Galaxy found to float in a calm sea of ​​halo gas

Using a cosmic mystery to explore another, astronomers analyzed the signal of a fast radio burst, an enigmatic explosion of cosmic radio waves lasting less than a millisecond, in order to characterize the diffuse gas in the halo of a gigantic galaxy.

A vast halo of low-density gas extends well beyond the luminous part of a galaxy where stars are concentrated. Although this hot and diffuse gas is more the mass of a galaxy than the stars, it is almost impossible to see. In November 2018, astronomers detected a rapid radio burst crossing the halo of a gigantic galaxy before heading towards the Earth, thus allowing them to discover, for the first time, the nature of the halo gas generated by a signal evasive radio.

"The signal of the rapid radio explosion revealed the nature of the magnetic field around the galaxy and the structure of the halo gas.The study demonstrates a new transformation technique to explore the nature of halos of galaxies", said Professor J. Xavier Prochaska. from astronomy and astrophysics to Santa Cruz UC and lead author of an article on new discoveries published online September 26 Science.

Astronomers still do not know what is causing rapid radio bursts and it is only recently that they have been able to trace some of these very short and very bright radio signals to the galaxies they are at. l & # 39; origin. The burst of November 2018 (named FRB 181112) was detected and located by the instrument behind this technique, CSIRO's Australian Square Kilometer Array Pathfinder (ASKAP). Follow-up observations with other telescopes made it possible to identify not only its host galaxy, but also a bright galaxy in front of it.

"When we superimposed the radio and optical images, we immediately saw that the rapid burst of the radio had pierced the halo of this galaxy in the foreground and that, for the first time, we had a direct way of to investigate this otherwise invisible matter that surrounded this galaxy, "said co-author Cherie Day at Swinburne University of Technology in Australia.

A galactic halo contains both dark matter and ordinary matter ("baryonic"), which should be mainly hot ionized gas. While the luminous part of a massive galaxy may be around 30,000 light-years away, its roughly spherical halo is ten times larger. A halo gas feeds star formation when it falls to the center of the galaxy, while other processes (such as supernova explosions) can eject material out of the formation regions of the galaxy. Stars and in the galactic halo. One of the reasons why astronomers want to study halo gas is to better understand these ejection processes, which can prevent the formation of stars.

"Halo gas is a fossil record of these ejection processes, so our observations can shed light on theories of how matter is ejected and how magnetic fields are inserted into galaxies," said Prochaska. .

Contrary to expectations, the results of the new study indicate a very low density and low magnetic field in the halo of this intermediate galaxy.

"The halo of this galaxy is surprisingly quiet," said Prochaska. "The radio signal was largely undisturbed by the galaxy, which is in stark contrast to what previous models predicted."

The FRB 181112 signal consisted of several pulses, each lasting less than 40 microseconds (ten thousand times shorter than a blink of an eye). The short duration of the pulses imposes an upper limit on the density of the halo gas, because a passage in a denser medium would lengthen the radio signals. The researchers calculated that the density of the halo gas must be less than one-tenth of an atom per cubic centimeter (equivalent to several hundreds of atoms in a child-sized balloon volume).

"Like the glittering air on a hot summer day, the tenuous atmosphere of this gigantic galaxy should distort the signal of the fast radio explosion. Instead, we received a impulse so pure and so clear that there is no signature of this gas, "said Jean-Pierre Macquart, co-author, astronomer at the International Center for Radioastronomy Research at Curtin University, Australia.

Density constraints also limit the possibility of turbulence or cold gas clouds in the halo ("cold" being a relative term, referring here to temperatures around 10,000 kelvins, compared to hot halo gas nearing one million kelvins). "A favored model is that the halos are invaded by coarse gas clouds, and we do not find any evidence of these clouds," said Prochaska.

The FRB signal also provides information on the magnetic field in the halo, which affects the polarization of the radio waves. The analysis of the polarization as a function of frequency gives a "measure of rotation" of the halo, which the researchers found very weak. "The weak magnetic field in the halo is a billion times weaker than that of a refrigerator magnet," prochaska said.

At this point, with the results of a single galactic halo, researchers can only tell whether the density and intensity of the unexpected magnetic field are unusual or whether previous studies on galactic halos have overestimated these properties. ASKAP and other radio telescopes will use fast radio gusts to study many other galactic halos and solve their properties.

"This galaxy is perhaps special," said Prochaska. "We will have to use the FRB to study dozens or hundreds of galaxies on different masses and ages in order to evaluate the whole population."