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

31 May 2012

Galactic star formation and accretion histories from matching galaxies to dark matter haloes

Authors: Moster et al (2012)
Paper: here

The authors perform a 'multi-epoch abundance matching' model (MEAM) for 0<z<4. Subhaloes are taken from the Millennium I and II simulations, and satellites are forced to have the same stellar mass as centrals at the time of infall (i.e. self-consistent treatment, but with no SF for satellites). The results are then combined with merger trees extracted from the dark matter simulations to predict the average assembly histories of galaxies, separated into star formation within the galaxies (in-situ) and accretion of stars (ex-situ).

Results: The peak star formation efficiency decreases with redshift from 23% at z=0 to 9% at z=4 while the corresponding halo mass increases from 10^11.8 Msun to 10^12.5 Msun. The star formation rate of central galaxies peaks at a redshift which depends on halo mass; for massive haloes this peak is at early cosmic times while for low-mass galaxies the peak has not been reached yet. In haloes similar to that of the Milky-Way about half of the central stellar mass is assembled after z=0.7. In low-mass haloes, the accretion of satellites contributes little to the assembly of their central galaxies, while in massive haloes more than half of the central stellar mass is formed ex-situ with significant accretion of satellites at z<2.

The sizes, masses and specific star-formation rates of massive galaxies

Authors: McLure et al, 2012
Text: here

The authors in this paper present results from recent observations based on a mass complete (M > 6*10^10 M_sun) high redshift (z~1.4) sample of galaxies. The focus of the paper is on the sizes of these galaxies and their correlation with other physical properties such as specific star formation (sSFR,) stellar ages and morphology of the galaxies. The paper shows convincingly that galaxies at z~1.4 are more compact then the local counterparts by a factor 1.6 - 2.4 depending on their sSFR. Based on simple toy models they then go on to claim, that the size-evolution is dominated by minor mergers.

A very interesting result of this work is that morphology and sSFR do not correlate strongly. The authors conclude based on this that two separate process must be responsible for the morphological transformation and the shutting down of the SFR.

The latter conclusion is still somewhat debated in the community.

24 May 2012

Runaway Stars and the Escape of Ionizing Radiation from High-Redshift Galaxies

Authors: Conroy & Kratter
Paper: here

In this paper, the effect of runaway stars on the escape fraction of ionising radiation of galaxies around redshift 10 (so during the epoch of reionisation) is investigated. Runaway stars are stars with high velocities that are able to migrate from the dense environment in which they were formed. This would significantly increase the fraction of photons produced by these stars that is able to escape into the IGM. The two main formation channels are through dynamical ejections from young stellar clusters and through the explosion of companion star. The first channel is most important, since the stars that produce most ionising photons are short-lived. The fraction of massive stars that are runaways is highly uncertain, ranging from 10% to 50%. The authors adopt a value of 30%.

To assess the enhancement of the escape fraction due to runaway stars the authors adopt a simple analytic model for the galaxies, similar to Ricotti & Shull (2000). The two models for the gas distribution are a spherical (exponential) number density profile and an isothermal disc profile. The escape fraction is computed by considering the distance and line-of-sight column density from the position of the star to the virial radius, integrated over all solid angles. The distribution of the stars follows that of the gas.

With this simple model, the authors find that for galaxies at redshift 10, the escape fraction can be higher up to a factor of 7 when runaway stars are included. The reason is that the non-runaway stars have escape fractions of less than 10% due to the dense environment of the galaxy. The enhancement peaks around a halo mass of 1e8 solar masses, because runaways travel a fixed distance inside the galaxy and higher mass galaxies have larger radii.

The authors discuss the effects of various assumptions in their model, by changing the redshift, the dust attenuation, the minimum and maximum stellar mass, the runaway velocity, the ionising luminosity function and the gas density profile. The latter had the most significant effect, with enhancement scaling different with halo mass for spherical and disc-like galaxies.

This study provides a rough first estimate of what the effect of runaway stars on the escape fraction could be, but, as the authors show, the significant dependence on the gas distribution makes the conclusions not very robust.

11 May 2012

PISN at the Epoch of Reionization

Authors: Pan, Kasen, & Loeb
Link: here

This paper discusses the observability of PISN, Core Collapse, and Type Ia Supernovae with JWST. They use light curves and spectra of the explosions using a time dependent radiative transfer code for a wide range of Pop III stellar masses and evolutionary states. One key point they argue based on the luminosity of various explosions is that for detecting large numbers of PISN, a wide rather than deep survey will be better with JWST. In addition, spectra of the explosions will help determine the stellar progenitor and it's evolutionary state prior to the explosion.

After obtaining the spectra, they attempt to quantify how many SN are produced by using a very simple analytic argument for the SFR in the early universe. The SFR is calibrated to ensure the Universe is reionized by z=6. They have two classes of models: one where only Pop III stars reionize and one where only Pop II stars provide the reionizing photons. Two different IMF's are assumed (Salpeter and a flat IMF) for the Pop III stars, and a Salpeter IMF is used for the Pop II case. With this constraint, they make predictions for the detectability of PISN and CCSN, and find that it would be possible with JWST to distinguish between the two Pop III IMF models used here.

The radius of baryonic collapse in disc galaxy formation

Authors: Kassin et al.
Link: here.

The authors propose a simple solution for the angular momentum catastrophe: measure the DM specific angular momentum (j) at the radius of baryonic collapse, defined as R_BC in their work, which is smaller than the R_vir and hence has a higher concentration of DM encompassed in it. Assuming the standard picture of galaxy formation where the baryons collapse from inside to outside in a DM halo, they are able to make a reasonable case for their R_BC parameter. The measurement of j at an inner radius leads to an agreement in the baryonic and DM j which they are able to constrain from observational data. Although they only run a DM-only simulation, certain assumptions about the j of the observed spiral galaxies (relating their circular velocity to the angular momentum) allow them to relate their simulated DM haloes and observed galaxies.

How long does it take to form a molecular cloud?

Authors: Clark, Glover, Klessen & Bonnell
Link: here

The authors use a 3D hydrodynamical code (GADGET2) coupled with a chemical network (including modeling of dust and the interstellar radiation field), to self-consistently study the formation of molecular clouds at the interface between colliding flows of the warm neutral medium. It has already been shown that this formation mechanism can produce bound gas clouds in 3D simulations, but no one has yet followed the chemistry (coupled with 3D hydro) in order to establish how much of this gas could be molecular. In particular, the authors wish to establish how much CO would be produced in this scenario, since it is this which is observed, not H2.

The authors have two simulations, one fast flow (with Mach number, M=2.62) and one slow flow (with M=1.22), which both start with all gas at 5000K, 1 atm/cm^3 and solar metallicity. The fast flow case results in star formation around 10 Myr earlier than the slow flow case and there appears to be a different mechanism at work in the formation of protostellar cores (modeled by sink particles here). In the fast flow case there is a greater mass of very cold (<30K, the temperature at which we observe most CO emission), very dense (>10^4 atm/cc, the typical density of protostellar cores) gas and both H2 and CO formation occur earlier in time than in the slow flow case. However, note that the time delay between the development of H2 regions and the onset of star formation is shorter for the fast flow.

The most interesting results, however, lie in the similarities in the cloud/star formation in these two simulations. In both cases some small regions develop completely molecular hydrogen early on, and well in advance of the onset of star formation (although the exact time delay is different in the two simulations). Even more striking is the fact that, in both simulations, the fraction of CO only becomes significant 1-2 Myrs before the onset of SF i.e. the actual time delay is about the same, despite the different physical conditions. At this point there is a very rapid increase in the mass of CO, taking it quickly from undetectable to observable levels. This supports models in which there are “dark” molecular clouds; clouds containing large reservoirs of H2 that we cannot observe due to their lack of CO.

Systematic variation of the stellar IMF in early-type galaxies

Authors: Cappellari et al.
Link: here

The authors present results on the shape of the initial mass function of stars in a complete sample of early-type galaxies observed with the integral-field spectroscopy. The novel aspect is that for the first time they manage to disentangle the contribution from dark matter and a changing IMF by detailed dynamical modeling in combination with stellar population modeling. The main result is that they find evidence for an IMF shape that is either top-heavy or bottom-heavy. This is an interesting and important result contributing to the general discussion on a changing IMF in galaxies.