Leibniz-Institut Für Astrophysik Potsdam (AIP)
My current research topics of interest are related to galaxy formation and evolution. Up to know, this was mostly from the local Universe point-of-view, "digging up" the "fossil" records of past formation events in nearby early-type galaxies, but I am starting to venture into the vastness of redshift space. Things that interest me are supermassive black holes, motions and orbits of stars in galaxies, dynamics of galaxies, their shapes and sub-components, stellar populations and similar. I am a co-PI of a small, but cool, survey of most massive galaxies called M3G, using MUSE, the amazing integral-field spectrograph for the VLT. Things are still "cooking", but watch this space, as they say. I was 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 and I am involved in the MOSAIC, an instrument for the E-ELT (European Extremely Large Telescope).
An astro highlight: Prolate-like rotation in most massive galaxies (past highlights can be found here):
Measuring the way stars move within galaxies is a very powerfull way of learning about the internal structure of galaxies, especially properties such as their three-dimensional shape and, ultimately, what is their gravitational potential like. The recent Gaia results show this very well for our own Galaxy. For galaxies at distances at which it is not possible to resolve individual stars and their motions, one has to look at the average motions of stars within certain regions, but crucial is to cover a large fraction of the body of the galaxy. This is what integral-field spectrographs are good at, and most of my past highlights topics are based on such data. This one is no different in that respect, but it is about something we didn't expect.
MUSE Most Massive Galaxies (M3G for short) is a small survey of 25 galaxies that can be characterised in two ways: they are the most massive galaxies in the Universe (more or less), and they live in the densest regions of the Universe (more or less, of course). The first claim is supported by the fact that these galaxies are brighter than -25.7 mag in K-band, which would make them more massive than about 10^12 Msun. Quite something, especially if you consider that the stellar mass of the Milky Way is a few times 10^10 Msun. Figure 1 shows the comparison of our sample galaxies with those of two other surveys, the ATLAS3D survey that targeted early-type galaxies (Cappellari et al. 2011) and the MASSIVE survey that targeted the most massive galaxies within 100 Mpc (Ma et al. 2014). Such galaxies are very rare, and one can only find them in very dense regions, for example in clusters of galaxies. So, to some extent that first characteristic defines the other, but we looked for a particularly dense regions, such as the Shapley Super Cluster.
Figure 1. The mass - size relation for galaxies from the ATLAS3D survey (open green circles; Cappellari et al. 2011), MASSIVE Survey (open blue squares; Ma et al. 2014) and our M3G galaxies (as red filled circles). M3G galaxies are selected to be more massive than ATLAS3D galaxies, but they turned out to be a bit more massive compared to MASSIVE galaxies as well.
We observed these galaxies with MUSE, the wonderful integral-field spectrograph on the ESO's VLT (on Cerro Paranal in Chile). MUSE is my work horse these days and I've written about some other cool results with it here and here and here. This time the observations took quite some time to finish up the survey (3 yr), but the sample is complete and the first results are out here. Well, we talked about it on various conference already using partial data (you can see the slides here, here, here and here), but once the sample was complete, we were able to publish a proper paper as well.
And what is that unexpected result? Well, first I need to tell you about a work we did in 2011... Using the magnitude limited ATLAS3D sample of early-type galaxies, we looked at the types of stellar kinematics these galaxies have. About 85% of galaxies had very regular looking stellar motions, very similar to those of discs. This is an example (a bit special, I agree, but still representative) of such stellar motions. Except that the rotation is very ordered, it also has a very well defined sense of rotation, around the short axis of the object; the angular momentum vector is aligned with the minor axis of an oblate spheroid.
The remaining 15% had irregular kinematics, showing fascinating features such as KDC (as in this case) or even not showing much rotation. For such galaxies, the angular momentum vector is often not aligned with any of the principle axes of the galaxy (e.g. minor or major). In a few cases, there was, however, an interesting alignement: the angular momentum vector was aligned with the long axis of the galaxy. Fig 2, shows these two possibilities.
Figure 2. These two galaxies are examples of a regular rotation around the minor (short) axis of the galaxy (left) and a "prolate-like" rotation around the major (long) axis of the galaxy (right). The type of rotation for the galaxy on the left is typical for galaxies (85% or so galaxies rotate like this, regardless of their mass). The prolate-like rotation of the galaxy on the right is unusual, and only a small fraction of galaxies has it, but the fraction increases as galaxies become more massive.
When stars rotate around the long axis, we say that the galaxy exhibits a "prolate-like" rotation (some people prefere "minor-axis" rotation), because such rotation is expected of galaxies that have prolate shape (for the definition of the oblate, triaxial and prolate shape have a look at Fig. 3). However, it is not true that prolate-like rotation requires the prolate shape. Such rotation is also possible in triaxial systems, but it is very indicative of a special configuration of the stellar orbits that make such a body.
Figure 3. Galaxies (when they are settled, non-interacting and undisturbed systems) can have these shapes (from left to right): triaxial, oblate, prolate or spherical. An object with the most general, triaxial, shape is characterised by three principal axes, which are all of different lengths, a<b<c. Oblate (axisymmetric) shape is characterised when a=b < c, while prolate shape is somewhat similar, but with different axes of the equal length: a >b=c. The simplest shape is that of a sphere: a=b=c. The topic of this post are galaxies which have kinematics that is expected from systems with prolate shapes. However, as this is a very special condition, our galaxies are more likely to be triaxial.
Anyway, out of our 25 M3G galaxies, 11 show prolate-like rotation, 1 exhibits no net rotation and 13 have more or less regular rotation. Fig. 4 shows the angle between the minor axis od the galaxy and the (projection) of the angular momentum vector. When this angle is large (close to 90), galaxies have prolate-like rotation. Such bi-modality in the distribution of this angle was not seen before in works that investigated somewhat less massive galaxies, and this is why the results were unexpected. They are, however, quite in agreement with some other pieces of evidence relating to the formation of massive galaxies. For example, that they have centrally flat surface brightness profiles (this was already a topic on these pages), they are slow rotators and found where numerical simulations predict them to be.
Figure 4. Distribution of the kinematic misalignment angle as a function of the observed galaxy ellipticity. The misalignment angle is defined as the angle between the minor axis of the galaxy and the projected angular momentum vector. There are two types of galaxies in our sample: Brightest Cluster Galaxies (BCGs) and "Satellites" (SAT). BCGs, shown in red here, as their name says, are the brightest galaxies in a cluster, which also makes them the most massive and the largest galaxies. "SAT" galaxies in this case are all other galaxies in our sample that are not BCGs. They are not the brightest galaxies in their clusters, but they are still more massive than most other galaxies. The way the M3G sample is constructed, all SAT galaxies are members of the Shapley Super Cluster. The histogram on the right is there to highlight the bi-modality of the distribution of the kinematic misalignment angle. Taken from Krajnović et al. (2018).
The result is interesting as it points to a very specific formation scenario for such massive galaxies. Numerical simulations indicate that prolate-like rotation can be created in a merger of massive and similar size (and mass) galaxies that are mostly gas free (no new stars are made), when they are on a special trajectories: sort of a head on collisions of "trucks", in space. Our finding, therefore, are consistent with the idea that the most massive galaxies are made in such gas free major mergers. But there are some open questions, for example: why only half of the most massive galaxies and why only half of the BCGs (see Fig. 4) show such kinematics?
To finish up this post, here is also a collage of the MUSE velocity maps for the 25 sample galaxies. The red ellipse marks the half-light radius of these systems, while the green and brown lines indicate the orientation of the kinematic and the photometric axes (when they are at an angle of close to 90 degree, we are seeing the prolate-like rotation.) These data have much more in them, and I am hoping for more similar surprises in future work.
Figure 5. A collage of the mean stellar velocity maps of the M3G sample galaxies. Galaxies are divided in two groups, the BCGs and SAT (non-BCGs). BCGs are plotted in the first 14 panels, starting from top left, followed by SAT galaxies. Both groups are ordered by the decreasing brightness (in K-band). The values in the lower right corner of each panel indicate the range of the velocities, where the negative values are shown with blue color and positive with red. Taken from Krajnović et al. (2018).