We analyse the CO-dark molecular gas content of simulated molecular clouds from the SILCC-Zoom project .
The simulations reach a resolution of 0.1 pc and include H _ { 2 } and CO formation , radiative stellar feedback and magnetic fields .
CO-dark gas is found in regions with local visual extinctions A _ { n } { V, 3 D } \sim 0.2 – 1.5 , number densities of 10 – 10 ^ { 3 } cm ^ { -3 } and gas temperatures of few 10 K – 100 K. CO-bright gas is found at number densities above 300 cm ^ { -3 } and temperatures below 50 K. The CO-dark gas fractions range from 40 % to 95 % and scale inversely with the amount of well-shielded gas ( A _ { n } { V, 3 D } \gtrsim 1.5 ) , which is smaller in magnetised molecular clouds .
We show that the density , chemical abundances and A _ { n } { V, 3 D } along a given line-of-sight can not be properly determined from projected quantities .
As an example , pixels with a projected visual extinction of A _ { n } { V, 2 D } \simeq 2.5 – 5 can be both , CO-bright or CO-dark , which can be attributed to the presence or absence of strong density enhancements along the line-of-sight .
By producing synthetic CO ( 1-0 ) emission maps of the simulations with RADMC-3D , we show that about 15 – 65 % of the H _ { 2 } is in regions with intensities below the detection limit .
Our clouds have X _ { n } { CO } -factors around 1.5 \times 10 ^ { 20 } cm ^ { -2 } ( K km s ^ { -1 } ) ^ { -1 } with a spread of up to a factor \sim 4 , implying a similar uncertainty in the derived total H _ { 2 } masses and even worse for individual pixels .
Based on our results , we suggest a new approach to determine the H _ { 2 } mass , which relies on the availability of CO ( 1-0 ) emission and A _ { n } { V, 2 D } maps .
It reduces the uncertainty of the clouds ’ overall H _ { 2 } mass to a factor of \lesssim 1.8 and for individual pixels , i.e .
on sub-pc scales , to a factor of \lesssim 3 .