Davor Krajnović's Research

Current Research

My research focuses on studying nearby galaxies, where nearby is a loose term. Most of the objects I collected data on, or have published papers about, are within 50 Mpc (~150 million light years), but I am also pursuing projects where objects under study are much more distant than that. Still, for modern extra-galactic astronomy and cosmology this is indeed all within the nearby Universe, but as such it offers an end point-of-view of the galaxy evolution and the formation of structures in the Universe.

I am interested in morphological, kinematic and dynamical properties of galaxies, as well as in history of formation of stars in these objects. My goal is to understand what processes led to formation of stars and to the assembly of these stars into present day (nearby) galaxies. There are many types of galaxies in the nearby Universe: faint dwarfs, spectacular discs with spiral arms and massive elliptical giants, indicating there are many ways one can make a galaxy. The challenge is to recognise these ways, describe them and explain their physical origin.

Galaxies are made of stars (and their plantes), gas clouds and dust grains of various sizes. There are also very exotic objects: remnants of dead stars such as neutron stars, pulsars and black holes. Moreover, in the centers of galaxies there are surprisingly massive black holes: typically a million times more massive than a black hole that remains after a cataclysmic death of a massive star. These are responsible for creating spectacularly bright quasars that one can see far in the early Universe, but they can also be quietly sitting and influencing their environments through gravity only, as in our own Milky Way. Wherever and however we find them, super-massive black holes and their host galaxies seem to evolve in (a near) unison. And then, there is also supposed to be dark matter in galaxies. Wast quantities of mater that is not of the same sort as the quarks and electrons that combine to build us, but some other type of matter that, if one assumes our understanding of gravity is correct, can be detected only as an extra and non-visible mass.

I am interested in understanding the link between the super-massive black holes and the galaxies that host them, and I am working on determination of the properties of dark matter in and around galaxies. The way I am pursuing these research interests is mostly by interpreting the observations, which means: I get data, use them to constrain parameters of models and that teaches me something about galaxies. For example, to determine how massive is a supermassive black hole sitting in the centre of a nearby galaxy, I take spectra of this galaxy, covering as much as possible of its area. From spectra I can determine the motion of stars (i.e. their velocities). I also take an image of the galaxies which gives me the information on the distribution of stars. Using the information on the distribution of stars I create dynamical models. Their properties are constrained by the information on the speeds of stars (from spectra). To explain the motion of the stars (especially those close in the centre of the galaxy) one needs to guess the mass of the black hole. The mass that gives the best agreement between the velocities predicted by the dynamical model and those observed (obtained from spectra), is the mass we were looking for. Fun!

I use various telescopes (e.g. ESO VLT and GEMINI) and instruments (SINFONI and NIFS). I am especially fond of the William Herschel Telescope of the Isaac Newton Group of Telescopes on La Palma. This telescope is the home of SAURON, the best (and simplest) instrument in the world.

SAURON is an integral-field spectrograph (or unit) based on the lenslet concept. It was constructed in Lyon in 1999 under leadership of Roland Bacon and was used in the SAURON project (in which I landed at the start of my PhD). It is a private instrument, made for a specific SAURON project, but since the first light in 1999 it took more than 600 000 spectra, which results in 96 refereed publications and about 5000 citations (by September 2012).

Last few years, I am a co-Principal Investigator of an international ATLAS3D project comprising about 36 scientists in 16 institutes and 7 countries (3 continents + middle of Pacific). This project started around observations with SAURON of all early-type galaxies (ellipticals and lenticulars) visible from La Palma and not further away than 42 Mpc (135 Million light years), but it expanded into a multi-wavelength survey covering from optical to radio wavelengths, probing stars and various phases of gas (ionised, atomic and molecular) in these nearby galaxies. To get all the observations we use various facilities around the globe: Westerbork Radio Synthesis Telescope in The Netherlands, IRAM 30m single dish in France, CARMA array in California and MegaCam camera on Canada-France-Hawaii Telescope on the Big Island, Hawaii. In addition to the observational effort ATLAS3D team members do different type of numerical simulations, such as: semi-analytic modeling, cosmological simulations, binary mergers and hydrodynamic simulations. The goal of the survey is to link the various informations together and derive a benchark in the properties of nearby galaxies, which can then be compared with what is seen in the early Universe.
A recent merger early-type galaxy NGC0474. The spectacular shell-like features are a evidence of a relatively recent collision with another galaxy. Some of the features are very faint (down to 29 mag/arsec2). Image credit: P-A. Duc + CFTH, as part of ATLAS3D deep imaging campaign with MegaCam.

Can we see the influence of the supermassive black hole? The first image on the left is the map of the random motions (velocity dispersion) of stars in the central galaxy NGC0524 (a star moves under influence of the gravity exerted by the black hole and all other stars). The other three images on the right are three models with three different black holes in the centers (from left to right): the one that fits the data best, and a less and a more massive black hole. The data are good enough to put robust limits on the mass of the black hole in the centre of NGC524 to about 830 million solar masses. From Krajnović et al. 2009, MNRAS, 399,1839 and based on observations with SAURON (WHT) and NICFS (Gemini North).

A mozaic of velocity maps of 260 early-type galaxies observed with SAURON from William Herschel Telescope on La Palma. Velocity maps are ordered with increasing angular momentum λR measured within one effective radius. From Krajnović et al. 2011, MNRAS, 414,2923)   

Last modified: 17 Sept 2012