My research primarily involves the study of spiral galaxies that are edge-on to the line of sight. Spiral galaxies (like the Milky Way) have very thin disks compared to their diameters and these disks are delineated mainly by stars. A face-on disk shows these stars, often with beautiful spiral structures. If you 'turn the disk on its side', however, then the stars would appear more like a thin line in the sky. For example, compare the face-on (left) and edge-on (right) systems shown below.
By looking at edge-on galaxies, such as the galaxy UGC 10288 on the right, one can have a better view of the galaxy's halo. Now there are two usages of the word, halo. One is the well-known dark-matter halo whose make-up is currently unknown in astronomy. The second is the gaseous halo which is what I study. By gaseous halo, I mean warm (10,000 degrees Kelvin) ionized gas, hot (millions of degrees K) ionized gas, cool (100 K) neutral hydrogen, cold (20 K) molecular hydrogen, magnetic fields, dust, and cosmic rays. As this list suggests, there are many components to the gaseous halos of galaxies, both cold and hot. In fact, the components in a galaxy's halo are the same (though weaker in intensity) as the components that are found in a galaxy's disk between the stars that are collectively known as the interstellar medium (ISM). The images above are optical images and that's where we see mostly starlight, not the ISM. In order to see the gaseous components, it is necessary to change the frequency of the observation away from the optical and look at other wavebands. Which waveband depends on which component you are interested in.
This picture shows the galaxy from above right (UGC 10288) again, but radio wavelengths are shown in blue and the stars are shown at infrared wavelengths (coloured reddish). I have other images which show the radio halo fo UGC 10288 a little better, but this particular picture reveals a background quasar (which is a distant active galaxy) with a vertical jet pointing upwards. There's another jet pointing down too but it is obscured by the disk of the foreground galaxy. This was a surprise and resulted in a press release about this system. We really weren't expecting to detect a radio galaxy right behind UGC 10288. In fact there is some interesting science that can uniquely be done because the background radio emission is passing through the foreground galaxy's halo.
Over the years, I have observed every component of a galaxy's ISM as seen in galaxy halos, but my current project is to focus on magnetic fields and cosmic rays. Why these components? Cosmic rays (CRs) are very high energy particles that are ejected mainlyfrom supernovae in the disk. By studying CRs we can link the halo emission to energetic activity in the underyling disk. But, very importantly, CRs are tied to magnetic field lines. Cosmic ray particles will flow along magnetic fields just the way they do around the Earth or near sunspots. The radiation that results is observed at radio wavelengths and is called synchrotron radiation. When this radiation is observed, we can be sure that both CRs and magnetic fields are present. We believe that magnetic fields are the key to the dynamics of the disk-halo interface and may even be the key link between galaxies and the intergalactic medium (the low density medium between galaxies). Let me now show you a picture of a galaxy with a significant halo, namely NGC 4631.
On the far left, we see the optical galaxy (mainly starlight) in greyscale. Superimposed are green vectors showing the direction of the magnetic fields in the galaxy. Noticie that there is an X-shaped structure to the field. The reason for this isn't entirely understood but may have to do with the magnetic dynamo and how it is generated in the rotating disk.
On the right we see the enormous radio halo around this galaxy, indicating that both CRs and magnetic fields exist very far from the galaxy's disk. If our eyes were tuned to radio (rather than optical) wavelengths, galaxies -- and in fact the entire sky -- would look very different from what we are used to.