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 next generation 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: Two channels of supermassive black hole growth (past highlights can be found here):
Suppermassive black holes are large dark objects residing in centres of galaxies. They are likely related to stellar black holes, which are the end products of the evolution of very massive stars, but the formation and evolution of supermassive black holes is still not well understood. They are called "supermassive" as their masses are well above million solar masses, up to tens of billions solar masses. There are also "supermassive" black holes with somewhat lower masses, and there is an expectation that also "intermediate" mass black holes exists (their masses would be of the order of tens of thousands solar masses), but these are not yet securely detected (a nicely controversial field to work on!). The supermassive black holes (I'll refer to them as SMBH from now on), next to their sole existence, have another puzzling property: their masses seem to be in proportion to the masses, and a number of other properties, of galaxies in which they live. The correlation between the SMBH masses and host galaxies are often called black hole scaling relations. These are very exciting as they connect objects which have very different sizes. SMBH, although very big, are comparable with the Solar System in size, perhaps 10 times as big. Galaxies, however, are very large objets. Their size is measured in 1000 parsecs (where 1pc ~ 3e13 km). So, there is something like 6-7 orders of magnitude in the size difference between SMBH and the host galaxy. Nevertheless, they seem to know of each other: large, bright and more massive galaxies have more massive SMBH (see Figure 1).
Figure 1. The tightest black hole - host galaxy scaling relation, relating the mass of SMBH and the velocity dispersion within an effective radius for a sample of nearby galaxies presented in van den Bosch (2016). Yellow symbols are early-type galaxies, while green symbols are spirals. Squares are galaxies with new SMBH mass estimates from a paper still to be published (Krajnović et al. 2018). The solid line is the best fit relation (from van den Bosch 2016), dashed and dotted lines show the scatter around the mean relation.
A simple way to explain this connection is that an SMBH and its host galaxy influence each other during their evolution. In other words, each "know" how the other grows. They could grow synchronous, or SMBH could grow faster, with galaxy catching up, or the other way around. This is one of the fundamental questions in modern astrophysics, especially, as the SMBH, in the form or Active Galactic Nuclei (AGN), can radiate huge amounts of energy, creating jets of plasma that pass through galaxies and illuminate the intergalactic space. This makes them as potential sources that regulate the growth of galaxies. But, what is meant by the "growth"?
Like humans, SMBH grow by eating. Black holes accrete matter via their strong gravitational fields. As the matter falls in the SMBH, it also forms an accretion disc around it, and the friction and complex interplay of friction, plasma physics and magnetic fields create the AGN phenomena. The more material falls in the SMBH, the more energy will be released, up to a point where the energy that is begin released starts blowing away the material (this is called Eddington luminosity, after Arthur Eddington who derived it to explain the physics of stars). So, SMBH can grow by "feeding" of the ambient material (gas and dust), but only until they blow it out.
Galaxies grow by increasing the number of their stars; they need to convert gas into stars. Similarly to SMBHs, they need to "capture" gas, which can come from another galaxy, or from what is called a "cold stream", a stream of gas from intergalactic space. Once captured, the gas has to cool and compress into giant dense clouds where the conditions are right for star formation. The more gas, the more stars can be made, but once stars are ignited, their radiation can heat the rest of the gas, evaporate the dust and stop further star formation.
This is why people making cosmological simulations like SMBHs and AGNs: they provide a lot of energy which can be used to stop galaxies forming stars, and evolve galaxies "passively" (without making new stars) the way observations are suggesting. The "tightest" black hole scaling relation (meaning the data points scatter the least from the mean relation) is between the SMBH mass and the "velocity dispersion" of the host galaxies (Figure 1). This velocity dispersion is a measure of the total binding energy of the galaxy. The way it is measured (by taking spectra within a certain large area of the galaxy) it actually includes both the ordered and random motion of stars and in this way it traces the gravitational potential as well as gives an estimate of the kinetic energy of the system. This is useful as it can be related to the Eddington luminosity and the energy feedback from the active SMBH which regulates the rate at which a galaxy can form stars and grow. So far so good, but the devil is in the details.
First problem is that very massive SMBHs (10 billion solar masses!) are found to exist when the Universe was something like 800 Myr old. These high redshift quasars (very luminous AGNs) pose problems how such big black holes are made in such a short time since the Big Bang. This is a fascinating problem (have a look at this semi-popular article by Smith, Bromm and Loeb for possible solutions), but I'll not go into this further.
The other problem is that it is actually very difficult to find direct observational evidence for the connection between SMBH activity (AGN) and the suppression of star formation. And finally, the black hole scaling relations do not seem to be exactly universal, in the sense that all types of galaxies follow the same scaling relation. There are indication that different types of galaxies follow different type of scalings, that some galaxies are more strongly correlated than the others, suggesting SMBH influence different galaxies in different ways. (And not all astronomers agree on how one should interpret these varied scaling relations. For example, here and here are two recent summaries of the SMBH scaling relations, with not exactly the same conclusions.)
And then there is another channel of growth for galaxies: they can collide and merge. When this happens in galaxies without any gas, new stars can't be born. If the initial galaxies have similar masses, the final galaxy will then be twice as massive as either of the progenitors. And if they had SMBHs (as it seems is the case for every massive galaxy), their SMBHs will also merger, and again just double the mass. The interesting thing is, however, that when such a mergers between galaxies happens, because stars actually don't collide, the velocity dispersion of the final galaxy will remain the same.
Figure 2. Mass-size diagram, a non-orthogonal projection of the virial mass plane, for galaxies with measured SMBH masses. The mass of the galaxy is calculated from the total K-band luminosity of the galaxy, while the size is also measured in the K-band. Both quantities come from the 2MASS Observations, but the final numbers are obtained using various relations and values from van den Bosch et al. (2014), van den Bosch (2016) and Cappellari (2016). The diagonal lines are lines of constant velocity dispersion, and the red line is the so-called "Zone-of-Exclusion", beyond which galaxies are not normally found. Colour scale shows the mass of the SMBH. These values are adaptively smoothed using LOESS method (Cleveland & Devlin 1988). Note how along a dashed line the colour of the symbols is constant. This seems to be breaking once the mass of the galaxy M* > 2×1011 MSUN, when along a velocity dispersion line there seem to be a change of colour from orange to red, or red to white. Investigating this deviation, subtle as it is, is the topic of this "astro highlight" and this paper.
So, both the galaxy and SMBH mass doubles, but the velocity dispersion does not change. This implies that at some point, the relation between SMBH mass and the galaxy velocity dispersion, the tightest known black hole scaling relation, should break down. What are the evidence for this. Well, they are not rally clear, but its has been known for some time now that very massive galaxies, those that could expect to go through such mergers, actually have black holes with masses which often deviate from the predicted scaling relation. The problem is that there are not so many such galaxy-SMBH systems: they are simply rare in the nearby Universe, where we can measure their black hole masses. Nevertheless, we went to investigate this a bit further and checked where do galaxies with measured SMBH masses fall in the mass - size diagram (Figure 2).
In this diagram the lines of constant velocity distribution are sort of diagonal (as mass and the galaxy size are linked via the velocity dispersion through the Virial Theorem, and galaxies seem to obey it). The colours indicate the measured black hole masses, smoothed using a statistical technique called LOESS (for some practical details have a look here). And indeed, if you are a galaxy with mass less then about 2e11 Msun, your black hole will sit where is it expected based on your velocity dispersion. But once you are a more massive galaxies, the black holes don't seem to follow this trend.
This can be explained using a simple toy model (Figure 3) where the growth of black holes has two channels. For lower mass galaxies, SMBHs grow through accretion of gas, and they regulate the growth of the host by energy feedback. This is why SMBH masses correlate with the velocity dispersion of galaxies. For higher mass galaxies, which do not have any gas, and their SMBH can't grow by accretion, nor galaxies can grow by star-formation, the channel of growth is different: they grow by merging. This, however, changes the dependence of the scaling relations, and SMBH mass does not correlate anymore with the galaxy velocity dispersion.
Figure 3. A toy model simulation of SMBH mass on the mass-size plane (left-hand panel), and the same model with over-plotted LOESS-smoothed black hole masses (right-hand panel). The toy model SMBH is based on a simple prescription in which SMBH masses are calculated based on the black hole mass - velocity dispersion relation for galaxies less massive than 2 × 1011 MSUN, and on a black hole mass - velocity dispersion relation modulated by the galaxy mass for higher mass galaxies. The colour scale of the model is limited to the same range as the LOESS-smoothed black hole masses of our galaxies, as shown on the colour bars. The red solid lines show the Zone-Of-Exclusion. Diagonal dashed lines are lines of constant velocity dispersion.
The model works well qualitativey, as can be seen from the right-hand panel of Figure 3, but the sample is still small. This is of course good, as to confirm/disprove the proposed conjecture, we need more data and more telescope time! Crucially, these usually also generate more ideas. For the moment, however, this model is also consistent with our understanding of the evolution of massive galaxies. The most massive galaxies, those that reside in the centres of groups of clusters of galaxies, are devoid of gas and can't grow via star formation. They however, capture smaller satellite galaxies, and by merging with similar size galaxies (this happens rarely, but it seems that is does happen often enough to be a factor). Such mergers of massive galaxies don't only change the black hole scaling relations, but also the internal structure of galaxies and the way stars orbit in them. So, for example, lower mass galaxies have ordered rotation, similar to those of spiral (disk) galaxies, but the most massive galaxies are all rotating very slow, and show very irregualr motions of stars (have a look at a few past "highlights"). Or, the most massive galaxies typically have something astronomers call a "core", a nuclear region where there is less light (less stars) than expected. The only model we have at the moment that can explain such cores are actually mergers of SMBHs, that kick the surrounding stars in the process of spiralling in. We haven't seen such a merger yet, but gravitational waves should find it, when the instrument become sensitive to it.