Journal Article

On the algorithms of radiative cooling in semi-analytic models

Yu Lu, Dušan Kereš, Neal Katz, H. J. Mo, Mark Fardal and Martin D. Weinberg

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 416, issue 1, pages 660-679
Published in print September 2011 | ISSN: 0035-8711
Published online August 2011 | e-ISSN: 1365-2966 | DOI: http://dx.doi.org/10.1111/j.1365-2966.2011.19072.x
On the algorithms of radiative cooling in semi-analytic models

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We study the behaviour of multiple radiative-cooling algorithms implemented in seven semi-analytic models (SAMs) of galaxy formation, including a new model we propose in this paper. We use versions of the models without feedback and apply them to dark matter haloes growing in a cosmological context, which have final virial masses that range from 1011 to 1014 M. First, using simplified smoothly growing halo models, we demonstrate that the different algorithms predict cooling rates and final cold gas masses that differ by a factor of ∼5 for massive haloes (≥1012 M). The algorithms are in better agreement for less massive haloes because they cool efficiently and, therefore, their cooling rates are largely limited by the halo accretion rate. However, for less massive haloes, all the SAMs predict less cooling than corresponding 1D hydrodynamic models. Secondly, we study the gas-accretion history of the central galaxies of dark matter haloes using merger trees. The inclusion of mergers alters the cooling history of haloes by locking up gas in galaxies within small haloes at early times. For realistic halo models, the dispersion in the cold gas mass predicted by the algorithms is 0.5 dex for high-mass haloes and 0.1 dex for low-mass haloes, while the dispersion in the accretion rate is about two times larger. Comparing to cosmological smoothed particle hydrodynamic simulations, we find that most SAMs systematically underpredict the gas-accretion rates for low-mass haloes but overpredict the gas-accretion rates for massive haloes. Although the models all include both ‘rapid’- and ‘slow’-mode accretion, the transition between the two accretion modes varies between models and also differs from the simulations. Finally, we construct a new phenomenological model that explicitly incorporates cold halo gas and a gradual transition between the cold and hot modes of gas accretion to illustrate that such a class of models can better match the results from cosmological hydrodynamic simulations. The large dispersion in cooling rates between different SAMs influences parameter choices for other galaxy physics, including star formation and feedback. Therefore, careful parametrizations of the multimode gas cooling and accretion mechanisms in simulations are necessary to ensure that the predictions from SAMs are reliable.

Keywords: methods: numerical; galaxies: evolution; galaxies: formation

Journal Article.  15675 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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