Galactic bubbles reveal new clues about the formation of the Milky Way

The findings

The bubbles present an opportunity to study the gas-filled area of the galaxy where they are located, known as the "circumgalactic medium," which extends 45,661 light-years out from the galactic center.

The researchers involved in the study, led by professor Anjali Gupta, wanted to compare the region containing the bubbles to regions far removed from the bubbles. They found that the temperature of the gas inside the bubbles or bubble regions is actually not that different from the area outside it.

It challenges the previously-assumed notion that they were heated by the shock of gas as it spreads out from the galaxy. 

In the press release from The Ohio State University, professor Gupta shared that the researchers were “surprised” at the similarity of the temperatures in the regions inside and outside the bubbles.

In fact, the scientists were also able to show that the brightness of the bubbles is not due to them being hotter than the surrounding environment, but because they are filled with very dense gas.

The observations carried out by Gupta and the study’s co-author Smita Mathur were made by the Suzaku satellite in a collaboration between NASA and the Japanese Aerospace Exploration Agency. 

The researchers were able to analyze 230 observations made between 2005 and 2014 in order to understand the electromagnetic radiation of the gases in the area, known as diffuse emission. 

Mystery of the formation

The effort hopefully brings scientists closer to figuring out how the bubbles are actually formed, a finding that has so far eluded astronomers.

One hypothesis has been that black holes are somehow involved. Great amounts of energy are emitted from the supermassive black hole at the center of our Milky Way galaxy and in other galaxies, similar black holes have been observed ingesting large amounts of matter to power high-energy jets.

Whereas, the study showed a large presence of non-solar neon-oxygen and magnesium-oxygen in the bubble shells, indicating that they were likely formed in the galactic center rather than through processes linked to a supermassive black hole but rather from the nuclear actions involved in forming stars.

Another possibility is that the bubbles were made from energy related to other huge stars and various astrophysical activities. 

“Our data supports the theory that these bubbles are most likely formed due to intense star formation activity at the galactic center, as opposed to black hole activity occurring at the galactic center,” Mathur shared in the press release.

The team looks forward to utilizing data from future space missions to further improve their models, with Gupta pointing out that “Scientists really do need to understand the formation of the bubble structure, so by using different techniques to better our models, we’ll be able to better constrain the temperature and the emission measures that we are looking for.”

Insight from the researchers

Interesting Engineering (IE) reached out to professor Anjali Gupta for more insight on this research.  

The following exchange has been lightly edited for clarity and flow.

Interesting Engineering: What can we learn by studying galactic bubbles?

Professor Gupta: Bubbles provide a great opportunity to understand the role of feedback in galaxy evolution due to their size and location. These are magnificent bubbles injecting energy and momentum into the galaxy.

IE: What is the difference between the Fermi bubbles and eRosita bubbles?

Fermi bubbles are emitting Gamma rays while eROSITA bubbles are X-ray bright. eROSITA bubbles are much larger and envelop the Fermi bubbles.

IE: What do we know about how these bubbles are formed? How can we find out more about their formation?

These bubbles are formed due to some shock that happened at the center of the Galaxy.

Measuring the bubbles' age, and cooling time of shock-heated gas can provide insights into their formation. To understand their formation, theorists and observers will need to work together. 

IE: How do the bubbles achieve their brightness if the temperature inside and outside of them is the same?

X-ray brightness depends on both temperature and density. eROSITA bubbles are brighter because of their large density, not due to higher temperature as claimed earlier. This is a paradigm shift in understanding the Galactic bubbles.

IE: What is next for your research?

So far, we are able to say bubbles are not formed due to adiabatic shocks. Next, we want to learn the formation process of these bubbles.

Read the study “Thermal and chemical properties of the eROSITA bubbles from Suzaku observations” in Nature Astronomy.

Abstract

The X-ray bright bubbles at the Galactic Centre provide an opportunity to understand the effects of feedback on galaxy evolution. The shells of the eROSITA bubbles show enhanced X-ray emission over the sky background. Previously, these shells were assumed to have a single temperature component and to trace the shock-heated lower-temperature halo gas. Using Suzaku observations, we show that the X-ray emission of the shells is more complex and best described by a two-temperature thermal model: one component close to the Galaxy’s virial temperature and the other at supervirial temperatures. Furthermore, we demonstrate that temperatures of the virial and supervirial components are similar in the shells and in the ambient medium, although the emission measures are significantly higher in the shells. This leads us to conclude that the eROSITA bubble shells are X-ray bright because they trace denser gas, not because they are hotter. Given that the pre- and postshock temperatures are similar and the compression ratio of the shock is high, we rule out that the bubble shells trace adiabatic shocks, in contrast to what was assumed in previous studies. We also observe non-solar Ne/O and Mg/O ratios in the shells, favouring stellar feedback models for the formation of the bubbles and settling a long-standing debate on their origin.