The Surface Brightness Fluctuations and Globular Cluster Populations of M87 and its Companions

Using the surface brightness fluctuations in HST WFPC-2 images, we determine that M87, NGC 4486B, and NGC 4478 are all at a distance of ~16 Mpc, while NGC 4476 lies in the background at ~21 Mpc. We also examine the globular clusters of M87 using archived HST fields. We detect the bimodal color distribution, and find that the amplitude of the red peak relative to the blue peak is greatest near the center. This feature is in good agreement with the merger model of elliptical galaxy formation, where some of the clusters originated in progenitor galaxies while other formed during mergers.Comment: 5 pages, 2 figure

The Globular Cluster System in the Inner Region of M87

1057 globular cluster candidates have been identified in a WFPC2 image of the inner region of M87. The Globular Cluster Luminosity Function (GCLF) can be well fit by a Gaussian profile with a mean value of m_V^0=23.67 +/- 0.07 mag and sigma=1.39 +/- 0.06 mag (compared to m_V^0=23.74 mag and sigma=1.44 mag from an earlier study using the same data by Whitmore it et al. 1995). The GCLF in five radial bins is found to be statistically the same at all points, showing no clear evidence of dynamical destruction processes based on the luminosity function (LF), in contradiction to the claim by Gnedin (1997). Similarly, there is no obvious correlation between the half light radius of the clusters and the galactocentric distance. The core radius of the globular cluster density distribution is R_c=56'', considerably larger than the core of the stellar component (R_c=6.8''). The mean color of the cluster candidates is V-I=1.09 mag which corresponds to an average metallicity of Fe/H = -0.74 dex. The color distribution is bimodal everywhere, with a blue peak at V-I=0.95 mag and a red peak at V-I=1.20 mag. The red population is only 0.1 magnitude bluer than the underlying galaxy, indicating that these clusters formed late in the metal enrichment history of the galaxy and were possibly created in a burst of star/cluster formation 3-6 Gyr after the blue population. We also find that both the red and the blue cluster distributions have a more elliptical shape (Hubble type E3.5) than the nearly spherical galaxy. The average half light radius of the clusters is ~2.5 pc which is comparable to the 3 pc average effective radius of the Milky Way clusters, though the red candidates are ~20% smaller than the blue ones.Comment: 40 pages, 17 figures, 4 tables, latex, accepted for publication in the Ap

Ultrasound-mediated optical tomography: a review of current methods

Ultrasound-mediated optical tomography (UOT) is a hybrid technique that is able to combine the high penetration depth and high spatial resolution of ultrasound imaging to overcome the limits imposed by optical scattering for deep tissue optical sensing and imaging. It has been proposed as a method to detect blood concentrations, oxygenation and metabolism at depth in tissue for the detection of vascularized tumours or the presence of absorbing or scattering contrast agents. In this paper, the basic principles of the method are outlined and methods for simulating the UOT signal are described. The main detection methods are then summarized with a discussion of the advantages and disadvantages of each. The recent focus on increasing the weak UOT signal through the use of the acoustic radiation force is explained, together with a summary of our results showing sensitivity to the mechanical shear stiffness and optical absorption properties of tissue-mimicking phantoms

Three-dimensional modeling of the HI kinematics of NGC 2915

The nearby blue compact dwarf, NGC 2915, has its stellar disc embedded in a large, extended (~ 22 B-band scale-lengths) HI disc. New high-resolution HI synthesis observations of NGC 2915 have been obtained with the Australia Telescope Compact Array. These observations provide evidence of extremely complex HI kinematics within the immediate vicinity of the galaxy's star-forming core. We identify and quantify double-peaked HI line profiles near the centre of the galaxy and show that the HI energetics can be accounted for by the mechanical energy output of the central high-mass stellar population within time-scales of 10^6-10^7 yr. Full three-dimensional models of the HI data cube are generated and compared to the observations to test various physical scenarios associated with the high-mass star-forming core of NGC 2915. Purely circular HI kinematics are ruled out together with the possibility of a high-velocity-dispersion inter-stellar medium at inner radii. Radial velocities of ~ 30 km/s are required to describe the central-most HI kinematics of the system. Our results lend themselves to the simple physical scenario in which the young stellar core of the galaxy expels the gas outwards from the centre of the disc, thereby creating a central HI under-density. These kinematics should be thought of as being linked to a central HI outflow rather than a large-scale galactic blow-out or wind.Comment: 11 pages, 6 figures, accepted for publication in MNRA

Onset of Phase Synchronization in Neurons Conneted via Chemical Synapses

We study the onset of synchronous states in realistic chaotic neurons coupled by mutually inhibitory chemical synapses. For the realistic parameters, namely the synaptic strength and the intrinsic current, this synapse introduces non-coherences in the neuronal dynamics, yet allowing for chaotic phase synchronization in a large range of parameters. As we increase the synaptic strength, the neurons undergo to a periodic state, and no chaotic complete synchronization is found.Comment: to appear in Int. J. Bif. Chao