This is the astro-ph blog of the Theoretical Modelling of Cosmic Structures group (TMoX) at the Max-Planck-Institute for Extraterrestrial Physics. We are an independent Max-Planck Research Group focusing on the various aspects in the formation and evolution of galaxies. Part of our focus is on the formation and evolution of early-type galaxies, super-massive black holes, the formation of the first structures in the universe and the enrichment history of the Universe. We are theoreticians using analytic modelling as well as numerical simulations in our work.

The CosmologyCake blog is dedicated to the discussion of research papers and current developments. We will regularly post interesting papers and comment on them. Feel free to leave your comments as well. We encourage authors of discussed papers to post replies if they wish to. Our aim is to provide a platform to discuss recent astro-ph papers within a wider audience. Please feel free to send papers you would like to be discussed to us at tmoxgroup@googlemail.com.

30 May 2011

The effects of a hot gaseous halo in galaxy major mergers



Authors: Benjamin P. Moster, Andrea V. Maccio', Rachel S. Somerville, Thorsten Naab, Thomas J. Cox

This paper presents the first attempt to simulate a major galaxy merger, including a component of hot gas. Galaxy properties including SFR, burst efficiency and B/T evolution are presented for mergers with and without a hot halo and with or without winds. These are compared with the equivalent simulation of an isolated galaxy.

The most interesting results are that, perhaps unsurprisingly, winds suppress the efficiency of the merger-induced starburst, causing the enhanced SFR to be suppressed but last longer, and that a hot gas halo leads to an enhanced SFR in both isolated and merger cases, but this leads to a reduced difference between the two. More interestingly perhaps, cooling from the hot halo is suppressed after the merger due to its increased temperature and spin-up (the latter effect may relate to the unusual coplanar merger geometry). With the enhanced "background" SFR from the cooling of the hot halo, and the winds suppressing any strong star burst, the strong dependence of burst efficiency on the progenitor cold gas fraction (as widely reported by Hopkins, Cox et al.) is reduced, or even negated (though it must be said, the gas fraction is defined at the beginning of the simulation, not directly before the merger).

The paper makes an important step in the direction of a multi-phase treatment of galaxy mergers. However the restriction to one (unusual) orbital configuration means these results cannot be used to provide any empirical relation describing the results of mergers. Indeed, the complexity of including winds and hot gas cooling in simulations, and dependence on the prescription used, implies that such empirical relations will be difficult to establish for sensitive parameters (e.g. star formation). On the other hand, the final bulge mass, and to a certain extent the internal dynamics of the merger remnant, appear to be fairly robust, and dependent primarily on the already formed stars in the progenitor galaxies with only a secondary dependence on the merger gas physics.

24 May 2011

The First Massive Black Hole Seeds and Their Hosts

Link to Paper
Authors: Jillian Bellovary, Marta Volonteri, Fabio Governato, Sijing Shen, Thomas Quinn, James Wadsley

The paper attempts to investigate the haloes in which massive seed black holes (MBH) can form by looking at local properties of the halo. The MBH are put in by using an efficiency factor on the same prescription that is used to make stars. In the study, MBH refers to 10^4 SM black holes which can be considered as direct collapse remnants or Pop III remnants that grew over time to reach this mass via mergers or accretion. Accretion onto black holes that are populated in this fashion is not accounted for. The aim is to track the formation histories of MBH seeds in haloes and make predictions for future surveys on the occupation fraction of MBH in various type of galaxies.

23 May 2011

Hierarchical formation of bulgeless galaxies II: Redistribution of angular momentum via galactic fountains

Link to paper here.
Authors: C.B. Brook, G.Stinson, B.K. Gibson, R. RoŇ°kar, J. Wadsley, T. Quinn

In this paper, the second in a suite, the authors are using cosmological zoom simulations to investigate the possibility of forming a bulgeless disc galaxy. Starting with initial conditions from the McMaster Unbiased Galaxy Simulations (MUGS) project, their programme is running disc galaxy re-simulations spanning 4 order of magnitude in stellar mass. After showing in their first paper that ejection of low angular momentum material enables the formation of bulgeless dwarf disc galaxies, this works introduces the mechanism of redistribution of momentum by galactic fountain as a possible solution to the angular momentum problem for intermediate mass disc galaxies.
After presenting the evolution of the main properties of the simulated object (especially the formation of a secular pseudo-bulge via bar/disc instabilities), they study in detail the evolution of the "bulge gas" (BG) since the last major merging epoch. They find that, after expelling most of this BG by efficient SNe activity during the starburst phase, by z=0, 71% of the BG has been brought back to the disk to form stars. The authors show that during the re-accretion of the BG via the galactic fountain, the material has gained angular momentum which prevent from accumulating gas to the centre of the galaxy, and hence prevent the formation of a too massive bulge.