The Luminosity Dependence of Quasar Clustering

We investigate the luminosity dependence of quasar clustering, inspired by numerical simulations of galaxy mergers that incorporate black hole growth. These simulations have motivated a new interpretation of the quasar luminosity function. In this picture, the bright end of the quasar luminosity function consists of quasars radiating nearly at their peak luminosities, while the faint end consists mainly of very similar sources, but at dimmer phases in their evolution. We combine this model with the statistics of dark matter halos that host quasar activity. We find that, since bright and faint quasars are mostly similar sources seen in different evolutionary stages, a broad range in quasar luminosities corresponds to only a narrow range in the masses of quasar host halos. On average, bright and faint quasars reside in similar host halos. Consequently, we argue that quasar clustering should depend only weakly on luminosity. This prediction is in qualitative agreement with recent measurements of the luminosity dependence of the quasar correlation function (Croom et al. 2005) and the galaxy-quasar cross-correlation function (Adelberger & Steidel 2005). Future precision clustering measurements from SDSS and 2dF, spanning a large range in luminosity, should provide a strong test of our model.Comment: 9 pages, 4 figures, submitted to Ap

Evolution in the black hole mass-bulge mass relation: a theoretical perspective

We explore the growth of super-massive black holes and host galaxy bulges in the galaxy population using the Millennium Run LCDM simulation coupled with a model of galaxy formation. We find that, if galaxy mergers are the primary drivers for both bulge and black hole growth, then in the simplest picture one should expect the mBH-mbulge relation to evolve with redshift, with a larger black hole mass associated with a given bulge mass at earlier times relative to the present day. This result is independent of an evolving cold gas fraction in the galaxy population. The evolution arises from the disruption of galactic disks during mergers that make a larger fractional mass contribution to bulges at low redshift than at earlier epochs. There is no comparable growth mode for the black hole population. Thus, this effect produces evolution in the mBH-mbulge relation that is driven by bulge mass growth and not by black holes.Comment: 6 pages, 3 figures, minor revisions, replaced with accepted MNRAS versio

Feedback from galactic stellar bulges and hot gaseous haloes of galaxies

We demonstrate that the feedback from stellar bulges can play an essential role in shaping the halo gas of galaxies with substantial bulge components by conducting 1-D hydrodynamical simulations. The feedback model we consider consists of two distinct phases: 1) an early starburst during the bulge formation and 2) a subsequent long-lasting mass and energy injection from stellar winds of low-mass stars and Type Ia SNe. An energetic outward blastwave is initiated by the starburst and is maintained and enhanced by the long-lasting stellar feedback. For a MW-like galactic bulge, this blastwave sweeps up the halo gas in the proto-galaxy and heats up the surrounding medium to a scale much beyond the virial radius of the halo, thus the accretion of the halo hot gas can be completely stopped. In addition, the long-lasting feedback in the later phase powers a galactic bulge wind that is reverse-shocked at a large radius in the presence of surrounding intergalactic medium and hence maintains a hot gaseous halo. As the mass and energy injection decreases with time, the feedback evolves to a subsonic and quasi-stable outflow, which is enough to prevent halo gas from cooling. The two phases of the feedback thus re-enforce each-other's impact on the gas dynamics. The simulation results demonstrate that the stellar bulge feedback may provide a plausible solution to the long-standing problems in understanding the MW type galaxies, such as the "missing stellar feedback" problem and the "over-cooling" problem. The simulations also show that the properties of the hot gas in the subsonic outflow state depend sensitively on the environment and the formation history of the bulge. This dependence and variance may explain the large dispersion in the X-ray to B-band luminosity ratio of the low $L_X/L_B$ Es.Comment: v2, discussions added, accepted for publication in MNRA

The Collision Between The Milky Way And Andromeda

We use a N--body/hydrodynamic simulation to forecast the future encounter between the Milky Way and the Andromeda galaxies, given current observational constraints on their relative distance, relative velocity, and masses. Allowing for a comparable amount of diffuse mass to fill the volume of the Local Group, we find that the two galaxies are likely to collide in a few billion years - within the Sun's lifetime. During the the interaction, there is a chance that the Sun will be pulled away from its present orbital radius and reside in an extended tidal tail. The likelihood for this outcome increases as the merger progresses, and there is a remote possibility that our Sun will be more tightly bound to Andromeda than to the Milky Way before the final merger. Eventually, after the merger has completed, the Sun is most likely to be scattered to the outer halo and reside at much larger radii (>30 kpc). The density profiles of the stars, gas and dark matter in the merger product resemble those of elliptical galaxies. Our Local Group model therefore provides a prototype progenitor of late--forming elliptical galaxies.Comment: accepted to MNRA

A Cosmological Framework for the Co-Evolution of Quasars, Supermassive Black Holes, and Elliptical Galaxies: I. Galaxy Mergers & Quasar Activity

(Abridged) We develop a model for the cosmological role of mergers in the evolution of starbursts, quasars, and spheroidal galaxies. Combining halo mass functions (MFs) with empirical halo occupation models, we calculate where major galaxy-galaxy mergers occur and what kinds of galaxies merge, at all redshifts. We compare with observed merger MFs, clustering, fractions, and small-scale environments, and show that this yields robust estimates in good agreement with observations. Making the simple ansatz that major, gas-rich mergers cause quasar activity, we demonstrate that this naturally reproduces the observed rise and fall of the quasar luminosity density from z=0-6, as well as quasar LFs, fractions, host galaxy colors, and clustering as a function of redshift and luminosity. The observed excess of quasar clustering on small scales is a natural prediction of the model, as mergers preferentially occur in regions with excess small-scale galaxy overdensities. We show that quasar environments at all observed redshifts correspond closely to the empirically determined small group scale, where mergers of gas-rich galaxies are most efficient. We contrast with a secular model in which quasar activity is driven by bars/disk instabilities, and show that while these modes probably dominate at Seyfert luminosities, the constraints from clustering (large and small-scale), pseudobulge populations, disk MFs, luminosity density evolution, and host galaxy colors argue that they must be a small contributor to the z>1 quasar luminosity density.Comment: 34 pages, 27 figures, submitted to ApJ. Fixed appearance of Figure

Gravitational Quenching in Massive Galaxies and Clusters by Clumpy Accretion

We consider a simple gravitational-heating mechanism for the long-term quenching of cooling flows and star formation in massive dark-matter haloes hosting ellipticals and clusters. The virial shock heating in haloes >10^12 Mo triggers quenching in 10^12-13 Mo haloes (Birnboim, Dekel & Neistein 2007). We show that the long-term quenching in haloes >Mmin~7x10^12 Mo could be due to the gravitational energy of cosmological accretion delivered to the inner-halo hot gas by cold flows via ram-pressure drag and local shocks. Mmin is obtained by comparing the gravitational power of infall into the potential well with the overall radiative cooling rate. The heating wins if the gas inner density cusp is not steeper than r^-0.5 and if the masses in the cold and hot phases are comparable. The effect is stronger at higher redshifts, making the maintenance easier also at later times. Clumps >10^5 Mo penetrate to the inner halo with sufficient kinetic energy before they disintegrate, but they have to be <10^8 Mo for the drag to do enough work in a Hubble time. Pressure confined ~10^4K clumps are stable against their own gravity and remain gaseous once below the Bonnor-Ebert mass ~10^8 Mo. They are also immune to tidal disruption. Clumps in the desired mass range could emerge by thermal instability in the outer halo if the conductivity is not too high. Alternatively, such clumps may be embedded in dark-matter subhaloes if the ionizing flux is ineffective, but they separate from their subhaloes by ram pressure before entering the inner halo. Heating by dynamical friction becomes dominant for massive satellites, which can contribute up to one third of the total gravitational heating. We conclude that gravitational heating by cosmological accretion is a viable alternative to AGN feedback as a long-term quenching mechanism.Comment: 24 pages, 20 figures, some improvements, MNRAS accepted versio

A Theoretical Interpretation of the Black Hole Fundamental Plane

We examine the origin and evolution of correlations between properties of supermassive black holes (BHs) and their host galaxies using simulations of major galaxy mergers, including the effects of gas dissipation, cooling, star formation, and BH accretion and feedback. We demonstrate that the simulations predict the existence of a BH 'fundamental plane' (BHFP), of the form M_BH sigma^(3.0+-0.3)*R_e^(0.43+-0.19) or M_BH M_bulge^(0.54+-0.17)*sigma^(2.2+-0.5), similar to relations found observationally. The simulations indicate that the BHFP can be understood roughly as a tilted intrinsic correlation between BH mass and spheroid binding energy, or the condition for feedback coupling to power a pressure-driven outflow. While changes in halo circular velocity, merger orbital parameters, progenitor disk redshifts and gas fractions, ISM gas pressurization, and other parameters can drive changes in e.g. sigma at fixed M_bulge, and therefore changes in the M_BH-sigma or M_BH-M_bulge relations, the BHFP is robust. Given the empirical trend of decreasing R_e for a given M_bulge at high redshift, the BHFP predicts that BHs will be more massive at fixed M_bulge, in good agreement with recent observations. This evolution in the structural properties of merger remnants, to smaller R_e and larger sigma (and therefore larger M_BH, conserving the BHFP) at a given M_bulge, is driven by the fact that bulge progenitors have characteristically larger gas fractions at high redshifts. Adopting the observed evolution of disk gas fractions with redshift, our simulations predict the observed trends in both R_e(M_bulge) and M_BH(M_bulge).Comment: 22 pages, 19 figures, replaced with version accepted to ApJ. Companion paper to arXiv:0707.400

The Kennicutt-Schmidt Star Formation Relation at z~2

Recent observations of excited CO emission lines from z~2 disc galaxies have shed light on the Kennicutt-Schmidt relation at high-z via observed SFR-CO (J=2-1) and (J=3-2) relations. Here, we describe a novel methodology for utilising these observations of high-excitation CO to derive the underlying Schmidt (SFR-rho^N) relationship. To do this requires an understanding of the potential effects of differential CO excitation with SFR. If the most heavily star-forming galaxies have a larger fraction of their gas in highly excited CO states than the lower SFR galaxies, then the observed molecular SFR-CO^alpha index, alpha, will be less than the underlying (volumetric) Schmidt index, N. Utilising a combination of SPH models of galaxy evolution and molecular line radiative transfer, we present the first calculations of CO excitation in z~2 disc galaxies with the aim of developing a mapping between various observed SFR-CO relationships and the underlying Schmidt relation. We find that even in relatively luminous z~2 discs, differential excitation does indeed exist, resulting in alpha < N for highly excited CO lines. This means that an observed (e.g.) SFR-CO (J=3-2) relation does not map linearly to SFR-H2 relation. We utilise our model results to provide a mapping from alpha to N for the range of Schmidt indices N=1-2. By comparing to recent observational surveys, we find that the observed SFR-CO (J=2-1) and SFR-CO (J=3-2) relations suggest that an underlying SFR rho^1.5 relation describes z~2 disc galaxies.Comment: Accepted by MNRAS, a few figure additions in response to referee repor

Predictions of Quasar Clustering: Redshift, Luminosity and Selection Dependence

We show that current clustering observations of quasars and luminous AGN can be explained by a merger model augmented by feedback from outflows. Using numerical simulations large enough to study clustering out to 25 comoving h^{-1} Mpc, we calculate correlation functions, biases, and correlation lengths as a function of AGN redshift and optical and X-ray luminosity. At optical wavelengths, our results match a wide range of current observations and generate predictions for future data sets. We reproduce the weak luminosity dependence of clustering over the currently well-measured range, and predict a much stronger dependence at higher luminosities. The increase in the amplitude of binary quasar clustering observed in the Sloan Digital Sky Survey (SDSS) is also reproduced and is predicted to occur at higher redshift, an effect that is due to the one halo term in the correlation function. On the other hand, our results do not match the rapid evolution of the correlation length observed in the SDSS at z\simeq 3, a discrepancy that is at least partially due to differences in the scales probed by our simulation versus this survey. In fact, we show that changing the distances sampled from our simulations can produce changes as large as 40% in the fitted correlation lengths. Finally, in the X-ray, our simulations produce correlation lengths similar to that observed in the Chandra Deep Field (CDF) North, but not the significantly larger correlation length observed in the CDF South.Comment: 13 page, 7 figures. Accepted for publication in Ap

The Stellar Mass Components of Galaxies: Comparing Semi-Analytical Models with Observation

We compare the stellar masses of central and satellite galaxies predicted by three independent semianalytical models with observational results obtained from a large galaxy group catalogue constructed from the Sloan Digital Sky Survey. In particular, we compare the stellar mass functions of centrals and satellites, the relation between total stellar mass and halo mass, and the conditional stellar mass functions, which specify the average number of galaxies of stellar mass M_* that reside in a halo of mass M_h. The semi-analytical models only predict the correct stellar masses of central galaxies within a limited mass range and all models fail to reproduce the sharp decline of stellar mass with decreasing halo mass observed at the low mass end. In addition, all models over-predict the number of satellite galaxies by roughly a factor of two. The predicted stellar mass in satellite galaxies can be made to match the data by assuming that a significant fraction of satellite galaxies are tidally stripped and disrupted, giving rise to a population of intra-cluster stars in their host halos. However, the amount of intra-cluster stars thus predicted is too large compared to observation. This suggests that current galaxy formation models still have serious problems in modeling star formation in low-mass halos.Comment: 12 pages, 6 figures, accepted for publication in Ap