7 ± 0.3
	C18O(2–1)	0.04 ± 0.01	558.1 ± 1.0	9.9 ± 2.4	0.4 ± 0.1
	C18O(1–0)	<0.04	⋯	⋯	⋯
	HCO+(1–0)	<0.07	⋯	⋯	⋯
	HCN(1–0)	<0.06	⋯	⋯	⋯
	CN(1–0)	<0.04	⋯	⋯	⋯
	HNC(1–0)	<0.06	⋯	⋯	⋯
	CCH(1–0)	<0.07	⋯	⋯	⋯

Note. For non-detected features, a 1σ upper limit is given. Spectral features attributed to the CND are in bold.

A machine-readable version of the table is available.

Download table as:  DataTypeset images: 1 2 3 4

Table 3.  Ratios of Selected Molecular Transitions

ID	$\tfrac{{}^{12}\mathrm{CO}(2-1)}{{}^{13}\mathrm{CO}(2-1)}$	$\tfrac{{}^{12}\mathrm{CO}(2-1)}{{{\rm{C}}}^{18}{\rm{O}}(2-1)}$	$\tfrac{{}^{13}\mathrm{CO}(2-1)}{{}^{13}\mathrm{CO}(1-0)}$	$\tfrac{{{\rm{C}}}^{18}{\rm{O}}(2-1)}{{{\rm{C}}}^{18}{\rm{O}}(1-0)}$	$\tfrac{{}^{13}\mathrm{CO}(2-1)}{{{\rm{C}}}^{18}{\rm{O}}(2-1)}$	$\tfrac{{}^{13}\mathrm{CO}(1-0)}{\mathrm{HCN}(1-0)}$	$\tfrac{\mathrm{HCN}(1-0)}{{\mathrm{HCO}}^{+}(1-0)}$	$\tfrac{\mathrm{HCN}(1-0)}{\mathrm{HNC}(1-0)}$	$\tfrac{\mathrm{HCN}(1-0)}{\mathrm{CN}(1-0)}$	$\tfrac{\mathrm{HCN}(1-0)}{\mathrm{CCH}(1-0)}$
A-1	1.5 ± 0.3	6.4 ± 1.6	1.4 ± 0.2	>4.2	4.2 ± 0.9	>7.2	⋯	⋯	⋯	⋯
A-2	1.2 ± 0.4	>14.8	1.5 ± 0.4	⋯	>12.8	>3.1	⋯	⋯	⋯	⋯
A-3	>7.7	>7.4	<1.0	⋯	⋯	>0.3	<0.8	⋯	⋯	⋯
B-1	5.1 ± 0.8	111.9 ± 61.8	1.5 ± 0.3	>0.5	21.8 ± 12.1	3.5 ± 1.4	0.6 ± 0.3	>1.4	>1.8	>1.3
B-2	2.2 ± 0.7	>25.1	>3.1	⋯	>11.7	⋯	⋯	⋯	⋯	⋯
B-3	20.0 ± 3.3	>144.1	>1.9	⋯	>7.2	<0.2	0.5 ± 0.1	1.7 ± 0.4	4.3 ± 1.3	2.6 ± 0.9
C-1	<4.5	⋯	>0.7	⋯	>2.9	⋯	⋯	⋯	⋯	⋯
C-2	1.1 ± 0.5	>21.2	>5.1	⋯	>19.6	⋯	⋯	⋯	⋯	⋯
C-3	24.2 ± 4.7	>114.9	>1.2	⋯	>4.7	<0.1	0.6 ± 0.1	2.0 ± 0.3	1.7 ± 0.2	3.9 ± 0.8
D-1	23.5 ± 4.4	>99.4	>1.1	⋯	>4.2	<0.1	0.6 ± 0.1	3.0 ± 0.4	2.4 ± 0.4	>7.0
D-2	>42.9	>41.4	⋯	⋯	⋯	⋯	⋯	⋯	⋯	⋯
D-3	<13.1	⋯	>0.5	⋯	>2.0	⋯	⋯	⋯	⋯	⋯
D-4	>15.9	>15.4	⋯	⋯	⋯	⋯	⋯	⋯	⋯	⋯
E-1	16.0 ± 2.6	>101.7	>1.9	⋯	>6.4	<0.2	0.5 ± 0.1	3.5 ± 0.9	2.2 ± 0.5	2.7 ± 0.7
E-2	<2.2	⋯	>3.0	⋯	>10.3	⋯	<0.7	⋯	⋯	⋯
E-3	11.4 ± 4.4	>36.2	>0.9	⋯	>3.2	⋯	⋯	⋯	⋯	⋯
F-1	<0.8	⋯	1.2 ± 0.3	⋯	>19.4	>2.9	⋯	⋯	⋯	⋯
F-2	3.5 ± 0.6	31.1 ± 8.2	1.1 ± 0.2	>1.6	8.8 ± 2.3	>8.4	⋯	⋯	⋯	⋯
G-1	5.6 ± 3.1	>25.8	>1.2	⋯	>4.6	⋯	⋯	⋯	⋯	⋯
G-2	1.8 ± 0.3	22.9 ± 5.7	2.4 ± 0.3	>2.6	12.9 ± 3.2	3.7 ± 0.8	0.5 ± 0.1	>2.4	>3.2	>2.1
H-1	1.6 ± 0.3	5.7 ± 1.0	1.2 ± 0.2	>9.8	3.5 ± 0.6	8.4 ± 2.7	0.7 ± 0.3	>1.9	>3.1	>1.8
H-2	⋯	<2.1	<0.1	>1.1	<0.3	>1.5	⋯	⋯	⋯	⋯

Note. T_pk ratios of the emission features. Limits are given for values where only a single transition was detected. Spectral features attributed to the CND are in bold.

Download table as:  ASCIITypeset image

Regions A, F, G, and H represent the molecular arm components. These regions typically contain a couple of features that have narrow line widths on the order of 10–15 km s⁻¹. These features are prominent in CO and its isotopologues, but nearly non-existent in the dense gas tracers. HCO⁺(1 − 0) and HCN(1 − 0) show faint narrow features in regions B, G, and H. In regions B and G, HCO⁺(1 − 0) and HCN(1 − 0) are at about 1/2 and 1/4 of the corresponding ¹³CO(1 − 0) peak temperature values, and in region H they are about 1/6 and 1/8 the ¹³CO(1 − 0) value. In region A there is a feature near 615 km s⁻¹ present in three of the transitions that have broad line widths, comparable to features in the CND. This feature corresponds to emission from the edge of the CND contributing to region A.

Regions C, D, and E show the emission primarily from the CND. These regions contain spectral features that are prominent in the dense gas tracers and are nearly absent in CO. ¹²CO(2 − 1) and ¹³CO(2 − 1) both contain these features from the CND, but the features are not seen in ¹³CO(1 − 0) or either of the C¹⁸O transitions. These features all have much broader line widths than the features on the molecular arms, with widths on the order of 50 km s⁻¹.

Region B contains spectral features attributed to the northern molecular arm as well as the CND. This is probably best seen in the ¹²CO(2 − 1) and ¹³CO(2 − 1) spectra. Components B-1 and B-2 have narrow line widths and closely resemble the features of regions A, F, G, and H, which correspond to the molecular arms. Component B-3 has a much broader line width, more similar to the features in regions C, D, and E. Region B, therefore, shows a spatial region where the molecular arm component and the CND overlap.

3.4.2. Absorption Spectra

The absorption spectra (Figure 15) were made by taking a spectrum through the central point of an image cube for each transition. Each spectrum was then converted to units of optical depth (τ) based on the continuum flux from Table 1, assuming a continuum source covering factor of unity (see Muller et al. 2009, their Section 4.1, for the size scale of the continuum) and the equation $\tau =-\mathrm{ln}[1-(\tfrac{-{F}_{\mathrm{spec}}}{{F}_{\mathrm{cont}}})]$, where F_spec and F_cont are the continuum subtracted line flux and the continuum flux, respectively. There are two groups of absorption lines detected in the various molecular lines. There is one group near the systemic velocity of Cen A (550 km s⁻¹; Section 3.3) that has three main components within 10 km s⁻¹ and multiple blended or weaker components called the low velocity complex (LV complex). The other group is redshifted by 20–70 km s⁻¹ and contains many blended components called the high velocity complex (HV complex). Both the low and high velocity complexes were previously detected in HCO⁺(1 − 0), HCN(1 − 0), and HNC(1 − 0) at a lower velocity resolution and sensitivity by Wiklind & Combes (1997). We restrict our analysis of the absorption spectra to three of the narrow components (539.7, 543.3, and 549.7 km s⁻¹) in the LV complex and a median value taken from 576–604 km s⁻¹ of the HV complex and only include the molecular transitions C¹⁸O(1 − 0) (where no emission was detected) and those also detected in emission.

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We do not detect emission connecting the narrow line width emission components of the molecular arms and the broad line width emission from the molecular CND. Region B and partially E in Figure 4 mark the locations where the CND and the molecular arms appear to spatially overlap. According to the moment 2 maps of ¹³CO(2 − 1), regions B and E correspond to high velocity dispersion regions. However, looking at the spectra taken toward these regions (see Figures 8 and 11), the cause of the high dispersion is multiple separate kinematic components along the lines of sight. In region B there is a weak broad component and two narrow components that are kinematically distinct. In region E there is a single narrow component and a kinematically distinct broad component. In the PV diagrams for slices 1 and 2 (Figure 6), there does not appear to be any connection between the broad, high velocity gradient component, corresponding to the CND, and the narrow, low velocity gradient component, corresponding to the molecular arms. However, due to the missing short spacings of the observations, there may be large scale structure that is resolved out, possibly hiding the connection described in Espada et al. (2009). An absence of an observed connection between the two main emission components means the method for transporting gas from the molecular arms to the CND is still not clear.

4.2.1. CO Isotopic Ratios

Intensity ratios between isotopologue transitions with the same J states, I[¹²CO(2 − 1)]/I[¹³CO(2 − 1)], I[¹²CO(2 − 1)]/I[C¹⁸O(2 − 1)], and I[¹³CO(2 − 1)]/I[C¹⁸O(2 − 1)], contain information on the opacities and abundance ratios of CO and its isotopologues, assuming each pair of transitions has a common excitation temperature T_x. For example, the intensity ratio, I[¹²CO(2 − 1)]/I[¹³CO(2 − 1)], with the same line width for 2–1 and 1-0 and in the absence of fractionation or selective photodissociation, gives

$$\begin{eqnarray*}&&\displaystyle \frac{{}^{12}{{\rm{T}}}_{21}}{{}^{13}{{\rm{T}}}_{21}}\simeq \displaystyle \frac{{}^{12}{f}_{a}{(}^{12}{J}_{21}({T}_{x}){-}^{12}{J}_{21}({T}_{{bg}}))(1-{e}^{{-}^{12}\tau })}{{}^{13}{f}_{a}{(}^{13}{J}_{21}({T}_{x}){-}^{13}{J}_{21}({I}_{{bg}}))(1-{e}^{-{A}^{12}\tau })},\end{eqnarray*}$$

with ${}^{13}\tau$ replaced by ${A}^{12}\tau$, where A is the isotopic abundance ratio [¹²CO/¹³CO], and with

$$\begin{eqnarray*}&&{J}_{\nu }(T)=\displaystyle \frac{\tfrac{{\rm{h}}\nu }{{\rm{k}}}}{{e}^{\tfrac{{\rm{h}}\nu }{{\mathrm{kT}}_{{\rm{x}}}}}-1}.\end{eqnarray*}$$

In the high temperature limit, ${}^{12}{J}_{{ij}}({T}_{{ij}})$ simply becomes T_ij, and assuming similar area filling factors, ${}^{i}{f}_{a}$, and the same T_x, this can be simplified to ${}^{12}{{\rm{I}}}_{21}{/}^{13}{{\rm{I}}}_{21}=(1-{e}^{{-}^{12}\tau })/(1-{e}^{-{A}^{12}\tau })$ (e.g., Aalto et al. 1995).

Similarly, taking ratios of the rotational transitions of ¹³CO (and C¹⁸O), between different J states of the same isotopologue like I[¹³CO(2 − 1)]/I[¹³CO(1 − 0)], constrains T_x. Again, assuming high temperature limit and identical line widths and filling factors, the line temperature ratio is approximated by the equation

$$\begin{eqnarray*}&&\displaystyle \frac{{}^{13}{T}_{\mathrm{mb}}^{21}}{{}^{13}{T}_{\mathrm{mb}}^{10}}\simeq \displaystyle \frac{({T}_{21}-{T}_{21}^{\mathrm{bg}})(1-{e}^{{-}^{13}{\tau }_{21}})}{({T}_{10}-{T}_{10}^{\mathrm{bg}})(1-{e}^{{-}^{13}{\tau }_{10}})}.\end{eqnarray*}$$

In the optically thick limit, this becomes

$$\begin{eqnarray*}&&\displaystyle \frac{{}^{13}{T}_{\mathrm{mb}}^{21}}{{}^{13}{T}_{\mathrm{mb}}^{10}}\simeq \displaystyle \frac{({T}_{21}-{T}_{21}^{\mathrm{bg}})}{({T}_{10}-{T}_{10}^{\mathrm{bg}})}\simeq 1,\end{eqnarray*}$$

and in the optically thin limit, we obtain

$$\begin{eqnarray*}&&\displaystyle \frac{{}^{13}{T}_{\mathrm{mb}}^{21}}{{}^{13}{T}_{\mathrm{mb}}^{10}}\simeq \displaystyle \frac{{\tau }_{21}({T}_{21}-{T}_{21}^{\mathrm{bg}})}{{\tau }_{10}({T}_{10}-{T}_{10}^{\mathrm{bg}})}.\end{eqnarray*}$$

The emission ratios between isotopologue transitions with the same J states do not give unique values for the opacities and the abundance ratios separately, so in order to get values to compare, we assume that the [¹²CO/¹³CO] abundance ratio is near 40, the values found toward the centers of NGC 253 (Henkel et al. 2014) and IC 342 (Meier & Turner 2000). We observe an emission line ratio ¹³CO(2 − 1)/¹³CO(1 − 0) of 1.1–1.5. The emission ratios are consistent with the single dish data from Israel et al. (2014).

We discuss five spectral features along the molecular arms that contain detections of ¹²CO(2 − 1), ¹³CO(2 − 1), C¹⁸O(2 − 1), and ¹³CO(1 − 0): A-1, B-1, F-2, G-2, and H-1. By starting near an abundance ratio [¹²C/¹³C] = 40, we adjust the abundance ratio and opacity for the isotopologue ratios until a consistent solution is found with the observed intensity ratios. We find best fit values for the abundance ratios for features A-1 and H-1 of [¹²CO/¹³CO] = 50, slightly raised from the value near the center of NGC 253 and IC 342 and [¹⁶O/¹⁸O] of about 300. The [¹⁶O/¹⁸O] abundance ratio for the nucleus of NGC 253 was found to be ∼200 by Henkel et al. (1993), which is consistent with the inner Galaxy (Wilson & Rood 1994). Opacities for ¹²CO(2–1) of around 50 and for ¹³CO(2–1) of around unity are implied for both A-1 and H-1. Spectral feature F-2 requires the [¹⁶O/¹⁸O] abundance ratio to change from [¹⁶O/¹⁸O] = 300 to [¹⁶O/¹⁸O] = 350 to maintain agreement with the [¹²CO/¹³CO] abundance. For F-2, ¹²CO opacities of around 15 are implied. The isotopic ratios for spectral features B-1 and G-2 exhibit more extreme values. To match the ratios with [¹²CO/¹³CO] around 40, [¹⁶O/¹⁸O] would be pushed to >800, while keeping [¹⁶O/¹⁸O] around 300 would require [¹²CO/¹³CO] to drop to <20. Opacities for ¹²CO for features B-1 and G-2 are around 2 and 12, respectively.

Using the ¹³CO(2 − 1) to ¹³CO(1 − 0) ratio, we can constrain the excitation temperature of the gas (Aalto et al. 1995; Meier & Turner 2000). Spectral features A-1, B-1, F-2, and H-1 have ¹³CO excitation temperatures over 10 K, while the feature G-2 has an excitation temperature of ∼20 K in the optically thin limit. For the CND, the limits from the ¹³CO(2 − 1) to ¹³CO(1 − 0) ratio give lower limits on the excitation temperature of the gas of about 15 K for the outer regions of the CND and 10 K for the inner regions assuming optically thin gas.

4.2.2. Dense Gas in the CND

4.2.3. The Influence of the AGN on Gas Chemistry

4.3.1. Dynamical Masses

4.3.2. Conversion Factor Analysis

Our observations include absorption as well as emission. A detailed analysis of the individual absorption components and their chemistry will be discussed in a forthcoming paper. Here we focus on the line ratios in absorption that directly confront the emission in order to address which emission component is most likely responsible for the absorption features. Absorption depth is converted to line opacity (Section 3.4.2); then optical depth ratios are calculated for comparison with the emission line ratios. As long as temperature/excitation does not change significantly between the two transitions making up the ratios and assuming the transitions are optically thin, then the τ absorption ratio and the T emission ratios may be compared robustly. Hence if the emission ratios and the absorption ratios are similar, this indicates the same gas component is being sampled.

Ratios of the calculated optical depth τ were taken from the absorption spectra shown in Figure 15 and are reported in Table 4. Three deep absorption lines in the low velocity complex (LV complex), located at 539.7, 543.4, and 549.7 km s⁻¹, and the median of the spectra from velocities 576–604 km s⁻¹ to represent the broad, high velocity absorption complex (HV complex) are discussed. The opacity ratio τ[¹³CO(2–1)]/τ[¹³CO(1–0)] ranges from about 1 to 2 in the absorption, and the emission intensity ratio ranges from 1 to 2 in the molecular arms, but the non-detection of ¹³CO(1–0) in the CND puts the opacity ratio at a lower limit of about 2 (emission intensity ratios taken from Table 3).

Table 4.  Opacity Ratios for Selected Absorption Components

Ratio	Abs. Comp. 1	Abs. Comp. 2	Abs. Comp. 3	Broad Median
	(539.7 km s−1)	(543.4 km s−1)	(549.7 km s−1)	(576–604 km s−1)
$\tfrac{{}^{12}\mathrm{CO}(2-1)}{{}^{13}\mathrm{CO}(2-1)}$	1.05a ± 0.01	0.50a ± 0.01	1.82 ± 0.02	∼12.5
$\tfrac{{}^{13}\mathrm{CO}(2-1)}{{}^{13}\mathrm{CO}(1-0)}$	1.45 ± 0.01	2.08 ± 0.02	1.01 ± 0.01	>2.5
$\tfrac{{}^{12}\mathrm{CO}(2-1)}{{{\rm{C}}}^{18}{\rm{O}}(2-1)}$	46a ± 8	17a ± 1	53 ± 2	>31
$\tfrac{{}^{13}\mathrm{CO}(2-1)}{{{\rm{C}}}^{18}{\rm{O}}(2-1)}$	44 ± 8	34 ± 2	29 ± 1	>2.5
$\tfrac{{}^{12}\mathrm{CO}(2-1)}{{\mathrm{HCO}}^{+}(1-0)}$	0.11a ± 0.01	0.14a ± 0.01	0.22 ± 0.01	0.27 ± 0.01
$\tfrac{{}^{13}\mathrm{CO}(1-0)}{\mathrm{HCN}(1-0)}$	0.22 ± 0.01	0.12 ± 0.01	0.50 ± 0.01	<0.04
$\tfrac{\mathrm{HCN}(1-0)}{{\mathrm{HCO}}^{+}(1-0)}$	0.34 ± 0.01	1.17a ± 0.01	0.24a ± 0.01	0.21a ± 0.01
$\tfrac{\mathrm{HCN}(1-0)}{\mathrm{HNC}(1-0)}$	2.89 ± 0.01	3.77a ± 0.04	3.00a ± 0.03	3.3a ± 0.3
$\tfrac{\mathrm{HCN}(1-0)}{\mathrm{CN}(1-0)}$	1.11 ± 0.01	1.74a ± 0.02	1.23a ± 0.01	0.71a ± 0.03
$\tfrac{\mathrm{HCN}(1-0)}{\mathrm{CCH}(1-0)}$	3.00 ± 0.01	6.56a ± 0.07	3.32a ± 0.03	1.54a ± 0.08
$\tfrac{\mathrm{CCH}(1-0)}{\mathrm{CN}(1-0)}$	0.37 ± 0.01	0.26a ± 0.01	0.37a ± 0.01	0.46a ± 0.02

Notes. Table of the absorption ratios taken to specific velocities. Peak opacities were used for the narrow components and for the broad absorption component, a median was calculated between velocities 576 and 604 km s⁻¹. For the transitions that contain hyperfine structure, the ratios only include the strongest component of the hyperfine.

^aRefers to a case where one or both of the absorption components have a hyperfine component blended with it. In the case of ¹²CO(2–1), the first two absorption components are blended together due to poor velocity resolution of the data.

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The opacity ratio of τ[¹³CO(2–1)]/τ[C¹⁸O(2–1)] is around 30 in the narrow LV complex components of the absorption and >2.5 in the HV complex (Table 4). In emission, this ratio ranges from about 4 to 20 in the molecular arms and a lower limit of 4 in the CND (Table 3). For spectral features B-1 and G-2, the optical depth ratios from the isotopologue analysis would be ∼20. Based on this CO isotopologue ratio, the narrow LV complex could be associated with the molecular arms and the HV complex with the CND, but this conclusion remains ambiguous from CO isotopologues alone and runs into problems when considering the dense gas tracers.

The ¹³CO(1–0)/HCN(1–0) optical depth ratio is very low in all absorption cases, ranging from 0.04 in the HV complex to about 0.1–0.5 in the narrow LV complex lines. The emission components from the molecular arms all have an emission ratio greater than about 3.5 for ¹³CO(1–0)/HCN(1–0), while the ratio for the CND is an upper limit around 0.2 for the outer peaks and 0.1 for the inner peaks.

The single dish data of Israel et al. (2014) have beam sizes of 47'' and 57'' for ¹³CO(1 − 0) and HCN(1 − 0), respectively. Assuming emission is uniform over the HCN beam, the ratio is 4.6. If the emission is more compact, this ratio could be as low as 3. This suggests that there is ≲25% extended emission that is not recovered by our interferometric ALMA observations. Therefore we consider the uncertainties of ratios between diffuse CO and the more concentrated high density tracers to be at most 30%. Ratios between different CO transitions or between dense gas tracers should be more certain than this.

There is a strong bimodality between the CND and the molecular arms in the ¹³CO(1–0)/HCN(1–0) emission ratio, with values of less than 0.2 for the CND and values of 3 or larger for the molecular arms. All of the other ratios seem to be roughly consistent between the two gas regions (Tables 3 and 4). So based on the ¹³CO(1–0)/HCN(1–0) ratio, the ratios of the opacities in the LV absorption components follow the CND values more closely than the corresponding values for the molecular arms, suggesting that the gas in the LV complex absorption is also more likely related to the CND rather than the molecular arms. However, there is an important caveat—namely, absorption is sensitive to molecules in the J = 0 state while emission is sensitive to gas in the J = 1 state. Therefore, it is possible that if there is a large amount of very cold/subthermal gas, then the HCN absorption will be significantly elevated relative to ¹³CO. If this is the case, then it may still be possible for the absorption components to result from molecular gas farther out than the CND. However, this effect should not significantly affect the comparison of the ratios between the dense gas tracers.

The absorption component centered on roughly 543 km s⁻¹ seems to be different from the other two narrow LV complex components in a number of ratios. This component has a much narrower line width than the other two and is the only region in both absorption or emission, where HCN(1–0) is stronger than HCO⁺(1–0). The differences in the ratios of the other dense gas tracers for this component suggest that HCN(1–0) is enhanced compared to the other narrow LV complex components. The pencil beam for the absorption could be picking out a specific region of a molecular cloud that happens to deviate from the average that is observed in emission.

The optical depth ratio of HCN(1–0)/CN(1–0) toward the narrow LV components are 1.1–1.7, which is similar to the CND emission values of 1.7 (C-3). Likewise for the HCN(1–0)/CCH(1–0) opacity ratios, the narrow LV components are 3.0–6.6, consistent with the emission value of 3.9. Therefore these ratios also suggest that the narrow line LV components could be associated with the CND. The HV component is further enhanced in CN and CCH relative to the CND/LV (HV: HCN(1–0)/CN(1–0) ≃ 0.7 and HCN(1–0)/CCH(1–0) ≃ 1.5). CN and CCH are both molecules that are present in PDR/XDRs, which indicates that the HV complex component is located in a region of stronger radiation field, possibly closer to the AGN.

Combining this with the previous statement that the gas seen in absorption seems to be similar to the gas that is observed in the CND suggests the absorption features may be placed entirely in the CND. The narrow LV complex components, which are at velocities much closer to systemic, must be coming from gas crossing in front of the AGN with small noncircular motions. The redshifted HV complex component of absorption then would come from gas much closer to the AGN. In fact, Espada et al. (2010) used VLBA observations of H i to determine that the broad component is very close to the AGN by finding a significant change in the HV complex on scales of 0.4 pc. Muller et al. (2009) do not observe HCO⁺(4 − 3) or HCN(4 − 3) in the HV complex and constrain the density of the LV absorption gas to be a few 10⁴ cm⁻³, consistent with the gas chemistry measured in the CND.

Quillen et al. (2010) have compiled a warped disk model of NGC 5128 that ranges from 2 pc up to 6500 pc using 2.122 μm molecular hydrogen line data (inner 3''; Neumayer et al. 2007), SMA CO observations (inner 1'; Espada et al. 2009), Spitzer 8 μm (>40''; Quillen et al. 2006), and H i observations (>60''; Struve et al. 2010). We do not detect the entirety of the dust lane, so we do not compare our data to the large scale portions of the model. We do detect the entire CND, which is about 22'' × 11'' in size at a position angle of 155° and an inclination of about 60° for the outer components compared to the position angle of 150° and inclination of ∼70° reported in Quillen et al. (2010). The inner components of the CND are at a radius of about 65 pc from the AGN.

Using the warped disk model from Quillen et al. (2010), we develop a potential picture of the inner region of Cen A. The inner 200 pc of NGC 5128 contains the CND, which is abundant in dense gas. Beyond the CND, there appears to be a gap in the molecular gas, before reaching the warped rings that create the parallel molecular arms, seen prominently in CO and its rare isotopologues. These warps create a ridge of overlapping molecular gas in the southeastern and northwestern regions explaining the molecular arms. The front and back sides of the turnover in the warp could possibly explain the splitting along the arms seen in the PV diagrams (see Figure 6, Slice 3). The regions where fewer of the molecular rings overlap are too faint to be detected by our observations.

The jet from the AGN is roughly perpendicular to the outer part of the CND, and we assume that the northern jet is approaching and is brighter due to Doppler boosting (Jones et al. 1996; Tingay et al. 1998; see also Figure 1). This would mean that the CND, which is at a position angle of roughly 150° and inclined at about 60°, has the northeastern side of the disk oriented away from Earth and the southwestern side closer to Earth. This would put the redshifted side of the CND (the northwestern side), which have velocities of ≥580 km s⁻¹, behind the AGN meaning that the HV complex component in the absorption cannot be coming from the redshifted side of the CND, assuming the CND is purely flat with only circular motions. However, from comparing the chemistry in the absorption to the emission components, the LV and HV absorption components appear more similar to the CND than the molecular arms.

Based on our data, we present two different scenarios that could result in the observed absorption and emission. In the first scenario, all of the narrow LV complex components of the absorption come from where individual clouds in the CND cross the AGN. The broad component would then come from material closer to the middle of the CND that is falling in toward the AGN, which may ultimately fuel the jet. If the gas is falling in toward the black hole, one would expect that the gas would be stretched in velocity space, as the gas nearer to the black hole is moving at a higher velocity than gas that is farther away. This could be consistent with the HV complex being similar to one of the narrow components but stretched and redshifted over a range of 25–60 km s⁻¹ as the gas falls in toward the black hole. Eckart et al. (1999) described a scenario in which both of the LV and HV complexes originate from gas farther away from the AGN than the CND; however, based on the chemistry of the absorption components, the gas in absorption appears to be more closely related to the CND than gas farther away.

In the second scenario, the warps in the molecular gas of Cen A continue into the innermost part of the CND, and it is possible that the warps are severe enough to place part of the redshifted side of the CND in front of the AGN to create the broad HV complex component, while some other non-warped parts of the CND remain in front to make the narrow roughly systemic LV complex components.

The velocities of the inner peaks of the CND are at about 470 and 660 km s⁻¹ and at a radius of about 65 pc, which means that the rotational velocity of that tilted ring is 1/2 the difference of those velocities, around 95 km s⁻¹. The maximum redshifted velocity that we observe in the absorption is about 60 km s⁻¹ off the systemic velocity, meaning that gas interior to the inner peaks of the CND would be responsible for parts of the HV complex component. However, this gas would need an extreme warp for these high velocities, so noncircular orbits are likely present. In fact, large noncircular motions are found in the CND by Espada et al. (2017). The chemistry of the HV complex component is enhanced in the PDR/XDR tracers CN and CCH. This would be expected of gas that is getting closer to the radiation field from the AGN. This is also consistent with the fact that we detect CN emission almost exclusively from the inner CND.