Luminosity Function Of Active Galactic Nuclei Research Articles

Cosmological Evolution of the Hard X-Ray AGN Luminosity Function: Formation History of Supermassive Black Holes

The cosmological evolution of the hard X-ray luminosity function (HXLF) of Active Galactic Nuclei (AGN) is investigated in the 2-10 keV luminosity range of 10 4 1 . 5 - 10 4 6 . 5 erg s - 1 as a function of redshift up to3. From a combination of surveys conducted at photon energies above 2 keV with HEAO1, ASCA and Chandra, we construct a highly complete (>96%) sample consisting of 247 AGNs over the wide flux range of 10 - 1 0 - 3.8 x 10 - 1 5 erg cm - 2 s - 1 (2-10 keV). We find that (i) the fraction of X-ray absorbed AGNs decreases with the intrinsic luminosity and (ii) the evolution of the HXLF of the whole AGNs (including both type-I and type-II AGNs) is best described with a luminosity dependent density evolution where the cutoff redshift increases with the luminosity. Our results directly constrain the evolution of AGNs that produce a major part of the hard X-ray background, thus solving its origin quantitatively. Based on these results, we discuss the formation history of supermassive black holes in galactic centers. The total accreted mass traced by the HXLF with an accretion mass-to-energy conversion factor of 0.1 is consistent with an estimate of SMBH mass density from the demography of galaxies in nearby universe.

Cosmological Evolution of the Hard X‐Ray Active Galactic Nucleus Luminosity Function and the Origin of the Hard X‐Ray Background

We investigate the cosmological evolution of the hard X-ray luminosity function (HXLF) of active galactic nuclei (AGNs) in the 2-10 keV luminosity range of 1041.5-1046.5 ergs s-1 as a function of redshift up to 3. From a combination of surveys conducted at photon energies above 2 keV with HEAO 1, ASCA, and Chandra, we construct a highly complete (>96%) sample consisting of 247 AGNs over the wide flux range of 10-10 to 3.8 × 10-15 ergs cm-2 s-1 (2-10 keV). For our purpose, we develop an extensive method of calculating the intrinsic (before absorption) HXLF and the absorption (NH) function. This utilizes the maximum likelihood method, fully correcting for observational biases with consideration of the X-ray spectrum of each source. We find that (1) the fraction of X-ray absorbed AGNs decreases with the intrinsic luminosity and (2) the evolution of the HXLF of all AGNs (including both type I and type II AGNs) is best described with a luminosity-dependent density evolution (LDDE) where the cutoff redshift increases with the luminosity. Our results directly constrain the evolution of AGNs that produce a major part of the hard X-ray background, thus solving its origin quantitatively. A combination of the HXLF and the NH function enables us to construct a purely observation-based population synthesis model. We present basic consequences of this model and discuss the contribution of Compton-thick AGNs to the rest of the hard X-ray background.

AV‐Band Survey for Variable Galactic Nuclei in the Hubble Deep Field

We present the results of a two-epoch variability survey in the Hubble Deep Field with the goal of investigating the population of active galactic nuclei (AGNs) to z 1. The high resolution and stability of the Hubble Space Telescope allow accurate photometry to be obtained within subarcsecond apertures, resulting in the ability to probe much lower AGN/host galaxy luminosity ratios than can be done from the ground. The primary data sets analyzed for galactic variability are the original Hubble Deep Field observations obtained in 1995 December and a second V-band (F606W) image obtained almost exactly 5 yr later in 2000. We find evidence for nuclear variability in 16 of 217 galaxies brighter than Vnuc = 27.5. Correcting for incompleteness and spurious detections, variable nuclei make up ~8% of the surveyed galaxies. These sources have a redshift range of 0.09 < z < 1.8 and cover the full range of galaxy nuclear V-I colors. Seven of our variable sources are coincident with X-ray sources detected in the 2 Ms Chandra survey, six from the main catalog and one from the supplementary catalog. We find that 44% of the variable nuclei are associated with mid-IR detections at 15 μm, and 31% are detected at 1.4 GHz. Optical spectra are available for 13 of the 16 variables. One is a broad-line AGN, and two others show weak evidence of type 2 AGNs. With the assumption that these variables are all active nuclei, we estimate the AGN luminosity function (LF) at 0.4 < z < 1.1 extending to MB -15. We find evidence for an increase in the number density of faint AGNs when comparing to the local Seyfert luminosity function. The LF for optically varying nuclei appears to rise in number density, with no evidence of turning over at these faint magnitudes.

Cosmic Star Formation History Associated with Quasar Activity: An Approach Using the Black Hole–to–Bulge Mass Correlation

The tight correlation between the masses of central black holes and their host spheroids in nearby galaxies and active galactic nuclei (AGNs) suggests that black hole growth is closely related to their spheroid formation. Based on our previous work regarding such a joint evolutionary scheme and the consequential black hole-to-bulge mass correlation, we use the X-ray luminosity function of AGN and the cosmological evolution rate, which are from ROSAT X-ray surveys, to estimate the cosmic star formation history associated with the black hole growth. By the basic assumption that the major black hole growth occurs during the luminous AGN phase, the luminosity function of AGNs as a function of redshift traces not only the accretion history of the black holes but also the cosmic star formation history of the spheroids. Although the space density of the especially luminous quasars is very low, we show that the total amount of star formation associated with the massive black hole growth is almost the same as that of Lyman break galaxies detected by the current optical deep surveys. We thus argue that the optical deep surveys may miss about half of the net star formation in our universe. This is probably due in part to significant dust extinction, as well as the small field of view of previous optical surveys, which cannot sample such rare events with relatively short timescale. However, the far-infrared emission from the dust heated by ongoing star formation during the black hole growth could sufficiently account for the observed SCUBA number counts and would be the probable dominating energy source of the SCUBA population.

Optical spectroscopic followup of serendipitous Chandra sources: the Chandra Multiwavelength Project (ChaMP)

AbstractA primary goal of the Chandra Multiwavelength Project (ChaMP) is to investigate the evolution of Active Galactic Nuclei (AGN) and quasars by measuring the luminosity function out to z ≥ 4 including obscured AGN that have been missed by previous surveys. To do so, we are acquiring spectra of optical counterparts to serendipitious X‐ray sources detected by Chandra for classification and redshift determination. We have amassed about 200 spectra using the FLWO 1.5m, WIYN 3.5m, CTIO/4m, Magellan and the MMT 6.5m. We have classified the majority of optical counterparts as 65% broad line AGN, 14% narrow emission line galaxies and 12% absorption line galaxies. The remaining counterparts are either stars (8%) or galaxy clusters (1%). We have acquired a sample of 120 AGN in 12 Chandra fields.With a limiting magnitude of r′ = 22 for spectroscopic followup, we are able to detect quasars out to z ∼ 5 and galaxies out to z ∼ 0.7. This preliminary sample forms the foundation for our measurement of the X‐ray luminosity function of AGNs out to z ∼ 4.

Hubble Space TelescopeImaging in the Chandra Deep Field–South. II. WFPC2 Observations of an X‐Ray Flux–limited Sample from the 1 Million Second Chandra Catalog

We present Hubble Space Telescope/WFPC2 observations of a well-defined sample of 40 X-ray sources with X-ray fluxes above the detection threshold of the full 1 Ms Chandra Deep Field-South (CDF-S). The sensitivity and spatial resolution of our HST observations are sufficient to detect the optical counterparts of 37 of the X-ray sources, yielding information on their morphologies and environments. In this paper we extend the results obtained in our previous study on the 300 ks CDF-S X-ray data (Schreier et al.). Specifically, we show that the optical counterparts to the X-ray sources are divided into two distinct populations: (1) an optically faint group with relatively blue colors, similar to the faint blue field galaxy population, and (2) an optically brighter group, including resolved galaxies with average colors significantly redder than the corresponding bright field galaxy population. The brighter objects comprise a wide range of types, including early- and late-type galaxies, starbursts, and active galactic nuclei (AGNs). By contrast, we show that the faint blue X-ray population is most consistent with being predominantly type 2 AGNs of low to moderate luminosity, located at higher redshifts (z ~ 1-2). This conclusion is supported by luminosity function models of the various classes of objects. Hence, the combination of deep X-ray data with the high spatial resolution of HST are for the first time allowing us to probe the faint end of the AGN luminosity function at cosmologically interesting redshifts.

Soft X-ray AGN luminosity function from ROSAT surveys

This is the second paper of our investigation of the 0.5-2 keV soft X-ray luminosity function (SXLF) of active galactic nuclei (AGN) using results from ROSAT surveys of various depth. The large dynamic range of the combined sample, from shallow large-area ROSAT All-Sky Survey (RASS)-based samples to the satellite's deepest pointed observation on the Lockman Hole, enabled us to trace the behavior of the SXLF. While the first paper (Miyaji, Hasinger, Schmidt 2000, hereafter paper I) emphasized the global behavior of the SXLF, cosmological evolution and contribution to the soft X-ray background, this paper presents actual numerical values for practical use of our results. To present the binned SXLF, we have used an improved estimator, which is free from biases associated with the conventional 1/Va estimator.

Prospects for Detecting Clusters of Galaxies through X‐Ray Absorption

We investigate the feasibility of detecting high-redshift clusters in absorption against background active galactic nuclei (AGNs) with the next generation of X-ray telescopes. The four limiting factors for detecting clusters in absorption are the sensitivity of the X-ray spectrometers, the abundance of X-ray-bright AGNs, the absorption equivalent width, and the sky covering factor of the hot gas in clusters. Based on isothermal and polytropic temperature profiles for the gas in clusters, we compute the absorption equivalent widths of the most abundant heavy elements in clusters with emission-weighted temperatures between 1 keV (typical of groups) and 12 keV (typical of the hottest known clusters). For typical AGN X-ray spectra, we find that the easiest lines to detect in absorption arise from He-like Si and S and Fe XX in groups, He-like Fe in 2-10 keV clusters, and H-like Fe in the hottest clusters. The sky covering factor of the hot gas is then determined as a function of absorption equivalent width for a nonevolving cluster population and for evolving cluster populations in an open (Ω = 0.3) and flat (Ω = 1.0) universe. In an open universe, approximately 5% of the sky is covered by hot gas with an equivalent width greater than 1 eV for He-like Fe absorption. In a high-density universe, the corresponding sky covering factor is only 1%. Based on the X-ray luminosity function of AGNs, we find that there are several thousand AGNs beyond z = 1 and several hundred AGNs beyond z = 2 that are bright enough to produce detectable absorption features in clusters with future X-ray calorimeters. These results show that an observing program of high-redshift, X-ray-bright AGNs will have the added benefit of detecting a significant sample of high-redshift clusters. High-redshift clusters can easily be missed in X-ray imaging surveys, since their X-ray surface brightness can be below the X-ray background.

The ROSAT Wide Field Camera Extragalactic Survey

We report the results of a new analysis of the ROSAT Wide Field Camera (WFC) all-sky survey data, designed to detect extragalactic sources of extreme ultraviolet (EUV) radiation in regions of low Galactic NH. We identify a total of 19 active galactic nuclei (AGN), more than double the number of confirmed AGN in the published WFC (2RE) survey. Our sample contains eight narrow-line Seyfert 1 galaxies, making this the first reasonably sized, complete sample of these extreme AGN, along with six broad-line Seyfert 1s and five BL Lacertae objects. These EUV-selected sources typically have steep soft X-ray spectra with a mean power-law energy index αX≈−2. The derived AGN luminosity function is rather flat and appears to cut off above a luminosity of 1046 erg s−1 keV−1, measured monochromatically at 200 eV. Narrow-line Seyfert 1s account for roughly half of the local (z≈ 0) volume emissivity in the EUV band.

Compact Nuclei in Galaxies at Moderate Redshift. II. Their Nature and Implications for the Active Galactic Nucleus Luminosity Function

This study explores the space density and properties of active galaxies to z0.8. We have investigated the frequency and nature of unresolved nuclei in galaxies at moderate redshift as indicators of nuclear activity such as active galactic nuclei (AGNs) or starbursts. Candidates are selected by fitting imaged galaxies with multicomponent models using maximum-likelihood estimate techniques to determine the best model fit. We select those galaxies requiring unresolved point-source components in the galaxy nuclei, in addition to disk and/or bulge components, to adequately model the galaxies' light. We have searched 70 WFPC2 images, primarily from the Medium Deep Survey, for galaxies containing compact nuclei. In our survey of 1033 galaxies, the fraction containing unresolved nuclear components contributing ≥5% of the total galaxy light is 9%±1% corrected for incompleteness. In this second of two papers in this series, we discuss the nature of the compact nuclei and their hosts. We present the upper limit luminosity function (LF) for low-luminosity AGNs (LLAGNs) in two redshift bins to z=0.8. Mild number-density evolution of the form ∝(1+z)1.9 is detected for nuclei at -18MB-14. The LFs appear to flatten at MB≥-16 and this flatness, combined with the increase in number density, is inconsistent with pure luminosity evolution. Based on the amount of density evolution observed for these objects, we find that almost all present-day spiral galaxies could have hosted LLAGNs at some point in their lives. We also comment on the likely contribution of these compact nuclei to the soft X-ray background.

The X-Ray Background and the AGN Luminosity Function

ROSAT deep and shallow surveys have provided an almost complete inventory of the constituents of the soft X-ray background which led to a population synthesis model for the whole X-ray background with interesting cosmological consequences. According to this model the X-ray background is the “echo” of mass accretion onto supermassive black holes, integrated over cosmic time. A new determination of the soft X-ray luminosity function of active galactic nuclei (AGN) is consistent with pure density evolution, and the comoving volume density of AGN at redshift 2–3 approaches that of local normal galaxies. This indicates that many larger galaxies contain black holes and it is likely that the bulk of the black holes was produced before most of the stars in the universe. However, only X-ray surveys in the harder energy bands, where the maximum of the energy density of the X-ray background resides, will provide the acid test of this picture.

Possible High-Redshift, Low-Luminosity Active Galactic Nuclei in the Hubble Deep Field

In the Hubble Deep Field (HDF), 12 candidate sources of high-redshift (z > 3.5) active galactic nuclei (AGN) have been identified. The color selection criteria were established by passing spectra of selected quasars and Seyfert galaxies (appropriately redshifted and modified for Lyman forest absorption), as well as stars, observed normal and starburst galaxies, and galaxy models for various redshifts through the filters used for the HDF observations. The actual identification of AGN candidates also involved convolving a Laplacian-of-Gaussian filter with the HDF images, thereby removing relatively flat galactic backgrounds and leaving only the pointlike components in the centers. Along with positions and colors, estimated redshifts and absolute magnitudes are reported, with the candidates falling toward the faint end of the AGN luminosity function. One candidate has been previously observed spectroscopically, with a measured redshift of 4.02. The number of sources reported here is consistent with a simple extrapolation of the observed quasar luminosity function to magnitude 30 in Johnson B. Implications for ionization of the intergalactic medium and for gravitational lensing are discussed.

An X-Ray–Luminous, Dwarf Seyfert Companion of Markarian 273

We report the discovery of the brightest X-ray source hosted by a faint (MB=-16) dwarf galaxy in the immediate vicinity of the ultraluminous IRAS merging galaxy Markarian 273. The dwarf galaxy, 13 away from Mrk 273, is at the tip of a faint northeast plume of Mrk 273. Its spectrum exhibits strong [O III], Hα, and [N II] emission lines, which establish the redshift of the dwarf galaxy, z=0.0376, the same as that of Mrk 273. The emission-line ratios are typical of Seyfert galaxies. The X-ray emission is consistent with a pointlike source coincident with the center of the dwarf galaxy. The intrinsic X-ray luminosity, 6.3×1041 ergs s−1, in the 0.1-2.4 keV energy range, is about 7 times larger than the B-band luminosity. The X-ray spectrum of the source can be fitted with a power law. All the evidence is consistent with the source being a Seyfert galaxy. Out of ~10 faint objects in the same field, only one is detected by ROSAT, and its ratio of soft X-ray to optical luminosity is as high as those for BL Lacertae objects and few active galactic nuclei (AGNs). If there is a population of such dwarf AGNs hidden as companions of major merger galaxies (such as Mrk 273), they may contribute to the luminosity function of AGNs and the cosmic X-ray background at the faint end.

A Search for “Dwarf'' Seyfert Nuclei. V. Demographics of Nuclear Activity in Nearby Galaxies

We use the sample of emission-line nuclei derived from a recently completed optical spectroscopic survey of nearby galaxies to quantify the incidence of local (z ≈ 0) nuclear activity. Particular attention is paid to obtaining accurate measurements of the emission lines and reliable spectral classifications. The resulting database contains the largest collection of star-forming nuclei and active galactic nuclei (AGNs) currently known for nearby galaxies. It consists of 420 emission-line nuclei detected from a nearly complete, magnitude-limited sample of 486 galaxies with BT ≤ 12.5 mag and declination greater than 0°; the equivalent width detection limit of the brightest emission line, usually Hα, is ~0.25 A. As is consistent with previous studies, we find detectable amounts of ionized gas in the central few hundred parsecs of most galaxies (86%); emission lines are present in essentially every spiral galaxy and in a large fraction of ellipticals and lenticulars. Based on their narrow-line spectra, half of the objects can be classified as H II or star-forming nuclei and the other half as some form of AGN, of which we distinguish three classes: Seyfert nuclei, low-ionization nuclear emission-line regions (LINERs), and transition objects that we assume to be composite LINER/H II-nucleus systems. The population of AGNs is consequently very large; approximately 43% of the galaxies in our survey can be regarded as active, although, for a number of reasons, this fraction is still rather uncertain. Most of the objects have much lower luminosities than commonly studied AGNs; the median luminosity of the narrow Hα line, after correcting for extinction, is only 2 × 1039 ergs s-1. Our sample therefore occupies the extreme faint end of the AGN luminosity function. We detect signatures of a broad-line region, as revealed by visible broad Hα emission, in ~20% of the AGN sample. Seyfert nuclei, both type 1 and type 2, reside in ~10% of all galaxies. LINERs make up the bulk (1/2-3/4) of the AGN population and a significant fraction (1/5-1/3) of all galaxies. A nonnegligible subset of LINERs emit broad Hα emission, furnishing direct evidence that a least some LINERs are indeed physically related to the AGN phenomenon. The dominant ionization mechanism of the nuclear emission depends strongly on the morphological type and luminosity of the host galaxy. AGNs are found predominantly in luminous, early-type (E-Sbc) galaxies, while H II nuclei prefer less luminous, late-type (Sbc and later) systems. The various AGN subclasses have broadly similar host galaxies.

Compact Nuclei in Moderately Redshifted Galaxies

The Hubble Space Telescope (HST) Wide Field Planetary Camera 2 is being used to obtain high-resolution images (≤015 FWHM) in the V (F606W) and I (F814W) bands for several thousand distant galaxies as part of the Medium Deep Survey (MDS). An important scientific aim of the MDS is to identify possible active galactic nucleus (AGN) candidates from these images in order to measure the faint end of the AGN luminosity function, as well as to study the host galaxies of AGNs and nuclear starburst systems. We are able to identify candidate objects based on morphology. Candidates are selected by fitting bulge + disk models and bulge + disk + point-source nuclei models to HST-imaged galaxies and determining the best model fit to the galaxy light profile.

The cosmic X-ray background-IRAS galaxy correlation and the local X-ray volume emissivity

We have cross-correlated the galaxies from the IRAS 2 Jy redshift survey sample and the 0.7 Jy projected sample with the all-sky cosmic X-ray background (CXB) map obtained from the High Energy Astronomy Observatory (HEAO) 1 A-2 experiment. We have detected a significant correlation signal between surface density of IRAS galaxies and the X-ray background intensity, with W(sub xg) = (mean value of ((delta I)(delta N)))/(mean value of I)(mean value of N)) of several times 10(exp -3). While this correlation signal has a significant implication for the contribution of the local universe to the hard (E greater than 2 keV) X-ray background, its interpretation is model-dependent. We have developed a formulation to model the cross-correlation between CXB surface brightness and galaxy counts. This includes the effects of source clustering and the X-ray-far-infrared luminosity correlation. Using an X-ray flux-limited sample of active galactic nuclei (AGNs), which has IRAS 60 micrometer measurements, we have estimated the contribution of the AGN component to the observed CXB-IRAS galaxy count correlations in order to see whether there is an excess component, i.e., contribution from low X-ray luminosity sources. We have applied both the analytical approach and Monte Carlo simulations for the estimations. Our estimate of the local X-ray volume emissivity in the 2-10 keV band is rho(sub x) approximately = (4.3 +/- 1.2) x 10(exp 38) h(sub 50) ergs/s/cu Mpc, consistent with the value expected from the luminosity function of AGNs alone. This sets a limit to the local volume emissivity from lower luminosity sources (e.g., star-forming galaxies, low-ionization nuclear emission-line regions (LINERs)) to rho(sub x) less than or approximately = 2 x 10(exp 38) h(sub 50) ergs/s/cu Mpc.

Quasar evolution and the growth of black holes

We present an attempt to relate the observed evolution of quasars to physical models of active galactic nuclei (AGN). Specifically, we suppose that a newly formed galaxy quickly grows or acquires a black hole which accretes at a rate that is ultimately limited by radiation pressure to a value close to the Eddington limit. Thereafter, black holes are presumed to accrete only intermittently at an average rate that is a universal function of black hole mass and time. We use simple limiting prescriptions for the AGN luminosity function to infer this function and to derive the current mass distribution of relict black holes in the nuclei of nearby galaxies. We deduce that the mean accretion rate |$\langle\dot M\rangle$| scales as M−1.5t−6.7 and, for our most conservative model, that the number of relict black holes per decade declines only as M−0.4 for black hole masses in the range |$3\times 10^7\lesssim M\lesssim 3\times 10^9\text M_\odot$|⁠. If it is further supposed that all sufficiently massive galaxies pass through a quasar phase, with asymptotic black hole mass a monotonic function of the galaxy mass, then it is possible to compare the space density of galaxies with estimated central masses to that of distant quasars. Although there are too few galaxies with estimated central masses to draw any firm conclusions from this comparison, the very rough agreement that we find suggests that such a comparison could be useful in the future when more central masses are measured. Interpretations of other types of AGN are briefly discussed.

Synthesis of accretion disk and nonthermal source models for active galactic nuclei

view Abstract Citations (30) References (49) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Synthesis of Accretion Disk and Nonthermal Source Models for Active Galactic Nuclei Band, David L. ; Malkan, Matthew A. Abstract We fitted multiwavelength observations of 12 active galactic nuclei (AGNs) with a model consisting of a nonthermal source surrounding an accretion disk around a massive black hole. The accretion disk radiates the optical and ultraviolet continua. The nonthermal source produces the infrared and soft X-ray continua by synchrotron emission, and the hard X-ray spectrum by inverse Compton scattering of photons from the accretion disk. By fitting the multiwavelength spectrum of a given AGN, we derive the black hole mass and accretion rate, and the nonthermal source size, magnetic field, and relativistic electron distribution. We find that the typical AGN radiates at nearly the Eddington luminosity and that the radius of the nonthermal source is of order 30-100 Schwarzschild radii. The photon energy density is comparable to the magnetic field energy density, while the relativistic electron energy density is much smaller. The electron lifetime due to radiative losses is very short, and electrons must be (re)accelerated throughout the source. We show that relativistic electrons are probably not produced as secondaries in proton-proton collisions because this would require large scattering optical depths and high relativistic proton energy densities. The gamma-ray compactness indicates that the source becomes optically thick to photon-photon pair production, but a pair cascade will not be produced. The small black-hole masses are more consistent with some density evolution rather than pure luminosity evolution of the AGN luminosity function. Publication: The Astrophysical Journal Pub Date: October 1989 DOI: 10.1086/167886 Bibcode: 1989ApJ...345..122B Keywords: Accretion Disks; Active Galactic Nuclei; Black Holes (Astronomy); Nonthermal Radiation; Spectrophotometry; Electron Energy; Luminosity; Pair Production; Relativistic Electron Beams; Spectral Energy Distribution; X Ray Spectra; Astrophysics; BLACK HOLES; GALAXIES: NUCLEI; RADIATION MECHANISMS; SPECTROPHOTOMETRY full text sources ADS | data products SIMBAD (12) NED (12) MAST (1)

Galaxy Mergers and Active Nuclei - Part Two - Cosmological Evolution

Galaxy mergers may produce active galactic nuclei (AGNs) by repopulating stellar loss-cone orbits around a central black hole. In the companion paper we derived a local bolometric luminosity function of AGNs based on this process. In this paper we interpret the observed cosmological evolution of the luminosity function of AGNs as due to evolution of the merging rate among galaxies after their formation at a redshift of approx.3. An important difference between our model and previous (empirical) models is that the evolution depends on galactic (stellar) luminosity instead of central nonthermal luminosity. The radio counts at 1.4 GHz and optical counts are reproduced by the model if the merging rate of the galaxies at the bright end of the galaxy luminosity function evolves considerably faster than the merging rate of the smaller galaxies. The theoretical and observed luminosity functions at high redshift have similar characteristics: (i) at high luminosity the evolution is best described by luminosity evolution, and (2) the luminosity function has a maximum at approx.10/sup 3/ Gpc/sup -3/, which is the space density of the most massive galaxies. A large fraction of these galaxies are presumably formed in the precursors of rich clusters. Their merger rate is highmore » initially and declines rapidly on a time scale of a few billion years. If the initial density fluctuation spectrum for protoclusters of mass M/sub cl/ has the form deltarho/rhoproportionalM/sup( -1+n//3)/2/sub cl/, then the steep evolution of the most luminous galaxies suggests nroughly-equal-1.3 at a redshift of approx.3, which is consistent with the observed clustering of galaxies.« less