17629793
Limits on Reddening and Gas-to-Dust Ratios for Seven Intermediate Redshift Damped Lyα Absorbers from Diffuse Interstellar Bands
We present equivalent width measurements and limits of six diffuse interstellar bands (DIBs, λ4428, λ5705, λ5780, λ5797, λ6284, and λ6613) in seven damped Lyα absorbers (DLAs) over the redshift range 0.091 ≤ z ≤ 0.524, sampling 20.3 ≤ log N(H I) ≤ 21.7. DIBs were detected in only one of the seven DLAs, that which has the highest reddening and metallicity. Based upon the Galactic DIB-N(H I) relation, the λ6284 DIB equivalent width upper limits in four of the seven DLAs are a factor of 4-10 times below the λ6284 DIB equivalent widths observed in the Galaxy, but are not inconsistent with those present in the Magellanic Clouds. Assuming the Galactic DIB-E(B – V) relation, we determine reddening upper limits for the DLAs in our sample. Based upon the E(B – V) limits, the gas-to-dust ratios, N(H I)/E(B – V), of the four aforementioned DLAs are at least ~5 times higher than that of the Galactic interstellar medium. The ratios of two other DLAs are at least a factor of a few times higher. The best constraints on reddening derive from the upper limits for the λ5780 and λ6284 DIBs, which yield E(B – V) ≤ 0.08 for four of the seven DLAs. Our results suggest that, in DLAs, quantities related to dust, such as reddening and metallicity, appear to have a greater impact on DIB strengths than does H I gas abundance; the organic molecules, likely responsible for DIBs in DLA-selected sight lines, are underabundant relative to sight lines in the Galaxy of similarly high N(H I). With regard to the study of astrobiology, this could have implications for the abundance of organic molecules in redshifted galaxies. However, since DLAs are observed to have low reddening, selection bias likely plays a role in the apparent underabundance of DIBs in DLAs.
18850797
Detection of Diffuse Interstellar Bands in the z=0.5 Damped Lyman alpha system towards AO 0235+164
We report the first detection of the 5705 and 5780 A Diffuse Interstellar Bands (DIBs) in a moderate redshift Damped Lyman alpha (DLA) system. We measure a rest frame equivalent width of 63.2 +- 8.7 mA for the 5705 and 216+-9 mA for the 5780 A feature in the z_abs 0.524 DLA towards AO 0235+164 and derive limits for the equivalent widths of the bands at 5797, 6284, and 6613 A. The equivalent width of the 5780 band is lower than would be expected based on the Galactic correlation of DIB strength with N(HI), but is in good agreement with the correlation with E(B-V). The relative strengths of the 5780 and 6284 A DIBs are inconsistent with all Galactic and extragalactic sightlines, except one Small Magellanic Cloud wing sightline towards Sk 143. However, the relative strengths of the 5705 and 5780 A DIBs are consistent with the Galactic relation, indicating that the relative strengths of these bands may be less sensitive to environment or that they may be associated with a similar carrier. The detection of DIBs at z~0.5 demonstrates that the organic compounds usually assumed to be the band carriers were already present in the universe some five gigayears ago.
16316489
Constraints on Galaxy Formation from Stars in the Far Outer Disk of M31
Numerical simulations of galaxy formation within the cold dark matter (CDM) hierarchical clustering framework are unable to produce large disk galaxies without invoking some form of feedback to suppress gas cooling and collapse until a redshift of unity or below. An important observational consequence of delaying the epoch of disk formation until relatively recent times is that the stellar populations in the extended disk should be of predominantly young-to-intermediate age. We use a deep Hubble Space Telescope/Wide Field Planetary Camera 2 archival pointing to investigate the mean age and metallicity of the stellar population in a disk-dominated field at 30 kpc along the major axis of M31. Our analysis of the color-magnitude diagram reveals the dominant population to have a significant mean age (greater than or similar to8 Gyr) and a moderately high mean metallicity ([Fe/H] similar to -0.7); tentative evidence is also presented for a trace population of ancient (greater than or equal to 10 Gyr) metal-poor stars. These characteristics are unexpected in CDM models, and we discuss the possible implications of this result as well as alternative interpretations.
16513414
The Stellar populations of the M31 Halo substructure
We present the first results from our survey of stellar substructure in the outskirts of M31 using the Advanced Camera for Surveys on board the Hubble Space Telescope. We discuss the stellar populations associated with five prominent stellar overdensities discovered during the course of our panoramic ground-based imaging survey with the Isaac Newton Telescope Wide-Field Camera; a sixth pointing targets a region of "clean" halo. The color-magnitude diagrams, which contain between approximate to 10,000 and 90,000 stars and reach 4 mag below the horizontal branch, reveal clear variations in morphology between most fields, indicating that the age and/or metallicity mix of stars is not constant at large radius. This directly confirms the existence of large-scale population inhomogeneities within the halo of M31 and lends further support to the notion that M31 has formed, at least in part, through satellite accretions. We find a striking similarity between the populations of the giant stellar stream and those of another overdensity, the NE shelf, which lies northeast of the galaxy center. If these overdensities are associated with the same population, then the difference in their red clump magnitudes implies that the NE shelf lies in front of the stream by several tens of kiloparsecs, in good agreement with recent orbit calculations for the stream progenitor. (Less)
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.
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.
55027916
Design and initial commissioning of beam diagnostics for the PEP-II B factory
PEP II is a 2.2-km-circumference collider with a 2.1-A, 3.1-GeV positron ring (the Low-Energy Ring) 1 m above a 1-A, 9-GeV electron ring (the High-Energy Ring); both are designed for 3 A maximum. We describe the beam diagnostics for HER commissioning, starting in May 1997. LER commissioning will follow in 1998. The beam size and pulse duration are measured using near-UV synchrotron light extracted by grazing-incidence mirrors that must withstand 200 W/cm.
118623913
A Blind Search for Pulsars in EGRET Data using a Time Differencing Technique
Most of the sources in the galactic plane detected by the EGRET instrument on board the Compton Gamma‐Ray Observatory still remain unidentified. The energy spectra of some of the sources suggest that the gamma‐ray emission originates from pulsars. Detecting pulsed gamma‐ray emission without prior knowledge of the pulsar spin parameters is exceedingly difficult. Standard pulsar blind searches rely on the execution of long FFTs, which use a large amount of computational power. We present a pulsar blind search, based on our Time Differencing Technique, that can be carried out more economically, in this example on a standard office PC. To prove the concept of this search method we conducted a blind periodicity search on all 271 sources from the 3rd EGRET catalog.
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.
122985602
BABAR SILICON VERTEX TRACKER
Abstract A silicon-strip detector tracking system is being constructed for the BaBar experiment at the SLAC B-Factory. The background environment at the B-Factory and the demands imposed by the objective of studying CP violation in the B-meson system lead to a set of requirements and a design that are substantially different from those of existing silicon-strip systems. Those requirements are explained, and a design is presented which is believed to robustly satisfy all of them.
15325639
A Fresh Look at Diffuse Gamma-ray Emission from the Inner Galaxy
The Energetic Gamma-Ray Experiment Telescope (EGRET) experiment onboard the Compton Gamma-Ray Observatory (CGRO) has provided the most precise measurements of the -ray sky to date. The EGRET measurements of the diffuse emission across the sky show an excess above 1 GeV. This "GeV excess" has been a topic of great debate and interest since its original discovery by Hunter et al. in 1997. We have modified the GLAST simulation and reconstruction software to model the EGRET instrument. This detailed modeling has allowed us to explore the parameters of the EGRET instrument, in both its beam-test configuration and in-orbit on CGRO, in greater detail than has previously been published. We have found that the GeV excess is significantly increased when previously unaccounted for instrumental effects are considered. We will present a new measurement of diffuse -ray emission in the inner Galaxy.