Dusty Cloud Acceleration by Radiation Pressure in Rapidly Star-Forming Galaxies

TL;DR

This study uses radiation hydrodynamic simulations to show that radiation pressure can effectively accelerate cold, dense clouds in star-forming galaxies, with longer survival times than hot wind entrainment, depending on cloud properties.

Contribution

It demonstrates the effectiveness of radiation pressure in accelerating cold clouds and compares their survival to hot wind entrainment, highlighting the influence of cloud optical depth and initial conditions.

Findings

Radiation pressure can accelerate clouds to hundreds of km/s.

Cloud survival time is longer under radiation pressure than hot wind entrainment.

Cloud morphology depends on optical depth and initial conditions.

Abstract

We perform two-dimensional and three-dimensional radiation hydrodynamic simulations to study cold clouds accelerated by radiation pressure on dust in the environment of rapidly star-forming galaxies dominated by infrared flux. We utilize the reduced speed of light approximation to solve the frequency-averaged, time-dependent radiative transfer equation. We find that radiation pressure is capable of accelerating the clouds to hundreds of kilometers per second while remaining dense and cold, consistent with observations. We compare these results to simulations where acceleration is provided by entrainment in a hot wind, where the momentum injection of the hot flow is comparable to the momentum in the radiation field. We find that the survival time of the cloud accelerated by the radiation field is significantly longer than that of a cloud entrained in a hot outflow. We show that the dynamics of the irradiated cloud depends on the initial optical depth, temperature of the cloud, and the intensity of the flux. Additionally, gas pressure from the background may limit cloud acceleration if the density ratio between the cloud and background is $\lesssim 10^{2}$. In general, a 10 pc-scale optically thin cloud forms a pancake structure elongated perpendicular to the direction of motion, while optically thick clouds form a filamentary structure elongated parallel to the direction of motion. The details of accelerated cloud morphology and geometry can also be affected by other factors, such as the cloud lengthscale, the reduced speed of light approximation, spatial resolution, initial cloud structure, and the dimensionality of the run, but these have relatively little affect on the cloud velocity or survival time.