The Origin of [C II] 158 m Emission toward the H II Region Complex S235
Although the transition of [C II] at m 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 [C II] emission toward the H II region complex Sh2-235 (S235) to better understand in detail the origin of [C II] 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 [C II] 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 [C II] intensity is strongly correlated with WISE 12 intensity across the entire complex, indicating that both trace ultra-violet radiation fields. The 22 and radio continuum intensities are only correlated with [C II] intensity in the ionized hydrogen portion of the S235 region and the correlations between the [C II] 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 [C II] emission, only WISE 12 m emission is correlated on small-scales of the individual H II region S235 and also has a decent correlation at the scale of entire galaxies. Future studies of a larger sample of Galactic H II regions would help to determine whether these results are truly representative.
0000-0001-8800-1793]L. D. Anderson \move@AU\move@AF\@affiliationDepartment of Physics and Astronomy, West Virginia University, Morgantown WV 26506 \move@AU\move@AF\@affiliationAdjunct Astronomer at the Green Bank Observatory, P.O. Box 2, Green Bank WV 24944 \move@AU\move@AF\@affiliationCenter for Gravitational Waves and Cosmology, West Virginia University, Chestnut Ridge Research Building, Morgantown, WV 26505
0000-0002-1397-546X]Z. Makai \move@AU\move@AF\@affiliationDepartment of Physics and Astronomy, West Virginia University, Morgantown WV 26506
0000-0001-8061-216X]M. Luisi \move@AU\move@AF\@affiliationDepartment of Physics and Astronomy, West Virginia University, Morgantown WV 26506 \move@AU\move@AF\@affiliationCenter for Gravitational Waves and Cosmology, West Virginia University, Chestnut Ridge Research Building, Morgantown, WV 26505
0000-0002-5306-4089]M. Andersen \move@AU\move@AF\@affiliationGemini South, Casilla 603, La Serena, Chile 0000-0002-5306-4089
Aix Marseille University, CNRS, CNES, LAM, Marseille, France 13388
0000-0002-9431-6297]M. R. Samal \move@AU\move@AF\@affiliationPhysical Research Laboratory, Navrangpura, Ahmedabad, Gujarat 380009, India
0000-0003-3485-6678]N. Schneider \move@AU\move@AF\@affiliationI. Physikalisches Institut der Universität zu Köln, Zülpicher Strae 77, 50937, Köln, Germany
0000-0001-6172-3403]P. Tremblin \move@AU\move@AF\@affiliationCEA-Saclay, Gif-sur-Yvette, France 91191
0000-0001-9509-7316]A. Zavagno \move@AU\move@AF\@affiliationAix Marseille University, CNRS, CNES, LAM, Marseille, France 13388
0000-0003-4338-9055]M. S. Kirsanova \move@AU\move@AF\@affiliationInstitute of Astronomy of the Russian Academy of Sciences, Moscow, Russia 119017 \move@AU\move@AF\@affiliationUral Federal University, Astronomical Observatory, Lenin 51, Ekaterinburg, Russia, 620083
0000-0002-8351-3877]V. Ossenkopf-Okada \move@AU\move@AF\@affiliationI. Physikalisches Institut der Universität zu Köln, Zülpicher Strae 77, 50937, Köln, Germany
Ural Federal University, Astronomical Observatory, Lenin 51, Ekaterinburg, Russia, 620083
The [C II] line is one of the most important transitions in the interstellar medium (ISM). This line arises from the transition of ionized carbon at (THz), at an equivalent temperature of K. Between 0.1 and of the total FIR-luminosity of galaxies is provided by this emission line (crawford1985; malhotra97; boselli2002), a result that also holds for Galactic star formation regions (stacey1991; schneider98). 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 (pineda2013; pabst2017). Since carbon has a lower ionization potential than hydrogen (eV vs. eV), ionized carbon exists in a variety of environments, and can trace the H/H/H transition layer.
[C II] emission is a good tracer of galactic star-formation rates (SFRs) in galaxies (delooze2011). 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. crawford1985 showed a strong linear relationship between the intensities of [C II] and CO within gas-rich galaxies. wolfire1989 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 pabst2017 shows strong correlation between [C II] and Spitzer 8.0 m emission from PDRs. Some additional information comes from the Galactic Observations of Terahertz C (“GOT C” langer2011) survey, a Herschel (pilbratt2010) open time key project333More 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, pineda2013 found that about half of [C II] emission (47%) is produced in regions of dense PDRs, 28% in dark H 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 to depending on the electron density and ionizing radiation strength (abel2006). [C II] emission also arises in regions of diffuse neutral gas (madden1993). 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., jones04; relano16). Therefore, this process is sometimes referred to as “photon-destruction” (e.g., swamy67). The resulting lack of available H-ionizing photons has been shown to reduce the size of “dusty” H II regions (sarazin77). Within an H II region, radiation pressure from the central source accelerates the dust grains outwards (draine11b; akimkin15; akimkin17).
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 (ossenkopf2007). 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 and 11.3 . At the dissociation front, H 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 CO, creating a transition layer of C/C/CO (cf. hollenbach1999, 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, pabst2017 report that nearly all [C II] emission (95%) originates from the irradiated molecular cloud, with only a small (5%) contribution from the adjacent H II region (see also pabst19). Unlike pineda2013, they do not make a clear distinction between PDRs and dark molecular gas. Their result is in rough agreement with goicoechea2015a 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 () of the [C II] emission comes from a gas component not associated with CO. goicoechea2018 also found support for [C II] emission arising from dense PDR gas in OMC1. simon2012 observed the H II region complex S106 with the Stratospheric Observatory for Infrared Astronomy (SOFIA; young2012) 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 (schneider18). graf2012 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 continuum.
Here, we present SOFIA observations of the [C II] 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;” sharpless1959) is located toward the Galactic anti-center. The distance to a water maser in the complex is (burns15). This is roughly consistent with the recent GAIA DR2 parallax of the ionizing source of S235 BD+351201 (brown18), which corresponds to a distance of . Here, we adopt a distance of 1.6 for the region. Since its first appearance in scientific literature (minkowski1946), it has been extensively studied from the optical through radio regimes (e.g., evans1981; nordh1984; allen2005; kirsanova2008; boley2009; camargo2011; kirsanova2014; bieging2016; dewangan2017).
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 (georgelin1973). Active star formation is continuing in S235MAIN, as it hosts more than young stellar objects (YSOs; dewangan2011). S235MAIN hosts the IR sources IRS1 and IRS2 (evans1981), both of which are created by B-type stars.
There are two smaller H II regions to the south of S235MAIN: S235A and S235C (israel1978). S235A is located () south of S235MAIN, and is also known as IRS3 (evans1981) and radio source G173.72+2.70 (israel1978). S235A has methanol and water masers (see chavarria2014, 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., felli1997). Near to S235A is the reflection nebula S235B (IRS4) caused by a B-type star (boley2009). 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 () south of S235A and is ionized by a B0.5 star (see Table 1 in bieging2016). dewangan2017 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., kirsanova2008; camargo2011). kirsanova2014 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.
3.1 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 (risacher2016). upGREAT is an enhanced version of the German Receiver for Astronomy at Terahertz Frequencies (GREAT; heyminck2012). We used the upGREAT LFA channel (a 7 pixel array in 2016 and a 27 pixel array in 2017) to tune to [C II] and the L1 (single pixel) channel to tune to the [N II] 205 m (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 (), centered at = (5h41m02.5s, 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 , the velocity resolution is 0.385 , and the full velocity range is to . The spatial resolution of the [N II] data is , the velocity resolution is 0.500 , and the full velocity range is to .
The final data cubes provided by the SOFIA Science Center (in units of main beam temperature ) were processed using the Grenoble Image and Line Data Analysis Software (GILDAS)444http://iram.fr/IRAMFR/GILDAS/ (pety2005). The data were first scaled to , 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 . 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 radiometer noise) baselines were removed from all spectra.
Here, we frequently use the integrated [C II] intensity, “moment 0,” from the velocity range to (Figure 3.1). All significant [C II] emission associated with S235 is found within this velocity range (see Figure LABEL:fig:av_specs). 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 LABEL:tab:regions.