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 June 2012

Galaxy disks do not need to survive in the L-CDM paradigm

Full title: Galaxy disks do not need to survive in the L-CDM paradigm: the galaxy merger rate out to z~1.5 from morpho-kinematic data

Authors: Puech, M., et al
Paper: here

The main result of the paper is the derivation of the time evolution of the galaxy merger rate using a sample of intermediate mass (M_stellar=10^10-10^11 M_sun) emission-line galaxies for which they have both kinematic (IMAGES survey) and morphological (from HST) data. These galaxies are chosen because they are the most likely progenitors of local spirals (see justifications in the paper). A large fraction of these galaxies are not relaxed morphologically and/or kinematically and the authors demonstrate that the disturbances are most likely triggered by major mergers.

The authors then match as many as possible of these observed, disturbed galaxies to simulated major mergers by comparing the real and simulated velocity fields and morphology etc. This allows them to associate a time since the merger began with each observation and they group them into “pre-fusion”, “post-fusion” (the main starburst period) and “relaxation” phases. They plot the observed SFR (normalised by gas mass) versus the time since the start of merger to illustrate that the IMAGES galaxies sample all the phases of an average merger well. They can then derive a merger rate from: rate = ngal in merger phase/time period for this phase, and a corresponding redshift for this rate: z= average start time of mergers in this phase. This gives them merger rates of 5.5, 10.1, 11.1 %/Gyr at z=0.72, 1.12, 1.55 respectively. They show that these merger rate values are in agreement with those from the Hopkins et al. 2009 semi-empirical model to within a factor of 2-3.

The authors draw some additional conclusions, including:
· Given the high gas fractions they infer for the major mergers occurring at z>0.6, a significant disc should be able to reform by z=0.
· The merger rate from the semi-empirical model is not in conflict with the fraction of bulgeless galaxies observed locally.
· Since cold streams have been shown to be suppressed below z~1.5, gas-rich major mergers could potentially take over as a channel for massive, thin disk formation at this redshift.
· Overall: there is no disc survival problem, but a period in which discs could/should be rebuilt at z<1.5.

15 June 2012

CANDELS: The contribution of the observed galaxy population to cosmic reinisation

Authors: Finkelstein, S. et al (2012)
Paper: here

Summary: In this paper, the contribution of the observed galaxies at redshifts 6, 7 and 8 to the reionisation of the IGM is studied. Taking only the observed galaxy population, an escape fraction of 30% is necessary to sustain reionisation at z=6, while if the luminosity function is integrated down to fainter galaxies, an escape fraction of 10% is sufficient. At redshifts 7 and 8 the observed galaxies do not produce enough ionising photons and fainter galaxies are needed to complete reionisation. The inferred volume ionised fraction from the ionising emissivity of these galaxy populations is consistent with observations of quasar spectra and Lyman-alpha spectroscopy, but inconsistent with the WMAP measurements of the Thomson scattering optical depth.

Birth of a Galaxy II: The Role of Radiation Pressure

Authors: Wise, J. et al (2012)
Paper: here

Summary: Using the AMR code ENZO, coupled with a ray-tracing radiative transfer solver, they study how radiation pressure affects the star formation history and growth of an early low-mass galaxy. They run three models, one with only primordial cooling (base model), one which also includes metal cooling (MC model), and one which includes metal cooling, momentum transfer from ionizing radiation, and an H2 dissociating radiation background (RP model). They then study how the metal cooling and radiation pressure affect the internal gas dynamics, the chemo-thermal state of the ISM, the ejecta and outflows, and the star formation history in the three models.

They find that the MC model overcools the gas, which traps the metals in a small region and creates an extremely metal rich dwarf galaxy, which is at odds with observed local dwarfs. When including the RP, however, the SFR is reduced, and the RP adds turbulence, and mixes the SNe ejecta throughout the ISM, preventing over-cooling. This model creates a galaxy which matches the local dwarf mass-metallicity relation.

7 June 2012

Monster Black Holes

2 letters:
McConnell et al. 2011
"Two ten-billion-solar-mass black holes at the centres of giant elliptical galaxies"
Paper: here

The authors report on the two new BHs found in the nearby universe, that are now the most massive BHs known to us. They explain their model which they used to infer the mass of these objects and talk about the underlying uncertainty. As a result of their high mass, they are outliers on the Mbh-sigma relation and hence, interesting to the BH-galaxy coevolution question.

Cappellari 2011
"Astrophysics: Monster black holes"
Paper: here

The implication of the above paper on the dry-merger-driven growth scenario for BHs is explored. The author argues that the two outliers on the Mbh-sigma relation could be a result of coalescing BHs, keeping the velocity dispersion on the resultant galaxy the same, but growing the central BH to double its initial mass. A speculative yet interesting insight into the BH-galaxy coevolution.

Star Formation in Galaxy Mergers with Realistic Models of Stellar Feedback & the Interstellar Medium

Authors: Hopkins et al. 2012
Paper: here

The authors discuss the results of high-resolution hydrodynamic simulations of galaxy mergers. They employ a sophisticated implementation of stellar feedback coupled to a `realistic' modelling of the multiphase ISM. They show that most of the stellar mass that ends up in the bulge is produced in situ, contrary to other recent simulations. The reason is the efficient disruption of GMC by stellar feedback which supplies more gas to the bulge than in other models. They compare their simulations to models assuming an effective equation of state.

1 June 2012

Stellar Disks in Aquarius Dark Matter Haloes

Authors: DeBuhr et al (2012)
Paper: here

The paper (Stellar Disks in Aquarius Dark Matter Haloes) shows results for stellar-bar formation in 4 Milky-Way-size dark matter haloes of the Aquarius project. The find that (i) the presence of a stellar component in the center of the haloes induces inner-halo contraction; (ii) 3/4 of the disks form a central bar; (iii) almost all of them experience a buckling instability with a boost of the stellar vertical velocity dispersion; (iv) when mass is reduced by at least a factor 2 no bar is formed; (v) re-orientation of the central part of the disk and halo determines warp formation.

There is not much new: just usual stellar dynamics analysis on the new Aquarius haloes.

Similar results about warps and bars might be reproduced by different phenomena (e.g. infalling satellite galaxies; dark-matter substructures; cold gas clumps; etc.)