[C II] 158 μm and [N II] 205 μm emission from IC 342. Disentangling the emission from ionized and photo-dissociated regions

A&A 591, A33 (2016)

DOI:10.1051/0004-6361/201526267 c

ESO 2016

Astronomy

&

Astrophysics

[C ^II ] 158 µ m and [N ^II ] 205 µ m emission from IC 342

Disentangling the emission from ionized and photo-dissociated regions

M. Röllig¹, R. Simon¹, R. Güsten², J. Stutzki¹, F. P. Israel³, and K. Jacobs¹

1 I. Physikalisches Institut der Universität zu Köln, Zülpicher Straße 77, 50937 Köln, Germany e-mail: roellig@ph1.uni-koeln.de

2 Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany

3 Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands Received 7 April 2015/ Accepted 22 March 2016

ABSTRACT

Context. Atomic fine-structure line emission is a major cooling process in the interstellar medium (ISM). In particular the [CII] 158 µm line is one of the dominant cooling lines in photon-dominated regions (PDRs). However, it is not confined to PDRs but can also originate from the ionized gas closely surrounding young massive stars. The proportion of the [CII] emission from HIIregions relative to that from PDRs can vary significantly.

Aims.We investigate the question of how much of the [CII] emission in the nucleus of the nearby spiral galaxy IC 342 is contributed by PDRs and by the ionized gas. We examine the spatial variations of starburst/PDR activity and study the correlation of the [C^II] line with the [NII] 205 µm emission line coming exclusively from the HIIregions.

Methods. We present small maps of [CII] 158 µm and [NII] 205 µm lines recently observed with the GREAT receiver on board SOFIA.We present different methods to utilize the superior spatial and spectral resolution of our new data to infer information on how the gas kinematics in the nuclear region influence the observed line profiles. In particular we present a super-resolution method to derive how unresolved, kinematically correlated structures in the beam contribute to the observed line shapes.

Results.We find that the emission coming from the ionized gas shows a kinematic component in addition to the general Doppler signature of the molecular gas. We interpret this as the signature of two bi-polar lobes of ionized gas expanding out of the galactic plane. We then show how this requires an adaptation of our understanding of the geometrical structure of the nucleus of IC 342.

Examining the starburst activity we find ratios I([CII])/I(¹²CO(1−0)) between 400 and 1800 in energy units. Applying predictions from numerical models of HIIand PDR regions to derive the contribution from the ionized phase to the total [CII] emission we find that 35−90% of the observed [CII] intensity stems from the ionized gas if both phases contribute. Averaged over the central few hundred parsec we find for the [CII] contribution a HII-to-PDR ratio of 70:30.

Conclusions.The ionized gas in the center of IC 342 contributes more strongly to the overall [CII] emission than is commonly observed on larger scales and than is predicted. Kinematic analysis shows that the majority of the [CII] emission is related to the strong but embedded star formation in the nuclear molecular ring and only marginally emitted from the expanding bi-polar lobes of ionized gas.

Key words. galaxies: ISM – galaxies: individual: IC 342 – radio lines: galaxies – radio lines: ISM – galaxies: starburst

1. Introduction

The [CII] 158 µm emission line is one of the strongest cooling lines in the interstellar medium (ISM) as long as most of the carbon exists as C⁺. This is true for the ionized phase, e.g. in HIIregions, as well as in the outer regions of molecular clouds, in so-called photo-dissociation regions (PDR). For PDRs this is particularly interesting because this line, owing to its not too high optical depths, traces almost the entire carbon content of a molecular cloud. Spatially, the [CII] emission of a PDR origi- nates from parts that are CO-dark. Consequently, it should also trace the fraction of molecular hydrogen gas that is spatially not coexistent with CO and therefore complements the standard CO-H2correlation in regions of high UV flux. The [CII] line also carries important information on the energetic state of the cloud.

Unfortunately, it is almost impossible to observe a PDR without

? The spectra as FITS files are only available at the CDS via anonymous ftp tocdsarc.u-strasbg.fr(130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/591/A33

picking up contributions from the accompanying HII region.

Discriminating between [CII] emission coming from the HIIre- gion and coming from the PDR is not easy, but crucial because a HII-pollution of the [CII] signal could significantly affect the conclusion of any emission line analysis.Abel(2006) presented numerical calculations of models of ionized and PDR gas show- ing that up to 50% of a detected [CII] line intensity can come from the HIIregion. One suggestion to clean a [CII] signal from HIIcontamination is to compare it with emission lines that are exclusively produced in the ionized gas, such as [NII] emission lines. Atomic nitrogen has an ionization potential of 14.53 eV, prohibiting N⁺ production below the Lyman edge. [NII] emis- sion is therefore only produced in the HIIregion and because of the comparable excitation conditions and critical densities of C⁺ and N⁺it should be an excellent tracer of [CII] emitted from the HIIregion.

IC 342, a face-on spiral galaxy at a distance of 3.9 ± 0.1 Mpc (Tikhonov & Galazutdinova 2010), has a nuclear region with active star formation. Downes et al. (1992) showed that five

giant molecular clouds (GMC) with masses of ∼10⁶ Mare sur- rounding a young central star cluster in a ring of dense molec- ular gas. Two molecular arms of a mini-spiral originate from the molecular ring, north and south of the galaxy center (see also Fig.1, left panel). The nuclear star cluster illuminates the molecular ring with intense far-ultraviolet (FUV) radiation pro- ducing photo-dissociation regions (PDRs) on the inner side fac- ing the central cluster. The nucleus of IC 342 shows a great similarity to the center of our Galaxy. In particular, the spa- tial size of the central GMCs as well as the infrared luminos- ity of the central few hundred pc of IC 342 are comparable to the Milky Way. In an earlier paper (Röllig et al. 2012) we pre- sented early Stratospheric Observatory For Infrared Astronomy (SOFIA, Young et al. 2012) observations of the [CII] 158 µm fine-structure transition of C⁺ at 1900.536900 GHz and the

12CO(11−10) transition at 1496.922909 GHz at two GMCs in the central molecular ring of IC 342. Using KOSMA-τ PDR model calculations (Störzer et al. 1996;Röllig et al. 2006,2013) we were able to distinguish between a strong PDR/star-burst emission in the southern GMC E and the much more quiescent conditions in the cooler and denser GMC C in the northern arm, confirming the findings ofMeier & Turner(2005).

However, the relative contribution of the diffuse material to the overall [CII] emission was unknown. With the upgraded spectral capabilities of the German REceiver for Astronomy at Terahertz frequencies (GREAT¹, Heyminck et al. 2012) re- ceiver on SOFIA, we are for the first time able to investigate the [NII] 205 µm and [CII] 158 µm fine-structure emission of the ionized material from HIIand PDRs simultaneously. The aim of this paper is to study how the relative contributions from these two ISM phases vary spatially.

2. Observations

We used the dual-channel receiver GREAT on SOFIA to per- form pointed observations close to the nucleus of IC 342. We used the L1/L2 GREAT configuration with the L1 channel tuned to the [NII] 205 µm fine-structure line [NII]³P1→³P0 (ν = 1461.13 GHz) and the L2 channel tuned to the [CII] 158 µm fine- structure line [CII]³P3/2→³P1/2(ν= 1900.5369 GHz). For the rest of this paper [NII] will always refer to the 205 µm line. The observations were done in dual beam-switch mode (chop rate 1 Hz; the chop throw was 100⁰⁰ both for [CII] and [NII], at an angle of 20 deg counterclockwise from the RA axis. ) toward selected positions centered around the nucleus of IC 342 on a half-beam sampled 7⁰⁰ grid. We do not see any signs of self- chopping in our data, but the 100⁰⁰chop throw does not exclude possible weak contamination of the two off-source positions. We made sure not to chop onto the spiral arms, but hardware lim- itations did not allow us to chop out of the galaxy. The cen- ter positions for all observations is RA, Dec (J2000) 03:46:48.5 68:05:47 (offset: 0⁰⁰, 0⁰⁰). We observed a rectangular 3 × 3 grid centered around the (0⁰⁰, 0⁰⁰) position plus an additional point- ing at (0⁰⁰, −14⁰⁰). The observations took place in February 2014 during three flights. In total we present data for ten positions.

The total observing time per position is between 2.5 and 7.5 min on-source, T_sys(SSB) varied between about 1300 K and 1400 K for [NII] and between 1900 K and 3200 K for the [CII] line de- pending on the date of observations.Heyminck et al.(2012) de- scribed the overall pointing accuracy as a combination of the

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

accuracy of the boresight determination (within 1−2⁰⁰) and the stability during flight, controlled with the optical guide cam- eras to 3−5⁰⁰. However, since then the pointing accuracy has im- proved considerably. Pointing instabilities due to drifts no longer occur and the total pointing accuracy (boresight determination and optical camera) is now below 1⁰⁰. Therefore, it is unlikely that a systematic pointing error may have contaminated the ob- served line profiles.

We used a fast Fourier transform spectrometer (XFFTS, Klein et al. 2012) with 32768 channels. The XFFTS provides 2.5 GHz bandwidth and about 88.5 kHz spectral resolution.

The data were converted to line brightness temperature T_B = ηf × T_A^∗/ηc applying a beam efficiency ηc ≈ 0.67(L1) and 0.65(L2) and a forward efficiency (ηf) of 0.97. Baselines were corrected with polynomials up to the fourth order. The reduction of these calibrated data were made with the GILDAS² package CLASS90. The data analysis and most of the figures in this paper were made using Mathematica³. In this paper we use integrated line intensities in units of energy, [I] = erg s⁻¹cm⁻² sr⁻¹, and temperature,hR

Tdvi = K km s⁻¹, as is common in the literature.

The conversion between the two is achieved with the following formula:R

Idν erg s⁻¹cm⁻²sr⁻¹ = ^2kν_c3³

R Tdv K km s⁻¹. When discussing intensity ratios we will specify the underlying inten- sity units.

3. Data overview

All [CII] and [NII] spectra were smoothed to a spectral resolu- tion of 2 and 4 km s⁻¹, respectively. The baseline noise RMS is between 21 and 62 mK for [NII] and between 42 and 98 mK for [CII]. We present the [CII] and [NII] in their native spatial res- olution of 14⁰⁰and 18.3⁰⁰, respectively, in Fig.1. The 3 × 3 grid covers the nucleus of IC 342, while the (0⁰⁰, −14⁰⁰) position cov- ers a position off the southern mini-spiral arm.

The [CII] emission is strongest at (0⁰⁰, −7⁰⁰) and weakest at the positions (−7⁰⁰, 7⁰⁰) and (0⁰⁰, −14⁰⁰) off the spiral arm. The positions (0⁰⁰, 0⁰⁰) and (+7⁰⁰, 0⁰⁰) are about 30% weaker than the strongest [CII] position. The line shape is Gaussian to a good de- gree. Overall the [CII] emission follows the¹²CO(1−0) emission showing a correlation between molecular gas and PDR.

The [NII] emission is weaker than [CII], between 1/3 and 1/10 at the peak level, and has a lower signal-to-noise ratio than the stronger [CII] signal. Comparing the [CII] and [NII] spectra in Fig.1we note that [NII] shows a slightly broader line width than [CII]. This is not surprising given the very different physical conditions in HIIregions compared to PDRs/GMCs. Generally, the line centers of [NII] are in good agreement with [CII] with a recognizable shift of ∼8 km s⁻¹at (0⁰⁰, 7⁰⁰) and (−7⁰⁰, 0⁰⁰).

To characterize the overall emission of the nucleus of IC 342, we averaged all spectra from the central 3 × 3 grid of observed positions. Each spectrum was equally weighted. Figure2shows the resulting averaged spectra of [CII] and [NII]. We fitted Gaus- sian line profiles to both average spectra. For [CII] the peak in- tensity is 670 ± 7 mK, for [NII] we find 102 ± 3 mK. Both lines have similar central velocities v0 of 36.6 ± 0.3 km s⁻¹ ([CII]) and 37.0 ± 1.0 km s⁻¹ ([NII]). The average [CII] line is nar- rower (FWHM) than the [NII] line: 66.6 ± 0.8 km s⁻¹vs. 79.8 ± 2.4 km s⁻¹. To summarize, the averaged [NII] emission is slightly redshifted with respect to the [CII] line and shows a line profile

2 http://www.iram.fr/IRAMFR/GILDAS

3 Wolfram Research, Inc., Mathematica, Version 10.0, Champaign, IL (2014),http://www.wolfram.com

-20 -10

0 10

-20 0 20

RA offset (arcsec)

DECoffset(arcsec)

A B C

D

E

Arm Off-Arm

Central trough

Fig. 1.Line-integrated map of the¹²CO(1−0) transition from the BIMA-SONG sample. The yellow points show the observed GREAT positions in IC 342. The (0, 0) position corresponds to (RA, Dec) (J2000) (03:46:48.5 68:05:47). The red points indicate named GMCs and other structures according to Table 3 inMeier & Turner(2001). Triangles facing up and right indicate HIIregions and super nova remnants, respectively, as given byTsai et al.(2006). The [CII] and [NII] spectra are shown on the right side. The [NII] intensity scale is multiplied by a factor of 5.

that is about 13 km s⁻¹broader than [CII], otherwise both aver- age spectra are well represented by Gaussians (dashed lines in Fig.2).

Following the approach inRöllig et al.(2012) we also com- pare the SOFIA data with complementary emission line data of CO and atomic carbon. In Fig.C.1we show for each position an overlay of the fine-structure lines presented in this paper with the available data. A direct comparison is complicated by the differ- ent spatial resolution and spatial sampling of the various lines (seeRöllig et al. 2012, for details of whether and how the data was smoothed and/or re-sampled).

Generally speaking, the agreement between [CII] and the molecular gas is strongest on the molecular ring and the spiral arms. Positions away from the spiral arms, e.g. (7⁰⁰, −7⁰⁰) show a significant difference in line width and central velocity indicat- ing a different kinematic origin. We note that the various lines in Fig.C.1show significantly different line profiles at some po- sitions. This occurs because the shown data only partly cover the positions observed with SOFIA. When re-sampling was not possible we chose the nearest neighbor spectrum. An exception is the¹²CO (1−0) data with a beam size and spatial sampling superior to SOFIA data. Hence, in this paper, we do not per- form a detailed comparison of the [CII] and [NII] lines with all

Fig. 2.Sum spectra of [CII] and [NII] and¹²CO (1−0) averaged across the central 3 × 3 grid. Individual positions were equally weighted. The [NII] intensity scale is multiplied by a factor of 5 and CO is divided by a factor of 5. The dashed lines show the result of Gaussian fits to the lines.

the additional emission lines, but select the¹²CO (1−0) BIMA- SONG data⁴(Helfer et al. 2003) as kinematic reference.

4 http://ned.ipac.caltech.edu/level5/March02/SONG/

SONG.html

Fig. 3.Comparison of the SOFIA fine-structure spectra with the corresponding¹²CO (1−0) line profiles scaled down to match either the line wings or peak strength of the [CII] (left panel) and [NII] (right panel) profiles such that the scaled CO emission is weaker than the [CII] and [NII] at all velocities. For easier comparison, we smoothed the [NII] spectra with a moving average across three spectral channels and also show the Gaussian fits to the lines. The CO scaling factors are given at each position individually.

4. Kinematics in the nucleus of IC 342

Meier & Turner(2005) combined multi-line millimeter observa- tions to derive an overall scenario of the structure and dynam- ics of the nucleus of IC 342. In response to the central barred gravitational potential, the molecular gas forms a mini-spiral with trailing arms (in their scenario), which ends in a circum- nuclear ring hosting several GMCs. Gas flows along the spiral arms onto the nuclear ring, triggering star formation at the rate of

∼0.1 Myr⁻¹. Recently,Rabidoux et al.(2014) used thermal and nonthermal 33 GHz luminosities to derive star formation rates of 0.4−0.6 Myr⁻¹ within the central 23⁰⁰. The volume inside the ring is dominated by the massive central nuclear star cluster. Its intense radiation gives rise to an expanding bi-conical outflow of hot, ionized gas, similar to the Fermi Bubbles observed in the Milky Way (Su et al. 2010).

To visualize the kinematic differences between the SOFIA data and molecular gas we show in Fig.3a comparison between the [CII] and [NII] line profiles (left and right panel, respectively) with scaled down¹²CO (1−0) line profiles. The scaling was done such that the downscaled CO emission is never stronger than the SOFIA line profiles. This allows us to immediately iden- tify C⁺and N⁺gas with different kinematics than the molecular gas. We note that the [CII] line profiles show a good agreement with the CO along the mini-spiral (diagonally from top left to bottom right). The same is not true for the [NII] line profiles where we see a significant difference in line shapes compared to the CO. The lower left quadrant of the [CII] data (positions (+7⁰⁰, 0⁰⁰), (+7⁰⁰, −7⁰⁰), and (0⁰⁰, −7⁰⁰)) shows a significant red- shifted part that is not visible in the CO data. The same red- shifted gas is also visible in [NII]. The topright position shows

Fig. 4.Comparison of the Gaussian line center velocities of¹²CO (1−0), [CII], and [NII]. The white numbers are the corresponding rounded ve- locities. The spatial resolution is 14⁰⁰and 18⁰⁰ for [CII] and [NII], re- spectively.

additional blueshifted C⁺gas, while the right position does not show any blueshifted material, but a weak redshifted contribu- tion. These two positions look different in [N^II]. The topright shows a significantly broader line centered at the ¹²CO (1−0) peak, and the right position does not show any component that is kinematically different to the CO.

In Fig. 4 we compare the Gaussian line center velocities of ¹²CO (1−0), [CII], and [NII] for all ten SOFIA positions.

12CO (1−0) shows a clear velocity gradient from the southwest

Fig. 5.Geometrical and kinematic scenarios as explained in Sect.4(originally suggested byMeier & Turner 2005, and slightly modified in this work). The red and blue lobe structures indicate the bi-conic expanding HIIgas mentioned in the text. Blue gas is moving toward the observer, red gas is moving away. The mini-spiral (green) and ring (yellow) are surrounding the central nuclear star cluster (white stars), which gives rise to the expanding HIIregions as well as to the intense PDR emission from the gas in the ring/spiral facing the cluster. The black arrows indicate a potential direction of motion consistent with the velocity structure observed in¹²CO (1−0). a) Standard geometry as proposed byMeier & Turner (2005) with arms in a trailing configuration. b) Proposed new geometry, spiral/arm plane flipped by 90^◦and a leading arm pattern. (The observer is located directly above the plane of paper.)

to the northeast, a signature of spiral/ring rotational kinematics⁵. It is consistent with a general nuclear rotation of an inclined spi- ral (the rotation axis is oriented from the SE to the NW) together with the aforementioned gas flow along the spiral arms. Which of the two motions is dominating is unclear. However, when ob- serving only tracers that are arranged spatially within the rotat- ing plane, thus following the nuclear rotation and gas flow along the arms, one suffers from a degeneracy of inclination angle of the rotating plane and the direction of rotation. To actually dis- tinguish between the two possible configurations it is necessary to observe a physically associated motion perpendicular to the plane, i.e., hot, ionized gas in the scenario of an expanding out- flow due to the central star cluster.

Given the scenario above, the two lobes of expanding HIIre- gions should emit significantly in [CII] and [NII] and allow us to derive a more detailed geometric model of the nuclear region of IC 342. First of all, we note that the general rotation signature is also visible in [CII] and [NII] (Figs.3and4) as of course some of the ionized gas will be associated with and follow the motion of the bulk of the gas in the central region. Superimposed on the general kinematics, we also note some deviations originat- ing from the very different physical conditions in the emitting regions.

The [NII] velocities show a stronger redshift to the south and to the southeast and a blue-shift in the north (middle panel in Fig. 4). The stronger redshift of [NII] in the SE is in agreement with the general kinematic scenario presented by Meier & Turner (2005). However, in their geometry (compare Fig. 10 inMeier & Turner 2005), the southeastern lobe is mov- ing toward the observer and should therefore be blueshifted. The SOFIA [NII] data clearly shows the opposite behavior. This re- quires us to modify the geometrical model by flipping it by 90^◦ around an axis along the spiral arms (see Fig.5). Now, the SE lobe is facing away from the observer, leading to the observed redshift in the [NII] emission while the NW lobe is expected to be blueshifted, just as it is at the (−7⁰⁰, 7⁰⁰) position.

5 The spatial resolution of the data is 14⁰⁰and 18⁰⁰for [CII] and [NII], respectively. Accordingly, at each of our positions we pick up emission from the neighboring pixel.

The [CII] velocities in the south and the southeast are also redshifted with respect to¹²CO (1−0). The C⁺gas, moving away from the observer is clearly visible as additional redshifted gas in the [CII] at the SE.

We present two possible interpretations of the velocity in- formation of¹²CO (1−0), [NII], and [CII]: (1) The radius of the ring is about 4⁰⁰ (Montero-Castaño et al. 2006, and references therein). Accordingly, within our central 3 × 3 grid of 7⁰⁰spaced observations we pick up information from both the ring and the S-shaped mini-spiral. If the kinematic signature of the molecu- lar ring that revolves around the central cluster is weaker than the gas inflow along the spiral arms, then the velocity pattern in¹²CO (1−0) is consequently also dominated by the spiral arm gas. In order to produce the observed Doppler shifts this requires a large inclination angle (edge-on) of the spiral-arm plane rela- tive to the observer (see Fig.5, panel a) and a slow ring rota- tion velocity. (2) If the velocity signature is mainly produced by the general rotational motion of the gas in the ring, then the pro- jected rotation direction needs to be clockwise. We note that both scenarios lead to a configuration where the mini-spiral is mov- ing in a leading-arm pattern. With the angular resolution of the data at hand a distinction between these two possibilities is not obvious.

The redshifted component observed to the SE of our small maps is not visible in any other molecular line data. It is there- fore unlikely that this redshift is the result of shocks. It is clearly not associated with any denser material and we con- tribute it to the ionized gas moving in a wide-angle outflow/lobe.

The (−7⁰⁰, 7⁰⁰) position shows a blueshifted component in [CII] as well as a much broader line profile in [NII]. The position (−7⁰⁰, 0⁰⁰) does not show any strong kinematic deviations from the CO gas in either [CII] or [NII]. At (0⁰⁰, 7⁰⁰) the [NII] line is visibly blueshifted compared to the CO line. The blueshifted gas in the NW could be emission from the approaching lobe of ionized gas. An alternative interpretation could be a shock- related origin. Montero-Castaño et al. (2006) presented detec- tions of hot NH3 with an emission peak close to our (−7⁰⁰, 0⁰⁰) position.Usero et al.(2006) also detected strong SiO emission, a typical shock tracer. We probably see a combination of out- flow/expansion of the H^II gas in the NW together with some

shock-related motions. It is unclear why the nearby portion of the bi-polar outflow is much less pronounced than the far-side, red- shifted lobe. A possible cause is an asymmetry in the ring/spiral structure hindering the gas flow in our direction.

4.1. Discussion

The current standard geometry of IC 342 is based on a number of previous observations. Generally these observations fall into two different categories: in-plane and off-plane. In-plane tracers are situated in the plane of the mini-spiral/ring and take part in the general rotational dynamic. Examples are molecular line ob- servations, such as¹²CO(1−0). Examples of an off-plane struc- ture are the proposed bi-polar lobes of expanding ionized gas, traced by Hα emission, moving perpendicular to the ring/arm plane. Generally, if only in-plane Doppler data is available it is not possible to distinguish between, for example a counterclock- wise rotation with trailing spiral arms and a clockwise rotation with leading arms in a plane that is 90^◦flipped. Only the combi- nation with additional data can resolve this degeneracy.

Earlier studies combined in-plane CO emission line maps with assumed off-plane Hα emission (Meier & Turner 2001, 2005). The apparent lack of Hα emission at the southern arm of the mini-spiral is interpreted as extinction due to the fore- ground arm while the 3 mm continuum emission (predominantly thermal free-free emission) from optically thin HIIregions re- mains unattenuated. This indicates strong star formation activity at the SW portion of the ring shielded by a significant column of foreground material. Assuming that the emission is from the cluster facing side of the ring and both are situated in the same plane, this would suggest an orientation similar to the left panel in Fig.5. If the same scenario were true for the second peak in the 3 mm map at the NE part of the arm one would expect to observe unextincted Hα emission. A comparison with Fig. 1 in Meier & Turner(2005) also shows an Hα deficit visible as a sig- nificantly darker lane following the molecular arm. Following this line of reasoning, both scenarios in Fig.5are possible.

Instead we argue that observed Hα and 3 mm continuum emission trace two different regimes. The Hα emission shown byMeier & Turner(2001,2005) is predominantly emitted by the ionized gas in the cavity and the expanding lobes, while the 3mm continuum stems from the current but embedded star formation activity in the molecular ring triggered by the inflowing gas. In our flipped geometry scenario the ring would account for the foreground extinction visible as significantly darker lane follow- ing the northern arm. The HIIemission from the NW is much weaker, most likely due to a strong asymmetry between the two lobes. We note that neither the standard nor the flipped geom- etry explains the absence of ring emission in the northern ring quadrant. Most likely the ring is broken up or fragmented.

The flipped geometry proposed here would imply a leading spiral arm structure within the inner Inner Lindblad Resonance (iILR) then transitioning into a trailing arm outside of the outer Inner Lindblad Resonance (oILR). The possibility of such a con- figuration has been shown in numerical computations assuming a weak barred potential (Wada 1994;Piñol-Ferrer et al. 2012). It is important to remember that leading/trailing arms are just tran- sient, rotating patterns not subject to shear, etc. Fundamentally there is no reason to disregard such a configuration.

The flipped geometry is problematic in the sense that it pro- poses a tilt between the plane of rotation of the mini-spiral within the ILR and the global plane of the galaxy. The study of three-dimensional orbits in a tri-axial potential is just at its beginning. Three-dimensional N-body simulations of orbits

in rotating potentials show the existence of complex three- dimensional orbits with various, sometimes interchanging tilt angles (Pfenniger 1984; Pfenniger & Friedli 1991). In a recent work,Portail et al.(2015) showed N-body simulations of orbits in the Galactic bulge, demonstrating the existence of bent or tilted orbits in barred discs. We conclude that a possible tilt angle of the mini-spiral/ring plane in IC 342 is neither supported nor prohibited by present theoretical models. A significantly differ- ent inclination compared to the general orientation of the plane of IC 342 can therefore not be ruled out a priori. Meidt et al.

(2009) derived the detailed velocity structure of IC 342 from CO and HI intensities up to a galactocentric distance of 15 kpc. Their first moment map shows indications of a warped outer disk most likely due to tidal interaction with a close companion galaxy. The velocity pattern of the inner disk seems to show some asymmetry very close to the nucleus that could indicate a changing tilt angle, but the spatial resolution of the data is not sufficient to support or discard this scenario.Schinnerer et al.(2003) showed that the CO gas in the central 300 pc shows noncircular motion and they suggest that this could be due to a nuclear CO disk tilted rela- tive to the large stellar disk. Later they discard this scenario and argue that streaming motions along the arms are a more plau- sible explanation. Fathi et al. (2009) studied the pattern speed in late-type barred spirals and derived their ellipticity profiles.

Their analysis showed that IC 342 shows a steep increase in el- lipticity at a radius of about 2 kpc. This could indicate a different tilt angle of the inner disk.

An argument in favor of the standard geometry (Meier & Turner 2005, see also Fig.5, panel a) is the presence of HNCO and CH₃OH emission, presumably shock excited, at the front side of the trailing arms. However, the spatial resolution of the data (∼6⁰⁰ × 5⁰⁰) make accurate localization difficult.

Comparing the maps of the shock tracers with the various CO isotopologue maps (Fig. 2 in Meier & Turner 2005), the displacement between the two appears marginal. Nevertheless, any high-resolution data tracing shocked gas or triggered star formation on either side of the spiral arms would be a good test on the underlying geometry and dynamics.

Another scenario preserving the standard geometrical inter- pretation would be a very wide-angle SE outflow together with an arm/ring plane that is significantly tilted with respect to the plane of sky. In this case blueshifted emission is expected close to the cluster and redshifted emission in the SE. Comparison of the line profiles of CO and the ionized gas in Fig.3shows a very weak blueshifted component in [CII] at the (0⁰⁰, 0⁰⁰) position. The [NII] profile at (0⁰⁰, 0⁰⁰) is wider compared to CO which could be interpreted as an overlay of blue- and redshifted emission.

However, the same is also found at almost all other observed positions and is more likely the signature of a larger overall ve- locity in the ionized gas. Furthermore, the wide-angle outflow scenario would also affect the entire NW lobe and lead to red- shifted emission signatures in [NII]. Again, the larger linewidths in [NII] and the overall lack of ionized gas emission in the NW inhibit a verification of this scenario.

We conclude that the [CII] and [NII] data are difficult to ex- plain within the current standard geometry of IC 342’s nucleus.

We suggest a modification of the current image by flipping the ring/arm plane which leads to a leading arm configuration better explaining the kinematics of the molecular gas in the spiral arm and the ring as well as the ionized gas. A disadvantage of the pro- posed geometry is the leading arm configuration with a strongly tilted axis with respect to the global galactic disk. Theoretical work on orbits in such a configuration as well as high-resolution observations are both required to resolve this uncertainty.

Fig. 6.Spatial distribution of the integrated line intensitiesR

Tmbdv of [CII] (left),¹²CO (1−0) (middle), and the I([CII] )/I(¹²CO (1-0)) line ratio (right). The colorbar of the right panel specifies the values for the line ratio calculated by using temperature units (T) and energy units (E).

To convert T units to E units, multiply T by 4510. The spatial resolution is 14⁰⁰and 18⁰⁰for [CII] and [NII], respectively.

5. The [CII] to¹²CO (1−0) ratio

The intensity ratio I([CII] )/I(¹²CO (1−0)) is often used to char- acterize the energetic state and star formation activity of the ISM (e.g.Stacey et al. 1991). The [CII] intensity strongly scales with the intensity of the ambient FUV intensity, which is mainly pro- duced by young massive stars. The¹²CO (1−0) line, on the other hand, is emitted from cooler, better shielded material. A high [CII]/¹²CO (1−0) ratio is indicative of strong PDR emission and star-burst activity. Common values range from a few hundred up to a few 10⁴ in extreme regions, such as 30 Doradus. In Table2we compare the integrated line intensity ratio for all ob- served positions and for the sum spectra, averaged over the cen- tral 3 × 3 grid. The values range from 400 to 1800. In Fig.6we show the spatial distribution of the ratios. The highest values are found along the lower left corner of our 3 × 3 grid, partly cor- responding to the GMC A (Meier & Turner 2001). The strong star formation in GMC B (and to a lesser degree also GMC E) is most likely causing the slightly increased line ratio at the offsets (0⁰⁰, 0⁰⁰) and (−7⁰⁰, 0⁰⁰) (compare with Fig.1).

Usually, the [CII]/¹²CO (1−0) ratio is used to deduce the global star formation activity of an object. On global scales, [CII] emission from HIIregions only contributes about 20% to the total [CII] intensity (Pineda et al. 2014), but an increased active star formation gives rise to a higher FUV flux on larger scales and to an enhanced [CII]/CO line ratio accordingly. The much higher angular resolution of the SOFIA [CII] data com- pared to the data available toStacey et al.(1991) reveals signif- icant local variations in the line ratio when pointing on or off sites of active star formation. This is apparent when comparing the position dependent [CII]/¹²CO (1−0) line ratio with the av- erage value of ∼855 (Table2, last line). From Fig.6we see that the high ratio at (7⁰⁰, −7⁰⁰) is mostly driven by the lack of CO emission, while at (0⁰⁰, −7), the high ratio is directly caused by the strong [CII] emission.

Our data shows only a spatial correlation between the [CII] and¹²CO (1−0) integrated line intensities (Pearson correlation coefficient ρ = 0.80). The [C^II] line peaks more around the edges of the CO distribution. Our angular resolution is too low to spa- tially attribute the [CII] emission to geometrical structures as de- picted in Fig. 5. The original interpretation ofMeier & Turner (2005) who locate the PDR activity on the inside of the molec- ular ring facing the nuclear cluster remains valid even in the proposed new geometry. The only difference is that the PDR

Fig. 7.Correlation between I([NII]) (205 µm) and I([CII]) (159 µm).

The data points are labeled with their position (in offsets of⁰⁰). The black line corresponds to the best fit line provided byAbel(2006) and the solid gray line shows the fit given byHeiles(1994). The dashed and dot-dashed lines show how the relation byAbel(2006) is affected by an enhanced elemental (N/O) ratio ((1.86 × (N/O)) and by beam size effects (I([N^II]) × (14⁰⁰/18⁰⁰)²), respectively.

surface, i.e. the cluster facing side of the ring, is facing away from the observer.

We note that the average line ratio derived in this paper is significantly lower than the value of 3250 (Stacey et al. 1991, corrected for a main beam efficiency of 0.65). Given the much lower area filling factor of the cool CO gas compared to more diffuse and widespread C⁺, we expect the line ratio to increase with increasing beam sizes.

6. [NII] and [CII] analysis

Because of their different ionization potentials, [N^II] and [CII] can originate from different environments. While both tracers can be emitted from HIIregions, the lack of photons with en- ergies above the Lyman limit in photon-dominated regions pro- hibits the emission of nitrogen fine-structure lines from PDRs.

An interesting question is, to what degree is it possible to use the observed [NII] emission to disentangle what fraction of [CII] emission stems from HIIregions and from PDRs? Based on numerical models of photo-ionization gas and PDRs using the Cloudy model (Ferland et al. 2013),Abel(2006) presented the following correlation,

log(I([CII])H⁺)= 0.937 log(I([N^II]))+ 0.689, (1) where I([NII]) and I([CII])H⁺ are the integrated line intensities of [NII] and [CII] from the HIIregion (intensities given in en- ergy units). This equation agrees well with a similar expression given byHeiles(1994). In Fig.7we compare our data with the expected [CII] from the HIIregions. Points on the theoretical curves correspond to observed [CII] intensities that are produced exclusively by HIIgas. Data points to the left of the lines cor- respond to a combination of PDR and HII gas; data points to the right of the curves are weaker than is expected from HIIre- gions only. This is the case for three of our ten positions, but the deviation from the theoretical curve is not too strong. The de- tailed [CII]-[NII] correlation is critically dependent on the over- all metallicity and the elemental carbon-to-nitrogen abundance ratio. Local variations of the elemental abundances will alter the numerical values in Eq. (1) and shift the theoretical line along the I([CII]) axis. Recently,Florido et al.(2015) showed that the

nuclei of barred spiral galaxies with star formation have a sig- nificantly enhanced (N/O) ratio. They find log(N/O) = −0.49, which is a factor of 1.86 higher than the value assumed byAbel (2006). Assuming a linear scaling between N⁺column density and [NII] emission, this would shift the solid black line in Fig.7 to the right. Similarly, if [CII] and [NII] is emitted from the same local volume then [NII] needs to be corrected for beam size ef- fects, I([NII]) × (14⁰⁰/18⁰⁰)²= I([N^II]) × 0.6, shifting the relation to the left. In the case of IC 342 these two opposite effects will most likely cancel each other out to some degree. However, this suggests a significantly lower [CII]/[N^II] ratio in sources with solar or subsolar metallicity due to the area beam filling⁶φ_a 1.

Table3lists the [CII] and [NII] intensities together with the theoretically expected value from Eq. (1) and their ratios and dif- ferences. It is remarkable that the fraction of [CII] emission com- ing from the PDR is only between 10 and 65%, which means that approximately 35−90% of all the [CII] is from the HII. This is a much higher fraction than the often assumed range of 10−60%

(Abel 2006).Pineda et al.(2014) estimate that in the Milky way the contribution from different gas phases to the [C^II] luminos- ity is dense PDRs (30%), cold HI(25%), CO-dark H2(25%), and ionized gas (20%). These are averaged values. They also show that the [CII] luminosity in the inner few kpc of the Milky Way is dominated by ionized gas and only a minor fraction is con- tributed by the PDRs. These results are in agreement with our findings from the inner few hundred pc of IC 342 making it a possible template for our Galactic Center.

Averaged over the central 3 × 3 we observed a ratio I([CII])/I([NII]) = 12 in energy units. Applying Eq. (1) to the averaged line intensities instead, we find hI([CII])_H+i = 2.62 × 10⁻⁴erg cm⁻²s⁻¹sr⁻¹corresponding to about 80% of the total [CII] intensity that is contributed by the ionized gas.

Pineda et al. (2014) have also studied the use of the [CII] luminosity as a tracer for the star formation rate. They give a fit of the form

log(SFR)= m log L[CII] + b. (2)

If L[CII] is the total [C^II] luminosity stemming from all the various contributing phases, then m = 0.98 ± 0.07 and b =

−39.80 ± 2.94. However, if most of the [CII] luminosity is pro- duced in the ionized gasPineda et al.(2014) find m= 0.91±0.06 and b = −36.30 ± 2.36, resulting in a higher SFR for a given L[CII]. We estimate the total [C^II] luminosity from L[CII] = 4π R²hI([CII])i, with the radius of the emitting nuclear region R ≈250 pc, and the mean [CII] intensity hI([CII])i from Table3.

Across our ten positions we find L[CII] = 2.7 × 10⁵−1.3 × 10⁶L. Using Eq. (2) and assuming an HIIdominated [CII] lu- minosity this corresponds to star formation rates between 0.16 and 0.65 Myr⁻¹within the central 500 pc, similar to the range of 0.4−0.6 Myr⁻¹derived byRabidoux et al.(2014). If we as- sume that the [CII] luminosity is produced not only in the ion- ized gas, but stems from all phases, we find significantly lower star formation rates of 0.03 and 0.13 Myr⁻¹. Reversing the ar- gument, we can take the SFR derived byRabidoux et al.(2014) and compute the expected [CII] luminosities using Eq. (2). We already expect the [CII] emission to be dominated by emission

6 There are different beam filling factors that are often confused. Here we use the following: If A_i is the projected area of object i then φ_a = Asource/Abeammeasures the coverage of the source extent by the beam.

Thus, factor φa corrects fluxes for source extents larger than the beam (where surface brightness is not affected) as well as surface brightness for source extents smaller than the beam (where flux is not affected).

from the ionized phase, thus assuming contributions from all phases in Eq. (2) we will overestimate the [CII] luminosities.

Putting in the numbers we find L[C^II] = 4.2 − 6.4 × 10⁶L, significantly more than observed. Assuming that most [CII] is emitted from HIIgas, we find L[CII] = 7.4 × 10⁵−1.2 × 10⁶L. This agrees well with our observations and confirms that the [CII] emission appears to be dominated by emission from the ionized gas.

6.1. [CII] and [NII] estimation from thermal emission

As a comparison we also derive an upper limit to the ex- pected [CII] and [NII] emission based on thermal continuum measurements in this section. FromRabidoux et al.(2014) we find that in a 21.3⁰⁰ beam IC 342 has a thermal flux of 15.4 mJy at 33 GHz, corresponding to 19 mJy at 5 GHz. Based on Mezger & Henderson (1967) we can calculate the emis- sion measure E M(5 GHz) = 4.85 × 10³S(Jy)5 GHz × T_e^0.35 × θ(⁰)⁻² pc cm⁻⁶. Meier et al. (2011) estimated T_e = 8000 K.

Assuming that the thermal emission comes from a solid angle θ  21.3⁰⁰, then S (Jy)_{5 GHz}= const. for smaller beams. Depend- ing on the angular extent of this ionized gas we can calculate its properties, such as the scale length L, mean electron density hn_ei, and its mean electron column density hN_ei. Table4summa- rizes the results for different H^IIsizes.

We note that the six HIIregions given byTsai et al.(2006) (see also Fig.1) have a total projected area of 2.26 sr, equiva- lent to an aggregated diameter of 1.7⁰⁰, which implies an area filling factor φ_a(21.3⁰⁰) = 1.7²/21.3² ≈ 1/160 in the 5 GHz beam, φa(18⁰⁰)= 1.7²/18²≈ 1/110 in the SOFIA 205 µm beam, and φa(14⁰⁰) = 1.7²/14² ≈ 1/70 in the SOFIA 158 µm beam⁷. This is an upper limit for the total HIIregion area because of the varying coupling to the beam depending on the position of the HIIregions. Accounting for the beam coupling at the (0⁰⁰, 0⁰⁰) position we find effective areas of 1.89 and 2.02 sr in a 14⁰⁰and 18⁰⁰beam, respectively. This corresponds to effective diameters θ = 1.55⁰⁰ and θ = 1.61⁰⁰ for [CII] and for [NII], respectively (see Table.4). We note that to determine the local physical prop- erties, such as hnei and hN_ei, θ= 1.7⁰⁰should be used instead of the effective diameters. We also note that for θ = 1.7⁰⁰, we find hn_ei ≈ 350 cm⁻³, smaller than the value of 700 cm⁻³found by Meier et al.(2011).

We assume in the center of IC 342 a metallicity twice so- lar (12+ log([O]/[H]) = 8.5) and a carbon depletion fac- tor of 0.4. However, in an HIIregion the gas-phase abundance equals the elemental abundance because all dust is destroyed.

We also assume that the elemental abundance of nitrogen scales linearly with the metallicity, even though there is some evi- dence that [N]/[H] increases more quickly under high-metallicity conditions (see e.g. Liang et al. 2006). Thus, gas-phase = el- emental [C]/[H] = 2 × 3.16 × 10⁻⁴ (Simón-Díaz & Stasi´nska 2011). For nitrogen we assume [N]/[H] = 2 × 8.32 × 10⁻⁵ (Simón-Díaz & Stasi´nska 2011). The total column density then is hNei × [X]/[H] assuming that all carbon and nitrogen are in singly ionized form.

Based on updated electron collision strengths for N⁺(Tayal 2011) Goldsmith et al. (2015) analyzed the [NII] fine struc- ture emission in the Galactic plane. Using their expression for the level population ratios we can derive the optically thin

7 Propagating the errors on the HIIsize estimates (∼30%) gives an uncertainty of∆φa/φa= 42%. The errors on S (5 GHz) of ∼1/30 result in∆hnei/hnei= 15% and ∆hNei/hNei ≈ 15%. BecauseR

Tdv= const.×

φa× N, we find∆Tobs/Tobs= 45%.

[NII] emission assuming LTE conditions. For the 205 µm line we find

Z

T_{205 µm}^[CII] dv= 2.1 × 10⁻¹⁶N(N⁺) K km s⁻¹ (3)

with the total N⁺column density N(N⁺); assuming Te= 8000 K and taking n_e = 350 cm⁻³ from Table 4 (see AppendixD for details and TableD.1for different values of neand Te). Table4 also gives the expected intensities for different sizes θ. Coupling the spatial distribution of HIIregions to the [NII] beam gives θ = 1.61⁰⁰ leading toR

T_205µm^[NII] dv = 6.5 K km s⁻¹, significantly lower than the observed value of 9.4 K km s⁻¹. However, here we assumed [N]/[O]∝[O]/[H]. If the elemental nitrogen abundance scales super-linearly, then the N⁺ column density and intensity is enhanced accordingly and could, at least partially, explain the discrepency.

Again assuming T = 8000 K and n = 350 cm⁻³ and a collisional de-excitation coefficient with electrons at 8000 K of 4.9 × 10⁻⁷(Wilson & Bell 2002), we findR

T_158µm^[CII] dv= 2.17 × 10⁻¹⁶× N(C⁺) K km s⁻¹using Eq. (1) fromPineda et al.(2013).

Accounting for the effective extent of the H^IIregions in the [CII] beam due to coupling to a Gaussian beam gives θ= 1.55⁰⁰. Then the net result is the predicted contribution to the [CII] emission from the HIIregionsR

T_{158 µm}^[CII] dv = 39.2 K km s⁻¹. Comparison with the observed value at the central position of 62.9 K km s⁻¹ then shows that a fraction of about 62% of the [CII] intensity is contributed by the compact HIIregions consistent with our ear- lier estimates (see Table3).

The assumption that all the thermal emission comes from the compact HIIregions might be wrong. A fraction a < 1 of S(Jy)5 GHz could be contributed by large-scale, extended (φa = 1) ionized gas. By adding this second component we can esti- mate how much it would contribute to the fine-structure emis- sion for a given value of a. We find that a cannot exceed the per- cent level. Otherwise such an extended contribution to the [CII] and [NII] emission would be much too high because it would not suffer from any area filling effect. Therefore we do not ex- pect the extended ionized gas to contribute significantly to the thermal radio emission, but it might still contribute to the fine- structure emission. However, when doing the same analysis for the other positions, we find that an additional component, pos- sibly extended and clumpy, is required to explain the observed [NII] intensities because of the even weaker coupling of the com- pact HIIemission to the beam at the off-center positions.

Another uncertainty is the distance to IC 342. Assuming a smaller distance would lower our estimates for the [CII] and [NII] intensities because the same angular extent would corre- spond to lower values of L and therefore to higher hn_ei but lower hNei. Summarizing, we find a high fraction of the [CII] emission expected from the compact HIIregions in the center of IC 342 based on its thermal emission.

6.2. Kinematic [CII]–[NII] correlation

The [CII]/¹²CO (1−0) line ratio and the [CII]−[NII] correlation in Eq. (1) are both based on integrated line intensities, discarding any additional kinematic information, but [CII] emission origi- nating from the HIIregion should carry the same kinematic sig- nature as the pure HIItracer, the [NII] line. The SOFIA/GREAT data has sufficient spectral resolution and signal-to-noise ratio to allow a more detailed analysis of the [CII]−[NII] correlation.

The [NII] emission shows different peak velocities and FWHM linewidths. We quantify the [CII] emission coming from

CO 1-0 ÷ 6 CII

CII residual=CII-I(CII)HII mod

NII I(CII)PDR

mod+I(CII)HII mod

I(CII)HIImod I(CII)PDR mod

-100 -50 0 50 100 150

-0.5 0.0 0.5 1.0 1.5

v(km/s) Tmb[K]

(0, 0)

Fig. 8. Comparison of the observed [CII] (red), [NII] (green), and

12CO (1–0) (black, gray filling, suppressed by a factor 6) lines at the center position (0⁰⁰, 0⁰⁰) together with the simulated [CII] spectrum (cyan) derived from the [NII] line and the residual [CII] line (orange) corresponding to “pure” PDR emission. The dashed lines give the re- spective Gaussian lines.

the HIIregion using the following approach: (1) assuming that Eq. (1) correctly predicts the amount of [CII] being emitted from the ionized gas, we simulate a [CII] spectral line assum- ing a Gaussian with line center velocity and FWHM line width taken from the Gaussian fit of the corresponding [NII] spectrum⁸ (Table1) and with an integrated line intensity corresponding to I([CII])H⁺ from Table2. This simulated [CII] line is subtracted from our observed [CII] for each velocity channel and gives the residual [CII] intensity, cleaned of HIIcontributions⁹. (2) A Gaussian is fitted to the residual [CII] line. The line parameters are then correlated to the¹²CO (1−0) line parameters.

Figure 8 compares the observed data from the center po- sition (0⁰⁰, 0⁰⁰) with the simulated [CII] data. The line shapes of the modeled [CII] and the residual [CII] lines are signifi- cantly different. The Gaussian line parameters for the [C^II]}res

are T_pk = 0.46 ± 0.02 K, v0 = 29.9 ± 1.2 km s⁻¹, and σ_FWHM= 54.3 ± 2.9 km s⁻¹. The line shape and position is close to the CO and C line shapes at this position (compare with Table1).

We performed the above analysis for all ten positions.

Figure9shows the spatial distribution of the derived line prop- erties of the residual [CII] line. The left panel shows the inte- grated line intensity of [CII]res. The intensities are stronger to the SE than to the NW and we find no spatial correlation to the

12CO (1−0) data. We note that scaling up the [NII] emission also increases the error of I([CII])H⁺and consequently also of [CII]res. It is also possible that the [CII]-[NII] correlation from Eq. (1) varies spatially. We attribute the remaining [CII] emission in the southeast to pick up from PDRs in the ring regions within the beam.

The three diagonally hatched positions in Fig. 9 show no residual [CII] emission, i.e. [CII] emitted from PDRs. For the NW position the most likely reason is that the simple scenario of an HIIregion neighboring a PDR is probably not applicable.

The majority of the [NII] observed there can be attributed to the expanding lobe of ionized gas that is not associated with a tran- sition to a PDR/GMC. Hence, we do not expect strong [C^II]res

emission. For the other two positions the explanation are less obvious. At the NE position we find the lowest signal-to-noise

8 We did not scale the [NII] spectrum directly to avoid the effects of noise amplification.

9 The baseline RMS is conserved during subtraction.