Higher Clustering Amplitude Research Articles

The impact of the dusty torus on obscured quasar halo mass measurements

Recent studies have found that obscured quasars cluster more strongly and are thus hosted by dark matter haloes of larger mass than their unobscured counterparts. These results pose a challenge for the simplest unification models, in which obscured objects are intrinsically the same as unobscured sources but seen through a dusty line of sight. There is general consensus that a structure like a "dusty torus" exists, meaning that this intrinsic similarity is likely the case for at least some subset of obscured quasars. However, the larger host halo masses of obscured quasars implies that there is a second obscured population that has an even higher clustering amplitude and typical halo mass. Here, we use simple assumptions about the host halo mass distributions of quasars, along with analytical methods and cosmological $N$-body simulations to isolate the signal from this population. We provide values for the bias and halo mass as a function of the fraction of the "non-torus obscured" population. Adopting a reasonable value for this fraction of $\sim$25% implies a non-torus obscured quasar bias that is much higher than the observed obscured quasar bias, because a large fraction of the obscured population shares the same clustering strength as the unobscured objects. For this non-torus obscured population, we derive a bias of $\sim$3, and typical halo masses of $\sim3\times10^{13}$ M$_{\odot}/h$ at $z=1$. These massive haloes are likely the descendants of high-mass unobscured quasars at high redshift, and will evolve into members of galaxy groups at $z=0$.

The nature of assembly bias – III. Observational properties

We analyse galaxies in groups in the Sloan Digital Sky Survey (SDSS) and find a weak but significant assembly-type bias, where old central galaxies have a higher clustering amplitude (61 $\pm$ 9 per cent) at scales > 1 Mpc than young central galaxies of equal host halo mass ($M_{h} \sim 10^{11.8} h^{-1}$ $M_{\odot}$). The observational sample is volume-limited out to z=0.1 with $M_r -$ 5 log$(h) \le -19.6$. We construct a mock catalogue of galaxies that shows a similar signal of assembly bias (46 $\pm$ 9 per cent) at the same halo mass. We then adapt the model presented by Lacerna & Padilla (Paper I) to redefine the overdensity peak height, which traces the assembly bias such that galaxies in equal density peaks show the same clustering regardless of their stellar age, but this time using observational features such as a flux limit. The proxy for peak height, which is proposed as a better alternative than the virial mass, consists in the total mass given by the mass of neighbour host haloes in cylinders centred at each central galaxy. The radius of the cylinder is parametrized as a function of stellar age and virial mass. The best-fitting set of parameters that make the assembly bias signal lower than 5$-$15 per cent for both SDSS and mock central galaxies are similar. The idea behind the parametrization is not to minimize the bias, but it is to use this method to understand the physical features that produce the assembly bias effect. Even though the tracers of the density field used here differ significantly from those used in paper I, our analysis of the simulated catalogue indicates that the different tracers produce correlated proxies, and therefore the reason behind this assembly bias is the crowding of peaks in both simulations and the SDSS.

Power spectrum of the maxBCG sample: detection of acoustic oscillations using galaxy clusters

We use the direct Fourier method to calculate the redshift-space power spectrum of the maxBCG cluster catalog -- currently by far the largest existing galaxy cluster sample. The total number of clusters used in our analysis is 12,616. After accounting for the radial smearing effect caused by photometric redshift errors and also introducing a simple treatment for the nonlinear effects, we show that currently favored low matter density "concordance" LCDM cosmology provides a very good fit to the estimated power. Thanks to the large volume (~0.4 h^{-3}Gpc^{3}), high clustering amplitude (linear effective bias parameter b_{eff} ~3x(0.85/sigma_8)), and sufficiently high sampling density (~3x10^{-5} h^{3}Mpc^{-3}) the recovered power spectrum has high enough signal to noise to allow us to find evidence (~2 sigma CL) for the baryonic acoustic oscillations (BAO). In case the clusters are additionally weighted by their richness the resulting power spectrum has slightly higher large-scale amplitude and smaller damping on small scales. As a result the confidence level for the BAO detection is somewhat increased: ~2.5 sigma. The ability to detect BAO with relatively small number of clusters is encouraging in the light of several proposed large cluster surveys.

Galaxy morphology, kinematics and clustering in a hydrodynamic simulation of a �� cold dark matter universe

We explore galaxy properties and their link with environment and clustering using a population of ∼1000 galaxies formed in a high-resolution hydrodynamic simulation of the Λ cold dark matter cosmology. At the redshift we concentrate on, z= 1, the spatial resolution is 1.4 proper h−1 kpc and the mass resolution is such that Milky Way sized disc galaxies contain ∼105 particles within their virial radii. The simulations include supermassive black hole accretion and feedback as well as a multiphase model for star formation. Overall, we find that a number of familiar qualitative relationships hold approximately between galaxy properties, for example we observe galaxies as lying between two broad extremes of type, where ‘late’ types tend to be smaller in size, have lower circular velocities, younger stars, higher star formation rates, larger disc to bulge ratios and lower Sersic indices than ‘early types’. We find that as in previous studies of small numbers of resimulated galaxies the stellar component of disc galaxies is not as rotationally supported as in observations. Bulges contain too much of the stellar mass, although exponential discs do have scalelengths compatible with observations, and the stellar mass Tully–Fisher relation at z= 1 is reproduced. The addition of black hole physics to the simulations does not appear to have made a large impact on the angular momentum results compared to these other studies, nor do we find that running an identical simulation with significantly lower mass resolution affects this aspect (although we are unable to test higher mass resolution, which other authors have shown can alleviate the problem). Despite this, we can profitably use the rank order of either disc-to-total ratio, Sersic index, or galaxy age to separate galaxies into morphological classes and examine the density–morphology relation and morphology dependence of clustering. We find that while at redshift z= 0 the well-known preponderance of early types in dense environments is seen, at z= 1 the density–morphology relation becomes flatter and late-type galaxies are even seen to have a higher clustering amplitude than early types.

Luminous red galaxy clustering atz≃ 0.7 - first results using AAOmega

We report on the AAT-AAOmega LRG Pilot observing run to establish the feasibility of a large spectroscopic survey using the new AAOmega instrument. We have selected Luminous Red Galaxies (LRGs) using single epoch SDSS riz-photometry to i =0.5 LRGs. Exposure times varied from 1 - 4 hours to determine the minimum exposure for AAOmega to make an essentially complete LRG redshift survey in average conditions. We show that LRG redshifts to i<20.5 can measured in approximately 1.5hr exposures and present comparisons with 2SLAQ and COMBO-17 (photo-)redshifts. Crucially, the riz selection coupled with the 3-4 times improved AAOmega throughput is shown to extend the LRG mean redshift from z=0.55 for 2SLAQ to z=0.681+/- 0.005 for riz-selected LRGs. This extended range is vital for maximising the S/N for the detection of the baryon acoustic oscillations (BAOs). Furthermore, we show that the amplitude of LRG clustering is s_0 = 9.9+/-0.7 h^-1 Mpc, as high as that seen in the 2SLAQ LRG Survey. Consistent results for the real-space amplitude are found from projected and semi-projected correlation functions. This high clustering amplitude is consistent with a long-lived population whose bias evolves as predicted by a simple ``high-peaks'' model. We conclude that a redshift survey of 360 000 LRGs over 3000deg^2, with an effective volume some 4 times bigger than previously used to detect BAO with LRGs, is possible with AAOmega in 170 nights.

Constraints on the correlation between QSO luminosity and host halo mass from high-redshift quasar clustering

Recent measurements of high-redshift quasar (QSO) clustering from the Sloan Digital Sky Survey indicate that QSOs at z∼ 4 have a bias 〈b〉≃ 14. We find that this extremely high clustering amplitude, combined with the corresponding space density, constrains the dispersion in the L–Mh relation to be less than 50 per cent at 99 per cent confidence for the most conservative case of a 100 per cent duty cycle. This upper limit to the intrinsic dispersion provides as strong a constraint as current upper limits to the intrinsic dispersion in the local MBH–σ relation and the ratio of bolometric to Eddington luminosity of luminous QSOs.

The Clustering of Active Galactic Nuclei in the Sloan Digital Sky Survey

We present the two-point correlation function (2PCF) of narrow-line active galactic nuclei (AGNs) selected within the First Data Release of the Sloan Digital Sky Survey. Using a sample of 13,605 AGNs in the redshift range 0.055 < z < 0.2, we find that the AGN autocorrelation function is consistent with the observed galaxy autocorrelation function on scales from 0.2 to greater than 100 h-1 Mpc. The AGN hosts trace an intermediate population of galaxies and are not detected in either the bluest (youngest) disk-dominated galaxies or many of the reddest (oldest) galaxies. We show that the AGN 2PCF is dependent on the luminosity of the narrow [OIII] emission line (L[OIII] with low L[OIII] AGNs having a higher clustering amplitude than high L[OIII] AGNs. This is consistent with lower activity AGNs residing in more massive galaxies than higher activity AGNs, and L[OIII] providing a good indicator of the fueling rate. Using a model relating halo mass to black hole mass in cosmological simulations, we show that AGNs hosted by ~1012 M⊙, dark matter halos have a 2PCF that matches that of the observed sample. This mass scale implies a mean black hole mass for the sample of MBH ~ 108 M⊙.

Modified Newtonian dynamics of large-scale structure

We examine the implications of modified Newtonian dynamics (MOND) on the large-scale structure in a Friedmann-Robertson-Walker universe. We employ a 'Jeans swindle' to write a MOND-type relationship between the fluctuations in the density and the gravitational force, g. In linear Newtonian theory, ‖g‖ decreases with time and eventually becomes <g 0 , the threshold below which MOND is dominant. If the Newtonian initial density field has a power-law power spectrum of index n < -1, then MOND-domination proceeds from the small to the large scale. At early times MOND tends to drive the density power spectrum towards k - 1 , independent of its shape in the Newtonian regime. We use N-body simulations to solve the MOND equations of motion, starting from initial conditions with a cold dark matter (CDM) power spectrum. MOND with the standard value go = 10 - 8 cm s - 2 yields a high clustering amplitude that can match the observed galaxy distribution only with strong (anti-) biasing. A value of g 0 10 - 9 cm s - 2 , however, gives results similar to Newtonian dynamics and can be consistent with the observed large-scale structure.