New research gets to the bottom of "extreme scattering events" in the interstellar medium.
Our galaxy may be populated with weird, largely invisible noodle-shaped structures, according to a team of Australian astronomers. These structures, observed via CSIRO's Compact Array telescope in eastern Australia, together comprise unexpected "lumps" in the expanses of thin gas found between stars. The find, described in a paper published today in Science, may demand a radical revision of how we understand the Milky Way.
The structures are more properly known as extreme scattering events (ESEs). An ESE is the result of a wide fluctuation in the density of the extremely slight "atmosphere" of dust, gas, radiation, and plasma found between star systems in a galaxy, aka the interstellar medium (ISM). The fluctuations in density or pressure result in lenses that deflect radio waves in ways detectable here on Earth.
The ESEs described in the current study indicate that pressures within the lenses are as high as 1,000 times that of the normal ambient pressure in the ISM.
"Although such pressures have been observed in a very small fraction of interstellar gas, the fact that ESEs are relatively common raises the question of how such structures can form and live long enough to be detected as ESEs," the CSIRO group, led by astrophysicist Keith Bannister, writes.
Since ESEs were first detected in 1987 by astronomers using the Green Bank Interferometer at the National Radio Astronomy Observatory in West Virginia, two main theories have been advanced as to what ESEs actually are. The first suggests that what we're really seeing in the ISM are large, flat sheets of plasma. The second suggests that ESEs are cold, self-gravitating gas clouds. In both models, the size of the lenses are roughly equal to the distance from Earth to the Sun.
"The fact that [extreme scattering events] are relatively common raises the question of how such structures can form and live long enough to be detected..."
Neither of these theories are ideal. As Barrister and his group explain, the plasma sheet idea gives the requisite density fluctuations, but the light curves that should result from such structures don't match our data. The cloud theory solves the problem, but it comes with the suggestion that clouds like these make up a very large portion of a galaxy's total mass.
Some large part of the problem in explaining ESEs is that we don't have very much data to go on. The lensing effects seen here on Earth have until now only been seen in radio waves—no detections have been made in optical, X-ray, or infrared wavelengths.
"Historically, ESEs have been difficult to find, requiring large investments in telescope time for only a limited number of detections," Bannister and co. note. The largest available dataset for hunting ESEs—consisting of distant radio signals whose distortions might reveal intervening ESEs between their quasar origins and Earth—consists of 149 sources recorded about every two days for 17 years. Some 1,200 source-years of observations have revealed only 15 ESEs.
What the Australian group offers is the first real-time observation of an ESE event. Using a new surveying technique, they were able to catch one in the act on June of 2014 following two months of scanning. Bannister and his team were then able to make follow-up observations at both optical and radio frequencies.
"The key to our technique is as follows: When a plasma lensing event is in progress, the radio spectrum of the background source changes from a featureless continuum to one that is highly structured," the astrophysicists explain.
Crucially, the ESE observed by the group is diverging—growing apart. The analysis revealed a "double-horned light curve" changing quickly with time. The conclusion is that the lenses are most likely not solid lumps, but are more likely to be sheets of ionized gas with so-far unknown geometries. The sheets may be flat like a thick piece of paper, warped around like a hazelnut shell, or, yes, wrapped cylindrically like a noodle. Determining which of those geometries is correct will require more ESE data.Fortunately, the technique developed by Barrister and co. will allow for much easier detection and observation of ESEs in the future. These surveys will finally allow for the geometries to be "unambiguously determined," according to the group. "Such surveys may unveil a substantial component of the Milky Way ISM."