Richard N. Henriksen

On Universal Halos and the Radial Orbit Instability

The radial orbit instability drives the density profile of a collapsing nearly spherical density perturbation toward the universal form found in cosmological simulations. This conclusion, first noted by Huss and coworkers, is explored in detail through a series of numerical experiments involving isolated halos. Simulations are run with and without the nonradial forces responsible for the instability. In the absence of the instability, the density profile is a pure power law, the differential energy distribution is close to a Boltzmann distribution, and the orbits are radially biased. The instability transforms the density profile to the NFW form, induces an energy cutoff at high binding energy, and isotropizes the orbits near the center of the system. New insights into the underlying physics of the radial orbit instability are presented.

Black Hole Growth in Dark Matter and the MBH-σ Relation

In this paper, we consider the growth of seed black holes immersed in dark matter halos. We investigate first the adiabatic growth in various initial distribution functions (isothermal; power law; and Navarro, Frenk, and White) and find the resulting density, radial velocity, and anisotropy profiles. In addition, we estimate the growth rate for a given black hole mass in the corresponding adiabatically modified dark matter distribution function. Only in the isothermal case is there a convincing black hole mass-age relation. By calculating the line-of-sight velocity dispersion for the various cases as a function of the black hole mass, we find the predicted adiabatic MBH-σ relation, which never approaches the recently observed power law. We conclude by abandoning adiabaticity, suggesting that the black hole grows proportionally to the dark matter halo itself on a dynamic timescale. This allows us to relate the observed MBH-σ relation to the cosmological power spectrum on galactic scales by using dimensional scaling arguments.

A first use of wavelet analysis for molecular clouds

In the present wavelet analysis of the C-13O spectral data from L1551s outflow region, the fractal Hausdorff dimension of the data space consisting of the centroid velocities over the x-y plane is noted to be definable as 2.35 + or - 0.01. An analysis of surface density yields a dimensional value close to 3, rather than Prasad and Sreenivasans (1990) 2.36 + or - 0.05 for the Kolmogorov range of the turbulence associated with a water jet in quiet surroundings; this suggests that there is gas between the emissivity peaks, as supported by the larger dominant structure found for the surface density. 13 refs.

Continuum halos in nearby galaxies: an EVLA survey (CHANG-ES) - I. Introduction to the survey

We introduce a new survey to map the radio continuum halos of a sample of 35 edge-on spiral galaxies at 1.5 GHz and 6 GHz in all polarization products. The survey is exploiting the new wide bandwidth capabilities of the Karl G. Jansky Very Large Array (i.e., the Expanded Very Large Array) in a variety of array configurations (B, C, and D) in order to compile the most comprehensive data set yet obtained for the study of radio halo properties. This is the first survey of radio halos to include all polarization products. In this first paper, we outline the scientific motivation of the survey, the specific science goals, and the expected improvements in noise levels and spatial coverage from the survey. Our goals include investigating the physical conditions and origin of halos, characterizing cosmic-ray transport and wind speed, measuring Faraday rotation and mapping the magnetic field, probing the in-disk and extraplanar far-infrared-radio continuum relation, and reconciling non-thermal radio emission with high-energy gamma-ray models. The sample size allows us to search for correlations between radio halos and other properties, including environment, star formation rate, and the presence of active galactic nuclei. In a companion paper (Paper II) we outline the data reduction steps and present the first results of the survey for the galaxy, NGC 4631.

Relaxation of a Collisionless System and the Transition to a New Equilibrium Velocity Distribution

In this paper we present our conclusions from the numerical study of the collapse of a destabilized collisionless stellar system. We use both direct integration of the Vlasov-Poisson equations and an N-body tree code to obtain our results, which are mutually confirmed. We find that spherical and moderately nonspherical collapse configurations evolve to new equilibrium configurations in which the velocity distribution approaches a Gaussian form, at least in the central regions. The evolution to this state has long been an open question, and in this work we are able to clarify the process responsible and to support predictions made from statistical considerations by Lynden-Bell and Nakamura. The simulations of merging N-body systems show a transition to a Gaussian velocity distribution that is increasingly suppressed as the initial separation of centers is increased. Possible reasons for this are discussed.

Hydrogen clouds and the MACHO/EROS events

We propose that the recently reported MACHO/EROS events correspond to gravitational amplification by dark clouds rather than compact objects. These clouds must be very dense with

‘Cored apple’ bipolarity: a global instability to convection in radial accretion?

M\sim 0.1 ~M_\odot

A Global Jet/Circulation Model for Young Stars

and

Coarse‐graining the distribution function of cold dark matter

R\la 10^{14}~{\rm cm}

Continuum Halos in Nearby Galaxies: An EVLA Survey (CHANG-ES). II. First Results on NGC 4631

. In all likelihood, the clouds will be members of a family of objects with different sizes and masses. We therefore expect events of longer duration than the ones reported by the MACHO and EROS groups but with light curves very different from the ones derived assuming point mass lenses. We suggest that one such event has already been observed in radio measurements of the quasar 1502+106. The abundances of free electrons, metals, complex molecules, and dust grains are constrained to be very small suggesting that the clouds are formed from a primordial mixture of hydrogen and helium. Cosmic rays and background UV radiation ionize a halo around the cloud. Radio waves from distant sources will be scattered by the electrons in this halo, an effect which may have already been observed in quasars such as 1502+106. We argue that dark clouds are a viable alternative to compact objects for baryonic dark matter in the halo.