Investigating the Physical Conditions in Extended System Hosting Mid-infrared Bubble N14

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Published 2020 July 21 • © 2020. The American Astronomical Society. All rights reserved.
, , Citation L. K. Dewangan et al 2020 ApJ 898 41 DOI 10.3847/1538-4357/ab964c

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lokeshd@prl.res.in

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1 Physical Research Laboratory, Navrangpura, Ahmedabad—380 009, India; lokeshd@prl.res.in

2 Kavli Institute for Astronomy and Astrophysics, Peking University, 5 Yiheyuan Road, Haidian District, Beijing 100871, People's Republic of China

3 Institute of Applied Physics of the Russian Academy of Sciences, 46 Ulyanov st., Nizhny Novgorod 603950, Russia

4 Department of Astronomy and Astrophysics, Tata Institute of Fundamental Research, Homi Bhabha Road, Mumbai 400 005, India

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  1. Received 2019 December 16
  2. Revised 2020 May 18
  3. Accepted 2020 May 18
  4. Published

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0004-637X/898/1/41

Abstract

To observationally explore physical processes, we present a multiwavelength study of a wide-scale environment toward l = 13fdg7–14fdg9 containing a mid-infrared bubble N14. The analysis of 12CO, 13CO, and C18O gas at [31.6, 46] km s−1 reveals an extended physical system (extension ∼59 pc × 29 pc), which hosts at least five groups of the APEX Telescope Large Area Survey of the Galaxy (ATLASGAL) 870 μm dust clumps at d ∼3.1 kpc. These spatially distinct groups/subregions contain unstable molecular clumps and are associated with several Class I young stellar objects (mean age ∼0.44 Myr). At least three groups of ATLASGAL clumps associated with the expanding H ii regions (including the bubble N14) and embedded infrared dark clouds, devoid of the ionized gas, are found in the system. The observed spectral indices derived using the Giant Metrewave Radio Telescope (GMRT) and THOR radio continuum data suggest the presence of nonthermal emission with the H ii regions. The high-resolution GMRT radio continuum map at 1280 MHz traces several ionized clumps powered by massive B-type stars toward N14, which are considerably young (age ∼103–104 yr). Locally, the early stage of star formation is evident toward all the groups of clumps. The position–velocity maps of 12CO, 13CO, and C18O exhibit an oscillatory-like velocity pattern toward the selected longitude range. Considering the presence of different groups/subregions in the system, the oscillatory pattern in velocity is indicative of the fragmentation process. All these observed findings favor the applicability of the global collapse scenario in the extended physical system, which also seems to explain the observed hierarchy.

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1. Introduction

It has been well accepted that molecular gas converts into young stellar objects (YSOs) of different masses (including massive OB stars (≳8 M⊙)) and their clusters in a giant molecular cloud (GMC), where several complex physical processes may operate. However, the mechanisms responsible for the birth of young stellar clusters and massive stars are still incompletely understood (Zinnecker & Yorke 2007; Tan et al. 2014). The study of a GMC allows us to explore the ongoing star formation mechanisms, such as global gravitational contraction (Hartmann et al. 2012, and references therein), and triggered star formation scenarios (i.e., "globule squeezing," "collect and collapse," and "cloud–cloud collision (CCC)"; Elmegreen 1998). Such study a requires the knowledge of the physical conditions in promising massive star-forming sites (e.g., H ii regions) associated with a GMC, which can be inferred through the analysis of the multiwavelength data. In this context, the present paper deals with an extended and a single physical system hosting several massive star-forming regions, which are situated toward l = 13fdg7–14fdg9 and b = −0fdg5 to +0fdg1. This extended physical system located in the Galactic arm(s) is identified via the reliable information of distance and radial velocity (Vlsr) of its different subregions, which helps to disentangle the target system against its background and foreground clouds.

Figure 1(a) shows the APEX Telescope Large Area Survey of the Galaxy (ATLASGAL; beam size ∼19farcs2; Schuller et al. 2009) 870 μm dust continuum map covering the wide-scale area (∼1fdg2 × 0fdg6) around the mid-infrared (MIR) bubble N14. The ATLASGAL map is also overlaid with 53 ATLASGAL clumps (taken from Urquhart et al. 2018). All these clumps are traced in a velocity range of [34.5, 43] km s−1, and are located at a single distance of ∼3.1 kpc (Urquhart et al. 2018). The spatial distribution of these clumps has enabled us to find an extended physical system. Based on the visual inspection, one can also arbitrarily depict at least five groups of clumps (i.e., group1, group2, group3, group4, and group5) in Figure 1(a), which are indicated by broken curves. The MIR bubble N14 has been characterized as a complete or closed ring with an average radius and thickness of 1farcm22 and 0farcm38, respectively (Churchwell et al. 2006; Dewangan & Ojha 2013; Yan et al. 2016). The bubble N14 also contains the ionized emission at its center (e.g., Dewangan & Ojha 2013). Using the Multi-Array Galactic Plane Imaging Survey (MAGPIS; beam size ∼6''; Helfand et al. 2006) radio continuum flux at 20 cm of the bubble N14, Beaumont & Williams (2010) computed the Lyman continuum photons (log Nuv) to be 48.36 (see also Dewangan & Ojha 2013), which is explained by a single O9V–O8.5V star (Panagia 1973) or a single O7.5V–O8V star (Martins et al. 2005) or at least six O9.5V stars (Beaumont & Williams 2010). In addition to the bubble N14, some other previously known sources (such as, G014.194−00.194, G14.427−00.075HII, G14.47−0.20, IRAS 18141−1615, and G14.71−0.19) are also labeled in Figure 1(a). Figure 1(b) displays the overlay of the MAGPIS 20 cm continuum emission contours on the ATLASGAL map at 870 μm. In the direction of at least three groups of clumps (i.e., group2, group3, and group4 in Figure 1(a)), the MAGPIS 20 cm contours reveal the presence of H ii regions powered by massive OB stars.

Figure 1.

Figure 1. (a) The panel displays the ATLASGAL contour map at 870 μm (area ∼1fdg2 × 0fdg6 (∼65 pc × 32.5 pc at a distance of 3.1 kpc); central coordinates: l = 14fdg3; b = −0fdg2) overlaid with the ATLASGAL dust continuum clumps at 870 μm (from Urquhart et al. 2018) (see diamonds). The ATLASGAL contours are shown with the levels of 3.9 Jy beam−1 × (0.067, 0.08, 0.1, 0.15, 0.2, 0.25, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, and 0.95). All the ATLASGAL clumps are located at a distance of ∼3.1 kpc. The broken box (in magenta) refers to the area shown using the MAGPIS 20 cm continuum data in Figure 1(b). Some known regions (e.g., bubble N14, G014.194-00.194, G14.427-00.075HII, G14.47-0.20, IRAS 18141-1615, and G14.71-0.19) are also labeled in the figure. At least five groups of ATLASGAL clumps are indicated in the figure (see broken curves). (b) Overlay of the MAGPIS 20 cm continuum contours (in red; resolution ∼6'') on the ATLASGAL contour map at 870 μm. The ATLASGAL map is the same as in Figure 1(a). The MAGPIS contours are shown with the levels of 2.2, 2.8, 3.3, 4.0, 5.5, and 8.0 mJy beam−1. The MAGPIS map is smoothed using a Gaussian function with a radius of four pixels. Hexagon and pentagon symbols represent the radio continuum sources from the THOR survey (l > 14fdg3; Bihr et al. 2016; Wang et al. 2018). Pentagon symbols show the sources with spectral index (α) > 0, while the sources with α < 0 are marked by hexagon symbols. (c) The panel displays the Sagittarius, Scutum, and Norma arms toward l = 0fdg0–35fdg0 (from Reid et al. 2016) in a longitude–velocity plot. The near and far sides of the arms are presented by broken and solid curves, respectively. The Vlsr of each ATLASGAL clump against its longitude is also marked in the plot. In panels (a) and (b), the scale bar referring to 10 pc (at a distance of 3.1 kpc) is drawn.

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However, the formation processes of the selected different groups of clumps as well as massive OB stars have yet to be investigated in the extended physical system (see Figure 1(a)). No attempt is made to examine the velocity structure of molecular gas and the identification of YSOs toward the entire selected longitude range. Such analysis is required for exploring the ongoing star formation mechanisms in the extended physical system containing several embedded clumps and H ii regions. It also helps us to observationally understand the origin of the large-scale configuration/system toward l = 13fdg7–14fdg9. In this context, a multiwavelength approach is adopted in this paper, which is a very useful and effective utility to gain the quantitative and qualitative physical information in the target site. The present work is benefited by the existing large-scale FOREST Unbiased Galactic plane Imaging survey with the Nobeyama 45 m telescope (FUGIN; Umemoto et al. 2017) molecular line data (i.e., 12CO, 13CO, and C18O) along with the Spitzer and Herschel infrared maps. New radio continuum maps observed by the Giant Metrewave Radio Telescope (GMRT) facility are also presented toward the MIR bubble N14.

This paper is arranged as follows. Section 2 presents the details of the adopted data sets in this paper. Section 3 provides new outcomes derived using a multiwavelength approach in the selected longitude range. The discussion of the observational outcomes is presented in Section 4. Finally, Section 5 gives the summary of the major findings obtained in this work.

2. Data and Analysis

The present paper utilizes the existing multiwavelength data sets obtained from various large-scale surveys (see Table 1). The selected target area (∼1fdg2 × 0fdg6 (or ∼65 pc × 32.5 pc); centered at l = 14fdg3; b = −0fdg2) in this paper is presented in Figure 1(a). The 12CO(J = 1−0), 13CO(J = 1−0), and C18O(J = 1−0) line data were obtained from the FUGIN survey, and are calibrated in main beam temperature (Tmb) (Umemoto et al. 2017). The typical rms noise level5 (Tmb) is ∼1.5 K, ∼0.7 K, and ∼0.7 K for 12CO, 13CO, and C18O lines, respectively (Umemoto et al. 2017). To improve sensitivities, each FUGIN molecular line data cube is smoothened with a Gaussian function having a half power beamwidth of 35''. Additionally, in the direction of the bubble N14, our unpublished GMRT radio continuum maps are also examined in this work.

Table 1.  List of the Different Surveys Adopted in This Paper

Survey Band/Line(s) Resolution ('') Reference
Multi-Array Galactic Plane Imaging Survey (MAGPIS) 20 cm ∼6 Helfand et al. (2006)
The H i/OH/Recombination line survey of the inner Milky Way (THOR) 1–2 GHz ∼25 Beuther et al. (2016)
GMRT observations (Proposal Code: 23_054) 610 MHz, 1280 MHz ∼5–6 PI: L. K. Dewangan
FUGIN survey 12CO, 13CO, C18O (J = 1–0) ∼20 Umemoto et al. (2017)
APEX Telescope Large Area Survey of the Galaxy (ATLASGAL) 870 μm ∼19.2 Schuller et al. (2009)
Herschel Infrared Galactic Plane Survey 70–500 μm ∼5.8–37 Molinari et al. (2010a)
Spitzer MIPS Inner Galactic Plane Survey 24 μm ∼6 Carey et al. (2005)
Spitzer Galactic Legacy Infrared Mid-Plane Survey Extraordinaire (GLIMPSE) 3.6–8.0 μm ∼2 Benjamin et al. (2003)

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Radio continuum observations at 610 and 1280 MHz were performed with the GMRT facility on 2012 December 28 (Proposal Code: 23_054; PI: L. K. Dewangan). The GMRT data were reduced using the Astronomical Image Processing System (AIPS) package, and the detailed reduction procedures can be found in Mallick et al. (2012, 2013). We flagged bad data from the UV data by multiple rounds of flagging using the tvflg task of the AIPS. After several rounds of self-calibration, the final maps at 610 and 1280 MHz were produced with the synthesized beams of 5farcs6 × 5farcs2 and 6'' × 6'', respectively. In general, due to the Galactic background emission, the antenna temperature of the sources located toward the Galactic plane is expected to be increased. It is found more severe in the GMRT low-frequency bands (i.e., 610 MHz), where the contribution from the background emission is generally much higher compared to the 1280 MHz band. A more detailed description concerning the system temperature correction can be found in Baug et al. (2015, and references therein). Such a correction is also applied to the GMRT 610 MHz data, before performing any scientific analysis. The final rms sensitivities of both the maps at 610 and 1280 MHz are ∼1 mJy beam−1. The unit of brightness in Jy beam−1 is adopted in this paper. However, the conversion from Jy beam−1 to Jy sr−1 is Jy beam−1 ×$\,{\left(\tfrac{\theta }{206,265}\right)}^{2}\times \tfrac{\pi }{4\mathrm{ln}2}$ = Jy sr−1, where θ is the beam size in arcseconds.

3. Results

In the selected target area, the multiwavelength data sets are analyzed to study the distribution of molecular gas, YSOs, embedded clumps, H ii regions, and dust temperature as well as velocity structure.

3.1. Extended Physical System Hosting H ii Regions

In the target longitude range (i.e., l = 13fdg7–14fdg9), different groups of the ATLASGAL 870 μm dust continuum clumps (at Vlsr range ∼[34.5, 43] km s−1) are presented in Section 1, and are labeled in Figure 1(a). In the direction of some of these ATLASGAL groups, the radio continuum emission is observed (see Figure 1(b)). The dust continuum emission at 870 μm may depict cold dust, while the ionized gas is traced by the radio continuum emission. As highlighted earlier, the extended physical system hosts several embedded clumps and H ii regions powered by massive OB stars. Hence, Figure 1(b) helps us to examine the spatial association between the dust clumps and the ionized gas in the system.

In Figure 1(b), we have marked the positions of the radio continuum sources (for l > 14fdg3) from the THOR survey (Bihr et al. 2016; Wang et al. 2018), which are shown by hexagons and pentagons. The THOR radio sources are detected toward the ATLASGAL clumps and the MAGPIS radio continuum emission. Each THOR radio source has a spectral index (α) value, which is defined as Fν ∝ να. Here, ν is the frequency of observation, and Fν is the corresponding observed flux density. The spectral indices of THOR radio sources are derived using the radio peak fluxes at 1.06, 1.31, 1.44, 1.69, 1.82, and 1.95 GHz (e.g., Bihr et al. 2016). Note that the areas around the bubble N14 are not observed in the THOR survey. THOR sources with α < 0 are shown by hexagons, while pentagons indicate THOR sources with α > 0. These sources are G14.779−0.333 (α ∼ 2.25), G14.457−0.185 (α ∼ 0.58), G14.477−0.005 (α ∼ −0.097), G14.490+0.021 (α ∼ −0.026), G14.598+0.019 (α ∼ −0.042), G14.668+0.013 (α ∼ −0.599), G14.390−0.021 (α ∼ −0.074), and G14.440−0.056 (α ∼ −0.344). In general, the positive and negative values of α allow us to distinguish the thermal and nonthermal radio continuum emission in a given massive star-forming region, respectively. A positive or near zero spectral index refers to thermally emitting sources (Bihr et al. 2016). For example, supernova remnants display nonthermal emission with α ≈ −0.5, while a steeper α ≈ −1 is expected in extragalactic objects (e.g., Rybicki & Lightman 1979; Longair 1992; Bihr et al. 2016). Based on the radio morphology, all eight selected THOR radio sources appear to be Galactic H ii regions. Six out of eight THOR radio sources show α < 0, suggesting the nonthermal radio continuum emission, and the remaining two THOR sources exhibit thermal radio continuum emission (or free–free emission).

In the catalog of ATLASGAL clumps (e.g., Urquhart et al. 2018), one can obtain the integrated flux and effective radius (Rc) of each ATLASGAL clump as well as other parameters, such as distance, Vlsr, dust temperature (Td), bolometric luminosity (Lbol), clump mass (Mclump), and H2 column density (N(H2)). Table 2 lists the positions and physical parameters of all these clumps. Additionally, we have also included the average volume density (${n}_{{{\rm{H}}}_{2}}$ = 3Mclump/(4π${R}_{\mathrm{clump}}^{3}{\mu }_{{{\rm{H}}}_{2}}{m}_{{\rm{H}}}$)) of each clump in the table. In the calculation, we assume that each clump has a spherical geometry. The mean molecular weight ${\mu }_{{{\rm{H}}}_{2}}$ is adopted to be 2.8, and mH is the mass of a hydrogen atom.

Table 2.  Summary of the Properties of 53 ATLASGAL Dust Clumps at 870 μm

ID l b S870 Vlsr Rc Td Lbol Mclump $N({{\rm{H}}}_{2})$ ${n}_{{{\rm{H}}}_{2}}$ Mvir Association
  (degree) (degree) (Jy) (km s−1) (pc) (K) (102 L⊙) (M⊙) (1022 cm−2) (103 cm−3) (M⊙)  
c1 13.786 −0.237 1.96 38.7 0.15 12.4 0.4 224.4 2.9 229.8 ⋯ group1
c2 13.867 −0.299 2.15 39.8 0.15 15.8 1.6 162.6 1.7 166.5 ⋯ group1
c3 13.882 −0.369 1.82 39.7 0.15 11.3 0.2 247.7 2.3 253.8 ⋯ group1
c4 13.906 −0.292 2.88 39.7 0.34 12.4 0.9 329.6 2.6 29.0 ⋯ group1
c5 13.982 −0.144 16.60 40.0 0.48 32.2 74.3 459.2 1.8 14.4 ⋯ group2
c6 13.992 −0.121 7.94 38.1 0.15 33.7 20.1 207.5 0.9 212.5 ⋯ group2
c7 13.997 −0.156 17.33 40.2 0.44 27.6 30.6 586.1 1.4 23.8 ⋯ group2
c8 14.001 −0.109 5.60 37.9 0.15 33.0 10.8 150.3 0.7 154.0 ⋯ group2
c9 14.009 −0.106 4.13 40.2 0.15 33.0 19.5 110.9 0.7 113.6 ⋯ group2
c10† 14.011 −0.176 11.23 41.0 0.44 20.6 14.6 570.2 4.1 23.1 337 group2
c11 14.017 −0.161 6.76 41.2 0.30 19.6 7.3 369.0 2.4 47.2 ⋯ group2
c12 14.019 −0.134 22.12 40.6 0.87 26.9 162.2 774.5 3.3 4.1 ⋯ group2
c13 14.026 −0.202 1.18 37.1 0.15 16.2 0.8 86.5 1.2 88.6 ⋯ group2
c14 14.184 −0.227 9.23 39.7 0.61 14.6 10.5 790.7 3.6 12.0 ⋯ group3
c15 14.187 −0.151 12.47 38.1 0.46 15.7 2.3 948.4 2.1 33.7 ⋯ group3
c16 14.192 −0.166 10.48 35.6 0.40 15.9 3.6 779.8 2.0 42.1 ⋯ group3
c17† 14.194 −0.194 42.86 39.4 1.12 16.1 37.2 3126.1 13.7 7.7 1694 group3
c18 14.197 −0.214 1.07 39.2 0.15 12.1 0.3 127.1 5.2 130.1 ⋯ group3
c19 14.204 −0.207 4.04 39.8 0.15 13.0 2.2 422.7 5.0 432.9 ⋯ group3
c20 14.206 −0.109 1.57 35.1 0.15 29.2 13.6 48.8 0.7 49.9 ⋯ group3
c21† 14.231 −0.176 8.40 37.5 0.61 15.0 4.5 687.1 3.1 10.5 333 group3
c22 14.236 −0.166 2.90 35.8 0.21 10.1 0.2 488.7 4.2 182.4 ⋯ group3
c23† 14.312 −0.189 3.45 38.8 0.27 15.4 1.4 270.4 2.4 47.5 184 group4
c24 14.319 −0.147 2.27 38.6 0.15 17.5 1.1 145.2 1.3 148.7 ⋯ group4
c25 14.364 −0.102 2.84 39.7 0.15 13.9 0.4 264.2 1.8 270.7 ⋯ group4
c26 14.397 −0.181 2.63 42.6 0.21 19.4 4.4 143.9 1.1 53.7 ⋯ group4
c27 14.406 −0.052 8.22 37.7 0.82 24.0 44.4 331.9 1.2 2.1 ⋯ group4
c28 14.414 −0.069 3.53 38.0 0.58 25.7 93.1 129.7 1.2 2.3 ⋯ group4
c29 14.419 −0.056 2.23 37.7 0.15 22.0 2.7 101.6 1.3 104.1 ⋯ group4
c30 14.427 −0.096 5.11 39.6 0.37 19.9 9.2 269.2 1.4 18.4 ⋯ group4
c31 14.431 −0.056 6.96 37.7 0.31 25.0 39.5 265.5 0.8 30.8 ⋯ group4
c32† 14.449 −0.101 25.78 40.2 0.76 19.1 44.8 1445.4 6.0 11.4 1247 group4
c33 14.449 −0.161 4.17 37.9 0.15 12.6 1.0 460.3 2.7 471.4 ⋯ group4
c34 14.454 −0.142 1.51 39.5 0.15 17.8 2.0 94.2 1.8 96.5 ⋯ group4
c35 14.459 −0.192 4.47 39.5 0.15 15.9 0.7 332.7 1.7 340.7 ⋯ group4
c36 14.462 −0.109 9.18 35.9 0.40 20.1 12.9 476.4 4.9 25.7 ⋯ group4
c37† 14.469 −0.084 17.96 38.1 0.72 13.2 7.3 1828.1 7.2 16.9 662 group4
c38 14.469 −0.101 3.02 36.6 0.15 17.8 3.2 188.4 4.2 192.9 ⋯ group4
c39 14.476 −0.126 4.44 40.5 0.28 16.9 2.2 299.9 4.6 47.2 ⋯ group4
c40 14.484 −0.192 9.05 38.2 0.39 13.5 1.0 885.1 2.4 51.6 ⋯ group4
c41† 14.492 −0.139 29.46 39.7 1.12 16.6 34.2 2046.4 9.0 5.0 1837 group4
c42 14.499 −0.027 1.90 38.0 0.15 15.1 0.6 153.8 1.6 157.6 ⋯ group4
c43 14.592 −0.122 1.72 39.0 0.15 27.5 6.4 57.7 0.6 59.1 ⋯ group5
c44 14.622 −0.131 2.43 39.7 0.30 13.7 1.1 231.7 2.1 29.7 ⋯ group5
c45 14.644 −0.117 5.02 40.8 0.46 12.0 2.7 606.7 4.1 21.5 ⋯ group5
c46 14.652 −0.001 4.36 37.1 0.30 12.1 0.7 518.8 3.6 66.4 ⋯ group5
c47† 14.686 −0.222 12.92 37.7 0.78 14.5 5.4 1119.4 3.6 8.2 495 group5
c48 14.696 −0.137 2.59 38.4 0.25 10.6 0.3 396.3 3.1 87.7 ⋯ group5
c49† 14.707 −0.156 9.20 40.6 0.66 14.3 5.0 814.7 3.7 9.8 638 group5
c50 14.711 −0.224 4.66 36.4 0.42 9.7 0.4 853.1 4.9 39.8 ⋯ group5
c51† 14.726 −0.202 18.98 37.5 0.70 13.0 3.0 1981.5 3.8 20.0 156 group5
c52 14.756 −0.174 8.96 38.9 0.24 19.3 0.7 494.3 1.1 123.6 ⋯ group5
c53 14.789 −0.197 8.53 39.2 0.49 8.5 0.5 2079.7 6.7 61.1 ⋯ group5

Note. These clumps are situated at a distance of ∼3.1 kpc. In the table, we have provided ID, Galactic coordinates (l, b), 870 μm integrated flux density (S870), radial velocity (Vlsr), clump effective radius (Rc), dust temperature (Td), bolometric luminosity (Lbol), clump mass (Mclump), H2 column density ($N({{\rm{H}}}_{2})$), average volume density (${n}_{{{\rm{H}}}_{2}}$), and virial mass (Mvir). Using different symbols, five groups are indicated in Figure 3(a). These groups are group1 (c1–c4; up down triangles), group2 (c5–c13; circles), group3 (c14–c22; squares), group4 (c23–c42; triangles), and group5 (c43–c53; stars). The clumps highlighted by a dagger have the NH3 line width, and have the ratio Mclump/Mvir > 1 (see Urquhart et al. 2018 for more details).

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Figure 1(c) presents a plot of Vlsr of 53 ATLASGAL clumps versus Galactic longitude range of l = 0°–35°. The locations of various spiral arms (i.e., near and far sides of the Sagittarius, Scutum, and Norma arms) of the Milky Way (from Reid et al. 2016) are also marked in Figure 1(c). This analysis suggests that the cloud associated with the extended physical system is located toward the near sides of the Scutum and Norma arms. In Figure 2, we present the observed 12CO, 13CO, and C18O spectra toward the selected target area. These profiles are obtained by averaging the selected target area as presented in Figure 1(a). In Figure 2, we find three velocity peaks around 23, 40, and 60 km s−1. The extended physical system, containing H ii regions (including the bubble N14), is associated with the velocity component around 40 km s−1, and is well depicted in a velocity range of [31.6, 46] km s−1. Note that the observed velocities of all the selected ATLASGAL clumps fall well within this velocity range. This exercise also indicates that the extended physical system is not physically associated with the other two velocity components around 23 and 60 km s−1, which are not examined in this paper.

Figure 2.

Figure 2. FUGIN 12CO profile (broken red curve), 13CO spectrum (thin magenta curve), and C18O profile (thick blue curve). The profiles are produced by averaging the target area shown in Figure 1(a). Ranges of the longitude and latitude of the target area are also labeled in the figure. The 12CO and 13CO spectra have been divided by factors of 5 and 17, respectively.

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To display five groups of clumps, Figure 3(a) shows the positions of 53 ATLASGAL clumps at 870 μm using different symbols (i.e., upside down triangles (group1), circles (group2), squares (group3), triangles (group4), and stars (group5)). The sites N14, G014.194−00.194, G14.427−00.075HII (and G14.47−0.20), G14.71−0.19 are seen toward group2, group3, group4, and group5, respectively. Figure 3(b) shows the distribution of Vlsr of 53 clumps against Galactic longitude. We find a noticeable velocity spread toward all the ATLASGAL groups (except group1), which is further explored using the molecular line data in Section 3.2. In Figure 3(c), we display the distribution of the dust temperatures of clumps against the Galactic longitude, showing a dust temperature range of ∼8–34 K. In group3 and group5, the clumps are found with Td < 15 K. In the direction of the bubble N14, the clumps associated with the group2 have Td > 25 K. One can find the masses and bolometric luminosities of the clumps associated with different ATLASGAL groups in Figures 3(d) and (e), respectively. All the ATLASGAL groups (except group1 and group5) have clumps with Lbol > 103 L⊙. In each group, at least two dense clumps (with ${n}_{{{\rm{H}}}_{2}}$ > 105 cm−3) are found (see Table 2).

Figure 3.

Figure 3. (a) Spatial distribution of 53 ATLASGAL dust continuum clumps at 870 μm toward the selected site. The ATLASGAL dust continuum clumps at 870 μm (from Urquhart et al. 2018) are highlighted by different symbols (i.e., upside down triangles, circles, squares, triangles, and stars), which are used to show five groups of clumps (see Table 2). These groups are labeled as group1 (up down triangles), group2 (circles), group3 (squares), group4 (triangles), and group5 (stars). (b)–(e) Distribution of the Vlsr, dust temperature, mass, and bolometric luminosity of clumps against the Galactic longitude.

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3.2. Kinematics of Molecular Gas

To study the distribution of molecular gas, the integrated FUGIN 12CO, 13CO, and C18O intensity maps are presented in Figures 4(a), (b), and (c), respectively. In each intensity map, the molecular gas is integrated over a velocity range of [31.6, 46] km s−1 (see also Figure 2). In the direction of the extended physical system, the distribution of molecular gas allows us to infer the existence of a GMC (extension ∼59 pc × 29 pc), which contains several dense regions traced using the C18O gas. In Figure 4(b), a shell-like feature is highlighted by a broken ellipse, and is prominently seen in the 13CO map. Using the C18O line data, we have selected 11 molecular clumps in the direction of the bubble N14 (see three clumps in ATLASGAL group2), G014.194−00.194 (see seven clumps in ATLASGAL group3), and G14.427−00.075 (see one clump in ATLASGAL group4) (see squares in Figure 4(c)). Using the zoomed-in maps of C18O, Figures 5(a), (b), and (c) show the position(s) of the selected molecular clump(s) in the direction of group2, group3, and group4, respectively (see also Table 3). In Figure 6, we display the integrated 13CO velocity channel maps (at intervals of 1 km s−1), revealing several clumpy regions in the GMC. In each velocity panel, the location of the bubble N14 is highlighted by a radio continuum contour. The shell-like feature is also marked in two velocity channel panels by a broken ellipse.

Figure 4.

Figure 4. (a) FUGIN 12CO(J = 1−0) map of intensity (moment-0) in the direction of the selected area around l = 13fdg7–14fdg9 and b = −0fdg5 to +0fdg1. (b) FUGIN 13CO(J = 1−0) map of intensity (moment-0). A shell-like feature is also highlighted in the panel (see the broken ellipse in white). (c) FUGIN C18O(J = 1−0) map of intensity (moment-0). Some selected molecular clumps are shown by squares. In each moment-0 map, the molecular emission is integrated from 31.6 to 46 km s−1.

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Figure 5.

Figure 5. Selected molecular clumps in the direction of group2, group3, and group4 using the FUGIN C18O(J = 1−0) map (see Figure 4(c) and also Table 3). In each panel, the axes are offsets (in arcmin) with respect to the central position (i.e., l = 14fdg901; b = −0fdg499).

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Figure 6.

Figure 6. Integrated velocity channel maps of 13CO(J = 1−0) (at velocity intervals of 1.3 km s−1). Bubble N14 is indicated by the MAGPIS 20 cm continuum contour (in blue) with a level of 3.7 mJy beam−1. The 13CO contours are shown with levels of 4.5, 5, 6, 7, 8, 9, 10, 11, 13, 15, 18, 21, and 23 K km s−1. A shell-like feature is also highlighted in two panels (see the broken ellipse in magenta). The gray-scale bar in the first left panel is applicable to all the other maps.

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Table 3.  Physical Parameters of Selected Molecular Clumps in the Direction of group2, group3, and group4 Using the C18O Line Data, which Are Indicated in Figure 5

ID l b Dc Mmc ΔV Mvir $\bar{n}$ Association
  (degree) (degree) (pc) (M⊙) (km s−1) (M⊙) (104 cm−3)  
g2clm1 14.015 −0.138 2.22 4265 3.77 3360 1.30 group2
g2clm2 14.015 −0.169 1.43 3930 3.03 1380 4.46 group2
g2clm3 13.992 −0.146 2.50 4930 3.02 2390 1.06 group2
g3clm1 14.185 −0.228 1.39 3575 2.93 1250 4.46 group3
g3clm2 14.201 −0.191 1.30 3530 3.85 2040 5.27 group3
g3clm3 14.198 −0.173 1.43 2200 3.57 1910 2.52 group3
g3clm4 14.191 −0.158 0.60 1210 3.40 733 18.20 group3
g3clm5 14.188 −0.139 1.87 1630 ⋯ ⋯ 0.84 group3
g3clm6 14.195 −0.129 1.11 1470 ⋯ ⋯ 3.59 group3
g3clm7 14.220 −0.125 1.27 1890 4.01 2140 3.07 group3
g4clm1 14.450 −0.102 1.99 16230 5.43 6130 6.92 group4

Note. Column 1 shows the IDs assigned to the molecular clump(s). The table also contains central positions (l, b), C18O clump diameter (Dc), mass derived from C18O (Mmc), FWHM (C18O ΔV), Mvir, and mean number density ($\bar{n}$) calculated from Mmc and Dc. Note that the C18O spectra toward the clumps g3clm5 and g3clm6 contain two closely located velocity components, which do not allow us to determine their line widths. Hence the values of Mvir are not computed for the clumps g3clm5 and g3clm6.

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Using the optically thin C18O line data, the total molecular masses (${M}_{\mathrm{mc}}({{\rm{H}}}_{2})$) and the virial masses (Mvir) for the molecular clumps highlighted in Figures 4(c) and 5 are estimated. For the calculations, we have used the procedures and equations given in Dewangan et al. (2019; see also Frerking et al. 1982; MacLaren et al. 1988; Mangum & Shirley 2016, for equations). Adopting the values of mass and clump diameter for each molecular clump, the mean number density ($\bar{n}$) is also estimated. The derived physical parameters are tabulated in Table 3, which shows that all of these molecular clumps are massive (>103 M⊙) and dense (>104 cm−3). One can notice that the value of Mvir is calculated for the case of a spherically symmetric clump with a constant density, no external pressure, and no magnetic field. Our calculations enable us to determine the ratio of Mmc and Mvir for all the selected molecular clumps (see Table 3). Note that the uncertainties of both mass estimates are due to the combination of several factors (e.g., Dewangan et al. 2019), some of which are unknown (such as clump density profiles, the C18O excitation temperature, etc.). We can consider an uncertainty in the mass calculation to be typically ∼20% and at largest ∼50%. Taking into account a value of ∼50% for both mass uncertainties, one could conclude that at least 5 of the 11 C18O clumps (two in the group2, two in the group3, and 1 in the group4) with Mmc/Mvir ≥ 2 should be unstable. This implies that these molecular clumps are unstable against gravitational collapse.

Figures 7(a), (b), and (c) display the longitude–velocity maps of 12CO, 13CO, and C18O, respectively. These molecular emissions exhibit a large spread in velocities over a range of ∼10–12 km s−1 over the entire physical system. In the direction of the extended physical system, continuous velocity structures are seen, where velocity gradients are also evident. The velocity appears to be oscillating along the longitude in all the position–velocity maps. Overall, the analysis of molecular gas confirms the spatial and velocity connections of all the selected groups. This indicates that due to some physical processes, the extended physical system breaks into smaller groups in a hierarchical manner (see Section 4 for more details).

Figure 7.

Figure 7. Longitude–velocity maps of (a) 12CO, (b) 13CO, and (c) C18O. (d) FUGIN 13CO integrated intensity map toward G14.427-00.075HII. (e) Latitude–velocity map of 13CO in the direction of G14.427-00.075HII. The molecular emission is integrated over the longitude range, which is highlighted in the panel. (f) Longitude–velocity map of 13CO toward G14.427-00.075HII. The molecular emission is integrated over the latitude range, which is labeled in the panel. In panels (a), (b), and (c), the molecular emission is integrated over the latitude range from −0fdg5 to 0fdg1, and a scale bar corresponding to 1 km s−1 pc−1. In panels (a), (b), and (c), the observed velocity structures toward the bubble N14 and G14.427-00.075HII are highlighted by a dotted box and a dashed box, respectively. A scale bar corresponding to 1 km s−1 and 1 pc in the horizontal and vertical axes, respectively, is shown in panels (a) and (b), while a slope of 1 pc per km s−1 is in panel (c).

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In the integrated molecular map of 13CO, we have highlighted a shell-like feature toward l = 14fdg3–14fdg5 or the site G14.427−00.075HII. In this selected longitude direction or the site G14.427−00.075HII, an arc-like feature is seen in velocity (see the dashed box in Figures 7(a), (b), and (c)). In Figure 7(d), we display a zoomed-in view of the cloud associated with the site G14.427−00.075HII. To further examine the velocity structure toward the site G14.427−00.075, Figures 7(e) and (f) present the latitude–velocity and longitude–velocity maps of 13CO, respectively. The latitude–velocity map clearly reveals the arc-like feature in the velocity space, which is also seen in Figure 7(f) (see the broken curve in Figures 7(e) and (f)). Hence, both the position–velocity maps favor the presence of an expanding H ii region toward the site G14.427−00.075. Earlier, the semi-ring-like or C-like or arc-like structure in velocity has been observed in massive star-forming regions, such as the Orion nebula (Wilson et al. 2005), the Perseus molecular cloud (Arce et al. 2011), W42 (Dewangan et al. 2015), and S235 (Dewangan et al. 2016b). Using a modeling of expanding bubbles in a turbulent medium, Arce et al. (2011) proposed that an expanding shell associated with the cloud should be responsible for the semi-ring-like or C-like structure in velocity. Considering the signature of the expanding H ii region, the observed radio continuum emission as well as the extended temperature structures, we suggest the impact of massive star(s) associated with the site G14.427−00.075HII to its vicinity.

The noticeable velocity gradient (i.e., ∼1 km s−1 pc−1) is also clearly seen toward the bubble N14 (see the solid white line in Figure 7(c)), where the extended and spherical-like temperature feature is evident (see Figure 9(a)). Earlier, Yan et al. (2016) reported the molecular maps toward the bubble N14 using different molecular lines (see Figure 5 in their paper). Sherman (2012) published the observational data at 3.3 mm continuum and several molecular lines toward the bubble N14. Based on the N2H+ line data, they pointed out that the bubble N14 is expanding into a very inhomogeneous cloud. Our findings favor this interpretation.

In order to highlight the oscillatory-like velocity pattern, in Figures 8(a) and (b), an arbitrarily chosen curve is drawn in the longitude–velocity maps of 13CO. Figure 8(a) is the same as Figure 7(b), but the 13CO emission is shown for higher contour levels. In Figure 8(b), the molecular emission is integrated over a small range of latitude (i.e., −0fdg228 to −0fdg065). In this selected latitude range, most of the molecular emission is observed toward the extended physical system (see the broken lines in Figure 8(c)). In addition to the oscillatory-like velocity pattern, in Figure 8(b), velocity gradients are also evident toward the selected groups/subregions, as discussed above. In Figure 8(c), we display the first moment map of C18O, showing the intensity-weighted mean velocity of the emitting gas. In the first moment map, one can clearly find noticeable velocity spread in the direction of the selected groups/subregions. We have also shown the distribution of the Vlsr of the ATLASGAL clumps and the locations of Galactic arms in Figure 8(a). We also find the information of the NH3 line widths toward at least 10 ATLASGAL clumps (see the red diamonds in Figure 8(a) and also Table 2) from Urquhart et al. (2018), which are used to compute the virial masses of the clumps. Based on the ratio of Mclump and Mvir, we find that these clumps (with Mclump > Mvir) are unstable against gravitational collapse.

Figure 8.

Figure 8. (a) Longitude–velocity map of 13CO (see also Figure 7(b)). The 13CO emission is integrated over the latitude from −0fdg5 to 0fdg1, and is shown between 540 and 1012 deg K. The panel also shows the Scutum and Norma arms (from Reid et al. 2016). The near side of the arms is presented by broken curves. The Vlsr of each ATLASGAL clump against its longitude is also displayed in the plot (see diamonds and also Figure 1(a)). Urquhart et al. (2018) reported the NH3 line width for some ATLASGAL clumps, which are highlighted by red diamonds in the figure (see also Table 2). (b) Longitude–velocity map of 13CO. The 13CO emission is integrated over the latitude from −0fdg228 to −0fdg065. In panels (a) and (b), an arbitrarily chosen solid curve (in yellow) shows an oscillatory-like velocity pattern along the longitude, and a scale bar corresponding to 1 km s−1 pc−1 is also shown. (c) Intensity-weighted mean velocity map (or the first moment map) of C18O. The color bar indicates the mean velocity (in km s−1). Broken lines are shown at b = −0fdg228 and −0fdg065.

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3.3. Temperature Map, Column Density Map, and Embedded Protostars

In Figures 9(a) and (b), we have presented the Herschel temperature and column density (N(H2)) maps (resolution ∼12'') of our selected target area, respectively. These maps6 were generated for the EU-funded ViaLactea project (Molinari et al. 2010b) using the Bayesian PPMAP method (Marsh et al. 2015, 2017), which was applied on the Herschel images at wavelengths of 70, 160, 250, 350, and 500 μm. In Figure 9(a), the Herschel temperature map is also superimposed with the MAGPIS 20 cm continuum contour. Radio continuum emission or H ii regions are found toward the areas with a relatively warm dust emission (Td > 21 K). In the extended physical system, at least three highlighted sites (i.e., G14.71−0.19, G14.47−0.20, and G014.194−00.194) are associated with the areas of cold dust emission (i.e., Td ∼ 16.5–18 K; see also Figure 1(a)). The most prominent feature in the Herschel temperature map is seen toward the bubble N14. The temperature structure of the bubble N14 is almost spherical, which is in agreement with the radio morphology. It seems that the feedback from massive stars (such as, stellar wind, ionized emission, and radiation pressure) might have heated the surroundings and is responsible for the extended temperature structure.

Figure 9.

Figure 9. (a) Herschel temperature map of the selected area toward l = 13fdg7–14fdg9 and b = −0fdg5 to +0fdg1. The MAGPIS contour with a level of 2.2 mJy beam−1 is also shown in the figure. (b) Herschel column density (N(H2)) map of the selected area in this paper. The box encompasses the area shown in Figures 10(a), (b), and (c).

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The column density map shows the distribution of materials with high column densities (>2.4 × 1022 cm−2) toward the highlighted sites (e.g., N14, G014.194−00.194, G14.427−00.075HII (and G14.47−0.20), G14.71−0.19) in the extended physical system. Using the Spitzer 8.0 μm image, Herschel column density map, and Herschel temperature map, Figures 10(a), (b), and (c) display a zoomed-in view of the area hosting some highlighted sources (e.g., IRAS 18141−1615, G14.427−00.075HII, and G14.47−0.20), respectively. The positions of the THOR radio sources are marked in Figures 10(a), (b), and (c). The MAGPIS 20 cm continuum contours are also overplotted on the Spitzer 8.0 μm image. The Spitzer image reveals extended bright emission as well as infrared dark clouds (IRDCs). The IRDCs are depicted as the absorption features against the Galactic background in the 8.0 μm image. The IRDCs are found with cold dust emission as well as high column density materials (see Figures 10(a), (b), and (c)). Previously, the extended emission at Spitzer 8.0 μm has been observed toward the bubble N14 (Churchwell et al. 2006; Dewangan & Ojha 2013; Yan et al. 2016). In general, it is known that the Spitzer 8.0 μm band contains polycyclic aromatic hydrocarbon (PAH) features at 7.7 and 8.6 μm. Considering the extended warm dust emission, PAH features and molecular gas surrounding the ionized emission, one can infer the existence of photon dominant regions (PDRs) in the extended physical system. The PDRs are traced by the PAH emission and indicate the molecular/ionized gas interface where one can expect the influence of UV radiation liberated by a nearby massive star.

Figure 10.

Figure 10. Zoomed-in view of the area highlighted by a broken box in Figure 9(b). (a) Overlay of the MAGPIS contours and the THOR radio sources on the Spitzer 8.0 μm image. The MAGPIS contours are shown with the levels of 2.2, 2.8, 3.3, 4.0, 5.0, 6.0, and 6.7 mJy beam−1. (b) Overlay of the THOR radio sources on the Herschel column density map. (c) Overlay of the THOR radio sources on the Herschel temperature map. In each panel, hexagon and pentagon symbols are the same as those in Figure 1(b).

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To infer different subregions in the extended physical system, the clumpfind algorithm is adopted with the $N({{\rm{H}}}_{2})$ of 2.4 × 1022 cm−2 as an input parameter. At least 21 subregions are identified, which are also marked and labeled in Figure 11(a). The total mass of each subregion is estimated using an equation, ${M}_{\mathrm{area}}={\mu }_{{{\rm{H}}}_{2}}{m}_{{\rm{H}}}{\mathrm{Area}}_{\mathrm{pix}}{\rm{\Sigma }}N({{\rm{H}}}_{2})$, where ${\mu }_{{{\rm{H}}}_{2}}$ (= 2.8) is defined earlier, Areapix is the area subtended by one pixel (i.e., 6'' pixel−1), and ${\rm{\Sigma }}N({{\rm{H}}}_{2})$ is the total column density (see also Dewangan et al. 2017). In Table 4, the mass and the radius of each Herschel subregion are listed. The masses of the Herschel subregions vary between 335 and 42,975 M⊙.

Figure 11.

Figure 11. (a) The boundaries of several extended regions are shown in the Herschel column density map using the $N({{\rm{H}}}_{2})$ contour level of 2.4 × 1022 cm−2. Each selected extended region is highlighted by an asterisk along with its corresponding clump ID (see Table 4). The cyan box area shows a zoomed-in version of Figures 12(a), (b), (c), and (d). (b) The positions of the identified Class I YSOs are shown toward the extended regions traced in the Herschel column density map. In this paper, the Class I YSOs, highlighted by open circles, are selected using the Spitzer color–color plot ([4.5]−[5.8] vs. [3.6]−[4.5]; see the text for more details). (c) The positions of the ATLASGAL clumps at 870 μm are displayed toward the extended regions traced in Figure 11(a). Different symbols are the same as those in Figure 3(a).

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Table 4.  Physical Parameters of Extended Regions Identified in the Herschel Column Density Map, which Are Highlighted in Figure 11(a)

ID l b Rc Mclump Association
  (degree) (degree) (pc) (M⊙)  
h1 13.789 −0.238 0.70 880 group1
h2 13.870 −0.301 0.74 1040 group1
h3 13.910 −0.293 0.53 525 group1
h4 14.014 −0.176 1.68 6175 group2
h5 14.197 −0.195 1.51 5400 group3
h6 14.234 −0.176 0.60 665 group3
h7 14.399 −0.186 0.47 420 group4
h8 14.494 −0.140 4.44 42975 group4
h9 14.532 −0.073 0.49 425 group4
h10 14.610 0.010 1.40 4045 group5
h11 14.689 0.024 0.56 635 group5
h12 14.682 −0.045 0.43 335 group5
h13 14.759 −0.081 0.86 1405 group5
h14 14.714 −0.155 2.55 13135 group5
h15 14.647 −0.118 0.92 1670 group5
h16 14.604 −0.138 0.77 1085 group5
h17 14.615 −0.203 0.76 1055 group5
h18 14.624 −0.326 0.86 1395 group4
h19 14.562 −0.361 0.90 1415 group4
h20 14.589 −0.408 0.60 710 group4
h21 14.692 −0.416 0.52 490 group4

Note. Column 1 lists the IDs given to the extended region. Table also contains positions, deconvolved effective radius (Rc), and clump mass (Mclump).

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Figure 11(b) shows the distribution of the selected Class I YSOs (mean age ∼0.44 Myr; Evans et al. 2009) toward the Herschel subregions, tracing the early phases of star formation activities in the extended physical system (see white circles). In other words, star formation activities are traced toward all the groups of the ATLASGAL clumps. Previously, using the Spitzer 3.6–5.8 μm photometric data, Hartmann et al. (2005) and Getman et al. (2007) applied the infrared color conditions (i.e., [4.5]−[5.8] ≥ 0.7 mag and [3.6]−[4.5] ≥ 0.7 mag) to identify Class I YSOs in a given star-forming region. We have used this selection scheme to select Class I YSOs in the extended physical system. In this context, photometric magnitudes of point-like objects at 3.6–5.8 μm were obtained from the Spitzer GLIMPSE-I Spring' 07 highly reliable catalog. In this work, we considered only objects with a photometric error of less than 0.2 mag in the selected Spitzer bands.

In Figure 11(c), one can examine the distribution of ATLASGAL clumps at 870 μm toward the Herschel subregions. The majority of ATLASGAL clumps at 870 μm (see Table 2) are mainly found in the direction of four Herschel subregions (see IDs # h4, h5, h8, and h14 in Table 4; mass range: 5400–42,975 M⊙), where signposts of active star formation are investigated (see Figure 11(c)). Using the C18O line data, dense molecular clumps (see Table 3) are also identified toward the Herschel subregions (see IDs # h4, h5, and h8 in Table 4).

3.4. GMRT Radio Continuum Maps

We find that the MAGPIS 20 cm continuum map reveals the compact radio continuum emission toward the bubble N14, while the diffuse radio emission is seen away from the bubble (see the white contour in Figure 9(a)). To further explore the ionized emission, low-frequency radio continuum maps of the bubble N14 are examined in this paper. In Figures 12(a) and (b), we display high-resolution GMRT radio continuum maps at 610 MHz (beam size ∼5farcs56 × 5farcs22) and 1280 MHz (beam size ∼6''), respectively. The GMRT 610 MHz continuum map is also superimposed with the 610 MHz continuum contour (see Figure 12(a)). In Figure 12(b), the GMRT 1280 MHz continuum map is also overlaid with the 1280 MHz continuum contour. Figure 12(c) displays the overlay of the GMRT 1280 MHz continuum emission contours on the Spitzer 8.0 μm image. The spherical-like radio morphology observed in the GMRT maps is found well within the bubble boundary. In Figures 12(a), (b), and (c), different color contours are used to show the inner radio morphology within the bubble N14.

Figure 12.

Figure 12. (a) A zoomed-in view of the bubble N14 using the GMRT 610 MHz continuum map (resolution ∼5farcs56 × 5farcs22). Cyan contours are 4 and 4.8 mJy beam−1. White contours are 6.2, 8.0, and 9.5 mJy beam−1. Black contours are 11, 12.5, 14, and 16 mJy beam−1. (b) A zoomed-in view of the bubble N14 using the GMRT 1280 MHz continuum map (resolution ∼6''). Cyan contours are 4.05, 4.5, 5.0, 5.5, and 6.0 mJy beam−1. White contours are 7.6, 8.15, and 8.7 mJy beam−1. Black contours are 9.25, 9.8, 10.35, 10.9, 11.45, 12.0, and 12.55 mJy beam−1. The broken box (in magenta) encompasses the area shown in Figure 12(d). (c) Overlay of the GMRT 1280 MHz continuum contours on the Spitzer 8.0 μm image. Cyan contours are 4.05, 4.5, 5.0, 5.5, and 6.0 mJy beam−1. Yellow contours are 7.6, 8.15, 8.7, 9.25, and 9.8 mJy beam−1. Blue contours are 10.35, 10.9, 11.45, 12.0, and 12.55 mJy beam−1. (d) The panel shows the boundaries of several radio continuum sources at 1280 MHz, which are identified using the radio continuum contours at 1280 MHz (see Figure 12(b)). Each ionized clump is highlighted by an asterisk along with its corresponding clump ID. In panel (a), two radio clumps "A" and "B" are identified using the GMRT radio continuum maps at 610 and 1280 MHz.

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Using the GMRT 1280 MHz continuum map, at least 17 radio clumps are identified toward the bubble N14, which are shown in Figure 12(d) (see the broken box in Figure 12(b)). In this analysis, the clumpfind algorithm (Williams et al. 1994) was employed. Adopting similar procedures as those given in Dewangan et al. (2017), the Lyman continuum photons (see also Matsakis et al. 1976, for equation) and spectral type of each radio source are computed. In the calculation, we used a distance of ∼3.1 kpc, an electron temperature of 104 K, and the models of Panagia (1973, see his Table 2). The analysis suggests that all the ionized clumps are powered by massive B-type stars. We have tabulated the derived physical properties of the ionized clumps (i.e., deconvolved effective radius of the ionized clump (RH ii), total flux (Sν), Lyman continuum photons ($\mathrm{log}{N}_{\mathrm{uv}}$), dynamical age (tdyn), and radio spectral type) in Table 5. Using the equation given in Dyson & Williams (1980), we have computed the values of tdyn for each radio clump (see Dewangan et al. 2017 for more details). The ages of all these radio clumps are found between ∼103 and 104 yr for an initial particle number density (ni) of 103 cm−3, indicating that they are very young. Furthermore, the values of Sν and Nuv are also computed for the entire extended spherical emission using the GMRT 1280 MHz continuum map. The integrated flux is estimated to be Sν ∼ 3.54 Jy at 1280 MHz, which yields $\mathrm{log}{N}_{\mathrm{uv}}$ to be ∼48.42. This implies that the observed extended radio emission can be explained by a single ionizing star of spectral type O8.5V (e.g., Panagia 1973). It is also in agreement with the analysis of the GMRT 610 MHz data (not presented here) as well as the previously reported value of $\mathrm{log}{N}_{\mathrm{uv}}$ (= 48.36; Beaumont & Williams 2010).

Table 5.  Physical Parameters of 17 Radio Clumps Traced in the GMRT 1280 MHz Continuum Map, which Are Labeled in Figure 12(d)

ID l b RH ii Sν $\mathrm{log}{N}_{\mathrm{uv}}$ tdyn Spectral Type
  (degree) (degree) (pc) (mJy) (s−1) (×103 yr) (dwarf main sequence (V))
g1 13.999 −0.112 0.10 26 46.29 1.1 B0.5-B1
g2 14.006 −0.123 0.27 238 47.25 9.8 B0-B0.5
g3 14.007 −0.127 0.17 98 46.87 4.0 B0-B0.5
g4 14.008 −0.130 0.21 142 47.03 5.8 B0-B0.5
g5 13.995 −0.118 0.20 121 46.96 6.3 B0-B0.5
g6 13.997 −0.123 0.17 97 46.86 4.2 B0-B0.5
g7 13.994 −0.121 0.15 74 46.75 3.0 B0-B0.5
g8 13.997 −0.127 0.16 77 46.76 3.7 B0-B0.5
g9 14.000 −0.135 0.13 47 46.55 2.6 B0-B0.5
g10 14.002 −0.136 0.14 58 46.64 3.3 B0-B0.5
g11 13.987 −0.119 0.20 133 47.00 5.8 B0-B0.5
g12 13.987 −0.125 0.14 68 46.71 2.2 B0-B0.5
g13 13.988 −0.127 0.14 62 46.66 2.3 B0-B0.5
g14 13.990 −0.129 0.11 35 46.42 1.5 B0.5-B1
g15 13.990 −0.134 0.20 127 46.98 5.1 B0-B0.5
g16 13.990 −0.140 0.11 34 46.41 1.7 B0.5-B1
g17 13.982 −0.125 0.13 46 46.54 2.3 B0-B0.5

Note. Table contains ID, Galactic coordinates (l, b), deconvolved effective radius of the H ii region (RH ii), total flux (Sν), Lyman continuum photons (log Nuv), dynamical age (tdyn), and radio spectral type.

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In general, radio continuum data at low frequencies are very useful to trace the nonthermal emission in a given astrophysical object (e.g., De Becker 2018). In order to infer the spectral indices of the radio clumps toward the bubble N14, the GMRT radio continuum maps at 610 and 1280 MHz are convolved to the same (lowest) resolution, and at least two radio clumps (i.e., clump A and clump B) are identified. Figures 13(a) and (b) show the radio spectral index plots of clump A and clump B, respectively. The spectral indices of the radio clumps are also labeled in Figures 13(a) and (b). The positions of these clumps are also highlighted in Figure 12(a) (see clump A (α ∼ −0.73) and clump B (α ∼ −0.14)). This exercise indicates the presence of nonthermal emission in bubble N14. Additionally, in the direction of l > 13fdg4, we also find that several H ii regions or THOR radio sources show nonthermal emission. It has been reported that H ii regions powered by massive OB stars are normally associated with thermal emission (e.g., Wood & Churchwell 1989; Kurtz 2005; Sánchez-Monge et al. 2008, 2011; Hoare et al. 2012; Purcell et al. 2013; Wang et al. 2018; Yang et al. 2019). In the literature, we also find some examples of H ii regions (i.e., IRAS 17160−3707 Nandakumar et al. 2016 and IRAS 17256−3631 Veena et al. 2016), which emit both thermal and nonthermal radiation. The observed nonthermal emission in H ii regions indicates the presence of relativistic electrons. More recently, it has been suggested that the nonthermal emission in H ii regions might refer to synchrotron radiation from locally accelerated electrons restrained in a magnetic field (see Padovani et al. 2019 for more details).

Figure 13.

Figure 13. (a) Radio spectral index plot of the radio clump "A," which is indicated in Figure 12(a). (b) Radio spectral index plot of the radio clump "B" (see Figure 12(a)). In each panel, filled circles (in black) are the flux densities at 610 and 1280 MHz.

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4. Discussion

Based on the analysis of the molecular gas and the distribution of the ATLASGAL clumps, an extended physical system (∼59 pc × 29 pc) is identified toward l = 13fdg7–14fdg9 containing the bubble N14. The system is found to host at least five groups or subregions. The spatial and velocity connections of these subregions are also found in the analysis of the molecular line data (see Section 3.2). Hence, the present work focuses on exploring the physical processes responsible for the observed hierarchy in the extended physical system. These groups host unstable clumps (see Tables 2 and 3), and are associated with the C18O emission. These dense clumps are associated with Class I YSOs, which trace the early phase of star formation (see Section 3.3). Furthermore, some of the subregions (e.g., group2 and group4) harbor massive OB stars, and their associated H ii regions are found to be expanding in their surroundings (see Section 3.2). In the direction of the H ii regions, the extended structures in the Herschel temperature map are also found, illustrating the signatures of the impact of massive stars via their energetics (i.e., stellar wind, ionized emission, and radiation pressure). Hence, to explain the observed hierarchy, the application of "globule squeezing" and "collect and collapse" processes can be examined. These two processes are explained by the expanding H ii regions powered by massive stars (e.g., Elmegreen & Lada 1977; Whitworth et al. 1994; Elmegreen 1998; Deharveng et al. 2005; Dale et al. 2007; Bisbas et al. 2015; Walch et al. 2015; Kim et al. 2018; Haid et al. 2019, and references therein).

In general, the impact of a massive star can be studied with the knowledge of the pressure of an H ii region (PH ii), the radiation pressure (Prad), and the stellar wind ram pressure (Pwind) (e.g., Dewangan et al. 2015, 2017). Based on the values of different pressure components, the influence of massive stars to their surroundings is found to be more significant up to a projected distance of a few parsecs (e.g., Dewangan et al. 2015, 2016a, 2017; Baug et al. 2019). Our observational results indicate that these processes might have influenced star formation activities locally in the extended physical system. However, these two processes may not explain the hierarchy extended up to about 59 pc in the selected target area.

In one of the theoretical models, the observed star formation can be explained by convergent gas flows (Ballesteros-Paredes et al. 1999; Vázquez-Semadeni et al. 2007; Heitsch et al. 2008) or the CCC process (e.g., Habe & Ohta 1992). The model predicts that the convergence of streams of neutral gas can produce molecular clouds (e.g., Ballesteros-Paredes et al. 1999; Vázquez-Semadeni et al. 2007; Heitsch et al. 2008). With time, one also expects the merging/converging/collision of the molecular clouds. However, we do not find distinct multiple velocity components associated with the extended physical system. In Figure 8(c), noticeable velocity spreads toward all the selected groups (except group5) are evident in the first moment map of C18O. In Sections 3.1 and 3.2, our observational results also show that the extended physical system is situated toward the near sides of the Scutum and Norma arms. Hence, locally, one cannot completely discard a triggered star formation scenario by convergent gas flows or the CCC process (e.g., Habe & Ohta 1992).

In recent years, it has been suggested that star-forming clouds seem to be in a state of global gravitational contraction (e.g., Hartmann et al. 2012; Vázquez-Semadeni et al. 2019). It is also reported that the global collapse leads in a chaotic and hierarchical manner, yielding gravitationally driven fragmentation in star-forming molecular clouds (e.g., Burkert & Hartmann 2004, 2007; Heitsch & Hartmann 2008; Galván-Madrid et al. 2009; Heitsch et al. 2009; Peretto et al. 2013; Beuther et al. 2015; Liu et al. 2015, 2016; Friesen et al. 2016; Jin et al. 2016; Csengeri et al. 2017; Hacar et al. 2017; Yuan et al. 2018; Barnes et al. 2019; Jackson et al. 2019; Vázquez-Semadeni et al. 2019). The modeling results based on the global hierarchical gravitational collapse in molecular clouds by Vázquez-Semadeni et al. (2019) indicate late birth of massive OB-stars and later loss of molecular gas feeding by feedback of massive stars. It also suggests that after the onset of global collapse, one can expect local collapse events in molecular clouds. Therefore, in the extended physical system, one can examine the observed hierarchy as the outcome of gravitational fragmentation (see the review article by Hennebelle & Falgarone 2012).

In three-dimensional models of molecular cloud formation in large-scale colliding flows including self-gravity, Heitsch et al. (2008) reported the formation of large-scale filaments due to global collapse of a molecular cloud. In favor of this physical process, we find the important observational evidence in the velocity space of the molecular gas. The position–velocity maps of 12CO, 13CO, and C18O show the oscillatory-like velocity pattern (with a period of ∼8–13.5 pc and an amplitude of ∼2 km s−1) in the direction of the entire longitude range (see Section 3.2). The position–velocity maps have been produced for different ranges of latitude (i.e., b = [−0fdg5, 0fdg1], and b = [−0fdg228, −0fdg065]; see Figure 8(a) and (b)). In the direction of the entire longitude range and b = [−0fdg228, −0fdg065], most of the molecular emission and IRDCs are observed (see broken lines in Figure 8(c)). In the direction of such a selected area, the oscillatory-like velocity pattern is still evident (see Figure 8(b)). It implies that the extended physical system shows a sinusoidal-like (i.e., oscillatory) velocity structure with a significant fragmentation. In the direction of l = 14fdg4–14fdg5 or the G14.427−00.075HII region, an arc-like configuration is found in the velocity space, which shows signatures of an expansion (see Section 3.2).

Previously, in the case of the filament L1571 (length ∼0.35–0.70 pc at a distance of 144 pc), Hacar & Tafalla (2011) performed a modeling of velocity oscillations as sinusoidal perturbations. Based on their analysis, the observed velocity oscillations along the filament (with a period of ∼0.19–0.24 pc and an amplitude of ∼0.04 km s−1) were explained by the filament fragmentation process via accretion along the filament. L1517 is known as the site of low-mass star formation. Recently, Dewangan et al. (2019) also found an oscillatory pattern (with a period of ∼6–10 pc and an amplitude of ∼0.5 km s−1) in velocity toward the S242 filamentary structure (length ∼30 pc at a distance of 2.1 kpc) hosting an H ii region excited by a B-type star, and suggested the fragmentation of the filament. Figure 8(b) in this paper looks similar to the published plot (i.e., Figure 7(b) in Dewangan et al. 2019) of S242, implying the onset of a similar fragmentation process. One can keep in mind that the extended physical system (at a distance of ∼3.1 kpc; size in longitude direction ∼59 pc) is not an extended filament, but it contains several IRDCs associated with the cold dust emission and high column density materials (see Section 3.3). It is also possible that the expansion of an H ii region (i.e., local event) may diminish the collapse signature. However, taking into account the existence of different groups/subregions in the extended physical system, the oscillatory pattern in velocity is suggestive of the fragmentation process. It may also favor the multiscale collapse, resulting in a hierarchical configuration. We find the higher values of velocity amplitude and period of oscillations in the extended physical system compared to the filaments L1517 and S242, which could be explained by the existence of more massive and larger clumps associated with massive star formation at different stages of evolution.

Taken together all these observed results, the concept of the global collapse seems to be applicable in the extended physical system, and can explain the presence of all spatially distinct groups/subregions in the extended physical system.

5. Summary and Conclusions

This paper deals with a multiwavelength study of a wide-scale environment toward l = 13fdg7–14fdg9 containing the bubble N14, allowing the examination of the ongoing physical processes. The major outcomes of this work are presented below.

  • 1.  
    The study of the FUGIN 12CO, 13CO, and C18O gas at [31.6, 46] km s−1 shows the presence of an extended physical system or molecular cloud (extension ∼59 pc× 29 pc) toward l = 13fdg7–14fdg9.
  • 2.  
    53 ATLASGAL 870 μm dust clumps at d ∼3.1 kpc are distributed toward the cloud. At least five spatially distinct groups of the ATLASGAL clumps are selected through a visual inspection in the extended physical system.
  • 3.  
    In the direction of four groups, gravitationally unstable clumps are identified, which are massive (>103 M⊙) and dense (>104 cm−3).
  • 4.  
    Considering the distribution of Class I YSOs (mean age ∼0.44 Myr), locally the early stage of star formation activity is observed toward each group of clumps.
  • 5.  
    In the direction of the extended physical system, the position–velocity maps reveal continuous velocity structures, where velocity gradients are also evident. The study of molecular gas displays the spatial and velocity connections of all the selected groups. These findings show a hierarchy in the extended physical system.
  • 6.  
    The radio continuum and Herschel maps trace at least three groups of clumps associated with the expanding H ii regions (including the bubble N14).
  • 7.  
    In the direction of the H ii regions, the warm dust emission (Td ∼ 21–26 K) spatially coincides with the ionized emission. This implies that the molecular cloud appears to be influenced by massive OB stars.
  • 8.  
    The observed spectral indices determined using the GMRT and THOR radio continuum data indicate the existence of nonthermal emission with the H ii regions.
  • 9.  
    The ionized emission traced in the GMRT 610 and 1280 MHz continuum maps shows an almost spherical morphology toward the bubble N14, which is found well within the bubble morphology depicted at 8.0 μm. A similar morphology is also observed in the Herschel temperature map (with Td ∼ 21–26 K) toward the bubble N14. The ionizing photon flux values computed at both the GMRT bands refer to a single ionizing star of O8.5V spectral type.
  • 10.  
    Using the GMRT 1280 MHz continuum map, at least 17 radio clumps, powered by B-type stars, are identified toward the bubble N14, which are found to be considerably young (age ∼103–104 yr for ni = 103 cm−3).

The analysis of molecular gas exhibits the oscillatory-like velocity pattern in the direction of the entire longitude range. Keeping in mind the presence of different groups/subregions, on a wide-scale, this velocity structure hints the onset of the fragmentation process in the extended physical system. Considering all the observational evidences presented in this work, the global collapse scenario seems to be operated in the extended physical system, which may also explain the observed hierarchy.

We thank the anonymous reviewer for several useful comments and suggestions, which greatly improved the scientific contents of the paper. The research work at Physical Research Laboratory is funded by the Department of Space, Government of India. This work is based (in part) on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology under a contract with NASA. This publication makes use of data from FUGIN, FOREST Unbiased Galactic plane Imaging survey with the Nobeyama 45 m telescope, a legacy project in the Nobeyama 45 m radio telescope. T.B. is supported by the National Key Research and Development Program of China through grant 2017YFA0402702. T.B. also acknowledges support from the China Postdoctoral Science Foundation through grant 2018M631241. L.E.P. acknowledges support of the Russian Foundation for Basic Research (project 18-02-00660). D.K.O. acknowledges the support of the Department of Atomic Energy, Government of India, under project No. 12-R&D-TFR-5.02-0200.

Footnotes

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10.3847/1538-4357/ab964c