Tracing Gas in Distant Galaxies

Stars are forming in galaxies near and far, fueled by massive clouds of gas. What can carbon monoxide ­— a familiar but dangerous gas on Earth — tell us about star formation in distant galaxies?

Fuel for Star Formation

A representative-color image of a star-forming region. Huge clouds of molecular gas and dust are punctuated by hundred of background stars. At the center and right-hand side of the image, newly formed stars glow brightly in the cavities they have carved out.

Molecular clouds, like those in the Vela Molecular Cloud Ridge complex pictured here, are the sites of star formation. [NASA/JPL-Caltech/UCLA]

Stars form in huge clouds of molecular hydrogen gas, which are difficult to observe directly. Luckily, these clouds contain smaller amounts of other gases that are easier to detect, like carbon monoxide. While you wouldn’t want to find it in your home, it’s a helpful thing to find in another galaxy; the spectral lines of carbon monoxide are sensitive to the density of the surrounding gas cloud.

The photons emitted by molecules like carbon monoxide make up just one small part of the light we observe from other galaxies. Most of the interstellar radiation field comes from the combined photons from stars, gas, and dust (as well as any background radiation that has passed through the galaxy). The interstellar radiation field contains a huge amount of information about a galaxy’s reservoir of star-forming gas — if we can find a way to extract it.

Gathering Galaxies

In today’s article, a team led by Daizhong Liu (Max Planck Institute for Astronomy, Germany) explores the relationship between emissions from carbon monoxide, the interstellar radiation field, and factors that determine the rate of star formation. 

A plot of flux density, in units of milli-Janskys, versus observed-frame wavelength in microns, for the galaxy PACS-819 at a redshift of 1.4451. The fitted spectral energy distribution peaks at approximately 200 microns and 70 milli-Janskys. The components of the fit peak at 5 microns (stellar), 100 microns (warm dust), and 200 microns (cold dust).

An example of the team’s fitting routine, as applied to observations (blue circles) of distant galaxy PACS-819. The best-fitting model of the galaxy’s spectra energy distribution (black line) is composed of several emission components: stellar (cyan), warm dust (red), cold dust (blue), and radio (purple). [Adapted from Liu et al. 2021]

Liu and coauthors assembled a sample of 76 galaxies for which at least two emission lines from carbon monoxide have been observed, as well as continuum emission from optical to submillimeter wavelengths. Their sample contains galaxies of all kinds, from relatively nearby galaxies (~25 million light-years away) with average rates of star formation to high-redshift starburst galaxies alight with new stars.

In order to understand how emission lines are related to a galaxy’s star-forming capabilities, the authors modeled each galaxy’s spectral energy distribution — the amount of energy emitted at each wavelength — and the strength of its carbon monoxide emission lines. The spectral energy distribution modeling determined the intensity of the interstellar radiation field, while the carbon monoxide modeling demonstrated how the relative strength of the molecule’s emission lines depend on the density and temperature of the surrounding molecular cloud.

A New Item in Our Toolkit

A cigar-shaped band of soft, diffuse light from the plane of M82 is crisscrossed by dark dust clouds.

Spectral lines from carbon monoxide molecules may serve as a useful probe of starburst galaxies, like M82, which have unusually high rates of star formation. [NASA, ESA and the Hubble Heritage Team (STScI/AURA)]

What does this mean for our understanding of star formation in other galaxies? Based on their modeling, Liu and collaborators found that the average intensity of the interstellar radiation field is tightly correlated with the temperature of the reservoir of star-forming gas, which they determined from the observations of carbon monoxide.

While the correlation between radiation field intensity and temperature was linear, the correlation with density was not. This implies that while the star-forming rate may increase smoothly with density and temperature in typical star-forming galaxies, galaxies experiencing bursts of star formation while merging with another galaxy might show huge increases in density without a large increase in temperature. The authors hope that studying carbon monoxide will allow us to probe the star-forming conditions in distant galaxies — an impressive feat for a humble molecule!


“CO Excitation, Molecular Gas Density, and Interstellar Radiation Field in Local and High-redshift Galaxies,” Daizhong Liu et al 2021 ApJ 909 156. doi:10.3847/1538-4357/abd801