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

8 November 2010

The first galaxies: assembly of disks and prospects for direct detection

Authors: A. Pawlik, M. Milosavljevic and V. Bromm
Link to the paper: arXiv:1011.0438

The authors study the formation of a galaxy embedded in a 109 solar masses halo at redshift z=10. Galaxies of this mass should be possible to observe with the James Webb Space Telescope (JWST).

The authors do not include star formation and feedback, but concentrate on the formation history of the galaxy in the presence of primordial and molecular cooling. The first result is that the galaxy for a gas disk with and without the inclusion of molecular cooling. Indeed, molecular cooling seems to act only on the thickness and fragmentation of the disk, but not on its assembly and gas content. This is one of the first simulations showing that such a halo can host a disk.

The second result is that a simple modeling of the star formation history of the galaxy gives estimates of its detectability with JWST. The authors predict that JWST could distinguish between normal and top-heavy IMF in the starburts case, and between starburst and continuous star formation. Their model can be rescaled to different values of parameters like, e.g., the escape fraction of photons.

5 November 2010

Jet-powered molecular hydrogen emission from radio galaxies

Authors: P. Ogle et al.
Link to the paper: arXiv:1009.4533

The authors examine a sample of 55 radio galaxies with z < 0.22 and find 31% percent of those to exhibit particularly strong mid-IR emission from warm H2 (T = 100 - 1500 K, with masses in this phase typically around some 108, up to 2 x 1010 solar masses), putting them in a newly (re)defined class of "molecular hydrogen emission galaxies" (MOHEGs). They argue that the jets in these sources may interact with the cold H2 in these galaxies and shock-heat it, resulting in the observed emission. They find that X-rays originating from the AGN are incapable of powering the H2 emission, but that cosmic rays may still be an alternative explanation if a cosmic ray pressure 25 times higher than thermal pressure is deemed to be reasonable.

The radio MOHEGs show only low to moderate star formation rates (< 3 solar masses per year) and much less 7.7 µm PAH emission then normal star forming galaxies with respect to the mid-IR continuum (a factor of 10 - 100 lower). The H2 luminosity does not show much correlation with radio power, but several sources in clusters with X-ray cavities indicate that the ratio between H2 luminosity and kinetic jet power may lie around 10-4 to some 10-3. Most (14 of 17) radio MOHEGs belong to close galaxy pairs, groups or cluster and the authors conjecture that this environment or past gas-rich mergers may deliver large quantities of gas to the galaxies.

The authors conclude that jet-driven outflows may be responsible for the emission, although the details are yet very unclear. The jet powers, however, would more than sufficient for this.

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

27 September 2010

Transformations in the Fall: The Birth of the Early-Type Galaxies

The authors discuss in their paper (Feldmann et al. 2010; arXiv:1008.3386a recent high resolution cosmological hydro simulation of the formation of a galaxy group environment. The remarkable feature of their simulation is to be able to resolve the detailed formation history of individual group members with high mass resolution (SPH particle mass = 1E6 M_sun). Their main conclusion is that elliptical galaxies form by major mergers before the assembly of the group, and that gas cooling and associated star formation is strongly suppressed via ram-pressure stripping once galaxies enter the group environment.

17 September 2010

On the star formation rates in molecular clouds

Authors: C. Lada, M. Lombardo, J. Alveshttp;

The authors study the correlations between SFR and cloud mass in a sample of 8 molecular cloud complexes (11 clouds). They find that only the mass in the cores of the cloud is tightly linked to the SFR.
Total cloud mass and SFR do not correlate well, with variations which are considerable (more than one order of magnitude in scatter). Instead, the correlation between high-density mass (which is roughly 10 per cent the total one for almost all the objects in the sample) and SFR is linear and quite tight (with a scatter of about 2-3).
The only concern is the following: if the correlation between high-density mass and SFR is so tight, and if the high-density mass is roughly 10 per cent the total one, why is the correlation between total mass and SFR so bad?...

10 September 2010

Galaxy and Mass Assembly (GAMA): Dust obscuration in galaxies and their recent star formation histories

Authors: D. B. Wijesinghe, A. M. Hopkins et al.
Link to article: arXiv:1009.0616v1

Star formation rates derived through pan-spectral analysis of a sample of ~ 30.000 galaxies drawn from the Galaxy and Mass Assembly (GAMA) survey. In order to get the SFR in different bands (by means of linear scaling factors derived from population synthesis models), dust obscuration corrections are applied to the Hα, [OII] and UV luminosities by using a range of extinction laws drawn from the literature. 
The results of this study clearly show that the fine-tuned Fischera & Dopita (2005) obscuration curve (Rv = 4.5) give the best agreement when comparing the different corrected SFR indicators. One interesting finding of the paper is that the 2200 Å feature present in the obscuration curves of the Milky way, has to be removed in order to obtain complete consistency between all SFR indicators suggesting that this feature may not be common in the average integrated attenuation of galaxy emission.
The last part of this paper attempt to give constraints on the star formation history of local Universe galaxies by comparing the corrected data with the predictions of evolutionary synthesis models. The evolutionary paths are computed by assuming an exponentially decaying SFR and give the best fit to the data by using a Baldry & Glazebrook (2003) IMF with a characteristic SFR decay of 90 Myrs, which correspond to stellar population ages of the galaxies ranging from 200 to 500 Myrs.

30 August 2010

Evolution of Galaxy Stellar Mass Functions, Mass Densities, and Mass to Light ratio from z~7 to z~4

Authors: Gonzalez, Labbé, Bouwens, Illingworth, Franx, Kriek
Link to article: arXiv:1008.3901v1

This paper address a very important question about the built-up and evolution of galaxies at high redshift, by deriving stellar masses from SED fitting for ~400 LBG observed with Hubble-WFC3/IR and Spitzer-IRAC in the GOODS South field. Using Bruzual & Charlot models with a Salpeter IMf, masses are obtained assuming a constant Star Formation History for galaxies. 

By fitting the Mass vs Luminosity relation at z~4 - with a slope that makes the M/L ratio increase with luminosity - and finding no clear evidence for evolution with redshift, this work use the UV Luminosity Functions of Bouwens et al. (2007-2010) and Monte-Carlo simulations to derive Mass Functions (MF) at z~4,5,6,7. The results show a significant difference with theoretical expectations from numerical simulations, and especially a much more flatter faint end slope. 

The last part of this paper show a comparison of the stellar mass growth computed by 2 different approaches: a quite large disagreement is found between the integration of the SFR density over time [SFRD ~ (1+z)^-6.3] compared with the integration of MF for the 4 bins of redshift [SMD ~ (1+z)^-2.8]. The authors propose that dust extinction overestimates, low duty cycle for SF or shortcomings in the modeling of faint galaxies may explain this noticeable difference.

27 August 2010

Predicted UV properties of very metal-poor starbursts

Authors:  Raiter, Schaerer, Fosbury
Link to article: arXiv:1008.2114

The properties of the rest-frame UV emission from low-metallicity starbursts are studyied in detail, with the aim of providing a basis for the interpretation of observations of high redshift galaxies in which zero or low-metallicity stars are forming.  A number of important effects are included, which were not accounted for in the earlier work of Schaerer (2002) on the emission properties of low-metallicity galaxies.  Among these are that the nebular line emission can be enhanced appreciably when particularly massive, hot stars are illuminating the gas within the galaxy.  The authors also pay particular attention to the impact that 2-photon emission can have on the equivalent widths of emission lines, especially Ly_alpha and He II 1640.  

20 August 2010

A relationship between AGN jet power and radio power

Authors: Cavagnolo, McNamara, Nulsen, Carilli, Jones & Birzan
Link to article: arxiv:1006.5699

The powerlaw scaling relation between jet power (derived from X-ray cavities) and radio power by Birzan et al. (2008) for clusters is extended to giant ellipticals. A powerlaw scaling is found over 6 orders of magnitude in jet power and it is found consistent with the relations of Willott et al. (1999) if the energy in non-radiating particles is about 100 times larger than in synchrotron-emitting electrons (ranging between tens to thousands). The scatter for the relation still is considerable - more than 1 dex wide for 68% confidence. The authors furthermore discuss the behaviour of poorly confined sources, which are not included in the fits.

13 August 2010

Accretion Disks in Active Galactic Nuclei: Gas Supply Driven by Star Formation

Authors: Wang, Yan, Gao, Hu, Li & Zhang
Link to article: arXiv:1007.4060

An analytic model of star formation and gas accretion in the inner ~1pc region surrounding a black hole powering an AGN is presented.  Turbulence generated by supernovae is taken to be responsible for the transport of angular momentum that allows gas to accrete onto the central black hole via a Shakura-Sunyaev accretion disk, which resides between the star forming region and the black hole.  In this model, the black hole accretion rate varies as the star formation rate to the power ~5/7.  The authors further discuss how this model can explain the observed metallicity-luminosity relation of AGN and the formation of nuclear star clusters.