My research this summer is focusing on hydrogen gas in radio galaxies. Radio galaxies are special class of “active galaxies” which exhibit large amounts of emission in the form of (you guessed it..) radio waves. All massive galaxies are believed to harbor a supermassive black hole (SMBH) at their centers. A supermassive black hole is million to several tens of billions of times more massive than our sun! In roughly 10% of these galaxies, the SMBH is actively swallowing gas and stars, making them “active”. As the gas falls towards the SMBH, it emits large quantities of radiation. This very small region in the center of the galaxy can outshine the rest of the galaxy!
However, not all of the gas that falls towards the center actually gets eaten by the black hole. Some of it is ejected along the rotation axis of the SMBH in jets. These jets of plasma (charged particles) can travel close to the speed of light! When this happens, the particles interact with the magnetic fields in the galaxy and emit radiation in the radio wavelengths.
In addition to simply emitting radio waves, these jets can interact with gas clouds in the galaxy in violent ways. If, as the jet is expanding away from the center of the galaxy, it encounters a gas cloud, a few things can happen. As the jet slams into the cloud, shocks can go through the cloud, ionizing it and causing it to emit light. These shocks can also result in the gas cloud collapsing and forming stars. Also, the jet can push the cloud outwards.
I’ve been studying these radio-loud active galaxies for the past couple years. One project has involved studying the interaction between the radio jets and ambient gas in the galaxy. I (and my collaborators) looked at images of gas (typically Oxygen or Hydrogen) emitting visible light and compared them to radio images to see if the emitting gas was predominantly along the inferred location of the radio jets. The results from that study are currently being prepared for publication.
My summer research at ASTRON is also looking at this interaction. Neutral hydrogen gas has been detected (in absorption) in front of some radio galaxies. This means clouds of hydrogen gas absorb a narrow band of radio waves as the radiation passes through them. In some cases, the hydrogen gas cloud appears to be moving away from the galaxy. If you just think about gravity pulling things towards the center of a galaxy, one would think the cloud should either be rotating around the galaxy or falling inward, not moving outward! The explanation for this is that through the interaction of the radio jet with a gas cloud, the cloud has been pushed outwards.
These types of interactions are very important in understanding how galaxies evolve. Simulations of the evolution of galaxies fail to reproduce what we see unless the SMBH and the host galaxy interact through feedback mechanisms (jet-gas interaction, startbursts, infalling material, etc). So, by studying these itneractions, we can better understand how this feedback operates and gain better insight into the evolution of galaxies.
In a later post, I will describe a bit of the details of what steps I actually go through to study this phenomenon.
