Substructure in the Andromeda Galaxy Globular Cluster System

In the most prominent current scenario of galaxy formation, galaxies form hierarchically through the merger of smaller systems. Such mergers could leave behind dynamical signatures which may linger long after the event. In particular, the globular cluster system (GCS) of a merging satellite galaxy may remain as a distinct sub-population within the GCS of a massive galaxy. Using the latest available globular cluster velocities and metallicities, we present the results of a search for grouping in the GCS of our nearest large spiral galaxy neighbor, M31. A modified friends-of-friends algorithm is used to identify a number of possible merger remnants in projected position, radial velocity and [Fe/H] parameter space. Numerical simulations are used to check that such merger remnants are indeed plausible over the timescales of interest. The identification of stellar streams associated with these groups is required in order to confirm that they represent merger remnants.Comment: 19 pages, 3 figures, accepted for publication in the Ap

Raster-scan imaging with normal-incidence, midinfrared InAs/GaAs quantum dot infrared photodetectors

We demonstrate normal incidence infrared imaging with quantum dot infrared photodetectors using a raster-scan technique. The device heterostructure, containing multiple layers of InAs/GaAs self-organized quantum dots, were grown by molecular-beam epitaxy. Individual devices have been operated at temperatures as high as 150 K and, at 100 K, are characterized by λpeak = 3.72 μm,λpeak=3.72μm, Jdark = 6×10−10 A/cm2Jdark=6×10−10A/cm2 for a bias of 0.1 V, and D∗ = 2.94×109 cm Hz1/2/WD∗=2.94×109cmHz1/2/W at a bias of 0.2 V. Raster-scan images of heated objects and infrared light sources were obtained with a small (13×13)(13×13) interconnected array of detectors (to increase the photocurrent) at 80 K. © 2002 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70691/2/APPLAB-80-18-3265-1.pd

Fine Structure of Dark Matter Halos and its Effect on Terrestrial Detection Experiments

Terrestrial dark matter detection experiments probe the velocity-space distribution of dark matter particles in the vicinity of the Earth. We present a novel method, to be used in conjunction with standard cosmological simulations of hierarchical clustering, that allows one to extract a truly local velocity-space distribution in exquisite detail. Preliminary results suggest a new picture for this distribution which is decidedly non-Maxwellian but instead is characterized by randomly positioned peaks in velocity space. We discuss the implications of these results for both WIMP and axion detection experiments.Comment: 5 pages, 3 figure

Requirement for PBAF in transcriptional repression and repair at DNA breaks in actively transcribed regions of chromatin

Actively transcribed regions of the genome are vulnerable to genomic instability. Recently, it was discovered that transcription is repressed in response to neighboring DNA double-strand breaks (DSBs). It is not known whether a failure to silence transcription flanking DSBs has any impact on DNA repair efficiency or whether chromatin remodelers contribute to the process. Here, we show that the PBAF remodeling complex is important for DSB-induced transcriptional silencing and promotes repair of a subset of DNA DSBs at early time points, which can be rescued by inhibiting transcription globally. An ATM phosphorylation site on BAF180, a PBAF subunit, is required for both processes. Furthermore, we find that subunits of the PRC1 and PRC2 polycomb group complexes are similarly required for DSB-induced silencing and promoting repair. Cancer-associated BAF180 mutants are unable to restore these functions, suggesting PBAF's role in repressing transcription near DSBs may contribute to its tumor suppressor activity

The phase-space structure of a dark-matter halo: Implications for dark-matter direct detection experiments

We study the phase-space structure of a dark-matter halo formed in a high resolution simulation of a Lambda CDM cosmology. Our goal is to quantify how much substructure is left over from the inhomogeneous growth of the halo, and how it may affect the signal in experiments aimed at detecting the dark matter particles directly. If we focus on the equivalent of ``Solar vicinity'', we find that the dark-matter is smoothly distributed in space. The probability of detecting particles bound within dense lumps of individual mass less than 10^7 M_\sun h^{-1} is small, less than 10^{-2}. The velocity ellipsoid in the Solar neighbourhood deviates only slightly from a multivariate Gaussian, and can be thought of as a superposition of thousands of kinematically cold streams. The motions of the most energetic particles are, however, strongly clumped and highly anisotropic. We conclude that experiments may safely assume a smooth multivariate Gaussian distribution to represent the kinematics of dark-matter particles in the Solar neighbourhood. Experiments sensitive to the direction of motion of the incident particles could exploit the expected anisotropy to learn about the recent merging history of our Galaxy.Comment: 13 pages, 13 figures, Phys. Rev. D in press. Postscript version with high resolution figures available from http://www.mpa-garching.mpg.de/~ahelmi/research/lcdm_dm.html; some changes in the text; constraints on the effect of bound dark-matter lumps revised; remaining conclusions unchange

Solar Wakes of Dark Matter Flows

We analyze the effect of the Sun's gravitational field on a flow of cold dark matter (CDM) through the solar system in the limit where the velocity dispersion of the flow vanishes. The exact density and velocity distributions are derived in the case where the Sun is a point mass. The results are extended to the more realistic case where the Sun has a finite size spherically symmetric mass distribution. We find that regions of infinite density, called caustics, appear. One such region is a line caustic on the axis of symmetry, downstream from the Sun, where the flow trajectories cross. Another is a cone-shaped caustic surface near the trajectories of maximum scattering angle. The trajectories forming the conical caustic pass through the Sun's interior and probe the solar mass distribution, raising the possibility that the solar mass distribution may some day be measured by a dark matter detector on Earth. We generalize our results to the case of flows with continuous velocity distributions, such as that predicted by the isothermal model of the Milky Way halo.Comment: 30 pages, 8 figure

Mutations in pericentrin cause Seckel syndrome with defective ATR-dependent DNA damage signaling

Large brain size is one of the defining characteristics of modern humans. Seckel syndrome (MIM 210600), a disorder of markedly reduced brain and body size, is associated with defective ATR-dependent DNA damage signaling. Only a single hypomorphic mutation of ATR has been identified in this genetically heterogeneous condition. We now report that mutations in the gene encoding pericentrin (PCNT)--resulting in the loss of pericentrin from the centrosome, where it has key functions anchoring both structural and regulatory proteins--also cause Seckel syndrome. Furthermore, we find that cells of individuals with Seckel syndrome due to mutations in PCNT (PCNT-Seckel) have defects in ATR-dependent checkpoint signaling, providing the first evidence linking a structural centrosomal protein with DNA damage signaling. These findings also suggest that other known microcephaly genes implicated in either DNA repair responses or centrosomal function may act in common developmental pathways determining human brain and body size

High-performance mid-infrared quantum dot infrared photodetectors

Quantum dot infrared photodetectors (QDIPs) have emerged as attractive devices for sensing long wavelength radiation. Their principle of operation is based on intersublevel transitions in quantum dots (QDs). Three-dimensional quantum confinement offers the advantages of normal incidence operation, low dark currents and high-temperature operation. The performance characteristics of mid-infrared devices with three kinds of novel heterostructures in the active region are described here. These are a device with upto 70 QD layers, a device with a superlattice in the active region, and a tunnel QDIP. Low dark currents (1.59 A cm−2 at 300 K), large responsivity (2.5 A W−1 at 78 K) and large specific detectivity (1011 cm Hz1/2 W−1 at 100 K) are measured in these devices. It is evident that QDIPs will find application in the design of high-temperature focal plane arrays. Imaging with small QD detector arrays using the raster scanning technique is also demonstrated.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/48926/2/d5_13_009.pd

Intersubband absorption in annealed InAs/GaAs quantum dots: a case for polarization-sensitive infrared detection

We have studied the characteristics of intersubband absorption of polarized infrared (IR) radiation in as-grown and annealed self-organized InAs/GaAs quantum dots. It is observed that with the increase of annealing time and temperature, the dots tend to flatten and behave more like quantum wells. As a result, their sensitivity to TE (in-plane)-polarized light decreases and that to TM (out-of-plane)-polarized light increases. The effect could be utilized for the realization of polarization-sensitive IR detectors.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/48917/2/d31508.pd