The formation history of elliptical galaxies De Lucia, Gabriella; Springel, Volker; White, Simon D. M. ...
Monthly notices of the Royal Astronomical Society,
February 2006, Letnik:
366, Številka:
2
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ABSTRACT
We take advantage of the largest high‐resolution simulation of cosmic structure growth ever carried out – the Millennium Simulation of the concordance Λ cold dark matter (CDM) cosmogony – to ...study how the star formation histories, ages and metallicities of elliptical galaxies depend on environment and on stellar mass. We concentrate on a galaxy formation model which is tuned to fit the joint luminosity/colour/morphology distribution of low‐redshift galaxies. Massive ellipticals in this model have higher metal abundances, older luminosity‐weighted ages and shorter star formation time‐scales, but lower assembly redshifts, than less massive systems. Within clusters the typical masses, ages and metal abundances of ellipticals are predicted to decrease, on average, with increasing distance from the cluster centre. We also quantify the effective number of progenitors of ellipticals as a function of present stellar mass, finding typical numbers below two for M* < 1011 M⊙, rising to approximately five for the most massive systems. These findings are consistent with recent observational results that suggest ‘down‐sizing’ or ‘antihierarchical’ behaviour for the star formation history of the elliptical galaxy population, despite the fact that our model includes all the standard elements of hierarchical galaxy formation and is implemented on the standard, ΛCDM cosmogony.
COLD FLOWS AND THE FIRST QUASARS Di Matteo, T; KHANDAI, N; DEGRAF, C ...
Astrophysical journal. Letters,
02/2012, Letnik:
745, Številka:
2
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Observations of the most distant bright quasars imply that billion solar mass supermassive black holes (SMBHs) have to be assembled within the first 800 million years. Under our standard galaxy ...formation scenario such fast growth implies large gas densities providing sustained accretion at critical or supercritical rates onto an initial black hole seed. It has been a long standing question whether and how such high black hole accretion rates can be achieved and sustained at the centers of early galaxies. Here we use our new MassiveBlack cosmological hydrodynamic simulation covering a volume (0.75 Gpc) super(3) appropriate for studying the rare first quasars to show that steady high density cold gas flows responsible for assembling the first galaxies produce the high gas densities that lead to sustained critical accretion rates and hence rapid growth commensurate with the existence of ~10 super(9) M sub(middot in circle) black holes as early as z ~ 7. We find that under these conditions quasar feedback is not effective at stopping the cold gas from penetrating the central regions and hence cannot quench the accretion until the host galaxy reaches M sub(halo) > ~ 10 super(12) M sub(middot in circle). This cold-flow-driven scenario for the formation of quasars implies that they should be ubiquitous in galaxies in the early universe and that major (proto)galaxy mergers are not a requirement for efficient fuel supply and growth, particularly for the earliest SMBHs.
We have performed the largest ever particle simulation of a Milky Way sized dark matter halo, and present the most comprehensive convergence study for an individual dark matter halo carried out thus ...far. We have also simulated a sample of six ultrahighly resolved Milky Way sized haloes, allowing us to estimate the halo-to-halo scatter in substructure statistics. In our largest simulation, we resolve nearly 300 000 gravitationally bound subhaloes within the virialized region of the halo. Simulations of the same object differing in mass resolution by factors of up to 1800 accurately reproduce the largest subhaloes with the same mass, maximum circular velocity and position, and yield good convergence for the abundance and internal properties of dark matter substructures. We detect up to four generations of subhaloes within subhaloes, but contrary to recent claims, we find less substructure in subhaloes than in the main halo when regions of equal mean overdensity are compared. The overall substructure mass fraction is much lower in subhaloes than in the main halo. Extrapolating the main halo's subhalo mass spectrum down to an Earth mass, we predict the mass fraction in substructure to be well below 3 per cent within 100 kpc, and to be below 0.1 per cent within the solar circle. The inner density profiles of subhaloes show no sign of converging to a fixed asymptotic slope and are well fitted by gently curving profiles of Einasto form. The mean concentrations of isolated haloes are accurately described by the fitting formula of Neto et al. down to maximum circular velocities of 1.5 km s−1, an extrapolation over some five orders of magnitude in mass. However, at equal maximum circular velocity, subhaloes are more concentrated than field haloes, with a characteristic density that is typically ∼2.6 times larger and increases with decreasing distance from halo centre.
It is well known that cosmic rays contribute significantly to the pressure of the interstellar medium in our own Galaxy, suggesting that they may play an important role in regulating star formation ...during the formation and evolution of galaxies. We here discuss a novel numerical treatment of the physics of cosmic rays and its implementation in the parallel smoothed particle hydrodynamics code GADGET-2. In our methodology, the non-thermal cosmic ray population of each gaseous fluid element is approximated by a simple power law spectrum in particle momentum, characterized by an amplitude, a cut-off, and a fixed slope. Adiabatic compression and a number of physical source and sink terms are modelled which modify the cosmic ray pressure of each particle. The most important sources considered are injection by supernovae and diffusive shock acceleration, while the primary sinks are thermalization by Coulomb interactions, and catastrophic losses by hadronic interactions. We also include diffusion of cosmic rays. Using a number of test problems, we show that our scheme is numerically robust and efficient, allowing us to carry out the first cosmological structure formation simulations that account for cosmic ray physics, together with radiative cooling and star formation. In simulations of isolated galaxies, we find that cosmic rays can significantly reduce the star formation efficiencies of small galaxies, with virial velocities below ~$80~{\rm km\,s}^{-1}$, an effect that becomes progressively stronger towards low-mass scales. In cosmological simulations of the formation of dwarf galaxies at high redshift, we find that the total mass-to-light ratio of small halos and the faint end of the luminosity function are affected. The latter becomes slightly flatter. When cosmic ray acceleration in shock waves is followed as well, we find that up to $40\%$ of the energy dissipated at structure formation shocks can appear as cosmic ray pressure at redshifts around $z\sim 3{-}6$, but this fraction drops to ~$10\%$ at low redshifts when the shock distribution becomes increasingly dominated by lower Mach numbers. Despite this large cosmic ray energy content in the high-redshift intergalactic medium, the flux power spectrum of the Lyman-α forest is only affected on very small scales of $k>0.1~{\rm km^{-1}s}$, and at a weak level of $5{-}15\%$. Within virialized objects, we find lower contributions of CR-pressure, due to the increased efficiency of loss processes at higher densities, the lower Mach numbers of shocks inside halos, and the softer adiabatic index of CRs, which disadvantages them when a composite of thermal gas and cosmic rays is adiabatically compressed. The total energy in cosmic rays relative to the thermal energy within the virial radius drops from 20% for $10^{12}\,h^{-1}~{M}_\odot$ halos to 5% for rich galaxy clusters of mass $10^{15}\,h^{-1}~{M}_\odot$ in non-radiative simulations. Interestingly, the lower effective adiabatic index also increases the compressibility of the intrahalo medium, an effect that slightly increases the central concentration of the gas and the baryon fraction within the virial radius. We find that this can enhance the cooling rate onto central cluster galaxies, even though the galaxies in the cluster periphery become slightly less luminous as a result of cosmic ray feedback.
We investigate the effects of galaxy formation on the baryonic acoustic oscillation (BAO) peak by applying semi-analytic modelling techniques to the Millennium-XXL, a 3 × 1011 particle N-body ...simulation of similar volume to the future Euclid survey. Our approach explicitly incorporates the effects of tidal fields and stochasticity on halo formation, as well as the presence of velocity bias, spatially correlated merger histories, and the connection of all these with the observable and physical properties of galaxies. We measure significant deviations in the shape of the BAO peak from the expectations of a linear bias model built on top of the non-linear dark matter distribution. We find that the galaxy correlation function shows an excess close to the maximum of the BAO peak (r ∼ 110 h
−1 Mpc) and a deficit at r ∼ 90 h
−1 Mpc. Depending on the redshift, selection criteria and number density of the galaxy samples, these biased distortions can be up to 5 per cent in amplitude. They are, however, largely absorbed by marginalization over nuisance parameters in current analytical modelling of the BAO peak in configuration space, in particular into the parameter that controls the broadening due to non-linear evolution. As a result, the galaxy formation effects detected here are unlikely to bias the high-precision measurements planned by the upcoming generation of wide-field galaxy surveys.
We present a precise estimate of the bulk virial scaling relation of halos formed via hierarchical clustering in an ensemble of simulated cold dark matter cosmologies. The result is insensitive to ...cosmological parameters; the presence of a trace, dissipationless gas component; and numerical resolution down to a limit of similar to 1000 particles. The dark matter velocity dispersion scales with total mass log unk=log(1082.9 plus or minus 4.0 km s super(-1)) + (0.3361 plus or minus 0.0026)log h(z) unk/10 super(13)M unk, with h(z) being the dimensionless Hubble parameter. At fixed mass, the velocity dispersion likelihood is nearly lognormal, with scatter unk = 0.0426 plus or minus 0.015, except for a tail with higher dispersions containing 10% of the population that are merger transients. We combine this relation with the halo mass function in Lambda CDM models and show that a low normalization condition, unk( Omega m/0.3) super(0.35)=0.69, favored by recent WMAP and SDSS analysis requires that galaxy and gas-specific energies in rich clusters be 50% larger than that of the underlying dark matter. Such large energetic biases are in conflict with the current generation of direct simulations of cluster formation. A higher normalization, S sub(s) = 0.80, alleviates this tension and implies that the hot gas fraction within r sub(500) is (0.71 plus or minus 0.09) unk Omega sub(b)/ Omega sub(m), a value consistent with recent Sunyaev-Zel'dovlch observations.
Dark matter is the dominant form of matter in the Universe, but its nature is unknown. It is plausibly an elementary particle, perhaps the lightest supersymmetric partner of known particle species. ...In this case, annihilation of dark matter in the halo of the Milky Way should produce -rays at a level that may soon be observable. Previous work has argued that the annihilation signal will be dominated by emission from very small clumps (perhaps smaller even than the Earth), which would be most easily detected where they cluster together in the dark matter haloes of dwarf satellite galaxies. Here we report that such small-scale structure will, in fact, have a negligible impact on dark matter detectability. Rather, the dominant and probably most easily detectable signal will be produced by diffuse dark matter in the main halo of the Milky Way. If the main halo is strongly detected, then small dark matter clumps should also be visible, but may well contain no stars, thereby confirming a key prediction of the cold dark matter model.
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DOBA, IJS, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Galactic stellar haloes in the CDM model Cooper, A. P.; Cole, S.; Frenk, C. S. ...
Monthly notices of the Royal Astronomical Society,
August 2010, Letnik:
406, Številka:
2
Journal Article
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We present six simulations of galactic stellar haloes formed by the tidal disruption of accreted dwarf galaxies in a fully cosmological setting. Our model is based on the Aquarius project, a suite of ...high-resolution N-body simulations of individual dark matter haloes. We tag subsets of particles in these simulations with stellar populations predicted by the galform semi-analytic model. Our method self-consistently tracks the dynamical evolution and disruption of satellites from high redshift. The luminosity function (LF) and structural properties of surviving satellites, which agree well with observations, suggest that this technique is appropriate. We find that accreted stellar haloes are assembled between 1 < z < 7 from less than five significant progenitors. These progenitors are old, metal-rich satellites with stellar masses similar to the brightest Milky Way dwarf spheroidals (107–108 M⊙). In contrast to previous stellar halo simulations, we find that several of these major contributors survive as self-bound systems to the present day. Both the number of these significant progenitors and their infall times are inherently stochastic. This results in great diversity among our stellar haloes, which amplifies small differences between the formation histories of their dark halo hosts. The masses (∼ 108–109 M⊙) and density/surface-brightness profiles of the stellar haloes (from 10 to 100 kpc) are consistent with expectations from the Milky Way and M31. Each halo has a complex structure, consisting of well-mixed components, tidal streams, shells and other subcomponents. This structure is not adequately described by smooth models. The central regions (<10 kpc) of our haloes are highly prolate (c/a∼ 0.3), although we find one example of a massive accreted thick disc. Metallicity gradients in our haloes are typically significant only where the halo is built from a small number of satellites. We contrast the ages and metallicities of halo stars with surviving satellites, finding broad agreement with recent observations.
We study the population statistics of the surviving subhaloes of ΛCDM dark matter haloes using a set of very high resolution N-body simulations. These include both simulations of representative ...regions of the Universe and ultra-high resolution resimulations of individual dark matter haloes. We find that more massive haloes tend to have a larger mass fraction in subhaloes. For example, cluster size haloes typically have 7.5 per cent of the mass within R
200 in substructures of fractional mass larger than 10−5, which is 25 per cent higher than galactic haloes. There is, however, a large variance in the subhalo mass fraction from halo to halo, whereas the subhalo abundance shows much higher regularity. For dark matter haloes of fixed mass, the subhalo abundance decreases by 30 per cent between redshift 2 and 0. The subhalo abundance function correlates with the host halo concentration parameter and formation redshift. However, the intrinsic scatter is not significantly reduced for narrow ranges of concentration parameter or formation redshift, showing that they are not the dominant parameters that determine the subhalo abundance in a halo.
We use simple analytic reasoning to identify physical processes that drive the evolution of the cosmic star formation rate, , in cold dark matter universes. Based on our analysis, we formulate a ...model to characterize the redshift dependence of and compare it with results obtained from a set of hydrodynamic simulations that include star formation and feedback. We find that the cosmic star formation rate is described by two regimes. At early times, densities are sufficiently high and cooling times sufficiently short that abundant quantities of star-forming gas are present in all dark matter haloes that can cool by atomic processes. Consequently, generically rises exponentially as z decreases, independent of the details of the physical model for star formation, but dependent on the normalization and shape of the cosmological power spectrum. This part of the evolution is dominated by gravitationally driven growth of the halo mass function. At low redshifts, densities decline as the universe expands to the point that cooling is inhibited, limiting the amount of star-forming gas available. We find that in this regime the star formation rate scales approximately as , in proportion to the cooling rate within haloes. We demonstrate that the existence of these two regimes leads to a peak in the star formation rate at an intermediate redshift z=zpeak. We discuss how the location of this peak depends on our model parameters, and show that the peak cannot occur above a limiting redshift of z≈ 8.7. For the star formation efficiency adopted in our numerical simulations, zpeak≈ 5–6, with half of all stars forming at redshifts larger than z≃ 2.2. We derive analytic expressions for the full star formation history and show that they match our simulation results to better than ≃10 per cent. Using various approximations, we reduce the expressions to a simple analytic fitting function for that can be used to compute global cosmological quantities that are directly related to the star formation history. As examples, we consider the integrated stellar density, the supernova and gamma-ray burst rates observable on Earth, the metal enrichment history of the Universe, and the density of compact objects. We also briefly discuss the expected dependence of the star formation history on cosmological parameters and the physics of the gas.
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