Detailed new research shows that there is a distinct correlation between galaxies with large, oxygen-rich gas halos and active ongoing star formation. Although active star formation requires large amounts of available gas, what is surprising is that much, or perhaps even most, of the gas may be in the halo region outside of where most stars are found. Galaxies without substantial halos evidently do not have sufficient gas in the inner regions alone, where most stars exist, to sustain star formation.
Using the Hubble Space Telescope, a survey of 42 galaxies collected information about the distribution of gas in galactic halos that extend far beyond galaxies’ visible stars. Although the gas itself is not visible, its characteristics can be inferred from traces imprinted on the spectra of light from distant quasars. Two other large ground-based telescopes collected information for the same galaxies on their distances, masses due to visible stars, and rates of star formation.
When the two sets of data were compared, some surprises emerged. In galaxies where stars are actively forming, the halos are full of large quantities of oxygen-enriched gas, which may have at least as much mass as is present in the visible stars. However, galaxies not actively forming stars did not have such massive halos.
Keck, Magellan & Hubble Telescopes Find Galactic Recyclers
Among the key findings of the work is that the color and shape of a galaxy is largely controlled by gas flowing through an extended halo around it. All modern simulations of galaxy formation find that they cannot explain the observed properties of galaxies without modeling the complex accretion and “feedback” processes by which galaxies acquire gas and then later expel it after processing by stars. The three studies investigated different aspects of the gas recycling phenomenon.
“Our results confirm a theoretical suspicion that galaxies expel and can recycle their gas, but they also present a fresh challenge to theoretical models to understand these gas flows and integrate them with the overall picture of galaxy formation,” said Jason Tumlinson of the Space Telescope Science Institute in Baltimore, Maryland; a coauthor of one of the Science papers.
The implication is that there must be an efficient process of exchange between gas in a galaxy’s halo, the “circumgalactic medium” (CGM), and the inner region of the galaxy where most stars form and reside: the “interstellar medium” (ISM). Since stars need cold, relatively dense gas in order to form, they cannot form in the warmer, more diffuse CGM, so the gas from there must migrate inward, cool down, and become more dense.
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