ALMA view of the $^{12}$C/$^{13}$C isotopic ratio in starburst galaxies

TL;DR

This study uses ALMA observations of CN lines to measure $^{12}$C/$^{13}$C ratios in the central regions of three starburst galaxies, revealing spatial variations and signs of processed material and gas inflow.

Contribution

First measurement of $^{12}$C/$^{13}$C ratios in these galaxies using ALMA CN lines, highlighting spatial variations and inflow signatures.

Findings

Ratios range from 6 to 67 across the galaxies.

Higher ratios are found in outskirts, lower in centers.

Evidence of gas inflow in NGC 1068.

Abstract

We derive molecular-gas-phase $^{12}$C/$^{13}$C isotope ratios for the central few 100 pc of the three nearby starburst galaxies NGC 253, NGC 1068, and NGC 4945 making use of the $\lambda$ $\sim$ 3 mm $^{12}$CN and $^{13}$CN $N$ = 1--0 lines in the ALMA Band 3. The $^{12}$C/$^{13}$C isotopic ratios derived from the ratios of these lines range from 30 to 67 with an average of 41.6 $\pm$ 0.2 in NGC 253, from 24 to 62 with an average of 38.3 $\pm$ 0.4 in NGC 1068, and from 6 to 44 with an average of 16.9 $\pm$ 0.3 in NGC 4945. The highest $^{12}$C/$^{13}$C isotopic ratios are determined in some of the outskirts of the nuclear regions of the three starburst galaxies. The lowest ratios are associated with the northeastern and southwestern molecular peaks of NGC 253, the northeastern and southwestern edge of the mapped region in NGC 1068, and the very center of NGC 4945. In case of NGC 1068, the measured ratios suggest inflow from the outer part of NGC 1068 into the circum-nuclear disk through both the halo and the bar. Low $^{12}$C/$^{13}$C isotopic ratios in the central regions of these starburst galaxies indicate the presence of highly processed material.

Tables

Table 1. Table 1: Main properties of the galaxies and observational parameters.

Source	R.A.(J2000)	DEC.(J2000)	Distancea	Beam size		Position angle	Typeb
	h m s	° ′ ″	Mpc	”$\times$”	pc$\times$pc	°
NGC 253	00:47:33.14	–25:17:17.5	3.9	3.6$\times$1.7	$\sim$68$\times$32	82	SAB(s)c; Starburst
NGC 1068	02:42:40.70	–00:00:48.0	14.4	4.5$\times$2.2	$\sim$314$\times$154	75	(R)SA(rs)b; AGN+Starburst
NGC 4945	13:05:27.50	–49:28:06.0	3.8	3.6$\times$2.3	$\sim$66$\times$42	81	SB(s)cd; AGN+Starburst

Table 2. Table 2: 12 CN/ 13 CN line ratios, 12 CN opacities, and the 12 C/ 13 C isotope ratios toward the 13 CN peaks and outskirts.

Source	Offset	$I$(12CN)/$I$(13CN)	$I{(}^{12}$CN 3/2–1/2)/$I{(}^{12}$CN 1/2–1/2)	${\tau }_1{(}^{12}$CN 3/2–1/2)	12C/13C	Note
NGC 253	(6.3″, 3.9″)	26.4$\pm$1.2	1.67$\pm$0.03	0.79$\pm$0.08	33.1$\pm$3.0	Peak
	(2.7″, 1.5″)	20.0$\pm$0.5	1.49$\pm$0.01	1.42$\pm$0.06	31.1$\pm$1.6	Peak
	(–2.4″, –2.1″)	25.7$\pm$1.1	1.55$\pm$0.01	1.21$\pm$0.05	37.2$\pm$2.6	Peak
	(–4.5″, –3.6″)	27.3$\pm$2.2	1.53$\pm$0.02	1.28$\pm$0.06	40.6$\pm$4.6	Peak
	(3.9″, 5.7″)	30.3$\pm$4.0	1.50$\pm$0.05	1.40$\pm$0.21	46.7$\pm$10.6	Outskirts
	(–1.5″, –5.1″)	52.7$\pm$4.4	1.66$\pm$0.02	0.82$\pm$0.07	66.9$\pm$8.8	Outskirts
NGC 1068	(0.3″, 0.3″)	32.9$\pm$2.7	1.77$\pm$0.02	0.52$\pm$0.05	37.3$\pm$4.3	Peak
	(0.3″, –1.8″)	47.2$\pm$8.0	1.63$\pm$0.03	0.93$\pm$0.11	62.2$\pm$16.4	Outskirts
	(–0.6″, 1.2″)	35.3$\pm$3.3	1.60$\pm$0.09	1.03$\pm$0.33	48.2$\pm$13.2	Outskirts
	(3.3″, 1.8″)	20.0$\pm$2.3	1.72$\pm$0.05	0.66$\pm$0.14	24.0$\pm$4.9	Outskirts
	(–3.6″, –0.3″)	21.0$\pm$3.0	1.67$\pm$0.06	0.80$\pm$0.18	26.5$\pm$7.1	Outskirts
NGC 4945	(0.0″, 0.9″)	7.9$\pm$0.3	1.78$\pm$0.10	0.50$\pm$0.27	9.0$\pm$1.7	Peak
	(–3.0″, –4.5″)	26.6$\pm$3.6	1.39$\pm$0.11	1.86$\pm$0.65	43.6$\pm$20.8	Outskirts
	(–4.8″, –0.3″)	18.9$\pm$2.2	1.39$\pm$0.11	1.88$\pm$0.69	34.0$\pm$15.1	Outskirts

Table 3. Table 3: Averaged 12 C/ 13 C ratios and comparison with previous results.

Source	Range	Average	Previous result	Ref.
NGC 253	30–67	41.6$\pm$0.2	40$\pm$10	1, 2, 3
NGC 1068	24–62	38.3$\pm$0.4	$\sim$40a	4
NGC 4945	6–44	16.9$\pm$0.3	40–50	1, 5, 6

Equations

$$\frac{I{(}^{12}\mathrm{CN}J=3/2-1/2)}{I{(}^{12}\mathrm{CN}J=1/2-1/2)}=\frac{1-{\mathrm{e}}^{-{\tau }_1}}{1-{\mathrm{e}}^{-{\tau }_2}},$$

$$\frac{{}^{12}\mathrm{C}}{{}^{13}\mathrm{C}}\approx \frac{f_1\times I{(}^{12}\mathrm{CN}J=3/2-1/2)+f_2\times I{(}^{12}\mathrm{CN}J=1/2-1/2)}{1.082\times I{(}^{13}\mathrm{CN})},$$

Full text

1 Introduction

Even though interstellar carbon isotope ratios are locally understood (e.g., Wilson & Rood 1994; Henkel et al. 1994b; Wilson 1999), in extragalactic space beyond the Magellanic Clouds they are almost unexplored. We lack information on objects outside the Local Group of galaxies tracing environments that drastically differ from those in the Milky Way and the Large Magellanic Cloud (LMC). We do not know whether our Galaxy is typical for its class of objects or whether its isotopic properties are exceptional. Would they turn out to be peculiar, what would this imply? Moreover, will we see strong variations in isotopic ratios when observing nearby galaxies with high angular resolution?

In the past, observational data have been mostly obtained for the Galaxy and the Magellanic Clouds (e.g., Wouterloot & Brand 1996; Wouterloot et al. 2008; Wang et al. 2009). A surprising result is that the metal-poor outer Galaxy is not merely providing a “bridge” between the solar neighborhood and the even more metal-poor LMC. This is explained by the different age of the bulk of the stellar populations of the outer Galaxy and the LMC and can be exemplified by the 12C/13C and 18O/17O ratios, which are both a measure of “primary” versus “secondary” nuclear processing. 12C and 18O are produced on rapid timescales primarily via He burning in massive stars. 13C and 17O are predominantly synthesized via CNO processing of 12C and 16O seeds from earlier stellar generations. The latter occurs on a slower time scale during the red giant phase in low and intermediate mass stars or novae (e.g., Wilson & Rood 1994; Henkel et al. 1994a, b).

Molecular spectroscopy is fundamentally important to constrain stellar nucleosynthesis and the chemical evolution of galaxies. Atomic spectroscopy of stellar or interstellar gas does not allow us to discriminate between different isotopic species. However, isotopic abundances are readily obtained by spectroscopy of molecular isotopologues (Henkel et al., 1994b). Locally, emphasizing carbon, observational constraints show very high molecular-gas-phase 12C/13C ratios (12C/13C given here and elsewhere represent the molecular gas) from molecular spectroscopy in the outer Galaxy (¿100), high ratios in the local interstellar medium ($\sim$70), lower ones in the inner Galactic disk and LMC ($\sim$50), and a smaller value in the Galactic center region ($\sim$20–25) (e.g., Güsten et al. 1985; Wilson & Rood 1994; Henkel et al. 1994b; Wouterloot & Brand 1996; Wilson 1999; Wang et al. 2009). The solar system ratio ($\sim$89; Wilson & Rood 1994; Henkel et al. 1994b) can be interpreted to represent conditions at a time when the local disk was 4.6$\times$109 yr younger than today. Within the framework of “biased infall” (e.g., Chiappini et al. 2001) the Galactic disk is slowly formed from inside out, which is causing gradients in the abundances across the disk. The stellar 13C ejecta, reaching the interstellar medium with a time delay, are less dominant in the young stellar disk of the outer Galaxy than in the inner Galaxy and the older stellar body of the LMC (for the LMC, see Hodge 1989). The solar system ratio, referring to a younger disk with less 13C, is consequently higher than that measured in the present local interstellar medium.

While the 13C bearing molecular species can be safely assumed to be optically thin (with the possible exception of 13CO), a basic problem is the optical depth of the 12C bearing species (e.g., HCN, HCO+; Nguyen-Q-Rieu et al. 1992; Wild et al. 1992; Henkel et al. 1993; Gao & Solomon 2004; Jiang et al. 2011; Wang et al. 2014; Davis 2014; Jiménez-Donaire et al. 2017). Beam filling factors, $\tau$(12CX), are in most cases unknown when it comes to extragalactic sources. With this in mind, a useful tracer should possess the following properties:

• •

The tracer must be abundant, showing strong lines, to allow us to detect also the rare species, albeit not too strong (i.e., opacity $\sim$ 1) to cause optical thickness problems.

• •

A useful check of the opacity should be provided by transitions exhibiting fine and hyperfine structure (fs and hfs, respectively). In such cases the splitting should be wide enough for the different components to be separated for a line emission with a width of several 100 km s*-1*, commonly encountered in external galaxies.

• •

The components should show, in the optically thin limit, relative intensities as predicted by local thermodynamical equilibrium (LTE). Then line ratios deviating from these value, can be used for optical depth estimates.

• •

The tracer must be well understood theoretically and observationally, the former in terms of its physical and chemical properties related to photodissociation and fractionation, the latter by a systematic survey of 12C/13C ratios in Galactic star-forming regions.

• •

Finally, there should be no blend with any other potential strong line of another species.

Presently, there is only one molecule matching all these conditions, the cyanide radical (CN). C2H may come close but a systematic Galactic survey based of this molecule has not yet been conducted. Previous observations show that CN is widespread in Galactic molecular clouds (e.g., Rodríguez-Franco et al. 1998; Han et al. 2015; Gratier et al. 2017; Watanabe et al. 2017; Yamagishi et al. 2018) and a variety of other objects, including the circumstellar envelopes of evolved stars (e.g., Bachiller et al. 1997; Savage et al. 2002; Milam et al. 2005, 2009; Hily et al. 2008; Adande et al. 2012). CN was detected for the first time in extragalactic sources by Henkel et al. (1988). It exhibits strong lines and is therefore also easily detected outside the Galaxy. (e.g., Henkel et al. 1988, 1990, 1991, 1993, 1994a, 1998, 2014; Aalto et al. 2002, 2007; Wang et al. 2004, 2009; Fuente et al. 2005; Pérez-Beaupuits et al. 2007, 2009; García-Burillo et al. 2010; Chung et al. 2011; Martín et al. 2011; Aladro et al. 2013, 2015; Meier et al. 2014, 2015; Sakamoto et al. 2014; Watanabe et al. 2014; Ginard et al. 2015; Nakajima et al. 2015, 2018; Saito et al. 2015; König et al. 2016; Qiu et al. 2018; Wilson 2018).

CN spectra are complex. Each CN rotational energy level with N ¿ 0 is split into a doublet by spin-rotation interaction. Because of the spin of the nitrogen nucleus ($I_{1}$ = 1), each of these components is further split into a triplet of states. The 13CN spectrum is further complicated by the spin of the 13C nucleus ($I_{2}$ = 1/2). All this results in a very complex hfs splitting of the rotational lines. Numerically, it has been shown that carbon ratios resulting from CN measurements should not be affected by isotope selective photodissociation or chemical fractionation (Langer et al., 1984; Roueff et al., 2015). Observationally, this has been confirmed by Savage et al. (2002) and Milam et al. (2005).

Previous observations of CN in the Milky Way indicate a 12C/13C isotope ratio gradient with Galactocentric distance (Savage et al., 2002; Milam et al., 2005), which agrees rather well with the gradient derived from measurements of CO and H2CO (e.g., Henkel et al. 1980, 1982, 1983, 1985, 1994b; Langer & Penzias 1990, 1993; Wilson & Rood 1994; Giannetti et al. 2014; Yan et al. 2019). This suggests that the 12C/13C isotope ratios obtained from CN are an excellent indicator of Galactic chemical evolution (Milam et al., 2005). This is even more true for extragalactic targets, since CO and the mm-wave lines of H2CO do not allow for direct determinations of optical depths of the main species, while the cm-wave lines of H${}_{2}^{13}$CO are extremely weak.

So far, few 12C/13C isotope ratio determinations of extragalactic targets have been performed. In this paper, we have therefore carried out observations of three nearby starburst galaxies, NGC 253, NGC 1068, and NGC 4945. In Sects. 2 and 3, we introduce our targets, observations of CN, data reduction, and describe the main results. The resulting carbon isotope ratio derived from CN are then discussed in Sect. 4. Our main conclusions are summarized in Sect. 5.

2 Targets, observations, and data reduction

2.1 Targets

The three objects we selected, i.e. NGC 253, NGC 1068, and NGC 4945, are prominent nearby starburst galaxies, exhibiting particularly strong molecular lines (e.g., Martín et al. 2006; Chou et al. 2007; García-Burillo et al. 2010, 2014, 2016, 2017; Aladro et al. 2013; Meier et al. 2015; Henkel et al. 2018). The choice of the galaxies was made to have three of the strongest extragalactic line emitters and to cover a certain range of starbursts, i.e. the transition from ”moderate starbursts” (NGC 253 and NGC 4945) to more luminous infrared galaxies (LIRGs; NGC 1068). Single-dish spectral line surveys have been performed for all our selected starburst galaxies in the 3 mm band (e.g., Henkel et al. 1990, 1994a; Wang et al. 2004; Aladro et al. 2013, 2015; Nakajima et al. 2018). The 12C/13C isotope ratio is $\sim$40 estimated from CN and CS (Henkel & Mauersberger, 1993; Henkel et al., 1993, 2014), but $>$81 obtained from C2H (Martín et al., 2010), in the starburst galaxy NGC 253. Interferometric measurements of 12C18O/13C18O in NGC 253 indicate a low value of $\sim$21 (Martín et al., 2019). It is $\sim$50 obtained from CN in NGC 1068 (Aladro et al., 2013). A value of 40–50 was reported in NGC 4945 (Henkel & Mauersberger, 1993; Henkel et al., 1994a; Wang et al., 2004). Toward M 82 and IC 342, Henkel et al. (1998) found 12C/13C $>$40 and $>$30 from CN. For Arp 220 and Mrk 231 it appears to be 100 from CO and OH (González–Alfonso et al., 2012; Henkel et al., 2014), and for the Cloverleaf QSO it may be ¿100 from CO (Henkel et al., 2010). These determined values indicate a trend matching qualitative expectations of decreasing 12C/13C values with time and metallicity (Henkel et al., 2014). However, all classes of sources targeted so far only encompass one or two objects.

Below follows a brief description of the selected targets. NGC 253, the Sculptor galaxy, an almost edge-on barred spiral (Pence, 1981; Puche et al., 1991), is one of the most prolific infrared and molecular lighthouses of the entire extragalactic sky. At a distance of $\sim$3.9 Mpc (e.g., Mouhcine et al. 2005; Rekola et al. 2005), it is a prime example of a galaxy with a nuclear starburst devoid of an active galactic nucleus (e.g., Ulvestad & Antonucci 1997; Henkel et al. 2004). Because of the exceptional strength of its molecular lines, NGC 253 was selected as the target of choice for the first unbiased molecular line survey of an extragalactic source (Martín et al., 2006). It is therefore a highly suitable target for this study.

NGC 1068, a local LIRG and a prototypical Seyfert 2 galaxy with a starburst at a distance of $\sim$14.4 Mpc (Bland-Hawthorn et al., 1997), is one of the best extragalactic targets for studying the physical and chemical properties of the interstellar medium (ISM) in the vicinity of an active galactic nucleus (AGN). Numerous molecular line observations have targetted NGC 1068’s circumnuclear molecular ring and the effect of nuclear activity on its ISM (e.g., Schinnerer et al. 2000; Usero et al. 2004; García-Burillo et al. 2010, 2014, 2016, 2017; Krips et al. 2011; Aladro et al. 2013; Viti et al. 2014; Wang et al. 2014; Qiu et al. 2018). H2O maser emission has been observed forming an edge-on disk in the circumnuclear environment of NGC 1068 (e.g., Greenhill et al. 1996; Gallimore et al. 1996, 1997, 2001).

NGC 4945, at a distance of $\sim$3.8 Mpc (e.g., Karachentsev et al. 2007; Mould & Sakai 2008), has an active Seyfert 2 nucleus and is, like NGC 253, an almost edge-on spiral galaxy. Its central region is known to show a rich molecular spectrum hosting not only a nuclear starburst but also an AGN (e.g., Marconi et al. 2000; Yaqoob 2012). Past molecular single-dish and interferometric observational studies exist for CO, CS, CN, HCN, HNC, HCO+, CH3OH, H2CO (e.g., Henkel et al. 1990, 1994a, 2018; Dahlem et al. 1993; Mauersberger et al. 1996; Curran et al. 2001; Wang et al. 2004; Chou et al. 2007; Hitschfeld et al. 2008; Green et al. 2016; McCarthy et al. 2018). H2O megamaser emission has been observed in the nucleus of NGC 4945 (Greenhill et al., 1997).

2.2 Observations

Our observations were carried out from 2014 December to 2015 January with the Atacama Large Millimeter/submillimeter Array (ALMA) in Band 3 (Project: 2013.1.01151.S). During the observations, 36–40 12-m antennas were employed in a compact configuration with baselines ranging from 15 to 349 m. For each source, the observations took about 30 minutes and 1 hour for 12CN and 13CN, respectively. The CN $N$ = 1–0 transition consists of 9 hyperfine components blended into two groups, with the stronger group representing the $J$ = 3/2–1/2 transitions and the weaker group the $J$ = 1/2–1/2 transitions. The 12CN $N$ = 1–0 ($J$ = 1/2–1/2 and 3/2–1/2) and 13CN $N$ = 1–0 ($J$ = 1/2–1/2 and 3/2–1/2) transitions have intensity weighted rest frequencies of 113.191, 113.491, 108.658, and 108.780 GHz, respectively, when LTE line ratios and optically thin emission are adopted. On each, the 12CN and the 13CN line, a spectral window was centered, with a bandwidth of 1875 MHz and a frequency resolution of 7812.5 kHz, corresponding to a channel width of $\sim$21 km s*-1*. Basic observational parameters as well as phase center coordinates are listed in Table 1. The 12CN data of NGC 253 were not part of our project. Instead, Meier et al. (2015) observed NGC 253 in 12CN $N$ = 1–0 with ALMA Band 3 (Project: 2011.0.00172.S). Quality and, in particular, beam sizes of these data are well matching our ALMA observations. 12CN $N$ = 1–0 data of NGC 253 are therefore taken from Meier et al. (2015) in this work. The observed spectra toward the central positions of NGC 253, NGC 1068, and NGC 4945 are shown in Fig. 1.

2.3 Data reduction

The data were calibrated and imaged using the CASA222https://casa.nrao.edu 4.2 version pipeline (McMullin et al., 2007). Our 13CN data reduction for NGC 253 (see Table 1) follows that performed for the 12CN data set (beam size $\sim$3.8*′′$\times$2.7′′* and position angle –31$\degr$) by Meier et al. (2015). The spectral line images were analysed using GILDAS333http://www.iram.fr/IRAMFR/GILDAS. Typical rms noise levels in the 12CN and 13CN velocity-integrated intensity images are 1–2 and 0.3–0.7 mJy beam*-1* for NGC 253, 1–2 and 0.4–0.7 mJy beam*-1* for NGC 1068, and 2–4 and 0.4–0.9 mJy beam*-1* for NGC 4945, respectively.

2.4 Spatial filtering

As mentioned in Sect. 2.1, the CN $N$ = 1–0 transitions has previously been observed using the 15-m SEST (beam size $\sim$44*′′), 30-m IRAM (beam size $\sim$22′′), and 45-m NRO (beam size $\sim$15′′*) telescopes toward NGC 253, NGC 1068, and NGC 4945 (Henkel et al., 1990, 1994a, 2014; Wang et al., 2004; Aladro et al., 2013, 2015; Nakajima et al., 2018). From these observations 12C/13C isotope ratios of 40$\pm$10, $\sim$50, and 40–50, respectively, were determined. One should note that the 13CN $N$ = 1–0 features observed with the 30-m IRAM telescope by Aladro et al. (2013) are weak and show low signal-to-noise ratios. Both 12CN and 13CN $N$ = 1–0 lines of NGC 1068 were detected with slightly higher signal-to-noise ratios with the 45-m NRO telescope (Nakajima et al., 2018). A 12C/13C isotope ratio of $\sim$40 in NGC 1068 in calculated from Nakajima et al.’s integrated intensity ratio of $I$(12CN)/$I$(13CN) following our method mentioned in Sect. 3.4. For NGC 4945, the 13CN $N$ = 1–0 transition was not detected by Wang et al. (2004).

Due to spatial filtering, the missing flux, that is flux of large scale structures not sampled by the interferometer, may affect 12CN/13CN line ratios. To evaluate the missing flux we reconstruct our ALMA data with beams of $\sim$22*′′* (IRAM 30-m; Henkel et al. 2014; Aladro et al. 2015), $\sim$15*′′* (NRO 45-m; Nakajima et al. 2018), and $\sim$44*′′* (SEST 15-m; Wang et al. 2004) for NGC 253, NGC 1068, and NGC 4945, respectively. We find that $\sim$89% and $\sim$86% of the 12CN and 13CN integrated flux observed by single-dish telescope is recovered for NGC 253 by our ALMA data, respectively. Only $\sim$30% and $\sim$53% of the 12CN and 13CN single-dish integrated flux of NGC 1068 is recovered, respectively. However, as mentioned above, the 13CN $N$ = 1–0 features observed with the 45-m NRO telescope (Nakajima et al., 2018) are weak and show large uncertainties. For NGC 4945, $\sim$63% of the 12CN single-dish integrated flux is recovered. We do not evaluate the missing flux of 13CN in NGC 4945 because (as already mentioned) the 13CN $N$ = 1–0 transition was not detected with the 15-m SEST (Wang et al., 2004). In their Table 2, Wang et al. (2004) provide an upper limit (3 sigma) to the 13CN $N$ = 1–0 line intensity, so at least $\sim$40% is recovered by our ALMA data. Based on similar missing flux of our 12CN and 13CN data in NGC 253 and NGC 1068, we note that 13CN $N$ = 1–0 is likely showing a less extended morphology (see Sect. 3.1) only because it is more rapidly reaching intensities below the detection threshold outside the line peaks. So our 13CN data may cover a single-dish integrated flux fraction which is similar to that of 12CN in NGC 4945. With similar missing flux levels related to our 12CN and 13CN data in NGC 253 and NGC 1068, which is based on a comparison of our interferometric measurements with previously published single-dish observations, missing flux appears to affect 12CN/13CN line ratios in NGC 253, NGC 1068, and NGC 4945 only weakly.

There may be line blending with the CH3OH (00–1*-1* E) transition at 108.894 GHz, 114 MHz ($\sim$300 km s*-1*) off the group of 13CN ($N$ = 1–0; $J$ = 3/2–1/2) lines centered at 108.780 GHz (see Sect. 2 and Fig. 1). However, CH3OH can be well identified in most locations of NGC 253 and NGC 4945, which indicates that the $I$(12CN)/$I$(13CN) ratios are at most only weakly affected by CH3OH in these two galaxies. For NGC 1068, the typical linewidths of 13CN are broader ($\sim$170 km s*-1*) and CH3OH can not be identified in most locations. This could indicate that the $I$(12CN)/$I$(13CN) ratio may be underestimated from our ALMA data in NGC 1068. The slightly less extended 13CN distributions also addressed in Sect. 3.1 when compared to 12CN may merely indicate that the minimum detectable molecular H2 column density is higher than in case of 12CN with its higher fractional abundance (see also Sect. 3.3).

3 Results

3.1 Distribution of 12CN and 13CN

The integrated intensity distributions of 12CN and 13CN in NGC 253, NGC 1068, and NGC 4945 are shown in Fig. 2. In all three galaxies 12CN shows extended distributions, in agreement with previous observations of 12CN in galaxies (e.g., Henkel et al. 1988; García-Burillo et al. 2010; Meier et al. 2014; Sakamoto et al. 2014; Ginard et al. 2015; Nakajima et al. 2015; Saito et al. 2015; Wilson 2018). This is also consistent with previous observational results for other gas tracers as e.g., 13CO, C18O, CS, HCN, or HCO+ toward our sources (e.g., Krips et al. 2011; Sakamoto et al. 2011; García-Burillo et al. 2014; Viti et al. 2014; Meier et al. 2015; Henkel et al. 2018; Tan et al. 2018; Martín et al. 2019). Weak 12CN emission is detected in the spiral arms of NGC 1068 (see Fig. 2). 13CN is only detected in the central regions of NGC 253, NGC 1068, and NGC 4945, and shows slightly less extended distributions than 12CN (see also the end of Sect. 2.4).

3.2 12CN/13CN line ratios

12CN/13CN line ratio maps of NGC 253, NGC 1068, and NGC 4945 are shown in Fig. 3. The line ratios are calculated using velocity-integrated intensities where the 13CN lines are detected with signal-to-noise ratios of S/N $\gtrsim$ 5$\sigma$. $I$(12CN)/$I$(13CN) ratios range from 19 to 53 with an average of 30.0 $\pm$ 0.2 (errors given here and elsewhere are standard deviations of the mean) in NGC 253, from 20 to 47 with an average of 31.4 $\pm$ 0.3 in NGC 1068, and from 6 to 25 with an average of 11.5 $\pm$ 0.1 in NGC 4945. High ratios ($>$30) are obtained in the outskirts of the region analyzed by us in NGC 253. It shows that low ratios ($<$30) associate with 13CN peak emission in NGC 253. Gradients are seen from the center to the northeastern and to the southwestern region of NGC 1068. Two locations in the northeast and southwest (see Table 2 and Sect. 4.2) have a low ratio (¡25). For NGC 4945, low ratios ($<$15) associate with 13CN peak emission and the northeastern region and high ratios ($>$15) are located in the outskirts and the southwestern region. One should note that the 13CN emission is weak and shows low signal-to-noise ratios in the edge regions of our three targets. This may lead to large uncertainties of $I$(12CN)/$I$(13CN) ratios in these locations.

3.3 Opacities of 12CN and 13CN

Previous observations toward external galaxies suggest that optical depths of the 12CN $N$ = 1–0 transition are modest (e.g., Henkel et al. 1998, 2014; Wang et al. 2009; Meier et al. 2015; König et al. 2016; Nakajima et al. 2018). For the $N$ = 1–0 line of the rare 13CN isotopologue this holds of course as well. Opacities can be determined by an analysis of line intensity ratios because (as already mentioned in Sect. 1) the $N$ = 1–0 transition is split into several spectral features. If 12CN $N$ = 1–0 is optically thin and LTE prevails, line intensity ratios should be 2:1 for 12CN (3/2–1/2)/(1/2–1/2) and 1:1.225 for 13CN (3/2–1/2)/(1/2–1/2) (see Fig. 1). Previous single-dish observations of 12CN $N$ = 1–0 in galaxies indicate that the 12CN (3/2–1/2)/(1/2–1/2) line intensity ratio is indeed $\sim$2 (e.g., Henkel et al. 1998, 2014; Aladro et al. 2013, 2015; Watanabe et al. 2014; Nakajima et al. 2018).

We averaged all pixels in NGC 253, NGC 1068, and NGC 4945 for which the 13CN line is detected with signal-to-noise ratios of S/N $\gtrsim$ 5$\sigma$. The line integrated intensity ratios of the two 12CN $N$ = 1–0 features are 1.58 $\pm$ 0.11, 1.70 $\pm$ 0.02, and 1.68 $\pm$ 0.25 in NGC 253, NGC 1068, and NGC 4945, respectively, which is slightly lower than that for LTE and optically thin emission. Following the method applied by Wang et al. (2004), the optical depth of the 12CN $N$ = 1–0 lines can be obtained from the integrated intensity ratio of 12CN (3/2–1/2)/(1/2–1/2) as

[TABLE]

where $\tau_{1}$ and $\tau_{2}$ are the optical depths of 12CN $J$ = 3/2–1/2 and 1/2–1/2, respectively, and $\tau_{1}$ = 2$\tau_{2}$ (see Skatrud et al. 1983 for relative LTE intensities under optically thin conditions). From the above average integrated intensity ratios of the two 12CN $N$ = 1–0 features the derived optical depths of 12CN $J$ = 3/2–1/2 are 1.1, 0.7, and 0.8 in NGC 253, NGC 1068, and NGC 4945, respectively. Opacities of 12CN at 13CN peaks and outskirts are also calculated in Table 2. These suggest that the opacity of 12CN $N$ = 1–0 slightly affects the line intensity ratios of 12CN/13CN in our three selected starburst galaxies. For 13CN $N$ = 1–0, the corresponding average line integrated intensity ratios are 1.04 $\pm$ 0.08, 1.03 $\pm$ 0.14, and 1.25 $\pm$ 0.01, which is consistent with LTE and optically thin emission considering the uncertainties of our interferometric measurements. In this work we assume that the 13CN $N$ = 1–0 line is optically thin in all of our three studied objects.

3.4 The 12C/13C isotopic ratios from CN

As mentioned in Sect. 3.3, the 12CN $N$ = 1–0 line is slightly influenced by saturation effects in the starburst galaxies NGC 253, NGC 1068, and NGC 4945. Therefore, in these objects this line’s intensities are not exactly proportional to CN column densities. Nevertheless, the integrated intensity ratios of 12CN and 13CN can be used to determine the 12C/13C isotope ratios. Assuming the 13CN $N$ = 1–0 lines are optically thin and assuming that LTE holds in NGC 253, NGC 1068, and NGC 4945 and following the method applied by Henkel et al. (2014), the 12C/13C isotope ratio can be obtained from the integrated intensity ratio of 12CN/13CN $N$ = 1–0 as

[TABLE]

where $f_{1}$ = $\frac{\tau_{1}}{1-{\rm e}^{-\tau_{1}}}$ and $f_{2}$ = $\frac{\tau_{2}}{1-{\rm e}^{-\tau_{2}}}$, and $\tau_{1}$ and $\tau_{2}$ are the optical depths of 12CN $J$ = 3/2–1/2 and 1/2–1/2 (see Sect. 3.3), respectively. The integrated intensity $I$(13CN) contains the 13CN $J$ = 1/2–1/2 and 3/2–1/2 components. The factor of 1.082 is caused by the fact that a weak 13CN ($F_{1}$, $F_{2}$ = 1–0) hyperfine feature near 108.4 GHz is both below our detection threshold and offset from the considered frequency range (see Henkel et al. 2014). The 12C/13C isotope ratios derived from the CN line ratios following Equation 2 range from 30 to 67 with an average of 41.6 $\pm$ 0.2 in NGC 253, from 24 to 62 with an average of 38.3 $\pm$ 0.4 in NGC 1068, and from 6 to 44 with an average of 16.9 $\pm$ 0.3 in NGC 4945 (see Table 3). The uncertainty of these 12C/13C isotope ratio is $\sim$10%, which is mainly caused by the absolute flux calibration error of $\sim$5%.

4 Discussion

4.1 Comparison to previous 12C/13C isotopic ratio measurements

As deduced in Sect. 3.4, with ALMA we find average 12C/13C isotope ratios of 41.6 $\pm$ 0.2, 38.3 $\pm$ 0.4, and 16.9 $\pm$ 0.3 in the nuclear disks of NGC 253, NGC 1068, and NGC 4945, respectively (see Table 5). We compare our ALMA measured 12C/13C isotope ratios with previous results obtained from single-dish observations in Table 5. For NGC 253 and NGC 1068, our average measured 12C/13C isotope ratios agree well with those obtained from the single-dish observations (Henkel & Mauersberger, 1993; Henkel et al., 1993, 2014; Nakajima et al., 2018). However, our ALMA measured 12C/13C isotope ratio is significantly lower than that obtained from the single-dish observations in NGC 4945 (Henkel & Mauersberger, 1993; Henkel et al., 1994a; Wang et al., 2004). This may be caused by several reasons. Previous single-dish observations may include a lot of material from outside the nuclear disk, i.e. from the bar extending from galactocentric radii of 100–300 pc and from the spiral arms even farther out (see the sketch in Henkel et al. 2018). Our ALMA data cover a smaller region with galactocentric radii out to $\sim$175 pc. The 12CN distribution appears to be complex in NGC 4945 (see Sect. 3.1). The typical linewidths of 12CN are $\sim$50 km s*-1* at velocity $\sim$700 km s*-1* and $\sim$150 km s*-1* at velocity $\sim$450 km s*-1* in the northeastern and southwestern regions of its highly inclined ($i$ $\sim$ 75$\degr$) nuclear disk (see our Fig. 1 or Figs. 5, 11, and 13 in Henkel et al. 2018). Perhaps, the starburst is still young and the gas moving outwards through the nuclear disk is a remnant of formerly quiescent gas highly enriched in 13C by AGB stars through the CNO cycle (like in our Galactic center), while 12C enrichment from young massive stars has not yet taken over in a substantial way.

Considering the above mentioned uncertainties of our interferometric and previous single-dish observations in the 12CN and 13CN $N$ = 1–0 transitions, our averaged 12C/13C isotope ratios in NGC 253 ($\sim$400 pc) and NGC 1068 ($\sim$500 pc) confirm previous results from single-dish observations. We conclude that the averaged 12C/13C isotope ratios are $\sim$40–50 in NGC 253 and NGC 1068 and $\sim$20–50 in NGC 4945. The apparent discrepancy between the single-dish and our interferometric results in NGC 4945 will be further discussed in Sect. 4.3. For NGC 4945 ($\sim$350 pc), our ALMA data offer a first value for the $\sim$200 pc sized nuclear disk, while the bar, the inner spirals, and the nuclear $\sim$50 pc are still waiting for a dedicated measurement. So further observations combining our 12-m ALMA with 7-m ACA and Total Power (TP) data of CN and eventually also other molecules and their isotopologues could provide further progress in the determination of accurate 12C/13C isotope ratios for the various distinct morphological components.

Presently, high resolution ($\sim$3*′′) observations of C18O with ALMA toward NGC 253 suggest a low 12C/13C isotope ratio of $\sim$21$\pm$6 (Martín et al., 2019), which is a factor of $\sim$2 lower than our results obtained from CN. We compare our ALMA measured 12CN/13CN ratio map with their results from 12C18O/13C18O ratios in NGC 253 (see Fig. 3 in Martín et al. 2019). It shows that the lowest 12C18O/13C18O ratios are associated with the northeastern hotspot which agrees with our results derived from 12CN/13CN ratios, but the distributions of 12CN/13CN and 12C18O/13C18*O ratios are slightly different in other locations. The cause(s) of these discrepancies may be different beam sizes, different molecular distributions due to chemistry or critical densities and could be settled by the combination of 12-m, 7-m, and TP measurements, making use of all the instruments available at the ALMA site.

4.2 Variation of the 12C/13C isotopic ratio

In our Galaxy, previous observations of H2CO, CO, and CN indicate the presence of a 12C/13C isotope ratio gradient and further suggest a significant dispersion at a given galactocentric radius (e.g., Henkel et al. 1980, 1982, 1983, 1985, 1994b; Langer & Penzias 1990, 1993; Wilson & Rood 1994; Savage et al. 2002; Milam et al. 2005; Giannetti et al. 2014). This is expected in an inside-out formation scenario for our Galaxy and in view of radial gas streaming and potential cloud-to-cloud variations due to local supernovae or ejecta by late-type stars (e.g., Milam et al. 2005; Henkel et al. 2014). However, even in the Galactic center region isotope ratios are not everywhere the same as recently discovered by Riquelme et al. (2010) and Zhang et al. (2015). The data of the former authors indicate that gas from the halo is accreted to the disk and from the outskirts of the disk to regions closer to the Galactic center (Riquelme et al., 2010).

In analogy to the Galactic center region, our extragalactic ALMA data also reveal spatial variations of the 12C/13C isotopic ratio within the inner $\sim$400, $\sim$500, and $\sim$350 pc sized regions of NGC 253, NGC 1068, and NGC 4945, respectively. As mentioned in Sect. 3.4, higher line ratios of 12CN/13CN indicate higher 12C/13C isotopic ratios. Therefore, the ratio maps can be used as a proxy for the relative 12C/13C isotopic ratio. 12CN/13CN line ratio maps and histograms of NGC 253, NGC 1068, and NGC 4945 are shown in Figs. 3 and 4, respectively. Variations of the 12CN/13CN line ratio as mentioned in Sect. 3.2 indicate that highest 12C/13C isotopic ratios are located in some of the outskirts of the three starburst galaxies while lowest ratios associate with galactic central regions and/or 13CN peak emission. The velocity components in the southwest of NGC 253 and NGC 4945 (Henkel et al., 2014, 2018) suggest higher 12C/13C isotopic ratios than in the northeast. The 12C/13C isotopic ratio varies by factors of $\sim$2–3 in NGC 253 and NGC 1068 and even $\sim$7 in NGC 4945 (see Fig. 4 and Table 3). While these extreme differences may in part be due to low 13CN signal-to-noise ratios, the differences are too large to be entirely a consequence of this effect (see Table 2).

For NGC 253, integrated intensities of 13CN are highest in two hotspots, located symmetrically with respect to the nucleus, one in the northeast and the other one in the southwest. Lowest 12C/13C isotopic ratios associate with these two hotspots (see Fig. 3) corresponding to centimeter and millimeter continuum peak emission (e.g., Turner & Ho 1985; Ulvestad & Antonucci 1997; Sakamoto et al. 2011; Krips et al. 2016; Mangum et al. 2019). The gas at these locations may be CNO-processed by intermediate mass stars in the more distant past. Two locations with low 12C/13C isotopic ratios in the northeast (3.3″, 1.8″) (offsets relative to our reference position; see Table 1) and southwest (–3.6″, –0.3″) of NGC 1068 (see Figs. 3 and 5, and Table 2) associate with two outflow knots OUT-III and II (García-Burillo et al., 2014, 2017), respectively, so that the highly processed gas has moved away from the center, while less processed gas may be infalling from the outer part of NGC 1068 into (at least the outer part of) the circum-nuclear disk (CND) through both the halo and the bar, and the CND is dominated by the outflowing motion. The decline of the 12C/13C isotopic ratio in these locations may be influenced by the outflow from the AGN and/or by processed material near the AGN. One should note that the two outflow knots OUT-II and III are identified with $\sim$1″resolution C2H data by García-Burillo et al. (2017), which corresponds to a $\sim$4.5 times higher resolution (with respect to beam area) than the CN data presented here. Beam dilution effects may not be negligible. Low 12C/13C isotopic ratios in the northeast of NGC 4945 show similar values as those in the nuclear region (see Fig. 3), which indicates that it may be strongly affected by highly processed outflowing material from the nuclear region (for this, see Henkel et al. 2018) having undergone substantial star formation in the past.

4.3 The 12C/13C isotopic ratio evolution in starburst galaxies

The 12C/13C isotope ratio is a useful probe of the chemical evolution of galaxies (e.g., Milam et al. 2005; Martín et al. 2010; Henkel et al. 2014). It is believed to be a direct measure of primary to secondary nuclear processing (Wilson & Rood, 1994). The 12C/13C isotope ratio is expected to decline with time (e.g., Henkel & Mauersberger 1993; Prantzos et al. 1996; Hughes et al. 2008; Martín et al. 2010; Henkel et al. 2014; Romano et al. 2017). This leads to the very low ratios in our Galactic center region. However, in case of a starburst, triggered by a bar or by a merger, gas from outside with higher ratios is flowing into a galaxy’s central region, enhancing the 12C/13C isotope ratio. A few Myr after the start of a starburst, this effect will be strengthened by the ejecta from massive stars. A top-heavy stellar initial mass function could make this effect even more pronounced (e.g., Henkel & Mauersberger 1993; Romano et al. 2017; Zhang et al. 2018).

The inflow scenario discussed for starbursts may even lead to higher 12C/13C ratios for ultraluminous infrared galaxies (ULIRGs) since such objects have more powerful inflows (e.g., Toyouchi & Chiba 2015; Yabe et al. 2015; Falstad et al. 2017). Indeed, ULIRGs have not only a higher 12C/13C ratio, but are also deviating from the canonical mass-metallicity relation in the sense of having a lower metallicity for their mass or a higher mass for their metallicity (Pereira-Santaella et al., 2017). More moderate starburst galaxies have experienced less inflow, which may suggest lower 12C/13C isotope ratios. In comparison, our Galaxy shows only weak signs of inflow (e.g., Morris & Serabyn 1996; Riquelme et al. 2010), which is reflected in the lower 12C/13C isotope ratios measured in its central molecular zone. The CN data presented here indicate that the averaged 12C/13C isotope ratios in the nuclear regions of NGC 253 and NGC 1068 are higher than in our Galactic center region. Nevertheless, the averaged 12C/13C isotope ratios in our selected nearby starburst galaxies are lower than previous observational results in the well-studied ULIRGs Arp 220 and Mrk 231 ($\sim$100; González–Alfonso et al. 2012; Henkel et al. 2014), and also in the high-$z$ Cloverleaf ULIRG/QSO ($\gtrsim$100; Henkel et al. 2010). This confirms the trend of declining 12C/13C values with time and metallicity proposed by Henkel et al. (2014).

Measurements of the 12C/13C isotope ratio based on CN lines have the potential to reveal the degree of gas processing in the nuclear regions of starburst galaxies. More galaxies are needed to study nucleosynthesis, to constrain galaxy dynamics, and to discriminate between different evolutionary stages to follow in more detail the secular decline of 12C/13C ratios, occasionally interrupted by inflow and starburst activity. With ALMA it is possible to extend such studies to objects at greater distances.

5 Summary

We have measured the 12C/13C isotopic ratio in the nuclear regions of three nearby starburst galaxies NGC 253, NGC 1068, and NGC 4945 making use of the 12CN and 13CN $N$ = 1–0 lines in the ALMA Band 3 at frequencies near 110 GHz. The main results are the following:

The 12C/13C isotopic ratios derived from the 12CN and 13CN line ratios range from 30 to 67 with an average of 41.6 $\pm$ 0.2 in NGC 253, from 24 to 62 with an average of 38.3 $\pm$ 0.4 in NGC 1068, and from 6 to 44 with an average of 16.9 $\pm$ 0.3 in NGC 4945. The 12C/13C isotopic ratios vary by factors of $\sim$2–3 in NGC 253 ($\sim$400 pc) and NGC 1068 ($\sim$500 pc) and $\sim$7 in NGC 4945 ($\sim$350 pc). The large scatter of values, particularly in NGC 4945, is certainly in part a consequence of the limited sensitivity of our data. Neverthelss, the variations are too large to be only caused by this effect, suggesting the presence of real variations as they have recently been found in our Galactic center region. 2. 2.

The highest 12C/13C isotopic ratios are located in the outskirts of the three starburst galaxies’ nuclear regions. The lowest ratios are associated with the northeastern and southwestern molecular peaks of NGC 253, the northeastern and southwestern edge of the mapped region in NGC 1068, and the very center of NGC 4945. 3. 3.

The measured 12C/13C isotopic ratios in NGC 1068 indicate that the highly processed gas has moved away from the center and less processed gas may be infalling from the outer part of NGC 1068 into the CND through both the halo and the bar. 4. 4.

Low 12C/13C isotopic ratios in the central regions of these starburst galaxies indicate the presence of highly processed material. 5. 5.

Our results agree with the scenario of 12C/13C ratios slowly decreasing with time in galaxies.

Acknowledgements.

The authors thank the anonymous referee for helpful comments. This work acknowledges support by The Heaven Lake Hundred-Talent Program of Xinjiang Uygur Autonomous Region of China, The National Natural Science Foundation of China under grant 11433008, and The Collaborative Research Council 956, subproject A6, funded by the Deutsche Forschungsgemeinschaft (DFG). C. H. acknowledges support by Chinese Academy of Sciences President’s International Fellowship Initiative under Grant No. 2019VMA0039. This paper makes use of the following ALMA data: ADS/JAO.ALMA#2011.0.00172.S and 2013.1.01151.S. ALMA is a partnership of ESO (representing its member states), NSF (USA) and NINS (Japan), together with NRC (Canada), NSC and ASIAA (Taiwan), and KASI (Republic of Korea), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by ESO, AUI/NRAO and NAOJ. This research has made use of the NASA/IPAC Extragalactic Database (NED) which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. This research has used NASA’s Astrophysical Data System (ADS).