Deep millimeter spectroscopy observations toward NGC 1068 (cid:63)

Aims. We aim for a better understanding of gas properties in the circum-nuclear disk (CND) region of the nearby gas-rich Seyfert 2 galaxy NGC 1068. We focus on line identiﬁcation and the basic physical parameters estimation of molecular gas in the CND region. Methods. We used the IRAM 30m telescope to conduct deep millimeter spectroscopy observations toward the center of NGC 1068. Results. Thirty-two lines were detected in this galaxy, 15 lines of wich were detected for the ﬁrst time. With a sensitivity better by about a factor of 4 than observations in the literature for this source at 3 mm band, we detected several weak lines for the ﬁrst time in this source, such as lines from CH 3 CCH, CH 3 OCH 3 , and HC 18 O + . Column densities of these molecules were estimated based on line emissions. Some marginal detections in the literature, such as HN 13 C(1–0), were conﬁrmed. CH 3 OCH 3 was detected for the ﬁrst time in external galaxies. Lines from several carbon chain molecules and shock-related molecules were also detected in this source. help and support during our observations.


Introduction
As cold molecular clouds are opaque to the visible and UV radiation in the Milky Way and other galaxies, observations of molecular rotational transitions at millimeter wavelength are important to study the interstellar medium (ISM; Omont 2007). As a tracer of total molecular gas in galaxies, low-J CO transitions have been observed in more than one thousand galaxies (Sanders et al. 1991;Young & Scoville 1991;Kennicutt 1998), while lines of molecules with high dipole moments, such as HCN (1-0), have also been detected as dense gas tracers in many galaxies (Gao & Solomon 2004;Liu et al. 2015).
Multiple line observations, especially with lines of different molecules (Usero et al. 2004), can provide useful constraints on the gas properties. A broadband line survey toward nearby galaxies is a powerful tool for such a study, which has been performed for M82 (Aladro et al. 2011) and Arp 157 (Davis et al. 2013) at the 3 mm band, NGC 253 at the 2 mm band (Martín et al. 2006), and Arp 220 at the 1 mm band .
As a prototypical Seyfert 2 galaxy with starburst at a distance of 14.4 Mpc (1 = 72 pc, Bland-Hawthorn et al. 1997), NGC 1068 was observed at 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). High spatial resolution The reduced spectrum (FITS file) is only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/613/A3 CO (1-0) observations show two molecular spiral arms with a diameter of ∼40 and a northern half-bar, while a CO (2-1) map reveals a nuclear ring with two bright knots in the CND region (Schinnerer et al. 2000). The dense gas fraction as traced by HCN (1-0) (Tacconi et al. 1994;Helfer & Blitz 1995) and CS (2-1) Takano et al. 2014) in the nuclear region is higher than the two arms. Observations of CO (3-2) Tsai et al. 2012;García-Burillo et al. 2014) showed that the difference of molecular gas temperatures between the nuclear region and the two arms was not as large as that of densities. Dozens of molecular lines at millimeter wavelength were detected at CND with single-dish observations (Usero et al. 2004;Nakajima et al. 2011Nakajima et al. , 2013Aladro et al. 2013). Moreover, several molecules were detected and resolved toward NGC 1068 with interferometers in the past few years (Tosaki et al. 2017;Kelly et al. 2017;Furuya & Taniguchi 2016;Izumi et al. 2016;Imanishi et al. 2016;Nakajima et al. 2015;Viti et al. 2014;Takano et al. 2014;García-Burillo et al. 2014. The molecular gas in the CND region was denser and hotter than that in the starburst ring, while chemical properties in the two regions were also different (Viti et al. 2014). The highest molecular gas temperature was higher than 150 K, and the gas density was above 10 5 cm −3 in the CND region (Viti et al. 2014). The distribution of different species of molecules were also different: CO isotopic species, for instance, were enhanced in the starburst ring, while the shock/dust related molecules were enhanced in the CND region (Nakajima et al. 2015). The spatially resolved observations showed that the CND region was a complex dynamical system. For instance, the east and west We mark each identified spectral line, using its rest frequency. The original RMS is about 1.50 mK at a frequency resolution of 0.195 MHz. The RMS is 0.45 mK after smoothing to the frequency resolution of 5.273 MHz at the rest frequency of 86.847 GHz. Eighteen lines were identified in this band, except for C 2 H (1-0), which ranges from 87.284 to 87.447 GHz. Right upper: CH 3 OH (5 −1,5 -4 0,4 ) (filled yellow) and Gaussian fitting profile (red line). The RMS is 0.41 mK at a velocity resolution of 12.47 km s −1 . Left lower: SiO (2-1), H 13 CO + (1-0), and HCO (1 0,1 -0 0,0 ) (filled yellow), overlaid with Gaussian fitting profiles (red for SiO, blue for H 13 CO + , light blue for HCO, and green for the combination of the three components). The RMS is 0.37 mK at the velocity resolution of 24.27 km s −1 . Right lower: SiO (5-4) (blue line and filled yellow) overlaid with SiO (2-1) (red line). The RMS is 0.94 mK for SiO (5-4) at a velocity resolution of 21.85 km s −1 , while it is 0.32 mK for SiO (2-1) at a velocity resolution of 24.27 km s −1 . dots were dominated by a fast shock and a slower shock (Kelly et al. 2017), while the dust torus also showed complex kinematics (García-Burillo et al. 2016). Gas inflow was driven by a past minor merger (Furuya & Taniguchi 2016), while the outflow was AGN driven ). We conducted a deeper survey of millimeter lines toward the CND region of NGC 1068 with the IRAM 30 m telescope, with the goal to quantify the gas properties in the CND. Compared to previous single-dish observations, our data probe weaker transition lines, which could place more constraints on the physical and chemistry properties of the CND.
In this paper, we focus on the transition line identification as well as on the basic physical parameter estimation. The detailed analysis for the physical and chemical properties and discussion will be the focus of a future paper. This paper is organized as follows: in Sect. 2 we present observations and data reduction, and the main results of the detected lines are provided in Sect. 3, we discuss the properties of carbon chain molecules and shockrelated molecules in Sect. 4, and give a briel summary in Sect. 5. The RMS is 0.27 mK at a velocity resolution of 24.75 km s −1 . Left lower: CH 3 CCH (5 0 -4 0 ) and c-C 3 H 2 (2 1,2 -1 0,1 ) (filled yellow), overlaid with the Gaussian fitting profiles (red for CH 3 CCH, blue for c-C 3 H 2 , and green for the combination of the two components). The RMS is 0.36 mK at a velocity resolution of 24.67 km s −1 . Right lower: H42α (filled yellow) and Gaussian fitting profile (red line). The RMS is 0.43 mK at a velocity resolution of 24.60 km s −1 .

Observations and data reduction
The observations toward the center of NGC 1068 (RA: 02:42:40.70 Dec: -00:00:48.0 J2000) were made at the end of December 2011, using the IRAM 30 m telescope at Pico Veleta, Spain 1 . The Eight Mixer Receiver (EMIR) with dualpolarization and the Fourier Transform Spectrometers (FTS) backend, which gave the frequency channel spacing of 195 kHz and 8 GHz instantaneous frequency coverage per sideband and per polarization, were used. The standard wobbler-switching mode with a ±120 offset at 0.5 Hz beam throwing was used for the observations. Pointing was checked about every two hours with nearby strong millimeter-emitting quasi-stellar objects. The typical system temperatures were 110 K at the 3 mm band, 142 K at the 2 mm band, and 172-223 K at the 1 mm band. We read out each spectrum every 12 min, which had an effective on-source time of about 5 min.
The molecular line intensities indicated in the antenna temperature (T A ) were converted into the main beam temperature (T mb ) using T mb = T A × F eff B eff , with the parameters of each band listed in Table 1. The data were reduced with the CLASS software of the GILDAS package 2 . We inspected each spectrum visually, and qualified spectra by comparing the measured noise and the theoretical noise before and after a few times of the boxcar smoothing. None of the spectra was discarded during the qualification. We subtracted linear baselines for all spectra and averaged them with time weighting for each frequency coverage, which is listed in Table 1. We identified each line by referring to frequencies from the National Institute of Standards and Technology (NIST) database recommended rest frequencies for observed interstellar molecular microwave transitions 3 .

Results
Thirty-two lines, including 31 lines from 26 molecules and 1 hydrogen recombination line, were detected toward the nuclear region of NGC 1068. The detected C 2 H (1-0) (ethynyl) lines show a double-component profile instead of six hyperfine lines due to line broadening, which were counted as one line in our list. All the detected lines are listed in Table 2, including the information of velocity-integrated fluxes, line center velocities, and line widths, which were obtained from Gaussian fitting with CLASS. Fifteen lines were first detected in NGC 1068, which are noted in Table 2 in boldface. CH 3 OCH 3 was tentatively detected as the first detection of this molecule in galaxies. The molecules, which have previously been detected in other galaxies but were detected for the first time in NGC 1068, were HC 18 O + , CH 3 CCH, and H 2 CO. Some molecules have previously been detected in NGC 1068 with other transitions, but the lines were first detected, such as SiO (5-4), H42α, HNCO (11 0,11 -10 0,10 ), HC 3 N (18-17), SO(5 6 -4 5 ), and SO (5 5 -4 4 ).
We estimated column densities of the five newly detected species (CH 3 CCH, CH 3 OCH 3 , H 2 CO, SO 2 , and HC 18 O + ) toward the center of NGC 1068 under local thermal equilibrium (LTE) assumption with the following equation: where k is the Boltzmann constant in J K −1 , ν is the rest frequency of the transition line in Hz, Q(T ex ) is the partition function, h is the Planck constant in J s, c is the light speed in cm s −1 , A ul is the spontaneous emission coefficient in s −1 , g u is the total degeneracy of upper energy level, E u /k is the upper  (3.2 ± 3.2 ) × 10 15 HC 18 O + (7.1 ± 2.7) × 10 12 level energy in K, C τ is the factor of optical depth correction, and T MB dν is the detected transition line integrated intensity in K cm s −1 . The values of Q(T ex ), A ul , g u , and E u /k were taken from the Cologne Database for Molecular Spectroscopy (CDMS) catalog 4 and splatagogue astronomical spectroscopy database 5 . To simplify the problem and facilitate calculation, we assumed an average source size of 4 for the beam dilution correction for all these five species. We used the rotational temperature of 10 ± 5 K for four species (CH 3 CCH, CH 3 OCH 3 , H 2 CO, and HC 18 O + ; the explanation in Aladro et al. 2013), and 60 ± 30 K for SO 2 , which is equal to the rotational temperature of SO we derived.
Since the C 2 H (1-0) hyperfine transition lines are mainly optically thin (see Sect. 4 for details), the optical depth correction was not considered. The column densities of the five newly detected species are listed in Table 3.
The information of individual molecules are listed below: -Methyl alcohol -CH 3 OH CH 3 OH (5 −1,5 -4 0,4 ) was detected at the rest frequency of 84.521 GHz in NGC 1068 (see Fig. 1 (Usero et al. 2004;García-Burillo et al. 2010;Aladro et al. 2013). With about 25% noise level at same velocity resolution as in Aladro et al. (2013), we obtained reliable information for the three lines, with free parameters for Gaussian fitting instead of the fixed parameters that were used in Aladro et al. (2013). SiO (5-4, v = 0) was also detected at the rest frequency of 217.105 GHz and is overlaid with SiO (2-1, v = 0) in Fig. 1. -Isotopic oxomethyliums -HCO + , H 13 CO + and HC 18 O + and hydroxymethylidynium -HOC + HCO + (1-0) was detected at the rest frequency of 89.189 GHz with a non-Gaussian profile in this source (see Fig. 2), which is consistent with the results in the literature (Usero et al. 2004;Krips et al. 2008;Aladro et al. 2013). As the isomer molecule of HCO + , HOC + (1-0) line at the rest frequency of 89.487 GHz was also detected, which is consistent with the results in the literaure (Usero et al. 2004;Aladro et al. 2013). We overlaid HCO + (1-0) with H 13 CO + (1-0), and HOC + (1-0) in Fig. 2. HC 18 O + (1-0) at the rest frequency of 85.162 GHz (Fig. 2) was marginally detected in NGC 1068. This is the third detection of this molecule in galaxies, while the first detection was HC 18 O + (2-1) toward NGC 253 (Martín et al. 2006), and the second detection is HC 18 O + (1-0) in M 82 (Aladro et al. 2015). We obtained the lowest column density of a molecular survey so far toward the center of NGC 1068, with N HC 18 O + = (7.1 ± 2.7) × 10 12 cm −2 . -Propyne -CH 3 CCH and cyclopropenylidenec-C 3 H 2 CH 3 CCH (5 0 -4 0 ) and c-C 3 H 2 (2 1,2 -1 0,1 ) were detected at the rest frequencies of 85.457 GHz and 85.339 GHz. Twocomponent Gaussian fitting was used for these two lines, and they are overlaid with the spectrum in Fig. 2. This is the first detection of propyne in NGC 1068. Its column density is listed in Table 3. c-C 3 H 2 (2 1,2 -1 0,1 ), as a stronger line than CH 3 CCH, is consistent with the detections reported in the literature (Nakajima et al. 2011). -Hydrogen recombination lines -H42α, and acetonitrile -CH 3 CN H42α was detected at the rest frequency of 85.695 GHz (Fig. 2). The central velocity of H42α is redshifted by about 100 km s −1 more than most of molecular lines, which implies that the emission of H42α does not come from the same region as the molecular gas in the CND. It might be from HII regions in the arms with strong Hα emission (Scoville et al. 1988) or from the narrow-line region of central AGN. Since velocities of molecular lines, such as CS (2-1) and 13 CO (1-0), in spiral arms are more redshifted by about 100 km s −1 than those in the CND (Takano et al. 2014) and we did not detect either broad-or narrow -line emission of H26α toward the central region of NGC 1068 with ALMA , we suggest that H42α more likely comes from spiral arms than from the narrow-line region of the AGN. CH 3 CN (5 k -4 k ) was detected at the rest frequency of 91.987 GHz (Fig. 3), as has been reported in the literature (Aladro et al. 2013). CH 3 CN (12 k -11 k ) was also detected at the rest frequency of 220.747 GHz (Fig. 5).

Carbon-chain molecules
Several carbon-chain molecules, including C 2 H, c-C 3 H 2 , HC 3 N, CH 3 CCH, CH 3 CN, and CH 3 OCH 3 , were detected in NGC 1068. C 2 H and c-C 3 H 2 are the most abundant molecules with two and three carbon atoms in the interstellar medium, and they both have a tight correlation with the star-forming regions behind diffuse and translucent clouds (Lucas & Liszt 2000;Gerin et al. 2011). C 2 H was first detected by Tucker et al. (1974) in the Milky Way, while it was first detected in the extragalactic source M 82 by Henkel et al. (1988). Meier & Turner (2005) reported a high-resolution C 2 H (1-0) observation toward the nuclear region of nearby galaxy IC 342, which showed that C 2 H was abundant in the central ring and might be affected by photodissociation region (PDR) chemistry. The authors suggested that C 2 H is probably abundant where C + and FUV photons are profuse (Meier & Turner 2005).
Owing to the line broadening, the six hyperfine components of C 2 H (1-0) were in two groups. Two-component Gaussian fitting (Jiang et al. 2011) was used to obtain the line ratio of the two groups, which gave fluxes of 5.6 and 2.4 K km s −1 , respectively. The relative optical depth ratio probably is 4. 25 : 41.67 : 20.75 : 20.75 : 8.33 : 4.25 for the six hyperfine transition lines (Tucker et al. 1974). When we assume that the six lines have the same excitation temperature and filling factors, the relative intensity ratio of the two groups is which decreases from about 2.2 to 1.0 with increasing τ 0 . The measured ratio of the two groups is 2.25 ± 0.08, which means that the C 2 H (1 − 0) lines are mainly optical thin. C 2 H and c-C 3 H 2 are abundant in the diffuse and translucent matter (Lucas & Liszt 2000) and interstellar matter in the Galactic plane (Gerin et al. 2011). The emission line ratio of C 2 H (1-0) to c-C 3 H 2 (2 1,2 -1 0,1 ) in NGC 1068 is 6.78 ± 0.34, close to the value of 7.13 ± 5.49 in the star-forming regions of M 51 (Watanabe et al. 2014). Future high-resolution observations of C 2 H and c−C 3 H 2 lines will be useful to understand C 2 H and c−C 3 H 2 chemistry in the nuclear region and the spiral arms of NGC 1068. Figure 2 shows spectra of CH 3 CCH (5 0 -4 0 ) at the rest frequency of 85.457 GHz together with c-C 3 H 2 (2 1,2 -1 0,1 ) at the rest frequency of 85.339 GHz. c-C 3 H 2 (cyclopropenylidene). With the designation c-, which means cyclic, it is the most stable molecule and has three carbon atoms and two hydrogen atoms (Spezzano et al. 2012). The rotational spectral line of c-C 3 H 2 was first detected in Sgr B2 (Thaddeus et al. 1985), while the extragalactic c-C 3 H 2 was first detected in M 82 (Mauersberger et al. 1991),0 which was used as a good tracer of a PDR in galaxies (Martín et al. 2006).
Interstellar methylacetylene (CH 3 CCH) was first detected with the J K = 5 0 -4 0 transition in Sgr B2 (Snyder & Buhl 1973), and CH 3 CN (methyl cyanide) was first detected with J = 6-5, also in Sgr B2 (Snyder & Buhl 1973), while CH 3 CCH and CH 3 CN were first detected in extragalactic sources in M 82 and NGC 253 with multiple transitions (Mauersberger et al. 1991). As second-generation molecules, the formation of CH 3 CN was generally interpreted with the grain mantle evaporation scenario, such as for IRAS 16293-2422 (Bottinelli et al. 2004).
The line ratio R, defined as I CH 3 CN(J=5−4) , is 0.84 ± 0.27 in NGC 1068, but is greater than 2 in M 82 and is 0.33 ± 0.07 in NGC 253 (Mauersberger et al. 1991). These results imply that the properties of large carbon chain molecules, such as CH 3 CCH and CH 3 CN, in the CND of NGC 1068 are more similar to the properties in NGC 253 than to the properties in M 82. CH 3 OCH 3 (dimethyl), which was first detected in Orion with multiple transitions (Snyder et al. 1974), was also detected in NGC 1068. The high column density of CH 3 OCH 3 indicates that molecules with a methyl radical are enhanced in the CND region of NGC 1068, which makes this region an ideal candidate to search for large carbon molecules.
With deep observations toward the CND of NGC 1068, emissions from molecules with up to nine atoms have been detected, which means that these large molecules can survive in the CND regions even near AGN with a strong X-ray radiation field. Further high-resolution observations toward such sources with ALMA can better determine the chemical networks of these molecules, which is important for the formation of larger Left lower: CH 3 CN (12 0 -11 0 ) (red line) overlaid with SO (5 6 -4 5 ) (blue line). The pink window ranges the subtracted transition line 13 CO (2-1). Since 13 CO (2-1) is not a perfect Gaussian profile, some residual emission of 13 CO (2-1) is in the pink window. The RMS is 2.38 mK for CH 3 CN (12 0 -11 0 ) at a velocity resolution of 28.65 km s −1 . Right lower: SO (5 5 -4 4 ) (blue line filled yellow) overlaid with SO (5 6 -4 5 ) (red line, divided by 4). The RMS is 0.79 mK for SO (5 5 -4 4 ) at a velocity resolution of 22.04 km s −1 , while it is 2.22 mK for SO (5 6 -4 5 ) at a velocity resolution of 28.75 km s −1 . molecules and for comparing the astrochemical conditions of molecular gas near AGN with star-forming regions in galaxies.

Shock-related molecules
High spatial resolution of CO (3-2) observation suggests a massive molecular outflow, which was considered as a bow-shock in the molecular disk . Lines from three shock-related molecules (SiO, SO, and HNCO) in the CND of NGC 1068 were detected with our observations. With different transitions of the fast-shock tracer SiO, the 2-1 line shows a broader line width than the 5-4 line (see Fig. 4 and Table 2), which means that part of the shocked gas was not dense enough to reach the excitation conditions of SiO (5-4). On the other hand, HNCO, which is thought to be a tracer of slow shocks or to originate from a dense region without a shock, shows similar line widths of the 11-10 and 4-3 transitions (see Fig. 15 and Table 2). High-resolution observations of the SiO and HNCO lines (García-Burillo et al. 2010;Kelly et al. 2017) show a clear difference between these tracers with different spatial distributions. Even though high-resolution observations with millimeter interferometers will be more powerful to distinguish the shock gas tracers in these galaxies, single-dish observations with multiple transitions will also be a useful tool to study the physical properties of shocked gas.
central AGN, which needs to be explained with more chemical models for such molecular gas with a strong X-ray field. Based on the ratio of the C 2 H (1-0) hyperfine features, they are optically thin. Shock-related molecules (SiO, SO, and HNCO) have also been detected with multiple transitions with different line widths, which indicates that the shocked regions had complicated excitation conditions.