Kind of weird! What does a giant X-ray bubble blowing up from the center of the galactic plane indicate?

A new X-ray survey has revealed the hidden structure of a Milky Way galaxy—giant X-ray bubbles extending far above and below the galactic plane from the galactic center.

These X-ray bubbles are large enough to engulf known gamma-ray Fermi bubbles. A team of astrophysicists from the Max Planck Institute for Extraterrestrial Physics in Germany, led by Peter Predl, says the two phenomena are likely to be linked in some way.

The halo of the Milky Way (the halo) above and below the plane of the Milky Way is a great place to look for evidence of those mischief that happened at the center of the galaxy in the past. Because the material density of the galactic halo is very low compared to the galactic disk, any energetic activity from the galactic center could potentially be detected in the form of shock waves and ejecta structures in the interstellar medium.

First, let's take a look at how astronomers discovered and explained Fermi bubbles. Discovered by the Fermi Gamma-ray Telescope in 2010, Fermi bubbles are filled with hot air and a magnetic field that emits gamma rays. The Fermi bubble extends 9 kiloparsecs (about 29,354 light-years) from the galactic center to all sides, and its total length is 18 kilopascals.

These bubbles are thought to be evidence of past activity in the region of the Milky Way, although we still don't know what that activity might have been. There are currently two possible speculations, one could be a stellar explosion, the birth of a large number of stars; the other could be the explosion of a supermassive black hole at the center of the Milky Way, although this black hole is currently in a quiescent state, but when the activity occurs it is in more active state. The new discovery provides scientists with background information that could help shed light on the mystery of the Fermi bubble.

The Fermi bubbles (red) and the eROSITA bubbles (cyan). (Predehl et al., Nature, 2020) Fermi bubbles (red) and the eROSITA bubbles (cyan). (Predehl et al., Nature, 2020) [eROSITA: extended ROentgen Survey with an Imaging Telescope Array]

This result comes from the X-ray telescope eROSITA of the Spektr-RG Space Observatory (Russian Astronomical Observatory). Launched in July 2019, it is tasked with an all-weather deep survey of the sky at X-ray wavelengths. With its incredible sensitivity, it imaged structures that other instruments could not clearly identify.

This structure is huge, extending 14 kiloparsecs (about 45,661 light-years) from the center of the Milky Way to both sides, which is huge enough to completely swallow the Fermi bubble. While we still don't know the exact cause of these bubbles, Predehl and his team believe they arose from the same event.

The "ROSAT" telescope is the predecessor of the "eROSITA" telescope array, and the X-ray bubbles are particularly interesting in the observations taken by "ROSAT". Although the resolution and sensitivity of "ROSAT" are relatively low, only part of the structure is clearly visible, but this result is enough to link it to Fermi bubbles. The "eROSITA" images provided data that made the connection between the two structures clearer.

"The large-scale X-ray radiation structures discovered by the Fermi bubble and the eROSITA telescope matrix show remarkable morphological similarities," the researchers wrote in their paper.

"We call the structure imaged by the eROSITA telescope matrix the 'eROSITA bubble', and we believe that the Fermi bubble and the eROSITA bubble are physically related, and our discovery confirms the speculation that the two structures have a common origin. "

Fermi bubbles in purple, eROSITA bubbles in yellow. (Predehl et al., Nature, 2020) The purple structures in the figure are Fermi bubbles, and the yellow structures are "eROSITA bubbles". (Predehl et al., Nature, 2020)

Even though they may have some connection, there are some differences between Fermi bubbles and eROSITA bubbles that cannot be ignored. First of all, Fermi bubbles are elliptical, and Erotica bubbles look almost spherical.

Second, the researchers say that eROSITA bubbles are much larger than Fermi bubbles, comparable in size to silver disks.

The researchers also observed two prominent structures in the eROSITA bubble. One is that the outer boundary of the eROSITA bubble is very bright in X-rays, which indicates that the gas at the boundary of the eROSITA bubble is hotter than the gas around it. This is consistent with a shock front propagating in the halo of the Milky Way (galactic halo), and is also associated with a forward shock wave that has been shown to be associated with the occurrence of some kind of energetic activity that forms the bubble of.

The second is that there is a boundary structure between the eROSITA bubble and the Fermi bubble enclosed by it, which the research team interprets as a contact discontinuity-this boundary separates the hot interstellar medium outside the Fermi bubble from the torrent inside the bubble. open.

Since expanding eROSITA bubbles would need to absorb more energy, eROSITA bubbles generated by the same event are able to limit the continued expansion of Fermi bubbles. For example, starburst activity may inflate the eROSITA bubble, but the bubble is just at the limit of energy produced by starburst activity.

On the other hand, they are in the energies of the Active Galactic Nucleus, a supermassive black hole that is actively accreting matter and producing powerful feedback streams in the form of jets and winds. Likewise, supernova activity associated with star formation can generate enough energy.

Either way, this new discovery provides us with some interesting new information, such as the fact that galaxies are thought to form and grow through the recondensation of cooling plasmas that are heated during the collapse of dark matter halos. The presence of eROSITA bubbles is direct evidence that the plasma can be reheated.

At the same time, we have a new perspective on the North Polar Spur, a hot and bright bubble of X-rays in the northern sky. Since it has always been difficult to determine how far this structure is from us, it has been difficult to determine its origin and related properties.

Some astronomers have linked it to relatively distant supernovae and the birth of stars, while others have suggested that it may be related to a more distant outflow from the center of the Milky Way. The team's results suggest that it may also be associated with eROSITA bubbles.

Using instruments other than X-rays, a deeper, closer look at these massive structures could help us find answers to these yet-to-be-clarified questions. At the same time, I also hope that more scientific observations can guide us into deeper research as always.

The research was published in the journal Nature.