Free Access
Issue
A&A
Volume 634, February 2020
Article Number A125
Number of page(s) 22
Section Catalogs and data
DOI https://doi.org/10.1051/0004-6361/201935800
Published online 21 February 2020

© ESO 2020

1. Introduction

Molecular gas is the fuel of star formation. So far, more than 60 molecular species have been detected in external galaxies (McGuire 2018). The relative abundances of different species vary with its surrounding astrophysical environment (Omont 2007). The molecular rotational transition lines at millimeter wavelength are sensitive to and trace the physical and chemical properties of the interstellar medium. An unbiased line survey could detect molecular rotation transitions with different energy levels and/or different species and provide an unbiased view on the molecular environment. Previous observations at millimeter band have shown that the properties of molecular gas are different between starbursts (SB) and active galactic nuclei (AGN) in external galaxies (Kohno et al. 2001; Krips et al. 2008; Izumi et al. 2013, 2016; Davis et al. 2013; Aladro et al. 2015; Nakajima et al. 2018; Li et al. 2019). The underlying causes of these variations, especially how the molecular composition might be effected by AGN, are still not fully clear.

NGC 1068 is one of the nearest (∼14.4 Mpc, 1″ = 72 pc, Bland-Hawthorn et al. 1997) and brightest (LIR = 3 × 1011L) Seyfert II galaxies with a starburst. It consists of a starburst ring (∼15″ from the central AGN) and a central nuclear disk (CND) (Schinnerer et al. 2000). It was established that the physical conditions between the starburst ring and the CND are different (Viti et al. 2014). The molecular gas in the CND is denser and hotter than the gas in the starburst ring. Multi-gas-phase components exist in the CND region (Viti et al. 2014). The gas temperature in the CND region is higher than 150 K, and the gas density is above 105 cm−3 (Viti et al. 2014). The CND region could be spatially resolved into an east and a west knot. These knots are dominated by a fast and a slow shock, respectively (Kelly et al. 2017). The mass of the central torus was estimated to be 2 × 105M (García-Burillo et al. 2014). With an AGN-driven outflow (García-Burillo et al. 2014) and a past inflow driven by a minor merger (Furuya & Taniguchi 2016), the torus shows a complex dynamical behavior (García-Burillo et al. 2016).

NGC 1068 is one of the best extragalactic targets for continuum and line observations in all wavelengths from X-rays to the radio domain (e.g., radio, Greenhill et al. 1996; millimeter, Schinnerer et al. 2000; infrared, Jaffe et al. 2004; optical, Antonucci & Miller 1985; UV, Antonucci et al. 1994; and X-ray, Kinkhabwala et al. 2002). These results show that the strong UV or X-ray field heavily influence the physical conditions, kinematics, and chemistry in the CND of NGC 1068 (Usero et al. 2004; Pérez-Beaupuits et al. 2009; García-Burillo et al. 2010; Nakajima et al. 2011, 2018; Aladro et al. 2011, 2015; Takano et al. 2014; Viti et al. 2014).

To better understand the chemistry in the CND of NGC 1068, we performed an observation of molecular lines at 3 mm bands (Qiu et al. 2017), and detected CH3OCH3 for the first time in external galaxies. As a supplement to the previous report, this paper presents observations of molecular lines at the 2 mm band toward NGC 1068. The organization of this paper is as follows: we present the observations and data reduction in Sect. 2, and the main results are provided in Sect. 3. We discuss the physical and chemical properties of molecules in Sect. 4, and give a brief summary in Sect. 5.

2. Observations and data reduction

The observations toward the center of NGC 1068 (RA: 02:42:40.70 Dec: −00:00:48.0 J2000) were carried out in January 2017 with the IRAM 30 m single-dish telescope at Pico Veleta Observatory (Spain)1. We used the Eight Mixer Receiver (EMIR) with dual polarization and the Fourier Transform Spectrometers (FTS) backend tuned. The frequency channel spacing was 195 KHz, and the instantaneous frequency coverage per sideband and polarization was 8 GHz. We used the standard wobbler-switching mode with a ±110″ offset and 1.5 s per phase. Each scan included eight subscans of 30 s. We checked pointing and focus every two hours.

The observations were performed using two tunings in the 2 mm window (160.7–168.6 GHz and 176.5–184.3 GHz) and in the 3 mm window (75.7–83.5 GHz and 91.4–99.2 GHz). The 3 mm spectra were obtained with a short integration time to supplement the line survey in the 2 mm window (Table 1). Figure 1 presents the full spectra. The molecular line temperature scale was converted from the antenna temperature into the main beam temperature Tmb. The conversion relation between them is , where Feff and Beff are the forward efficiency and main-beam efficiency of the telescope, respectively. The values assumed for Feff and Beff of each band are listed in Table 1. The main observation parameters, including band range, half-power bandwidth (HPBW), and spatial resolution, are also listed in Table 1.

thumbnail Fig. 1.

Full spectra of NGC 1068 obtained in the current observations. Red represents a coaddition of the current spectrum and that in Qiu et al. (2017). Blue represents the new detections in this galaxy.

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Table 1.

Band parameters.

The data reduction procedures are similar to those described in Qiu et al. (2017). Linear baseline subtraction and Gaussian profile fitting were made for all the detected lines. The data reduction was performed using the CLASS software of the GILDAS package2. We identified each molecular transition by referring to the frequencies from the NIST database recommended rest frequencies for observed interstellar molecular microwave transitions3 and from the splatalogue database for astronomical spectroscopy4.

We performed a literature survey of previous observations made with single-dish telescopes toward NGC 1068. The lines detected by other research groups, together with those detected by us, are listed in Table A.1. This table may serve as a reference source for future studies of this object. We assume a heliocentric systemic velocity vsys ∼ 1137 km s−1 (from NASA/IPAC Extragalactic Database (NED)5) for this source throughout.

3. Results

Overall, 15 lines belonging to 12 different molecular species are detected. Figure 2 shows the individual line profiles in velocity units, and Table A.1 summarizes the parameters derived from Gaussian fitting with CLASS to the observed lines. The main results include the detection of the J = 2 − 1 transitions of 13CCH, HCO+, HOC+, and HNC, and emission lines of CH3OH (J1, k − 1 − J0, k)E, SO2 (52, 4 − 51, 5), and CH3CN (9k − 8k). To our knowledge, these lines are detected for the first time in NGC 1068. In addition, we also detect the lines of CH3CN (5k − 4k), N2H+ (1 − 0), CH3OCH3 (32, 2 − 31, 3), CH3OH (2k − 1k), CS (2 − 1), HC3N (18 − 17), and HCN (2 − 1).

thumbnail Fig. 2.

Molecular transitions toward NGC 1068. The red curves represent Gaussian fitting profiles.

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CH3CN (5k − 4k), HC3N (18 − 17), and CH3OCH3 (32, 2 − 31, 3), have been detected or tentatively detected in previous observations (Qiu et al. 2017). To enhance the signal-to-noise (S/N), we combined the data of Qiu et al. (2017) for these three transitions. CH3OCH3 (32, 2 − 31, 3) was only marginally detected toward NGC 1068 by Qiu et al. (2017). No stronger line is known in this wavelength. For this line, we also combined the spectral data of Aladro et al. (2013) and found that the S/N increases from 1.6 to 2.3, probably supporting the potential existence of CH3OCH3 in this galaxy. We checked the raw data of Qiu et al. (2017), and found that the CH3CCH (140 − 130) line at 239 GHz is marginally detected, as shown in Fig. 2.

3.1. Line ratios

3.1.1. HCN/HCO+

Table 2 lists the velocity-integrated intensity ratio of the J = 2 − 1 transitions of five species, including 13CCH, HCN, HCO+, HOC+, and HNC, detected in the CND of NGC 1068. The flux ratio of HCN to HCO+ can be used to distinguish galaxies powered by starburst and AGN (Kohno et al. 2001; Krips et al. 2008; Imanishi et al. 2009; Izumi et al. 2016; Aladro et al. 2018; Li et al. 2019). AGN galaxies usually exhibit higher [HCN/HCO+] ratios than starburst galaxies because high X-ray radiation in AGNs can enhance the abundance of HCN (Kohno et al. 2001; Privon et al. 2017). Our observations show that the flux ratio of HCN-to-HCO+ (J = 2 − 1) is 1.82 ± 0.03 in the CND of NGC 1068. This is consistent with previous interferometer observations at other J-transitions. For example, the flux ratio of HCN-to-HCO+ is 1.6–2.0 at J = 1 − 0 (Viti et al. 2014), 1.6–3.3 at J = 3 − 2 (Imanishi et al. 2016), and 2.1–2.8 at J = 4 − 3 (Viti et al. 2014) in the nucleus of NGC 1068. Our single-dish and previous interferometer observational results confirm that the abundance of HCN is enhanced in the CND of NGC 1068, which maybe related to the outflow of AGN (García-Burillo et al. 2014).

Table 2.

Flux ratios (row/column).

3.1.2. HNC/HCN

HCN and its isomer HNC are tracers of dense gas, and commonly coexist in different environments (Gao & Solomon 2004; Aalto et al. 2012). Some galaxies exhibit a high HNC/HCN ratio, which may be caused by low temperature, high ion density, high optical depth, and/or IR pumping. Theoretical studies (Meijerink & Spaans 2005; Meijerink et al. 2007) suggest that the HNC/HCN ratio increases in X-ray dominated regions (XDRs) to a higher degree than that in photon-dominated regions (PDRs) and quiescent-cloud regions. Observations of three galaxies (Arp 220, NGC 4418, and Mrk 231), which have strong X-ray emission produced by embedded AGN, indicate a high HNC-to-HCN (J = 3 − 2) flux ratio of 1.9 ± 0.3 in Arp 220, 1.5 ± 0.2 in Mrk 231, and 2.3 ± 0.3 in NGC 4418 (Aalto et al. 2007). Our observations show that the integrated line ratio of HNC to HCN (J = 2 − 1) toward the CND of NGC 1068 is 0.16 ± 0.03. This is in agreement with J = 3 − 2 and J = 4 − 3 line ratios (0.15 ± 0.03 and 0.19 ± 0.02, respectively; Pérez-Beaupuits et al. 2007, 2009). This means that we do not find enhanced HNC emission in the CND of NGC 1068, and an XDR exists here. Because the HNC line is narrower than that of HCN, we exclude the IR pumping scenario: the IR pumping dominated scenario will show a broader line width of HNC than HCN, as described in Aalto et al. (2002). The ammonia (NH3) observations show that some components of the molecular cloud have a higher temperature of 140 ± 30 K in the CND of NGC 1068 (Ao et al. 2011). A reasonable explanation of the lower HNC-to-HCN ratio is that the warmer molecular cloud leads to the HNC deficiency.

3.2. Line profile

The line profile can be taken as a diagnostic tool to investigate the properties of CND molecular gas. Usero et al. (2004) found that all molecular line profiles in the CND of NGC 1068 are distributed asymmetrically. They therefore suggested that the east and west knots of the CND are chemically differentiated.

Previous interferometer observations of CO (1 − 0) showed that NGC 1068 has a circumnuclear molecular ring with diameter ranging from ∼20″ to 40″ (Schinnerer et al. 2000). The beam size of the IRAM 30 m telescope at the 3 mm band is larger than the inner diameter of the two spiral arms of NGC 1068. In contrast, the beam sizes at the 2 mm and 1 mm bands are smaller than the inner diameter of the two spiral arms. The HPBW of HOC+ (1 − 0) and (2 − 1), for instance, is about 27.5″ at 89.5 GHz and 13.7″ at 179.0 GHz, as shown in Fig. 3. High-resolution images show that the most prominent emission stems from the CND (Takano et al. 2014; Viti et al. 2014). Therefore, the molecular lines lying within the 2 mm and 1 mm bands can preclude most of the contamination from the two spiral arms. Using a similar method as Usero et al. (2004), we divided each line into its blue and red velocity components (see Fig. 4), which dominantly arise from the east and west knots, respectively. The line profiles in Fig. 4 show that the local systemic velocity of these lines ranges from v − vsys = −200 km s−1 to 200 km s−1. We defined the blue and red components as those lying within an interval of −200 <  v − vsys <  0 km s−1 and 0 <  v − vsys <  200 km s−1, respectively. The velocity-integrated intensities of the two components are listed in Table 3. As shown in Fig. 4, there are noticeable differences between the line shapes of the CND spectra. Some molecular lines, including HC3N (18 − 17), SiO (55 − 44), SO2 (143, 11 − 142, 12), HNC (2 − 1), HCN (2 − 1), HCO+ (2 − 1), HNCO (11 − 10), 13CCH (2 − 1), and CH3CN (9 − 8), show a stronger blue component, while the others, including SO (55 − 44), HOC+ (2 − 1), CS (5 − 4), CH3CCH (14k − 13k), and H2CO (31, 2 − 21, 1), show a stronger red component.

thumbnail Fig. 3.

CO (1 − 0) emission of NGC 1068 (gray scale and contours; taken from Schinnerer et al. (2000)). The solid and dashed circles represent the beams of HOC+ (1 − 0) and HOC+ (2 − 1), respectively. The central box marks the circumnuclear disk.

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Table 3.

Flux ratios of the blue and red components of the CDN lines.

Interferometer observations showed that the SiO (3 − 2) transition is dominantly distributed in the east knot and the HNCO (6 − 5) emission in the west knot (Kelly et al. 2017). Therefore, we hypothesize that the molecular species whose blue-to-red flux ratios are higher than that of SiO are mainly located in the east knot, while those with blue-to-red flux ratios lower than that of HNCO are mainly located in the west knot. Our single-dish observations of SiO (55 − 44) and HNCO (11 − 10) suggest a blue-to-red flux ratio (RE/W) of 2.1 and 1.3, respectively. The blue-to-red flux ratio of HC3N is higher than that of SiO, and thus is mainly located in the east knot. The blue-to-red flux ratios of the 13CCH (2 − 1), CH3CN (9 − 8), SO (55 − 44), HOC+ (2 − 1), CS (5 − 4), CH3CCH (14k − 13k), and H2CO (31, 2 − 21, 1) transitions are lower than that of the HNCO (11 − 10) lines, and thus mainly arise from the west knot. The 13CCH (2 − 1), CH3CN (9 − 8), and CH3CCH (14 − 13) transitions include several multiple components, which may affect the determination of their blue-to-red flux ratios. However, as shown in Fig. 4, the strongest components have small frequency splits, suggesting that this effect can be neglected.

thumbnail Fig. 4.

Profiles of the molecular lines at 1 mm and 2 mm bands, as detected in Qiu et al. (2017) and in the current observations. The three vertical dotted lines at v − vsys = −200 km s−1, v − vsys = 0 km s−1, and v − vsys = +200 km s−1 delimit the blue and red kinematical components. The wavelengths and relative intensities of the split components of 13CCH (2 − 1), CH3CN (9 − 8), and CH3CCH (14 − 13) are marked by red lines.

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In Fig. 5 we compare the line profiles of HCN, HNC, HCO+, and HOC+ and those of the 13C isomers. We find that the (J = 2 − 1)-to-(J = 1 − 0) flux ratios of HCN, HNC, and HCO+ are higher for the blue components than those for the red components, while there is no significant difference for HOC+. Under the assumption of 12C/13C, Wang et al. (2014) suggested that the redshifted parts of HCN (1 − 0) and HCO+ (1 − 0) have lower optical depths and more likely come from the spiral arms than from the nuclear region. Because the beam size of J = 2 − 1 is much smaller than that of J = 1 − 0, the different line profiles between the J = 2 − 1 and the J = 1 − 0 transitions may be attributed to the difference of their emission regions. The similar line profiles between the J = 2 − 1 lines of HCN and HCO+ and their isotopic lines (H13CN (1 − 0) and H13CO+ (1 − 0)) suggest that they may come from the same location. The J = 2 − 1 and J = 1 − 0 transitions of HOC+ show similar profiles, suggesting that the molecule might be distributed in the CND region.

thumbnail Fig. 5.

Upper left panel: HCN (2 − 1) (red line); H13CN (1 − 0) (blue line filled yellow, × 16); HCN (1 − 0) (black line, ×1.5). Upper right panel: HCO+ (2 − 1), H13CO+ (1 − 0)×18, HCO+ (1 − 0)×1.3. Middle left panel: HNC (2 − 1), HN13C (1 − 0)×35, HNC (1 − 0)×1.5. Middle right panel: HOC+ (2 − 1), HOC+ (1 − 0)×4. Lower left panel: HCN (2 − 1) (red line, ×3.2); HNC (2 − 1) (black line filled yellow, ×16); HCO+ (2 − 1) (black line, ×1.8); the black line curve is the Gaussian fitting line of HNC (2 − 1). Lower right panel: HCN (2 − 1) (red line); HOC+ (2 − 1) (black line filled yellow, × 20); HCO+ (2 − 1) (black line, ×1.8); the black line curve is the Gaussian fitting line of HOC+ (2 − 1).

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HCN and HCO+ are tracers of dense gas. Their J = 2 − 1 transitions show similar line profiles and comparable velocities (see the lower panels of Fig. 5). Compared to the HCN (2 − 1) line, the HNC (2 − 1) line has a similar line center velocity, but a noticeably narrower line width. The HOC+ (2 − 1)-to-HCO+ (2 − 1) flux ratio in the blue component is higher than that in the red component. However, this is not the case for HNC and its isomer HCN. A possible explanation is that the J = 2 − 1 transitions of HCN, HNC, and HCO+ are associated with each other in the CND of NGC 1068, while the HOC+ (2 − 1) transition does not peak at the same position. This is consistent with the latest interferometer observation toward the center of the Seyfert II galaxy Mrk 273 (Aladro et al. 2018), which shows that the peak of the HOC+ (3 − 2) emission is not at the same position as the dense gas of HCN and HCO+. Further observations with higher spatial resolution of HOC+ in the CND of NGC 1068 are required to investigate the origin of this offset.

3.3. Rotation diagram

We applied the rotation diagram to derive excitation temperatures (Tex) and column densities (Ntot) of the molecules we detected in our observations. With the assumption of local thermodynamic equilibrium (LTE), optically thin conditions, and negligible background temperature, we plot the populations of the upper levels (Nu) against the corresponding excitation energies (Eu) of the transitions (Fig. 6), using the equation

(1)

thumbnail Fig. 6.

Boltzmann diagrams. Tc is the rotation temperature of its cold component, and Tw is the rotation temperature of its warmer component. Transitions detected by the JCMT 15 m telescope are also marked in the panels (filled diamond). The empty circles are discarded for the linear fitting. Pound signs mark the lines that are detected for the first time in this work.

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where k is the Boltzmann constant, ν is the rest frequency of the transition, Q(Tex) is the partition function, h is the Planck constant, c is the light speed, Aul is the spontaneous emission coefficient, gu is the total degeneracy of upper energy level, and Eu/k is the upper level energy. The values of Q(Tex), Aul, gu, and Eu/k are taken from the Cologne Database for Molecular Spectroscopy (CDMS) catalog6 and the splatagogue astronomical spectroscopy database7. ∫Ts dν is the detected transition-integrated intensity. Ts is the source-averaged brightness temperature, corrected for beam dilution by , where θb is the antenna HPBW and θs is the source size. The antenna HPBW is estimated by HPBW (″) = 2460/ν (GHz). Following the assumption made by other researchers (e.g., Usero et al. 2004; Bayet et al. 2009; Krips et al. 2008; Aladro et al. 2013), we took a θs value of 4″ that was obtained based on the interferometric observations of 12CO, HCN, and 13CO (Helfer & Blitz 1995; Schinnerer et al. 2000).

The rotation diagrams of the 14 molecular species detected in our observations are shown in Fig. 6. We combined previously published data for this plot. The derived values of Tex and Ntot are listed in Table 4. Two components were considered to fit the rotation diagrams of 8 molecules, including CS, HCN, HCO+, HNC, SiO, CN, and CH3OH. Alternatively, the nonlinear data distribution in the rotation diagrams can be caused by finite optical depths (Goldsmith & Langer 1999). In this scenario, Eq. (1) is modified to

(2)

Table 4.

Rotational temperatures and column densities of the observed molecules.

where the optical depth correction factor ln Cτ is expressed with ln Cτ = ln [τ/(1 − eτ)]. However, using the method developed by Goldsmith & Langer (1999), we found that the Cτ value (< 0.005) is too low to fit the rotation diagrams.

The rotation temperatures of CS are consistent with the results derived by Aladro et al. (2013) within the errors, whereas the cold components of HC3N and HCN have slightly higher rotation temperatures than the values obtained by Aladro et al. (2013). For HCO+, HNC, and SiO, the rotation temperatures derived by Aladro et al. (2013) lie between those of the cold and warm components. Because the SO (56 − 45) and CH3CN (12 − 11) transitions are strongly blended with the wings of the strong 13CO (2 − 1) and C18O (2 − 1) transitions, their fluxes represent upper limits; see Qiu et al. (2017). These two transitions were excluded in fitting the rotational diagrams. The rotation temperature of SO is lower than that derived by Aladro et al. (2013; ∼22.8 ± 18.6 K). Because they deviate from the fitting lines, we did not use the transitions of SO (54 − 44), HNCO (61, 6 − 51, 5), (51, 4 − 41, 3), and (51, 5 − 41, 4) in the rotation diagrams.

3.4. Molecular transitions detected with different single-dish telescopes

Some of the molecular transitions have been detected by single-dish telescopes with different beam sizes. NGC 1068 consists of a starburst ring and a CND. As shown in Fig. 1 of Takano et al. (2019), the beam of the IRAM 30 m telescope covers the CND and starburst ring at the 3 mm band, while the beam of the NRO 45 m telescope only covers the CND. In the 1 and 2 mm windows, the JCMT beam covers the CND and starburst ring, while the beam of the IRAM 30 m telescope only covers the CND. By comparing the main-beam temperatures of a certain transition detected by different telescopes, we could roughly estimate the size of the regions from which this molecule arises. This approach has been used by Takano et al. (2019) to study the molecular emission regions in NGC 1068 and NGC 253. The authors concluded that the distributions of molecules between these two galaxies are significantly different. Combining these results with the data in the literature, we calculated the ratios between the observed intensities by different telescopes for 28 transitions (see Table 5).

Table 5.

Integrated intensity ratios shown in Fig. 7.

When the beams cover the same emission regions (i.e., the CND), the intensities measured by the telescope with the wider beam should be lower because of the beam-dilution effect, and thus the ratios shown in Fig. 7 should be higher than unity. However, we find that the ratios for 13CO (1 − 0), CH3OH (2k − 1k), HNC (1 − 0), 13CO (2 − 1), HCN (3 − 2), C18O (1 − 0), CO (1 − 0), SiO (2 − 1), HCO+ (1 − 0), and CS (2 − 1) are lower than unity, suggesting that the surrounding starburst ring may significantly contribute to these transitions. Theoretically, the line intensity ratio decreases with increasing percentage of the line emission from the starburst ring. The lines with intensity ratios much higher than unity may be mainly distributed in the CND. This is supported by interferometer observations. For instance, 13CN (11/2 − 01/2), HC3N (11 − 10), HNCO (50, 5 − 40, 4), and CH3CN (6k − 5k) have high intensity ratios. The ALMA observations of Takano et al. (2014) showed that they are concentrated in the CND. Significant contributions from the ring have been revealed by ALMA for the 13CO (1 − 0) emission, which has a low intensity ratio (Fig. 7). The intensity ratio of the CS (2 − 1) line is close to unity, again in agreement with the ALMA observations that reveal low contributions from the ring.

thumbnail Fig. 7.

Integrated intensity ratios of the molecular lines detected with different single-dish telescopes (lower panel). The calculated source sizes are shown in the upper panel.

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Assuming that the telescopes with different beam sizes detect the same molecular regions, we can derive the source size by the expression

(3)

where I1 and I2 are the integrated intensities detected with two different telescopes, and θb1 and θb2 are the beam sizes of the corresponding telescopes. As shown in Fig. 7 and Table 5, the estimated source sizes are generally larger than 4″. However, these calculations sensitively depend on the intensity ratio and on the assumption of a Gaussian brightness distribution. Low pollution from outside of the CND may substantially increase the estimated θs value. Nevertheless, the assumption of θs = 4″ does not significantly affect the results we obtain from the rotation diagrams. With θs increasing by a factor of two, the derived column densities would decrease by about 10%, while the rotation temperatures would be maintained at the same level.

4. Discussion

4.1. HOC+ in XDR

Based on the observations of HCN and its isomer HNC in interstellar clouds, Herbst et al. (1976) hypothesized that if HNC is present in interstellar clouds, then HOC+, the energetically disfavored HCO+ isomer, is also present. Subsequently, the HOC+ (1 − 0) transition was detected toward Sgr B2 by Woods et al. (1983) and Ziurys & Apponi (1995), based on the experimental work of Gudeman & Woods (1982). So far, HOC+ has been detected in diverse environments in our Galaxy and external galaxies, including diffuse clouds (Liszt et al. 2004), an ultracompact HII region (Rizzo et al. 2003), PDRs (Apponi et al. 1999; Fuente et al. 2003), dense molecular clouds (Apponi & Ziurys 1997), starburst galaxies (Fuente et al. 2005; Martín et al. 2009a; Aladro et al. 2015), and Seyfert galaxies (Usero et al. 2004; Aalto et al. 2015; Aladro et al. 2018). These observations suggest that HOC+ is widespread in the interstellar medium, with the [HOC+]-to-[HCO+] ratio varying from tens to some thousand. The formation routes of HOC+ and its isomer HCO+ were described by Aladro et al. (2018) and references therein. Interferometer observations toward Mrk 273, a nearby Seyfert II galaxy, show a global HCO+/HOC+ (J = 3 − 2) brightness temperature ratio of 9 ± 4 and nuclear ratio of 5 ± 3 (Aladro et al. 2018). HOC+ was tentatively detected toward the nucleus of Mrk 231, which hosts a powerful AGN, with the HCO+/HOC+ (J = 3 − 2) brightness temperature ratio ranging from 10 to 20 (Aalto et al. 2015). These values are generally lower than those in other sources, suggesting that the AGN might be associated with enhanced HOC+.

We detect the J = 2 − 1 transitions of both HCO+ and HOC+ in the CND of NGC 1068, allowing us to further investigate the association between AGN and the HOC+ enhancement. Figure 3 presents the CO (1 − 0) map overlaid by beam sizes of the IRAM 30 m telescope at the HOC+ (1 − 0) and (2 − 1) lines. The HPBW at the HOC+ (1 − 0) line is larger than the inner diameter of the starburst ring, while that of the HOC+ (2 − 1) line is smaller. This means that the HOC+ (2 − 1) emission dominantly arises from the CND region and not from the two spirals that are dominated by star formation. The situations are the same with the HCO+ (1 − 0) and (2 − 1) transitions. Figure 8 shows the transitions of J = 1 − 0 and J = 2 − 1 of HCO+ and HOC+, as well as the HCO+-to-HOC+ temperature ratio profiles. The HCO+-to-HOC+ temperature ratio of each velocity channel ranges from 32 to 109 and from 7 to 27 for the J = 1 − 0 and J = 2 − 1 transitions, respectively. These low values are consistent with the results of Usero et al. (2004), who suggested that the high electron density in the XDR may lead to a low [HCO+]/[HOC+] ratio. HCO+ and HOC+ in the XDR are produced through three chemical paths (CO + , CO+ + H2, and H2O + C+) (Maloney et al. 1996; Sternberg & Dalgarno 1995). The HCO+ and HOC+ fractions formed in these paths were illustrated by Usero et al. (2004; see their Fig. 8). Our detections suggest that the integrated intensity ratio of HCO+/HOC+ (J = 2 − 1) is 15.8 ± 3.0, which indicates an electron abundance with n(e)/n(H2) = 10−4.6 according to the XDR model of Usero et al. (2004). The CO + and H2O + C+ routes dominate the production of HCO+ and HOC+. The high ionization degree suggests strong X/UV irradiation powered by AGN.

thumbnail Fig. 8.

Top and middle panels: HCO+ and HOC+ spectra. Bottom panel: HCO+-to-HOC+ temperature ratio profile derived for channels fulfilling T(HOC+) > 1σ (left: J = 1 − 0 transitions; right: J = 2 − 1 transitions).

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As shown in Fig. 8, the profiles of the HOC+ (1 − 0) and (2 − 1) lines are noticeably asymmetrical with respect to vsys. The asymmetry can be quantified by the ratio between the integrated fluxes in the blue and red parts relative to the central frequency. In Table 3 we compare the blue-to-red flux ratio of HOC+ line with those of other lines. We find that the ratio for HOC+ (2 − 1) is 0.93, differing from those of HNC (2 − 1) (∼1.70), HCN (2 − 1) (∼1.47), and HCO+ (2 − 1) (∼1.38). This might suggest that HOC+ has a different distribution than HNC, HCN, and HCO+. We note that the spatial resolved observations toward Mrk 273 also showed different peak positions of the HOC+, HNC, HCN, and HCO+ lines (Aladro et al. 2018). Therefore, we hypothesize that the formation of HOC+ is more sensitive to the environments of the XDR than the formation of other molecules. Future interferometer observations of HOC+ in NGC 1068 would allow a firmer conclusion.

4.2. Comparison of the molecules in NGC 1068, M82, and NGC 253

To investigate the roles of AGN and starburst on molecular chemistry, we compared the molecular lines detected in NGC 1068 and in the two nearby typical starburst galaxies M 82 and NGC 253. M 82 is a prototype starburst galaxy with a high star formation rate (∼9 M yr−1, Strickland et al. 2004) at a distance of ∼3.6 Mpc (Freedman et al. 1994). NGC 253 is an almost edge-on barred spiral galaxy with an active nuclear starburst (∼3.6 M yr−1, Strickland et al. 2004 at a distance of ∼3 Mpc Mouhcine et al. 2005). The star formation in M 82 and NGC 253 is in the middle and later stages, respectively. The unbiased line surveys of NGC 253 (Martín et al. 2006) and M 82 (Aladro et al. 2011) at the millimeter band have revealed that the two galaxies have clearly different chemical characteristics. In Figs. 9 and 10 we compare the rotation temperatures, column densities, and fraction abundances of nine detected molecular species in three galaxies, including SiO, CH3CN, HC3N, SO2, SO, HNCO, CS, CH3OH, and CH3CCH. The molecular rotation temperatures in NGC 1068 are generally lower than the temperatures in M 82 and NGC 253. The only exception is the warm component of CS. CS is a dense-gas tracer. Its high temperature probably suggests that the IR-photon heating, resulting from dust emission, is effective inside of the CS molecular cloud. Therefore, we conclude that the interior of the dense molecular cloud in the CND of NGC 1068 is more severely obscured by dust than the interiors in M 82 and NGC 253. The heavy dust obscuration significantly shield molecules from a strong UV/X-ray radiation field, leading to the enhancement of complex molecules such as CH3OCH3.

thumbnail Fig. 9.

Comparison of the rotation temperatures in NGC 1068, M 82, and NGC 253. The molecular rotation temperatures of M 82 and NGC 253 are taken form Aladro et al. (2011) and Martín et al. (2006).

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thumbnail Fig. 10.

Upper panel: comparison of the column densities in NGC 1068, M 82, and NGC 253. Lower panel: comparison of the fraction abundances with respect to C34S. The column densities of the molecules in M 82 and NGC 253 are taken from Aladro et al. (2011) and Martín et al. (2006), respectively. The column density of C34S in NGC 1068 is taken from Aladro et al. (2013).

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As shown in the upper panel of Fig. 10, the column densities of molecular species in NGC 1068 are higher than those in M 82 and NGC 253 by one to two orders of magnitude, suggesting that NGC 1068 is a rich reservoir of molecules. The fraction abundance of SiO in NGC 1068 is clearly larger than those in M 82 and NGC 253 (see the lower panel of Fig. 10). This is consistent with the interferometer observations of SiO (3 − 2), which suggest that SiO is enhanced by a fast shock in the east knot of CND of NGC 1068 (Kelly et al. 2017).

The [c-C3H2]/[HC3N] ratio can be used to trace the evolutionary stages of the star formation (Fuente et al. 2005). Taking the column density of c-C3H2 obtained by Nakajima et al. (2011), we derive [c-C3H2]/[HC3N] = 0.07 for NGC 1068, which is lower than that in M 82 (∼2) and NGC 253 (∼0.2). This suggests that the UV radiation field plays a less important role in the chemistry of NGC 1068 than that in M 82 and NGC 253, where HC3N is significantly dissociated and the cyclic molecule c-C3H2 is more resistant to UV photons.

CH3CCH has a small dipole moment (0.75 Debye), and thus its rotation temperature should be close to the kinetic temperature of the molecular cloud. It follows that the kinetic temperature in NGC 1068 is the lowest of the three galaxies. Through a large velocity gradient (LVG) analysis, Krips et al. (2008) obtained that the kinetic temperature of M 82 is 60–100 K. However, there are two solutions for the kinetic temperature of NGC 1068 (20 K and 60–240 K). The lower solution is close to the rotational temperature of CH3CCH (28.0 ± 3.4 K).

The profile of the SO (55 − 44) line suggests that SO is concentrated in the west knot of CND. Based on models of ice evaporation process, Viti et al. (2004) suggested that SO is much more abundant than SO2 at the beginning of the high-mass star formation, while the reverse is the case in the hot core. The ALMA 1.1 mm continuum emission showed that an ongoing star formation signature existed in the southwestern direction of the AGN with a distance of about 2 arcsec (Imanishi et al. 2016). Therefore, it is likely that SO is enhanced by star formation in the west knot of the CND in NGC 1068. In ocntrast, the blue-to-red flux ratio of the SO2 (55 − 44) line (∼2.05) is close to that of the SiO (55 − 44) line (∼2.08). Therefore, we infer that SO2 is more concentrated in the east knot. Figure 10 shows that SO2 is more abundant than SO in both NGC 1068 and NGC 253. This can be explained by the chemical model of Viti et al. (2004), according to which SO2 is more abundant than SO within the post-shock gas. Previous observations have shown that the nuclear molecular clouds of NGC 253 are dominated by large-scale low-velocity shocks (Martín et al. 2006) and that the east and west knot of NGC 1068 are dominated by a fast and a slow shock, respectively (Kelly et al. 2017). Figure 10 also shows that the abundance ratio of SO/SO2 in NGC 1068 is lower than that in NGC 253. SO2 can be photodissociated by interstellar UV radiation to form SO (Willacy & Millar 1997). The lower [SO]/[SO2] ratio in NGC 1068 is consistent with our conclusion that the UV radiation field plays a less important role in the chemistry of NGC 1068 than in that of NGC 253.

5. Summary

We reported an observation toward the nuclear region of NGC 1068 at the 2 mm bands. The chemical properties of the CND in NGC 1068 were investigated. Our main conclusions are as follows:

  1. Fifteen emission lines toward the center of NGC 1068 are detected, eight of which are the first detection in this source. Based on the line profile diagnosis of molecular gas in CDN, we infer that HC3N and SO2 are mainly concentrated in the east knot of the CND, while 13CCH, CH3CN, SO, HOC+, CS, CH3CCH, and H2CO are in the west knot of the CND.

  2. The CND of NGC 1068 is highly ionized with an ionization degree of X(e) ∼ 10−4.6. The high ionization degree is consistent with the spacial distribution of HOC+, which is enhanced in XDR and is mainly distributed in the CND of NGC 1068 and not in the surrounding starburst ring.

  3. Based on the rotation-diagram, we derived the column densities and rotation temperatures of 14 molecular species in NGC 1068. With these results, we find that the physical conditions and chemical environments in NGC 1068 may significantly differ from those in NGC 253 and M 82: the UV radiation filed in NGC 1068 is lower than that in M 82 and NGC 253, and NGC 1068 has the lowest kinetic temperature.

It is clear that the CND of NGC 1068 has complex chemical environments. This paper demonstrates that the data obtained by single-dish telescopes can provide significant supplements to interferometer observations in investigating the physical and chemical environments of galaxies.


1

This publication is based on data acquired with the IRAM 30-m telescope. IRAM is supported by INSU/CNRS (France), MPG (Germany), and IGN (Spain).

Acknowledgments

We thank the anonymous referee for the useful comments that improved the manuscript. This work was supported by the Natural Science Foundation of China (NSFC) awarded to YZ (No. 11973099) and the China Postdoctoral Science Foundation funded project (No. 2019M653144) awarded to JJQ. JSZ thanks the support of NSFC (No. 11590782). LWJ appreciates support by the Guangzhou Education Bureau (No. 1201410593). JJQ wishes to thank Dr. Jun-Zhi Wang for his useful suggestions and Mr. Deng-Rong Lu for his help with the data reduction. XDT acknowledges support by the Heaven Lake Hundred-Talent Program of Xinjiang Uygur Autonomous Region of China. We wish to express my gratitude to the staff at the IRAM 30 m telescope for their kind help and support during our observations.

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Appendix A: Additional table

Table A.1.

The transitions detected with single dish telescope in NGC 1068.

All Tables

Table 1.

Band parameters.

Table 2.

Flux ratios (row/column).

Table 3.

Flux ratios of the blue and red components of the CDN lines.

Table 4.

Rotational temperatures and column densities of the observed molecules.

Table 5.

Integrated intensity ratios shown in Fig. 7.

Table A.1.

The transitions detected with single dish telescope in NGC 1068.

All Figures

thumbnail Fig. 1.

Full spectra of NGC 1068 obtained in the current observations. Red represents a coaddition of the current spectrum and that in Qiu et al. (2017). Blue represents the new detections in this galaxy.

Open with DEXTER
In the text
thumbnail Fig. 2.

Molecular transitions toward NGC 1068. The red curves represent Gaussian fitting profiles.

Open with DEXTER
In the text
thumbnail Fig. 3.

CO (1 − 0) emission of NGC 1068 (gray scale and contours; taken from Schinnerer et al. (2000)). The solid and dashed circles represent the beams of HOC+ (1 − 0) and HOC+ (2 − 1), respectively. The central box marks the circumnuclear disk.

Open with DEXTER
In the text
thumbnail Fig. 4.

Profiles of the molecular lines at 1 mm and 2 mm bands, as detected in Qiu et al. (2017) and in the current observations. The three vertical dotted lines at v − vsys = −200 km s−1, v − vsys = 0 km s−1, and v − vsys = +200 km s−1 delimit the blue and red kinematical components. The wavelengths and relative intensities of the split components of 13CCH (2 − 1), CH3CN (9 − 8), and CH3CCH (14 − 13) are marked by red lines.

Open with DEXTER
In the text
thumbnail Fig. 5.

Upper left panel: HCN (2 − 1) (red line); H13CN (1 − 0) (blue line filled yellow, × 16); HCN (1 − 0) (black line, ×1.5). Upper right panel: HCO+ (2 − 1), H13CO+ (1 − 0)×18, HCO+ (1 − 0)×1.3. Middle left panel: HNC (2 − 1), HN13C (1 − 0)×35, HNC (1 − 0)×1.5. Middle right panel: HOC+ (2 − 1), HOC+ (1 − 0)×4. Lower left panel: HCN (2 − 1) (red line, ×3.2); HNC (2 − 1) (black line filled yellow, ×16); HCO+ (2 − 1) (black line, ×1.8); the black line curve is the Gaussian fitting line of HNC (2 − 1). Lower right panel: HCN (2 − 1) (red line); HOC+ (2 − 1) (black line filled yellow, × 20); HCO+ (2 − 1) (black line, ×1.8); the black line curve is the Gaussian fitting line of HOC+ (2 − 1).

Open with DEXTER
In the text
thumbnail Fig. 6.

Boltzmann diagrams. Tc is the rotation temperature of its cold component, and Tw is the rotation temperature of its warmer component. Transitions detected by the JCMT 15 m telescope are also marked in the panels (filled diamond). The empty circles are discarded for the linear fitting. Pound signs mark the lines that are detected for the first time in this work.

Open with DEXTER
In the text
thumbnail Fig. 7.

Integrated intensity ratios of the molecular lines detected with different single-dish telescopes (lower panel). The calculated source sizes are shown in the upper panel.

Open with DEXTER
In the text
thumbnail Fig. 8.

Top and middle panels: HCO+ and HOC+ spectra. Bottom panel: HCO+-to-HOC+ temperature ratio profile derived for channels fulfilling T(HOC+) > 1σ (left: J = 1 − 0 transitions; right: J = 2 − 1 transitions).

Open with DEXTER
In the text
thumbnail Fig. 9.

Comparison of the rotation temperatures in NGC 1068, M 82, and NGC 253. The molecular rotation temperatures of M 82 and NGC 253 are taken form Aladro et al. (2011) and Martín et al. (2006).

Open with DEXTER
In the text
thumbnail Fig. 10.

Upper panel: comparison of the column densities in NGC 1068, M 82, and NGC 253. Lower panel: comparison of the fraction abundances with respect to C34S. The column densities of the molecules in M 82 and NGC 253 are taken from Aladro et al. (2011) and Martín et al. (2006), respectively. The column density of C34S in NGC 1068 is taken from Aladro et al. (2013).

Open with DEXTER
In the text

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