Dark Matter Halo Mass Function Research Articles

Dark matter halo mass functions and density profiles from mass and energy cascade

Halo abundance and structure play a central role for modeling structure formation and evolution. Without relying on a spherical or ellipsoidal collapse model, we analytically derive the halo mass function and cuspy halo density (inner slope of −4/3) based on the mass and energy cascade theory in dark matter flow. The hierarchical halo structure formation leads to halo or particle random walk with a position-dependent waiting time tau _g. First, the inverse mass cascade from small to large scales leads to the halo random walk in mass space with tau _gpropto m_h^{-lambda }, where m_h is the halo mass and lambda is a halo geometry parameter with predicted value of 2/3. The corresponding Fokker-Planck solution for halo random walk in mass space gives rise to the halo mass function with a power-law behavior on small scale and exponential decay on large scale. This can be further improved by considering two different lambda for haloes below and above a critical mass scale m_h^*, i.e. a double-lambda halo mass function. Second, a double-gamma density profile can be derived based on the particle random walk in 3D space with a position-dependent waiting time tau _g propto Phi (r)^{-1} propto r^{-gamma }, where Phi is the gravitational potential and r is the particle distance to halo center. Theory predicts gamma =2/3 that leads to a cuspy density profile with an inner slope of −4/3, consistent with the predicted scaling laws from energy cascade. The Press-Schechter mass function and Einasto density profile are just special cases of proposed models. The small scale permanence can be identified due to the scale-independent rate of mass and energy cascade, where density profiles of different halo masses and redshifts converge to the -4/3 scaling law (rho _h propto r^{-4/3}) on small scales. Theory predicts the halo number density scales with halo mass as propto m_h^{-1.9}, while the halo mass density scales as propto m_h^{4/9}. Results were compared against the Illustris simulations. This new perspective provides a theory for nearly universal halo mass functions and density profiles.

COSMOS2020: The galaxy stellar mass function

Context. How galaxies form, assemble, and cease their star formation is a central question within the modern landscape of galaxy evolution studies. These processes are indelibly imprinted on the galaxy stellar mass function (SMF), and its measurement and understanding is key to uncovering a unified theory of galaxy evolution. Aims. We present constraints on the shape and evolution of the galaxy SMF, the quiescent galaxy fraction, and the cosmic stellar mass density across 90% of the history of the Universe from z = 7.5 → 0.2 as a means to study the physical processes that underpin galaxy evolution. Methods. The COSMOS survey is an ideal laboratory for studying representative galaxy samples. Now equipped with deeper and more homogeneous near-infrared coverage exploited by the COSMOS2020 catalog, we leverage the large 1.27 deg2 effective area to improve sample statistics and understand spatial variations (cosmic variance) – particularly for rare, massive galaxies – and push to higher redshifts with greater confidence and mass completeness than previous studies. We divide the total stellar mass function into star-forming and quiescent subsamples through NUVrJ color-color selection. The measurements are then fit with single- and double-component Schechter functions to infer the intrinsic galaxy stellar mass function, the evolution of its key parameters, and the cosmic stellar mass density out to z = 7.5. Finally, we compare our measurements to predictions from state-of-the-art cosmological simulations and theoretical dark matter halo mass functions. Results. We find a smooth, monotonic evolution in the galaxy stellar mass function since z = 7.5, in general agreement with previous studies. The number density of star-forming systems have undergone remarkably consistent growth spanning four decades in stellar mass from z = 7.5 → 2 whereupon high-mass systems become predominantly quiescent (“downsizing”). Meanwhile, the assembly and growth of low-mass quiescent systems only occurred recently, and rapidly. An excess of massive systems at z ≈ 2.5 − 5.5 with strikingly red colors, with some being newly identified, increase the observed number densities to the point where the SMF cannot be reconciled with a Schechter function. Conclusions. Systematics including cosmic variance and/or active galactic nuclei contamination are unlikely to fully explain this excess, and so we speculate that they may be dust-obscured populations similar to those found in far infrared surveys. Furthermore, we find a sustained agreement from z ≈ 3 − 6 between the stellar and dark matter halo mass functions for the most massive systems, suggesting that star formation in massive halos may be more efficient at early times.

On the expected purity of photometric galaxy surveys targeting the Cosmic Dawn

ABSTRACT Over the last three decades, photometric galaxy selection using the Lyman-break technique has transformed our understanding of the high-z Universe, providing large samples of galaxies at $3 \lesssim z \lesssim 8$ with relatively small contamination. With the advent of the JWST, the Lyman-break technique has now been extended to z ∼ 17. However, the purity of the resulting samples has not been tested. Here, we use a simple model, built on the robust foundation of the dark matter halo mass function, to show that the expected level of contamination rises dramatically at $z \gtrsim 10$, especially for luminous galaxies, placing stringent requirements on the selection process. The most luminous sources at $z \gtrsim 12$ are likely at least 10 000 times rarer than potential contaminants, so extensive spectroscopic follow-up campaigns may be required to identify a small number of target sources.

Searching for Dwarf Hα Emission-line Galaxies within Voids. I. Survey Methods and First Observations

The population density of dwarf galaxies in low-density voids is likely determined by the dark matter halo mass function and how galaxy formation proceeds in smaller halos. This depends on the nature of dark matter itself, making the dwarf galaxy population a tracer of its properties. While dwarfs have been found in smaller, closer voids, they have proven difficult to find in larger, more distant voids through magnitude-limited spectroscopic surveys. This is because these surveys detect an overwhelmingly large number of objects behind the voids that must be verified spectroscopically, making void surveys prohibitively inefficient and expensive in terms of large-telescope time. Narrowband imaging for emission lines such as Hα reduces the number of background objects, although the overall number remains large. If imaging is done through a filter set with overlapping transmission wings, then object redshift can be estimated from photometry alone. The precision possible is an order of magnitude greater than single-band photometry, with the caveat that the captured line must be identified through other means. Broadband photometry can be used to reject enough objects with emission of an unwanted type to make obtaining spectra of the remaining objects feasible. In this study, we present an Hα survey for dwarf galaxies with M r ′ fainter than −14 mag through the center 4.3 square degrees of the void FN8. Using Sloan photometry, we exclude enough [O ii] and [O iii] emitters that follow-up spectra of only a few dozen objects are required to statistically estimate the void population density.

The north–south asymmetry of the ALFALFA H i velocity width function

ABSTRACT The number density of extragalactic 21-cm radio sources as a function of their spectral line widths – the H i width function (H i WF) – is a sensitive tracer of the dark matter halo mass function (HMF). The Lambda cold dark matter model predicts that the HMF should be identical everywhere provided it is sampled in sufficiently large volumes, implying that the same should be true of the H i WF. The Arecibo Legacy Fast ALFA (ALFALFA) 21-cm survey measured the H i WF in northern and southern Galactic fields and found a systematically higher number density in the north. At face value, this is in tension with theoretical predictions. We use the Sibelius-DARK N-body simulation and the semi-analytical galaxy formation model GALFORM to create a mock ALFALFA survey. We find that the offset in number density has two origins: the sensitivity of the survey is different in the two fields, which has not been correctly accounted for in previous measurements; and the 1/Veff algorithm used for completeness corrections does not fully account for biases arising from spatial clustering in the galaxy distribution. The latter is primarily driven by a foreground overdensity in the northern field within $30\, \mathrm{Mpc}$ , but more distant structure also plays a role. We provide updated measurements of the ALFALFA H i WF (and H i mass function) correcting for the variations in survey sensitivity. Only when systematic effects such as these are understood and corrected for can cosmological models be tested against the H i WF.

Estimating the warm dark matter mass from strong lensing images with truncated marginal neural ratio estimation

ABSTRACT Precision analysis of galaxy–galaxy strong gravitational lensing images provides a unique way of characterizing small-scale dark matter haloes, and could allow us to uncover the fundamental properties of dark matter’s constituents. Recently, gravitational imaging techniques made it possible to detect a few heavy subhaloes. However, gravitational lenses contain numerous subhaloes and line-of-sight haloes, whose subtle imprint is extremely difficult to detect individually. Existing methods for marginalizing over this large population of subthreshold perturbers to infer population-level parameters are typically computationally expensive, or require compressing observations into hand-crafted summary statistics, such as a power spectrum of residuals. Here, we present the first analysis pipeline to combine parametric lensing models and a recently developed neural simulation-based inference technique called truncated marginal neural ratio estimation (TMNRE) to constrain the warm dark matter halo mass function cut-off scale directly from multiple lensing images. Through a proof-of-concept application to simulated data, we show that our approach enables empirically testable inference of the dark matter cut-off mass through marginalization over a large population of realistic perturbers that would be undetectable on their own, and over lens and source parameter uncertainties. To obtain our results, we combine the signal contained in a set of images with Hubble Space Telescope resolution. Our results suggest that TMNRE can be a powerful approach to put tight constraints on the mass of warm dark matter in the multi-keV regime, which will be relevant both for existing lensing data and in the large sample of lenses that will be delivered by near-future telescopes.

How gas flows shape the stellar–halo mass relation in the eagle simulation

ABSTRACT The difference in shape between the observed galaxy stellar mass function and the predicted dark matter halo mass function is generally explained primarily by feedback processes. Feedback can shape the stellar–halo mass (SHM) relation by driving gas out of galaxies, by modulating the first-time infall of gas on to galaxies (i.e. preventative feedback), and by instigating fountain flows of recycled wind material. We present and apply a method to disentangle these effects for hydrodynamical simulations of galaxy formation. We build a model of linear coupled differential equations that by construction reproduces the flows of gas on to and out of galaxies and haloes in the eaglecosmological simulation. By varying individual terms in this model, we isolate the relative effects of star formation, ejection via outflow, first-time inflow, and wind recycling on the SHM relation. We find that for halo masses $M_{200} \lt 10^{12} \, \mathrm{M_\odot }$ the SHM relation is shaped primarily by a combination of ejection from galaxies and haloes, while for larger M200 preventative feedback is also important. The effects of recycling and the efficiency of star formation are small. We show that if, instead of M200, we use the cumulative mass of dark matter that fell in for the first time, the evolution of the SHM relation nearly vanishes. This suggests that the evolution is due to the definition of halo mass rather than to an evolving physical efficiency of galaxy formation. Finally, we demonstrate that the mass in the circumgalactic medium is much more sensitive to gas flows, especially recycling, than is the case for stars and the interstellar medium.

The physics of galactic winds driven by cosmic rays I: Diffusion

ABSTRACT The physics of Cosmic ray (CR) transport remains a key uncertainty in assessing whether CRs can produce galaxy-scale outflows consistent with observations. In this paper, we elucidate the physics of CR-driven galactic winds for CR transport dominated by diffusion. A companion paper considers CR streaming. We use analytic estimates validated by time-dependent spherically symmetric simulations to derive expressions for the mass-loss rate, momentum flux, and speed of CR-driven galactic winds, suitable for cosmological-scale or semi-analytic models of galaxy formation. For CR diffusion coefficients κ ≳ r0ci, where r0 is the base radius of the wind and ci is the isothermal gas sound speed, the asymptotic wind energy flux is comparable to that supplied to CRs, and the outflow rapidly accelerates to supersonic speeds. By contrast, for κ ≲ r0ci, CR-driven winds accelerate more slowly and lose most of their energy to gravity, a CR analogue of photon-tired stellar winds. Given CR diffusion coefficients estimated using Fermi gamma-ray observations of pion decay, we predict mass-loss rates in CR-driven galactic winds of the order of the star formation rate for dwarf and disc galaxies. The dwarf galaxy mass-loss rates are small compared to the mass-loadings needed to reconcile the stellar and dark matter halo mass functions. For nuclear starbursts (e.g. M82, Arp 220), CR diffusion and pion losses suppress the CR pressure in the galaxy and the strength of CR-driven winds. We discuss the implications of our results for interpreting observations of galactic winds and for the role of CRs in galaxy formation.

The mass function dependence on the dynamical state of dark matter haloes

Context. Galaxy clusters are luminous tracers of the most massive dark matter haloes in the Universe. To use them as a cosmological probe, a detailed description of the properties of dark matter haloes is required. Aims. We characterize how the dynamical state of haloes impacts the dark matter halo mass function at the high-mass end (i.e., for haloes hosting clusters of galaxies). Methods. We used the dark matter-only MultiDark suite of simulations and the high-mass objects M > 2.7 × 1013 M⊙ h−1 therein. We measured the mean relations of concentration, offset, and spin as a function of dark matter halo mass and redshift. We investigated the distributions around the mean relations. We measured the dark matter halo mass function as a function of offset, spin, and redshift. We formulated a generalized mass function framework that accounts for the dynamical state of the dark matter haloes. Results. We confirm the recent discovery of the concentration upturn at high masses and provide a model that predicts the concentration for different values of mass and redshift with one single equation. We model the distributions around the mean values of concentration, offset, and spin with modified Schechter functions. We find that the concentration of low-mass haloes shows a faster redshift evolution compared to high-mass haloes, especially in the high-concentration regime. We find that the offset parameter is systematically smaller at low redshift, in agreement with the relaxation of structures at recent times. The peak of its distribution shifts by a factor of ∼1.5 from z = 1.4 to z = 0. The individual models are combined into a comprehensive mass function model, which predicts the mass function as a function of spin and offset. Our model recovers the fiducial mass function with ∼3% accuracy at redshift 0 and accounts for redshift evolution up to z ∼ 1.5. Results. This new approach accounts for the dynamical state of the halo when measuring the halo mass function. It offers a connection with dynamical selection effects in galaxy cluster observations. This is key toward precision cosmology using cluster counts as a probe.

Estimating Lifetimes of UV-selected Massive Galaxies at 0.5 ≤ z ≤ 2.5 in the COSMOS/UltraVISTA Field through Clustering Analyses

Abstract To investigate the lifetimes of red sequence (RS), blue cloud (BC), and green valley (GV) galaxies, we derive their lifetimes using clustering analyses at 0.5 ≤ z ≤ 2.5 in the COSMOS/UltraVISTA field. Several essentials that may influence the lifetime estimation have been explored, including the dark matter (DM) halo mass function (HMF), the width of the redshift bin, the growth of DM halos within each redshift bin, and the stellar mass. We find that the HMF difference results in scatters of ∼0.2 dex on the lifetime estimation, adopting a redshift bin width of Δz = 0.5 is good enough to estimate the lifetime, and no significant effect on lifetime estimation is found due to the growth of DM halos within each redshift bin. The galaxy subsamples with higher stellar masses generally have shorter lifetimes, but the lifetimes in different subsamples at z > 1.5 tend to be independent of stellar mass. Consistently, the clustering-based lifetime for each galaxy subsample agrees well with that inferred using the spectral energy distribution modeling. Moreover, the lifetimes of the RS and BC galaxies also coincide well with their typical gas-depletion timescales attributed to the consumption of star formation. Interestingly, the distinct lifetime behaviors of the GV galaxies at z ≤ 1.5 and z > 1.5 cannot be fully accounted for by their gas-depletion timescales. Instead, this discrepancy between the lifetimes and gas-depletion timescales of the GV galaxies suggests that there are additional physical processes, such as feedback of active galactic nuclei, which accelerates the quenching of GV galaxies at high redshifts.

The Evolution of Star Formation Rate Density of Galaxies

We present a semi-analytical calculation of the global star formation density (SFD) by using the well constrained cold dark matter (CDM) halo mass function. Both, halo masses <I>M<SUB>H</SUB></I>(<i>z</i>) and stellar masses <I>M</I>_*(<i>z</i>) are taken from observations of Lyα emitter (LAEs) and/or Lyman break galaxies (LBGs). Most of them, spectroscopically selected, are characterized by high star formation rates. The view of galaxy formation is mainly based on the hierarchical (“botton-up”) cold dark matter model for structure formation. We have used the connection between the halo mass and the star formation rate in galaxies of the halo mass <I>M<SUB>H</SUB></I> at redshift <i>z</i>. Our model has the advantage that we are able to calculate the global star formation rate <i>ρ</i>_*(<i>z</i>) (in <I>M</I>ʘ<i>y</i>-1<i>Mpc</i>-3) by a closed equation. All parameters (<I>M<SUB>H</SUB></I>; <I>M</I>_* and <i>n</i>) have a well-defined physical meaning. From the CDM spectrum, the power law index of the halo mass function is well constrained. Our results are compiled in Table 1 and Figure 1. Here our results are compared with observations and hydrodynamical simulations. The physical meaning of the evolution of comoving cosmic star density as a function of redshift with three epochs is discussed. We find a good agreement between the SFD inferred from observations and our model in the range of redshifts <i>z</i> = 0 - 7.

Predicting dark matter halo formation in N-body simulations with deep regression networks

ABSTRACT Dark matter haloes play a fundamental role in cosmological structure formation. The most common approach to model their assembly mechanisms is through N-body simulations. In this work, we present an innovative pathway to predict dark matter halo formation from the initial density field using a Deep Learning algorithm. We implement and train a Deep Convolutional Neural Network to solve the task of retrieving Lagrangian patches from which dark matter haloes will condense. The volumetric multilabel classification task is turned into a regression problem by means of the Euclidean distance transformation. The network is complemented by an adaptive version of the watershed algorithm to form the entire protohalo identification pipeline. We show that splitting the segmentation problem into two distinct subtasks allows for training smaller and faster networks, while the predictive power of the pipeline remains the same. The model is trained on synthetic data derived from a single full N-body simulation and achieves deviations of ∼10 per cent when reconstructing the dark matter halo mass function at z = 0. This approach represents a promising framework for learning highly non-linear relations in the primordial density field. As a practical application, our method can be used to produce mock dark matter halo catalogues directly from the initial conditions of N-body simulations.

Stochastic Processes as the Origin of the Double Power-law Shape of the Quasar Luminosity Function

Abstract The quasar luminosity function (QLF) offers insight into the early coevolution of black holes and galaxies. It has been characterized observationally up to redshift z ∼ 6 with clear evidence of a double power-law shape, in contrast to the Schechter-like form of the underlying dark-matter halo mass function. We investigate a physical origin for the difference in these distributions by considering the impact of stochasticity induced by the processes that determine the quasar luminosity for a given host halo and redshift. We employ a conditional luminosity function and construct the relation between median quasar magnitude versus halo mass with log-normal in luminosity scatter Σ, and duty-cycle ϵ DC, and focus on high redshift z ≳ 4. We show that, in order to reproduce the observed QLF, the Σ = 0 abundance matching requires all of the brightest quasars to be hosted in the rarest most massive dark-matter halos (with an increasing in halo mass). Conversely, for Σ &gt; 0 the brightest quasars can be overluminous outliers hosted in relatively common dark-matter halos. In this case, the median quasar magnitude versus halo mass relation, M UV,c, flattens at the high-end, as expected in self-regulated growth due to feedback. We sample the parameter space of Σ and ϵ DC and show that M UV,c flattens above for . Models with ϵ DC ∼ 1 instead require a high mass threshold close to . We investigate the impact of ϵ DC and Σ on measurements of clustering and find there is no luminosity dependence on clustering for Σ &gt; 0.3, consistent with recent observations from Subaru HSC.

The normalization and slope of the dark matter (sub-)halo mass function on sub-galactic scales

ABSTRACT Simulations of cold dark matter make robust predictions about the slope and normalization of the dark matter halo and subhalo mass functions on small scales. Recent observational advances utilizing strong gravitational lensing have demonstrated the ability of this technique to place constraints on these quantities on subgalactic scales corresponding to dark matter halo masses of 106–$10^9\, \mathrm{M}_\odot$. On these scales the physics of baryons, which make up around 17 per cent of the matter content of the Universe but which are not included in pure dark matter N-body simulations, are expected to affect the growth of structure and the collapse of dark matter haloes. In this work, we develop a semi-analytic model to predict the amplitude and slope of the dark matter halo and subhalo mass functions on subgalactic scales in the presence of baryons. We find that the halo mass function is suppressed by up to 25 per cent, and the slope is modified, ranging from −1.916 to −1.868 in this mass range. These results are consistent with current measurements, but differ sufficiently from the expectations for a dark matter only universe that it may be testable in the near future.

Towards accurate rescaling of a halo mass function

Abstract We investigate the precision within which a simulated dark matter halo mass function can be rescaled to a different set of cosmological parameters. Our tests show that the accuracy almost linearly depends on the difference of the cosmological parameters and amounts to few percent in the case of WMAP5 and PLANCK parameters. The rescaling thus allows us to obtain a mass function with better precision than the one given by the Sheth-Mo-Tormen approximation and even more modern fits currently used in the literature.

Scale-dependent bias and bispectrum in neutrino separate universe simulations

Cosmic background neutrinos have a large velocity dispersion, which causes the evolution of long-wavelength density perturbations to depend on scale. This scale-dependent growth leads to the well-known suppression in the linear theory matter power spectrum that is used to probe neutrino mass. In this paper, we study the impact of long-wavelength density perturbations on small-scale structure formation. By performing separate universe simulations where the long-wavelength mode is absorbed into the local expansion, we measure the responses of the cold dark matter (CDM) power spectrum and halo mass function, which correspond to the squeezed-limit bispectrum and halo bias. We find that the scale-dependent evolution of the long-wavelength modes causes these quantities to depend on scale and provide simple expressions to model them in terms of scale and the amount of massive neutrinos. Importantly, this scale-dependent bias reduces the suppression in the linear halo power spectrum due to massive neutrinos by 13 and 26% for objects of bias $\bar{b}=2$ and $\bar{b} \gg1$, respectively. We demonstrate with high statistical significance that the scale-dependent halo bias ${\it cannot}$ be modeled by the CDM and neutrino density transfer functions at the time when the halos are identified. This reinforces the importance of the temporal nonlocality of structure formation, especially when the growth is scale dependent.

A new smooth-k space filter approach to calculate halo abundances

We propose a new filter, a smooth-k space filter, to use in the Press-Schechter approach to model the dark matter halo mass function which overcomes shortcomings of other filters. We test this against the mass function measured in N-body simulations. We find that the commonly used sharp-k filter fails to reproduce the behaviour of the halo mass function at low masses measured from simulations of models with a sharp truncation in the linear power spectrum. We show that the predictions with our new filter agree with the simulation results over a wider range of halo masses for both damped and undamped power spectra than is the case with the sharp-k and real-space top-hat filters.

The Dearth of z ∼ 10 Galaxies in All HST Legacy Fields—The Rapid Evolution of the Galaxy Population in the First 500 Myr*

Abstract We present an analysis of all prime HST legacy fields spanning &gt;800 arcmin2 in the search for z ∼ 10 galaxy candidates and the study of their UV luminosity function (LF). In particular, we present new z ∼ 10 candidates selected from the full Hubble Frontier Field (HFF) data set. Despite the addition of these new fields, we find a low abundance of z ∼ 10 candidates with only nine reliable sources identified in all prime HST data sets that include the HUDF09/12, the HUDF/XDF, all of the CANDELS fields, and now the HFF survey. Based on this comprehensive search, we find that the UV luminosity function decreases by one order of magnitude from z ∼ 8 to z ∼ 10 over a four-magnitude range. This also implies a decrease of the cosmic star formation rate density by an order of magnitude within 170 Myr from z ∼ 8 to z ∼ 10. We show that this accelerated evolution compared to lower redshift can entirely be explained by the fast build up of the dark matter halo mass function at z &gt; 8. Consequently, the predicted UV LFs from several models of galaxy formation are in good agreement with this observed trend, even though the measured UV LF lies at the low end of model predictions. The difference is generally still consistent within the Poisson and cosmic variance uncertainties. We discuss the implications of these results in light of the upcoming James Webb Space Telescope mission, which is poised to find much larger samples of z ∼ 10 galaxies as well as their progenitors at less than 400 Myr after the big bang.

Low-redshift Lyman limit systems as diagnostics of cosmological inflows and outflows

We use cosmological hydrodynamic simulations with stellar feedback from the FIRE project to study the physical nature of Lyman limit systems (LLSs) at z<1. At these low redshifts, LLSs are closely associated with dense gas structures surrounding galaxies, such as galactic winds, dwarf satellites, and cool inflows from the intergalactic medium. Our analysis is based on 14 zoom-in simulations covering the halo mass range M_h~10^9-10^13 Msun at z=0, which we convolve with the dark matter halo mass function to produce cosmological statistics. We find that the majority of cosmologically-selected LLSs are associated with halos in the mass range 10^10 < M_h < 10^12 Msun. The incidence and HI column density distribution of simulated absorbers with columns 10^16.2 < N_HI < 2x10^20 cm^-2 are consistent with observations. High-velocity outflows (with radial velocity exceeding the halo circular velocity by a factor >~2) tend to have higher metallicities ([X/H] ~ -0.5) while very low metallicity ([X/H] < -2) LLSs are typically associated with gas infalling from the intergalactic medium. However, most LLSs occupy an intermediate region in metallicity-radial velocity space, for which there is no clear trend between metallicity and radial kinematics. Metal-enriched inflows arise in the FIRE simulations as a result of galactic winds that fall back onto galaxies at low redshift. The overall simulated LLS metallicity distribution has a mean (standard deviation) [X/H] = -0.9 (0.4) and does not show significant evidence for bimodality, in contrast to recent observational studies but consistent with LLSs arising from halos with a broad range of masses and metallicities.

ON THE EVOLUTION OF HIGH-REDSHIFT ACTIVE GALACTIC NUCLEI

ABSTRACT We build a simple physical model to study the high-redshift active galactic nucleus (AGN) evolution within the co-evolution framework of central black holes (BHs) and their host galaxies. The correlation between the circular velocity of a dark halo V c and the velocity dispersion of a galaxy σ is used to link the dark matter halo mass and BH mass. The dark matter halo mass function is converted to the BH mass function for any given redshift. The high-redshift optical AGN luminosity functions (LFs) are constructed. At , the flattening feature is not shown at the faint end of the optical AGN LF. This is consistent with observational results. If the optical AGN LF at can be reproduced in the case in which central BHs have the Eddington-limited accretion, it is possible for the AGN lifetime to have a small value of . The X-ray AGN LFs and X-ray AGN number counts are also calculated at and , respectively, using the same parameters adopted in the calculation for the optical AGN LF at . It is estimated that about 30 AGNs per at can be detected with a flux limit of in the 0.5–2 keV band. Additionally, the cosmic reionization is also investigated. The ultraviolet photons emitted from the high-redshift AGNs mainly contribute to the cosmic reionization, and the central BHs of the high-redshift AGNs have a mass range of . We also discuss some uncertainties in both the AGN LFs and AGN number counts originating from the relation, Eddington ratio, AGN lifetime, and X-ray attenuation in our model.