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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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Satellite galaxies are commonly used as tracers to measure the line-of-sight velocity dispersion ($sigma_{rm LOS}$) of the dark matter halo associated with their central galaxy, and thereby to estimate the halos mass. Recent observational dispersion estimates of the Local Group, including the Milky Way and M31, suggest $sigmasim$50 km/s, which is surprisingly low when compared to the theoretical expectation of $sigmasim$100s km/s for systems of their mass. Does this pose a problem for $Lambda$CDM? We explore this tension using the {small{SURFS}} suite of $N$-body simulations, containing over 10000 (sub)haloes with well tracked orbits. We test how well a central galaxys host halo velocity dispersion can be recovered by sampling $sigma_{rm LOS}$ of subhaloes and surrounding haloes. Our results demonstrate that $sigma_{rm LOS}$ is biased mass proxy. We define an optimal window in $v_{rm LOS}$ and projected distance ($D_p$) -- $0.5lesssim D_p/R_{rm vir}lesssim1.0$ and $v_{rm LOS} lesssim0.5V_{rm esc}$, where $R_{rm vir}$ is the virial radius and $V_{rm esc}$ is the escape velocity -- such that the scatter in LOS to halo dispersion is minimised - $sigma_{rm LOS}=(0.5pm0.1)sigma_{v,{rm H}}$. We argue that this window should be used to measure line-of-sight dispersions as a proxy for mass, as it minimises scatter in the $sigma_{rm LOS}-M_{rm vir}$ relation. This bias also naturally explains the results from cite{mcconnachie2012a}, who used similar cuts when estimating $sigma_{rm LOS,LG}$, producing a bias of $sigma_{rm LG}=(0.44pm0.14)sigma_{v,{rm H}}$. We conclude that the Local Groups velocity dispersion does not pose a problem for $Lambda$CDM and has a mass of $log M_{rm LG, vir}/M_odot=12.0^{+0.8}_{-2.0}$.
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.
The leading tensions to the collisionless cold dark matter (CDM) paradigm are the small-scale controversies, discrepancies between observations at the dwarf-galactic scale and their simulational counterparts. In this work we consider methods to infer 3D morphological information on Local Group dwarf spheroidals, and test the fitness of CDM+hydrodynamics simulations to the observed galaxy shapes. We find that the subpopulation of dwarf galaxies with mass-to-light ratio $gtrsim 100 M_odot/L_odot$ reflects an oblate morphology. This is discrepant with the dwarf galaxies with mass-to-light ratio $lesssim 100 M_odot/L_odot$, which reflect prolate morphologies, and more importantly with simulations of CDM-sourced galaxies which are explicitly prolate. Although more simulations and data are called for, if evidence of oblate pressure-supported stellar distributions persists, we argue that an underlying oblate non-CDM dark matter halo may be required, and present this as motivation for future studies.
Understanding the formation and evolution of early-type, spheroid-dominated galaxies is an open question within the context of the hierarchical clustering scenario, particularly, in low-density environments. Our goal is to study the main structural, dynamical, and stellar population properties and assembly histories of field spheroid-dominated galaxies formed in a LCDM scenario to assess to what extend they are consistent with observations. We selected spheroid-dominated systems from a LCDM simulation that includes star formation, chemical evolution and Supernova feedback. A sample of 18 field systems with Mstar <= 6x10^10 Msun that are dominated by the spheroid component. For this sample we estimate the fundamental relations of ellipticals and then compared with current observations. The simulated spheroid galaxies have sizes in good agreement with observations. The bulges follow a Sersic law with Sersic indexes that correlate with the bulge-to-total mass ratios. The structural-dynamical properties of the simulated galaxies are consistent with observed Faber-Jackson, Fundamental Plane, and Tully-Fisher relations. However, the simulated galaxies are bluer and with higher star formation rates than observed isolated early-type galaxies. The archaeological mass growth histories show a slightly delayed formation and more prominent inside-out growth mode than observational inferences based on the fossil record method. The main structural and dynamical properties of the simulated spheroid-dominated galaxies are consistent with observations. This is remarkable since none of them has been tuned to be reproduced. However, the simulated galaxies are blue and star-forming, and with later stellar mass growth histories as compared to observational inferences. This is mainly due to the persistence of extended discs in the simulations. Abridged
We study the radial acceleration relation (RAR) between the total ($a_{rm tot}$) and baryonic ($a_{rm bary}$) centripetal acceleration profiles of central galaxies in the cold dark matter (CDM) paradigm. We analytically show that the RAR is intimately connected with the physics of the quasi-adiabatic relaxation of dark matter in the presence of baryons in deep potential wells. This cleanly demonstrates how the mean RAR and its scatter emerge in the low-acceleration regime ($10^{-12},{rm m,s}^{-2}lesssim a_{rm bary}lesssim10^{-10},{rm m,s}^{-2}$) from an interplay between baryonic feedback processes and the distribution of CDM in dark halos. Our framework allows us to go further and study both higher and lower accelerations in detail, using analytical approximations and a realistic mock catalog of $sim342,000$ low-redshift central galaxies with $M_rleq-19$. We show that, while the RAR in the baryon-dominated, high-acceleration regime ($a_{rm bary}gtrsim10^{-10},{rm m,s}^{-2}$) is very sensitive to details of the relaxation physics, a simple `baryonification prescription matching the relaxation results of hydrodynamical CDM simulations is remarkably successful in reproducing the observed RAR without any tuning. And in the (currently unobserved) ultra-low-acceleration regime ($a_{rm bary}lesssim 10^{-12},{rm m,s}^{-2}$), the RAR is sensitive to the abundance of diffuse gas in the halo outskirts, with our default model predicting a distinctive break from a simple power-law-like relation for HI-deficient, diffuse gas-rich centrals. Our mocks also show that the RAR provides more robust, testable predictions of the $Lambda$CDM paradigm at galactic scales, with implications for alternative gravity theories, than the baryonic Tully-Fisher relation.
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