28577443
STELLAR POPULATIONS IN THE CENTRAL 0.5 pc OF THE GALAXY. I. A NEW METHOD FOR CONSTRUCTING LUMINOSITY FUNCTIONS AND SURFACE-DENSITY PROFILES
We present new high angular resolution near-infrared spectroscopic observations of the nuclear star cluster surrounding the Milky Way's central supermassive black hole. Using the integral-field spectrograph OSIRIS on Keck II behind the laser-guide-star adaptive optics system, this spectroscopic survey enables us to separate early-type (young, 4-6 Myr) and late-type (old, >1 Gyr) stars with a completeness of 50% down to K' = 15.5 mag, which corresponds to ~10 M_☉ for the early-type stars. This work increases the radial extent of reported OSIRIS/Keck measurements by more than a factor of three from 4" to 14" (0.16 to 0.56 pc), along the projected disk of young stars. For our analysis, we implement a new method of completeness correction using a combination of star-planting simulations and Bayesian inference. We assign probabilities for the spectral type of every source detected in deep imaging down to K' = 15.5 mag using information from spectra, simulations, number counts, and the distribution of stars. The inferred radial surface-density profiles, Σ(R)∝ R^(-Γ), for the young stars and late-type giants are consistent with earlier results (Γ_(early) = 0.93 ± 0.09, Γ_(late) = 0.16 ± 0.07). The late-type surface-density profile is approximately flat out to the edge of the survey. While the late-type stellar luminosity function is consistent with the Galactic bulge, the completeness-corrected luminosity function of the early-type stars has significantly more young stars at faint magnitudes compared with previous surveys with similar depth. This luminosity function indicates that the corresponding mass function of the young stars is likely less top-heavy than that inferred from previous surveys.
11410109
Evidence for substructure in Ursa minor dwarf spheroidal galaxy using a Bayesian object detection method
We present a method for identifying localized secondary populations in stellar velocity data using Bayesian statistical techniques. We apply this method to the dwarf spheroidal galaxy Ursa Minor and find two secondary objects in this satellite of the Milky Way. One object is kinematically cold with a velocity dispersion of 4.25 ± 0.75 km s-1 and centred at (9.1 arcmin ± 1.5, 7.2 arcmin ± 1.2) in relative RA and Dec. with respect to the centre of Ursa Minor. The second object has a large velocity offset of -12.8+1.75-1.5 km s-1 compared to Ursa Minor and centred at (-14.0 arcmin+2.4-5.8,-2.5 arcmin+0.4-1.0). The kinematically cold object has been found before using a smaller data set, but the prediction that this cold object has a velocity dispersion larger than 2.0 kms-1 at 95 per cent confidence level differs from previous work. We use two- and three-component models along with the information criteria and Bayesian evidence model selection methods to argue that Ursa Minor has additional localized secondary populations. The significant probability for a large velocity dispersion in each secondary object raises the intriguing possibility that each has its own dark matter halo, that is, it is a satellite of a satellite of the Milky Way. © 2014 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society.
122373827
Pion reabsorption in heavy-ion collisions interpreted in terms of the Delta capture process
We have measured energy-differential cross sections for pi(0) production in Ar-36 + (197)AU collisions at 95 MeV/u. From an analysis of spectral features due to pion final-state interactions we have estimated the cross section of the capture process Delta + N --> N + N in the center-of-mass energy range root s similar or equal to 2.05-2.25 GeV. Within the frame of BUU calculations, our results support the extension of the detailed-balance principle to broad-width resonances.
18689235
Thermal hard-photons probing multifragmentation in nuclear collisions around the fermi energy
Hard-photon (Eγ>30 MeV) emission originating from photon-neutron bremsstrahlung collisions is investigated in four different heavy-ion reactions at intermediate bombarding energies (36Ar+197Au, 107Ag, 58Ni, 12C at 60 A MeV) coupling the TAPS photon spectrometer with two charged-particle multidetectors covering more than 80% of the solid angle. The hard-photon spectra of the three heavier targets result from the combination of two distinct exponential distributions with different slope parameters, a results which deviates from the behaviour expected for hard-photon production just in first-chance proton-neutron collisions. The thermal origin of the steeper bremsstrahlung component is confirmed by the characteristics of its slope and angular distribution. Such thermal hard-photons convey undisturbed information of the thermodynamical state of hot and excited nuclear systems undergoing multifragmentation.
117054684
Thermal Hard-Photons from Hot Fragmenting Nuclei
Hard-photon (Eγ⩾30 MeV) production has been studied in four different heavy-ion reactions (36Ar+197Au,107Ag,58Ni,12C at 60A MeV) with TAPS electromagnetic calorimeter, in coincidence with LCPs (Z⩽2) and IMFs (3⩽Z⩽20) detected by two phoswich multidetectors. Aside from the main known emission mechanism, pn bremsstrahlung during the first high-density stage, the existence of a component of lower energy bremsstrahlung photons emitted in secondary pn collisions from a thermalizing source is demonstrated. Thermal hard photons arise as a promising probe of the thermodynamical properties of nuclear systems undergoing multifragmentation.