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

Discovery of a Probable Physical Triple Quasar

We report the discovery of the first known probable case of a physical triple quasar (not a gravitational lens). A previously known double system, QQ 1429-008 at z = 2.076, is shown to contain a third, fainter QSO component at the same redshift within the measurement errors. Deep optical and IR imaging at the Keck and VLT telescopes has failed to reveal a plausible lensing galaxy group or a cluster, and moreover, we are unable to construct any viable lensing model which could lead to the observed distribution of source positions and relative intensities of the three QSO image components. Furthermore, there are hints of differences in broad-band spectral energy distributions of different components, which are more naturally understood if they are physically distinct AGN. Therefore, we conclude that this system is most likely a physical triple quasar, the first such close QSO grouping known at any redshift. The projected component separations in the restframe are ~ 30 - 50 kpc for the standard concordance cosmology, typical of interacting galaxy systems. The existence of this highly unusual system supports the standard picture in which galaxy interactions lead to the onset of QSO activity.Comment: Submitted to ApJL, LaTeX, 13 pages, 4 eps figures, all include

Globular clusters and dwarf galaxies in Fornax - I. Kinematics in the cluster core from multi-object spectroscopy

We acquired radial velocities of a significant number of globular clusters (GCs) on wide fields between galaxies in the nearby Fornax cluster of galaxies, in order to derive their velocity dispersion radial profile and to probe the dynamics of the cluster. We used FLAMES on the VLT to obtain accurate velocities for 149 GCs, within a ~500x150 kpc strip centered on NGC 1399, the Fornax central galaxy. These objects are at the very bright tail (M_V < -9.5) of the GC luminosity function, overlapping the so-called ``ultra-compact dwarfs'' magnitude range. Eight of the brightest FLAMES-confirmed members indeed show hints of resolution in the subarcsecond pre-imaging data we used for selecting the ~500 targets for FLAMES spectroscopy. Ignoring the GCs around galaxies by applying 3d_25 diameter masks, we find 61 GCs of 20.0 < V < 22.2 lying in the intra-cluster (IC) medium. The velocity dispersion of the population of ICGCs is 200 km/s at ~150 kpc from the central NGC 1399 and rises to nearly 400 km/s at 200 kpc, a value which compares with the velocity dispersion of the population of dwarf galaxies, thought to be infalling from the surroundings of the cluster.Comment: To be published in A&A Letters. 4 pages, 3 figures, 3 table

Crtc1-/- mice and mood disorders: Role of the CREB-CRTC1 pathway in depression and antidepressant treatment

Mood disorders, which include major depressive disorders and bipolar disorders, are complex neuropsychiatric diseases and are among the leading causes of disability worldwide. Studies over the past decades have highlighted the heterogeneity of these troubles and suggest that they result from complex interactions between genetic or molecular alterations and environmental stressors. Numerous findings have shown the importance of neuroplasticity- related processes, and it is hypothesized that a defective and dysfunctional neurocircuitry would underlie these diseases. The pleiotropic transcription factor CREB has been widely involved in mood disorders, as it is crucial for the expression of genes underlying synaptic plasticity, such as brain-derived neurotrophic factor (BDNF). The expression of this neurotrophin mainly relies on CREB, but also on CREB-regulated transcription coactivator 1 (CRTC1), a potent activator of CREB- dependent transcription involved in mechanisms such as synaptic plasticity and long-term potentiation. To further investigate the role of CRTC1 in the brain, our group generated a CRTC1-deficient mouse line that presented several behavioral and molecular phenotypes related to mood disorders. The aim of the present thesis was to further investigate the involvement of CRTC1 in mood disorders etiology, as well as in antidepressant response, through the use of the Crtc1-/- mice model. We found that these animals are resistant to the therapeutic effects of classical antidepressants. We indeed showed that CRTC1 was necessary for the antidepressant- induced upregulation of neurotrophic genes. Correspondingly, by pharmacologically acting on epigenetic gene regulation, we were able to restore Bdnf expression in Crtc1-/- mice, which was paralleled by a partial rescue of their depressive-like phenotype. We also demonstrated that Crtc1-/- mice present impairments of several systems, including the agmatinergic system which importantly contributes to their phenotype. Finally, we provided evidence that agmatine might be a new fast-acting antidepressant. Altogether, the results presented here confirm CRTC1 as a potent mediator of several molecular pathways, impairments of which participate in the establishment of mood disorders. Furthermore, they also provide new insights into the role of CRTC1 in such troubles, as well as in antidepressant response. -- Les troubles de l'humeur, comprenant la dépression et les troubles bipolaires, sont des troubles neuropsychiatriques complexes, et font partie des principales causes d'invalidité dans le monde. Les recherches menées durant les dernières décennies mettent en évidence l'hétérogénéité de ces troubles, et suggèrent qu'ils résultent d'interactions complexes entre altérations génétiques ou moléculaires et stress environnemental. De nombreuses études ont montré l'importance de la neuroplasticité et des mécanismes qui lui sont liés, car il semblerait que ces maladies découlent d'une circuiterie neuronale altérée et dysfonctionnelle. Le facteur de transcription CREB est très impliqué dans les troubles de l'humeur. Il est en effet crucial pour l'expression de gènes liés à la plasticité synaptique, tels que BDNF (brain-derived neurotrophic factor). L'expression de cette neurotrophine est principalement régulée par CREB, mais également par CRTC1 (CREB-regulated transcription coactivator 1), un important co-activateur de CREB. Il est aussi impliqué dans des mécanismes tels que la plasticité synaptique et la potentialisation à long terme. Afin d'étudier plus en détails le rôle de CRTC1, notre groupe a généré une lignée de souris déficientes en CRTC1 qui présentent plusieurs altérations comportementales et moléculaires liées aux troubles de l'humeur. L'objectif de cette thèse était d'investiguer plus en profondeur l'implication de CRTC1 dans l'étiologie des troubles de l'humeur ainsi que dans la réponse aux antidépresseurs, par l'utilisation des souris Crtc1-/-. Nous avons observé que ces animaux ne répondent pas aux antidépresseur classiques. En effet, nous avons vu que CRTC1 était nécessaire pour l'expression de gènes liés à la neuroplasticité induite par les antidépresseurs. En conséquences, nous avons démontré qu'en agissant sur le système épigénétique de ces souris, nous avons pu restaurer l'expression de Bdnf, en parallèle d'une restauration partielle de leur comportement. Nous avons également démontré que les souris Crtc1-/- présentent une dysfonction de certains systèmes physiologiques, incluant le système agmatinergique qui contribue grandement à leur phénotype. Finalement, nous avons aussi montré que l'agmatine pourrait être un nouvel antidépresseur à action rapide. D'une manière générale, ces résultats confirment que CRTC1 est un régulateur-clé de nombreux processus cellulaires dont la dysfonction pourrait contribuer à l'établissement des troubles de l'humeur. De plus, ils amènent également de nouvelles informations sur le rôle de CRTC1 dans ces troubles, ainsi que dans la réponse aux antidépresseurs

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

A G1-like globular cluster in NGC 1023

The structure of a very bright (MV = -10.9) globular cluster in NGC 1023 is analyzed on two sets of images taken with the Hubble Space Telescope. From careful modeling of King profile fits to the cluster image, a core radius of 0.55+/-0.1 pc, effective radius 3.7+/-0.3 pc and a central V-band surface brightness of 12.9+/-0.5 mag / square arcsec are derived. This makes the cluster much more compact than Omega Cen, but very similar to the brightest globular cluster in M31, G1 = Mayall II. The cluster in NGC 1023 appears to be very highly flattened with an ellipticity of about 0.37, even higher than for Omega Cen and G1, and similar to the most flattened clusters in the Large Magellanic Cloud.Comment: 14 pages, 3 figures, 1 table. Accepted for AJ, Oct 200

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

The Consistency of Fermi-LAT Observations of the Galactic Center with a Millisecond Pulsar Population in the Central Stellar Cluster

I show that the spectrum and morphology of a recent Fermi-LAT observation of the Galaxy center are consistent with a millisecond pulsar population in the nuclear Central stellar cluster of the Milky Way. The Galaxy Center gamma-ray spectrum is consistent with the spectrum of four of eight globular clusters that have been detected in the gamma-ray. A dark matter annihilation interpretation cannot be ruled out, though no unique features exist that would require this conclusion.Comment: 5 pages, 1 figure; v3: matches version to appear in JCA