FOREST unbiased Galactic plane imaging survey with the Nobeyama 45 m telescope (FUGIN). VI. Dense gas and mini-starbursts in the W 43 giant molecular cloud complex

Abstract

We performed new large-scale 12CO, 13CO, and C18O J = 1–0 observations of the W 43 giant molecular cloud complex in the tangential direction of the Scutum arm (l ∼30°) as a part of the FUGIN project. The low-density gas traced by 12CO is distributed over 150 pc × 100 pc (l × b), and has a large velocity dispersion (20–30 km s−1). However, the dense gas traced by C18O is localized in the W 43 Main, G30.5, and W 43 South (G29.96−0.02) high-mass star-forming regions in the W 43 giant molecular cloud (GMC) complex, which have clumpy structures. We found at least two clouds with a velocity difference of ∼10–20 km s−1, both of which are likely to be physically associated with these high-mass star-forming regions based on the results of high 13CO J = 3–2 to J = 1–0 intensity ratio and morphological correspondence with the infrared dust emission. The velocity separation of these clouds in W 43 Main, G30.5, and W 43 South is too large for each cloud to be gravitationally bound. We also revealed that the dense gas in the W 43 GMC has a high local column density, while “the current SFE” (star formation efficiency) of the entire GMC is low ($\sim\!\! 4\%$) compared with the W 51 and M 17 GMC. We argue that the supersonic cloud–cloud collision hypothesis can explain the origin of the local mini-starbursts and dense gas formation in the W 43 GMC complex.

CO-to-H2 conversion and spectral column density in molecular clouds: the variability of the XCO factor

“…and X12 CO = 2.0×10 20 [H2 (K km s −1 ) −1 ] is the widely used conversion factor (Bolatto et al 2013), and Y13 CO = (5.0 ± 2.5) × 10 5 is the abundance ratio of H2 to 13 CO molecules (Dickman 1978). We also adopt Y13 CO = 7.7 × 10 5 in section 3 only for comparison with the result of Kohno et al (2020). The column density of 13 CO molecules is given by (Pineda et al 2008)…”

“…Big circles are average and standard deviation of the plots in figure 3. Figure 1 shows plots of the mean H2column densities in giant molecular clouds (GMC) in W43 calculated using the X12 CO and LTE methods by Kohno et al (2020). Filled circles indicate the mean column densities for individual GMC and cloud components in W43 Main region, and red triangles are those for their intensity peaks.…”

“…Throughout the paper except for section 3.1, we adopted the H2 -to-13 CO abundance ratio of Y13 CO = (5.0 ± 2.5) × 10 5 (Dickman 1978), and even smaller ratios are often employed (Pineda et al 2008). However, figures 1 and 3, where was adopted Y13 CO = 7.7×10 5 (Kohno et al 2020), show a good agreement between the averaged values of NH 2 from X12 CO and LTE methods, as they lie on the dashed line showing that both are equal. On the other hand, if we adopt the lower abundance (Y13 CO = (5.0 ± 2.5) × 10 5 ), the plots are shifted upwards by a factor of 1.5, significantly displacing from the dashed line.…”

Dark Supernova Remnants Revealed by CO-Line Bubbles in the W43 Molecular Complex along the 4-kpc Galactic Arm

“…All the channel maps exhibit numerous bubbly features (hereafter, bubbles), arcs and filaments. Bubbles and arcs show up in the channel maps more pronounced than in the integrated intensity map of the same region [26]. Each of the bubbles appears in a narrow range of velocity of a few km s −1 , and the diameter is typically ∼0 • .1 (10 pc), ranging from ∼2 to ∼15 (3 to 24 pc).…”

“…In this paper, we extend the search for dSNRs in the form of molecular bubbles and/or cavities in the giant molecular clouds (GMC) surrounding the active star-forming region, W43 Main (G31+00+90 km s −1 ), in the tangential direction of the 4-kpc molecular arm. The molecular gas properties and star formation in the W43 region have recently been extensively studied using the FUGIN CO line data [25,26]. We adopt a distance of 5.5 kpc according to the literature, which is close to the near-side kinematical distance of 5.56 ± 0.46 kpc at v lsr = 93 km s −1 (center velocity of W43) for the most recent rotation curve of the Galaxy [27].…”