Glittering black holes reveal invisible cloud in our galaxy
“The reason rapid variability is interesting is that it’s usually a sign of something extreme that’s going on,” Murphy says. Detecting extreme events can mean spotting hidden supernovae or grabbing nearby stars releasing flares so large that they wipe out any potential for life on planets in their orbit. This rapid variability is difficult to observe, however, because it requires that a radio source be far away (small in our field of view) and anything obstructing it to be large and close to home.
In 2019, Murphy worked on an independent investigation into the radio wave aftermath of the merger of two neutron stars. The team used ASKAP to scan an expanse of the cosmos new to and 33 days after the merger. But after this analysis completed, the data remained a treasure for further analysis of variations in the night sky. “We have about 30,000 galaxies – 30,000 radio sources – in this area. So I had to face a lot data, ”says Wang.
Wang wanted to find the most capricious radio signals in the sky. She wrote a script to wipe out stagnant radio blip data that they didn’t care about, but still ended up with thousands of radio sources that seemed to vary. The vast majority had uninteresting explanations or were artifacts of the detection process. Still, Wang examined each of them. “So I just click, click, click, click for several days, “Wang said,” and finally I found it. “
Of the 30,000 distant galaxies, only six were scintillating rapidly. “Of those six, five were in a straight line,” says Murphy. “When you find out something like this, you think there is something strange going on here.”
For Wang and Murphy, something strange also meant that there could be something wrong. Their team had to confirm that their result was not just a one-off result. They reimagined the sky from a different angle so the cool feature appeared elsewhere, excluding unreliable pixels. But in the end, they couldn’t blame him on the telescope’s bad behavior. “So you’re left with the idea that it must be something astronomical,” Murphy said. “It must be real.”
Encouraged, Wang and Murphy collected more snapshots of the scintillating signals over 11 months – seven nights of observation in all. This time interval allowed them to determine the size and shape of what they believe to be an interfering gas cloud, as the backlights moved relative to Earth, the first example of such an approach. Their results show that the gas filament is thin and about a third of a light year long – 20,000 times longer than the distance between Earth and the Sun.
How did this strange cloud form? Murphy’s team can’t know for sure, but they believe the immense gravity of a star shredded a cloud of gas to these proportions. Black holes are known to create these gas streams, but none are nearby. “So rather than a black hole,” Murphy says, “we have some kind of plasma cloud that’s been disturbed by a star and stretched it out so that we have this long tidal current.”
One aspect of the cloud baffled Murphy’s team. She says only the hot charged gas, plasma, could cause the flicker. But based on his team’s models, they believe the cloud could only form its shape by moving quickly – around 30 kilometers per second – and that means more of it would actually be very cold. So cold, in fact, that the hydrogen droplets inside could freeze like snowflakes.
Françoise Combes, astrophysicist from the Collège de France not involved in the work, is sold on the discovery of the team. In fact, Combes’ work two decades ago hypothesized that not only cold clouds exist, but also that they constitute a large part of the missing baryons in the Milky Way. She thinks that this cloud is probably just the small tip of a much larger fractal cloud structure throughout the galactic disk. “Scintillations are the hallmark of the existence of this hierarchy of molecular cloud scales,” she wrote in an email to WIRED. “There is a lot of space to have a large fraction of dark baryons in the form of cold molecular clouds.”