Using CO line ratios to trace the physical properties of molecular clouds

Abstract

The carbon monoxide (CO) rotational transition lines are the most common tracers of molecular gas within giant molecular clouds (MCs). We study the ratio (R 2−1/1−0 ) between CO's first two emission lines and examine what information it provides about the physical properties of the cloud. To study R 2−1/1−0 we perform smooth particle hydrodynamic simulations with time dependent chemistry (using GADGET-2), along with post-process radiative transfer calculations on an adaptive grid (using RADMC-3D) to create synthetic emission maps of a MC. R 2−1/1−0 has a bimodal distribution that is a consequence of the excitation properties of each line, given that J = 1 reaches local thermal equilibrium (LTE) while J = 2 is still sub-thermally excited in the considered clouds. The bimodality of R 2−1/1−0 serves as a tracer of the physical properties of different regions of the cloud and it helps constrain local temperatures, densities and opacities. Additionally this bimodal structure shows an important portion of the CO emission comes from diffuse regions of the cloud, suggesting that the commonly used conversion factor of R 2−1/1−0 ∼ 0.7 between both lines may need to be studied further.

CO line ratios in molecular clouds: the impact of environment

“…As the ISRF increases the unshielded molecular gas is fully dissociated, destroying most of the CO in the already diffuse gas and therefore resulting in no CO emission from these regions of the cloud. This is consistent with the fact that lower values of R21 (∼ 0.3) are associated with diffuse and warm areas of the cloud (Sakamoto et al 1994;Peñaloza et al 2017). Similar to CG15-M5-G100 (see bottom left panel and Figure 1), values of R21 > 1 are correlated with CO emission originating from dense and hot gas at the edge of the cloud.…”

“…velocity in PPV space, we then create zeroth moment maps for each line. All the final maps have an imposed cut at emissions lower than 0.01 K km s −1 , this is motivated by our previous study (Peñaloza et al 2017). All these maps are 'ideal' synthetic observations since they do not include any noise or telescope effects.…”

“…It is important to note that this plot may look slightly different from Figure 4 of . This is because our radiative transfer approach here includes the refinement routine described in Peñaloza et al (2017) and the Sobolev-Gnedin approximation described in Appendix A. When compared, the results presented here Figure 7.…”

“…Typically, extragalactic studies adopt a value of R21 = 0.7 (Eckart et al 1990;Casoli et al 1991;Brand & Wouterloot 1995;Sakamoto et al 1997;Hasegawa 1997;Sawada et al 2001;Bigiel et al 2008;Leroy et al 2009;Barriault, Joncas & Plume 2011) to convert from W21 to W10. However, this could be inaccurate given the results shown in Peñaloza et al (2017), which suggest R21 has a bimodal distribution dependant on the physical conditions surrounding the emitting gas. Another example of a widely-used ratio is R31, which is mostly used to study star formation in high redshift galaxies (Aravena et al 2010;Bauermeister et al 2013;Aravena et al 2014;Daddi et al 2015).…”

Challenges and Techniques for Simulating Line Emission

“…As mentioned above, CLOUDY and DESPOTIC are restricted to problems in 1D whereas the remaining tools in Table 2 work in 3D. LIME, MOLLIE, and RADMC-3D do exact radiative transfer in 3D and are typically used on smaller scales where non-symmetric features such as filaments and turbulence become important [e.g 61,62]. ART 2 also does radiative transfer in 3D including the continuum from far-UV to radio wavelengths and incorporating the resonant line Lyα [e.g 29,63,64] (an updated version of the code including CO and some prominent far-infrared fine-structure lines such as CII, OI, OIII and NII will be out later this year).…”