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.
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.
At the same time, observations show that when stars are actively forming, the intense energy they emit sets up strong outflows of gas from the ISM to the CGM. It is likely that gas from the ISM is what replenishes the CGM, because gas from the external intergalactic medium (IGM) tends to have smaller quantities of elements (“metals”) heavier than hydrogen and helium than are observed in the CGM.
The role of oxygen in this research is somewhat incidental. A certain ionized form of oxygen (O VI, lacking 5 outer electrons) has a distinctive pair of absorption lines that provide a convenient indicator of warm gas that is enriched in metals in general. This is the signal that was sought in light from distant quasars as evidence of a metal-enriched galactic halo. Other observations on the target galaxies confirmed that the gas was actually associated with the galaxy it appeared to surround.
It is known that O VI accounts for less than 20% of oxygen in the CGM, and usually much less. Since O VI is used as the marker, the estimates of the amount of gas in the CGM are almost certainly low. All galaxies surveyed were confirmed to be at redshifts in the range 0.10≤z≤0.36, so that the O VI signal would not be confused with some other.
The spatial extent of observed halos was rather large, in some cases extending almost 500,000 light-years out from the galactic center. Although the size of the Milky Way’s halo is not known, most of its visible stars are within a radius of 50 to 60 light-years. On the other hand, dark matter halos of very large galaxies may extend as far as 1 million light-years.
All galaxies of any size have undergone rapid star formation in the past, so this research implies they very likely had prominent halos similar to those observed in galaxies currently forming stars actively. So the obvious question is: What becomes of those halos by the time star formation largely ceases?
The findings of this research place significant constraints on possible answers to that question. The research paper concludes:
The passive galaxies in our sample once formed stars; thus, it follows that they would once have possessed halos of ionized, metal-enriched gas visible in O VI. The relative paucity of O VI around these galaxies implies that this material was transformed by processes that plausibly accompany the quenching of star formation, such as tidal stripping in group environments, reaccretion onto the galaxy in ionized form, or heating or cooling to a temperature at which O VI is too rare to detect. Our findings present a quantitative challenge for theoretical models of galaxy growth and feedback, which must explain both the ubiquitous presence of massive, metal-enriched ionized halos around star-forming galaxies and the fate of these metals after star formation ends.
There are two other papers relevant to this topic in the same issue of Science. One paper concerns research that has identified ionized clouds of gas in the Milky Way within one galactic radius of the Sun that have velocities relative to the galaxy of less than 170 km/sec. Apparently there is enough mass in such clouds to sustain the current rate of star formation. There are also other clouds, farther out, with velocities above 170 km/sec, that may eventually be available to continue star formation.
A second paper deals with a post-starburst galaxy. Spectroscopic studies of the galaxy in the light of a distant quasar reveal a wind of “warm-hot” plasma (~300,000 K) that has 10 to 150 times more mass than the outflow of colder gas, The plasma wind extends out to at least 220,000 light-years. In a starburst galaxy stars may be forming at unusually high rates of 100 solar masses per year or more. The resulting outflow of hot plasma may be rapid enough that some of its mass escapes completely from the galaxy.
|Tumlinson, J., Thom, C., Werk, J., Prochaska, J., Tripp, T., Weinberg, D., Peeples, M., O’Meara, J., Oppenheimer, B., Meiring, J., Katz, N., Dave, R., Ford, A., & Sembach, K. (2011). The Large, Oxygen-Rich Halos of Star-Forming Galaxies Are a Major Reservoir of Galactic Metals Science, 334 (6058), 948-952 DOI: 10.1126/science.1209840|