A team of scientists has managed to measure the extension of the gases in galaxies that defines their boundaries.
Ask most people and they’ll tell you it’s made of stars. Our galaxy, the Milky Way, is home to between about 100 and 300 billion stars, and we can see thousands of them with the naked eye. But most of a galaxy’s mass is actually gas, and the extent of the gas has been difficult to measure. Now researchers have found a way to see how far this gas extends into the cosmos by discovering that galaxies are much larger than we thought. One of the fundamental questions about galaxies concerns their size. If we limit our observations to the stars, then our galaxy, for example, is about 87,000 light-years across. But do these measurements really define size? In new research published in Nature Astronomy, researchers measured the extent of gas extending beyond a galaxy’s stellar population.
Advertisement
The size of the halo
Galaxies serve as reservoirs of star-forming material called the circumgalactic medium (CGM). The CGM interfaces with the intergalactic medium (IGM), which is even more gas that exists between galaxies. CGM is notoriously difficult to observe because it is so widespread and extensive. But it makes up about 70% of a typical galaxy (ignoring dark matter) and plays an important role as an interface between a galaxy and the cosmic web that connects galaxies.
Astronomers rely on bright background objects to try to observe the CGM. Objects such as , pulsars or other distant galaxies can illuminate the gas and allow astronomers to measure its spectra. But this only works when things line up correctly and only produces a beam image of the galaxy.
In this new research, a team of astronomers has found a different way to observe the CGM. They used the Keck Cosmic Web Imager (KCWI) on the Keck 10-meter telescope in Hawaii to observe the gas around IRAS 08339+6517. Instead of a limited, beam-like look at the gas, they were able to detect clouds of gas well outside the typical boundaries of a galaxy, up to 100,000 beyond the edge of starlight that typically defines a galaxy.
IRAS 08339+6517 is a starburst galaxy approximately 182,000 light-years away. A starburst galaxy is a galaxy that generates stars at an extraordinarily high rate. They show that it is a frontal spiral galaxy and 90% of its starlight is contained within a radius of about 8,000 light years. Unlike normal spirals, it has rather extreme properties, with a star formation rate that is about 10 times higher than typical for its mass and stellar populations that are dominated by very young stars.
A larger border than expected
The researchers found that as the CGM extends beyond the galaxy, the physical properties of the hydrogen and oxygen in the gas change. The change is ubiquitous over a certain distance and indicates that the gas interacts with different energy sources. “We found it everywhere we looked, which was really exciting and kind of surprising,” said lead author Nielsen. “Now we are seeing where the galaxy’s influence ends, the transition where it becomes part of the galaxy’s surroundings, and, ultimately, where it joins the larger cosmic web and other galaxies. These are all boundaries that are usually poorly defined.”
“But in this case, it appears that we have found a fairly clear boundary in this galaxy between its and its circumgalactic medium,” Professor Nielsen said. In the CGM, the gas is heated by something other than typical conditions inside galaxies; this probably includes heating by diffuse emissions from collective galaxies in the Universe, and perhaps some contribution is due to shocks.
The boundary is where the gas is heated differently inside the galaxy than outside. Within the disk of the galaxy, gas is photoionized by HII (ionized atomic hydrogen) star-forming regions. At longer distances, the gas is ionized by shocks or the extragalactic UV background. “It is this interesting change that is important and provides some answers to the question of a,” he says.
The gas flows into galaxies and becomes fuel for further star formation. At the same time, gas escapes from a galaxy as part of stellar feedback. There are three broad types of galaxies: starburst galaxies with extreme amounts of star formation, quenched galaxies with very little star formation, and intermediate galaxies. The gas in the CGM and IGM plays a role in a galaxy’s gas budget.
Passione Astronomia has always tried to build, through scientific dissemination, a community of more informed and aware people. It is also the reason why many people like you have subscribed to Passione Astronomia and become an integral part of it every day. The more we are, the better off we are, and for this reason we also invite you to take it into consideration : your support helps us continue our project.
Gas flow
IRAS 08339+6517 has remarkably strong gas outflow, but its metallicity profile is flat and shallow. Astronomers usually assume that galaxies with these metallicities and high SFRs are acquiring significant amounts of gas. Other scientific observations of IRAS 08339+6517 indicate a rapid influx of gas into the center of the very strong starburst and consequently strong outflows.
However, IRAS08 is a complex object that also interacts with a nearby galaxy. VLA observations of HI gas around IRAS08 identified a filament that extends up to 130,00 light-years from the galaxy and contains 70% of the neutral gas in the system. This filament interacts with a nearby galaxy about 200,000 light-years away, which is only a tenth of the mass of IRAS-08. The authors state that this interaction with its neighbor could improve the , but there is no evidence that it is influencing the morphology of IRAS-08. This does not appear to be the first phase of a possible merger.
Finding the boundary between the CGM and IGM could be a critical step in understanding how gas enters and exits galaxies, and how it can interact with nearby galaxies without it. “The circumgalactic medium plays a huge role in that cycle of that gas,” says Dr. Nielsen. “So by being able to understand what the CGM looks like around galaxies of different types, those that are forming stars, those that are no longer forming stars, and those that are transitioning from one to another, we can observe differences in this gas, which could drive differences within galaxies, and changes in this reservoir may actually drive changes in the galaxy itself.”
Nature has few discrete boundaries: everything interacts with other things, including massive galaxies. And these interactions hold the key to their understanding. These findings could open a whole new window into how galaxies, gas and stars interact and how galaxies evolve.
Read more
