In Pursuit of the Least Luminous Galaxies

The dwarf galaxy companions to the Milky Way are unique cosmological laboratories. With luminosities as low as 10^-7 L_MW, they inhabit the lowest mass dark matter halos known to host stars and are presently the most direct tracers of the distribution, mass spectrum, and clustering scale of dark matter. Their resolved stellar populations also facilitate detailed studies of their history and mass content. To fully exploit this potential requires a well-defined census of virtually invisible galaxies to the faintest possible limits and to the largest possible distances. I review the past and present impacts of survey astronomy on the census of Milky Way dwarf galaxy companions, and discuss the future of finding ultra-faint dwarf galaxies around the Milky Way and beyond in wide-field survey data.Comment: Review article, 20 pages. Accepted to Advances in Astronomy, Dwarf Galaxy Cosmology issu

Investigating the origins of the CMB-XRB cross correlation

Recently, we presented evidence for a cross-correlation of the WMAP satellite map of the cosmic microwave background (CMB) and the HEAO1 satellite map of the hard X-ray background (XRB) with a dimensionless amplitude of 0.14 +- 0.05 normalized to the product of the rms fluctuations of the CMB and XRB (Boughn & Crittenden, 2004). Such a correlation is expected in a universe dominated by a cosmological constant via the integrated Sachs-Wolfe (ISW) effect and the level of the correlation observed is consistent with that predicted by the currently favored Lambda cold dark matter model of the universe. Since this offers independent confirmation of the cosmological model, it is important to verify the origin of the correlation. Here we explore in detail some possible foreground sources of the correlation. The present evidence all supports an ISW origin.Comment: 8 pages, 7 eps figure

Realizable Hamiltonians for Universal Adiabatic Quantum Computers

It has been established that local lattice spin Hamiltonians can be used for universal adiabatic quantum computation. However, the 2-local model Hamiltonians used in these proofs are general and hence do not limit the types of interactions required between spins. To address this concern, the present paper provides two simple model Hamiltonians that are of practical interest to experimentalists working towards the realization of a universal adiabatic quantum computer. The model Hamiltonians presented are the simplest known QMA-complete 2-local Hamiltonians. The 2-local Ising model with 1-local transverse field which has been realized using an array of technologies, is perhaps the simplest quantum spin model but is unlikely to be universal for adiabatic quantum computation. We demonstrate that this model can be rendered universal and QMA-complete by adding a tunable 2-local transverse XX coupling. We also show the universality and QMA-completeness of spin models with only 1-local Z and X fields and 2-local ZX interactions.Comment: Paper revised and extended to improve clarity; to appear in Physical Review

Telescoper: de novo assembly of highly repetitive regions.

MotivationWith advances in sequencing technology, it has become faster and cheaper to obtain short-read data from which to assemble genomes. Although there has been considerable progress in the field of genome assembly, producing high-quality de novo assemblies from short-reads remains challenging, primarily because of the complex repeat structures found in the genomes of most higher organisms. The telomeric regions of many genomes are particularly difficult to assemble, though much could be gained from the study of these regions, as their evolution has not been fully characterized and they have been linked to aging.ResultsIn this article, we tackle the problem of assembling highly repetitive regions by developing a novel algorithm that iteratively extends long paths through a series of read-overlap graphs and evaluates them based on a statistical framework. Our algorithm, Telescoper, uses short- and long-insert libraries in an integrated way throughout the assembly process. Results on real and simulated data demonstrate that our approach can effectively resolve much of the complex repeat structures found in the telomeres of yeast genomes, especially when longer long-insert libraries are used.AvailabilityTelescoper is publicly available for download at sourceforge.net/p/[email protected] informationSupplementary data are available at Bioinformatics online

The Observed and Predicted Spatial Distribution of Milky Way Satellite Galaxies

We review evidence that the census of Milky Way satellites similar to those known may be incomplete at low latitude due to obscuration and in the outer halo due to a decreasing sensitivity to dwarf satellites with distance. We evaluate the possible impact that incompleteness has on comparisons with substructure models by estimating corrections to the known number of dwarfs using empirical and theoretical models. If we assume that the true distribution of Milky Way satellites is uniform with latitude, then we estimate a 33% incompleteness in the total number of dwarfs due to obscuration at low latitude. Similarly, if we suppose that the radial distribution of Milky Way satellites matches that of M31, or that of the oldest sub-halos or the most massive sub-halos in a simulation, we estimate a total number of Milky Way dwarfs ranging from 1 -- 3 times the known population. Although the true level of incompleteness is quite uncertain, the fact that our extrapolations yield average total numbers of MW dwarfs that are realistically 1.5 -- 2 times the known population, shows that incompleteness needs to be taken seriously when comparing to models of dwarf galaxy formation. Interestingly, the radial distribution of the oldest sub-halos in a Lambda+CDM simulation of a Milky Way-like galaxy possess a close match to the observed distribution of M31's satellites, which suggests that reionization may be an important factor controlling the observability of sub-halos. We also assess the prospects for a new SDSS search for Milky Way satellites to constrain the possible incompleteness in the outer halo.Comment: 9 pages, 7 figures. Replaced with MNRAS accepted versio

The Mixmaster Universe in Five Dimensions

We consider a five dimensional vacuum cosmology with Bianchi type-IX spatial geometry and an extra non-compact coordinate. Finding a new class of solutions, we examine and rule out the possibility of deterministic chaos. We interpret this result within the context of induced matter theory.Comment: 13 page

Energy Distribution of a Charged Black Hole with a Minimally Coupled Scalar Field

Using three different energy-momentum complexes, the Einstein, Landau-Lifshitz, and Papapetrou prescriptions, we calculate the energy of an electrically charged black hole exact solution with a self-interacting, minimally-coupled scalar field and the asymptotic region locally an Anti-deSitter spacetime. Writing the metric in Kerr-Schild Cartesian coordinates, we demonstrate that this metric belongs to the Kerr-Schild class of solutions. Applying each of the three energy-momentum prescriptions and comparing the results, we find consistency among these complexes, suggesting their utility as localized measures of energy.Comment: 11 pages; To appear in Astrophysics and Space Scienc

The fraction of early-type galaxies in low redshift groups and clusters of galaxies

We examine the fraction of early-type (and spiral) galaxies found in groups and clusters of galaxies as a function of dark matter halo mass. We use morphological classifications from the Galaxy Zoo project matched to halo masses from both the C4 cluster catalogue and the Yang et al (2007) group catalogue. We find that the fraction of early-type (or spiral) galaxies remains constant (changing by less than 10%) over three orders of magnitude in halo mass (13<log MH/Msol/h<15.8). This result is insensitive to our choice of halo mass measure, from velocity dispersions or summed optical luminosity. Furthermore, we consider the morphology-halo mass relations in bins of galaxy stellar mass M*, and find that while the trend of constant fraction remains unchanged, the early-type fraction amongst the most massive galaxies (11<log M*/Msol/h <12) is a factor of three greater than lower mass galaxies (10<logM*/Msol/h<10.7). We compare our observational results with those of simulations presented in De Lucia et al (2011), as well as previous observational analyses, and semi-analytic bulge (or disc) dominated galaxies from the Millennium Simulation. We find the simulations recover similar trends as observed, but may over-predict the abundances of the most massive bulge dominated (early-type) galaxies. Our results suggest that most morphological transformation is happening on the group scale before groups merge into massive clusters. However, we show that within each halo a morphology-density relation remains: it is summing the total fraction to a self-similar scaled radius which results in a flat morphology-halo mass relationship.Comment: 9 page, 5 figures, modified to match accepted version (MNRAS

Fritz Hasenohrl and E = mc^2

In 1904, the year before Einstein's seminal papers on special relativity, Austrian physicist Fritz Hasenohrl examined the properties of blackbody radiation in a moving cavity. He calculated the work necessary to keep the cavity moving at a constant velocity as it fills with radiation and concluded that the radiation energy has associated with it an apparent mass such that E = 3/8 mc^2. Also in 1904, Hasenohrl achieved the same result by computing the force necessary to accelerate a cavity already filled with radiation. In early 1905, he corrected the latter result to E = 3/4 mc^2. In this paper, Hasenohrl's papers are examined from a modern, relativistic point of view in an attempt to understand where he went wrong. The primary mistake in his first paper was, ironically, that he didn't account for the loss of mass of the blackbody end caps as they radiate energy into the cavity. However, even taking this into account one concludes that blackbody radiation has a mass equivalent of m = 4/3 E/c^2 or m = 5/3 E/c^2 depending on whether one equates the momentum or kinetic energy of radiation to the momentum or kinetic energy of an equivalent mass. In his second and third papers that deal with an accelerated cavity, Hasenohrl concluded that the mass associated with blackbody radiation is m = 4/3 E/c^2, a result which, within the restricted context of Hasenohrl's gedanken experiment, is actually consistent with special relativity. Both of these problems are non-trivial and the surprising results, indeed, turn out to be relevant to the "4/3 problem" in classical models of the electron. An important lesson of these analyses is that E = mc^2, while extremely useful, is not a "law of physics" in the sense that it ought not be applied indiscriminately to any extended system and, in particular, to the subsystems from which they are comprised

Propagating front in an excited granular layer

A partial monolayer of ~ 20000 uniform spherical steel beads, vibrated vertically on a flat plate, shows remarkable ordering transitions and cooperative behavior just below 1g maximum acceleration. We study the stability of a quiescent disordered or ``amorphous'' state formed when the acceleration is switched off in the excited ``gaseous'' state. The transition from the amorphous state back to the gaseous state upon increasing the plate's acceleration is generally subcritical: An external perturbation applied to one bead initiates a propagating front that produces a rapid transition. We measure the front velocity as a function of the applied acceleration. This phenomenon is explained by a model based on a single vibrated particle with multiple attractors that is perturbed by collisions. A simulation shows that a sufficiently high rate of interparticle collisions can prevent trapping in the attractor corresponding to the nonmoving ground state.Comment: 16 pages, 9 figures, revised version, to appear in Phys. Rev. E, May 199