The density of the interstellar medium, the matter and radiation that exists between stars in space, determines where stars form and release energy and drives how galaxies change over time. Variations in this density result from how gases move (gas motion), but the nature of this motion across space and in different galaxies is unknown. Dense gas that forms stars is most likely formed by a combination of gas instability, collision, and turbulence. However, it is still difficult to understand the precise origin of stars because gas motion is difficult to quantify across the large spatial scale. In this study, the researchers measured the gas motion of the Milky Way and NGC 4321 galaxies, from which they obtained data ranging from 10-1-103 parsecs (pc). They detected fluctuations in velocity that were the same regardless of spatial scale and galaxy environments. Statistical analyses of these fluctuations showed how gases are densified to form stars. The researchers found oscillating gas flows with wavelengths from 0.3-400pc. These flows are most likely coupled with similarly oscillating densification mechanisms that most likely form through instabilities in gravity. These results demonstrate that the density of interstellar medium is controlled by gas flows and densification mechanisms, which are independent, and exist in many orders of magnitude in spatial scale.
The researchers wanted to understand how star formation is influenced by gas flows and interstellar medium density in different galaxies.
The researchers used observations of the structure of the interstellar medium (ISM) in a variety of galaxy environments that spans a wide range of spatial scales. They measured the velocity of the interstellar medium from scales as small as 0.1pc, the size of individual stars, up to over 1000pc, the size of giant molecular clouds. On the larger scale (100pc to 1000pc), they used observations from NHC 4321, a nearby galaxy. On the intermediate scale (1pc to 100pc), observations from the Galactic Disk and Central Molecular Zone, both different regions of the Milky Way, were used. Finally, on the smallest scale (0.1 to 10pc), observations from two molecular clouds were used: G035.39-00.3 and G0.253+0.016. Observations can be found in Extended Data Fig. 1.
Gas motion was calculated using spectral decomposition, an imaging technique used in astronomy based on how light is emitted from an object. Spectral decomposition allowed the researchers to describe all prominent features of spectral data. The researchers visualized these results (Fig. 1). When the researchers compared this figure, which visualizes gas velocity, and observations of the ISM structures of different galaxy environments, they found that they consistently similar across different spatial scales. Measuring this relationship between density and velocity enables scientists to understand how star-forming gas is formed.
The researchers selected smaller regions from each galaxy environment in Fig. 1 to measure the relationship between ISM density and gas velocity. They mathematically modeled functions of velocity and gas density in Fig. 2. As seen in figure 2, the functions are very similar. In addition, through analyzing these functions, the researchers found that gas velocity oscillates with a specific wavelength.
When the researchers examined the corresponding ISM density functions, they saw trends in the function that were similar to the oscillations in the gas velocity functions. In addition, the phase difference between the ISM density and gas velocity fluctuations varies between galaxy environments and shows how gas flows converge or diverge.
As observed in Fig. 2, spiral galaxies (Fig. 2a) show oscillations in their functions, while those for molecular clouds do not (Fig. 2b, e). In spiral arms, regions of stars that extend from the center of spiral galaxies, the wavelength of the velocity of gas decreases with an increase in wavelength in rotational velocity of the galaxy. The oscillations probably result from a combination of how gases and stars stream along the spiral arms and how gravity flows into the center of spiral galaxies.
On the other hand, molecular clouds form at the stagnation point of converging flows, where the local velocity of gas is zero. The kinetic energy cascading from large to small spatial scales in this event causes turbulence, which results in the complex and free-form structure of molecular clouds. Because this structure has no regularity, the structure functions of density and velocity as seen in Fig 2b and e resemble power laws, and show no regular oscillations.
Gravitational instabilities may drive the formation of ISM density fluctuations and their correlated gas flows. The researchers found that the separation of periodic density enhancements in the subregions of galaxy environments are 3-5 times greater than those in their respective galaxy environments as a whole. Periodic density enhancements are arranged like beads on a string along filaments extending from galaxies (such as spiral arms), and show separation-to-diameter ratios observed in this study. These findings show that this ratio in the velocity function of the ISM matches that of density enhancements.
In this study, the analysis of interstellar medium density and gas motion over vast spatial scales was a novel approach to understanding gas flows and how they create dense, star-forming gas. The results of this study indicate that star-forming gas is controlled by interdependent gas flows. The next steps to further this research is to replicate the study using more galaxy environments, and to create and observe simulations to further understand how these environments play a role in star formation in galaxies.
The researchers analyzed galaxy environments that covered about four orders of magnitude, from the kiloparsec (kpc) scale to 0.1pc scales. Environments included in the study were: NGC 4321, the Milky Way Disk, G035.39-0.33, CMZ, and G0.253+0.016.
Observations and data
The researchers collected data from various surveys of the environments studied.
SCOUSEPY was used to describe the spectral decomposition data for NGC 4321 and G035.39-0.33. GAUSSPY+ was used for data from the Galactic disk region.
Data selection for statistical analysis
Subregions from each of the five environments were chosen for analysis and can be found in Fig. 1. Three subregions on different scales but with similar structure were chosen for analysis: part of the southern spiral arm in NGC 4321, part of the CMZ gas stream, and a filament in a molecular cloud in the Galactic Disk. Two more molecular clouds in the Galactic Disk and in the Milky Way’s CMZ were also chosen.
Analysis of filamentary structures
Analyses were conducted along the crests of each subregion by applying FILFINDER in each environment to calculate ISM density, and modeled velocity using mathematical functions.