The authors goal with this paper is to investigate the range of kinematic properties of boxy bulges. Boxy bulges are believed to be bars, and not true bulges. In boxy bulge systems, there is often no rotional velocity gradient with height- this is called cylindrical rotation. The authors find that all bulges that exhibit cylindrical rotation are boxy bulges, but not all boxy bulges have cylindrical rotation and it is not a defining characteristic of boxy bulges. Galaxies that have strong cylindrical rotation have similar metalicity profiles in their 'bulge' as in their disk, showing that these galaxies are likely to be pure disk.
Results From Paper: The paper analyzed a number of edge on galaxies that exhibited a boxy or peanut bulge shape. They took spectra with the slit across the bulge, and at varying heights in the bulge to see any velocity and metallicity gradients with height in the bulge. The goal is to characterize properties of the boxy bulge system and see if there is any defining charactertistic. The authors have three main catagories of bulges:
The radial velocity measurements have a resolution of 1 angstrom, but as height incrases above the plane there is severe signal to noise issues, that complicate interpretation of results. Dispite this, the authors find that NGC 3390 is cylindrically rotating with no velocity gradient with respect to height above the disk. On the opposite end, IC 4767 exhibited a velocity gradient and therefore has no cylindrical rotation. This galaxy was not a boxy bulge but a peanut bulge. The other results were somewhere in the middle regarding cylindrical rotation but noise becomes an issue in interpreting their results.
The rest of the paper touched on the metallicity of the populations within these systems. With the signal-to-noise issues the authors were only confident enough in the data to apply two of the galaxies into the GANDALF program to find metallicity. The results show that in NGC 3390, there is no metallicty gradient with height and the population is in fact identical to the disk population. However, in NGC 1381 it was found that the population was quite old and showed alpha enhancement with increasing height- one of the characteristics of a classical bulge. It was unclear whether NGC 1381 exhbited cylindrical rotation: at low Z heights it did seem to rotate cylindrically, but at larger Z's there were large discrepencies in the profile.The end results from the paper were that if a galaxy exhibits clear cylindrical rotation, that it is likely a pure disk galaxy and the boxy bulge is due purely to the bar, as the stellar population and dynamics of the system match that of the disk. But in systems that exhibit partial cylindrical rotation the situation becomes much more complicated, and the chemdynamical bar formation models do not exist that can probe solutions to this issue. There are several scenarios in which a bar origin for the boxy bulge is still possible even with an alpha and metallicity gradient with Z, so better models of bar formation and evolution are needed to probe the intermediate cases.
On an aside, this paper had several references to models tailored to individual galaxies, specifically the Milky Way. I was under the assumption the Milky Way had a normal bulge, but many of the papers given state that the Milky Way is nearly a pure disk galaxy, with the bulge being similar to the boxy bulge found in NGC 3390. If there is any classical bulge present, the referenced papers state that it is extremely small in mass, at most a few % of the total. How does this tie in with class? The primary area this paper was focused on that we covered in class was stellar populations of galaxies. However, there was a lot of overlap with Astro 615, as we covered boxy bulges/cores and the kinematics of galaxies. Cume Question: What is cylindrical rotation? (*note: coming up with a question for this was rather hard, as there is no way that any non galaxy person would know much about the differences between classical and boxy bulges, or the specifics of gradients within the bulge). Answer: A constant rotational velocity within the bulge of a galaxy independant of height. Stated another way: the absence of a velocity gradient within the bulge of the galaxy with respect to height above the midplane. Follow up research/project: The sample size in the paper was small, and plagued by signal-to-noise issues. It is difficult to have any robust conclusion with such a small sample. Extending the work done in the paper to a larger sample of nearby edge on galaxies that exhibit a boxy bulge, combined with higher signal-to-noise observations would be a nice project to work on. One could see if galaxies with nearly total cylindrical rotation all have the same stellar population makeup as their disks, or if NGC 3390 was an isolated case.