Max Planck Institute for Astronomy - Annual Report 2005

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46 II.8 Dynamics, Dust, and Young Stars Computer Simulations of Merging Galaxies During close encounters or collisions of galaxies huge tidal forces occur that whirl up and compress the dust and gas in the galaxies. This induces a steep increase of star formation in dense dust filaments that absorb a major part of the galaxy light. In some nearby galaxy pairs this process can be studied in detail. In distant stellar systems, however, this is impossible. Here, one has to infer the star formation rate from luminosities in certain wavelengths ranges averaged over the entire galaxy. With the help of extensive computer simulations, a theorist at MPIA and her colleagues showed how the star formation rate, dust absorption, and observed appearance of the galaxy change during a merger. They derived an analytical formula based on the simulations, which can be used to predict the dust absorption. These results will considerably improve galaxy evolution models. While stars in the Milky Way virtually never collide, galaxy collisions, by comparison, are rather frequent. In fact, they are a central mechanism of galaxy evolution. The most famous example of two merging galaxies are NGC 4038 and NGC 4039 at a distance of 63 million light years, jointly called the Antennae (Fig. II.8.1). In the collision zone, huge clouds of dust are stirred up. In many places the collision caused the clouds to condense and to form new stars. In this kind of galaxy interaction, the star formation rate can increase from a few solar masses per year to 30 to 50 solar masses per year; for a short time the rate can be even higher. Fig. II.8.1: The Antennae in an overall view (left) and imaged with the hubble Space Telescope in detail (right). (Photo: B. Whitmore, naSa/eSa).
SFR [M � /a] 80 60 40 20 0 0 0.5 1 1.5 Time [Gyr] In these starburst galaxies, the most massive and luminous stars dominate the radiation in the UV and blue spectral range. But at the same time, dust clouds absorb a large fraction of the light. Therefore the regions of the most intense star formation may not be visible at all in the UV or optical. But these young stars heat the dust, which then glows in the infrared. These galaxies then appear as ultra luminous infrared galaxies (ulirg) with total luminosities of more than 10 12 solar luminosities. The peak of the dust emission is at around 100 microns, which in distant galaxies is shifted towards longer wavelengths, so that these galaxies glow brightly in the submillimeter and millimeter wavelength range. Observationally, high star formation rates are always linked to the presence of dense dust clouds. In numerous simulations, theorists have tried to account for this effect in order to be able to interpret the observations or to support simulations of galaxy evolution. However, previous attempts either neglected the presence of dust or else assumed very simplified geometries for the galaxies, which clearly could not apply to the complex merging systems. This new work uses a full radiative transfer model within detailed hydrodynamic simulations, so that for the first time astronomers can study the detailed appearance of merging galaxies at many wavelengths, including the effect of dust scattering, absorption, and re-emission. Another improvement in these simulations is a better treatment of the feedback of energy into the gas from supernovae with respect to previous work. The simula- II.8 Dynamics, Dust, and Young Stars – Computer Simulations of Merging Galaxies 47 2 2.5 3 Fig. II.8.2: Star formation rate as a function of time for the merger of two Milky Way-like Sbc galaxies. The small figures illustrate the distribution of the gas at various stages of the merger event from a viewpoint perpendicular to the orbital plane. tions assume an empirical relationship between the gas density and the resulting star formation rate. In addition, the simulations track the production of heavy elements (»metals«) produced by supernovae. The goal of this work was to study the fraction of light absorbed by dust as a function of physical properties of the galaxy such as mass, gas fraction, metallicity, and star formation rate. The starting point of the simulations is models of individual galaxies with properties chosen to match spiral galaxies in the nearby universe. They span a range of properties such as mass, gas fraction, and metallicity. These galaxies are then set up on a parabolic orbit and allowed to collide with one another. A large number of simulations was performed in order to explore variations due to the galaxy orientation and orbit. During the interaction, the distribution of stars, gas, and dust was determined at 50 different points in time and used as input to the detailed radiative-transfer calculations. This procedure produced images of the galaxies at 22 wavelengths between 21nm and 5 mm and from eleven different viewpoints. The radiative transfer code is based on a »ray tracing« method, in which »photon packets« emitted by stars are followed as they are scattered or absorbed by dust grains, and then re-radiated at longer wavelength. Figure II.8.2 shows an example of the evolution of the star formation rate during the merger of two gas-rich spiral galaxies viewed perpendicular to the orbital plane. The small figures illustrate the distribution of the gas at various stages. Tidal forces compress the gas, inducing a high star formation rate during the first close passage. These forces become weaker as the galaxies separate, Fig. II.8.3: Time evolution of the bolometric luminosity and the fraction of it attenuated by dust (upper curves). The variation of the UV and blue-band fraction is smaller for each of the eleven viewing angles (lower curves) considered. Lominosity [L � ] 10 12 10 11 Bolometric Absorbed Not absorbed 10 10 0 0.5 1 1.5 Time [Gyr] 2 2.5 3
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Page 5: Contents Preface ..................
Page 8 and 9: 6 I. General I.1 Scientific Goals U
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