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.

21 July 2011

Adaptive gravitational softening in GADGET

Authors: Francesca Iannuzzi and Klaus Dolag

Link to paper

The authors describe the implementation of adaptive gravitational softening in GADGET. The softening lenght is a function of the local density, as for standard SPH. A correction to the formulation of the Lagrangian is necessary to reduce noise and preserve the translational invariance.

Applied to cosmological simulations, the enhanced resolution at high densities shows in the increase of particle clustering at small scales, amplitude of the correlation function, and inner profile of massive haloes. This allows to recover the results of higer resolution simulations at much lower computational costs.

1 June 2011

On the effects of microphysical grain properties on the yields of carbonaceous dust from type II SNe


http://arxiv.org/abs/1105.4631
The paper studies the carbonaceous dust yields and properties from SN II, by theoretically exploring the parameter space {shape, sticking coefficient}.
The authors do not attempt to explain the discrepancies of some order of magnitude with the observed data, but try to constrain the effects of the different model parameters (based on nucleation theory).
They find that grain condensation times and grain size distributions are affected by the choice of different parameters, and, e.g.,
1. - larger sticking coefficients imply larger grains;
2. - more spherical grains have larger dimensions;
3. - larger sticking coefficient imply shorter condensation times (grains form earlier).
However, the total amount of dust prodiced (i.e. the dust yield) is quite independent from them.

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.

27 April 2011

Galaxy and Mass Assembly (GAMA): The star formation rate dependence of the stellar initial mass function

Link to paper on ADS here

Observations of galaxies found in the Galaxy and Mass Assembly (GAMA) survey are presented and analyzed. It is argued that the observations suggest a high-mass IMF slope which evolves with star formation rate. For galaxies with larger star formation rates, the trend is for flatter IMF slope, meaning a larger portion of high mass stars than in less actively star forming galaxies. If correct, the implications of this finding are potentially far-reaching.

24 March 2011

Recoiling Black Holes in Merging Galaxies...

Link to paper here.
Authors: Laura Blecha, Thomas J. Cox, Abraham Loeb, Lars Hernquist

This proceeding paper present a parameter study of the effect of Gravitational-Wave (GW) recoil of merging supermassive black holes (SMBH). Using a suite of merger SPH simulations, the authors identify systematic trends in the behavior of recoiling BHs. The main results from this study are the following: i) as a function of the escape velocity, kicked BHs may remain confine in gas-rich galaxy merger. ii) recoil events reduce the lifetime of bright AGN, which can still be detectable either as kinematical or spatial offsets. iii) recoiling BHs may be up to 5 time less massive than their stationary counterparts. iv) the displacement of AGN feedback by a recoil causes higher central star formation, hence extending the starburst phase.

An Evolving Stellar Initial Mass Function and the Gamma-ray Burst Redshift Distribution

Link to paper here.
Authors: Wang & Dai

Whether or not the gamma-ray burst rate tracks the overall star formation rate in the universe has been a topic of much interest and debate. In particular, with the ever-growing sample of GRBs at high redshift, it appears that the GRB rate rises faster with redshift than the star formation rate. It has been suggested that this is due to GRB progenitors being of preferentially low metallicity, but some observations show GRBs exploding in metal-rich galaxies so this may not provide a clear explanation for the dicrepancy. In this paper, the authors show that an IMF, such as that suggested by Dave (2010), which becomes more top-heavy at higher redshifts can explain the discrepancy between the star formation rate and the GRB rate.

18 March 2011

Relativistic jet feedback in evolving galaxies

http://arxiv.org/abs/1012.1092
Authors: Alexander Y. Wagner, Geoffrey V. Bicknell

The actual physics of radio-mode AGN feedback is still largely mysterious and so far can be treated in cosmological simulations only by subgrid models. The authors examine the interaction of young radio sources with a fractal two-phase ISM (1 kpc, up to ~105 years) by high resolution simulations with a physical jet implementation. They discuss effects of enhanced or inhibited star formation as well as the momentum and energy budget.

17 March 2011

PCA for Halo properties

http://arxiv.org/abs/1103.1641
Authors: Ramin A. Skibba, Andrea V. Maccio'

The authors study the correlations between the structural parameters of dark matter haloes using Principal Component Analysis (PCA). They find three important components: halo mass, concentration, and halo 'relaxedness'.

10 March 2011

Stellar black holes at the dawn of the universe

http://arxiv.org/abs/1102.1891v1
Authors: Mirabel, Dijkstra, Laurent, Loeb, Pritchard


The authors propose that feedback from accreting black holes in high-mass X-ray binaries are an important and so-far overlooked source in the process of reionisation. They present an estimate of the total number of ionising photons produced by these sources and discuss some implications for reionisation models.

Disentangling galaxy environment and host halo mass

http://arxiv.org/abs/1103.0547
Authors: Marcel R. Haas (1 and 2), Joop Schaye (1), Akila Jeeson-Daniel

A lot of indicators of galaxy environment used in the (observational) literature are strong indicators of the mass of the halo in which the galaxy resides, the 'internal' environment. These authors give methods to compute a halo mass independent parameter to quantify the 'external' environment of a galaxy.

3 March 2011

On the relative abundance of LiH and LiH+ molecules in the early universe

Authors: Stefano Bovino, Mario Tacconi, Franco A. Gianturco, Daniele Galli, Francesco Palla
full paper:  http://arxiv.org/abs/1102.3812

The authors revisit Li chemistry evolution and perform full
ab-initio computations of reaction rates and cross sections. They find
significant differences with respect to previous works, since, with the
updated data LiH become more aboundant of LiH+ at all redshift.
At z~1, differences with previous works are up to 2 orders of magnitude.

25 February 2011

The Delay of Population III Star Formation by Supersonic Streaming Velocities

Greif, Thomas; White, Simon; Klessen, Ralf; Springel, Volker
http://adsabs.harvard.edu/abs/2011arXiv1101.5493G

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
Paper: http://arxiv.org/abs/1102.2903


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:  http://arxiv.org/abs/1102.3210

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
http://arxiv.org/abs/1102.1437

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: http://arxiv.org/abs/1102.1481

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
http://arxiv.org/abs/1101.5007

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.

27 January 2011

Title: Dynamical Delays Between Starburst and AGN Activity in Galaxy Nuclei

Author: Philip F. Hopkins
Link to paper: http://arxiv.org/abs/1101.4230

The author investigates the observed (possible) delay between the onset of AGN activity and the peak of star formation in star-burst galaxies. He shows that the delay can be reproduced in simulations of merging galaxies, and give a physical explanation of it. He argues of implications for the role of the AGN feedback in suppressing star formation.

On Lyman-limit Systems

On Lyman-limit Systems and the Evolution of the Intergalactic Ionizing Background
Authors: Matthew McQuinn, S. Peng Oh, Claude-Andre Faucher-Giguere
http://arxiv.org/abs/1101.1964v1

The authors use cosmological simulations to study the properties of Lyman-limit systems at 3 < z < 6 and the relation between the ionising emissivity of galaxies and the amplitude of the intergalactic ionising background. This could explain the rapid decrease in the Ly-alpha forest transmission observed around z=6 without invoking reionisation.

19 January 2011

Conformal Cyclic Cosmology in WMAP data?

Authors: V.G.Gurzadyan, R.Penrose
Link to Paper: arxiv.org/abs/1011.3706

The Conformal Cyclic Cosmology model. A dynamic and controversial model (R. Penrose) that argues for various aeons preceding and following our current universe. The authors also explain that the validity of this model can be tested via observations (CMB anisotropies) and even claim to have found proof for the same!

An Actively Accreting Massive Black Hole in the Dwarf Starburst Galaxy Henize 2-10

Authors: Reines et al.
Link to the paper: arXiv:1101.1309

The authors report observations strongly suggesting the presence of a central black hole in a blue compact dwarf galaxy. The mass of the black hole is estimated to be ~10^6 MSun, which is intriguing given that the galaxy itself has no bulge, nucleus, or nuclear stellar cluster. In turn, the authors suggest that in the early universe black holes may have formed before the stellar components of the first dwarf galaxies.