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Extragalactic Astrophysics &
Observational Cosmology Group
 
Lucio Mayer
Postal Address:
University of Zurich
Institute for Theoretical Physics
CH-8057 Zurich
AND
ETH Hoenggerberg Campus
Physics Department, HPT D11
CH-8093 Zurich
Switzerland
Phone: +41 (0)44 633-3280 (ETH)
Phone: +41 (0)44 635-6197 (Uni Zurich)
Fax: +41 (0)44 633-1238 (ETH)
Fax: +41 (0)44 635-5704 (Uni Zurich)
email: lucio@phys.ethz.ch
Curriculum Vitae
Education:
2001 Ph.D in Astronomy, Universita' degli Studi di Milano, Milano, Italy
1997 Laurea degree in Physics; Universita` degli Studi di Milano, Milano, Italy
Employment:
Oct 2006 -   present Assistant Professor (SNF Professorship), Institute fuer Theoretische Physik, University of Zurich, and ETH Zurich
Sep 2005 - Sep 2006   Zwicky Prize Fellow, Institute of Astronomy, ETH Zurich
2003 - 2005   Post-Doctoral Research Associate, University of Zurich, Zurich
2001 - 2003 Research Associate, University of Washington, Seattle, USA
October-January 2000 - Visiting Scientist, Max Planck Institut fur Astrophysik, Garching bei Munchen, Germany
Awards:
Sep 2005 -   Zwicky Prize Fellow, Institute of Astronomy, ETH Zurich
June-July 1999   Research Fellowship, University of Massachusetts, Amherst (USA)
Research Interests

Cosmological structure formation and galaxy formation

Stellar dynamics and galaxy interactions

Planet formation

Interstellar medium and star formation

Origin and evolution of supermassive black holes

Computational astrophysics



Research Highlights


THE ORIGIN OF DWARF SPHEROIDAL GALAXIES

Since the PhD I have been developing a new model for the origin of the faintest and most dark matter dominated galaxies in the Universe, dwarf spheroidals. Our own Galaxy and its neighbor, M31, are surrounded by a swarm of dwarf spheroidal satellites. They are almost completely devoid of gas, have a dominant old stellar population and are supported by velocity dispersion. This is in contrast with dwarf galaxies at much larger distance from the primaries, that are gas-rich, rotating disks with on going star formation. In 2001-2003 we showed that repeated tidal shocks sufferred by a dwarf similar to a dwarf irregular galaxy as it orbits the primary turn it into an object resembling a dwarf spheroidal in less than a Hubble time. Heating by tides and removal of angular momentum by induced bar/buckling instabilities in the stellar disk chenge the kinematics from predominant rotation to predominant pressure support. The process is a natural consequence of the infall of smaller galaxies towards more massive galaxies in hierarchical structure formation. The generality of the mechanism, dubbed "tidal stirring", explains why this morphology-density relation is seen in all known galaxy groups, not just the Local Group. In 2004-2006 I have been refining the model by including the effect of ram pressure. Diffuse hot halos of gas surrounding massive galaxies such as the Milky Way are a prediction of cold dark matter models, the current structure formation paradigm, and observations of the nearby Universe also strongly indicate that such halo is present around our Galaxy. I have shown that ram pressure combined with tides is able to remove most of the gas content of dwarfs infalling at higher redshift, while none of them can achieve that individually. The heating and ionizing radiation from the cosmic UV background also plays a major role at high redshift. It keeps the gas hotter and ionized, suppressing star formation and increasing gas mass loss. The preduction is thus that dSphs with the lowest gas content and oldest stars are those that entered the sphere of influence of the Milky Way or M31 earlier, at z > 2, when the cosmic UV radiation was still energetically relevant with respect to the other processes. Below we can see a sequence of images showing the gradual transformation of the stellar (left) and gas component (right) when both ram pressure and tides are included. This is taken from the latest paper on the subject which is submitted for publication to MNRAS.



COEVOLUTION OF GALAXIES AND SUPERMASSIVE BLACK HOLES

The dynamical evolution during the interaction of two systems containing central SMBHs can be divided into three main phases: (1) the two black holes sink to the center of the common mass distribution by a process called dynamical friction that slows down the relative motions of the host galactic cores and causes the SMBHs to form a pair once the two galaxies merge; (2) the orbital radius of the SMBH pair shrinks as three-body interactions between the black holes and other components of the galaxies, such as stars and gas, extract energy from the orbit; (3) the black holes come close enough for gravitational radiation to become an efficient mechanism for further angular momentum loss, causing the eventual coalescence of the pair. It is of primary importance to establish the necessary conditions leading to the merger of two SMBHs since coalescing SMBH binaries constitute the most powerful sources of gravitational wave emission that the Laser Interferometer Space Antenna (LISA) will be able to detect. To gain insight in the physical processes which determine the fate of SMBHs during galactic collisions, with my collaborators Stelios Kazantzidis from the University of Chicago, Monica Colpi from the University Milano-Bicocca, Piero Madau from UCSC, Victor P. Debattista and Thomas Quinn from the University of Washington, Ben Moore, Joachim Stadel from the University of Zurich and James Wadsley from McMaster University, performed high-resolution supercomputer simulations of galaxy mergers. The simulations employ the popular astrophysics technique of Smooth Particle Hydrodynamics in which the gaseous component of galaxies is modeled as a collection of discrete particles and include the effects of radiative cooling and star formation. One of the most intriguing findings of this study is that gaseous dissipation facilitates the process of SMBH pairing and merging by increasing the resilience of the interacting galactic cores to tidal disruption. This result supports scenarios of hierarchical build-up of SMBHs, due to collisions and gas accretion, following the merger hierarchy from early times until present. The higher SMBH pairing efficiency reported by us has interesting implications for the probability of observing coalescence events whose gravitational radiation emission would be detectable up to high redshift by LISA. A paper describing these results was published in the April 2005 issue of The Astrophysical Journal. In a complementary study that was presented in the ESO/MPE Conference ''Relativistic Astrophysics and Cosmology: Einstein's Legacy'' and appeared recently on the Astrophysics abstracts, Mayer, Kazantzidis, Madau, Colpi, Quinn and Wadsley showed that at very small scales the details of SMBH binding are extremely sensitive to gas thermodynamics. The figure presents the relative separation of two black holes as a function of time in merger simulations with different prescriptions for the equation of state (EOS) of the gas which are motivated by both theoretical models and observations of interacting galaxies. The first case (blue line) approximates well the balance between radiative heating and cooling in a galaxy that is forming stars at a prodigious rate (''starburst galaxy''). The second case (red line) pertains to galaxies, known as ''active galactic nuclei'' (AGN), the nucleus of which produces more radiation than the rest of the galaxy and which are thought to harbor SMBHs at their centers. The results of the simulations suggest that the coalescence of the two black holes will occur when the merger remnant is a powerful starburst galaxy, such as an Ultraluminous Infrared Galaxy (ULRIG), rather than an AGN.



The simulations that we performed allowed for a considerable dynamic range to be resolved in the same calculation: from scales of hundreds of kiloparsecs at which the galaxies begin their cosmic dance to scales of tens of parsecs that correspond to the sizes of nuclear disks. These simulations employ the technique of ''particle splitting'' to greatly improve the resolution of hydrodynamical computations and are among the most expensive calculations ever performed on this topic, using up to 200000 hours of CPU time each at various supercomputer centers around the world. The first results of this endeavor were presented recently on the ESO/MPE Conference ''Relativistic Astrophysics and Cosmology: Einstein's Legacy'' and a journal paper is currently in progress. With my collaborators showed that nuclear disks are produced by strong gas inflows generated by tidal torques during the merger event. These inflows can proceed to scales below 100 parsecs and slow down considerably at a scale of about 50 parsecs, forming a compact disk embedded in a larger disk of a few hundred parsecs in size. The panel below illustrates the complexity of dynamical evolution in a typical collision between two equal-mass disk galaxies. The simulation follows dark matter, stars, gas, and supermassive black holes, but only the gas component is visualized. Brighter colors indicate regions of higher gas density and the time corresponding to each snapshot is given by the labels. The first 10 images measure 100 kpc on a side, roughly five times the diameter of the visible part of the Milky Way galaxy. The next five panels represent successive zooms on the central region. The final frame shows the inner 300 pc of the nuclear region at the end of the simulation. During the interaction violent tidal forces tear the galactic disks apart, generating spectacular tidal tails, plumes and prominent bridges of material connecting the two galaxies. The ultimate outcome of a series of increasingly close encounters is the inevitable merger of the disk galaxies into a single structure and the formation of a nuclear disk as shown in the last panel. The simulated nuclear disks have masses of approximately a billion solar masses and exhibit prominent non-axisymmetric features known to produce strong gas inflows. The gas inflows are likely responsible for fueling the central black hole, but even higher resolution will be needed to study this process in detail. Nevertheless, the simulations carried out by Mayer and his collaborators provide the first direct evidence that gas originally in galaxies separated by hundreds of kiloparsecs is collected to parsec scales simply as a result of the dynamics and hydrodynamics involved in the merger.



Selected Papers