Title: The Rhythm of the ISM: Tracing the Timescales of Gas Evolution and Star Formation across Galactic Environments
Authors: Zuzanna Kocjan, Vadim A. Semenov
First Author’s Institution: Department of Astronomy, University of Maryland
Status: Submitted to the Astrophysical Journal [open]
The connection between gas and star formation in galaxies
Stars are born in the interstellar medium (ISM), when cold, dense clouds of gas become unstable and collapse under gravity. Despite their plentiful reservoirs of gas, galaxies convert only a small fraction of this material into stars, making star formation a surprisingly inefficient process. A key factor behind this inefficiency is stellar feedback: radiation, stellar winds, and supernova explosions from young stars can inject energy and momentum into the surrounding gas, heating, stirring, and dispersing it. In this way, it shapes future star formation across scales ranging from individual regions in the ISM to whole galaxies.
An important tool astronomers use to study star formation is the Kennicutt–Schmidt relation, which links the amount of gas in a galaxy to the rate at which stars form. More specifically, it relates the gas surface density, Σgas, to the star formation rate surface density, ΣSFR (for more on this relation, see this Astrobites article on a classic paper). However, this law is underpinned by a crucial ingredient: timescales. In particular, the pace of star formation depends both on how quickly galactic gas is cycled into star-forming material and on how rapidly those regions convert gas into stars once they form. Today’s paper investigates the physical origin of star formation scaling relations behind the Kennicutt-Schmidt law by building on a simple theoretical framework for the ISM.
Consider a kettle of boiling water…
In a given region of a galaxy, the ISM consists of gas in either an actively star-forming state or an inert, non-star-forming state, depending on whether it is dense and unstable enough to collapse (as illustrated in Figure 1). The transition of non-star-forming gas into the star-forming state occurs on the supply timescale, τ+. Conversely, the dispersal of star-forming gas back into a non-star-forming state is characterized by the removal timescale, τ–. As a helpful analogy, the authors compare this process to water boiling in a kettle: the ISM is continuously “boiling,” with gas moving between active and inactive states. Meanwhile, the total gas reservoir gradually decreases, analogous to water slowly evaporating as the kettle boils. The gas depletion time, τ*, therefore represents the timescale over which the available gas would be exhausted by star formation.
Figure 1: Schematic of the gas cycling framework used in the author’s theoretical model for how gas is converted into stars, which takes into account a supply, removal, and depletion timescale. Figure 1 in today’s paper.
Building on this picture of gas cycling, the authors turn to simulated galaxies, where the motion and evolution of the gas can be followed directly. By tracking how gas flows through the ISM in the simulations, they derive the three characteristic timescales above—τ+, τ–, and τ*—that describe how gas is supplied to, removed from, and ultimately consumed by star formation. The goal is to connect the small-scale physics of the ISM to the large-scale star formation efficiencies and scaling relations observed across galaxies.
The timescales of the interstellar medium
To explore how gas and star formation are connected across different galactic environments, the authors analyze three simulated systems: a dwarf galaxy, a Milky Way–like galaxy, and a gas-rich starburst galaxy as shown in Figure 2. Despite spanning very different physical regimes, the galaxies exhibit remarkably similar trends in the fraction of gas actively forming stars as a function of Σgas. This suggests that the amount of star-forming gas is governed primarily by local interstellar conditions, since regions with higher gas surface densities tend to contain denser, more strongly self-gravitating gas.
Figure 2: Using simulations (left to right) of an isolated intermediate mass dwarf galaxy, Milky Way-like galaxy, and gas-rich galaxy, the authors measure the star-forming gas fraction versus the gas surface density (right). Adapted from Figures 2 and 4 in today’s paper.
Figure 3: Scaling relations between the supply (left), depletion (middle), and removal (right) times in kiloparsec-scale regions of the simulated galaxies (shown in different colors). Based on the measurements from the simulations, the authors introduce a parameterized model for the gas cycling framework, as defined in the equations. Adapted from Figure 5 in today’s paper.
To further understand this trend, this study then applies their theoretical framework to determine the timescales of ISM gas cycling on the scales of individual star-forming regions. These include the timescales for the formation, dispersal, and local depletion of star-forming gas as described above: τ+, τ−, and τ∗, and which also exhibit strong correlations with the gas surface density (see Figure 3). Specifically, the timescale for supplying gas into the star-forming state is linked to the rate at which turbulence redistributes material through the galactic disk. The depletion timescale, over which star-forming gas is turned into stars, decreases at higher Σgas, since denser regions more efficiently collapse and form stars. By contrast, the removal timescale is comparatively short, reflecting how quickly feedback and changes in local equilibrium can disrupt star-forming clouds. In this way, the “boiling kettle” framework is able to capture the major processes driving small-scale gas evolution.
The cycling of gas through different phases of the ISM offers a useful framework for understanding how galaxies form stars. In the relatively well-ordered systems studied here, this balance can be described in simple terms; however, it is likely to become more complex in extreme environments where additional physical processes—such as those operating in the early stages of galaxy formation—play a significant role. Even so, these results highlight how key galactic properties can emerge naturally from the interplay between galaxy-scale dynamics, ISM turbulence, and the state of star-forming gas.
Article edited by Jayde Willingham.
Featured image credit: Adapted from “Where Did the Interstellar Medium Come From?” (The Cosmos At Your Doorstep).
