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 interests, CV, publications, my astronomy (free) software, and other interestes 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 stellar kinematics data (kinematics extraction was presented in Cappellari et al. 2011 and velocity maps in Krajnović et al. 2011). 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 science team of MUSE, a next generation integral-field spectrograph for the VLT.

Here is a highlight of my recent work based on ATLAS3D data (old "highlights" can be found here). These two diagrams are from Krajnović et al. 2013 (MNRAS, 433, 2812K) and with colours and symbols show the nuclear shape of the radial surface brightness profiles in the λR - ε space of nearby early-type galaxies. (λR and ε respectively measure the specific angular momentum of stars and the apparent shape of galaxies within the half-light radius). The shape of the light profiles can be related to the formation of galaxies. They either continue increasing at the last resolution point (typically about 10 pc for galaxies in this study) or they turn downwards at some point and stay flat until the last observable point (so, it is critical to use Hubble Space Telesope observations). The nuclear regions in galaxies where the light profile turns down and stays flat are called "cores". The other type of light profiles, those that keep increasing at the smallest spatial scales we can probe, are sometimes called "core-less" (or power-laws). The idea is that cores are made by massive black holes which are bound in a bianry system and kick far out all stars that cross their paths. These black holes are brought together in a merger of two galaxies of simular sizes. However, if there is a lot of gas present during the merger (i.e. brought by one of the galaxies), even if cores are created by the massive black hole binary, new stars will be made from the gas and the light profile will be core-less.

There are several ways to determine which galaxy has a core or a core-less profiles. One can, for example, fit an outer piece of the light profile with a function (such as the Sersic 1968 profile) and then estimate how much does the nuclear light profile deviate from this outer fit (e.g. Kormendy et al. 2009), in other words shows an "excess" (e.g. core-less) or a "deficit" (core). Anotehr way is to fit a more specific function, such as the so-called core-Sersic function (Graham et al. 2003). In this work, we used the so-called "Nuker-law" (Lauer et al. 1995), which is a double power-law function meant to be applied only to the inner regions of the light profiles (not on the full galaxy). Each method gives somewhat different results (as it measures different things), but, as we show in the paper, they are not very different.

The two diagrams above show the angular momentum versus the ellipticity for 260 ATLAS3D galaxies. On both panels, small open symbols are galaxies with no available HST observations (we do not have data to investigate their nuclear light profiles) and filled small symbols are galaxies for which the classification was not possible (it is uncertain due to significant presence of nuclear dust which masks the central light). Also on both panels, colours of symbols indicate the class of the nuclear profiles: red -- core, blue -- core-less. The green solid line separates fast from slow rotators (divsion of early-type galaxies into slow and fast rotators is described in Emsellem et al. (2007) and Emsellem et al. (2011) papers). Left: Core galaxies are shown with squares and core-less galaxies with circles. The grid of dashed and dotted lines show the region where one can expect to find galaxies that are oblate and axisymmetric, and of certain anisotropy in the distribution of velocities (more details can be found in the paper by Cappellari et al. 2007). Right: Shapes of symbols indicate the kinematic group (Krajnović et al. 2011): group a -- non-rotating galaxies, group b -- featureless non-regularly rotating galaxies, group c -- kinematically distinct cores, group d -- 2σ galaxies made of two counter-rotating discs, and group e - regularly rotating (disc-like) galaxies. Kinematic classification is not provided for galaxies with no HST data. The contours show the distribution of a family of oblate objects with an intrinsic shape of εintr= 0.7 ± 0.2 (from Emsellem et al. (2011)).

Conclusion: fast rotators are typically core-less while slow rotators are cores, but there are significant exceptions to this rule, with an implication that either the accepted formation of cores may not be the only path for their formation, or that there are some additonal core perservations mechanisms at play. Perhaps the most interesting finding is that there are some core-less slow rotator galaxies (blue symbols below the green line), pointing to more complex formation of these galaxies (i.e. formation in gas rich mergers). Unfortunately, a significant number of slow rotators which could be in this regime has not HST imaging. This will be changed soon, as we were awarded HST/WFC3 observations during the Cycle 21 of 12 galaxies in this region, which will allow us to determine the formation paths for these galaxies.