Davor Krajnović

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


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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....


An astro highlight: Mass assembly of NGC3115 (past highlights can be found here):

One of the standard galaxies to target with new instruments is NGC3115, and inevitably we observed it with MUSE during the commissioning. It is a large galaxy on the sky, with a half light radius of about 44 arcsec, and as such it was used as a test of making a large mosaic with MUSE without using an optimal strategy for the sky fields (these are necessary to take out the contribution of the Sun spectrum from the object) and general calibrations. It was envisaged as a quick and dirty try on a target with lots of available imaging to test MUSE image quality and mosaicing capabilities. Seeing the data in 2014, we decided that they are worthy of more then just a small performance test for MUSE. The results is a wonderful paper led by Adrien Guérou with a title: "Exploring the mass assembly of the early-type disc galaxy NGC3115 with MUSE".


Figure 1. Image of NGC3115 and the 4 MUSE pointings mapping the disc.

NGC3115 was observed with 4 adjacent MUSE pointings tracing the extent of its disc (Fig. 1). To maximise the coverage, MUSE was oriented such that the disc was along the diagonal of the square(ish) field-of-view, hence the toothy pattern of the data. The quality of the data are best seen in Fig. 2, which shows the stellar kinematics, from top to bottom: the mean velocity, the velocity dispersion, and the higher order moments of the line-of-sight velocity distribution. The stars in this galaxy rotate fast and in a regular manner. There is a rather thin disc (also visible on the images), but the rotation is present above the disc as well. It seems the whole galaxy is spinning rapidly.


Figure 2. Maps of the first four moments of the line-of-sight velocity distribution: (a) the mean velocity, (b) the velocity dispersion, (c) the third order Gauss-Hermite moment, and (d) the fourth order Gauss-Hermite moment. The colour bar and the numbers around it give the range of the values on each map. Note the thin and fast rotating structure in the mid-plane of the galaxy, corresponding to the optical thin disk. The velocity dispersion is small in the disc and rises only in the very central region.

MUSE can give us more than just the motions of stars. We can also investigate the chemical properties of the stars, and in comparison with models of stellar populations (and stellar evolution) can give us a decent handle on how old are the stars. This is called the star formation history and it is shown in Fig. 3., where Adrien separated stars by age in six panels. On each you can see the distribution of stars of a certain age (as written in upper left corner). Each coloured dot on these panels represent a region in NGC3115 where the total mass of stars (in solar masses) of a particular age is as given by a value that can be read of the colour bar on top (and the numbers in the lower left corner). There are three important conclusiones here: 1) most of the stars are very old, born within the first few giga years of the age of the Universe. But, 2) stars have been forming in this galaxy ever since, not much, but continuously, and 3) they were born in the disc like structure. The distribution of stars of different ages in different structures is a very interesting results. It shows that NGC3115 is made of multiple structural components (which is also suggested by the imaging and kinematics), and it seems these different components can be age tagged, enabling to date their formation and therefore follow the (structural) evolution of this galaxy.


Figure 3. Maps of the stellar mass distribution in NGC3115. These maps show the present day stellar mass distribution as a function of the (mass-weighted) age. Each pannel shows the mass in stars of a certain age (as indicated in the upper left corner), thile the total stellar mass within a given age range is given in the lower right corner. The difference in the distribution of the stellar mass between panels (e) and (f) suggest a different type of mass assembly: for age>12 Gyr the distribution is more centrally concentrated and therefore it can be linked with a dissipative process (such as a collapse of a gas cloud). Panles (a) to (c) suggest an extended star formation within the disc of NGC3115.

The last figure in this highlight shows what can be found out when you combine both ages of stars and their chemistry (or as astronomers like to call it: metallicity, which measures the fraction of say iron with respect to hydrogen). The metallicity is an important tool as metals (all elements except hydrogen and helium) are only made in stars or supernova explosions, and, hence, more metal rich the stars were born from material that was already pre-processed in stars that are no more. Fig. 4 shows that young and metal rich stars are found in the thinnest structure (how much is something thin or thick depends how far it extends above the midplane of the galaxy), the thin disc, which is also most rapidly rotating (Fig. 2). Young but metal poorer stars are born in a thicker component. This components is similar to the thick disc of the Milky Way, a still rapidly rotating structure, in which stars have drifted away from their original birth place within the thin mid-plane of the galaxy due to mutual gravitational interaction. Finally, there are also old stars of different metallicity. Those that are poor are in an extended spheroidal structure that surrounds NGC3115, while those that are metal rich are in the very centre of the galaxy.


Figure 4. Stellar mass fraction of NGC3115 in four bins of age and metallicity pairs: young (age<12 Gyr), old (age>12Gyr), metal-rich ([Z/H]=0,2) and metal-poor ([Z/H]<0.2). The old and metal-poor stars in NGC3115 are mostly located in the spheroid, while the young and metal-rich stars are mostly in the disc.

These fascinating MUSE data can be used to piece a picture of the formation and subsequent evolution of NGC3115. The galaxy was likely created some 1-2 Gyr after the Big Bang (redshift of ~3), from a collapse of a gas cloud. Most of stars were born in that event. Then over next 1-2 Gyr (to redshift of about 2) this proto galaxy went through a few mergers with other similar systems, containing already chemically enriched gas. Finally, over the last 10 Gyr, NGC3115 was evolving passively, meaning without any large galactic collision with a similar size galaxy (although accretion of smaller galaxies were possible at larger radii beyond the MUSE data). This evolution was dominated by the so-called secular processes: the stars age and die and new stars are born from the stellar material (and not some new gas from outside of the galaxy), while structure of the galaxy is shaped by processes associated with disc instabilities such as bars and spiral arms (you'll have to look at Adrien's paper to check out those).


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