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

25 February 2011

The Delay of Population III Star Formation by Supersonic Streaming Velocities

Greif, Thomas; White, Simon; Klessen, Ralf; Springel, Volker

The authors take into account the streaming velocities between gas and dark matter in their simulations, and present results on how this affects the virialization of minihaloes and hence the star formation rate of Pop III stars.

24 February 2011

Dissecting the size evolution of elliptical galaxies since z~1: puffing up vs minor merging scenarios

Authors: Ignacio Trujillo, Ignacio Ferreras, Ignacio G. de la Rosa

The authors discuss the size-evolution of early-type galaxies over cosmic time and discuss what can be learned from that in terms of the assembly history of early-type galaxies.

17 February 2011

X-ray mass proxies from hydrodynamic simulations of galaxy clusters (paper I)

Authors: Fabjan, Borgani, Rasia, Bonafede, Dolag, Murante & Tornatore

Two sets of zoom hydrodynamical simulations of galaxy clusters is used in this work to test the robustness and evolution of scaling relations between the total cluster mass and 3 mass proxies: the gas mass, the temperature of the intra-cluster medium and the product of the two: YX = MgasT. The largest set of about 140 galaxy clusters offers the opportunity to statistically test the intrinsic scatter of the scaling relations between the different proxies. A second smaller set allows to quantify the robustness of those relations against the effects of changing the physical processes included into the simulations. The following mechanisms have thus been compared: (i) thermal conduction, (ii) artificial viscosity, (iii) cooling and star formation, (iv) galactic winds, and (v) AGN feedback.
As found in the previous study of Kravtsov et al. (2006), the authors confirm that the relation with the YX parameter is the least sensitive to the variation of the ICM physics and stay lose to the predictions of the self-similar model along its redshift evolution.

Chromospheres in Metal-Poor Stars Evidenced from the He I 10830 Angstrom Line

Authors: Takeda & Takada-Hidai
Link to paper:

Observations are presented which reveal the presence of hot chromospheres in extremely metal-poor stars.  This raises a challenge to the conventional view that old low-mass stars should have little chromospheric activity as a result of magnetic braking (which slows the rotation of the star, and hence could shut off the dynamo process that produces the magnetic activity which, in turn, powers the chromosphere).  Interestingly, this result also strongly suggests that old metal-free (Pop III) stars may also exhibit substantial chromospheric activity.  If this is the case, then these stars should launch a weak solar-like wind, which we have found (Johnson & Khochfar 2011) would likely prevent the pollution of their surfaces by ISM material.  This provides reason for optimism about the possibility of detecting, unequivocally, Pop III stars in the Galaxy today, if such low-mass primordial stars formed (as suggested by recent high-resolution cosmological simulations; Clark et al. 2011, Greif et al. 2011).

11 February 2011

The Halo Occupation Distribution of Black Holes

Authors: Colin DeGraf, Matthew Oborski, Tiziana Di Matteo, Suchetana Chatterjee, Daisuke Nagai, Zheng Zheng, Jonathan Richardson

This paper investigates the halo occupation distribution (HOD) of black holes within a hydrodynamic cosmological simulation that directly follows black hole growth. Functional fits are derived for the number of BH per halo mass,  the conditional mass function of BHs, and the radial distribution of BHs.

Synthetic Observations of Simulated AGN Jets: X-ray Cavities

Authors: Peter J. Mendygral, Sean M. O'Neill, Thomas W. Jones
Link to paper:

X-ray cavities in galaxy clusters are now commonly used to determine the mechanical energy input from radio sources. The authors provide a detailed assessment of these observational methods by computing realistic X-ray emission maps for the simulated sources, determining the cavity enthalpy as done for real observations and comparing the results to the known and directly measureable properties in the simulation. Determined cavity enthalpies are typically within a factor of 2 of the simulation values; cavity ages seem more vulnerable to projection effects and problems of the methods, in particular the buoyant rise time.

4 February 2011

Clustering of HI galaxies

Clustering of HI galaxies in HIPASS and ALFALFA
Authors: S. S. Passmoor, C. M. Cress, A. Faltenbacher

This paper studies the clustering of HI galaxies. It presents
results for the correlation functions, and compare them to dark-matter.
This gives constraints on galaxy formation and its relation to dark-matter.