IC 5146 dark Streamer: is a first reliable candidate of edge collapse, hub-filament systems, and intertwined sub-filaments?

Figures 1a and 1b display the Herschel column density ($N(\mathrm{H}_{2})$) and temperature ($T_{\mathrm{d}}$) maps (resolution $\sim$13.^′′5) of our selected target area around the IC 5146 dark Streamer. For the first time, we present these high resolution maps of our target site, which have been produced using the getsf-hires method as discussed in Section 2. Visual inspection of the column density map shows several extended structures. Previously reported two HFSs (i.e., E-HFS and W-HFS) and filament skeletons are also shown in the Herschel column density map (see Figure 1c), which were identified using the HGBS column density map at the resolution of $\sim$18.^′′2 (e.g., Arzoumanian et al., 2019). In earlier works, a single long filament, designated as fl, hosting E-HFS and W-HFS at its opposite ends was mainly discussed. We have also marked several small circular regions (radii = 25^′′) along this long filament (see magenta circles in Figure 1c), where some physical parameters (such as column density, temperature etc.) are determined (see Section 3.3). In this paper, the Herschel column density map at the resolution of $\sim$13.^′′5 reveals the presence of two intertwined sub-filaments (i.e., fl-A and fl-B; $T_{\mathrm{d}}$ $\sim$11–15 K), which are indicated by arrows in Figure 1a. We have considered the long filament fl as the main filament, which seems to be composed of two intertwined sub-structures. This is a new result in the IC 5146 dark Streamer. The implication of this outcome is discussed in Section 4.2.

Figure 2a shows the boundaries of different structures traced in the column density map. In order to identify these structures, we used the $N(\mathrm{H}_{2})$ contour at 5.22 $\times$ 10²¹ cm^-2 and the IDL based clumpfind algorithm (Williams et al., 1994). The clumpfind algorithm divides two- and three-dimensional data into distinct emission clumps by contouring of data with a multiple of the rms noise (see more information in Williams et al., 1994). The identified structures primarily trace the eastern filament and the western HFS. We have also identified several clumps toward the structures as presented in Figure 2a. We used the $N(\mathrm{H}_{2})$ contours at [5.22, 9.3, and 12] $\times$ 10²¹ cm^-2 to trace these clumps. In $N(\mathrm{H}_{2})$ map, we define the clumps as non-filamentary arbitrarily shaped structures (size $\sim$0.5 pc), while the cores are rather circularly shaped structures (size $\leq$0.1 pc; see Section 3.2). In standard terminology, clumps have a lower density ($\sim$10⁴ cm^-3) than the embedded cores ($>$10⁵ cm^-3; Onishi et al., 2002; Saito et al., 2006). The locations and boundaries of the clumps are displayed in Figure 2b. Filled upside-down triangles show the clumps distributed toward the long filament fl, while the clumps located away from this structure are highlighted by open upside-down triangles. Figure 2c displays the distribution of the Herschel clumps, the ionized emission traced by the NVSS 1.4 GHz continuum contour, and the Planck 353 GHz or 850 $\mu$m continuum emission against the structures as presented in Figure 2a. We do not find any noticeable radio continuum emission toward the long filament fl, including both the HFSs. The elongated appearance is also evident in the Planck 353 GHz or 850 $\mu$m continuum map, which does not show any inner structures due to its coarse beam size.

We have also computed the mass of each Herschel clump using the equation, $M_{clump}=\mu_{H_{2}}m_{H}A_{pix}\Sigma N(H_{2})$, where $\mu_{H_{2}}$ is the mean molecular weight per hydrogen molecule (i.e., 2.8), $A_{pix}$ is the area subtended by one pixel (i.e., 3^′′/pixel), and $\Sigma N(\mathrm{H}_{2})$ is the total column density (see also Dewangan et al., 2017). Figure 2d presents the mass distribution against the position of all the clumps, allowing us to examine the mass distribution of clumps. Filled symbols show the clumps distributed toward the long filament fl, and massive ones appear to be present at its opposite ends.

In order to highlight two sub-filaments, a false color map is produced using the Herschel image at 250 $\mu$m (see Figure 3a). Previously reported positions of the Class I YSOs, flat-spectrum sources, and Class II YSOs (from Harvey et al., 2008) are shown on the Herschel image. We also employed the “Edge-DoG” algorithm on the Herschel image at 250 $\mu$m, and its outcome is presented in Figure 3b. The IDL based “Edge-DoG” filter enhances the extended brightness inhomogeneities (e.g., sharp edges) based on the Difference of Gaussians filters technique (Assirati et al., 2014). The positions of HFSs (i.e., E-HFS and W-HFS) and two possible sub-filaments are marked in Figure 3b. Figure 3c exhibits a zoom-in view of the Herschel column density map toward the IC 5146 dark Streamer, where the two sub-filaments fl-A and fl-B are indicated by arrows. Based on the visual inspection of Herschel images, we have presented a cartoon diagram displaying the possible configuration of fl-A and fl-B in Figure 3d. The overlapping areas of the sub-filaments are shown by filled hexagons, where either the YSOs or dense sources are identified. The implication of this configuration is discussed in Section 4.2.

To identify the dust continuum sources (or cores) and filament skeletons present in our target site IC 5146 Streamer, we used the getsf tool presented in Men’shchikov (2021). The getsf method is capable of extracting the sources and filaments from an astronomical image and requires only a single user input of the maximum size of the structure to extract. We employed getsf utility on the N(H₂) map at 13.^′′5. The maximum source and filament size were set to be 20^′′ and 200^′′, respectively. Figure 4a presents the overlay of getsf extracted sources on the N(H₂) map. The size of sources indicates their footprint size, which is plotted by their estimated major and minor axes. We have only selected the sources which lie within the N(H₂) contour value of 5.22$\times$10²¹ cm^-2. A total of 67 sources were found and displayed in Figure 4. The overlay of filament skeletons on the N(H₂) map at the spatial scale of $\sim$14^′′ is shown in Figure 4b. The presence of at least two close filaments along the major axis of fl is marked by arrows. We have overlaid the getsf sources on the image. The size of dots is proportional to the footprint area of getsf sources. The color scheme of sources infers their mass distribution. The mass of the sources is estimated by the same method presented in Section 3.1. This analysis suggests that the more massive cores are located at the hub locations (i.e., E-HFS and W-HFS) compared to the other areas. We cleaned the getsf identified filament skeletons by removing the spurious structures and the structures outside of our target region.

In the introduction section, we have highlighted the candidate filaments experiencing edge collapse. Apart from the IC 5146 dark Streamer (d $\sim$600 pc), we find three other EDC filaments (i.e., Mon R1 (d $\sim$760 pc), S242 (d $\sim$2.1 kpc), and NGC 6334 (d $\sim$1.3 kpc)), which are nearby EDC filamentary systems (d $\lesssim$ 2 kpc). The existing observations of dust polarised emission from the Planck telescope have been employed to study the large-scale magnetic field of the four EDC filaments. We used the Planck 353 GHz Stokes I, Q, and U maps to estimate the linear polarization angles (PAs) of polarized dust emission caused by the anisotropic dust grains in our target sites. The stokes I, Q, and U maps were converted from the cosmic microwave background (CMB) temperature (K_cmb) scale to MJy sr^-1 using the unit conversion factor of 246.54 (e.g., Planck Collaboration et al., 2016b). We also smoothed the stokes maps by astropy based Gaussian 2D-kernel (input parameter x_stddev=2) to increase the signal-to-noise ratio. We estimated the PAs in Galactic coordinates using the conventional relation of $\theta_{\rm GAL}=0.5\times~{}{\rm arctan2(-U,Q)}$, where $-$U is used to follow the IAU convention (see more details in Planck Collaboration et al., 2015) and a two-argument function arctan2 is used to avoid the $\pi$-ambiguity in the estimation of PAs. The magnetic field orientations were then computed by adding 90$\arcdeg$ in the electric field polarization PAs (e.g., Planck Collaboration et al., 2016c, d). We term this angle as B_Gal throughout the paper, which is measured from galactic north to east along the counter-clockwise direction.

The distribution of the plane of sky (POS) magnetic field in the direction of our selected EDC filaments IC 5146 dark Streamer, S242, Mon R1, and NGC 6334 is displayed by streamlines in Figure 10. We used streamplot in matplotlib to display the magnetic field orientations toward our selected targets. The magnetic field direction is nearly perpendicular to all the filaments, consistent with the observations of other targets (e.g., Palmeirim et al., 2013; Planck Collaboration et al., 2016a; Cox et al., 2016). The ¹³CO emission at the velocity range of [0.8, 2.5] km s^-1 (see Figure 8c) appears to be parallel to the B-field orientation shown in Figure 10a. It is reported that faint filaments (striations) are well aligned to the magnetic fields, and the main filament can gain its mass from these striations (e.g., Palmeirim et al., 2013; Zhang et al., 2020). Therefore, the ¹³CO emissions parallel to the B-field show that IC 5146 dark Streamer is not isolated but interacts with the natal clouds. However, interestingly the B-field direction is curved at the edges of our target filaments. As discussed by Wang et al. (2019) and Chung et al. (2022), the B-field directions in the EDC filaments make a shape of “)” and “(” at the filament edges (hereafter, the bending effect). This is because of the material pile up at the edges, which itself moves toward the center of the filament (e.g., Clarke & Whitworth, 2015). This longitudinal motion of gaseous material from the ends to the center of the filament can have sufficient ram pressure that pinches the magnetic field lines and forms the U-shaped magnetic field morphology (or bending effect; see more details in Gómez et al., 2018; Wang et al., 2019, 2020a). This effect is most prominently seen in the S242 and Mon R1 filament than in the other sources (e.g., IC 5146 dark Streamer and NGC 6334; see arrows in Figure 10). We suspect that the bending effect at one of the edges of the IC 5146 dark Streamer is not prominent because of the contamination caused by the presence of the HFS. However, still, this effect is significant toward the south-eastern clump of the IC 5146 dark Streamer (see also Figure 7 of Chung et al., 2022). Similarly, the NGC 6334 also shows the signature of HFSs at its edges (see Figure 2 of Tigé et al., 2017a) and does not show the strong bending effect in Figure 10. However, the high-resolution polarization map (beam $\sim$14^′′) of NGC 6334 by Arzoumanian et al. (2021) indeed shows the bending effect at the dense eastern region (Figure 10) revealing the longitudinal gas motion along the filament. The direction of the magnetic field at the other end of the NGC 6334 is randomly oriented in the high-resolution map of Arzoumanian et al. (2021). However, our Planck magnetic field map shows the signature of curvature at this end, too (see Figure 10). It is discussed in the literature that the magnetic field distortions can be caused by the outflow-driven shocks, feedback from expanding ionized fronts, and gravity-driven gas flows (e.g., Arzoumanian et al., 2021; Eswaraiah et al., 2021). Also, recent observations signify that the curved magnetic field can be originated by the effect of gravity and the collision of clouds (e.g., Wang et al., 2020a, 2022). Therefore, it is quite possible that the intense star formation activity and the presence of HFSs at the filament edges can distort the initial bending effect in EDC filaments. Although our target filaments are promising EDC candidates, their non-linearity can diminish the bending effect at low-resolution Planck magnetic field maps. Thus, we confirm that the other linear EDC filaments (if HFSs are not present) should show the bending effect.

Figure 11 displays the spatial distribution of magnetic field position angle (B_Gal) toward selected EDC filaments. The derived B_Gal maps are masked out for regions of low intensity and used to obtain the histograms of B_Gal (see column 2 of Figure 11). The mean B_Gal is found to be 48.37, 87.96, 96.44, and 56.70 degrees for the IC 5146 dark Streamer, S242, Mon R1, and NGC 6334, respectively. To derive the B_Gal distribution in the IC 5146 dark Streamer, we have considered only the region containing elongated filament. Hence we have masked out the southern-west region seen in Figure 10. Interestingly, these two filaments are nearly perpendicular to the Galactic plane. Mon R1, however, is a highly curved filament having its ends nearly perpendicular and parallel to the Galactic plane. A positive gradient in B_Gal can be seen from where the Mon R1 filament gets perpendicular to the Galactic plane. The distribution of B_Gal along the long axis of selected filaments is presented by blue curves in Figure 11. To quantify the distribution of the magnetic field toward the target filaments, we estimated the magnetic field position angle with respect to the filament’s major axis from the direction of its head (B_Filament). The distribution of B_Filament vs filament’s length (B_Filament–$L$) is shown by red curves in Figure 11. Interestingly, for all the filaments, the global trend follows a negative slope in a range of [$-$0.04, $-$0.02] degree arcsec^-1. We estimated the slope by fitting a straight line (see black dotted line) on the B_Filament–$L$ plot. The negative slope in B_Filament–$L$ distribution agrees well with the idea of magnetic field bending effect by Wang et al. (2019). Ideally, a straight filament having magnetic field lines perpendicular toward its central regions and curved magnetic fields (i.e., shapes of “)” and “(”) toward the respective edges (see Figure 13 of Wang et al., 2019), should show the global increasing or decreasing trend in B_Filament–$L$ plot. The negative or positive trend will depend upon the choice of reference direction (i.e., filament’s head or tail) from which the magnetic field angle is measured. In our target filaments, a global linear trend in B_Filament–$L$ plot hints at the EDC signature. However, despite the linear trend, we witness local oscillations as well. This is possible because of the non-linearity of the filament (or sky-projection effect) and the contamination caused by other ongoing processes.

Based on the examination of the TRAO molecular line data, a collision of turbulent converging flows was suggested to explain the existence of HFSs, which also includes the role of mass flow along the filaments to the dense cores in IC 5146 (Chung et al., 2021). In order to further explore the collision process in the Streamer, we revisited the TRAO ¹³CO and C¹⁸O line data (see Section 3.3). Our analysis of the molecular line data shows the existence of two cloud components around 2, and 4 km s^-1 in the direction of the IC 5146 dark Streamer, and a velocity connection of these components is also found. The areas of the E-HFS and W-HFS, where the higher level of clumpiness is observed, are seen toward the common zones of the cloud components (see Section 3.3.2). Earlier works on the cloud-cloud collision (CCC) recommend a spatial and velocity connection of two cloud components as a reliable tracer of CCC (see Fukui et al., 2021; Maity et al., 2022, and references therein for more details). In general, colliding gas flows are thought to be operated in the low-density medium, while the high-density phase of colliding gas flows could refer to CCC (e.g., Beuther et al., 2020; Dewangan, 2022).

Central hubs of HFSs, hosting massive stars and clusters of YSOs, are thought to gain the inflow material from very large scales of 1-10 pc, which can be funneled along molecular filaments (see Tigé et al., 2017b; Motte et al., 2018, for more details). Theoretical works favor the existence of HFSs by the colliding clouds or large-scale colliding flows (Balfour et al., 2015; Inoue et al., 2018), and this scenario is also supported in the recent review article on CCC (Fukui et al., 2021). Based on our findings, it is likely that the IC 5146 dark Streamer may be influenced by the collision process, and both the HFSs appear to be originated in the shock-compressed interface layer by the colliding clouds/flows (see also Chung et al., 2021). The massive sources are exclusively found to reside at the hub locations (see Figure 4).

Now, we have examined why HFSs are formed at the edges of the filament fl? It was already explained by the onset of the edge-driven collapse in the filament fl, which has a higher aspect ratio (see Wang et al., 2019; Chung et al., 2022, and references therein). In the EDC process, density enhancement is expected at each end of the filament due to high gas acceleration (Bastien, 1983; Pon et al., 2012; Clarke & Whitworth, 2015; Hoemann et al., 2022).

The present work reveals the existence of two coupled (or intertwined) sub-filaments (i.e., fl-A and fl-B) toward the main filament fl, showing almost a double helix-like pattern (see Figures 3d, 4, and Section 3.1). The TRAO ¹³CO and C¹⁸O line data do not reveal this configuration due to a coarse beam size. However, the JCMT C¹⁸O (3–2) map seems to support the detected structures in the Herschel maps. Hence, such intertwined configuration is seen in both the dust and molecular emissions. A cartoon diagram displaying the physical configuration of sub-filaments fl-A and fl-B (Figure 3d) is motivated by their simultaneous detection in the dust continuum and the molecular emission maps. However, the existing molecular line data do not allow us to trace the velocity information along each sub-filament. Previously, Dewangan et al. (2021) found the velocity oscillation along two intertwined filaments in site LBN 140.07+01.64. The physical configuration of sub-filaments in IC 5146 Streamer appears very similar to the sub-filaments identified in LBN 140.07+01.64 (see Figures 3 and 7 therein). Therefore, due to such limited sites in literature, the presence of intertwined filaments or their double helix-like pattern and their role in star formation demand more observational as well as theoretical insights. The sub-filaments fl-A and fl-B appear to spatially overlap with each other along the major axis, forming multiple common areas where the Herschel clumps and YSOs are seen. This is another new outcome of this work. In this relation, we examined the existing “fray and fragment” scenario of the formation of intertwined sub-structures (Tafalla & Hacar, 2015; Clarke et al., 2017), which firstly predicts the formation of the main filament by a collision of two supersonic turbulent gas flows, and then the scenario favors the origin of the intertwined system of velocity-coherent sub-structures in the main filament due to residual turbulent motions and self-gravity (see also Smith et al., 2014; Shimajiri et al., 2019). This scenario was proposed in the NGC 6334 filament (Shimajiri et al., 2019) and the Taurus filament (Tafalla & Hacar, 2015).

Additionally, we have also investigated a noticeable velocity oscillation along the filament fl (see Section 3.3.2). Previously, in the case of a filament G350.5-N associated with the cloud G350.54+0.69, Liu et al. (2019) reported a large-scale periodic velocity oscillation. To explain this aspect, on the basis of gravitational-instability-induced core formation models, they proposed a proposal with the combination of longitudinal gravitational instability and a large-scale physical oscillation along the filament. Considering two sub-filaments and the distribution of the Herschel clumps toward the long filament fl, the observed velocity oscillations/variations seem to support the presence of two coupled or intertwined sub-filaments and fragment/clump formation along the filament, where the non-thermal (or turbulent) pressure seems to be dominated (see Section 3.3.2).

Taken together, our findings reveal the onset of multiple physical processes in the IC 5146 dark Streamer, which includes the edge collapse, CCC, accretion flows, and “fray and fragment” scenario.