SiO emission in the filamentary Infrared Dark Cloud G035.39-00.33: An ALMA view

TL;DR

The study targets the filamentary infrared dark cloud G035.39-00.33 with ALMA observations of SiO (2-1), H$^{13}$CO$^+$ (1-0), CH$_3$OH (2-1), and CS (2-1) at ~3.5$^{\prime\prime}$ resolution to trace shock activity and current star formation. It uncovers three protostellar outflows and two large-scale arc-like shock structures evident in SiO, CS, and CH$_3$OH, with narrow, extended emission suggesting large-scale shocks at the cloud edges. Column density and abundance analyses show elevated SiO in outflows (N$_{SiO}$ up to ~2×10$^{13}$ cm$^{-2}$; χ$_{SiO}$ up to ~1×10$^{-9}$) compared to arcs (lower χ$_{SiO}$ by factors of ~3), indicating different shock origins. By integrating radio continuum and CO data, the authors propose that shocks from SNR G35.6-0.4 expansion and/or cloud–cloud collisions may have shaped the filament and potentially triggered star formation, although a decisive mechanism remains open for follow-up observations. The results illuminate how external shocks can sculpt IRDCs and influence their star formation activity, highlighting the need for deeper, higher-sensitivity ALMA studies to disentangle these processes.

Abstract

Filamentary infrared dark clouds (IRDCs) are believed to represent the initial conditions for massive star and cluster formation. We investigate the IRDC G035.39-00.33 using SiO, H13CO+, CH3OH, and CS emission observed with ALMA at 3.5\arcsec\ resolution (0.05 pc). The SiO emission traces shock activity within the cloud, providing insights into current star formation and cloud formation mechanisms. We identify several regions with broad SiO emission clearly associated with outflows, pinpointing the locations of ongoing star formation across the cloud. The ALMA images also reveal a series of spatially extended SiO emission spots with narrow line profiles, aligned along an arc-like path that is also seen in CS and CH3OH emission. While the broad SiO emission is mainly associated with the main cloud filament, as seen in visual extinction, the narrow SiO arch is located at the edge of the cloud, far from the identified sites of star formation activity. The presence of these arc-like morphologies suggests that large-scale shocks may have compressed the gas in the surroundings of the G035.39-00.33 cloud, shaping its filamentary structure. By inspecting the large-scale radio continuum emission around G035.39-00.33, we find that this IRDC is part of a larger star-forming complex where the densest and coolest material appears at the interacting regions between a Supernova Remnant (SNR) and an expanding HII region. In particular, we hypothesize that this IRDC may be spatially coincident with the ionized expanding gas associated with the previously identified SNR G35.6-0.4. We suggest that collisions between giant molecular clouds and expanding gas flows from interacting SNRs and HII regions may be responsible for the observed arc-like structures. Such shock compressions could play an important role in the formation of IRDCs and in the potential triggering of star formation.

Figures (11)

• Figure 1: Left panel: Three-color image (red color = 8 $\upmu$m, green color = 4.5 $\upmu$m, blue = 3.6 $\upmu$m) of Cloud H, obtained from GLIMPSE benjamin2003Carey2009. The white contour highlights the dense material in the IRDC as shown by the mass surface extinction map of KainulainenTan2013, corresponding to an extinction level of $A_{\rm V}$= 10 mag. The dashed yellow and white rectangles are the fields of view for the ALMA and IRAM-30 m observations, respectively. The physical scale is shown in the lower-right corner. Right panels: The grey background colour scale shows the mass surface density, with contour levels of $A_{\rm V}$ = 10, 20, 30, and 40 mag. The black contours represent SiO (2-1) and H$^{13}$CO$^+$ (1-0) emission from ALMA observations. The SiO data are spatially smoothed to a resolution of 6$^{\prime\prime}$, and the spectral resolution was degraded from 0.21 to 0.6 km s$^{-1}$. The SiO contours are integrated over the velocity range from -20 to 120 km s$^{-1}$, and levels are 21 (3$\sigma$), 210, 630, 1050, and 2300 (peak value) mJy beam$^{-1}$ km s$^{-1}$. The H$^{13}$CO$^+$ contours are integrated over the velocity range from 40 to 50 km s$^{-1}$, with the levels of 15 (3$\sigma$), 50, 100, 151, 200, 253 (peak value) mJy beam$^{-1}$ km s$^{-1}$. Red open circles and medium green rectangles indicate the positions of 8 $\upmu$m sources and 24 $\upmu$m sources, respectively jimenez2010parsec. The marker sizes for the 8 $\upmu$m and 24 $\upmu$m sources are scaled by the source flux. Magenta crosses mark the locations of the cores identified by Rathborne2006 using 1.2 mm continuum emission. Dark blue dashed and dotted lines trace the arc-like structures observed in the SiO emission. The physical scale is shown in the lower-left corner of each panel.
• Figure 2: The middle upper panel shows the SiO (2-1) integrated intensity map, with the SiO spectral grid overlaid. The grid size corresponds to 12$^{\prime\prime}\times12^{\prime\prime}$. Gray contours represent the dense material, with contour levels of $A_{\rm V}$ = 10, 20, 30, and 40 mag. Dark blue dashed and dotted lines trace the arc-like structures. Green dashed rectangles indicate the regions from which the SiO and H$^{13}$CO$^+$ spectra are extracted (shown in the left, middle lower, and right panels). The beam size (4.32$^{\prime\prime}\times$2.93$^{\prime\prime}$) and the physical scale are shown in the lower-left corner. The left, middle lower, and right panels show the averaged spectra of SiO (black lines) and H$^{13}$CO$^+$ emission (red lines) extracted from the green dashed regions in the middle upper panel. The H$^{13}$CO$^+$ intensities have been divided by a factor of two for clarity. The green numbers in the upper left corner of each panel correspond to the spectra extracted from the matching numbered green rectangles in the middle upper panel. The purple circles indicate regions with no detectable SiO emission. The gray dashed parallel line in each spectrum indicates the 3rms noise levels of the SiO emission. The vertical dashed black line marks the central velocity of Cloud H (45.5 km s$^{-1}$).
• Figure 3: a): The background shows the 850 $\upmu$m continuum emission within the ALMA field of view, with contour levels at 55($\sim$5 $\sigma$), 220, 440 mJy beam$^{-1}$. Blue and red contours represent the blue- and red-shifted SiO emission integrated over velocity ranges of 30-44 km s$^{-1}$ and 46-60 km s$^{-1}$, respectively, as indicated in the upper-right corner. The SiO contour levels start at 3$\sigma$ (16 mJy beam$^{-1}$ km s$^{-1}$) and increase by 0.2$I_\mathrm{peak}$ up to $I_\mathrm{peak}$ (910 and 620 mJy beam$^{-1}$ km s$^{-1}$ for the blue and red contours, respectively). Yellow dashed rectangles mark the regions corresponding to the green rectangles in Figure \ref{['fig2']}, where outflows are identified. The dashed black lines indicate the zoomed-in view of the outflow region. Dark red open circles and medium green rectangles indicate the positions of 8 $\upmu$m sources and 24 $\upmu$m sources, respectively. Other symbols are the same as in Figure \ref{['fig1']}. b): Zoom-in view of the outflow region at positions 3 and 4. The blue and red velocity ranges are shown in the upper-right corner. Yellow lines indicate the extents of the blue and red lobes, and the cyan star marks the position of the potential driving protostar, located centrally between the lobes. The two right panels indicated by dashed black lines show the SiO spectra with Gaussian fitting extracted from positions 3 and 4. The symbols are the same as Figure \ref{['figA2']}. c): Zoom-in view of the outflow region at position 7. The velocity range of blue-shifted SiO emission is noted in the upper-right corner. The right panel presents the corresponding SiO spectra with Gaussian fitting. d): Zoom-in view of the outflow region at positions 19 and 20. The velocity ranges of blue- and red-shifted SiO emission are shown in the upper-right corner. The two right panels display the SiO spectra with Gaussian fitting extracted from positions 19 and 20.
• Figure 4: The red background represents the CS integrated intensity, overlaid with SiO (2-1) emission (black contours) and C$^{18}$O (2-1) emission (gray contours). The integrated velocity ranges from 40 to 44.5 km s$^{-1}$ (blue-shifted gas; left panel), 44.5 to 45.5 km s$^{-1}$ (ambient gas; central panel), and 45.5 to 50 km s$^{-1}$ (red-shifted gas; right panel). SiO (2–1) black contour levels start at 3$\sigma$ and increase by 0.2$I_\mathrm{peak}$ up to $I_\mathrm{peak}$. The rms noise levels ($\sigma$) for the different velocity components are 7.4, 7.5, and 7.4 Jy beam$^{-1}$ km s$^{-1}$, and the corresponding peak intensities ($I_\mathrm{peak}$) are 1010, 288, and 877 Jy beam$^{-1}$ km s$^{-1}$, respectively. The C$^{18}$O gray contour levels in the left panel are 0.738, 1.107, 1.476, and 1.738 K km s$^{-1}$ (peak value). In the central and right panels, the contour levels are 0.322, 0.805, 1.127, and 1.44 K km s$^{-1}$ (peak value), and 0.624, 0.936, 1.248, 1.56, and 1.75 K km s$^{-1}$ (peak value), respectively. The blue contours display 610 $\mathrm{MHz}$ emission at 1.1 (5$\sigma$) and 1.54 (7$\sigma$) mJy beam$^{-1}$ from Paredes2014AA. Green crosses mark the locations of the identified cores. Green dashed and dotted lines indicated the arc-like structures. The beam size and physical scale are displayed in each lower right corner.
• Figure 5: The red background represents the CH$_3$OH integrated intensity, overlaid with SiO (2-1) emission (black contours) and C$^{18}$O (1-0) emission (gray contours). The integrated velocity ranges from 43.8 to 44.5 km s$^{-1}$, which represents the blue-shifted gas. SiO black contour levels are 21 (3$\sigma$), 210, 630, 1050, and 2300 (peak value) mJy beam$^{-1}$ km s$^{-1}$. The C$^{18}$O gray contour levels are 0.738, 1.107, 1.476, and 1.738 K km s$^{-1}$ (peak value). The blue contours display 610 $\mathrm{MHz}$ emission at 1.1 (5$\sigma$) and 1.54 (7$\sigma$) mJy beam$^{-1}$ from Paredes2014AA. Green crosses mark the locations of the identified cores. Green dashed and dotted lines indicated the arc-like structures. The beam size is displayed in the lower right corner.