A. J. Benson

Carnegie Institution for Science, Washington, West Virginia, United States

Are you A. J. Benson?

Claim your profile

Publications (139)538.72 Total impact

  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We describe the methodology to include nonlinear evolution, including tidal effects, in the computation of subhalo distribution properties in both cold (CDM) and warm (WDM) dark matter universes. Using semi-analytic modeling, we include effects from dynamical friction, tidal stripping, and tidal heating, allowing us to dynamically evolve the subhalo distribution. We calibrate our nonlinear evolution scheme to the CDM subhalo mass function in the Aquarius N-body simulation, producing a subhalo mass function within the range of simulations. We find tidal effects to be the dominant mechanism of nonlinear evolution in the subhalo population. Finally, we compute the subhalo mass function for $m_\chi=1.5$ keV WDM including the effects of nonlinear evolution, and compare radial number densities and mass density profiles of subhalos in CDM and WDM models. We show that all three signatures differ between the two dark matter models, suggesting that probes of substructure may be able to differentiate between them.
    The Astrophysical Journal 07/2014; 792(1). · 6.73 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We compare predictions of cooled masses and cooling rates from three stripped-down Semi-Analytic Models (SAMs) of galaxy formation with the results of N-body+SPH simulations with gas particle mass of 3.9x10^6 Msun, where radiative cooling of a gas of primordial composition is implemented. We also run a simulation where cooling is switched on at redshift ~2, in order to test cooling models in a regime in which their approximations are expected to be valid. We confirm that cooling models implemented in SAMs are able to predict the amount of cooled mass at z=0 to within ~20 per cent. However, some relevant discrepancies are found. (i) When the contribution from poorly resolved halos is subtracted out, SAMs tend to under-predict by ~30 per cent the mass that cools in the infall-dominated regime. (ii) At large halo masses SAMs tend to over-predict cooling rates, though the numerical result may be affected by the use of SPH. (iii) As found in our previous work, cooling rates are found to be significantly affected by model details: simulations disfavour models with large cores and with quenching of cooling at major mergers. (iv) When cooling is switched on at z~2, cold gas accumulates very quickly in the simulated halos. This accumulation is reproduced by SAMs with varying degrees of accuracy.
    04/2014; 441(3).
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Well-motivated particle physics theories predict the existence of particles (such as sterile neutrinos) which acquire non-negligible thermal velocities in the early universe. These particles could behave as warm dark matter (WDM) and generate a small-scale cutoff in the linear density power spectrum which scales approximately inversely with the particle mass. If this mass is of order a keV, the cutoff occurs on the scale of dwarf galaxies. Thus, in WDM models the abundance of small galaxies, such as the satellites that orbit in the halo of the Milky Way, depends on the mass of the warm particle. The abundance also scales with the mass of the host galactic halo. We use the \galform semi-analytic model of galaxy formation to calculate the properties of galaxies in universes in which the dark matter is warm. Using this method, we can compare the predicted satellite luminosity functions to the observed data for the Milky Way dwarf spheroidals, and determine a lower bound on the thermally produced WDM particle mass. This depends strongly on the value of the Milky Way halo mass and, to some extent, on the baryonic physics assumed; we examine both of these dependencies. For our fiducial model we find that for a particle mass of 3.3 keV (the 2$\sigma$ lower limit found by Viel et al. from a recent analysis of the Lyman-$\alpha$ forest) the Milky Way halo mass is required to be $> 1.4 \times 10^{12}$ \msun. For this same fiducial model, we also find that all WDM particle masses are ruled out (at 95% confidence) if the halo of the Milky Way has a mass smaller than $1.1 \times 10^{12}$ \msun, while if the mass of the Galactic halo is greater than 1.8 $\times 10^{12}$ \msun, only WDM particle masses larger than 2 keV are allowed.
    10/2013;
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We study the oscillations and stability of self-gravitating cylindrically symmetric fluid systems and collisionless systems. This is done by studying small perturbations to the equilibrium system and finding the normal modes, using methods similar to those used in astroseismology. We find that there is a single sequence of purely radial modes that become unstable if the adiabatic exponent is less than 1. Nonradial modes can be divided into p modes, which are stable and pressure-driven, and g modes, which are are gravity driven. The g modes become unstable if the adiabatic exponent is greater than the polytrope index. These modes are analogous to the modes of a spherical star, but their behavior is somewhat different because a cylindrical geometry has less symmetry than a spherical geometry. This implies that perturbations are classified by a radial quantum number, an azimuthal quantum number, and wavelength in the z direction, which can become arbitrarily large. We find that decreasing this wavelength increases the frequency of stable modes and increases the growth rate of unstable modes. We use use variational arguments to demonstrate that filaments of collisionless matter with ergodic distribution functions are stable to purely radial perturbations, and that filaments with ergodic power-law distribution functions are stable to all perturbations.
    Monthly Notices of the Royal Astronomical Society 05/2013; · 5.52 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We present a clustering analysis of X-ray selected AGN by compiling X-ray samples from the literature and re-estimating the dark matter (DM) halo masses of AGN in a uniform manner. We find that moderate luminosity AGN (Lx(2-10 keV)=10^42-10^44 erg/sec) in the z=0-1.3 Universe are typically found in DM haloes with masses of ~10^13 Msun. We then compare our findings to the theoretical predictions of the coupled galaxy and black hole formation model GALFORM. We find good agreement when our calculation includes the hot-halo mode of accretion onto the central black hole. This type of accretion, which is additional to the common cold accretion during disk instabilities and galaxy mergers, is tightly coupled to the AGN feedback in the model. The hot-halo mode becomes prominent in DM haloes with masses greater than ~10^12.5 Msun, where AGN feedback typically operates, giving rise to a distinct class of moderate luminosity AGN that inhabit rich clusters and superclusters. Cold gas fuelling of the black hole cannot produce the observationally inferred DM halo masses of X-ray AGN. Switching off AGN feedback in the model results in a large population of luminous quasars (Lx(2-10 keV) > 10^44 erg/sec) in DM haloes with masses up to ~10^14 Msun, which is inconsistent with the observed clustering of quasars. The abundance of hot-halo AGN decreases significantly in the z~3-4 universe. At such high redshifts, the cold accretion mode is solely responsible for shaping the environment of moderate luminosity AGN. Our analysis supports two accretion modes (cold and hot) for the fuelling of supermassive black holes and strongly underlines the importance of AGN feedback in cosmological models both of galaxy formation and black hole growth.
    Monthly Notices of the Royal Astronomical Society 05/2013; 435(1). · 5.52 Impact Factor
  • Source
    Arya Farahi, Andrew J. Benson
    [Show abstract] [Hide abstract]
    ABSTRACT: We present a new method to compute the first crossing distribution in excursion set theory for the case of correlated random walks. We use a combination of the path integral formalism of Maggiore & Riotto, and the integral equation solution of Zhang & Hui, and Benson et al. to find a numerically robust and convenient algorithm to derive the first crossing distribution in terms of a perturbative expansion around the limit of an uncorrelated random walk. We apply this methodology to the specific case of a Gaussian random density field filtered with a Gaussian smoothing function. By comparing our solutions to results from Monte Carlo calculations of the first crossing distribution we demonstrate that our method accurate for power spectra $P(k)\propto k^n$ for $n=1$, becoming less accurate for smaller values of $n$. It is therefore complementary to the method of Musso & Sheth, which will therefore be more useful for standard $\Lambda$CDM power spectra. Our approach is quite general, and can be adapted to other smoothing functions, and also to non-Gaussian density fields.
    Monthly Notices of the Royal Astronomical Society 03/2013; 433(4). · 5.52 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We study the impact of star formation and stellar feedback prescriptions on galaxy properties predicted by means of "stripped-down" versions of independently developed semi-analytic models (SAMs). These include cooling, star formation, feedback from supernovae (SNe) and simplified prescriptions for galaxy merging, but no chemical evolution, disc instabilities or AGN feedback. We run these versions on identical samples of dark matter (DM) haloes extracted from high-resolution N-body simulations in order to perform both statistical analysis and object-by-object comparisons. We compare our results with previous work based on stripped-down versions of the same SAMs including only gas cooling, and show that all feedback models provide coherent modifications in the distribution of baryons between the various gas phases. In particular, we find that the predicted hot gas fractions are considerably increased by up to a factor of three, while the corresponding cold gas fractions are correspondingly decreased, and a significant amount of mass is ejected from the DM halo. Nonetheless, we also find relevant differences in the predicted properties of model galaxies among the three SAMs: these deviations are more relevant at mass scales comparable to that of our own Galaxy, and are reduced at larger masses, confirming the varying impact of stellar feedback at different mass scales. We also check the effect of enhanced star formation events (i.e. starbursts modes), defined in connection with galaxy mergers. We find that, in general, these episodes have a limited impact in the overall star formation histories of model galaxies, even in massive DM halos where merger-driven star formation has often been considered very important.
    01/2013;
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: The escape of ionizing radiation from galaxies plays a critical role in the evolution of gas in galaxies, and the heating and ionization history of the intergalactic medium. We present semi-analytic calculations of the escape fraction of ionizing radiation for both hydrogen and helium from galaxies ranging from primordial systems to disk-type galaxies that are not heavily dust-obscured. We consider variations in the galaxy density profile, source type, location, and spectrum, and gas overdensity/distribution factors. For sufficiently hard first-light sources, the helium ionization fronts closely track or advance beyond that of hydrogen. Key new results in this work include calculations of the escape fractions for He I and He II ionizing radiation, and the impact of partial ionization from X-rays from early AGN or stellar clusters on the escape fractions from galaxy halos. When factoring in frequency-dependent effects, we find that X-rays play an important role in boosting the escape fractions for both hydrogen and helium, but especially for He II. We briefly discuss the implications of these results for recent observations of the He II reionization epoch at low redshifts, as well as the UV data and emission-line signatures from early galaxies anticipated from future satellite missions.
    The Astrophysical Journal 12/2012; 770(1). · 6.73 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We present a spectro-photometric survey of 2522 extragalactic globular clusters (GCs) around twelve early-type galaxies, nine of which have not been published previously. Combining space-based and multi-colour wide field ground-based imaging, with spectra from the Keck DEIMOS instrument, we obtain an average of 160 GC radial velocities per galaxy, with a high velocity precision of 15 km/s per GC. After studying the photometric properties of the GC systems, such as their spatial and colour distributions, we focus on the kinematics of metal-poor (blue) and metal-rich (red) GC subpopulations to an average distance of ~8 effective radii from the galaxy centre. Our results show that for some systems the bimodality in GC colour is also present in GC kinematics. The kinematics of the red GC subpopulations are strongly coupled with the host galaxy stellar kinematics. The blue GC subpopulations are more dominated by random motions, especially in the outer regions, and decoupled from the red GCs. Peculiar GC kinematic profiles are seen in some galaxies: the blue GCs in NGC 821 rotate along the galaxy minor axis, whereas the GC system of the lenticular galaxy NGC 7457 appears to be strongly rotation supported in the outer region. We supplement our galaxy sample with data from the literature and carry out a number of tests to study the kinematic differences between the two GC subpopulations. We confirm that the GC kinematics are coupled with the host galaxy properties and find that the velocity kurtosis and the slope of their velocity dispersion profiles is different between the two GC subpopulations in more massive galaxies.
    Monthly Notices of the Royal Astronomical Society 09/2012; 428(1). · 5.52 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We describe a methodology to accurately compute halo mass functions, progenitor mass functions, merger rates and merger trees in non-cold dark matter universes using a self-consistent treatment of the generalized extended Press-Schechter formalism. Our approach permits rapid exploration of the subhalo population of galactic halos in dark matter models with a variety of different particle properties or universes with rolling, truncated, or more complicated power spectra. We make detailed comparisons of analytically derived mass functions and merger histories with recent warm dark matter cosmological N-body simulations, and find excellent agreement. We show that, once the accretion of smoothly distributed matter is accounted for, coarse-grained statistics such as the mass accretion history of halos can be almost indistinguishable between cold and warm dark matter cases. However, the halo mass function and progenitor mass functions differ significantly, with the warm dark matter cases being strongly suppressed below the free-streaming scale of the dark matter. We demonstrate the importance of using the correct solution for the excursion set barrier first-crossing distribution in warm dark matter - if the solution for a flat barrier is used instead the truncation of the halo mass function is much slower, leading to an overestimate of the number of low mass halos.
    Monthly Notices of the Royal Astronomical Society 09/2012; 428(2). · 5.52 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: We predict the formation histories, properties and descendants of Lyman-break galaxies (LBGs) in the Λ cold dark matter cosmology. In our model, which incorporates a top-heavy initial mass function in starbursts, we find that most LBGs are starbursts triggered by minor mergers of galaxies. The duration of the LBG phase is predicted to be quite short, ˜ 20-60 Myr. We investigate the distributions of stellar and halo masses and morphologies for bright (?) and faint (?) LBGs at z= 3, 6 and 10 [where we classify LBGs according to their rest-frame ultraviolet (UV) luminosities relative the observed characteristic luminosity ? at z≈ 3]. Bright LBGs at z= 3 are predicted to have median stellar masses ˜ 1 × 109 h-1 M⊙ and host halo masses ˜ 3 × 1011 h-1 M⊙, and be typically mildly disc dominated in stellar mass. On the other hand, faint LBGs at z= 10 are predicted to have median stellar masses of only ˜ 1 × 107 h-1 M⊙ and host halo masses ˜ 2 × 1010 h-1 M⊙, and be generally bulge dominated. Bright LBGs at z= 3 evolve to present-day galaxies with median stellar mass ˜ 5 × 1010 h-1 M⊙ (comparable to the Milky Way), consisting of roughly equal numbers of disc- and bulge-dominated systems, and hosted by haloes with median mass ˜ 2 × 1013 h-1 M⊙ (corresponding to medium-size galaxy groups). The model predicts that 40 per cent of Milky Way mass galaxies at the present day have a bright LBG progenitor in the redshift range 3 < z < 4, while 95 per cent have a faint LBG progenitor in the same redshift range and 7 per cent have a faint LBG progenitor at 10 < z < 11. With our multiwavelength model, we also investigate the overlap between the LBG population and that of selected submillimetre galaxies (SMGs); at z= 3, only ˜ 1 per cent of bright LBGs are also predicted to be bright SMGs (with an ?m flux in excess of 5 mJy).
    Monthly Notices of the Royal Astronomical Society 07/2012; 423(4):3709-3726. · 5.52 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We introduce a method for constructing end-to-end mock galaxy catalogues using a semi-analytical model of galaxy formation, applied to the halo merger trees extracted from a cosmological N-body simulation. The mocks that we construct are lightcone catalogues, in which a galaxy is placed according to the epoch at which it first enters the past lightcone of the observer, and incorporate the evolution of galaxy properties with cosmic time. We determine the position between the snapshot outputs at which a galaxy enters the observer's lightcone by interpolation. As an application, we consider the effectiveness of the BzK colour selection technique, which was designed to isolate galaxies in the redshift interval 1.4<z<2.5. The mock catalogue is in reasonable agreement with the observed number counts of all BzK galaxies, as well as with the observed counts of the subsample of BzKs that are star-forming galaxies. We predict that over 75 per cent of the model galaxies with K_{AB}<=23, and 1.4<z<2.5, are selected by the BzK technique. Interloper galaxies, outside the intended redshift range, are predicted to dominate bright samples of BzK galaxies (i.e. with K_{AB}<=21). Fainter K-band cuts are necessary to reduce the predicted interloper fraction. We also show that shallow B-band photometry can lead to confusion in classifying BzK galaxies as being star-forming or passively evolving. Overall, we conclude that the BzK colour selection technique is capable of providing a sample of galaxies that is representative of the 1.4<z<2.5 galaxy population.
    Monthly Notices of the Royal Astronomical Society 06/2012; 429(1). · 5.52 Impact Factor
  • R. G. Bower, A. J. Benson, Robert A. Crain
    [Show abstract] [Hide abstract]
    ABSTRACT: The observed stellar mass function (SMF) is very different to the halo mass function predicted by Λ cold dark matter (ΛCDM), and it is widely accepted that this is due to energy feedback from supernovae and black holes. However, the strength and form of this feedback is not understood. In this paper, we use the phenomenological model GALFORM to explore how galaxy formation depends on the strength and halo mass dependence of feedback. We focus on 'expulsion' models in which the wind mass loading, β, is proportional to ?, with n= 0, 1, 2 and contrast these models with the successful Bower et al. model (B8W7), for which ?. A crucial development is that our code explicitly accounts for the recapture of expelled gas as the system's halo mass (and thus gravitational potential) increases. While models with high wind speed and mass loading result in a poor match to the observed SMF, a model with slower wind speed matches the flat portion of the SMF at M★˜ 109-1011 h-1 M⊙. When combined with active galactic nucleus feedback, the model provides a good description of the observed SMF above 109 h-1 M⊙. In order to explore the impact of different feedback schemes further, we examine how the expulsion models compare with a further range of observational data, contrasting the results with the B8W7 model. In the expulsion models, the brightest galaxies are assembled more recently, and the specific star formation rates of galaxies decrease strongly with decreasing stellar mass. The expulsion models tend to have a cosmic star formation density that is dominated by lower mass galaxies at z= 1-3, and dominated by high-mass galaxies at low redshift. These trends are in conflict with observational data, but the comparison highlights some deficiencies of the B8W7 model also. The experiments in this paper not only give us important physical insight into the impact of the feedback process on the formation histories of galaxies, but the strong mass dependence of feedback adopted in B8W7 still appears to provide the most promising description of the observed Universe.
    Monthly Notices of the Royal Astronomical Society 06/2012; 422(4):2816-2840. · 5.52 Impact Factor
  • Aparna Venkatesan, A. Benson
    [Show abstract] [Hide abstract]
    ABSTRACT: We present calculations of the escape fraction of ionizing radiation from galaxies in the early universe. Such galaxies are the formation sites of the first stars and quasars, which are strong sources of hard ionizing radiation. The escape fraction is a critical input parameter for any cosmological code or simulation that tracks hydrogen or helium reionization at high redshifts. We present our results on the escape fractions for HI, HeI and HeII across a representative range of galaxy properties including varying source spectral indices, stellar mass functions, metallicities and galaxy density profiles. We also examine these effects on the thermal and ionization history of the intergalactic medium, including the critical role of X-rays from first-light sources.
    05/2012;
  • Source
    R. G. Bower, A. J. Benson, R. A. Crain
    [Show abstract] [Hide abstract]
    ABSTRACT: The observed stellar mass function (SMF) is very different to the halo mass function predicted by Lambda-CDM, and it is widely accepted that this is due to energy feedback from supernovae and black holes. However, the strength and form of this feedback is not understood. In this paper, we use the phenomenological model GALFORM to explore how galaxy formation depends on the strength and halo mass dependence of feedback. We focus on 'expulsion' models in which the wind mass loading, beta, is proportional to 1/\vdisk^n, with n=0,1,2 and contrast these models with the successful Bower et al.\ 2008 model (B8W7). A crucial development is that our code explicitly accounts for the recapture of expelled gas as the system's halo mass (and thus gravitational potential) increases. We find that a model with modest wind speed but high mass loading matches the flat portion of the SMF. When combined with AGN feedback, the model provides a good description of the observed SMF above 10^9 h^-1 Msol. However, in the expulsion models, the brightest galaxies are assembled more recently than in B8W7, and the specific star formation rates of galaxies decrease strongly with decreasing stellar mass. The expulsion models also tend to have a cosmic star formation density that is dominated by lower mass galaxies at z=1-3, and dominated high mass galaxies at low redshift. These trends are in conflict with observational data, but the comparison highlights some deficiencies of the B8W7 model also. The experiments in this paper give us important physical insight to the impact of the feedback process on the formation histories of galaxies, but the strong mass dependence of feedback adopted in B8W7 still appears to provide the most promising description of the observed universe.
    12/2011;
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We study the evolution of the cold gas content of galaxies by splitting the interstellar medium into its atomic and molecular hydrogen components, using the galaxy formation model galform in the Λ cold dark matter framework. We calculate the molecular-to-atomic hydrogen mass ratio, H2/H i, in each galaxy using two different approaches, the pressure-based empirical relation of Blitz & Rosolowsky and the theoretical model of Krumholz, McKeee & Tumlinson, and apply them to consistently calculate the star formation rates of galaxies. We find that the model based on the Blitz & Rosolowsky law predicts an H i mass function, 12CO (1–0) luminosity function, correlations between H2/H i and stellar and cold gas mass, and infrared–12CO molecule luminosity relation in good agreement with local and high-redshift observations. The H i mass function evolves weakly with redshift, with the number density of high-mass galaxies decreasing with increasing redshift. In the case of the H2 mass function, the number density of massive galaxies increases strongly from z= 0 to 2, followed by weak evolution up to z= 4. We also find that H2/H i of galaxies is strongly dependent on stellar and cold gas mass, and also on redshift. The slopes of the correlations between H2/H i and stellar and cold gas mass hardly evolve, but the normalization increases by up to two orders of magnitude from z= 0 to 8. The strong evolution in the H2 mass function and H2/H i is primarily due to the evolution in the sizes of galaxies and, secondarily, in the gas fractions. The predicted cosmic density evolution of H i agrees with the observed evolution inferred from damped Lyα systems, and is always dominated by the H i content of low- and intermediate-mass haloes. We find that previous theoretical studies have largely overestimated the redshift evolution of the global H2/H i due to limited resolution. We predict a maximum of  at z≈ 3.5.
    Monthly Notices of the Royal Astronomical Society 12/2011; 418(3):1649 - 1667. · 5.52 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: We use a coupled model of the formation and evolution of galaxies and black holes (BHs) to study the evolution of active galactic nuclei (AGNs) in a cold dark matter universe. The model is embedded in the galaxy formation code galform and predicts the masses, spins and mass accretion histories of BHs in tandem with the formation of their host galaxies. BHs grow by accretion during starbursts, triggered by discs becoming dynamically unstable or by galaxy mergers, and accretion from quasi-hydrostatic hot gas haloes. Using an empirical law for AGN obscuration, our model matches the observed luminosity functions (LFs) of AGNs over a wide range of redshifts. Due to the suppression of cooling in massive haloes by AGN feedback, at low redshift (z≲ 2), the brightest quasars (Lbol≳ 1046 erg s−1) are predicted preferentially to inhabit haloes with masses . The model predicts a hierarchical buildup of BH mass, with the typical mass of actively growing BHs increasing with decreasing redshift. Nevertheless, the model displays clear ‘downsizing’ as reflected in the differential evolution of the space density of faint and bright AGNs. This arises naturally from the interplay between the starburst and hot gas halo accretion modes. The faint end of the LF is dominated by massive BHs accreting at low rates via a thick disc, primarily during the hot-halo mode. The bright end is populated by BHs accreting close to or above the Eddington limit during the starburst mode. Obscuration plays a central role in determining the observed abundance of AGNs and, hence, in their implied cosmic evolution.
    Monthly Notices of the Royal Astronomical Society 11/2011; 419(4):2797 - 2820. · 5.52 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: We present a model for the satellites of the Milky Way in which galaxy formation is followed using semi-analytic techniques applied to the six high-resolution N-body simulations of galactic haloes of the Aquarius project. The model, calculated using the galform code, incorporates improved treatments of the relevant physics in the Λ cold dark matter cosmogony, particularly a self-consistent calculation of reionization by ultraviolet (UV) photons emitted by the forming galaxy population, including the progenitors of the central galaxy. Along the merger tree of each halo, the model calculates gas cooling (by Compton scattering off cosmic microwave background photons, molecular hydrogen and atomic processes), gas heating (from hydrogen photoionization and supernova energy), star formation and evolution. The evolution of the intergalactic medium is followed simultaneously with that of the galaxies. Star formation in the more massive progenitor subhaloes is suppressed primarily by supernova feedback, while for smaller subhaloes, it is suppressed primarily by photoionization due to external and internal sources. The model is constrained to match a wide range of properties of the present-day galaxy population as a whole, but at high redshift it requires an escape fraction of UV photons near unity in order to completely reionize the universe by redshift z≳ 8. In the most successful model, the local sources photoionize the pre-galactic region completely by z≃ 10. In addition to the luminosity function of Milky Way satellites, the model matches their observed luminosity–metallicity relation, their radial distribution and the inferred values of the mass within 300 pc, which in the models increase slowly but significantly with luminosity. There is a large variation in satellite properties from halo to halo, with the luminosity function, for example, varying by a factor of ∼2 among the six simulations.
    Monthly Notices of the Royal Astronomical Society 09/2011; 417(2):1260 - 1279. · 5.52 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: 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.
    Monthly Notices of the Royal Astronomical Society 07/2011; · 5.52 Impact Factor
  • Source
    Aparna Venkatesan, Andrew J. Benson
    [Show abstract] [Hide abstract]
    ABSTRACT: The first stars and quasars are known sources of hard ionizing radiation in the first billion years of the Universe. We examine the joint effects of X-rays and hard UV radiation from such first-light sources on the hydrogen and helium reionization of the intergalactic medium (IGM) at early times, and the associated heating. We study the growth and evolution of individual HII, HeII and HeIII regions around early galaxies with first stars and/or QSO populations. We find that in the presence of helium-ionizing radiation, X-rays may not dominate the ionization and thermal history of the IGM at redshifts, z, of 10-20, contributing relatively modest increases to IGM ionization, and heating up to about 10^3--10^5 K in IGM temperatures. We also calculate the 21 cm signal expected from a number of scenarios with metal-free starbursts and quasars at these redshifts. The peak values for the spin temperature reach about 10^4 to 10^5 K in such cases. The maximum values for the 21 cm brightness temperature are around 30-40 mK in emission, while the net values of the 21 cm absorption signal range from about a few to 60 mK on scales of 0.01-1 Mpc. We find that the 21 cm signature of X-ray versus UV ionization could be distinct, with the emission signal expected from X-rays alone occurring at smaller scales than that from UV radiation, resulting from the inherently different spatial scales at which X-ray and UV ionization/heating manifest. This difference is time-dependent, and becomes harder to distinguish with an increasing X-ray contribution to the total ionizing photon production. Such differing scale-dependent contributions from X-ray and UV photons may therefore "blur" the 21 cm signature of the percolation of ionized bubbles around early halos (depending on whether a cosmic X-ray or UV background built up first), and affect the interpretation of 21 cm data constraints on reionization.
    07/2011;

Publication Stats

5k Citations
538.72 Total Impact Points

Institutions

  • 2014
    • Carnegie Institution for Science
      Washington, West Virginia, United States
  • 2001–2012
    • California Institute of Technology
      • Department of Astronomy
      Pasadena, California, United States
  • 2004–2008
    • University of Oxford
      • Department of Physics
      Oxford, ENG, United Kingdom
  • 2006
    • Technion - Israel Institute of Technology
      H̱efa, Haifa District, Israel
    • University of Victoria
      • Department of Physics and Astronomy
      Victoria, British Columbia, Canada
  • 1998–2004
    • Durham University
      • Department of Physics
      Durham, ENG, United Kingdom
  • 2002
    • Princeton University
      • Department of Astrophysical Sciences
      Princeton, New Jersey, United States
    • University of Chicago
      • Department of Astronomy and Astrophysics
      Chicago, IL, United States
  • 1999–2001
    • Università degli Studi di Trieste
      • Department of Physics
      Trst, Friuli Venezia Giulia, Italy