arXiv subjects

Rupert Croft

Publications and source records attributed to Rupert Croft.

At least 55 records · Page 3Linked to original sources

arXiv2017

The descendants of the first quasars in the BlueTides simulation

Supermassive blackholes with masses of a billion solar masses or more are known to exist up to $z=7$. However, the present-day environments of the descendants of first quasars is not well understood and it is not known if they live in massive galaxy clusters or more isolated galaxies at $z=0$. We use a dark matter-only realization (BTMassTracer) of the BlueTides cosmological hydrodynamic simulation to study the halo properties of the descendants of the most massive black holes at $z=8$. We find that the descendants of the quasars with most massive black holes are not amongst the most massive halos. They reside in halos of with group-like ($\sim 10^{14}M_{\odot}$) masses, while the most massive halos in the simulations are rich clusters with masses $\sim 10^{15} M_{\odot}$. The distribution of halo masses at low redshift is similar to that of the descendants of least massive black holes, for a similar range of halo masses at $z=8$, which indicates that they are likely to exist in similar environments. By tracing back to the $z = 8$ progenitors of the most massive (cluster sized) halos at $z=0$; we find that their most likely black hole mass is less than $10^7 M_{\odot}$; they are clearly not amongst the most massive black holes. We also provide estimates for the likelihood of finding a high redshift quasar hosting a black hole with masses above $10^{7} M_{\odot}$ for a given halo mass at $z=0$. For halos above $10^{15} M_{\odot}$, there is only $20 \%$ probability that their $z=8$ progenitors hosted a black hole with mass above $10^{7} M_{\odot}$.

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arXiv2017

The properties of the first galaxies in the BLUETIDES simulation

We employ the very large cosmological hydrodynamical simulation BLUETIDES to investigate the predicted properties of the galaxy population during the epoch of reionisation ($z>8$). BLUETIDES has a resolution and volume ($(400/h\approx 577)^{3}\,{\rm cMpc^3}$) providing a population of galaxies which is well matched to depth and area of current observational surveys targeting the high-redshift Universe. At $z=8$ BLUETIDES includes almost 160,000 galaxies with stellar masses $>10^{8}\,{\rm M_{\odot}}$. The population of galaxies predicted by BLUETIDES closely matches observational constraints on both the galaxy stellar mass function and far-UV ($150\,{\rm nm}$) luminosity function. Galaxies in BLUETIDES are characterised by rapidly increasing star formation histories. Specific star formation rates decrease with redshift though remain largely insensitive to stellar mass. As a result of the enhanced surface density of metals more massive galaxies are predicted to have higher dust attenuation resulting in a significant steepening of the observed far-UV luminosity function at high luminosities. The contribution of active SMBHs to the UV luminosities of galaxies with stellar masses $10^{9-10}\,{\rm M_{\odot}}$ is around $3\%$ on average. Approximately $25\%$ of galaxies with $M_{*}\approx 10^{10}\,{\rm M_{\odot}}$ are predicted to have active SMBH which contribute $>10\%$ of the total UV luminosity.

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arXiv2016

The Photometric Properties of Galaxies in the Early Universe

We use the large cosmological hydro-dynamic simulation BlueTides to predict the photometric properties of galaxies during the epoch of reionisation ($z=8-15$). These properties include the rest-frame UV to near-IR broadband spectral energy distributions, the Lyman continuum photon production, the UV star formation rate calibration, and intrinsic UV continuum slope. In particular we focus on exploring the effect of various modelling assumptions, including the assumed choice of stellar population synthesis model, initial mass function, and the escape fraction of Lyman continuum photons, upon these quantities. We find that these modelling assumptions can have a dramatic effect on photometric properties leading to consequences for the accurate determination of physical properties from observations. For example, at $z=8$ we predict that nebular emission can account for up-to $50\%$ of the rest-frame $R$-band luminosity, while the choice of stellar population synthesis model can change the Lyman continuum production rate up to a factor of $\times 2$.

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arXiv2016

The Lyman-continuum photon production efficiency in the high-redshift Universe

The Lyman Continuum photon production efficiency ($ξ_{\rm ion}$) is a critical ingredient for inferring the number of photons available to reionise the intergalactic medium. To estimate the theoretical production efficiency in the high-redshift Universe we couple the BlueTides cosmological hydrodynamical simulation with a range of stellar population synthesis models. We find Lyman Continuum photon production efficiencies of $\log_{10}(ξ_{\rm ion}/{\rm erg^{-1}\, Hz})\approx 25.1-25.5$ depending on the choice of stellar population synthesis model. These results are broadly consistent with recent observational constraints at high-redshift though favour a model incorporating the effects of binary evolution

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arXiv2015

Luminosity function of [OII] emission-line galaxies in the MassiveBlack-II simulation

We examine the luminosity function (LF) of [OII] emission-line galaxies in the high-resolution cosmological simulation MassiveBlack-II (MBII). From the spectral energy distribution of each galaxy, we select a sub-sample of star-forming galaxies at $0.06 \le z \le 3.0$ using the [OII] emission line luminosity L([OII]). We confirm that the specific star formation rate matches that in the GAMA survey. We show that the [OII] LF at z=1.0 from the MBII shows a good agreement with the LFs from several surveys below L([OII])=$10^{43.0}$ erg/s while the low redshifts ($z \le 0.3$) show an excess in the prediction of bright [OII] galaxies, but still displaying a good match with observations below L([OII])=$10^{41.6}$ erg/s. Based on the validity in reproducing the properties of [OII] galaxies at low redshift ($z \le 1$), we forecast the evolution of the [OII] LF at high redshift ($z \le 3$), which can be tested by upcoming surveys such as the HETDEX and DESI. The slopes of the LFs at bright and faint ends range from -3 to -2 showing minima at z=2. The slope of the bright end evolves approximately as 1/(z+1) at z=2 while the faint end evolves as ~3/(z+1) at $0.6 \le z \le 2$. In addition, a similar analysis is applied for the evolution of [OIII] LFs, which is to be explored in the forthcoming survey WFIRST-AFTA. Finally, we show that the auto-correlation function of [OII] and [OIII] emitting galaxies shows a rapid evolution from z=2 to 1.

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arXiv2015

Investigating Galaxy-Filament Alignments in Hydrodynamic Simulations using Density Ridges

In this paper, we study the filamentary structures and the galaxy alignment along filaments at redshift $z=0.06$ in the MassiveBlack-II simulation, a state-of-the-art, high-resolution hydrodynamical cosmological simulation which includes stellar and AGN feedback in a volume of (100 Mpc$/h$)$^3$. The filaments are constructed using the subspace constrained mean shift (SCMS; Ozertem & Erdogmus (2011) and Chen et al. (2015a)). First, we show that reconstructed filaments using galaxies and reconstructed filaments using dark matter particles are similar to each other; over $50\%$ of the points on the galaxy filaments have a corresponding point on the dark matter filaments within distance $0.13$ Mpc$/h$ (and vice versa) and this distance is even smaller at high-density regions. Second, we observe the alignment of the major principal axis of a galaxy with respect to the orientation of its nearest filament and detect a $2.5$ Mpc$/h$ critical radius for filament's influence on the alignment when the subhalo mass of this galaxy is between $10^9M_\odot/h$ and $10^{12}M_\odot/h$. Moreover, we find the alignment signal to increase significantly with the subhalo mass. Third, when a galaxy is close to filaments (less than $0.25$ Mpc$/h$), the galaxy alignment toward the nearest galaxy group depends on the galaxy subhalo mass. Finally, we find that galaxies close to filaments or groups tend to be rounder than those away from filaments or groups.

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arXiv2015

The formation of Milky Way-mass disk galaxies in the first 500 million years of a cold dark matter universe

Whether among the myriad tiny proto-galaxies there exists a population with similarities to present day galaxies is an open question. We show, using BlueTides, the first hydrodynamic simulation large enough to resolve the relevant scales, that the first massive galaxies to form are %in fact predicted to have extensive rotationally-supported disks. Although their morphology resembles in some ways Milky-way types seen at much lower redshifts, these high-redshift galaxies are smaller, denser, and richer in gas than their low redshift counterparts. From a kinematic analysis of a statistical sample of 216 galaxies at redshift $z=8-10$ we have found that disk galaxies make up 70\% of the population of galaxies with stellar mass $10^{10} M_\odot$ or greater. Cold Dark Matter cosmology therefore makes specific predictions for the population of large galaxies 500 million years after the Big Bang. We argue that wide-field satellite telescopes (e.g. WFIRST) will in the near future discover these first massive disk galaxies. The simplicity of their structure and formation history should make possible new tests of cosmology.

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arXiv2014

Observational Requirements for Lyman-alpha Forest Tomographic Mapping of Large-Scale Structure at z ~ 2

The z > 2 Lyman-alpha (Lya) forest traces the underlying dark-matter distribution on large scales and, given sufficient sightlines, can be used to create 3D maps of large-scale structure. We examine the observational requirements to construct such maps and estimate the signal-to-noise as a function of exposure time and sightline density. Sightline densities at z = 2.25 are n_los = [360, 1200,3300] deg^{-2} at limiting magnitudes of g =[24.0, 24.5,25.0], resulting in transverse sightline separations of d_perp = [3.6, 1.9, 1.2] h^{-1} Mpc, which roughly sets the reconstruction scale. We simulate these reconstructions using mock spectra with realistic noise properties, and find that spectra with S/N = 4 per angstrom can be used to generate maps that clearly trace the underlying dark-matter at overdensities of rho/ ~ 1. For the VLT/VIMOS spectrograph, exposure times t_exp = [4, 6, 10] hrs are sufficient for maps with spatial resolution epsilon_3d = [5.0, 3.2, 2.3] h^{-1} Mpc. Assuming ~ 250 h^{-1} Mpc is probed along the line-of-sight, 1 deg^2 of survey area would cover a comoving volume of ~ 10^6 h^{-3} Mpc^3 at =2.3, enabling efficient mapping of large volumes with 8-10m telescopes. These maps could be used to study galaxy environments, detect proto-clusters, and study the topology of large-scale structure at high-z.

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arXiv2014

The MassiveBlack-II Simulation: The Evolution of Halos and Galaxies to z~0

(Abridged for arXiv)We investigate the properties of halos, galaxies and blackholes to z=0 in the high resolution hydrodynamical simulation MassiveBlack-II (MBII) which evolves a LCDM cosmology in a comoving volume Vbox=100(Mpc/h)^3. MBII is the highest resolution simulation of this size which includes a self-consistent model for star formation, black hole accretion and associated feedback. We provide a simulation browser web application which enables interactive search and tagging of halos, subhalos and their properties and publicly release our galaxy catalogs. Our analysis of the halo mass function (MF) in MBII reveals that baryons have strong effects, with changes in the halo abundance of 20-35% below the knee of the MF (Mhalo < 10^13.2 Msun/h at z=0) when compared to fits based on dark matter only simulations. We provide a fitting function for the halo MF out to redshift z=11 and discuss how the onset of non-universality in the MF limits the accuracy of our fit. We study the halo occupation distribution and clustering of galaxies, in particular the evolution and scale dependence of stochasticity and bias finding reasonable agreement with observational data. The shape of the cosmic spectral energy distribution predicted by MBII is consistent with observations, but lower in amplitude. The Galaxy Stellar Mass Function (GSMF) function is broadly consistent with observations at z>=2. At z<2, the population of passive low mass (for M*<10^9 Msun) galaxies in MBII makes the GSMF too steep compared to observations whereas at the high mass end (M*>10^11 Msun) galaxies hosting bright AGNs make significant contributions to the GSMF. The quasar bolometric luminosity function is also largely consistent with observations. We note however that more efficient AGN feedback (beyond simple thermal coupling used here) is likely necessary for the largest, rarest objects/clusters at low redshifts.

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arXiv2013

High redshift supermassive blackholes: accretion through cold flows

We use zoom-in techniques to re-simulate three high-redshift (z > 5.5) halos which host 10^9 solar mass blackholes from the ~ Gpc volume, MassiveBlack cosmological hydrodynamic simulation. We examine a number of factors potentially affecting supermassive blackhole growth at high redshift in cosmological simulations. These include numerical resolution, feedback prescriptions and formulation of smoothed particle hydrodynamics. We find that varying the size of the region over which feedback energy is deposited directly, either for fixed number of neighbours or fixed volume makes very little difference to the accretion history of blackholes. Changing mass resolution by factors of up to 64 also does not change the blackhole growth history significantly. We find that switching from the density-entropy formulation to the pressure-entropy formulation of smoothed particle hydrodynamics slightly increases the accretion rate onto blackholes. In general numerical details appear to have small effects on the main fueling mechanism for blackholes at these high redshifts. We examine the fashion by which this occurs, finding that the insensitivity to simulation technique seems to be a hallmark of the cold flow feeding picture of these high-z supermassive blackholes. We show that the gas that participates in critical accretion phases, in these massive objects at z > 6~7 is in all cases colder, denser, and forms more coherent streams than the average gas in the halo. This is also mostly the case when the blackhole accretion is feedback regulated (z < 6), however the distinction is less prominent. For our resimulated halos, cold flows appear to be a viable mechanism for forming the most massive blackholes in the early universe, occurring naturally in LambdaCDM models of structure formation. Not requiring fine tuning of numerical parameters, they seem to be physically inevitable in these objects.

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arXiv2013

Theoretical predictions for the effect of nebular emission on the broad band photometry of high-redshift galaxies

By combining optical and near-IR observations from the Hubble Space Telescope with NIR photometry from the Spitzer Space Telescope it is possible to measure the rest-frame UV-optical colours of galaxies at z=4-8. The UV-optical spectral energy distribution of star formation dominated galaxies is the result of several different factors. These include the joint distribution of stellar masses, ages, and metallicities, and the subsequent reprocessing by dust and gas in the ISM. Using a large cosmological hydrodynamical simulation we investigate the predicted spectral energy distributions of galaxies at high-redshift with a particular emphasis on assessing the potential contribution of nebular emission. We find that the average pure stellar UV-optical colour correlates with both luminosity and redshift such that galaxies at lower-redshift and higher-luminosity are typically redder. Assuming the escape fraction of ionising photons is close to zero, the effect of nebular emission is to redden the UV-optical 1500-V_w colour by, on average, 0.4 mag at z=8 declining to 0.25 mag at z=4. Young and low-metallicity stellar populations, which typically have bluer pure stellar UV-optical colours, produce larger ionising luminosities and are thus more strongly affected by the reddening effects of nebular emission. This causes the distribution of 1500-V_w colours to narrow and the trends with luminosity and redshift to weaken. The strong effect of nebular emission leaves observed-frame colours critically sensitive to the source redshift. For example, increasing the redshift by 0.1 can result in observed frame colours changing by up to ~0.6. These predictions reinforce the need to include nebular emission when modelling the spectral energy distributions of galaxies at high-redshift and also highlight the difficultly in interpreting the observed colours of individual galaxies without precise redshifts.

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arXiv2013

Interpreting the observed UV continuum slopes of high-redshift galaxies

The observed UV continuum slope of star forming galaxies is strongly affected by the presence of dust. Its observation is then a potentially valuable diagnostic of dust attenuation, particularly at high-redshift where other diagnostics are currently inaccesible. Interpreting the observed UV continuum slope in the context of dust attenuation is often achieved assuming the empirically calibrated Meurer et al. (1999) relation. Implicit in this relation is the assumption of an intrinsic UV continuum slope ($β=-2.23$). However, results from numerical simulations suggest that the intrinsic UV continuum slopes of high-redshift star forming galaxies are bluer than this, and moreover vary with redshift. Using values of the intrinsic slope predicted by numerical models of galaxy formation combined with a Calzetti et al. (2000) reddening law we infer UV attenuations ($A_{1500}$) $0.35-0.5\,{\rm mag}$ ($A_{V}$: $0.14-0.2\,{\rm mag}$ assuming Calzetti et al. 2000) greater than simply assuming the Meurer relation. This has significant implications for the inferred amount of dust attenuation at very-high ($z\approx 7$) redshift given current observational constraints on $β$, combined with the Meurer relation, suggest dust attenuation to be virtually zero in all but the most luminous systems.

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arXiv2013

Confronting predictions of the galaxy stellar mass function with observations at high-redshift

We investigate the evolution of the galaxy stellar mass function at high-redshift ($z\ge 5$) using a pair of large cosmological hydrodynamical simulations: {\em MassiveBlack} and {\em MassiveBlack-II}. By combining these simulations we can study the properties of galaxies with stellar masses greater than $10^{8}\,{\rm M_{\odot}}\,h^{-1}$ and (co-moving) number densities of $\log_{10}(ϕ\, [{\rm Mpc^{-3}\,dex^{-1}}\,h^{3}])>-8$. Observational determinations of the galaxy stellar mass function at very-high redshift typically assume a relation between the observed UV luminosity and stellar mass-to-light ratio which is applied to high-redshift samples in order to estimate stellar masses. This relation can also be measured from the simulations. We do this, finding two significant differences with the usual observational assumption: it evolves strongly with redshift and has a different shape. Using this relation to make a consistent comparison between galaxy stellar mass functions we find that at $z=6$ and above the simulation predictions are in good agreement with observed data over the whole mass range. Without using the correct UV luminosity and stellar mass-to-light ratio, the discrepancy would be up to two orders of magnitude for large galaxies $>10^{10}\,{\rm M_{\odot}}\,h^{-1}$. At $z=5$, however the stellar mass function for low mass $<10^{9}\,{\rm M_{\odot}}\,h^{-1}$ galaxies is overpredicted by factors of a few, consistent with the behaviour of the UV luminosity function, and perhaps a sign that feedback in the simulation is not efficient enough for these galaxies.

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arXiv2012

Growth of Early Supermassive Black Holes and the High-Redshift Eddington Ratio Distribution

Using a new large-scale (~ 0.75 Gpc)^3 hydrodynamic cosmological simulation we investigate the growth rate of supermassive black holes in the early universe (z > 4.75). Remarkably, we find a clear peak in the typical Eddington ratio at black hole masses of 4-8 * 10^7 solar masses (typically found in halos of ~7 * 10^11 to 10^12 solar masses), independent of redshift and indicative that most of BH growth occurs in the cold-flow dominated regime. Black hole growth is by and large regulated by the evolution of gas density. The typical Eddington ratio at a given mass scales simply as cosmological density (1+z)^3 and the peak is caused by the competition between increased gas density available in more massive hosts, and a decrease due to strong AGN feedback that deprives the black hole of sufficient gas to fuel further rapid growth in the high mass end. In addition to evolution in the mean Eddington ratio, we show that the distribution of Eddington ratio among both mass-selected and luminosity-selected samples is approximately log-normal. We combine these findings into a single log-normal fitting formula for the distribution of Eddington ratios as a function of (M_BH,z). This formula can be used in analytic and semi analytic models for evolving black hole populations, predicting black hole masses of observed quasars, and, in conjunction with the observed distribution of Eddington ratios, can be used to constrain the black hole mass function.

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arXiv2011

Dark matter halo occupation: environment and clustering

We use a large dark matter simulation of a LambdaCDM model to investigate the clustering and environmental dependence of the number of substructures in a halo. Focusing on redshift z=1, we find that the halo occupation distribution is sensitive at the tens of percent level to the surrounding density and to a lesser extent to asymmetry of the surrounding density distribution. We compute the autocorrelation function of halos as a function of occupation, building on the finding of Wechsler et al. (2006) and Gao and White (2007) that halos (at fixed mass) with more substructure are more clustered. We compute the relative bias as a function of occupation number at fixed mass, finding a strong relationship. At fixed mass, halos in the top 5% of occupation can have an autocorrelation function ~ 1.5-2 times higher than the mean. We also compute the bias as a function of halo mass, for fixed halo occupation. We find that for group and cluster sized halos, when the number of subhalos is held fixed, there is a strong anticorrelation between bias and halo mass. Such a relationship represents an additional challenge to the halo model.

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arXiv2011

The Lyman-alpha forest in three dimensions: measurements of large scale flux correlations from BOSS 1st-year data

Using a sample of approximately 14,000 z>2.1 quasars observed in the first year of the Baryon Oscillation Spectroscopic Survey (BOSS), we measure the three-dimensional correlation function of absorption in the Lyman-alpha forest. The angle-averaged correlation function of transmitted flux (F = exp(-tau)) is securely detected out to comoving separations of 60 Mpc/h, the first detection of flux correlations across widely separated sightlines. A quadrupole distortion of the redshift-space correlation function by peculiar velocities, the signature of the gravitational instability origin of structure in the Lyman-alpha forest, is also detected at high significance. We obtain a good fit to the data assuming linear theory redshift-space distortion and linear bias of the transmitted flux, relative to the matter fluctuations of a standard LCDM cosmological model (inflationary cold dark matter with a cosmological constant). At 95% confidence, we find a linear bias parameter 0.16<b<0.24 and redshift-distortion parameter 0.44<beta<1.20, at central redshift z=2.25, with a well constrained combination b(1+β)=0.336 +/- 0.012. The errors on beta are asymmetric, with beta=0 excluded at over 5 sigma confidence level. The value of beta is somewhat low compared to theoretical predictions, and our tests on synthetic data suggest that it is depressed (relative to expectations for the Lyman-alpha forest alone) by the presence of high column density systems and metal line absorption. These results set the stage for cosmological parameter determinations from three-dimensional structure in the Lyman-alpha forest, including anticipated constraints on dark energy from baryon acoustic oscillations.

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arXiv2011

Cold flows and the first quasars

Observations of the most distant bright quasars imply that billion solar mass supermassive black holes (SMBH) have to be assembled within the first eight hundred 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 cosmological hydrodynamic simulation (MassiveBlack) covering a volume (0.75 \Gpc)^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 ~10^9 solar mass 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_halo > 10^{12} solar masses. 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.

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arXiv2011

Early Black Holes in Cosmological Simulations: Luminosity Functions and Clustering Behaviour

We examine predictions for the quasar luminosity functions (QLF) and quasar clustering at high redshift (z > 4.75) using MassiveBlack, our new hydrodynamic cosmological simulation which includes a self-consistent model for black hole growth and feedback. We show that the model reproduces the Sloan QLF within observational constraints at z >= 5. We find that the high-z QLF is consistent with a redshift-independent occupation distribution of BHs among dark matter halos (which we provide) such that the evolution of the QLF follows that of the halo mass function. The sole exception is the bright-end at z=6 and 7, where BHs in high-mass halos tend to be unusually bright due to extended periods of Eddington growth caused by high density cold flows into the halo center. We further use these luminosity functions to make predictions for the number density of quasars in upcoming surveys, predicting there should be ~119+-28 (~87+-28) quasars detectable in the F125W band of the WIDE (DEEP) fields of the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS) from z=5-6, ~19+-7 (~18+-9) from z=6-7, and ~1.7+-1.5 (~1.5+-1.5) from z=7-8. We also investigate quasar clustering, finding that the correlation length is fully consistent with current constraints for Sloan quasars (r_0~17 h^{-1} Mpc at z=4 for quasars above m_i = 20.2), and grows slowly with redshift up to z=6 (r_0~22 h^{-1} Mpc). Finally, we note that the quasar clustering strength depends weakly on luminosity for low L_BH, but gets stronger at higher L_BH as the BHs are found in higher mass halos.

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