Speaker
Jonathan Heiner
(CRyA, UNAM Morelia)
Description
Cold, self-absorbing atomic hydrogen can be seen as absorption features in the broader HI profile. However, unless emission of molecular gas (typically CO) at the same velocity as the absorption feature is detected, it is not always possible to distinguish between self-absorption and the lack of HI emission. Ideally one would want to dispense with the need to detect molecular emission, since not all molecular gas is visible in tracer lines such as CO. It is also important to distinguish whether gas is flowing into, or exiting a gas cloud.
We present simulations featuring decaying turbulence, where molecular clouds form in filamentary structures that resemble observed molecular clouds. In order to determine when and where molecular gas has formed, we used a simple recipe involving the local visual extinction and temperature. Then, we produced synthetic observations of the simulations.
After separating the absorption features from those due to the absence of atomic gas, we compared to what extent and on what scale the HI self-absorption (HISA) correlates with the molecular column density. In actual observations it is possible to use the second derivative of the brightness temperature profile to find potential HISA features. We verified how well the second derivative traces the molecular gas. While it does a rather poor job at the sub-parsec scale, the agreement improves significantly at scales of several parsec, although it does not become a perfect tracer.
Additionally, we show the evolution of the probability density functions within the simulation volume, which informs us of the state of the gas (for example whether it is forming stars). Our simulations are dominated by colliding gas flows, which can be seen in position/velocity cuts through the simulation volume. Finally, I will discuss what improvements to the model are needed in order to truly distinguish between situations where atomic gas flows into the cloud and where atomic gas is leaving, for example due to photodissociation of the molecular gas.
Author
Jonathan Heiner
(CRyA, UNAM Morelia)