We present results for a galaxy formation model that includes a simple treatment for the disruption of dwarf galaxies by gravitational forces and galaxy encounters within galaxy clusters. This is implemented a posteriori in a semi-analytic model by considering the stability of cluster dark matter subhaloes at z= 0. We assume that a galaxy whose dark matter substructure has been disrupted will itself disperse, while its stars become part of the population of intracluster stars responsible for the observed intracluster light. Despite the simplicity of this assumption, our results show a substantial improvement over previous models and indicate that the inclusion of galaxy disruption is indeed a necessary ingredient of galaxy formation models. We find that galaxy disruption suppresses the number density of dwarf galaxies by about a factor of 2. This makes the slope of the faint end of the galaxy luminosity function shallower, in agreement with observations. In particular, the abundance of faint, red galaxies is strongly suppressed. As a result, the luminosity function of red galaxies and the distinction between the red and the blue galaxy populations in colour–magnitude relationships are correctly predicted. Finally, we estimate a fraction of intracluster light comparable to that found in clusters of galaxies.
We present results for a galaxy formation model which includes a simple treatment for the disruption of dwarf galaxies by gravitational forces and galaxy encounters within galaxy clusters. This is implemented a posteriori in a semi–analytic model by considering the stability of the cluster dark matter haloes at z = 0. We assume that a galaxy whose dark matter substructure has been disrupted will itself disperse, while its stars become part of the population of intracluster stars responsible for the observed intracluster light. Despite the simplicity of this assumption, our results show a substantial improvement over previous models and indicate that the inclusion of galaxy disruption is indeed a necessary ingredient of galaxy formation models. We find that galaxy disruption suppresses the number density of dwarf galaxies by about factor of two. This makes the slope of the faint end of the galaxy luminosity function shallower, in agreement with observations. In particular, the abundance of faint, red galaxies is strongly suppressed. As a result, the luminosity function of red galaxies and the distinction between the red and the blue galaxy populations in colour–magnitude relationships are correctly predicted. Finally, we estimate a fraction of intracluster light comparable to that found in clusters of galaxies.
Emission lines from metals offer one of the most promising ways to detect the elusive warm-hot intergalactic medium (WHIM; 10^5 K10^6 K). We find that the OVIII 18.97A is the strongest emission line, with a predicted maximum surface brightness of ~10^2 photon/s/cm^2/sr, but a number of other lines are only slightly weaker. All lines show a strong correlation between the intensity of the observed flux and the density and metallicity of the gas responsible for the emission. On the other hand, the potentially detectable emission consistently corresponds to the temperature at which the emissivity of the electronic transition peaks. The emission traces neither the baryonic nor the metal mass. In particular, the emission that is potentially detectable with proposed missions, traces overdense (rho>10^2rho_mean) and metal-rich (Z>0.1Z_sun) gas in and around galaxies and groups. While soft X-ray line emission is therefore not a promising route to close the baryon budget, it does offer the exciting possibility to image the gas accreting onto and flowing out of galaxies.
We present a detailed analysis of predicted galaxy-galaxy merger fractions and rates in the Millennium simulation and compare these with the most up to date observations of the same quantities up to z~3. We carry out our analysis by considering the predicted merger history in the Millennium simulation within a given time interval, as a function of stellar mass. This method, as opposed to pair fraction counts, considers mergers that have already taken place, and allows a more direct comparison with the observed rates and fractions measured with the concentration-asymmetry-clumpiness (CAS) method. We examine the evolution of the predicted merger fraction and rate in the Millennium simulation for galaxies with stellar masses M_* ~ 10^9 - 10^12 M_sun. We find that the predicted merger rates and fractions match the observations well for galaxies with M_* > 10^11 M_sun at z<2, while significant discrepancies occur at lower stellar masses, and at z>2 for M_* > 10^11 M_sun systems. At z>2 the simulations underpredict the observed merger fractions by a factor of 4-10. The shape of the predicted merger fraction and rate evolutions are similar to the observations up to z~2, and peak at 1 10^11 M_sun. We discuss possible reasons for these discrepancies, and compare different realisations of the Millennium simulation to understand the effect of varying the physical implementation of feedback. We conclude that the comparison is potentially affected by a number of issues, including uncertainties in interpreting the observations and simulations in terms of the assumed merger mass ratios and merger time-scales. (abridged)
We investigate the physics driving the cosmic star formation (SF) history using the more than fifty large, cosmological, hydrodynamical simulations that together comprise the OverWhelmingly Large Simulations (OWLS) project. We systematically vary the parameters of the model to determine which physical processes are dominant and which aspects of the model are robust. Generically, we find that SF is limited by the build-up of dark matter haloes at high redshift, reaches a broad maximum at intermediate redshift, then decreases as it is quenched by lower cooling rates in hotter and lower density gas, gas exhaustion, and self-regulated feedback from stars and black holes. The higher redshift SF is therefore mostly determined by the cosmological parameters and to a lesser extent by photo-heating from reionization. The location and height of the peak in the SF history, and the steepness of the decline towards the present, depend on the physics and implementation of stellar and black hole feedback. Mass loss from intermediate-mass stars and metal-line cooling both boost the SF rate at late times. Galaxies form stars in a self-regulated fashion at a rate controlled by the balance between, on the one hand, feedback from massive stars and black holes and, on the other hand, gas cooling and accretion. Paradoxically, the SF rate is highly insensitive to the assumed SF law. This can be understood in terms of self-regulation: if the SF efficiency is changed, then galaxies adjust their gas fractions so as to achieve the same rate of production of massive stars. Self-regulated feedback from accreting black holes is required to match the steep decline in the observed SF rate below redshift two, although more extreme feedback from SF, for example in the form of a top-heavy IMF at high gas pressures, can help.
Rest-frame UV emission lines offer the possibility to directly image the gas around high-redshift galaxies with upcoming optical instruments. We use a suite of large, hydrodynamical simulations to predict the nature and detectability of emission lines from the intergalactic medium at 2
In this paper we review the current predictions of numerical simulations for the origin and observability of the warm hot intergalactic medium (WHIM), the diffuse gas that contains up to 50 per cent of the baryons at z∼0. During structure formation, gravitational accretion shocks emerging from collapsing regions gradually heat the intergalactic medium (IGM) to temperatures in the range T∼105–107 K. The WHIM is predicted to radiate most of its energy in the ultraviolet (UV) and X-ray bands and to contribute a significant fraction of the soft X-ray background emission. While O vi and C iv absorption systems arising in the cooler fraction of the WHIM with T∼105–105.5 K are seen in FUSE and Hubble Space Telescope observations, models agree that current X-ray telescopes such as Chandra and XMM-Newton do not have enough sensitivity to detect the hotter WHIM. However, future missions such as Constellation-X and XEUS might be able to detect both emission lines and absorption systems from highly ionised atoms such as O vii, O viii and Fe xvii.
Approximately half the baryons in the local Universe are thought to reside in the warm-hot intergalactic medium (WHIM), i.e. diffuse gas with temperatures in the range 105 < T < 107 K. Emission lines from metals in the UV band are excellent tracers of the cooler fraction of this gas, with T≲ 106 K. We present predictions for the surface brightness of a sample of UV lines that could potentially be observed by the next generation of UV telescopes at z < 1. We use a subset of simulations from the OverWhelmingly Large Simulations project to create emission maps and to investigate the effect of varying the physical prescriptions for star formation, supernova and active galactic nuclei (AGN) feedback, chemodynamics and radiative cooling. Most models agree with each other to within a factor of a few, indicating that the predictions are robust. Of the lines we consider, C iii (977 Å) is the strongest line, but it typically traces gas colder than 105 K. The same is true for Si iv (1393,1403 Å). The second strongest line, C iv (1548,1551 Å), traces circumgalactic gas with T∼ 105 K. O vi (1032,1038 Å) and Ne viii (770,780 Å) probe the warmer (T∼ 105.5 and 106 K, respectively) and more diffuse gas that may be a better tracer of the large-scale structure. N v (1239,1243 Å) emission is intermediate between C iv and O vi. The intensity of all emission lines increases strongly with gas density and metallicity, and for the bright emission it is tightly correlated with the temperature for which the line emissivity is highest. In particular, the C iii, C iv, Si iv and O vi emission that is sufficiently bright to be potentially detectable in the near future (surface brightness ≳103 photon s−1 cm−2 sr−1) comes from relatively dense (ρ > 102ρmean) and metal rich (Z≳ 0.1 Z⊙) gas. As such, emission lines are highly biased tracers of the missing baryons and are not an optimal tool to close the baryon budget. However, they do provide a powerful means to detect the gas cooling on to or flowing out of galaxies and groups.
We present a new semi–analytic treatment of the evolution of galactic winds within high resolution, large scale cosmological N–body simulations of a and we make predictions for the volume filling factor of winds as a function of our model parameters. We then verify this prediction by extracting a set of synthetic spectra along random lines of sight through our simulated box and by calculating the probability distribution function (PDF) of the spectral flux. We find that galactic winds do not significantly modify the PDF. We finally argue that the increased flux transmissivity found by Adelberger et al. (2003) around a small sample of Lyman break galaxies may be explained by the presence of hot ionised bubbles due to pressure–driven winds outflowing from the galaxies. However, this effect cannot be explained by cooled, momentum–driven winds. We conclude that the result of Adelberger et al. (2003) may be the outcome of a selection effect.
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