$\rm H_2CO$ and $\rm H110\alpha$ Observations toward the Aquila Molecular Cloud

The formaldehyde $\rm H_2CO(1_{10} - 1_{11})$ absorption line and H$110\alpha$ radio recombination line (RRL) have been observed toward the Aquila Molecular Cloud using the Nanshan 25 m telescope operated by the Xinjiang Astronomical Observatory CAS. These first observations of the $\rm H_2CO$ $(1_{10} - 1_{11})$ absorption line determine the extent of the molecular regions that are affected by the ongoing star formation in the Aquila molecular complex and show some of the dynamic properties. The distribution of the excitation temperature $T_{ex}$ for $\rm H_2CO$ identifies the two known star formation regions W40 and Serpens South as well as a smaller new region Serpens 3. The intensity and velocity distributions of $\rm H_2CO$ and $\rm ^{13}CO(1-0)$ do not agree well with each other, which confirms that the $\rm H_2CO$ absorption structure is mostly determined by the excitation of the molecules resulting from the star formation rather than by the availability of molecular material as represented by the distribution. Some velocity-coherent linear $\rm ^{13}CO(1-0)$ structures have been identified in velocity channel maps of $\rm H_2CO$ and it is found that the three star formation regions lie on the intersect points of filaments. The $\rm H110\alpha$ emission is found only at the location of the W40 H II region and spectral profile indicates a redshifted spherical outflow structure in the outskirts of the H II region. Sensitive mapping of $\rm H_2CO$ absorption of the Aquila Complex has correctly identified the locations of star-formation activity in complex molecular clouds and the spectral profiles reveal the dominant velocity components and may identify the presence of outflows.


INTRODUCTION
The Aquila Molecular Cloud (AMC) or the Aquila Rift complex is located along the Galactic plane and stretches from 20 • to 40 • in longitude and from -1 • to 10 • in latitude, as revealed by CO and H I, observations (Dame et al. 2001;Prato et al. 2008). The western part of Aquila Rift contains several active starforming regions: Serpens Main, Serpens South, W40, and MWC 297. Here, we focus on part of the Aquila Rift complex that harbors two known sites of star formation: the western Serpens South is a young embedded cluster which suggests that arcs and large-scale expanding bubbles and/or flows affect the velocity fields and play a role in the formation and evolution of the these clouds (Nakamura et al. 2017).
In this study we consider the H 2 CO absorption at 4.830 GHz and H110α recombination line emission at 4.874 GHz in the Aquila Rift. The distribution of H 2 CO absorption in the Galaxy and against 262 Galactic radio sources has been surveyed showing that H 2 CO absorption is associated with most of the H II regions (Davies & Few 1979;Downes et al. 1980;Pipenbrink & Wendker 1988). Since H 2 CO is only seen in absorption against a background continuum it only samples the physical conditions in the foreground of the H II region, while other mm and sub-mm spectral lines are observed both in front and behind the source. The correlation between the distribution of 13 CO emission and H 2 CO absorption is found to be very strong such that both components arise from similar regions (Tang et al. 2013).
In the present paper, we show results of the first H 2 CO and H110α observations toward the W40 and Serpens South regions of the Aquila Rift. In Section 2, we present the details of our observations. The results and discussion of the observations are described in Section 3. Finally, the conclusions are summarised in Section 4.

H 2 CO and H110α observations
The H 2 CO (1 10 -1 11 ) absorption line (λ = 6 cm, ν 0 = 4829.6594 MHz) and the H110α RRL (ν 0 = 4874.1570 MHz) have been observed in the Aquila molecular cloud during February 2015 using the Nanshan 25-m radio telescope of the Xinjiang Astronomical Observatory of Chinese Academy of Sciences. The 25-m radio telescope has an HPBW (half power beam width) of 10 ′ at this wavelength. The observations were performed in an On-The-Fly mode with an average integration time of one minute for each position. The central position of the observing pattern is 18 h 30 m 03 s -2 • 02 ′ 40 ′′ (J2000). The 6 cm low noise receiver had a system temperature of about 23 K during the observations. In order to observe the H 2 CO and the H110α RR lines simultaneously, the center frequency of the spectrometer was set at 4851.9102 MHz. A Digital Filter Bank was used with 8192 channels and 64 MHz bandwidth, corresponding to a velocity resolution of 0.48 km s −1 at 4.852 GHz. The sensitivity of the system (DPFU, Degrees Per Flux Unit) was 0.116 K Jy −1 and the main beam efficiency at this wavelength is 65%.
We used CLASS and GREG (parts of GILDAS) to process the H 2 CO and H110α line data. The area of the Aquila molecular cloud observed is 100 ′ × 100 ′ . The average sigma noise level of these maps is 0.020 K. The signal to noise ratio of all detected points was better than 3. Assuming a distance of 436 pc for the Aquila complex, the spatial scale of the maps is 0.124 pc arcmin −1 .

Archival data
The 13 CO(1 − 0) and 12 CO(1 − 0) data observed with the 13.7 m millimeter wave telescope of Purple Mountain Observatory in Delingha in April and May 2011 have been taken from the Millimeter Wave Radio Astronomy Database 1 . The velocity resolution of this data is 0.17 km s −1 and the system temperature of these onthe-fly mode observations ranged from 250 to 310 K. The 13 CO(J = 1 − 0) data to 10 ′ has been resampled onto the H 2 CO observing grid. The sigma noise levels of the 13 CO(1 − 0) and 12 CO(1 − 0) data are 0.056 K and 0.122 K, respectively. The 6 cm continuum data for the Aquila Rift region has been obtained from the Sino-German λ6 polarization survey of the Galactic Plane using the Urumqi 25 m telescope of the National Astronomical Observatories, CAS, were taken by Sun et al. (2011). The central frequency of the data was 4.8 GHz, and the observing bandwidth was 600 MHz. The resolution of the data is 9.5 ′ and the system temperature was about 22 K at the zenith.

The Formaldehyde Absorption
The intensity map of the H 2 CO absorption towards the Aquila Molecular Cloud integrated over the velocity interval of 3 -11 km s −1 is shown in Fig.1a. The observation parameters are shown in Tab.1. Two concentrations can be seen in the map, that correspond to the W40 H II region and Serpens South with maximum flux values of the H 2 CO absorption of -1.097 K km s −1 and -1.007 K km s −1 . The velocities at the offsets of (20,-5) and (0,0) are 7.125 km s −1 and 6.216 km s −1 , respectively. The color-scale map of the 13 CO(J = 1−0) emission is shown together with the H 2 CO contours in Fig.1b. The 13 CO emission map at Serpens South shows several elongated structures that only partially follow the H 2 CO absorption structure and there is enhanced emission on the north side o W40 that could result from heating by the H II region.
The H 2 CO absorption and the 13 CO emission structures are not in good agreement with each other in both the Serpens South and the H II region. The 13 CO emission at W40 is offset and there is no clear concentration   or a change in optical depth corresponding to Serpens South in Fig.1b. The velocity distributions of H 2 CO is smooth and shows a gradient towards the East of the AMC (Fig.2a). In the W40 region the velocity of ∼ 7 km/s corresponds to the systematic velocity of cold gas surrounding the entire region (Shimoikura et al. 2015) and agrees with the velocity found at the periphery of this region by Shimoikura et al. (2018). The velocity of H 2 CO ( ∼ 6 km/s) at the southern part of main body of Serpens South agrees with the 13 CO velocity distribution. On the other hand, the velocity structure of 13 CO (Fig.2b) shows significant substructure and a gradient towards the North-East. The low intensity regions at the boundary between W40 and Serpens South may be related to an interaction between the regions as suggested by Shimoikura et al. (2018). In detail, the H 2 CO absorption and 13 CO emission display different distributions and there is no clear evidence that the Serpens South region is superposed on the sky with the W40 region.
The 5 GHz radio continuum image within AMC is displayed in Fig.3. The continuum temperature T c in the H II region ranges from 1 to 3.3 K, and is less than 0.04 K in Serpens South (in Tab.1, col.6). The strong H 2 CO absorption at W40 is clearly related to the radio continuum of the H II region, while the continuum is significantly weaker at Serpens South. Because the 13 CO emission does not show any enhancement at Serpens South, the H 2 CO absorption there should result from an en-hanced continuum background within Serpens South in addition to the cosmic microwave background (CMB). Therefore the absorption contours in W40 and Serpens South define the extent of the region that is affected by the ongoing star formation, which is well beyond the sites of star formation.
The formaldehyde line is in absorption across the whole region of W40 and Serpens South. This can only happen when everywhere the excitation temperature of the 6-cm line is less than the brightness temperature of the radio continuum sources plus the microwave background. While the excitation temperature T ex may be uniformly low (≤ 1K) in cold clouds (Heiles 1973), the excitation conditions may vary strongly across star formation regions. Therefore, a simple determination of the absorbing H 2 CO column density would not be accurate. Instead we perform an approximate determination of the excitation temperature of H 2 CO across the region. Making the imperfect assumption that the column densities of H 2 CO and 13 CO(1 − 0) are correlated, we may estimate the H 2 CO column density from the column density of 13 CO(1 − 0), which may be obtained from Sato et al. (1994). Assuming an H 2 CO to H 2 abundance ratio of 3 × 10 −9 (Evans et al. 1975) and a 13 CO(1 − 0) to H 2 abundance ratio of 2.4 × 10 −6 (Ripple et al. 2013), the column density ratio of between H 2 CO and 13 CO(1 − 0) may be calculated as: (1) Using the optical depth τ app of H 2 CO as calculated using the formulation in Pipenbrink & Wendker (1988): which already assumed a mean value for T ex = 2 K. The excitation temperature of H 2 CO may then be calculated using Heiles (1973) as follows: where T L is the antenna temperature of the absorption line in degrees kelvin, and T c is the continuum brightness temperature of the radio background plus the CMB. The results for N (H 2 CO) and T ex are presented in Tab.1. The distribution of T ex determined in this manner shows an enhanced temperature around W40 H II region ranging from 2 -5 K and around Serpens South ranging from 1 -2 K (Fig.4). This enhanced T ex in Serpens South confirms the presence of local heating source and ongoing star formation activity. In addition, there is another Serpens 3 region in the formaldehyde absorption structure south of W40 that shows similar conditions of (prestellar) star-formation and inferred weak radio continuum (Fig.4). It appears that the extent of the H 2 CO absorption is determined mostly by the excitation of the molecules in regions that are affected by the star formation rather than by the availability of molecular material as represented by the 13 CO(J = 1 − 0) distribution. In order to connect the Serpens 3 region with evidence of ongoing star formation, the locations of protostellar cores (Könyves et al. 2015) in the region have been added to Fig. 4. While there are protostellar cores associated with W40 and Serpens South, there are no cores (yet) present around Serpens 3. This would suggest that the Serpens 3 region is still less evolved than the other regions of star-formation. On the other hand, protostellar cores are found around the north-west extension of the H 2 CO absorption, which shows less enhancement of T ex but may also qualify as a star-forming region.
The map of the H 2 CO line width superposed on the H 2 CO integrated intensity contours and spectral plots at four selected regions are shown in Fig.5. The spectra at locations (A) around W40 H II and (B) around Serpens South show a single feature with a multicomponent substructure. The spectral feature at Serpens South is slightly weaker and broader than the feature at W40 and shows shallow redshifted and blueshifted wings, possibly resulting from outflows or the superposition of multiple components. The W40 spectrum shows a possible blueshifted feature in at -8.5 km s −1 that could have a counterpart in the H110α RL at that location (see section below). The spectra at locations (C) and (D) show a broader multicomponent structure. In the (C) region at a distance of of 0.7 pc West of the W40 H II the spectrum is broadened relative to the W40 spectrum (A) and has a low velocity shoulder similar to that found in the 13 CO(1 − 0) spectrum and the map of Figure 2b. This lower velocity component may suggest the presence of an outflow component associated with the W40 H II region. The southeastern region (D) of Serpens 3 also shows line broadening toward lower velocities as compared with the spectrum at W40 (A) as well as a high-velocity wing that may result from a superposition of cloud filaments moving to the northeast. In addition, there may be a separate high-velocity component at 15 km s −1 .
The H 2 CO channel maps are presented in Fig.6 with velocity interval of 1 km s −1 . Most of the Serpens South part has a velocity of 6 km s −1 , while most of the W40 part has velocity of 7 km s −1 . In the 5 km s −1 panel an east-west and a southeast-northwest velocity-coherent linear structure are present, which resemble the linear structures in 13 CO(1 − 0) intensity map (Fig.1b). Similarly at 8 km s −1 there is a northeast-southwest structure running through the W40 region. In order to emphasize these structures, we indicate with dashed curves the locations of apparently velocity-coherent structures superposed on the H 2 CO intensity map integrated over the intervals of 5 -6 km s −1 in Fig.7. Assuming the distance of 436 pc, these linear structures are about 5 -10 pc in length and they may be remnants of the super-bubbles converging in our observed area (see also Nakamura et al. (2017)). It should be noted that our three star formation regions coincide with intersection points of these linear structures.

The H110α Recombination Line
The H110α RRL is only detected at the W40 H II region. The spectrum of the H110α line at W40 is presented in Fig.8 together with the 13 CO(1 − 0) profile at the same location. This much broader profile comes from the distribution across the circular image of W40, starting at 10 km s −1 and tapering off at -20 km s −1 . However, the H110α spectrum at W40 does not have a counterpart in the 13 CO(1 − 0) profile at lower velocities, but there may be weak spectral component (to be confirmed) in the H 2 CO absorption spectrum at -8.5 km s −1 . With a peak continuum temperature of 3.33 K, the optical depth (T L /T c ) has a peak value of 0.038, with an average value of 0.15. The profile of the H110α line resembles a spherical (blueshifted) outflow with an The local thermodynamic equilibrium electron temperature T * e is determined using recombination lines by Brown et al. (1978) discussions as follows: where the T * e indicates a value of 7300 K assuming the peak optical depth and a total line width of 35 km s −1 . This value is meaningful when all H110α emission originates within the outskirts of a typical H II region.

CONCLUSION
The H 2 CO (1 10 −1 11 ) absorption line and H110α RRL emission line have been mapped for the first time toward the Aquila molecular cloud containing the W40 H II region and the Serpens South region. The map of the integrated intensity of the H 2 CO absorption clearly defines the boundaries of the area that is affected by the embedded star formation. There is prominent absorption at the location of the W40 H II region and weaker absorption in the Serpens South region. A third star formation region Serpens 3 has been identified 1.4 pc south of W40. No substantial correspondence has been found between the three star formation regions and the emission map of 13 CO(1 − 0). Except for the W40 region, the intensity and velocity distributions of H 2 CO and 13 CO(1 − 0) do not agree well with each other.
The radio continuum structure of the region shows a T c peak at the W40 H II region, but only estimated values of 0.04 K in the Serpens South region and at Serpens 3. Therefore, the H 2 CO absorption at W40 comes from the H II region plus the CMB background, while the weaker components at Serpens South and the Serpens 3 regions result from a weak continuum background plus the CMB. The extent of the H 2 CO absorption confirms that it is mostly determined by the excitation of the molecules in regions that are affected by the star formation rather than the availability of molecular gas as depicted by 13 CO(J = 1 − 0).
Instead of determining the absorbing column density of the formaldehyde across the region with clearly varying environmental conditions, we determined the excitation temperature of the H 2 CO. Assuming that the column densities of H 2 CO relates to that of 13 CO(1 − 0) (see Tang et al. (2013)), and that there is fixed abundance ratio of H 2 CO and 13 CO(1 − 0), the column density of 13 CO(1 − 0) may be used to determine the T ex distribution. While this procedure may not be very reliable, the T ex shows enhancements in the W40 H II region ranging from 2 to 5 K, and > 2 at Serpens South. This also identifies the new star formation region Serpens 3 region with T ex = 2 K. The results show that the local conditions strongly affect the excitation of H 2 CO and that assuming a constant value for T ex would not be appropriate across this region.
The velocity structure of H 2 CO is very smooth while the velocity structure of 13 CO(1−0) has much fine structure and a gradient in a different direction. Therefore, the presence of H 2 CO absorption may be correlated with significant substructure within the 13 CO(1 − 0) integrated emission structure but there is no global correlation. The integrated intensity map of 13 CO(1−0) shows several fine structure regions with discrepant velocities. One of these regions at about 1.2 pc west of W40 has a larger linewidth for H 2 CO and shows a lower 13 CO(1−0) velocity. This region may be associated with an outflow found in a near-infrared survey by Zhang et al. (2015).
Some velocity-coherent filamentary structures have been identified in velocity channel maps of H 2 CO that are possible remnants of earlier super-bubble structures. Assuming the distance of 436 pc, these linear structures range between about 5 -10 pc in length. The three star formation regions are found to lie close to intersection points of these filaments, which may suggest a causal relation. The observed velocity substructure of the H 2 CO absorption lines may also relate to the presence of such filaments.
The H110α RRL is only detected in the W40 H II region. The emission spectrum of H110α shows a broad profile covering a velocity range of 30 km s −1 . The shape of the spectral profile suggests that this emission originates in a shell in front of W40 that is spherically expanding at 26 km s −1 . The LTE electron temperature corresponding to the line strength is estimated at 7900 K, which is typical value for the outskirts of an H II region. Sensitive mapping of H 2 CO absorption has been able to correctly identify star-formation activity in complex molecular clouds such as the Aquila Complex. In addition, the detailed structure of the absorption lines may reveal discrepant velocity components resulting from outflow regions. Sato, F., Mizuno, A., Nagahama, T., et al. 1994, ApJ, 435, 279 Shimoikura, T., Dobashi, K., Nakamura, F., Shimajiri, Y., & Sugitani, K. 2018, PASJ, arXiv:1809 Shimoikura, T., Dobashi, K., Nakamura, F., et al. 2015, ApJ, 806, 201 Smith, J., Bentley, A., Castelaz, M., et al. 1985 Table 1 continued   Table 1 continued