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Home All issues Volume 542 (June 2012) A&A, 542 (2012) L22 Full HTML
Issue
A&A
Volume 542, June 2012
GREAT: early science results
Article Number L22
Number of page(s) 4
Section Letters
DOI https://doi.org/10.1051/0004-6361/201218935
Published online 10 May 2012

© ESO, 2012

1. Introduction

IC 342 is a gas-rich spiral galaxy with active star formation in its nucleus. IC 342 is located behind the Galactic plane und is therefore highly obscured; the distance to the nearly face-on galaxy is still debated. Tikhonov & Galazutdinova (2010) give a distance of 3.9 ± 0.1 Mpc derived from stellar photometry. Observations of planetary nebulae (Herrmann et al. 2008) and cepheids (Saha et al. 2002) give distances of 3.4 ± 0.2 Mpc.

Within its central 30′′ two molecular arms of a mini-spiral end in a clumpy central ring of dense gas, which surrounds a young star cluster. Downes et al. (1992) showed the presence of five giant molecular clouds, A to E, around the nucleus of IC 342 with masses of  ~106 M⊙. This structure can be seen in Fig. 1, which shows a map of line-integrated 12CO(1–0) emission observed with BIMA (Berkeley Illinois Maryland Association). The central molecular ring surrounds a nuclear star cluster with active star formation and strong far-ultraviolett (FUV) radiation, illuminating the molecular ring and producing photo-dissociation regions (PDRs) on the side facing the central cluster. Energetiv FUV photons dissociate and ionize molecules and atoms in the gas and effectively heat the gas and dust via photoelectric heating. Consequently, PDRs strongly emit radiation from species that are abundant and excited under these conditions, like [C II]. Meier & Turner (2005) find that the chemistry in the ring is a mixture of PDR gas and regions of denser, more shielded material. Especially the GMC complex B appears to show properties of PDRs produced by the surrounding starburst environment (Downes et al. 1992; Israel & Baas 2003). The spatial sizes of the central GMCs and the infrared luminosity of the inner 400 pc of IC 342 are both similar to the center of our Galaxy. Especially the relative contribution of the diffuse material to the overall [C II] emission remains unknown but it appears that the PDRs remain largely confined to the central ring.

Detailed knowledge of the H2 column densities, excitation temperatures, and densities in IC 342 exists from CO and its isotopomers as a basis for comparison with the [C II] lines (e.g. Ishizuki et al. 1990; Downes et al. 1992; Turner & Hurt 1992; Wright et al. 1993; Turner et al. 1993; Meier et al. 2000; Meier & Turner 2001; Israel & Baas 2003; Meier & Turner 2005). Comparison with this complementary data, particularly with their spectral shape, allows us to distinguish different kinematic components from the [C II] lines and to assess the interaction of the star formation activity on the gas.

In this work we present velocity-resolved spectra of ionized atomic ([C II] 158 μm) gas, with spatial resolutions of  ~156. The observations were performed with the German REceiver for Astronomy at Terahertz frequencies (GREAT1, Heyminck et al. 2012) on board the Stratospheric Observatory For Infrared Astronomy (SOFIA).

In Sect. 2 we describe the observations. The spectra are presented in Sect. 3. The analysis of the data and ambient conditions are discussed in Sect. 4, and conclusions are drawn in Sect. 5.

thumbnail Fig. 1

Line-integrated map of the 12CO(1–0) transition from the BIMA-SONG sample. The black dots denote the postitions of our two [C II] detections in IC 342. The positions correspond to giant molecular clouds C and E. The (0, 0) position corresponds to (RA, Dec) (J2000) (03:46:48.5 68:05:47).

2. Observations

We used the dual-channel receiver GREAT on SOFIA in September 2011 to perform pointed observations of the [C II] 158 μm fine-structure transition of C+ at 1900.536900 GHz and the 12CO(11–10) transition at 1496.922909 GHz near the center of IC 342. The observations were performed in dual beam-switch mode (chop rate 1 Hz) toward selected positions, see Table 1 for details. The chopper offset was 100′′ with a chopper angle of 110 degrees against north (counter-clockwise). The observations were taken during the transfer flight from the US to Europe. Owing to unknown technical errors on the flight we had to discard one of the observed positions.

The center position is RA, Dec (J2000) 03:46:48.5 68:05:47. We observed 2 GMCs at the following offsets in arcsecond: GMC C (+3.6, +3.6), and GMC E (−5.5,  −3.7) using the designation by Downes et al. (1992) (see also Fig. 1). The integration times were 9.3, and 3.7 min, respectively. [C II] emission was detected at both positions. We did not detect any 12CO(11–10) emission. The rms of the baseline for position C and E was 0.10 and 0.21 K km s-1 , respectively, with a channel width of 4 km s-1.

We used a fast Fourier transform spectrometer (FFTS), with 8192 channels providing 1.5 GHz bandwidth and about 212 kHz of spectral resolution. Calibration was performed with the standard pipeline (Guan et al. 2012). Using the beam efficiency ηc ≈ 0.51 and the forward efficiency (ηf) of 0.95 (Heyminck et al. 2012), we converted all data to line brightness temperature scale, . The reduction of these calibrated data, as well as the maps shown throughout the paper, were made with the GILDAS2 package CLASS90. The pointing was established with the optical guide cameras to about 1′′ precision, and was found to have deviated by about 3–5′′ after 20 min.

3. Results

thumbnail Fig. 2

Averaged [C II] spectra (dark gray, filled) toward the GMC C (top), and GMC E (bottom) positions. The 12CO(2–1) spectra are divided by 2, the [CI]  and 13CO spectra are multiplied with 2.

In Fig. 2 we show the [C II] emission observed with GREAT (dark gray, filled) for both positions. On each spectrum we overlay 12CO(1–0) data from BIMA-SONG3 (Helfer et al. 2003), as well as 12CO(2–1), 12CO(3–2), 12CO(4–3), 13CO(2–1), 13CO(3–2), and [CI]  spectra from Israel & Baas (2003). Because of the different beam sizes, gridding, and map coverage it was not always possible to regrid and convolve all spectra to the positions and beam sizes of the GREAT observations.

Where possible, we smoothed the other data sets to the 15′′ resolution of our [C II] line observations. We kept the 12CO(2–1) and 13CO(2–1) spectra on their native resolution of 22′′ and 23′′. We kept 12CO(3–2) and 13CO(3–2) on their native resolution of 15′′. 12CO(1–0), 12CO(4–3), and [C I]3P1–3P0 spectra were smoothed to the resolution of our [C II] data and interpolated at the two positions C and E. For the remaining ground-based spectra at the positions C and E we averaged the data points closest to our [C II] observations.

The [C II] emission is strongest at position E. At both positions the different transitions show a good correlation of the central line velocities and line widths. The spectra at position C exhibit a consistent central velocity of 40 km s-1 and similiar line widths of about 50 km s-1 . Only the [C II] line has a broader line width and is blueshifted by a few km s-1 . Position E shows slight variations in the different spectral lines. All lines have a comparable line width of slightly less than 50 km s-1. Most lines appear to be centered around 24 km s-1 , except for 12CO(2–1) and (3–2) with slightly redshifted central velocities.

We performed a Gaussian fit for all lines at the two positions to determine their respective line parameters such as peak- and line-integrated intensity and , line width ΔV, and line center velocity v0. For position C we fitted a single-component Gaussian to the data without any additional constraints. The results are given in Table 1. At position E we already noted the slight asymmetry in several of the CO lines, possibly resulting from two different, spatially and kinematically unrelated gas components. We used the optically thinnest lines from our data set, the 13CO lines (Israel & Baas 2003), to specify the line center velocity of 24 km s-1 of the main component by fitting a single Gaussian to their profiles. To quantify the contributions of two separate components to the spectra, we fitted two separate Gaussian components to the spectra while keeping the line center velocity of the main component fixed to 24 km s-1 . Only for 12CO(4–3) and [C I] it was not possible to find a second line-component, therefore we performed an unconstrained single-component fit that we can attribute to the main component because of the comparable velocities. For 12CO(1–0) we fixed the velocity and width of the high-velocity component to 53 and 47 km s-1 , respectively, and derived the line parameters for the low-velocity component. The decomposition into two different components is also supported by the BIMA 12CO(1–0) data at 5.5′′ resolution, which shows a line center velocity v0 ≈ 50 km s-1 at our center position. Therefore, emission from the PDR-dominated gas close to the central cluster is expected at v0 ≈ 50 km s-1.

As an independant method to derive the [C II] line parameters, we also calculated the zeroth, first and second moment of the spectra, i.e. the integrated intensity, mean velocity and line width. The derived emission line parameters are summarized in Table 1 (moments are given as italic numbers).

The GMC E and B are both located southwest of the central position of IC 342 with an angular separation of only 3.5′′. Accordingly, the 15′′ [C II] beam will always pick up emission of both GMCs. The [C II] spectrum of position E shows the strongest emission from both positions most likely because of the already known PDR/starburst contribution from GMC B (Meier & Turner 2005). Position C is situated in the molecular ring, closest to the central cluster and will most likely contain contributions from dense molecular gas as well as from PDRs.

Israel & Baas (2003) showed the existence of strong velocity gradients in the central region of IC 342. To quantify what pointing shift could alternatively mimic the observed line shift, we convolved the BIMA 12CO (1-0) data to 15′′. We found that a pointing offset of 7 ′′, significantly higher than the quoted accumulative pointing accuracy after 20 min, exactly in the direction of the central cluster could explain the [C II] line profile. Therefore, it is unlikely, though it cannot be fully excluded, that a systematic pointing error may have caused the observed velocity shift.

Table 1

IC 342 line parameters derived from Gaussian fits.

4. Discussion

Table 2 lists line ratios of [C II] and [C I] to 12CO(1–0) and 12CO(2–1) and 12CO(4–3)/12CO(1–0) (if available, the numbers for position E are given in the order high/low velocity component). Stronger PDR emission will be reflect in a higher ratio of the fine structure lines to the molecular lines. Apparently, GMC E shows the strongest PDR contribution. Stacey et al. (1991) give a [C II]/12CO(1–0) ratio of 5000 (corrected for main beam efficiency and assuming unity beam filling). They cite [C II] intensities (beamsize 55′′) of 3 × 10-4 erg s-1 cm-2 sr-1, but only 40 K km s-1 for the 12CO(1–0) line intensity in a 60′′ beam. The [C II] and 12CO(1–0) data were taken using the Kuiper Airborn Observatory (KAO) and the Owens Valley Radio Observatory (OVRO), respectively. Given the larger beam sizes and unknown filling factors, their values are consistent with our data.

Stacey et al. (1991) showed that starburst galaxies have a high [C II]/12CO(1–0) ratio of  4100 while cooler, less active galaxies show much weaker ratios. Decomposition of the 12CO(1–0) data into a low- and high-velocity component at 24 and 53 km s-1 , respectively, gives [C II]/12CO(1–0) ratios of 600 and 4000, which indicates contributions from quiescent, weakly excited gas plus a strong, starburst-like contribution from the local PDRs.

The [C II] emitting level lies 91 K above the ground state and has a critical density of  ≳3500 cm-3. Assuming optically thin emission and a level population in the high-temperature, high-density limit, we can calculate a lower limit of the C+ column density from the integrated intensity (Crawford et al. 1985). The results are given in Table 2. The beam-averaged column densities have values between 0.9−1.6 × 1017 cm-2, relatively low numbers for massive PDRs. The source intrinsic column density will be accordingly higher for a compact source, lower than the beam: a beam filling factor of 1/10, not unlikely given the small size of the cores visible in the interferometric 12CO(1–0) map, will give a column density that is a factor of 10 higher. Column densities of 1−2 × 1017 cm-2 and a [C+]/[H] ratio of 10-4 gives a total mass in the 15′′ beam of 5−10 × 106 M⊙ at a distance of 3.9 Mpc. Israel & Baas (2003) give comparable central masses of IC 342 of 5−7 × 106   M⊙, given the uncertainties in the relative abundance of ionized carbon, filling factors, and distance.

Table 2

Line ratios and estimated column densities of the observed GMCs.

To confine the local gas parameters we used the KOSMA-τ PDR model code (Störzer et al. 1996; Röllig et al. 2006) to model the emission of an ensemble of clumpy PDRs in a beam of 15′′ (Cubick et al. 2008). We fitted the model to the absolute intensities therefore making the total model gas mass sensitive to the distance of the PDR. As a result from the fit we receive the mean gas density in the beam, the total gas mass, and the FUV intensity that illuminates the PDRs in units of the Draine field (Draine 1978). The derived parameters for all positions are given in Table 2 assuming a distance of 3.9 Mpc. Employing a shorter distance will reduce the total gas mass estimates accordingly.

To perform a consistent fit across both positions we excluded the 13CO lines from the fitting. Note that the 13CO lines show relative weak emission, which requires lower model densities of  ~1000 cm-3 for both positions. Without fitting to the 13CO lines the densities are higher by a factor of a few. Furthermore, the [C II] and [C I] line intensities relative to the CO emission can only be explained by densities below 104 cm-3 and low FUV fields. Note that the model computations assume solar metallicities, while Engelbracht et al. (2008) give somewhat higher values for IC 342. Higher metallicities will lead to lower [C II]/CO ratios.

The fit results from GMC E support the decomposition into two separate gas components. The lower velocity of 24 km s-1 would be consistent with assuming that the emission originates in the trailing, spiral arm region, which is connected to the molecular ring. The emission of the high-velocity component resembles the PDR signature of a starburst/PDR environment with mean FUV fields of 250–300 (in units of the Draine field) and densities of 104 cm-3. If we assume that the observations at position E were mispointed (see discussion in the previous section), we derive the following PDR model parameters: ⟨n⟩ = 1000 cm-3, Mtot = 19 × 106 M⊙ and χ = 5. These gas parameters are in conflict with a scenario where a PDR/starburst dominates at the center of the galaxy.

5. Conclusions

We used the dual-band heterodyne receiver GREAT on board the airborne telescope SOFIA to observe two giant molecular clouds situated around the nucleus of IC 342 in the 12CO J = 11 → 10 transition and the [C II] 158 μm fine-structure line. We detected [C II] emission at both positions but could not detect any 12CO J = 11 → 10 emission.

The new SOFIA/GREAT spectra reveal a spectral distribution of the [C II] emission that follows the distribution of the neutral and molecular gas, [C I] and CO. The [C II] spectrum observed at the position of GMC E shows two velocity components, a high-velocity component that we attribute to emission from a PDR/starburst region in the molecular ring close to the central cluster with densities of 104 cm-3, FUV field of 250–300 and a total mass of 2 × 106   M⊙ and a cooler, low-velocity component with densities of 2 × 103 cm-3, FUV fields of a few and a total mass 7–8 times higher than the starburst component. The model for GMC C gives model parameters of densities of 5 × 103 cm-3, FUV field of 7 and a total mass of 2 × 107   M⊙.

Despite the challenges that one might expect in an early transfer flight of SOFIA to Germany, we were able to deduce important astrophysical results primarily owing to the high spectral resolution available with the GREAT receiver. These data demonstrate the promise of the GREAT/SOFIA facility for future work, such as detailed mapping of the [C II] emission from the central regions of IC 342 and other nearby galaxies.

1

GREAT is a development by the MPI für Radioastronomie and cooperation with the MPI für Sonnensystemforschung and the DLR Institut für Planetenforschung.

Acknowledgments

We thank the SOFIA engineering and operations teams, whose tireless support and good-spirit teamwork has been essential for the GREAT accomplishments during Early Science, and say Herzlichen Dank to the DSI telescope engineering team. Based [in part] on observations made with the NASA/DLR Stratospheric Observatory for Infrared Astronomy. SOFIA Science Mission Operations are conducted jointly by the Universities Space Research Association, Inc., under NASA contract NAS2-97001, and the Deutsches SOFIA Institut under DLR contract 50 OK 0901. The research presented here was supported by the Deutsche Forschungsgemeinschaft, DFG through project number SFB956C.

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All Tables

Table 1

IC 342 line parameters derived from Gaussian fits.

In the text
Table 2

Line ratios and estimated column densities of the observed GMCs.

In the text

All Figures

thumbnail Fig. 1

Line-integrated map of the 12CO(1–0) transition from the BIMA-SONG sample. The black dots denote the postitions of our two [C II] detections in IC 342. The positions correspond to giant molecular clouds C and E. The (0, 0) position corresponds to (RA, Dec) (J2000) (03:46:48.5 68:05:47).
In the text
thumbnail Fig. 2

Averaged [C II] spectra (dark gray, filled) toward the GMC C (top), and GMC E (bottom) positions. The 12CO(2–1) spectra are divided by 2, the [CI]  and 13CO spectra are multiplied with 2.
In the text

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