arXiv subjects

Aaron D. Ludlow

Publications and source records attributed to Aaron D. Ludlow.

45 records · Page 3Linked to original sources

arXiv2014

Atomic carbon as a powerful tracer of molecular gas in the high-redshift Universe: perspectives for ALMA

We use a high-resolution hydrodynamic simulation that tracks the non-equilibrium abundance of molecular hydrogen within a massive high-redshift galaxy to produce mock Atacama Large Millimeter Array (ALMA) maps of the fine-structure lines of atomic carbon, CI 1-0 and CI 2-1. Inspired by recent observational and theoretical work, we assume that CI is thoroughly mixed within giant molecular clouds and demonstrate that its emission is an excellent proxy for H2. Nearly all of the H2 associated with the galaxy can be detected at redshifts z<4 using a compact interferometric configuration with a large synthesized beam (that does not resolve the target galaxy) in less than 4 h of integration time. Low-resolution imaging of the \CI lines (in which the target galaxy is resolved into three to four beams) will detect ~80 per cent of the H2 in less than 12 h of aperture synthesis. In this case, the resulting data cube also provides the crucial information necessary for determining the dynamical state of the galaxy. We conclude that ALMA observations of the CI 1-0 and 2-1 emission are well-suited for extending the interval of cosmic look-back time over which the H2 distributions, the dynamical masses, and the Tully-Fisher relation of galaxies can be robustly probed.

astro-ph.GA↗

arXiv2013

The Mass Profile and Accretion History of Cold Dark Matter Halos

We use the Millennium Simulation series to study the relation between the accretion history (MAH) and mass profile of cold dark matter halos. We find that the mean density within the scale radius, r_{-2} (where the halo density profile has isothermal slope), is directly proportional to the critical density of the Universe at the time when the main progenitor's virial mass equals the mass enclosed within r_{-2}. Scaled to these characteristic values of mass and density, the mean MAH, expressed in terms of the critical density of the Universe, M(ρ_{crit}(z)), resembles that of the enclosed density profile, M(<ρ>), at z=0. Both follow closely the NFW profile, suggesting that the similarity of halo mass profiles originates from the mass-independence of halo MAHs. Support for this interpretation is provided by outlier halos whose accretion histories deviate from the NFW shape; their mass profiles show correlated deviations from NFW and are better approximated by Einasto profiles. Fitting both M(<ρ>) and M(ρ_{crit}) with either NFW or Einasto profiles yield concentration and shape parameters that are correlated, confirming and extending earlier work linking the concentration of a halo with its accretion history. These correlations also confirm that halo structure is insensitive to initial conditions: only halos whose accretion histories differ greatly from the NFW shape show noticeable deviations from NFW in their mass profiles. As a result, the NFW profile provides acceptable fits to hot dark matter halos, which do not form hierarchically, and for fluctuation power spectra other than CDM. Our findings, however, predict a subtle but systematic dependence of mass profile shape on accretion history which, if confirmed, would provide strong support for the link between accretion history and halo structure we propose here.

astro-ph.CO↗

arXiv2013

The Mass-Concentration-Redshift Relation of Cold Dark Matter Halos

We use the Millennium Simulation series to investigate the mass and redshift dependence of the concentration of equilibrium cold dark matter (CDM) halos. We extend earlier work on the relation between halo mass profiles and assembly histories to show how the latter may be used to predict concentrations for halos of all masses and at any redshift. Our results clarify the link between concentration and the ``collapse redshift'' of a halo as well as why concentration depends on mass and redshift solely through the dimensionless ``peak height'' mass parameter, $ν(M,z)=δ_{\rm crit}(z)/σ(M,z)$. We combine these results with analytic mass accretion histories to extrapolate the $c(M,z)$ relations to mass regimes difficult to reach through direct simulation. Our model predicts that, at given $z$, $c(M)$ should deviate systematically from a simple power law at high masses, where concentrations approach a constant value, and at low masses, where concentrations are substantially lower than expected from extrapolating published empirical fits. This correction may reduce the expected self-annihilation boost factor from substructure by about one order of magnitude. The model also reproduces the $c(M,z)$ dependence on cosmological parameters reported in earlier work, and thus provides a simple and robust account of the relation between cosmology and the mass-concentration-redshift relation of CDM halos.

astro-ph.CO↗

arXiv2013

The Dynamical State and Mass-Concentration Relation of Galaxy Clusters

We use the Millennium Simulation series to study how the dynamical state of dark matter halos affects the relation between mass and concentration. We find that a large fraction of massive systems are identified when they are substantially out of equilibrium and in a particular phase of their dynamical evolution: the more massive the halo, the more likely it is found at a transient stage of high concentration. This state reflects the recent assembly of massive halos and corresponds to the first pericentric passage of recently-accreted material when, before virialization, the kinetic and potential energies reach maximum and minimum values, respectively. This result explains the puzzling upturn in the mass-concentration relation reported in recent work for massive halos; indeed, the upturn disappears when only dynamically-relaxed systems are considered in the analysis. Our results warn against applying simple equilibrium models to describe the structure of rare, massive galaxy clusters and urges caution when extrapolating scaling laws calibrated on lower-mass systems, where such deviations from equilibrium are less common. The evolving dynamical state of galaxy clusters ought to be carefully taken into account if cluster studies are to provide precise cosmological constraints.

astro-ph.CO↗

arXiv2012

The spatial and velocity bias of linear density peaks and proto-haloes in the Lambda cold dark matter cosmology

We use high resolution N-body simulations to investigate the Lagrangian bias of cold dark matter haloes within the LCDM cosmology. Our analysis focuses on "proto-haloes", which we identify in the simulation initial conditions with the subsets of particles belonging to individual redshift-zero haloes. We then calculate the number-density and velocity-divergence fields of proto-haloes and estimate their auto spectral densities. We also measure the corresponding cross spectral densities with the linear matter distribution. We use our results to test a Lagrangian-bias model presented by Desjacques and Sheth which is based on the assumption that haloes form out of local density maxima of a specific height. Our comparison validates the predicted functional form for the scale-dependence of the bias for both the density and velocity fields. We also show that the bias coefficients are accurately predicted for the velocity divergence. On the contrary, the theoretical values for the density bias parameters do not accurately match the numerical results as a function of halo mass. This is likely due to the simplistic assumptions that relate virialized haloes to density peaks of a given height in the model. We also detect appreciable stochasticity for the Lagrangian density bias, even on very large scales. These are not included in the model at leading order but correspond to higher order corrections.

astro-ph.CO↗

arXiv2011

The Peaks Formalism and the Formation of Cold Dark Matter Haloes

We use two cosmological simulations of structure formation to study the conditions under which dark matter haloes emerge from the linear density field. Our analysis focuses on matching sites of halo collapse to local density maxima, or "peaks", in the initial conditions of the simulations and provides a crucial test of the central ansatz of the peaks formalism. By identifying peaks on a variety of smoothed, linearly extrapolated density fields we demonstrate that as many as ~70% of well-resolved dark matter haloes form preferentially near peaks whose characteristic masses are similar to that of the halo, with more massive haloes showing a stronger tendency to reside near peaks initially. We identify a small but significant fraction of haloes that appear to evolve from peaks of substantially lower mass than that of the halo itself. We refer to these as "peakless haloes" for convenience. By contrasting directly the properties of these objects with the bulk of the proto-halo population we find two clear differences: 1) their initial shapes are significantly flatter and more elongated than the predominantly triaxial majority, and 2) they are, on average, more strongly compressed by tidal forces associated with their surrounding large scale structure. Using the two-point correlation function we show that peakless haloes tend to emerge from highly clustered regions of the initial density field implying that, at fixed mass, the accretion geometry and mass accretion histories of haloes in highly clustered environments differ significantly from those in the field. This may have important implications for understanding the origin of the halo assembly bias, of galaxy properties in dense environments and how environment affects the morphological transformation of galaxies near groups and rich galaxy clusters.

astro-ph.CO↗

arXiv2011

The Density and Pseudo-Phase-Space Density Profiles of CDM halos

Cosmological N-body simulations indicate that the spherically-averaged density profiles of cold dark matter halos are accurately described by Einasto profiles, where the logarithmic slope is a power-law of adjustable exponent, γ=dlnρ/dlnr ~ r^α$. The pseudo-phase-space density (PPSD) profiles of CDM halos also show remarkable regularity, and are well approximated by simple power laws, Q(r)=ρ/σ^3 ~ r^-χ. We show that this is expected from dynamical equilibrium considerations, since Jeans' equations predict that the pseudo-phase-space density profiles of Einasto halos should resemble power laws over a wide range of radii. For the values of αtypical of CDM halos, the inner Q profiles of equilibrium halos deviate significantly from a power law only very close to the center, and simulations of extremely high-resolution would be needed to detect such deviations unambiguously. We use an ensemble of halos drawn from the Millennium-II simulation to study which of these two alternatives describe best the mass profile of CDM halos. Our analysis indicates that at the resolution of the best available simulations, both Einasto and power-law PPSD profiles (with adjustable exponents αand χ, respectively) provide equally acceptable fits to the simulations. A full account of the structure of CDM halos requires understanding how the shape parameters that characterize departures from self-similarity, like αor χ, are determined by evolutionary history, environment or initial conditions.

astro-ph.CO↗

arXiv2010

Secondary Infall and the Pseudo-Phase-Space Density Profiles of Cold Dark Matter Halos

We use N-body simulations to investigate the radial dependence of the density and velocity dispersion in cold dark matter (CDM) halos. In particular, we explore how closely Q rho/sigma^3, a surrogate measure of the phase-space density, follows a power-law in radius. Our study extends earlier work by considering, in addition to spherically-averaged profiles, local Q-estimates for individual particles, Q_i; profiles based on the ellipsoidal radius dictated by the triaxial structure of the halo, Q_i(r'); and by carefully removing substructures in order to focus on the profile of the smooth halo, Q^s. The resulting Q_i^s(r') profiles follow closely a power law near the center, but show a clear upturn from this trend near the virial radius, r_{200}. The location and magnitude of the deviations are in excellent agreement with the predictions from Bertschinger's spherical secondary-infall similarity solution. In this model, Q \propto r^{-1.875} in the inner, virialized regions, but departures from a power-law occur near r_{200} because of the proximity of this radius to the location of the first shell crossing - the shock radius in the case of a collisional fluid. Particles there have not yet fully virialized, and so Q departs from the inner power-law profile. Our results imply that the power-law nature of $Q$ profiles only applies to the inner regions and cannot be used to predict accurately the structure of CDM halos beyond their characteristic scale radius.

astro-ph.CO↗

arXiv2008

The Unorthodox Orbits of Substructure Halos

(Abridged) We use cosmological N-body simulations to study the properties of substructure halos in galaxy-sized dark matter halos. We extend prior work on the subject by considering the whole population of subhalos physically associated with the main system. These are defined as subhalos that have at some time in the past been within the virial radius of the halo's main progenitor and that have survived as self-bound entities to $z=0$. We find that this population extends beyond {\it three times} the virial radius, and contains objects on extreme orbits. We trace the origin of these unorthodox orbits to the tidal dissociation of bound groups of subhalos, which results in the ejection of some subhalos along tidal streams. Ejected subhalos are primarily low-mass systems, leading to mass-dependent biases in their spatial distribution and kinematics: the lower the subhalo mass at accretion time, the less centrally concentrated and kinematically hotter their descendant population. The bias is strongest amongst the most massive subhalos, but disappears at the low-mass end. Our findings imply that subhalos identified within the virial radius represent an incomplete census of the substructure physically related to a halo: only about {\it one half} of all associated subhalos are found today within the virial radius of a halo. These results may explain the age dependence of the clustering of low-mass halos, and has implications for (i) the interpretation of the structural parameters and assembly histories of halos neighboring massive systems; (ii) the existence of low-mass dynamical outliers in the Local Group; and (iii) the presence of evidence for evolutionary effects well outside the traditional virial boundary of a galaxy system.

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