59367290
The Metal-rich Globular Cluster NGC 6553: Observations with WFPC2, STIS, and NICMOS
We present a Hubble Space Telescope study of the metal-rich globular cluster NGC 6553 using WFPC2, NICMOS, and STIS. Our primary motivation is to calibrate the STIS broadband LP magnitude against V555 and I814 magnitudes for stars of known metallicity and absolute (visual) magnitude, for application to our study of LMC globular clusters. NGC 6553 has been shown in earlier studies to have a very unusual color-magnitude diagram, so we also use our data to investigate the reddening, distance, luminosity function, and structure of this cluster. We deduce a higher metallicity and smaller distance modulus than did some previous studies, but we emphasize that very large patchy extinction on small angular scales prohibits accurate determination of the parameters of this cluster. The horizontal branch of NGC 6553 in (V, V-I) is tilted at an angle close to that of the reddening vector. We show that extinction does not, however, explain the tilt, which is presumably a metallicity effect. The color-magnitude diagram shows an apparent second turnoff some 1.5 mag fainter than that of the cluster. We show that this is most likely the background Galactic bulge: however, in that case, the color-magnitude diagram of NGC 6553 is not a good match to that of the field bulge population. The cluster is probably more metal-rich than is the mean field bulge star.
6698984
The Bulge Globular Cluster NGC 6553: Observations with HST's WFPC2, STIS, and NICMOS
As part of a large HST project to study the formation and evolution of rich star clusters in the Large Magellanic Cloud, we present results for the Galactic bulge metal-rich globular cluster NGC6553. HST observations using WFPC2, NICMOS and STIS were obtained for this cluster. The primarily reason for studying NGC6553 is to allow the possibility to transform our NICMOS and STIS magnitudes directly into stellar masses. This is particularly important for determining the low mass end of the IMF. NGC 6553 was chosen because its metallicity, [Fe/H]$=-0.22\pm 0.05$, is representative of the metallicities
16439685
Primordial or Dynamical Mass Segregation in Young LMC Clusters
We present the detailed analysis of Hubble Space Telescope observations of the spatial distributions of different stellar species in two young compact star clusters in the Large Magellanic Cloud, NGC 1805 and NGC 1818. Based on a comparison of the characteristic relaxation times in their cores and at their half-mass radii with the observed degree of mass segregation, it is most likely that significant primordial mass segregation was present in both clusters, particularly in NGC 1805. Both clusters were likely formed with very similar initial mass functions.
17422475
Probing the Nature of the G1 Clump Stellar Overdensity in the Outskirts of M31
We present deep Hubble Space Telescope ACS observations of the G1 clump, a distinct stellar overdensity lying at similar to 30 kpc along the southwestern major axis of M31, close to the G1 globular cluster ( from the work of Ferguson and coworkers). Our well-populated color-magnitude diagram reaches similar to 7 mag below the red giant branch tip with 90% completeness, and allows the detection of various morphological features that can be used to derive detailed constraints on the age and metallicity of the constituent stellar population. We find that the color-magnitude diagram is best described spread in metallicity is constrained to be P = 0.5 dex. The star formation rate in this region has declined over time, with the bulk of the stellar mass having formed > 6 Gyr ago. Nonetheless, a nonnegligible mass fraction (approximate to 10%) of the population has formed in the last 2 Gyr. We discuss the nature of the G1 clump in light of these new stellar population constraints and argue that the combination of stellar content and physical size make it unlikely that the structure is the remnant of an accreted dwarf galaxy. Instead, the strong similarity between the stellar content of the G1 clump and that of the M31 outer disk suggests that the substructure is a fragment of the outer disk, perhaps torn off from the main body during a past accretion/merger event; this interpretation is consistent with extant kinematical data. If this interpretation is correct, our analysis of the stellar content provides further evidence that the outskirts of large disk galaxies have been in place for a significant time. (Less)
18564854
Searching for the Precursors of Life in External Galaxies
Are the organic molecules crucial for life on Earth abundant in early-epoch galaxies? To address this, we searched for organic molecules in extragalactic sources via their absorption features, known as diffuse interstellar bands (DIBs). There is strong evidence that DIBs are associated with polycyclic aromatic hydrocarbons (PAHs) and carbon chains. Galaxies with a preponderance of DIBs may be the most likely places in which to expect life. 
We use the method of quasar absorption lines to probe intervening early-epoch galaxies for the DIBs. We present the equivalent width measurements of DIBs in one neutral hydrogen (HI) abundant galaxy and limits for five DIB bands in six other HI-rich galaxies (damped Lyman-alpha systems--DLAs). Our results reveal that HI-rich galaxies are dust poor and have significantly lower reddening than known DIB-rich Milky Way environments. We find that DIBs in HI-rich galaxies do not show the same correlation with hydrogen abundance as observed in the Milky Way; the extragalactic DIBs are underabundant by as much as 10 times. The lower limit gas-to-dust ratios of four of the HI-rich early epoch galaxies are much higher than the gas-to-dust ratios found in the Milky Way. Our results suggest that the organic molecules responsible for the DIBs are underabundant in HI-rich early epoch galaxies relative to the Milky Way.
16449065
Stellar Abundances in Giant Stars in the Metal-Rich Globular Cluster NGC 6528
We present the first results of a detailed abundance analysis, based on VLT observations, of giant stars in the very metal-rich globular cluster NGC 6528. We will be able to tie the horizontal branch abundances (see e.g. Carretta et al. 2001) to those of the more luminous giants (see CMDs). For the very similar cluster NGC 6553 studies of different types of stars have yielded very disparate results. Our first analysis of three of our stars seem to indicate that indeed the different sorts of stars do show similar abundances if one homogeneous set of models and parameters are being used.
2889124
The silicon tracker/converter for the gamma-ray large area space telescope
The Silicon Tracker/Converter of the Gamma-ray Large Area Space Telescope (GLAST) will have an active area of 80 m2, representing one of the largest planned applications of the silicon-strip detector technology. The large number of channels (1.3 million) to read out, together with the requirement that the tracker provide the trigger to the data acquisition, force the readout electronics to be of very low noise. Furthermore, to "t into the power constraints of the satellite environment, the electronics must have an ultra-low power consumption. To ful"ll these requirements, plus others imposed by the space environment, such as redundancy, a mixed mode CMOS front-end readout chip and a digital readout controller chip have been designed and prototyped. In this article, we present the status of the readout electronics and the results from a test-beam study with a small GLAST tracker prototype. ( 1999 Elsevier Science B.V. All rights reserved.
122426348
Pion Charge Exchange Scattering at High-Energies
We report on a study of pion charge-exchange scattering carried out at Fermilab in the energy range 20 to 200 GeV. The results can be described remarkably well by a simple Regge-pole model. The charge-exchange cross sections in the forward direction lead to a prediction for the difference in total cross sections of ..pi../sup -/p and ..pi../sup +/p which is in satisfactory agreement with direct measurements of this difference in another experiment at Fermilab. (AIP)
23177219
Fermi-LAT Sensitivity to Dark Matter Annihilation in Via Lactea II Substructure
We present a study of the ability of the Fermi Gamma-ray Space Telescope to detect dark matter (DM) annihilation signals from the Galactic subhalos predicted by the Via Lactea II N-body simulation. We implement an improved formalism for estimating the boost factor needed to account for the effect of DM clumping on scales below the resolution of the simulation, and we incorporate a detailed Monte Carlo simulation of the response of the Fermi-LAT, including a simulation of its all-sky observing mode integrated over a 10 year mission. We find that for WIMP masses up to about 150 GeV c –2 in standard supersymmetric models with σv = 3 × 10–26 cm3 s–1, a few subhalos could be detectable with >5 standard deviation significance and would likely deviate significantly from the appearance of a point source.
111381772
Eight gamma-ray pulsars discovered in blind frequency searches of Fermi LAT data
We report the discovery of eight gamma-ray pulsars in blind frequency searches using the LAT, onboard the Fermi Gamma-ray Space Telescope. Five of the eight pulsars are young (tau_c 10^36 erg/s), and located within the Galactic plane (|b|<3 deg). The remaining three are older, less energetic, and located off the plane. Five pulsars are associated with sources included in the LAT bright gamma-ray source list, but only one, PSR J1413-6205, is clearly associated with an EGRET source. PSR J1023-5746 has the smallest characteristic age (tau_c=4.6 kyr) and is the most energetic (Edot=1.1E37 erg/s) of all gamma-ray pulsars discovered so far in blind searches. PSRs J1957+5033 and J2055+25 have the largest characteristic ages (tau_c~1 Myr) and are the least energetic (Edot~5E33 erg/s) of the newly-discovered pulsars. We present the timing models, light curves, and detailed spectral parameters of the new pulsars. We used recent XMM observations to identify the counterpart of PSR J2055+25 as XMMU J205549.4+253959. In addition, publicly available archival Chandra X-ray data allowed us to identify the likely counterpart of PSR J1023-5746 as a faint, highly absorbed source, CXOU J102302.8-574606. The large X-ray absorption indicates that this could be among the most distant gamma-ray pulsars detected so far. PSR J1023-5746 is positionally coincident with the TeV source HESS J1023-575, located near the young stellar cluster Westerlund 2, while PSR J1954+2836 is coincident with a 4.3 sigma excess reported by Milagro at a median energy of 35 TeV. Deep radio follow-up observations of the eight pulsars resulted in no detections of pulsations and upper limits comparable to the faintest known radio pulsars, indicating that these can be included among the growing population of radio-quiet pulsars in our Galaxy being uncovered by the LAT, and currently numbering more than 20.
208651245
A Monte Carlo Study of the Irreducible Background in the EGRET In- strument
The diffuse extragalactic γ-ray background (EGRB) has been derived by various groups from observations by the Energetic Gamma Ray Experiment Telescope (EGRET) instrument on the Compton Gamma Ray Observatory (CGRO). The derived EGRB consists of γ-rays that may come from astrophysical components, such as from unresolved extragalactic point sources (blazars, normal galaxies, etc.), true extragalactic diffuse emission, misattributed diffuse signals from the Galaxy and other celestial sources, and an irreducible instrumental background due to γ-rays produced by cosmic-ray (CR) interactions in the EGRET instrument. Using the Gamma Ray Large Area Space Telescope (GLAST) simulation and reconstruction software, we have investigated the magnitude of the irreducible instrumental background in the GLAST Large Area Telescope (LAT). We re-scale our results to the EGRET and present preliminary results of our study and its effect on current estimates of the EGRB.