The Origin of [CII] 158um Emission toward the HII Region Complex S235

TL;DR

This study uses SOFIA and GBT observations to investigate the origin of [CII] 158um emission in the S235 HII region, revealing its association with UV radiation fields and complex spatial relationships with ionized and molecular gas.

Contribution

It provides detailed spatial and spectroscopic analysis of [CII] emission origins in a Galactic HII region, highlighting the correlation with UV radiation indicators and the limited association with ionized hydrogen.

Findings

Approximately half of [CII] emission coincides with ionized hydrogen gas.

[CII] intensity correlates strongly with WISE 12um emission across the complex.

Correlations between [CII] and molecular gas tracers are weak.

Abstract

Although the 2P3/2-2P1/2 transition of [CII] at 158um is known to be an excellent tracer of active star formation, we still do not have a complete understanding of where within star formation regions the emission originates. Here, we use SOFIA upGREAT observations of [CII] emission toward the HII region complex Sh2-235 (S235) to better understand in detail the origin of [CII] emission. We complement these data with a fully-sampled Green Bank Telescope radio recombination line map tracing the ionized hydrogen gas. About half of the total [CII] emission associated with S235 is spatially coincident with ionized hydrogen gas, although spectroscopic analysis shows little evidence that this emission is coming from the ionized hydrogen volume. Velocity-integrated [CII] intensity is strongly correlated with WISE 12um intensity across the entire complex, indicating that both trace ultra-violet radiation fields. The 22um and radio continuum intensities are only correlated with [CII] intensity in the ionized hydrogen portion of the S235 region and the correlations between the [CII] and molecular gas tracers are poor across the region. We find similar results for emission averaged over a sample of external galaxies, although the strength of the correlations is weaker. Therefore, although many tracers are correlated with the strength of [CII] emission, only WISE 12um emission is correlated on small-scales of the individual HII region S235 and also has a decent correlation at the scale of entire galaxies. Future studies of a larger sample of Galactic HII regions would help to determine whether these results are truly representative.

Tables

Table 1. Table 1: Parameters of the regions of interest

Region	${\alpha }_{J2000}$	${\delta }_{J2000}$	Radius
	h:m:s	d:m:s	$″$
S235PDR	5:41:02.76	35:51:06.48	$284.4$aaThe S235PDR region is annular, with its inner radius equal to the radius of S235ION. The value listed here represents the outer radius.
S235ION	5:41:02.76	35:51:06.48	$187.2$
S235AB	5:40:51.24	35:42:23.04	$75.6$
S235C	5:40:51.24	35:38:42.36	$82.8$
bg1	5:41:20.40	35:46:19.20	$46.8$
bg2	5:41:22.20	35:55:32.52	$46.8$
bg3	5:40:42.24	35:55:22.80	$46.8$
bg4	5:40:53.40	35:45:00.72	$46.8$
bg5	5:40:43.68	35:40:33.60	$46.8$

Table 2. Table 2: Fluxes from regions of interest

Region	$F_{\left[C\mathrm{II}\right]}$	$F_{12\mathrm{\mu m}}$	$F_{22\mathrm{\mu m}}$	$F_{1.4\mathrm{GHz}}$	$F_{12CO21}$	$F_{12CO32}$	$F_{13CO21}$	Area
	${10}^{-12}$ W m-2	Jy	Jy	Jy	${10}^{-15}$ W m-2	${10}^{-15}$ W m-2	${10}^{-15}$ W m-2	sq. arcmin.
S235MAIN	2.9	310	1000	1.9	5.7	14	1.0	71
S235ION	1.5	160	520	1.9	2.7	6.9	0.54	31
S235PDR	1.4	150	520	0.022	3.0	6.8	0.46	40
S235AB	0.30	40	150	0.33	0.62	1.5	0.17	5.0
S235C	0.27	33	110	0.051	0.41	0.86	0.088	6.0

Table 3. Table 3: Correlations a a Fits made using I [ C ​ II ] = A + B × I λ subscript 𝐼 delimited-[] C II 𝐴 𝐵 subscript 𝐼 𝜆 I_{\rm[C\,{\footnotesize II}]}=A+B\times I_{\lambda} . between [C  II ] and other tracers

Tracer	Regions	$A$bbThe A-values for the WISE correlations are uncertain due to the absolute flux calibration of the WISE data.	$B$	$R^2$
		$K$  $\mathrm{km}{s}^{-1}$	$K$  $\mathrm{km}{s}^{-1}$/$\mathrm{mJy}$ or None
$F_{12\mu m}$	S235ION	$-5.01$	$0.89\pm 0.02$	$0.70$
	S235PDR	$-12.44$	$1.02\pm 0.02$	$0.59$
	S235AB	$0.64$	$0.71\pm 0.03$	$0.51$
	S235C	$-3.86$	$0.61\pm 0.02$	$0.85$
	S235BG	$-9.71$	$0.59\pm 0.02$	$0.57$
	Global	$-6.99$	$0.89\pm 0.01$	$0.67$
$F_{22\mu m}$	S235ION	$20.44$	$0.38\pm 0.01$	$0.44$
	S235PDR	$-3.61$	$0.89\pm 0.02$	$0.64$
	S235AB	$14.95$	$0.22\pm 0.01$	$0.54$
	S235C	$-13.94$	$0.86\pm 0.01$	$0.68$
	S235BG	$-5.73$	$0.79\pm 0.13$	$0.04$
	Global	$16.19$	$0.45\pm 0.00$	$0.55$
$F_{1.4\mathrm{GHz}}$	S235ION	$32.34$	$10.91\pm 0.30$	$0.36$
	S235PDR	$36.35$	$19.36\pm 0.89$	$0.15$
	S235AB	$28.55$	$14.86\pm 0.68$	$0.74$
	S235C	$26.69$	$62.88\pm 2.47$	$0.76$
	S235BG	$7.42$	$15.93\pm 6.66$	$0.04$
	Global	$36.74$	$11.14\pm 0.20$	$0.46$
$I_{12CO21}$	S235ION	$19.80$	$0.62\pm 0.01$	$0.56$
	S235PDR	$9.39$	$0.33\pm 0.02$	$0.07$
	S235AB	$2.86$	$0.46\pm 0.21$	$0.18$
	S235C	$1.75$	$0.58\pm 0.16$	$0.00$
	S235BG	$3.00$	$0.02\pm 0.03$	$0.04$
	Global	$14.15$	$0.37\pm 0.02$	$0.23$
$I_{13CO21}$	S235ION	$26.25$	$0.78\pm 0.02$	$0.57$
	S235PDR	$14.75$	$0.38\pm 0.02$	$0.12$
	S235AB	$-4.99$	$0.78\pm 0.23$	$0.20$
	S235C	$8.87$	$0.76\pm 0.20$	$0.00$
	S235BG	$5.18$	$-0.06\pm 0.11$	$0.01$
	Global	$16.08$	$0.47\pm 0.02$	$0.26$
$I_{12CO32}$	S235ION	$30.36$	$1.44\pm 0.05$	$0.45$
	S235PDR	$18.66$	$1.06\pm 0.06$	$0.09$
	S235AB	$11.58$	$0.97\pm 0.39$	$0.04$
	S235C	$19.53$	$0.82\pm 0.38$	$0.00$
	S235BG	$3.39$	$0.02\pm 0.04$	$0.01$
	Global	$21.84$	$1.37\pm 0.05$	$0.23$

Equations

$$\frac{F_{[\mathrm{C}\mathrm{II}]}}{\mathrm{W}{\mathrm{m}}^{-2}}=8.1\times 10^{-18}\frac{I_{[\mathrm{C}\mathrm{II}]}}{\mathrm{K}\mathrm{km}{\mathrm{s}}^{-1}}.$$

Full text

H II regions – infrared: ISM – radio continuum: ISM – techniques: photometric

††software: AstroPy (Astropy Collaboration et al., 2013; Price-Whelan et al., 2018), AplPy (Robitaille & Bressert, 2012; Robitaille, 2019), IDL

1 Introduction

The [C II] line is one of the most important transitions in the interstellar medium (ISM). This line arises from the $\mathrm{{}^{2}P_{3/2}-^{2}P_{1/2}}$ transition of ionized carbon at \lambda\sim 158$$\,\mu{\rm m} ($1.9\,$THz), at an equivalent temperature of $\Delta E/k_{\rm B}\simeq 91.2\,$K. Between 0.1 and $1\%$ of the total FIR-luminosity of galaxies is provided by this emission line (Crawford et al., 1985; Malhotra et al., 1997; Boselli et al., 2002), a result that also holds for Galactic star formation regions (Stacey et al., 1991; Schneider et al., 1998). We observe [C II] emission from diffuse clouds, the warm ionized medium (WIM), the surface of molecular clouds, dense photodissociation regions (PDRs), and cold H I clouds (Pineda et al., 2013; Pabst et al., 2017). Since carbon has a lower ionization potential than hydrogen ($11.3\,$eV vs. $13.6\,$eV), ionized carbon exists in a variety of environments, and can trace the H*+/H/H2* transition layer.

[C II] emission is a good tracer of galactic star-formation rates (SFRs) in galaxies (De Looze et al., 2011). They suggested, however, that [C II] emission cannot be used reliably as an SFR indicator for low-metallicity dwarf galaxies, and the scatter of the [C II]/SFR relationship increases as the galactic metallicity decreases. Since CO and [C II] emission are both correlated with the SFR, the connection between CO emission and [C II] intensities has been widely studied. Crawford et al. (1985) showed a strong linear relationship between the intensities of [C II] and 12CO within gas-rich galaxies. Wolfire et al. (1989) found a tight linear correlation between the intensities of [C II] and CO in observations of both Galactic and extragalactic sources, suggesting a common origin of these lines.

Despite the strong [C II]/SFR relationship, there is still some doubt about where exactly the [C II] emission originates. A detailed study by Pabst et al. (2017) shows strong correlation between [C II] and Spitzer 8.0 µm emission from PDRs. Some additional information comes from the Galactic Observations of Terahertz C+ (“GOT C+” Langer et al., 2011) survey, a Herschel (Pilbratt et al., 2010) open time key project111More information can be found on the GOT C+ web site: https://irsa.ipac.caltech.edu/data/Herschel/GOT_Cplus/overview.html. Using GOT C+ data, Pineda et al. (2013) found that about half of [C II] emission ($\sim$47%) is produced in regions of dense PDRs, 28% in dark H2 gas, 21% in cold atomic gas, and just 4% in ionized hydrogen gas. The fraction of [C II] emission originating from the ionized phases of the ISM varies widely, from $\sim 5\%$ to $50\%$ depending on the electron density and ionizing radiation strength (Abel, 2006). [C II]  emission also arises in regions of diffuse neutral gas (Madden et al., 1993). GOT C+ sparsely sampled the Galactic plane along 454 sight lines, in a variety of Galactic environments, but the lack of spatial information toward star formation regions makes their results difficult to generalize.

Since they make the ultraviolet (UV) photons that create C+, the locations of massive stars should be strongly correlated with the locations of intense [C II] emission. The UV radiation from massive stars frequently creates ionized hydrogen, or “H II,” regions. Dust within the regions absorbs and scatters high-energy photons. This leads to dust grain heating and subsequent emission of thermal photons in the mid- and far-infrared (MIR and FIR; e.g., Jones, 2004; Relaño et al., 2016). Therefore, this process is sometimes referred to as “photon-destruction” (e.g., Krishna Swamy & O’dell, 1967). The resulting lack of available H-ionizing photons has been shown to reduce the size of “dusty” H II regions (Sarazin, 1977). Within an H II region, radiation pressure from the central source accelerates the dust grains outwards (Draine, 2011; Akimkin et al., 2015, 2017).

Outside the ionized hydrogen zone of H II regions is a PDR, which is the boundary between the H II region and the interstellar medium. PDRs have a layered structure because interstellar dust shields species from far-UV (FUV) photons, and hence chemical stratifications are produced by the progressively weaker FUV-field (Ossenkopf et al., 2007). The “ionization front” is the boundary of the H II region, interior to which nearly all gas is ionized. Beyond the ionization front, hydrogen is predominantly neutral but carbon may be mostly ionized due to photons with energies between 13.6 $\,{\rm eV}$ and 11.3 $\,{\rm eV}$. At the dissociation front, H2 becomes the dominant species. Further from the ionizing source, where the material is more opaque to FUV-photons and the temperature is decreasing, ionized carbon recombines to produce atomic carbon and 12CO, creating a transition layer of C*+*/C/CO (cf. Hollenbach & Tielens, 1999, their Figure 3).

Although the connection between [C II] and PDRs is well-established, the origin and distribution of [C II] emission toward individual H II regions in the Milky Way has received relatively little study. The few studies have have been done suggest that most of the [C II] emission toward H II regions arises from dense PDRs. In a study of the Orion B molecular cloud, Pabst et al. (2017) report that nearly all [C II] emission ($\sim$95%) originates from the irradiated molecular cloud, with only a small ($\sim$5%) contribution from the adjacent H II region (see also Pabst et al., 2019). Unlike Pineda et al. (2013), they do not make a clear distinction between PDRs and dark molecular gas. Their result is in rough agreement with Goicoechea et al. (2015) who found that 85% of the [C II] emission in the Orion molecular cloud 1 (OMC1) is produced on the surface of the molecular cloud. A smaller amount ($\sim 15\%$) of the [C II] emission comes from a gas component not associated with CO. Goicoechea et al. (2018) also found support for [C II] emission arising from dense PDR gas in OMC1. Simon et al. (2012) observed the H II region complex S106 with the Stratospheric Observatory for Infrared Astronomy (SOFIA; Young et al., 2012) and found that part of the [C II] emission comes from the ionized hydrogen region since the locations of [C II] emission are similar to that of the cm continuum. A more recent study of S106 with SOFIA, however, argued that the [C II] emission is actually from the PDRs (Schneider et al., 2018). Graf et al. (2012) investigated the [C II] emission toward NGC2024 in the Orion B complex with SOFIA observations. They concluded that the observed ionized carbon comes from a highly clumpy interface between the molecular cloud and the H II region and shows a good spatial correlation with the 8$$\,\mu{\rm m} continuum.

Here, we present SOFIA observations of the [C II] 158$$\,\mu{\rm m} line toward the massive star-forming complex Sh2-235, with the goal of understanding the origin of [C II] emission. S235 is a rich complex, with three separate H II regions and prominent PDRs. It therefore has the environments associated with strong [C II] emission. We can thus use the results from S235 to provide context to the results from external galaxies. We deal mainly with velocity-integrated [C II] emission; most of the detailed kinematics of the region will be discussed in a forthcoming paper.

2 The Sh2-235 star formation complex

The Sh2-235 star forming complex (hereafter “S235;” Sharpless, 1959) is located toward the Galactic anti-center. The distance to a water maser in the complex is $1.56^{+0.09}_{-0.08}$ $\,{\rm kpc}$ (Burns et al., 2015). This is roughly consistent with the recent GAIA DR2 parallax of the ionizing source of S235 BD+351201 (Brown, 2018), which corresponds to a distance of $1.73\pm 0.13$ $\,{\rm kpc}$. Here, we adopt a distance of 1.6 $\,{\rm kpc}$ for the region. Since its first appearance in scientific literature (Minkowski, 1946), it has been extensively studied from the optical through radio regimes (e.g., Evans & Blair, 1981; Nordh et al., 1984; Allen et al., 2005; Kirsanova et al., 2008; Boley et al., 2009; Camargo et al., 2011; Kirsanova et al., 2014; Bieging et al., 2016; Dewangan & Ojha, 2017).

In this paper, we study three main regions of the S235 complex (Figure 1). The main S235 region H II region (Sh2-235) which we call “S235MAIN” is ionized by an O9.5V star BD+351201 (Georgelin et al., 1973). Active star formation is continuing in S235MAIN, as it hosts more than $200$ young stellar objects (YSOs; Dewangan & Anandarao, 2011). S235MAIN hosts the IR sources IRS1 and IRS2 (Evans & Blair, 1981), both of which are created by B-type stars.

There are two smaller H II regions to the south of S235MAIN: S235A and S235C (Israel & Felli, 1978). S235A is located $\sim 10^{\prime}$ ($\sim\!4\,\,{\rm pc}$) south of S235MAIN, and is also known as IRS3 (Evans & Blair, 1981) and radio source G173.72+2.70 (Israel & Felli, 1978). S235A has methanol and water masers (see Chavarría et al., 2014, and references therein), and is known as an expanding H II region ionized by stars of main sequence spectral types between B0 and O9.5 (e.g., Felli et al., 1997). Near to S235A is the reflection nebula S235B (IRS4) caused by a B-type star (Boley et al., 2009). We refer to S235A and S235B combined as the star forming region “S235AB” since they are not separated at the angular resolution of our data. S235C is located $\sim\!5\arcmin$ ($\sim\!2\,\,{\rm pc}$) south of S235A and is ionized by a B0.5 star (see Table 1 in Bieging et al., 2016). Dewangan & Ojha (2017) showed that these smaller star formation regions are interacting with the surrounding molecular clouds, and that star formation may be triggered by the expansion of the H II region (e.g., Kirsanova et al., 2008; Camargo et al., 2011). Kirsanova et al. (2014) suggested that the star formation in S235AB is not related to the expansion of S235MAIN.

The S235 complex is an ideal target for studies of [C II] emission. It is nearby, bright, and has been the focus of many previous studies. It also contains three separate H II regions, two of which are compact. The size of H II regions depends on their age and the intensity of ionizing radiation. Therefore, we can examine differences in [C II] emission for H II regions of different ages and ionizing radiation fields.

3 Data

3.1 SOFIA [C II] and [N II] data

We observed [C II] and [N II] emission toward the S235 complex in SOFIA Cycles 4 and 5 in November 2016 and February 2017 using the SOFIA upGREAT instrument (Risacher et al., 2016). upGREAT is an enhanced version of the German Receiver for Astronomy at Terahertz Frequencies (GREAT; Heyminck et al., 2012). We used the upGREAT LFA channel (a 7 pixel array in 2016 and a 2$\times$7 pixel array in 2017) to tune to [C II] and the L1 (single pixel) channel to tune to the ${\rm{}^{3}P_{1}-^{3}P_{0}}$ [N II] 205 µm (1.46 THz) line. The total observing time for both cycles was 3.5 hours. We observed in total power on-the-fly (OTF) mapping mode and mapped a total area of $0{}^{\circ}\mskip-9.0mu.\,21\times 0{}^{\circ}\mskip-9.0mu.\,37$ ($\alpha\times\delta$), centered at $(\alpha,\delta[J2000])$ = (5h41m02.5s, $35^{\circ}51$m57s). We employed a fast mapping mode for S235MAIN, which resulted in an undersampled map for [N II], and a slow mapping mode for S235AB and S235C, which resulted in a fully-sampled [N II] map. The spatial resolution of the [C II] data is $14.8\arcsec$, the velocity resolution is 0.385 $\,{\rm km\,s^{-1}}$, and the full velocity range is $-60$ to $20\,\,{\rm km\,s^{-1}}$. The spatial resolution of the [N II] data is $20.2\arcsec$, the velocity resolution is 0.500 $\,{\rm km\,s^{-1}}$, and the full velocity range is $-60$ to $20\,\,{\rm km\,s^{-1}}$.

The final data cubes provided by the SOFIA Science Center (in units of main beam temperature $T_{\rm MB}$) were processed using the Grenoble Image and Line Data Analysis Software (GILDAS)222http://iram.fr/IRAMFR/GILDAS/ (Pety, 2005). The data were first scaled to $\mathrm{T_{A}^{\ast}}$, the antenna temperature corrected for atmospheric opacity, using a forward efficiency of 0.97. Antenna temperature values were converted to main beam temperatures using the main beam efficiency of $\eta_{\rm MB}=0.69$. If the rms of an individual spectrum was higher than two times the radiometer noise, the spectrum was ignored. First order (if rms $<$ radiometer noise) or third order (if rms $<$ 2 $\times$ radiometer noise) baselines were removed from all spectra.

Here, we frequently use the integrated [C II] intensity, “moment 0,” from the velocity range $-29$ to $-12$ $\,{\rm km\,s^{-1}}$ (Figure 2). All significant [C II] emission associated with S235 is found within this velocity range (see Figure 5). This figure shows strong [C II] emission from the PDRs surrounding S235, but the most intense emission in the field is found toward S235AB and S235C.

[N II] is only weakly detected when averaged over the entire ionized hydrogen region. Because there is little spatial information on the distribution of [N II], we limit analyses using these data.

3.1.1 Regions of interest

Using the [C II] moment 0 map as a guide, we determine regions of interest in the S235 field. For S235MAIN, we define one region of interest spatially coincident with the ionized hydrogen gas (“S235ION”), and an annular region surrounding the ionized hydrogen gas that contains most of the plane-of-sky PDR emission (“S235PDR”). We define regions of interest for the two smaller H II regions located to the south of S235MAIN (“S235AB” and “S235C”). We also sample diffuse emission in the field using five smaller background regions (“bg1–5”). We list the parameters of these regions of interest in Table 1.

3.2 Green Bank Telescope Radio Recombination Line Maps

To understand the distribution and velocity structure of the ionized gas, we create a fully-sampled map in 6 $\,{\rm GHz}$ radio recombination line (RRL) emission using the Green Bank Telescope (GBT). The region has been observed previously in H-$\alpha$ by Lafon et al. (1983), who found a strong velocity gradient in the ionized hydrogen gas; the velocity is $\sim-15\,\,{\rm km\,s^{-1}}$ in the south-east of the region (near our “bg1” region) and $\sim-25\,\,{\rm km\,s^{-1}}$ in the north-west (near our “bg3” region). This strong gradient is unusual for an H II region and may be due in part to absorption of H-$\alpha$ emission or to a bulk “champagne flow” of ionized gas.

We follow the same observing setup as in Anderson et al. (2018) and Luisi et al. (2018), tuning to 64 different frequencies at two polarizations within the 4$-$8 GHz receiver bandpass. Of these 64 tunings, 22 are Hn$\alpha$ transitions from $n=95$ to $n=117$. Carbon and helium RRLs fall within the same bandpass as that of hydrogen, shifted by $-149.56$ $\,{\rm km\,s^{-1}}$ and $-122.15$ $\,{\rm km\,s^{-1}}$ from hydrogen, respectively. Carbon RRLs are thought to arise from PDRs surrounding H II regions, whereas helium RRLs come from the ionized hydrogen region.

There are 15 usable Hn$\alpha$ lines after removing those spoiled by radio frequency interference (RFI) or instrumental effects. Over the frequencies of the usable Hn$\alpha$ lines, the beam size ranges from $98\arcsec$ to $183\arcsec$, with an average of $141\arcsec$. We calibrate the intensity scale of our spectra using noise diodes fired during data acquisition and assume a main beam efficiency of 0.94.

The map is $0.8^{\circ}\times 0.7^{\circ}$, centered at $(\alpha,\delta[J2000])$ = (5h40m42s, $+35^{\circ}$48m52s). We create four complete maps, two in RA and two in Dec., to mitigate in-scan artifacts, and average all together.

We average all lines at a given position to make one sensitive spectrum (Balser, 2006), a technique that is well-understood (Anderson et al., 2011; Liu et al., 2013; Alves et al., 2015; Luisi et al., 2018). After removing transient RFI, we remove a polynomial baseline for each transition and shift the spectra so that they are aligned in velocity (Balser, 2006). We re-grid the 15 good Hn$\alpha$ lines to a velocity resolution of 0.5$\,{\rm km\,s^{-1}}$ and a spatial resolution of 1′. We then average the individual maps using a weighting factor of $t_{\rm intg}T^{-2}_{\rm sys}$ where $t_{\rm intg}$ is the integration time and $T_{\rm sys}$ is the system temperature.

We show the integrated intensity moment zero hydrogen RRL map in Figure 3. The emission is strongest for S235MAIN and peaked at the location of the ionizing source. S235AB also shows detected emission, but S235C does not. The other H II regions in the field have some detected RRL emission. We show the peak line velocity derived from Gaussian fits in Figure 5, although due to the relatively poor spatial resolution we do not show the fit for S235PDR. For S235ION, the fit is to the average spectrum from our RRL map integrated over the entire region of interest. For S235AB and S235C, we use the pointed RRL results from Anderson et al. (2015), because the signal to noise in the RRL map is rather poor and the observational setup in Anderson et al. (2015) is the same as ours used here.

3.3 Ancillary data

We also use WISE MIR, 1.4 $\,{\rm GHz}$ NVSS radio continuum, and CO rotational transitions to investigate the S235 region (Figure 4). For later comparisons, we regrid all ancillary data to $10\arcsec$ pixels, and do the same for the SOFIA [C II] data. The CO data in panels (d), (e), and (f) of this figure are of integrated intensity, integrated over $-29$ to $-12$ $\,{\rm km\,s^{-1}}$ as we did for [C II] (Figure 2).

3.3.1 WISE 12$\,\mu{\rm m}$ and 22$\,\mu{\rm m}$ data

WISE (Wright et al., 2010) mapped the entire sky at four wavelengths: 3.4$$\,\mu{\rm m}, 4.6$$\,\mu{\rm m}, 12$$\,\mu{\rm m} and 22$$\,\mu{\rm m}. The angular resolutions are $6{}^{\prime\prime}\mskip-9.0mu.\,1$, $6{}^{\prime\prime}\mskip-9.0mu.\,4$, $6{}^{\prime\prime}\mskip-9.0mu.\,5$ and $12^{\prime\prime}$ with $5\sigma$ point-source sensitivities of $0.08$ mJy, $0.11$ mJy, $1$ mJy and $6$ mJy at the native pixel scales, respectively. We use here the 12$$\,\mu{\rm m} and 22$$\,\mu{\rm m} bands. The 12$$\,\mu{\rm m} band is sensitive to the emission from polycyclic aromatic hydrocarbon (PAH) features at 11.2$$\,\mu{\rm m}, 12.7$$\,\mu{\rm m}, and 16.4$$\,\mu{\rm m} (e.g., Roser & Ricca, 2015; Tielens, 2008). Due to high expected optical depth in this wavelength range, the WISE 12 µm band is likely a surface, rather than volume tracer, in contrast to the some of the other ancillary data used here. The W4 22$$\,\mu{\rm m} bandpass is sensitive to stochastically-heated very small grains (VSGs) within the H II region plasma, and also to dust grains within the PDRs (PAHs are prominent contributors of 24$$\,\mu{\rm m} emission; see Robitaille et al., 2012).

The WISE data have “Digital Number” (DN) units and we used the DN-to-mJy conversion factors of $2.9045\times 10^{-3}$ and $5.2269\times 10^{-2}$ for the 12$$\,\mu{\rm m} and 22$$\,\mu{\rm m} data, respectively333For more information, see http://wise2.ipac.caltech.edu/docs/release/prelim/expsup/sec2_3f.html. We use the color-correction of Wright et al. (2010), assuming a spectral index of $\alpha=0$. This correction increases the 12 and 22 µm flux densities by $9.1\%$ and $1.0\%$, respectively.

The 12$$\,\mu{\rm m} data show the same spatial distribution as [C II] throughout the field, including for the smaller H II regions to the south (see panel (a) in Figure 4). In the PDR of S235, the 12$$\,\mu{\rm m} emission follows the filamentary distribution of the ionized carbon. The WISE 22$$\,\mu{\rm m} emission (panel (b) in Figure 4) is centrally concentrated for S235, with comparatively faint emission in the PDRs. The 22 µm emission for S235AB and S235C is compact.

3.3.2 NVSS 1.4 $\,{\rm GHz}$ continuum data

The ionized gas of H II regions emits in radio continuum due to free-free (Bremsstrahlung) radiation. We obtained $1.4\,$GHz radio continuum data from the NRAO VLA Sky Survey444https://www.cv.nrao.edu/nvss/ (NVSS; Condon et al., 1998). The NVSS FWHM synthesized beam size is $\sim 45\arcsec$ and its rms uncertainty is $\approx 0.45\,$mJy/beam. During the regridding process, we convert the NVSS data from their native units of Jy beam*-1* to mJy pixel*-1*, the same as we use for our WISE data.

Similar to the RRL and 22 µm band data, the radio continuum emission (panel (c) in Figure 4) is concentrated around the ionizing source. That the [C II] and radio continuum distributions differ shows that we observe two different ionized phases of the ISM along the line of sight.

3.3.3 CO data

The ancillary molecular line data used here of ${}^{12}\mathrm{CO}$ $2-1$, ${}^{12}\mathrm{CO}$ $3-2$, and ${}^{13}\mathrm{CO}$ $2-1$ are from Bieging et al. (2016). The observations were made with the Heinrich Hertz Submillimeter Telescope555For more information, see: http://aro.as.arizona.edu/smt_docs/smt_telescope_specs.htm in March to April, $2010$. The two $J=2-1$ lines were observed simultaneously. The $\mathrm{{}^{12}CO}$ emission line was observed in the upper sideband, while the $\mathrm{{}^{13}CO}$ line at $220.399\,\mathrm{GHz}$ was observed in the lower sideband. The maps cover $\sim 1{}^{\circ}\mskip-9.0mu.\,2\times 0{}^{\circ}\mskip-9.0mu.\,8$ ($\alpha,\delta$), have a velocity resolution of $0.3\,$km s*-1*, and have a spatial resolution of $38\arcsec$.

The ${}^{12}\mathrm{CO}$ $3-2$ line was observed in April, 2014, also with the Heinrich Hertz Submillimeter Telescope, using the multi-pixel focal plane array of superconducting mixers (“SuperCam”; Kloosterman et al., 2012). The FWHM beam width is $24\arcsec$. The total area covered is $\sim 0{}^{\circ}\mskip-9.0mu.\,3\times 1{}^{\circ}\mskip-9.0mu.\,0$ ($\alpha,\delta$) with 0.23\,$$\,{\rm km\,s^{-1}} velocity resolution.

Kirsanova et al. (2008) distinguished three main velocity components in the S235MAIN region using 13CO $1-0$ and CS $2-1$ observations: a “red” component ($-18<\mathrm{V_{lsr}}<-15\,$km s*-1*), a “central” component ($-21<\mathrm{V_{lsr}}<-18\,$km s*-1*) and a “blue” component ($-25<\mathrm{V_{lsr}}<-21\,$km s*-1*). With additional observations of NH3, Kirsanova et al. (2014) confirmed the existence of these different velocity components. Using 12CO $1-0$ data, Dewangan & Ojha (2017) reported that the emission toward S235MAIN peaks in the velocity range $-22$ to $-20$, while the smaller H II regions, S235A (“S235AB”) and S235C, are at \sim-17\,$$\,{\rm km\,s^{-1}}.

The 12CO $2-1$ data have a broad velocity component (centered at $\sim-11.5\,$km s*-1*) that is well-separated from the aforementioned velocity components, but is absent in the other molecular and [C II] line data (see Figure 5). This redshifted component is possibly related to more diffuse gas not associated with S235; the component is also prominent in EBHIS H I emission (Winkel et al., 2016). As this component is redshifted relative to the overall velocity of the star-forming region S235, we likely observe the diffuse molecular gas component foreground or background to the star formation complex.

The CO moment maps have similar spatial distributions (see panels (d), (e) and (f) in Figure 4). The molecular emission is more extended than that of [C II], but components associated with the ionized hydrogen zone and the PDR of S235MAIN are clearly visible. The spatial distributions of the observed molecular gas tracers shows a bridge between S235AB and S235C, especially in case of the denser gas tracer 13CO $2-1$. As these small H II regions are essentially at the same velocity, this indicates that S235AB and S235C might be physically connected by a dense gas component.

In the S235PDR region the spatial correlation of the observed [C II] and molecular gas is weaker than in the inner ionized hydrogen region. The CO emission appears clumpy compared to the emission of the other tracers. CO emission is absent toward the north. This “emission-free” part can also be seen in previous studies (e.g., Dewangan et al., 2016; Dewangan & Ojha, 2017). This molecular deficit may be due to escaping ionized hydrogen gas in this direction (Dewangan et al., 2016). Although there is no indication that ionized hydrogen gas fills the evacuated space (see radio continuum map, panel (c) of Figure 4), the gas may be too rarefied to produce strong radio continuum emission.

3.4 Velocity structure of [C II] emission

We compare the spectra of [C II] (Section 3.1), RRL (Section 3.2), and CO (Section 3.3.3) emission from the regions of interest in Figure 5. The [C II] emission peaks near $-20$ $\,{\rm km\,s^{-1}}$ for S235MAIN, and $-18\,\,{\rm km\,s^{-1}}$ for S235AB and S235C. All significant [C II] emission is in the range $-29$ to $-12\,\,{\rm km\,s^{-1}}$. Hydrogen RRL emission is much broader than that of the other tracers. It peaks near $-23\,\,{\rm km\,s^{-1}}$ for S235ION an near $-16\,\,{\rm km\,s^{-1}}$ for S235AB and S235C. Quireza et al. (2006) found that H RRLs toward S235ION peak at $-23.07\,\,{\rm km\,s^{-1}}$ and C RRLs peak at $-19.82\,\,{\rm km\,s^{-1}}$ (Silverglate & Terzian, 1978; Vallee, 1987, see also). Anderson et al. (2015) found that the RRL emission from S235A peaks at $-15.3\,\,{\rm km\,s^{-1}}$, and that of S235C peaks at $-16.6\,\,{\rm km\,s^{-1}}$. The CO emission has a similar velocity profile to that of [C II], and carbon RRLs also peak near the velocities of strongest CO emission. We conclude that there is a real offset between the H RRL emission and that of other tracers in S235ION. This offset is not seen in the other regions of interest.

The [C II] emission can be decomposed into three velocity ranges: $-23$ to $-20\,\,{\rm km\,s^{-1}}$ (“low”), $-20$ to $-18$ $\,{\rm km\,s^{-1}}$ (“middle”), and $-18$ to $-13$ $\,{\rm km\,s^{-1}}$ (“high”). These three ranges have distinct emission components. We show moment maps of these three velocity ranges in Figure 6. The low velocity range contains bright compact [C II] emission in the ionized hydrogen region of S235MAIN, emission from the eastern PDR, and a ridge of emission extending to the northwest. The middle velocity range contains multiple knots of compact [C II] emission in the ionized hydrogen region of S235MAIN, and emission from the PDR to the northeast and to the west. The high velocity range mainly contains [C II] emission from the western PDR of S235MAIN, and also compact emission from S235AB and S235C. Although S235AB and S235C are detected in the middle velocity range, this is just the blueshifted wing of emission (see Figure 5). All velocity components show excellent spatial agreement with 12CO $2-1$ emission.

4 Correlations between [C II] and ancillary data

Below, we determine correlations between [C II] intensity and the intensity of the ancillary data sets. We are dealing with emission integrated along the line of sight, and so in the direction of the ionized hydrogen gas we also detect emission from the front- and back-side PDRs. This is especially important for the 12 $\,\mu{\rm m}$ emission, as it is a strong PDR tracer. For example, the region S235ION contains dense molecular clumps S235 Central E and Central W (Kirsanova et al., 2014; Dewangan & Ojha, 2017). These clumps are presumably do not reside within the ionized hydrogen volume, but nevertheless the emission associated with the clumps is contained in the S235ION region of interest.

The NVSS and RRL data both trace the emission from ionized gas in the region. The spatial resolution of the NVSS is more than twice as fine as that of our RRL data. We therefore use the NVSS for all spatial analyses, and use the RRL data for all spectral analyses.

4.1 Total fluxes in regions of interest

We give the total fluxes for the regions of interest in Table 2. We find these values by integrating the emission over each aperture as defined in Table 1. For [C II] and the molecular tracers, the data are in units of main beam temperature, $T_{\rm MB}$, so the integrated intensity $I_{\rm[C\,{\footnotesize II}]}=\int T_{\rm MB}dv$ has units of $\,{\rm K}$  $\,{\rm km\,s^{-1}}$. We convert the [C II] and CO data to the more useful quantity of flux in units of W m*-2* per pixel. For our SOFIA [C II] maps with $7\arcsec$ pixels, the conversion between integrated intensity and flux for each pixel is

[TABLE]

We subtract a local background value from the two WISE bands, computed as the median value surrounding the region, but perform no background correction for the other data.

The $I_{\rm[C\,{\footnotesize II}]}$ and $F_{12\,\micron}$ values appear in roughly the same ratio for all regions of interest. This correlation is less strong for the other tracers. We will explore this in more detail in Section 4.3.

We see from Table 2 that about half the emission from S235MAIN comes from the S235PDR region of interest for [C II], the MIR bands, and all CO transitions. The [C II] emission is strong south of the ionizing source (see Figure 2). The [C II] emission in a central zone $1.5\arcmin$ in radius around the ionizing source has an integrated intensity of $\sim 0.6\times 10^{-12}$ W m*-2*, which is $\sim 40\%$ that of S235ION and $\sim 20\%$ that of the total emission from S235MAIN.

The values for all tracers are similar for S235AB and S235C, except for the radio continuum data. S235AB has $\sim 6$ times higher flux density than S235C, indicating a higher Lyman continuum photon production rate in S235AB, a higher optical depth in S235C, and/or stronger dust absorption in S235C.

For S235MAIN, roughly half of the integrated intensity in all CO transitions is coming from the S235ION region, and half from the S235PDR region. In all the observed molecular transitions, the S235AB region is marginally brighter than S235C.

4.2 [C II] emission from front- and back-side PDRs

Because of emission from front- and back-side PDRs along the line of sight, it is difficult to accurately determine the percentage of [C II] emission from the entire PDR of S235. Based on results from previous studies of H II regions in [C II] emission, we expect that nearly all the [C II] emission arises from dense PDRs (e.g., Pabst et al., 2017).

In Figure 7 we show position-velocity (p-v) diagrams for S235MAIN. These diagrams show [C II] emission from the S235ION region in red and from S235PDR in cyan. We create the orange (from H RRL data) and green (from C RRL data) crosses in this figure by integrating the RRL data along its position axes, and then fitting Gaussians spaxel by spaxel.

Based on its coincidence with CO and the velocity offset of the ionized hydrogen gas with respect to all other tracers, we believe that the [C II] emission seen toward the ionized hydrogen is from line-of-sight PDRs and not from the ionized hydrogen volume. The peak velocity of [C II], CO, and C RRL emission toward the ionized hydrogen gas is redshifted relative to that of the ionized hydrogen gas itself, whereas [C II] and CO emission from the PDR is at a similar velocity to that of the ionized hydrogen gas. In support of this, we note the excellent spatial agreement between [C II] and CO emission in the three velocity ranges shown in Figure 6. This is consistent with an expansion of the ionized hydrogen gas preferentially toward us. In this scenario, the [C II], CO, and C RRL emission are therefore due to back-side PDRs, as we draw in the cartoon model of Figure 8. This explanation is consistent with the fact that minimal absorption is seen across the face of the region in H$-\alpha$ maps (see Figure 1; also Dewangan & Ojha, 2017, their Figure 10b). If there were dense foreground PDR material, we would see H-$\alpha$ absorption across the face of the region, which we do for H II regions RCW120 (see Anderson et al., 2015, their Figure 1) and M20 (the Trifid Nebula; see Rho et al., 2006, their Figure 1). Our picture of S235 is similar to the model of Orion in Pabst et al. (2019, cf. their Figure 3). The main difference is that they observe a shell of [C II] expanding toward us, whereas in S235 we see no evidence for such a feature.

We can also use the measured [C II] and [N II] intensities to estimate the fraction of [C II] emission that arises from neutral hydrogen regions. Croxall et al. (2017) found that the [C II] to [N II] intensity ratio has a value of $I_{\rm[C\,{\footnotesize II}]}/I_{\rm[N\,{\footnotesize II}]}\simeq 4$ for ionized gas, independent of electron density. Values of $I_{\rm[C\,{\footnotesize II}]}/I_{\rm[N\,{\footnotesize II}]}>4$ indicate [C II] emission from regions of neutral hydrogen. From the S235ION region, we measure $I_{\rm[N\,{\footnotesize II}]}\simeq 2\times 10^{-13}$ W m*-2*, which gives us $I_{\rm[C\,{\footnotesize II}]}/I_{\rm[N\,{\footnotesize II}]}\simeq 10$ (see Table 2). Using Equation 1 from Croxall et al. (2017), this results in a fraction of [C II] emission from neutral hydrogen regions $f_{\rm{\rm[C\,{\footnotesize II}]},Neutral}\simeq 0.5$. Because of the low quality of our [N II] data, this value is only approximate. Our intensity ratio, however, is roughly consistent with that observed in the external galaxy sample of Croxall et al. (2017) and the study of Rigopoulou et al. (2013). If half of the [C II] emission towards S235ION comes from locations of neutral hydrogen, just 25% of the emission from all of S235MAIN is from the ionized hydrogen volume.

4.3 Correlations with [C II] intensities

We investigate the correlation between [C II] intensity and that of the other tracers to help determine the origin of the [C II] emission. We plot pixel-by-pixel correlations of the regridded data (Figures 9, 10, A.1, and A.2) to examine the relationships between the integrated intensity of [C II] and that of the other tracers in the S235MAIN (S235ION and S235PDR), S235AB, and S235C regions. The bottom panel of each figure contains all data points, one per pixel of the regridded maps.

To quantify the relationships between [C II] and other observed tracers, we fit linear regressions of the form $I_{{\rm[C\,{\footnotesize II}]}}=A+B\times I_{\lambda}$, where $I_{{\rm[C\,{\footnotesize II}]}}$ is the integrated intensity of [C II], $I_{\lambda}$ is the integrated intensity of CO or the flux density for the IR and radio continuum data. We use a robust least-squares method to determine the fit parameters, with a Cauchy loss function. The robust least squares fit minimizes the influence of outliers, which otherwise can skew the results. For data with few outliers, the choice of loss function has minimal impact on the fit parameters. We have left the data in their natural units, so $B$ has units of K $\,{\rm km\,s^{-1}}$/mJy for the WISE and NVSS data, and is unitless for the CO data. We compute the coefficient of determination $R^{2}$ and list these values and the fit parameters in Table 4.3.

Based on previous results, we expect that this linear relationship can approximate the correlation between [C II] and the other tracers, as Pabst et al. (2017) found between [C II] and 8.0 µm emission in L1630. For example, Malhotra et al. (1997) show that the [C II]/IR ratio for galaxies is roughly constant at $27\times 10^{-3}$ for $L_{\rm IR}<10^{11}L_{\odot}$. De Looze et al. (2011) claim that the $L_{{\rm[C\,{\footnotesize II}]}}/_{\rm FIR}$ ratio is typically linear within normal galaxies, but that it shows non-linearities for ultraluminous galaxies. Although more complicated forms of the relationship are also found in the literature (e.g., $\log(L_{{\rm[C\,{\footnotesize II}]}})=A\times\log(L_{\rm IR})+B$; Ibar et al., 2015), we prefer the simplicity of the linear relationship.