Davor Krajnović

Leibniz-Institut Für Astrophysik Potsdam (AIP)
An der Sternwarte 16
14482 Potsdam
Tel. : +49 331 7499 237
Fax. : +49 331 7499 429

publicationsCV and publications





Astronomy linksAstronomy links

Welcome to my web pages. More about my research, CV, publications, my astronomy (free) software, and other interests, you can find following the links on the left.

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....

Here is a highlight of my recent work based on the MUSE Science Verification data obtained at the end of June 2014 (past highlights can be found here). It comes from a paper by Eric Emsellem (ESO), Marc Sarzi (University of Hertfordshire) and myself (Emsellem, Krajnović & Sarzi, 2014, MNRAS, accepted). It spawned press releases in Germany and the UK, and it featured as a Picture of the Week by ESO.

Multi Unit Spectroscopic Explorer, or MUSE for short, is a next generation instrument on ESO's Very Large Telescope situated on Cerro Paranal in Chile. It was installed at the beginning of 2014 on UT4 and we were commissioning it throughout the spring. The first light and some of the commissioning data can be seen on this ESO press release. The standard procedure with the new instruments, before they are offered to the astronomical community, is to demonstrate their scientific capabilities. This is done during the Science Verification (SV) observations. The idea is that astronomers devise programmes which will test the instrument and all the tools that are required to use it, and bring some exciting science. Once MUSE was successfully commissioned, ESO announced a call for the SV proposals, selected a bunch of sound looking ideas and started observing in June (the SV run was about 2 weeks long, split between late June and mid August). The catch with the SV data is that they become immediately public and anyone in the world can use them.

We observed one of the biggest nearest galaxies. Its name is Messier 87 (M87 for short), or NGC4486 (if you prefer the New General Catalogue of Nebulae and Clusters of Stars of John Dreyer), and it is in the centre of the Virgo Cluster of galaxies, some 54 million light-years away. Galaxies like M87 are fascinating from many aspects. They are the largest galaxies, reside in the centres of galaxy clusters (bottoms of their gravitational wells), consists of old stars, have very massive black holes in their centres, which are typically very active, shooting out plasma jets and steering the inter-cluster medium very far beyond the extent of their host galaxies. Galaxies like M87 are often considered the end products of galaxy evolution, but the question still is: how have they become what they are now?

This is a copy of Fig. 2 from Emsellem, Krajnovic & Sarzi (2014). The first panel shows the SAURON velocity map, similar to the one presented in Emsellem et al. (2004), but now binned to a much higher signal-to-noise ratio (of 300, instead of 60 in that paper), which suggested that there is indeed some rotation in the central parts. The second panel presents the MUSE velocity map, and the third panel is a kinemetry reconstruction of the features on the middle panel, highlighting the KDC and the prolate-like rotation of the outer body. The color-bar on the right describes the rotation pattern: red are those regions that are receding from and blue are those that are approaching the observer. Green are the regions with no-net motion. The recession velocity of M87 was subtracted. A dashed magenta line marks the photometric major-axis of M87 (at large radii). The central 2" were masked as these kinematics are significantly influenced by the AGN (having very broad and strong emission-lines). North is up, East is left.

The first time I saw a map of stellar velocities of M87, the one coming from SAURON integral-field spectrograph observations, but not as good as the one shown on the left most panel on the figure, I was puzzled: why are stars not moving? (Have a look at the collage at the bottom of the page of stellar velocity maps typical of the galaxies in the nearby universe). The answer is that they are moving, actually very fast (often faster than a million km/h), but they do not move in a coherent way. Basically every star is moving in a direction different from that of the other stars and the mean motion observed at each point (what is plotted in the figure above) is very low. There was, however, something odd in the SAURON velocity map of M87 and we decided to have a second look at the centre of this galaxy, with an instrument of better characteristics. MUSE has a larger field-of-view than SAURON, covering about 1 arcmin x 1 arcmin on the sky. It also has a beter resolution, both in terms of probing smaller spatial scales (one square pixel is 0.2 arcseconds on a side), as well as in the resolving power of the spectrograph. Basically, with MUSE one can distinguish better than with SAURON between the various velocities of the stars. The improvement can be seen on the middle panel of the figure below.

The MUSE velocity map of M87 (middle panel above) is somewhat larger than the previous SAURON map. Notably it adds information in the top-left and bottom-right corners, or, in astro terms, in the North-East and South-West regions. This part, which was not covered by SAURON, is actually very important, as it shows that stars in M87 do coherently rotate about the center of the galaxy (evidence of this could also be seen in this paper). Another unexpected finding brought up by MUSE is that the stars in the very centre also rotate coherently, but their rotation is distinct from the large scale rotation (seen in the corners). It is distinct in two senses: the two rotations are not spatially connected and are around different axes, with some 140 degrees difference between them. The axis of the outer rotation is actually close to be aligned with the major axis of the galaxy (dashed magenta line above), the main symmetry axis which traces the orientation of the galaxy. There are very few galaxies that show this kind of rotation, often described as being "prolate-like", as a body with a prolate symmetry is expected to exhibit such a rotation.

Similar structures are know to exist, typically in massive elliptical galaxies. They are called "kinematically distinct cores" (KDCs). What makes this kinematic configuration even more surprising is that neither of the axis of rotation coincides with the minor axis of the galaxy. That kind of configuration is rarely found, and it very strongly suggest that the body of M87 is of a triaxial shape. As a matter of fact, now that we have detected the coherent rotation, albeit low, we can have some hope in determining the actual shape of M87. Before, with essentially zero net rotation, many models of very different shapes could be made to describe M87 (its light distribution and no rotation), but to hardly any one could attach a decent significance. Now, we have a possibility to change this.

To summarise, MUSE observations of M87 revelad that this central galaxy in the Virgo cluster has a very complex kinematic structure, comprising of an outer prolate-like rotation and a central KDC. The two rotation patterns seem distinct and with a mutual orientation of about 140 degrees. How does one build a galaxy like this? M87 is a product of a long history of galaxy mergers, some of which were between galaxies of smilar masses and most of which were between a big galaxy (M87) and many small satellites. By knowing the actual shape and kinematics, by knowing the chemistry and age of the stars, but also the properties of globular clusters and surrounding satellite galaxies, we may start deciphering the colourful history of M87.

And MUSE is a really impressive instrument!