Physical processes within a galaxy can launch gas into outflows, acting as conduits for the exchange of mass and energy between a galaxy and its surroundings.

Physical processes within a galaxy can launch gas into outflows, acting as conduits for the exchange of mass and energy between a galaxy and its surroundings. These outflows play a crucial role in the Baryon Cycle by regulating the amount of gas available for star-formation. They have been ubiquitously observed in galaxies of all masses and across various wavelengths, such as UV and X-rays, which reveal the warm-hot and hot components of these multiphase outflows, respectively. However, the cold component, particularly the molecular phase that dominates the mass balance of the outflows, is detectable in radio wavelengths but is difficult to observe and quantify due to its low surface brightness.  The high-density, low-temperature nature of this phase also makes it difficult to resolve in hydrodynamic simulations. As a result, there are several unanswered questions to surrounding the existence of this phase, most importantly- why do some galaxies show this phase and not others?

In Vijayan & Krumholz 2024, we propose an analytical model to examine the kinematical properties of the molecular phase in galactic outflows. This theoretical model describes the statistical properties of the molecular phase in an outflowing system, building upon the system of equations in Krumholz et al 2017.

The model explores the parameter space created by two quantities. The first is equation.pdf, the generalised Eddington ratio, that captures the strength of the outflows vis a vis the gravity. This is shown on the perpendicular axis. The second is equation_1.pdfwhich is the ratio between the escape time of a cloud from a galaxy and the dissociation time scale for photons incident from the inter-stellar radiation field. This figure plots the fraction of mass-flux of clouds that remains molecular and has not dissociated. For e.g.- a galaxy with equation_2.pdfand equation_3.pdfsuch that it lies in the red region of the plot will not host any molecular outflows.
We have overlaid the plot with data from a sample of actual galaxies. The galaxies in this sample have been differentiated as star-bursts (‘SB’, shown in pink stars) and non-SB (shown in black circles). Our model predicts that SB are likely to host molecules in their outflows.

Our model assumes that molecular clouds are continuously launched from a galaxy. It is agnostic to the process that propels these clouds into outflowing winds and can be applied to outflows driven by star formation or AGN activity. Once launched, the H₂ molecules in these clouds dissociate into H under the influence of photons from the interstellar radiation field. The survival of these clouds depends on how quickly they can escape the radiation field.

A key parameter in our model is the ratio of escape time to dissociation time. This ratio, along with the generalized Eddington ratio, which quantifies the strength of the outflow driver relative to gravity, determines whether a galaxy will host molecules in its outflows. We show that the Mach number of the disc, which sets the mass distribution of the clouds that will be launched, is relatively unimportant.

In our paper, we apply our model to a sample of galaxies and find that starbursts, with short escape times, are more likely to host molecular outflows. The virtue of our model lies in its reliance on observable galaxy properties, making it a useful tool for predicting targets for SKA observations.