Submillimetre galaxies as laboratories for dust grain coagulation

TL;DR

This study investigates dust grain growth in dense interstellar environments of submillimetre galaxies, concluding that physical coagulation limits prevent grains from exceeding a few micrometers, consistent with observed FIR properties.

Contribution

It demonstrates how SMGs serve as natural laboratories to constrain dust coagulation processes and establishes limits on grain sizes based on turbulence and velocity thresholds.

Findings

Grains can reach up to ~30 μm in unlimited coagulation models with high turbulence.

Observed FIR emissivity index indicates grains do not grow beyond a few micrometers.

Coagulation is limited by grain velocities exceeding threshold, preventing unlimited growth.

Abstract

Coagulation in the dense interstellar medium (ISM) is an important process that determines the size of the largest grains. We use submillimetre galaxies (SMGs) as laboratories of grain coagulation, since some of them host the densest ISM on a galactic scale among various populations of galaxies known. We examine how large the grains can be in such dense environments based on the mean ISM density estimated from the observed typical dust mass density in SMGs. We also consider local density enhancement based on a model of supersonic turbulence, which is expected from strong stellar feedback. In the unlimited coagulation model, in which we do not impose any coagulation threshold velocity, grains as large as $\sim 30~\mu$m can form under the observationally estimated mean gas density if the Mach number of turbulence is $\mathcal{M}\gtrsim 3$. We exclude this possibility since the observed emissivity index $\beta\simeq 2$ in the far infrared (FIR) indicates that such large grains cannot actively form in SMGs. This means that coagulation does not proceed in an unlimited way: 30-$\mu$m grains should have velocities larger than the coagulation threshold. If we use a coagulation threshold (upper limit) grain velocity ($\sim 0.08$ km s$^{-1}$) taken from a theoretical study, grains likely grow only up to $\mu$m size, which is small enough not to affect the FIR emissivity index. The above results indicate that SMGs can be used to constrain the physical processes relevant to coagulation.