Leibniz-Institut Für Astrophysik Potsdam (AIP)
I am a co-PI of the ATLAS3D Project. Check out the web page for the data and the paper products of the survey. Of interest might also be a popular article about the first results of the project, published in Physics World in November 2011 (Viewing galaxies in 3D). I am also a member of the consortium which built MUSE, the next generation integral-field spectrograph for the VLT. The very exciting capabilities of MUSE and its exploitation are starting to take most of my time....
An astro highlight (past highlights can be found here):
One of the first targets with MUSE, still during the early stages of commissioning of the instrument on Paranal, was an elliptical galaxy NGC5813. Why? Because it intrigued me for a long time... In 2003, we published a paper, led by Eric Emsellem and the SAURON team, showing integral-field kinematics of a representative sample of early-type galaxies. There were many famous object in that sample, and this one had another distinction: NGC5813 was the first known object to have a "kinematically distinct core", discovered by Geroge Efstathiou, Richard Ellis and David Carter in 1980. The SAURON data were very good, but they didn't reveal all secrets of NGC5813. The structure of the NGC5813 remained a mystery for good 35 years. The new MUSE data, however, allow us to start understanding the hidden nature of NGC5813 and provide a good topic for An Astro Highlight based on this paper.
Kinematically distinct cores, or kinematically decoupled cores, or kinematically peculiar cores, or most commonly only referred to as KDCs, are exciting features on the mean velocity maps of galaxies. As you can see from the first panel on the plot below, which shows the map of the mean velocities of stars in NGC5813 (essentially a map of ordered motions of stars), the centre of the galaxy, its core, seems to rotate, while the rest of the galaxy's body does not seem to have any ordered rotation. Why is that? How do you make structures like that? Are they stable and long lived?
The mean velocity and the velocity dispersion maps of NGC5813 as seen by MUSE, showing the famous KDC, but also a very specific structure on the velocity dispersion map (two peaks along the major axis). This figure is based on Fig 2. from a recently published paper in MNRAS (Krajnović et al. 2015). NGC5813 is the first know KDC, and the MUSE data, constraining dynamical models, allowed us to understand its internal structure. It is not a decoupled component, separated from the rest of the galaxy's body. Instead, it is made of two components of stars, each following a family of orbits with opposite net angular momentum: they are counter-rotating. This counter-rotation together with a specific distribution of stars within the components defines the structure we see on the kinematics maps. North is up, East is left. A specialist talk about KDCs in general, which I gave at the Macquarie University, Sydney, in February 2015, is available here (27Mb).
These are some of the question astronomers are trying to answer since the discovery of KDCs. There are many types of KDCs (velocity maps of a number of KDC you can see on the figure at the bottom of the page): some have similar velocity maps as NGC5813, where the core seems to rotate and the rest of the body doesn't, but there are even more spectacular cases, such as NGC4365, where the rotation axes of the core and the body are misaligned by about 90 degrees, or NGC5322 where the core is counter-rotating with respect to the rest of the body (180 degrees misalignment). Sometimes, but not very often, the misalignments is even different from the 90 and 180 degrees (can you spot such galaxy on the figure below?). When astronomers started discovering more and more KDCs in 1990s, they became the topic of the day as they look like kinematic proofs that galaxies indeed grow by interactions. KDCs are the smoking guns of mergers, but what kind of mergers and how are they actually created is still a bit of an uncharted territory (although there are plenty of ideas out there).
KDC are often found in large and massive elliptical galaxies. These objects are essentially balls, or even rugby balls, of stars embedded in dark matter halos and hot gas. Some have active galactic nuclei and large scale jets, which seem to stir the halos of hot gas, rising bubbles of gas around the central galaxy (all this is true for NGC5813). KDC are also found in smaller galaxies, but these seem to be different from the KDCs in big galaxies: they are made of young stars and they are likely to disappear as their stellar population ages, as it was elegantly shown by Richard McDermind and collaborators in this paper.
On the second panel of the figure above you can see what makes NGC5813 very special indeed. It shows the map of the velocity dispersion, a measure of random motions, and it is very unusual. Typically the velocity dispersion maps of early-type galaxies (including galaxies with KDCs) have a peak (highest values) in the centre of the galaxy, there the gravitational potential is the deepest. From that point outwards, velocity dispersion decreases pretty much steadily, following the distribution of the light. But the velocity dispersion map in NGC5813 is very different: it has a central peak, but this is then followed by a drop of some 100 km/s, followed by a rise of about 40-50 km/s and a subsequent drop. While general drops and rises in velocity dispersion are known to occur, typically in galaxies with small central stellar discs or bars, the structure of the velocity dispersion map in NGC5813 is very specific. The rise happens only on two sides of the nucleus (and not everywhere around it), along the major axis, and it coincides with the end of the KDC (of the region that exhibits clear ordered rotation). What could be the cause for that?
As I mentioned earlier, NGC5813 intrigued us for quite some time. The drop in the velocity dispersion was known since the discovery of the KDC, and the SAURON observations revealed part of its structure. But unfortunately the field-of-view of SAURON was too small, and the final mosaic of three SAURON pointings didn't cover the full structure (we didn't really know what to expect). The MUSE, however, did it in one shot. It revealed this special structure, which has been actually seen before, but in very different types of galaxies.
A small percentage of early-type galaxies, something like 7%, typically less massive and luminous and quite flat, disc-dominated systems, show what we call a double-sigma profile on their velocity dispersion maps: drop in the centre, followed by two peaks in the velocity dispersion along the major axis. Their velocity maps can be diverse: from very nice counter-rotating KDCs (180 degrees difference in angle between the central and the outer part of the body) to very messy velocity maps, with no ordered rotation. The most famous of these galaxies is NGC4550, and there is a very reasonable explanation for its structure (see this as well): it is made of two discs of stars, which rotate in opposite directions. The two peaks in the velocity dispersion are then the consequence of the counter-rotation. Imagine that in the same spectrum you are detecting one population of stars that go in one direction (quite fast) and another population that goes in the opposite direction (as fast). This will result in the mean observed velocity close to zero (the relative velocities cancels out), but the spread in velocities (the velocity dispersion) will be large. Depending on the type of these two counter-rotating discs, how many stars they have and how are they distributed, one can get a plethora of structures on the velocity and the velocity dispersion maps.
Once we saw the shape of the velocity dispersion maps of NGC5813, we had a good hunch that that NGC5813 could also be built of two populations of stars, which rotate in oposite directions. But NGC5813 is not a flat, disc-like galaxy, and the situation is a bit more complicated. Still with the help of dynamical models, we found that indeed, the galaxy can be described as having two components of stars. They couter-rotate, but not very fast; neither of them is a disc (as in a typical double-sigma galaxy). The distribution of stars is such that one component is dominant in the centre: within the KDC one component consists of 70% of the stars - that is why we see the rotation. Outside of the KDC, the components become more similar, contributing about 50-50% in mass. This 50-50 ratio also explains why there is no visible rotation: stars in both components steadily rotate, but the net motion gets canceled and we don't see it.
What does that mean for formation of the KDCs? Well, it seems KDC in NGC5813 should not be considered as a separate entity, divorced from the rest of the galaxy. It is not a ball of stars that has different kinematics from the rest of stars. It is not "decoupled". It is a result of mixing of different orbital families of stars. There are still many ways you can form this structures, such as accretion of gas from cosmological cold flows, turbulent dics of gas rich galaxies in the early universe (stars in NGC5813 are almost as old as the Universe), mergers of similar-in-mass galaxies.... Plenty of possibilities, and only by understanding the structure of other KDCs in a similar way as here, we can hope to understand their formation.
NGC5813 is a wonderful system. It has a complex kinematics which can be explained with a relatively simple, but very specific dynamical model. It also has very complex multi-phase gas structure, linked to the active galactic nucleus and its jet. But that might be another highlight, in the future.
The mean velocity maps of galaxies with KDCs from ATLAS3D Paper II (Krajnović et al. 2011). All velocity maps were obtained with the SAURON instrument. One before the last in the 2nd row is the SAURON velocity map of NGC5813, also published in 2004. North is up, East is left.