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

28 October 2010

SMBH formation by direct collapse: keeping protogalactic gas H2 free in dark matter haloes with Tvir>10^4 K

Authors: Shang et. al.

The paper presents the estimates of J(crit) values needed for a halo to undergo direct collapse. They perform 3-D hydrodynamical adaptive mesh refinement (AMR) simulations of gas collapse in three different protogalactic halos with Tvir >10e4 K, irradiated by a UV flux with various intensities and spectra.

They then determine the J(crit) required to suppress molecular (hydrogen) cooling in each of the three halos simulated above and find that;[i] for a hard spectrum (metal free stars): J(crit) is between 10e4 to 10e5,[ii] for a softer spectrum (normal stellar population) J(crit) lies between 30 to 300.

The values are ~ 3 to 10 percent lower than previous estimates. They argue that this improved estimate resulted from a better hydrogen molecule-dissociational rate that they adopted. As seen in the Dijkstra paper (see previous post), the reduction in J(crit) exponentially increases the number of rare halos exposed critical radiation; there by preventing fragmentation and ensuing direct collapse. This might give rise to 10e5 solar mass objects at the centre of these haloes- progenitors for SMBH.

Fluctuations in the high-z LW background: close halo pairs as the origins of SMBH

Author: Dijkstra et. al.

The paper discusses the variation of the LW background required to dissociate the hydrogen molecules and prevent fragmentation of gas clouds into stars-thereby producing SMBH candidates.

They take into account the (i) the clustering of DM halos, (ii) Poisson fluctuations in the number of corresponding star forming galaxies, and (iii) scatter in the LW luminosity produced by halos. Although most of the haloes would be exposed to global mean value of the LW background, some haloes could be exposed to the high critical LW flux (Jcrit) that can prevent fragmentation.

This fraction, although low ~ 10e-8 to 10e-6 of all the haloes with Tvir>10e4 K, has an exponential dependance on the value of the flux; i.e. any small change in the value of J(crit) will exponentially affect the number of haloes that are exposed to this critical flux.

22 October 2010

Lyman 'bump' galaxies - II. A possible signature of massive extremely metal-poor or metal-free stars in z = 3.1 Lya emitters

Authors: Inoue, et al.
Link to article: arXiv:1010.2582

Observations of both Lya emission and Lyman continuum emission (the Lyman 'bump') are presented for a sample of galaxies at z = 3.1.  Besides exhibiting the Lyman 'bump' (blueward of Lya), which could be a sign of a population of metal-poor stars which have hard spectra, some of the galaxies also show a spatial offset between the Lya emission and the Lyman continuum emission.  The authors argue that the observed properties of the galaxies may be best explained by the presence of massive, metal-free stars.  Interestingly, it is the same galaxies which show the spatial offset which also appear to require the largest fractions of Pop III stars.  Indeed, this would be consistent with a scenario in which Pop III star formation may take place at relatively low redshift, albeit only at the outskirts of metal-enriched regions.

19 October 2010

Dwarf archaeology

This paper examines two scenarios for the formation of ultra-faint dwarf galaxies (UFDs), representing the formation processes of the first galaxies. It is claimed that the first galaxies are chemical "one-shot" events, where only one (long-lived) stellar generation forms after the first, Population III, SN explosions. This conclusion is established using a comparison between the stellar abundance signatures as observed in the present-day UDFs, and the results of high-resolution hydro simulations.

Dwarf archaeology: Probing the first enrichment events with low-luminosity galaxies. Authors: Anna Frebel & Volker Bromm