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Register a new userHalo Occupation Distributions of Moderate X-ray AGNs through Major and Minor Mergers in a $Lambda$-CDM Cosmology
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Liliana Altamirano-D\\'evora
Publication date
2015
fields
Physics
and research's language is
English
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Motivated by recent inferred form of the halo occupation distribution (HOD) of X-ray selected AGNs, in the COSMOS field by Allevato et al. (2012), we investigate the HOD properties of moderate X-ray luminosity Active Galactic Nuclei (mXAGNs) using a simple model based on merging activity between dark matter halos (DMHs) in a $Lambda$-CDM cosmology. The HODs and number densities of the simulated mXAGNs at $z=0.5$, under the above scenarios to compare with Allevato et al. (2012) results. We find that the simulated HODs of major and minor mergers, and the observed for mXAGNs are consistent among them. Our main result is that minor mergers, contrary to what one might expect, can play an important role in activity mAGNs.
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The spatial clustering of active galactic nuclei (AGNs) is considered to be one of the important diagnostics for the understanding of the underlying processes behind their activities complementary to measurements of the luminosity function (LF). We analyse the AGN clustering from a recent semi-analytic model performed on a large cosmological $N$-body simulation covering a cubic gigaparsec comoving volume. We have introduced a new time-scale of gas accretion on to the supermassive black holes to account for the loss of the angular momentum on small scales, which is required to match the faint end of the observed X-ray LF. The large simulation box allows us accurate determination of the auto-correlation function of the AGNs. The model prediction indicates that this time-scale plays a significant role in allowing massive haloes to host relatively faint population of AGNs, leading to a higher bias factor for those AGNs. The model predictions are in agreement with observations of X-ray selected AGNs in the luminosity range $10^{41.5}~mathrm{erg} mathrm{s}^{-1} leq L_{2-10mathrm{keV}} leq 10^{44.5}~mathrm{erg} mathrm{s}^{-1}$, with the typical host halo mass of $10^{12.5-13.5} h^{-1},{rm M}_{odot}$ at $z lesssim 1$. This result shows that the observational clustering measurements impose an independent constraint on the accretion time-scale complementary to the LF measurements. Moreover, we find that not only the effective halo mass corresponding to the overall bias factor, but the extended shape of the predicted AGN correlation function shows remarkable agreement with those from observations. Further observational efforts towards the low luminosity end at $z sim 1$ would give us stronger constraints on the triggering mechanisms of AGN activities through their clustering.
We present a new method for embedding a stellar disc in a cosmological dark matter halo and provide a worked example from a {Lambda}CDM zoom-in simulation. The disc is inserted into the halo at a redshift z = 3 as a zero-mass rigid body. Its mass and size are then increased adiabatically while its position, velocity, and orientation are determined from rigid-body dynamics. At z = 1, the rigid disc is replaced by an N-body disc whose particles sample a three-integral distribution function (DF). The simulation then proceeds to z = 0 with live disc and halo particles. By comparison, other methods assume one or more of the following: the centre of the rigid disc during the growth phase is pinned to the minimum of the halo potential, the orientation of the rigid disc is fixed, or the live N-body disc is constructed from a two rather than three-integral DF. In general, the presence of a disc makes the halo rounder, more centrally concentrated, and smoother, especially in the innermost regions. We find that methods in which the disc is pinned to the minimum of the halo potential tend to overestimate the amount of adiabatic contraction. Additionally, the effect of the disc on the subhalo distribution appears to be rather insensitive to the disc insertion method. The live disc in our simulation develops a bar that is consistent with the bars seen in late-type spiral galaxies. In addition, particles from the disc are launched or kicked up to high galactic latitudes.
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New Chandra X-ray data and extensive optical spectroscopy, obtained with AAOmega on the 3.9 m Anglo-Australian Telescope, are used to study the complex merger taking place in the galaxy cluster Abell 2744. Combining our spectra with data from the literature provides a catalog of 1237 redshifts for extragalactic objects lying within 15 of the cluster center. From these, we confirm 343 cluster members projected within 3 Mpc of the cluster center. Combining positions and velocities, we identify two major substructures, corresponding to the remnants of two major subclusters. The new data are consistent with a post core passage, major merger taking place along an axis that is tilted well out of the plane of the sky, together with an interloping minor merger. Supporting this interpretation, the new X-ray data reveal enriched, low entropy gas from the core of the approaching, major subcluster, lying ~2 north of the cluster center, and a shock front to the southeast of the previously known bright, compact core associated with the receding subcluster. The X-ray morphology of the compact core is consistent with a Bullet-like cluster viewed from within ~45 degrees of the merger axis. An X-ray peak ~3 northwest of the cluster center, with an associated cold front to the northeast and a trail of low entropy gas to the south, is interpreted as the remnant of an interloping minor merger taking place roughly in the plane of the sky. We infer approximate paths for the three merging components.
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