Dark Matter Halo Merger Research Articles

Convergence of galaxy properties with merger tree temporal resolution

Dark matter halo merger trees are now routinely extracted from cosmological simulations of structure formation. These trees are frequently used as inputs to semi-analytic models of galaxy formation to provide the backbone within which galaxy formation takes place. By necessity, these merger trees are constructed from a finite set of discrete "snapshots" of the N-body simulation and so have a limited temporal resolution. To date, there has been little consideration of how this temporal resolution affects the properties of galaxies formed within these trees. In particular, the question of how many snapshots are needed to achieve convergence in galaxy properties has not be answered. Therefore, we study the convergence in the stellar and total baryonic masses of galaxies, distribution of merger times, stellar mass functions and star formation rates in the Galacticus model of galaxy formation as a function of the number of "snapshot" times used to represent dark matter halo merger trees. When utilizing snapshots between z=20 and z=0, we find that at least 128 snapshots are required to achieve convergence to within 5% for galaxy masses. This convergence is obtained for mean quantities averaged over large samples of galaxies - significant variance for individual galaxies remains even when using very large numbers of snapshots. We find only weak dependence of the rate of convergence on the distribution of snapshots in time - snapshots spaced uniformly in the expansion factor, uniformly in the logarithm of expansion factor or uniformly in the logarithm of critical overdensity for collapse work equally well in almost all cases. We provide input parameters to Galacticus which allow this type of convergence study to be tuned to other simulations and to be carried out for other galaxy properties.

Angular correlation function of 1.5 million luminous red galaxies: clustering evolution and a search for baryon acoustic oscillations

We present the angular correlation function measured from photometric samples comprising 1562 800 luminous red galaxies (LRGs). Three LRG samples were extracted from the Sloan Digital Sky Survey (SDSS) imaging data, based on colour-cut selections at redshifts, z≈ 0.35, 0.55 and 0.7 as calibrated by the spectroscopic surveys, SDSS-LRG, 2dF-SDSS LRG and QSO (quasi-stellar object) (2SLAQ) and the AAΩ-LRG survey. The galaxy samples cover ≈7600 deg2 of sky, probing a total cosmic volume of ≈5.5 h−3 Gpc3. The small- and intermediate-scale correlation functions generally show significant deviations from a single power-law fit with a well-detected break at ≈1 h−1 Mpc, consistent with the transition scale between the one- and two-halo terms in halo occupation models. For galaxy separations 1–20 h−1 Mpc and at fixed luminosity, we see virtually no evolution of the clustering with redshift and the data are consistent with a simple high peaks biasing model where the comoving LRG space density is constant with z. At fixed z, the LRG clustering amplitude increases with luminosity in accordance with the simple high peaks model, with a typical LRG dark matter halo mass 1013–1014h−1 M⊙. For r < 1 h−1 Mpc, the evolution is slightly faster and the clustering decreases towards high redshift consistent with a virialized clustering model. However, assuming the halo occupation distribution (HOD) and Λ cold dark matter (ΛCDM) halo merger frameworks, ∼2–3 per cent/Gyr of the LRGs are required to merge in order to explain the small scales clustering evolution, consistent with previous results. At large scales, our result shows good agreement with the SDSS-LRG result of Eisenstein et al. but we find an apparent excess clustering signal beyond the baryon acoustic oscillations (BAO) scale. Angular power spectrum analyses of similar LRG samples also detect a similar apparent large-scale clustering excess but more data are required to check for this feature in independent galaxy data sets. Certainly, if the ΛCDM model were correct then we would have to conclude that this excess was caused by systematics at the level of Δw≈ 0.001–0.0015 in the photometric AAΩ-LRG sample.

THE NON-CAUSAL ORIGIN OF THE BLACK-HOLE-GALAXY SCALING RELATIONS

We show that the MBH–Mbulge scaling relations observed from the local to the high-z universe can be largely or even entirely explained by a non-causal origin, i.e., they do not imply the need for any physically coupled growth of black hole (BH) and bulge mass, for example, through feedback by active galactic nuclei (AGNs). Provided some physics for the absolute normalization, the creation of the scaling relations can be fully explained by the hierarchical assembly of BH and stellar mass through galaxy merging, from an initially uncorrelated distribution of BH and stellar masses in the early universe. We show this with a suite of dark matter halo merger trees for which we make assumptions about (uncorrelated) BH and stellar mass values at early cosmic times. We then follow the halos in the presence of global star formation and BH accretion recipes that (1) work without any coupling of the two properties per individual galaxy and (2) correctly reproduce the observed star formation and BH accretion rate density in the universe. With disk-to-bulge conversion in mergers included, our simulations even create the observed slope of ∼1.1 for the MBH–Mbulge relation at z = 0. This also implies that AGN feedback is not a required (though still a possible) ingredient in galaxy evolution. In light of this, other mechanisms that can be invoked to truncate star formation in massive galaxies are equally justified.

How Does Feedback Affect Milky Way Satellite Formation?

We use sub-parsec resolution hydrodynamic resimulations of a Milky Way (MW) like galaxy at high redshift to investigate the formation of the MW satellite galaxies. More specifically, we assess the impact of supernova feedback on the dwarf progenitors of these satellite, and the efficiency of a simple instantaneous reionisation scenario in suppressing star formation at the low-mass end of this dwarf distribution. Identifying galaxies in our high redshift simulation and tracking them to z=0 using a dark matter halo merger tree, we compare our results to present-day observations and determine the epoch at which we deem satellite galaxy formation must be completed. We find that only the low-mass end of the population of luminous subhalos of the Milky-Way like galaxy is not complete before redshift 8, and that although supernovae feedback reduces the stellar mass of the low-mass subhalos (log(M/Msolar) < 9), the number of surviving satellites around the Milky-Way like galaxy at z = 0 is the same in the run with or without supernova feedback. If a luminous halo is able to avoid accretion by the Milky-Way progenitor before redshift 3, then it is likely to survive as a MW satellite to redshift 0.

Dark matter halo merger and accretion probabilities in the excursion set formalism

The merger and accretion probabilities of dark matter haloes have so far only been calculated for an infinitesimal time interval. This means that a Monte Carlo simulation with very small time-steps is necessary to find the merger history of a parent halo. In this paper, we use the random walk formalism to find the merger and accretion probabilities of haloes for a finite time interval. Specifically, we find the number density of haloes at an early redshift that will become part of a halo with a specified final mass at a later redshift, given that they underwent n major mergers, n = 0, 1, 2, ... . We reduce the problem into an integral equation which we then solve numerically. To ensure the consistency of our formalism, we compare the results with Monte Carlo simulations and find very good agreement. Though we have done our calculation assuming a flat barrier, the more general case can easily be handled using our method. This derivation of finite time merger and accretion probabilities can be used to make more efficient merger trees or implemented directly into analytical models of structure formation and evolution.

The atomic-to-molecular transition and its relation to the scaling properties of galaxy discs in the local Universe

We extend existing semi-analytic models of galaxy formation to track atomic and molecular gas in disk galaxies. Simple recipes for processes such as cooling, star formation, supernova feedback, and chemical enrichment of the stars and gas are grafted on to dark matter halo merger trees derived from the Millennium Simulation. Each galactic disk is represented by a series of concentric rings. We assume that surface density profile of infalling gas in a dark matter halo is exponential, with scale radius r_d that is proportional to the virial radius of the halo times its spin parameter $\lambda$. As the dark matter haloes grow through mergers and accretion, disk galaxies assemble from the inside out. We include two simple prescriptions for molecular gas formation processes in our models: one is based on the analytic calculations by Krumholz, McKee & Tumlinson (2008), and the other is a prescription where the H_2 fraction is determined by the kinematic pressure of the ISM. Motivated by the observational results of Leroy et al. (2008), we adopt a star formation law in which $\Sigma_{SFR}\propto\Sigma_{H_2}$ in the regime where the molecular gas dominates the total gas surface density, and $\Sigma_{SFR}\propto \Sigma_{gas}^2$ where atomic hydrogen dominates. We then fit these models to the radial surface density profiles of stars, HI and H_2 drawn from recent high resolution surveys of stars and gas in nearby galaxies. We explore how the ratios of atomic gas, molecular gas and stellar mass vary as a function of global galaxy scale parameters, including stellar mass, stellar surface density, and gas surface density. We elucidate how the trends can be understood in terms of three variables that determine the partition of baryons in disks: the mass of the dark matter halo, the spin parameter of the halo, and the amount of gas recently accreted from the external environment.

Merger-Induced Quasars, Their Light Curves, and Their Host Halos

AbstractWe revisit a basic model of quasar activation by major mergers of dark matter halos (with “galactic” masses of ≲1013M⊙h−1). This model usually consists of two main ingredients: the halo merger rate describing triggering, and a quasar light curve, which describes the evolution of individual quasars. We show how the matching between model predictions and a variety of new, independent data sets allows one to efficiently constrain several aspects of black hole growth and evolution that must be taken into account in future studies by more advanced models of galaxy formation. Our results can be summarized as follows: (1) A descending phase modelled such that quasars in more massive halos shut down faster than those in less massive ones allows a good description of the bright end of the AGN luminosity function at all epochs and is compatible with downsizing, with more massive galaxies shutting down star formation earlier. (2) We measure the average bias of type 2 AGNs in SDSS to be b = 1.233 ± 0.195, independent of luminosity in the range 42.5≤ log L(erg s−1) ≤ 45.5. Such a value of the bias implies that faint AGNs at z&lt;0.3 are mainly hosted by halos more massive than ~1011.5–12M⊙h−1. The black hole mass function predicted by this model is flatter than previously found. (3) The high clustering signal measured at z&gt;3 in SDSS forces successful models to be characterized by rather short delay times of tdelay≲108 yr between the triggering and the shining epochs, implying massive “seed” BHs ≳ 105M⊙h−1 and initial super-Eddington growth. (4) The low number counts of X-ray AGNs measured in recent deep surveys are better reproduced by models with a minimal post-peak phase and a higher minimum hosting halo mass at high redshifts. (5) Cross-correlating the feedback-constrained MBH–M relation, with the redshift-dependent Mstar–M relation obtained from the cumulative number-matching of the stellar and halo mass functions, we find a factor of ~2 larger BH-to-stellar mass ratio at high redshifts. We discuss the meaning of such trends in connection with the mild, positive evolution in the MBH-σstar relation, and the strong observed evolution in the sizes and velocity dispersions of their hosts.

Cosmic queuing: galaxy satellites, building blocks and the hierarchical clustering paradigm

Abstract We study the properties of building blocks (BBs; i.e. accreted satellites) and surviving satellites of present-day galaxies using the semi-analytic model of galaxy formation SAG (‚semi-analytic galaxies’) in the context of a concordance Λ cold dark matter (ΛCDM) cosmology. We consider large number of dark matter (DM) halo merger trees spanning a wide range of masses (∼1 × 1010–2.14 × 1015 M⊙). We find higher metallicities for BBs with respect to surviving satellites, an effect produced by the same processes behind the build up of the mass–metallicity relation. We prove that these metallicity differences arise from the higher peak height in the density fluctuation field occupied by BBs and central galaxies which have collapsed into a single object earlier than surviving satellites. BBs start to form stars earlier, during the peak of the merger activity in ΛCDM, and build up half of their final stellar mass (measured at the moment of disruption) up to four times faster than surviving satellites. Surviving satellites keep increasing their stellar masses rather quiescently down to z≃ 1. The difference between the metallicities of satellites, BBs and central galaxies depends on the host DM halo mass, in a way that can be used as a further test for the concordance cosmology.

THE ASSEMBLY OF SUPERMASSIVE BLACK HOLES AT HIGH REDSHIFTS

The supermassive black holes (SMBHs) massive enough to power the bright redshift ~6 quasars observed in the Sloan Digital Sky Survey (SDSS) are thought to have assembled by mergers and/or gas accretion from less massive "seed" BHs. If the seeds are the ~100 Msol remnant BHs from the first generation of stars, they must be in place well before redshift z=6, and must avoid being ejected from their parent proto-galaxies by the large (few hundred km/s) kicks they suffer from gravitational-radiation induced recoil during mergers with other BHs. We simulate the SMBH mass function at redshift z=6 using dark matter (DM) halo merger trees, coupled with a prescription for the halo occupation fraction, accretion histories, and radial recoil trajectories of the growing BHs. Our purpose is (i) to map out plausible scenarios for successful assembly of the z~6 quasar BHs by exploring a wide region of parameter space, and (ii) to predict the rate of low-frequency gravitational wave events detectable by the Laser Interferometer Space Antenna (LISA) for each such scenario. Our main findings are as follows: (1) ~100 Msol seed BHs can grow into the SDSS quasar BHs without super--Eddington accretion, but only if they form in minihalos by z~30 and subsequently accrete ~60% of the time; (2) the scenarios with optimistic assumptions required to explain the SDSS quasar BHs overproduce the mass density in lower--mass (10^5 to 10^7 Msol) SMBHs by a factor of 100-1000, unless seeds stop forming, or accrete at a severely diminished rates or duty cycles (e.g. due to feedback), at z < 20-30. We also present several successful assembly models and their LISA detection rates, including a "maximal" model that gives the highest rate (~30/yr at z~6) without overproducing the total SMBH density.

Constraining cold dark matter halo merger rates using the coagulation equations

We place additional constraints on the three parameters of the dark matter halo merger rate function recently proposed by Parkinson, Cole & Helly by utilizing Smoluchowski's coagulation equation, which must be obeyed by any binary merging process which conserves mass. We find that the constraints from Smoluchowski's equation are degenerate, limiting to a thin plane in the three dimensional parameter space. This constraint is consistent with those obtained from fitting to N-body measures of progenitor mass functions, and provides a better match to the evolution of the overall dark matter halo mass function, particularly for the most massive halos. We demonstrate that the proposed merger rate function does not permit an exact solution of Smoluchowski's equation and, therefore, the choice of parameters must reflect a compromise between fitting various parts of the mass function. The techniques described herein are applicable to more general merger rate functions, which may permit a more accurate solution of Smoluchowski's equation. The current merger rate solutions are most probably sufficiently accurate for the vast majority of applications.

Generating dark matter halo merger trees

We present a new Monte-Carlo algorithm to generate merger trees describing the formation history of dark matter halos. The algorithm is a modification of the algorithm of Cole et al (2000) used in the GALFORM semi-analytic galaxy formation model. As such, it is based on the Extended Press-Schechter theory and so should be applicable to hierarchical models with a wide range of power spectra and cosmological models. It is tuned to be in accurate agreement with the conditional mass functions found in the analysis of merger trees extracted from the LCDM Millennium N-body simulation. We present a comparison of its predictions not only with these conditional mass functions, but also with additional statistics of the Millennium Simulation halo merger histories. In all cases we find it to be in good agreement with the Millennium Simulation and thus it should prove to be a very useful tool for semi-analytic models of galaxy formation and for modelling hierarchical structure formation in general. We have made our merger tree generation code and code to navigate the trees available at this http URL .

The statistical properties of Λ cold dark matter halo formation

We present a comparison of the statistical properties of dark matter halo merger trees extracted from the Millennium Simulation with Extended Press–Schechter (EPS) formalism and the related galform Monte Carlo method for generating ensembles of merger trees. The volume, mass resolution and output frequency make the Millennium Simulation a unique resource for the study of the hierarchical growth of structure. We construct the merger trees of present-day friends-of-friends groups and calculate a variety of statistics that quantify the masses of their progenitors as a function of redshift, accretion rates, and the redshift distribution of their most recent major merger. We also look in the forward direction and quantify the present-day mass distribution of haloes into which high-redshift progenitors of a specific mass become incorporated. We find that the EPS formalism and its Monte Carlo extension capture the qualitative behaviour of all these statistics, but as redshift increases they systematically underestimate the masses of the most massive progenitors. This shortcoming is worst for the Monte Carlo algorithm. We present a fitting function to a scaled version of the progenitor mass distribution and show how it can be used to make more accurate predictions of both progenitor and final halo mass distributions.

Spatial clustering in the ESO-Sculptor survey: two-point correlation functions by galaxy type at redshifts 0.1–0.5

Context: Galaxy clustering shows segregation effects with galaxy type, color and luminosity, which bring clues on the relationship with the underlying density field. Aims: We explore these effects among the populations of giant and dwarf galaxies detected in the ESO-Sculptor survey. Methods: We calculate the spatial two-point auto and cross-correlation functions for the 765 galaxies with R_c≤ 21.5 and 0.1 ≤ z ≤ 0.51 and for subsets by spectral type and luminosity. Results: At separation of 0.3 h-1 Mpc, pairs of early-type galaxies dominate the clustering over all the other types of pairs. At intermediate scales, 0.3-5 h-1 Mpc, mixed pairs of dwarf and giant galaxies contribute equally as pairs of giant galaxies, whereas the latter dominate at ≃10 h-1 Mpc. Moreover, the correlation functions per galaxy type display the expected transition between the 1-halo and 2-halo regimes in the scenario of hierarchical merging of dark matter halos. The 1-halo component of the early-type galaxies largely outdoes that for the late spiral galaxies, and that for the dwarf galaxies is intermediate between both. In contrast, the 2-halo component of the early-type galaxies and late spiral galaxies are comparable, whereas that for the dwarf galaxies is consistent with null clustering. Conclusions: We link the clustering segregation of the early-type and late spiral galaxies to their spatial distribution within the underlying dark matter halos. The early-type galaxies are preferentially located near the centers of the most massive halos, whereas late spiral galaxies tend to occupy their outskirts or the centers of less massive halos. This appears to be independent of luminosity for the early-type galaxies, whereas faint late spiral galaxies might reside in less dense regions than their bright analogs. The present analysis also unveils unprecedented results on the contribution from dwarf galaxies: at the scale at which they significantly cluster inside the halos (≤0.3 h-1 Mpc), they are poorly mixed with the late spiral galaxies, and appear preferentially as satellites of early-type galaxies. Based on observations collected at the European Southern Observatory (ESO), La Silla, Chile. All appendices are only available in electronic form at http://www.aanda.org

Constraints on Elliptical Galaxy Formation from Dry Mergers

Recent observations suggest that dissipationless mergers of elliptical galaxies build up the population of massive early-type galaxies (Bell et al. 2004; Faber et al. 2006). This type of merger is observed in galaxy clusters (Tran et al. 2005) and predicted by semi-analytic models which find mass assembly times significantly later than star-formation times for the most massive elliptical galaxies (de Lucia &amp; Blaizot 2006). Here, we use a semi-analytic model of minor mergers of dark matter halos to examine the role of dry minor mergers in elliptical galaxy formation.

Orbital parameters of merging dark matter halos

In order to specify cosmologically motivated initial conditions for major galaxy mergers (mass ratios $\leq$ 4:1) that are supposed to explain the formation of elliptical galaxies we study the orbital parameters of major mergers of cold dark matter halos using a high-resolution cosmological simulation. Almost half of all encounters are nearly parabolic with eccentricities $e \approx 1$ and no correlations between the halo spin planes or the orbital planes. The pericentric argument $\omega$ shows no correlation with the other orbital parameters and is distributed randomly. In addition we find that 50 % of typical pericenter distances are larger than half the halo's virial radii which is much larger than typically assumed in numerical simulations of galaxy mergers. In contrast to the usual assumption made in semi-analytic models of galaxy formation the circularities of major mergers are found to be not randomly distributed but to peak around a value of $\epsilon \approx 0.5$. Additionally all results are independent of the minimum progenitor mass and major merger definitions (i.e. mass ratios $\leq$ 4:1; 3:1; 2:1).

Galics- VI. Modelling hierarchical galaxy formation in clusters

High-resolution N-body re-simulations of 15 massive (10 14 10 15 M⊙) dark matter haloes have been combined with the hybrid galaxy formation model GalICS (Hatton et al. 2003), to study the formation and evolution of galaxies in clusters, within the framework of the hierarchical merging scenario. This paper describes the high-resolution resimulation technique used to build the dark matter halo sample, and discusses its reliability. New features incorporated in GalICS include a better description of galaxy positioning after dark matter halo merger events, a more reliable computation of the temperature of the inter-galactic medium as a function of redshift, that also takes into account the re ionisation history of the Universe, and a semi-analytic description of the ram pressure strippi ng of cold gas from galactic discs, suffered by galaxies during their motion through the diffuse hot intra-cluster medium. Within the multitude of available model results, we choose to focus here on the luminosity functions, morphological fractions and colour distributions of galax ies in clusters and in cluster outskirts, at z = 0. No systematic dependency on cluster richness is found either for the galaxy luminosity functions, morphological mixes, or colour distributions. Moving from higher density (cluster cores), to lower density environments (clust er outskirts), we detect a progressive flattening of the luminosity functions, an increase of the fr action of spirals and a decrease of that of ellipticals and S0s, and the progressive emergence of a bluer tail in the distributions of galaxy colours, especially for spirals. As compared to cluster spirals, early-type galaxies show a flatter luminosity function, and more homogeneous and redder colours. An overall good agreement is found between our results and the observations, particularly in terms of the cluster luminosity functions and morphological mixes. However, some discrepancies are also apparent, with too faint magnitudes of the brightest cl uster members, especially in the B band, and galaxy colours tendentially too red (or not blue enough) in the model, with respect to the observations. Finally, ram pressure stripping appea rs to affect very little our results.

Spin, Accretion, and the Cosmological Growth of Supermassive Black Holes

If supermassive black holes (SMBHs) are the energy sources that power quasars and active galactic nuclei (AGNs), then QSO SDSS 1148+5251, the quasar with the highest redshift (zQSO = 6.43), hosts an SMBH formed within ~0.9 Gyr after the big bang. This requirement places constraints on the cosmological formation of SMBHs, believed to grow from smaller initial seeds by a combination of accretion and mergers. We focus on gas accretion onto seeds produced by the collapse of Population III stars at high redshift. We incorporate the results of recent relativistic, MHD simulations of disk accretion onto Kerr black holes to track the coupled evolution of the masses and spins of the holes. We allow for an additional amplification of ~104 in the mass of a typical seed due to mergers, consistent with recent Monte Carlo simulations of hierarchical mergers of cold dark matter halos containing black hole seeds. We find that the growth of Population III black hole remnants to ~109 M☉ by zQSO ≳ 6.43 favors MHD accretion disks over standard thin disks. MHD disks tend to drive the holes to a submaximal equilibrium spin rate a/M ~ 0.95 and radiation efficiency ϵM ~ 0.2, while standard thin disks drive them to maximal spin (a/M = 1) and efficiency (ϵM = 0.42). This small difference in efficiency results in a huge difference in mass amplification by accretion at the Eddington limit. The MHD equilibrium efficiency is consistent with the observed ratio of the QSO plus AGN luminosity density to the local SMBH mass density. Our prototype analysis is designed to stimulate the incorporation of results from relativistic stellar collapse and MHD accretion simulations in future Monte Carlo simulations of hierarchical structure formation to better determine the cosmological role of SMBHs and their mass and spin distributions.

Intracluster and Intragroup Entropy from Quasar Activity

We investigate how the hierarchical merging of dark matter halos, the radiative cooling of baryons, and the energy feedback from supernovae and active galactic nuclei or quasars combine to govern the amount and the thermal state of the hot plasma pervading groups and clusters of galaxies. We show that, by itself, supernova preheating of the external gas flowing into clusters falls short of explaining the observed X-ray scaling relations of the plasma luminosity L_X or the plasma entropy K versus the X-ray temperature T. To account for the scaling laws from rich to poor clusters takes preheating enhanced by the energy input from active galactic nuclei. In groups, on the other hand, the internal impacts of powerful quasars going off in member galaxies can blow some plasma out of the structure. So they depress L_X and raise K to the observed average levels; meanwhile, the sporadic nature of such impulsive events generates the intrinsic component of the wide scatter apparent in the data. The same quasar feedback gives rise in groups to entropy profiles as steep as observed, a feature hard to explain with simple preheating schemes. Finally, we argue a close connection of the L_X-T or the K-T relation with the M_{\bullet}-\sigma correlation between the host velocity dispersion and the masses of the black holes, relics of the quasar activity.

The Minimum Stellar Mass in Early Galaxies

The conditions for the fragmentation of the baryonic component during merging of dark matter halos in the early Universe are studied. We assume that the baryonic component undergoes a shock compression. The characteristic masses of protostellar molecular clouds and the minimum masses of protostars formed in these clouds decrease with increasing halo mass. This may indicate that the initial stellar mass function in more massive galaxies was shifted towards lower masses during the initial stages of their formation. This would result in an increase of the number of stars per unit halo mass, i.e., the efficiency of star formation.

Probing galaxy formation with TeV gamma ray absorption

We present here the extragalactic background light (EBL) predicted by semi-analytic models of galaxy formation, and show how measurements of the absorption of gamma rays of ∼ TeV energies via pair production on the EBL can probe cosmology and the formation of galaxies. Semi-analytic models permit a physical treatment of the key processes of galaxy formation - including gravitational collapse and merging of dark matter halos, gas cooling and dissipation, star formation, supernova feedback and metal production - and have been shown to reproduce key observations at low and high redshift. Using this approach, we investigate the consequences of variations in input assumptions such as the stellar initial mass function and the underlying cosmology. We conclude that observational studies of the absorption of ∼ 10 −2–10 2 TeV gamma rays will help to constrain the star formation history of the universe, and the nature and extent of the extinction of starlight due to dust and reradiation of the absorbed energy at infrared wavelengths.