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A&A
Volume 631, November 2019
Article Number L12
Number of page(s) 6
Section Letters to the Editor
DOI https://doi.org/10.1051/0004-6361/201936685
Published online 14 November 2019

© ESO 2019

1. Introduction

The [C II] 158 μm line (2P3/2–2P1/2 fine structure line) is one of the dominant cooling lines in photon-dominated regions (PDR, Tielens & Hollenbach 1985; Sternberg & Dalgarno 1995; Röllig et al. 2006). Because of its close link to the UV radiation from massive stars, the [C II] emission is often used to trace star formation in distant galaxies. PDR models show that the [C II] emission typically stems from a depth of up to AV  ∼  1 on the PDR surface, which corresponds to a [C II] optical depth of around unity1 (Ossenkopf et al. 2013). In order to quantify the optical depth of [12C II], we need to compare it with an optically thin line. This is naturally given by the [13C II] lines. Because the [13C II] 2P3/2–2P1/2 transition splits into three hyperfine components and they are within ±65 km s−1 of the [12C II] line (Cooksy et al. 1986), high resolution spectroscopy with a good sensitivity is needed to detect [13C II]. After detections of the [13C II] hyperfine components in M 42 (Boreiko et al. 1988; Stacey et al. 1991; Boreiko & Betz 1996), the Heterodyne Instrument for the Far-Infrared (HIFI) on Herschel and the German REceiver for Astronomy at Terahertz Frequencies (GREAT2) on board the Stratospheric Observatory for Infrared Astronomy (SOFIA) enable us to detect the [13C II] lines in more Galactic sources (Graf et al. 2012; Ossenkopf et al. 2013; Goicoechea et al. 2015; Guevara et al. 2019). Guevara et al. (2019) showed that a uniform excitation temperature model gives an optical depth of 2 to 7 in four Galactic sources, as a lower limit of the optical depth due to the limited signal-to-noise ratio (S/N) of the [13C II] spectra and the high excitation temperature assumption. A multi-layer model including an absorption layer requires higher optical depths. In this Letter, we report the first detection of [13C II] emission from the Large Magellanic Cloud (LMC).

2. Observation and data reduction

We observed [12C II] and the [13C II] hyperfine lines at selected positions in N159, 30 Dor, and N160 with upGREAT (Risacher et al. 2016; Heyminck et al. 2012) on board SOFIA (Young et al. 2012) in June 2018, as part of the guaranteed time in cycle 6 observations during two flights. The two polarizations of the Low Frequency Array (LFAH and LFAV) were tuned to the [C II] line at 1900.5369 GHz. The bandwidth of 4 GHz allows us to observe all three [13C II] hyperfine emission lines simultaneously. In all positions, the strongest [13C II] line (F = 2−1) is blended with the [12C II] line, so that we ignored this line in the analysis. In parallel, the High Frequency Array (HFAV) was tuned to the [O I] 63 μm line. Each array has seven pixels in a hexagonal configuration. The beam size is 14″ for [C II] and 6.3″ for [O I], and the pixel separation in the hexagonal configuration is scaled by the beam size in the two frequency arrays, sharing the position of the center pixels. The central pixels of LFAH and LFAV are aligned within ∼2″, and the central pixel of HFAV is about 3″ away from the LFA central pixels. In the first flight we started to observe N160 with single-phase chopped observations, and switched to the double beam switch mode later to have a better baseline in some of the pixels. In the second flight we observed N159, together with a short observation of 30 Dor in double beam switch mode. The position in 30 Dor had a lower priority because its [12C II] line profile is very broad and it overlaps with the [13C II] F = 1−0 line (see Fig. 1; the 30 Dor spectrum at > 270 km s−1 in [13C II] F = 1−0 shows a wing of [12C II].)

The data were calibrated by the standard GREAT pipeline (Guan et al. 2012), which converts the observed counts to the main beam temperature scale (Tmb). For the N160 data, we confirmed that the baseline structures in the single-phase observations match those in the spectra of the same phase (phase A) extracted from the double beam switch observations. Therefore, we averaged the spectra of the other phase (phase B) from the double beam switch observations, and added them to the single-phase observations to obtain better baselines in a few pixels. Although the S/N of the result is limited by the integration time of the phase B observations, we still gain S/N compared to ignoring the single phase data. We subtracted linear baselines and spectrally resampled the data to 0.5 km s−1 channel width. We then averaged the spectra at each position and each pixel weighting them by the baseline noise. In Table 1, the final baseline noise and the total integration times are listed for positions where [13C II] is detected. Figures A.1–A.3 present the observed positions.

thumbnail Fig. 1.

Spectra of [13C II], [12C II], and [O I] at selected positions in the LMC. Left: [12C II] spectra (blue) and [O I] 63 μm spectra when available (green). The vertical lines aid the comparison of the velocities of the line profiles. Middle two panels: [13C II] F = 1−0 and F = 1−1 spectra (red) and [C II] spectra (blue) scaled for optically thin emission and 12C+/13C+ = 49. The horizontal lines indicate the rms noise of the baseline. Right: combined [13C II] spectra (red) and the scaled [C II] spectra (blue). See text for the formula of the combined [13C II] spectra and the scaled [C II] spectra.

Table 1.

Summary of the pointed observations for the [13C II] line.

3. Combined [13C II] spectra and the [C II] optical depth

The left panels in Fig. 1 show the [12C II] spectra (blue) for positions where the [13C II] emission is detected (Table 1). For positions 1, 3, and 4, where the HFAV center pixel is observed at the same positions, we also show the obtained [O I] 63 μm spectra. We note that the beam size of [O I] is smaller by a factor of 2.2 (6.3″). In the middle two columns, the [13C II] F = 1−0 and F = 1−1 spectra are represented as red lines. The right panels show the combined [13C II] spectra following Guevara et al. (2019):

(1)

(2)

Here δvF → F′ is the velocity offset of the [13C II] lines relative to the [12C II] line, and sF → F′ are the relative intensities of the hyperfine components (ΣF, F′sF → F′  =  1 when using all three hyperfine components). We use δv1 → 0  =   − 65.2 km s−1, δv1 → 1  =  63.2 km s−1, s1 → 0  =  0.25, and s1 → 1  =  0.125 (Ossenkopf et al. 2013). We composed the [13C II] spectra only from the [13C II] F = 1−0 and F = 1−1 spectra because F = 2−1 is blended with the [12C II] line for all sources. The equation scales up the sum of the detected hyperfine components to represent the full [13C II] emission, taking into account that the F = 2−1 line should contribute 62.5%.

For optically thin [12C II] emission, the expected spectrum for the combined [13C II] is scaled as Tmb, 13, tot(v) = Tmb, 12(v)/α+ (blue lines in the right panels of Fig. 1). We assume that the isotopic ratio of carbon ions α+ = 12C+/13C+ equals the elemental abundance of 12C/13C = 49 (Wang et al. 2009) for the LMC (see Sect. 4.2). The expected spectrum of individual [13C II] hyperfine line is scaled by sF → F′.

We derived the [12C II] optical depth assuming that the excitation temperature of the [13C II] and [12C II] is the same (Ossenkopf et al. 2013; Guevara et al. 2019):

(3)

Here τ12 and τ13 are the optical depth of [12C II] and [13C II], respectively, and τ12 = α+τ13. We use the combined [13C II] spectra and derived [12C II] optical depth in each velocity bin. Figure 2 shows the derived optical depth (τ12) with the error bars.

thumbnail Fig. 2.

Optical depth of the [C II] emission (τ12) when assuming 12C+/13C+ = 49 for each velocity bin in the three regions with enhanced [13C II]. Asterisks indicate the derived τ12 together with the error bars. Blue lines show the [C II] emission profiles.

4. Discussion

4.1. Line profiles and optical depth

At the N159 W [C II] peak (position 1), N160 CO peak (position 4), and possibly N160 A (position 5) the [13C II] spectra show an enhancement over the scaled [12C II] spectra (Fig. 1), which indicates that either the [12C II] line is optically thick or the isotopic ratio is lower than 49. As discussed in the following, the former is more likely because (1) the enhancement varies over different velocity bins, while it is reasonable to assume that different velocity components have the same isotopic ratio; (2) the [O I] 63 μm profile indicates self-absorption at the velocity where [13C II] shows an enhancement (position 1 and 4); or (3) the peak velocity of the [13C II] profile is consistent with the peak velocity of the [C I] 492 GHz and 13CO(3–2) lines (position 5).

At the N159 W [C II] peak (position 1) there are two velocity components in the [12C II] spectra (around 231 km s−1 and 240 km s−1). For the stronger component around 231 km s−1, the [13C II] intensity is larger than the scaled [12C II] intensity, while the second velocity component around 240 km s−1 does not indicate an enhanced [13C II] emission; the peak intensity of the scaled [C II] intensity for this velocity component is consistent with the noise level of [13C II]. Since a variation of 12C/13C within a physical scale of a few pc (14″ of the beam size corresponds to 3.4 pc at the distance of the LMC; 50 kpc) has not been reported, we assume that the two velocity components have the same isotopic ratio, and attribute the difference in the [12C II]/[13C II] ratios between the two velocity components to the difference in their optical depths. The [O I] 63 μm emission indicates self-absorption around 231 km s−1, but none around 240 km s−1. This is consistent with [12C II] being optically thick only for the 231 km s−1 component.

The line ratio at the velocity bin of the [C II] peak of position 1 translates into (Fig. 2), and a few velocity bins around the peak indicate an optical depth of around unity. These values are similar to those of M 43 and Horsehead measured by Guevara et al. (2019) and the mean value in the Orion molecular cloud (1.3, Goicoechea et al. 2015), and are somewhat lower than that of the Orion Bar (∼3, Ossenkopf et al. 2013) and the bright shell confining Orion’s Veil bubble (3.5, Pabst et al. 2019).

The [13C II] profile is narrower than the [12C II] line for the velocity component around 231 km s−1. A single Gaussian fit gives a width of 3.6 km s−1 for [13C II] and 6.8 km s−1 for [12C II]. This difference is larger than expected from theoptical depth broadening. For a Gaussian velocity distribution and a line-center optical depth of , the increase in the line width can be approximated as (Ossenkopf et al. 2013). For , the optical depth broadens the line only by 20%. The excess of the measured [12C II] broadening compared to the optical depth broadening is consistent with the picture in Okada et al. (2019); our beam includes several PDR components that are spatially separated and/or are in different physical phases, and each component contributes to a certain velocity range in the observed line profiles depending on their dynamics. Individual components have different [C II] optical depths, and we see the enhancement of [13C II] only for the components with a significant [C II] optical depth. Thus, [13C II] is much narrower than [12C II]. From the dataset presented in Okada et al. (2019), we extracted the [C I] 492 GHz, CO(4–3), and 13CO(3–2) spectra at this position with 0.5 km s−1 velocity resolution and 20″ (for 13CO(3–2)) or 16″ (for two other lines) spatial resolution, and confirm that the line widths are also similar to that of [13C II] (Fig. A.4).

At the N160 CO peak (position 4) the [13C II] intensity is also higher than the scaled [12C II] intensity, and the [O I] 63 μm profile indicates a large optical depth at the peak velocity of the [12C II] line, although it is less clear than position 1. The derived τ12 at a few velocity bins around the peak is around unity. At N160 A (position 5) we do not see a clear enhancement of [13C II] compared to the scaled [12C II] at the peak velocity of [12C II], but one might be present around 235–237 km s−1. When fitting a single Gaussian, the central velocity is at 238.5 km s−1 for [12C II] and 237.4 km s−1 for [13C II], and the fit to [C I] 492 GHz and 13CO(3–2) gives a closer central velocity (237.5 km s−1 and 237.9 km s−1, respectively; see also Fig. A.4). This supports the suggestion that the [13C II] enhancement around 235–237 km s−1 is real. Figure 2 shows that the velocity bins around the [C II] peak indicates an optically thin [C II] emission, but τ12 of 1 to 4 is suggested at the blue wing (around 235 km s−1). At the other two positions (2 and 3), [13C II] is marginally detected, and it is consistent with optically thin [C II] emission within the noise level.

4.2. Isotopic ratio

The estimate of the optical depth of the [12C II] line is based on an assumption of the isotopic ratio 12C/13C, which is not as well studied in the LMC as in our Galaxy. Wang et al. (2009) obtain 12C/13C = 49 ± 5 in N113 in the LMC, which is presumably the most accurate carbon isotope ratio determined for the LMC because they use optically thin lines. It is consistent with previous measurements in other regions in the LMC: for N159 (Johansson et al. 1994) and 35 ± 21 for 30 Dor-10 (Heikkilä et al. 1999). In the Galaxy the 12C/13C ratio increases along the Galactocentric distance (Wilson & Rood 1994). The value in the LMC is lower than the value for the local ISM in the solar neighborhood and is close to the values in the inner Galaxy (Wilson & Rood 1994), which is inconsistent with a pure metallicity dependence (Wang et al. 2009). 13C is a secondary nuclear fusion product, which is converted from 12C at the red giant stage (Wilson 1999), and ejected into the ISM with a time delay. Therefore, a low 12C/13C can be explained by old stellar populations in the LMC (Wang et al. 2009).

In Fig. 3 we estimated α+ = 12C+/13C+ when assuming [12C II] is optically thin for each velocity bin at the three positions discussed in Sect. 4.1. As discussed above, we assume that α+ is constant over different velocity components. At positions 1 (N159 W) and 5 (N160 A), a value of α+ of 20–30 over the whole velocity range is not excluded when taking into account the noise level, but we do see a systematic trend across the velocity: a dip in the estimated α+ around the [C II] peak at position 1 and a gradient at 235–240 km s−1 at position 5. In addition, the estimated α+ of 20–30 is lower than the measured 12C/13C in N159 (Johansson et al. 1994). Therefore, the scenario of optically thick [12C II] discussed in Sect. 4.1 is more likely.

thumbnail Fig. 3.

Estimate of α+ = 12C+/13C+ when assuming [12C II] is optically thin. Asterisks indicate the derived α+ together with the error bars. Blue lines show the [C II] emission profiles.

4.3. Fractionation

Because the fractionation reaction from 13C+ to 12C+ is an exothermal reaction (the exothermicity is 35 K; Langer et al. 1984), 13C+ tends to be underabundant with respect to 12C+ at low temperatures, namely at higher AV in a PDR. Röllig & Ossenkopf (2013) calculated the fractionation in the clump integrated intensity of [12C II] and [13C II], and showed that only models with low UV fields (χ ≤ 100) and high densities (n ≥ 105 cm−3) trace the chemical fractionation, which would result in α <  α+, and that the derived [12C II] optical depth is a lower limit. On the other hand, it is unlikely that the fractionation is significant in the regions in this study because the estimated PDR properties are either low density and low UV field, or high density and high UV field (Okada et al. 2019).

4.4. Metallicity effect

Model predictions do not indicate a clear metallicity dependence of the [C II] optical depth. For a high density PDR clump where the C+ layer consists of a thin surface, the surface brightness of the [C II] emission is almost constant with the metallicity (Röllig et al. 2006) because the surface brightness is proportional to the thickness of the layer and the density of C+ ions, with the former being proportional to the inverse of the metallicity due to the dust extinction and the latter being proportional to the metallicity as a first-order approximation. The derived optical depths in the LMC regions are similar to those of some Galactic regions (M 43 and Horsehead measured by Guevara et al. 2019); however, this study does not provide enough statistics to conclude that we do not see a metallicity dependence in the [C II] optical depth.

4.5. Implications for the interpretation of the [C II] intensity

The impact of the optical depth effect depends strongly on the actual geometry of the sources. Most PDR models compute the [C II] intensity emerging from the surface of an externally illuminated clump or a face-on plane-parallel structure. For this geometry the radiative transfer computation takes the optical depth correctly into account. However, when the [C II] emission stems from a mixture of components including surfaces illuminated from the back (negative temperature gradient toward the observer) or an ensemble of clumps we can no longer simply sum up the intensity from all surfaces. Instead, we would have to run a full three-dimensional radiative transfer computation (e.g., Andree-Labsch et al. 2017). It makes a significant difference when many clumps overlap along the line of sight, even when the optical depth of each clump is about unity. The interpretation of the optically thick [C II] emission in terms of classical PDR model predictions will result in incorrect parameters.

To assess the impact of the optical depth on interpretations of the [C II] emission in distant galaxies, we averaged over the available large maps in Orion (Pabst et al. 2019) and M 17 (Guevara et al., in prep.) to derive an averaged optical depth of [12C II]. At the distance of LMC, the map sizes of about one degree (7.2 pc at Orion) and 2.5′ (1.4 pc at M 17) would correspond to 30″ and 6″, respectively. The average optical depths are ∼1 and ∼3, respectively. These values are lower than the results from smaller areas (Pabst et al. 2019; Guevara et al. 2019), but still moderately optically thick. To obtain representative values for other nearby galaxies we would need to average much larger areas. These data are not yet available; however, our study indicates that the [C II] emission in distant galaxies can have an optical depth of about unity, which leads to an underestimate of the [C II] intensity by a factor two when assuming an optically thin scenario.

5. Summary

We detected [13C II] F = 1−0 and F = 1−1 emissions in N159 and N160 in the LMC for the first time. Assuming an isotopic ratio 12C+/13C+ of 49, the optical depth of [12C II] is estimated as 1–3 at the peak velocities. Although the possibility of an optically thin [12C II] emission with a lower isotopic ratio is not quantitatively excluded, the fact that two velocity components in N159 have different intensity ratios of [12C II]/[13C II], a narrower line profile of [13C II] than [12C II], and a self-absorption of the [O I] 63 μm at the peak of the [13C II] line favor an interpretation with an optically thick [12C II] emission. This study indicates that the [C II] intensity in distant galaxies can be underestimated by about a factor of 2.

1

For example, the peak optical depth is 0.8 using an extinction cross section from Draine (2003) for RV = 4, a carbon abundance of 1.6  ×  10−4 (Sofia et al. 2004), an excitation temperature of 50 K, and the line width of 1.5 km s−1.

2

GREAT is a development by the MPI für Radioastronomie and the KOSMA/Universität zu Köln, in cooperation with the MPI für Sonnensystemforschung and the DLR Institut für Planetenforschung.

Acknowledgments

This work is based on observations made with the NASA/DLR Stratospheric Observatory for Infrared Astronomy (SOFIA). SOFIA is jointly operated by the Universities Space Research Association, Inc. (USRA), under NASA contract NAS2-97001, and the Deutsches SOFIA Institut (DSI) under DLR contract 50 OK 0901 to the University of Stuttgart. This work is carried out within the Collaborative Research Centre 956, sub-project A4 and C1, funded by the Deutsche Forschungsgemeinschaft (DFG) – project ID 184018867.

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Appendix A: Additional figures

thumbnail Fig. A.1.

Footprints of the LFA seven pixels in the [13C II] observations in N159 overlaid with integrated intensities of [C II] (color) and CO(4–3) (contours) (Okada et al. 2019). The labeled numbers correspond to the position IDs in Table 1.

thumbnail Fig. A.2.

Same as Fig. A.2, but for 30 Dor.

thumbnail Fig. A.3.

Same as Fig. A.1, but for N160.

Figures A.1–A.3 present the observed positions of the LFA seven pixels overlaid on the [C II] and CO(4–3) maps in three regions. Five positions where [13C II] is detected and discussed in this study are indicated.

Figure A.4 shows the normalized spectra of CO(4–3), 13CO(3–2), and [C I] 492 GHz obtained from the dataset presented in Okada et al. (2019) together with the [12C II] and [13C II] emissions in this study. The 13CO(3–2) spectra are extracted with the spatial resolution of 20″, the CO(4–3) and [C I] 492 GHz spectra are extracted with the spatial resolution of 16″ at individual positions where the deep [13C II] observations were executed.

thumbnail Fig. A.4.

Normalized spectra of CO(4–3), 13CO(3–2), and [C I] 492 GHz (Okada et al. 2019) together with the [12C II] and [13C II] emissions in this study at three positions with enhanced [13C II]. 13CO(3–2) has a spatial resolution of 20″, and CO(4–3) and [C I] 492 GHz have a spatial resolution of 16″.

All Tables

Table 1.

Summary of the pointed observations for the [13C II] line.

In the text

All Figures

thumbnail Fig. 1.

Spectra of [13C II], [12C II], and [O I] at selected positions in the LMC. Left: [12C II] spectra (blue) and [O I] 63 μm spectra when available (green). The vertical lines aid the comparison of the velocities of the line profiles. Middle two panels: [13C II] F = 1−0 and F = 1−1 spectra (red) and [C II] spectra (blue) scaled for optically thin emission and 12C+/13C+ = 49. The horizontal lines indicate the rms noise of the baseline. Right: combined [13C II] spectra (red) and the scaled [C II] spectra (blue). See text for the formula of the combined [13C II] spectra and the scaled [C II] spectra.
In the text
thumbnail Fig. 2.

Optical depth of the [C II] emission (τ12) when assuming 12C+/13C+ = 49 for each velocity bin in the three regions with enhanced [13C II]. Asterisks indicate the derived τ12 together with the error bars. Blue lines show the [C II] emission profiles.
In the text
thumbnail Fig. 3.

Estimate of α+ = 12C+/13C+ when assuming [12C II] is optically thin. Asterisks indicate the derived α+ together with the error bars. Blue lines show the [C II] emission profiles.
In the text
thumbnail Fig. A.1.

Footprints of the LFA seven pixels in the [13C II] observations in N159 overlaid with integrated intensities of [C II] (color) and CO(4–3) (contours) (Okada et al. 2019). The labeled numbers correspond to the position IDs in Table 1.
In the text
thumbnail Fig. A.2.

Same as Fig. A.2, but for 30 Dor.
In the text
thumbnail Fig. A.3.

Same as Fig. A.1, but for N160.
In the text
thumbnail Fig. A.4.

Normalized spectra of CO(4–3), 13CO(3–2), and [C I] 492 GHz (Okada et al. 2019) together with the [12C II] and [13C II] emissions in this study at three positions with enhanced [13C II]. 13CO(3–2) has a spatial resolution of 20″, and CO(4–3) and [C I] 492 GHz have a spatial resolution of 16″.
In the text

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