The Mass Function of Main Sequence Stars in NGC6397 from Near IR and Optical High Resolution HST Observations

We have investigated the properties of the stellar mass function in the globular cluster NGC6397 using a large set of HST observations that include WFPC2 images in V and I, obtained at ~4' and 10' radial distances, and a series of deep images in the J and H bands obtained with the NIC2 and NIC3 cameras of NICMOS pointed to regions located ~4.5' and ~3.2' from the center. These observations span the region from ~1 to ~3 times the cluster's half-light radius. All luminosity functions, derived from color magniutde diagrams, increase with decreasing luminosity up to a peak at M_I~8.5 or M_H~7 and then precipitously drop well before photometric incompleteness becomes significant. Within the observational uncertainties, at M_I~12 or M_H~10.5 (~0.09 Msun) the luminosity functions are compatible with zero. By applying the best available mass- luminosity relation appropriate to the metallicity of NGC6397 to both the optical and IR data, we obtain a mass function that shows a break in slope at \~0.3 Msun. No single exponent power-law distribution is compatible with these data, regardless of the value of the exponent. We find that a dynamical model of the cluster can simultaneously reproduce all the luminosity functions observed throughout the cluster only if the IMF rises as m**-1.6 in the range 0.8-0.3 Msun and then drops as m**0.2 below ~0.3 Msun. Adopting a more physical log-normal distribution for the IMF, all these data taken together imply a best fit distribution with characteristic mass m_c~0.3 and sigma~1.8.Comment: 18 pages, 6 figures (ps). Accepted for publication in Ap

Gene transfer into stimulated and unstimulated T lymphocytes by HIV-1-derived lentiviral vectors

Genetic modification of T lymphocytes holds great potential for treatments of cancer, T cell disorders and AIDS. While in the past recombinant murine retroviruses were the vectors of choice for gene delivery to T cells, vectors based on lentiviruses can provide additional benefits. Here, we show that VSV-G pseudotyped HIV 1 vector particles delivering the enhanced green fluorescent protein (EGFP) efficiently transduce human T lymphocytes. Transduction efficiency was optimal when infection included centrifugation of cells with concentrated vector supernatant in the presence of Polybrene. In contrast to previous reports describing murine retrovirus-mediated gene transfer to T lymphocytes, fibronectin did not improve the transduction efficiency of the VSVG-pseudotyped HIV-1 particles. Similar gene transfer efficiencies were observed following stimulation of cells with PHA/IL-2 or anti-CD3i/CD28i antibodies, although greater transgene expression was observed in the latter case. Interestingly, production of vectors in the absence of the accessory proteins Vif, Vpr, Vpu and Nef was accompanied by a 50% decrease in transduction efficiency in activated T cells. Transduction of T cells that were not stimulated before infection was achieved. No transduction of non-prestimulated cells was observed with a GAL V-pseudotyped murine retroviral vector. The requirement for accessory proteins in non-prestimulated cells was more pronounced. Our results have implications for lentiviral vector targeting of other cells of the hematopoietic system including stem cells

Ages and Metallicities of Young Globular Clusters in the Merger Remnant NGC 7252

UV-to-visual spectra of eight young star clusters in the merger remnant and protoelliptical galaxy NGC 7252, obtained with the Blanco 4-m telescope on Cerro Tololo, are presented. These clusters lie at projected distances of 3-15 kpc from the center and move with a velocity dispersion of 140+/-35 km/s in the line of sight. Seven of the clusters show strong Balmer absorption lines in their spectra [EW(H-beta)= 6-13 Angstrom], while the eighth lies in a giant HII region and shows no detectable absorption features. Based on comparisons with model-cluster spectra by Bruzual & Charlot (1996) and Bressan, Chiosi, & Tantalo (1996), six of the absorption-line clusters have ages in the range of 400-600 Myr, indicating that they formed early on during the recent merger. These clusters are globular clusters as judged by their small effective radii and ages corresponding to ~100 core crossing times. The one emission-line object is <10 Myr old and may be a nascent globular cluster or an OB association. The mean metallicities measured for three clusters are solar to within +/-0.15 dex, suggesting that the merger of two likely Sc galaxies in NGC 7252 formed a globular-cluster system with a bimodal metallicity distribution. Since NGC 7252 itself shows the characteristics of a 0.5-1 Gyr old protoelliptical, its second-generation solar-metallicity globulars provide direct evidence that giant ellipticals with bimodal globular-cluster systems can form through major mergers of gas-rich disk galaxies.Comment: 34 pages, incl. 6 figures in EPS format, AAS LaTeX, to be published in AJ, Vol. 116, Nov. 199

Palomar 13: An Unusual Stellar System in the Galactic Halo

We have measured Keck/HIRES radial velocities for 30 candidate red giants in the direction of Palomar 13: an object traditionally cataloged as a compact, low-luminosity globular cluster. From a sample of 21 confirmed members, we find a systemic velocity of 24.1 km/s and a projected, intrinsic velocity dispersion of 2.2 km/s. Although small, this dispersion is several times larger than that expected for a globular cluster of this luminosity and central concentration. Taken at face value, this dispersion implies a mass-to-light ratio of ~ 40 (in solar units) based on the best-fit King-Michie model. The surface density profile of Palomar 13 also appears to be anomalous among Galactic globular clusters -- depending upon the details of background subtraction and model-fitting, Palomar 13 either contains a substantial population of "extra-tidal" stars, or it is far more spatially extended than previously suspected. The full surface density profile is equally well-fit by a King-Michie model having a high concentration and large tidal radius, or by a NFW model. We examine -- and tentatively reject -- a number of possible explanations for the observed characteristics of Palomar 13 (e.g., velocity "jitter" among the red giants, spectroscopic binary stars, non-standard mass functions, modified Newtonian dynamics), and conclude that the two most plausible scenarios are either catastrophic heating during a recent perigalacticon passage, or the presence of a massive dark halo. Thus, the available evidence suggests that Palomar 13 is either a globular cluster which is now in the process of dissolving into the Galactic halo, or a faint, dark-matter-dominated stellar system (ABRIDGED).Comment: 31 pages, 13 postscript figures and 1 color gif image. Also available at http://www.physics.rutgers.edu/ast/ast-rap.html. Accepted for publication in the Astrophysical Journa

The Spectroscopic Age of 47 Tuc

High signal-to-noise integrated spectra of the metal-rich globular cluster 47 Tuc, spanning the H-gamma(HR) and Fe4668 line indices, have been obtained. The combination of these indices has been suggested (Jones & Worthey 1995, ApJ, 446, L31) as the best available mechanism for cleanly separating the age-metallicity degeneracy which hampers the dating of distant, unresolved, elliptical galaxies. For the first time, we apply this technique to a nearby spheroidal system, 47 Tuc, for which independent ages, based upon more established methods, exist. Such an independent test of the technique's suitability has not been attempted before, but is an essential one before its application to more distant, unresolved, stellar populations can be considered valid. Because of its weak series of Balmer lines, relative to model spectra, our results imply a spectroscopic ``age'' for 47 Tuc well in excess of 20 Gyr, at odds with the colour-magnitude diagram age of 14+/-1 Gyr. The derived metal abundance, however, is consistent with the known value. Emission ``fill-in'' of the H-gamma line as the source of the discrepancy cannot be entirely excluded by existing data, although the observational constraints are restrictive.Comment: 17 pages, 4 figures, LaTeX, accepted for publication in The Astronomical Journal, also available at http://casa.colorado.edu/~bgibson/publications.htm

Globular Cluster Systems in Brightest Cluster Galaxies: Bimodal Metallicity Distributions and the Nature of the High-Luminosity Clusters

We present new (B,I) photometry for the globular cluster systems in eight Brightest Cluster Galaxies (BCGs), obtained with the ACS/WFC camera on the Hubble Space Telescope. In the very rich cluster systems that reside within these giant galaxies, we find that all have strongly bimodal color distributions All the BCGs show population gradients, with much higher relative numbers of red clusters within 5 kpc of their centers, consistent with their having formed at later times than the blue, metal-poor population. A striking new feature of the color distributions emerging from our data is that for the brightest clusters (M_I < -10.5) the color distribution becomes broad and less obviously bimodal. we suggest that it may be a characteristic of many BCGs. Furthermore, the blue (metal-poor) clusters become progressively redder with increasing luminosity, following a mass/metallicity scaling relation Z ~ M^0.55. We argue that these GCS characteristics are consistent with a hierarchical-merging formation picture in which the metal-poor clusters formed in protogalactic clouds or dense starburst complexes with gas masses in the range 10^7 - 10^10 M_Sun, but where the more massive clusters on average formed in bigger clouds with deeper potential wells where more pre-enrichment could occur.Comment: 48 pages, 24 Figures, PDF, Submitted to Astrophys.J. and refereed. For complete pdf file with better figures, see: http://physwww.mcmaster.ca/%7Eharris/Preprints.htm

Detection of ionized gas in the globular cluster 47 Tucanae

We report the detection of ionized intracluster gas in the globular cluster 47 Tucanae. Pulsars in this cluster with a negative period derivative, which must lie in the distant half of the cluster, have significantly higher measured integrated electron column densities than the pulsars with a positive period derivative. We derive the plasma density within the central few pc of the cluster using two different methods which yield consistent values. Our best estimate of n_e = (0.067+-0.015)/cm^3 is about 100 times the free electron density of the ISM in the vicinity of 47 Tucanae, and the ionized gas is probably the dominant component of the intracluster medium.Comment: 5 pages, 3 included figures, accepted for publication by ApJ Letter

Three-body equations of motion in successive post-Newtonian approximations

There are periodic solutions to the equal-mass three-body (and N-body) problem in Newtonian gravity. The figure-eight solution is one of them. In this paper, we discuss its solution in the first and second post-Newtonian approximations to General Relativity. To do so we derive the canonical equations of motion in the ADM gauge from the three-body Hamiltonian. We then integrate those equations numerically, showing that quantities such as the energy, linear and angular momenta are conserved down to numerical error. We also study the scaling of the initial parameters with the physical size of the triple system. In this way we can assess when general relativistic results are important and we determine that this occur for distances of the order of 100M, with M the total mass of the system. For distances much closer than those, presumably the system would completely collapse due to gravitational radiation. This sets up a natural cut-off to Newtonian N-body simulations. The method can also be used to dynamically provide initial parameters for subsequent full nonlinear numerical simulations.Comment: 8 pages, 9 figure

Massive perturbers and the efficient merger of binary massive black holes

We show that dynamical relaxation in the aftermath of a galactic merger and the ensuing formation and decay of a binary massive black hole (MBH), are dominated by massive perturbers (MPs) such as giant molecular clouds or clusters. MPs accelerate relaxation by orders of magnitude relative to 2-body stellar relaxation alone, and efficiently scatter stars into the binary MBH's orbit. The 3-body star-binary MBH interactions shrink the binary MBH to the point where energy losses from the emission of gravitational waves (GW) lead to rapid coalescence. We model this process based on observed and simulated MP distributions and take into account the decreased efficiency of the star-binary MBH interaction due to acceleration in the galactic potential. We show that mergers of gas-rich galactic nuclei lead to binary MBH coalescence well within the Hubble time. Moreover, lower-mass binary MBHs (<10^8 Msun) require only a few percent of the typical gas mass in a post-merger nucleus to coalesce in a Hubble time. The fate of a binary MBH in a gas poor galactic merger is less certain, although massive stellar structures (e.g. clusters, stellar rings) could likewise lead to efficient coalescence. These coalescence events are observable by their strong GW emission. MPs thus increase the cosmic rate of such GW events, lead to a higher mass deficit in the merged galactic core and suppress the formation of triple MBH systems and the resulting ejection of MBHs into intergalactic space.Comment: 14 pages, 4 figures, 3 tables. More detailed explanations and changes in structure. Section on hypervelocity stars moved to another paper (in preparation). Results and conclusions unchanged. Accepted to Ap