Search Publications
The Discovery of Globular Clusters in the Protospiral Galaxy NGC 2915: Implications for Hierarchical Galaxy Evolution
Ford, H. C.; Infante, L.; Zheng, W. +30 more
We have discovered three globular clusters beyond the Holmberg radius in Hubble Space Telescope Advanced Camera for Surveys images of the gas-rich dark matter-dominated blue compact dwarf galaxy NGC 2915. The clusters, all of which start to resolve into stars, have MV606=-8.9 to -9.8 mag, significantly brighter than the peak of the lumi…
Anomalous Evolution of the Dwarf Galaxy HIPASS J1321-31
Knezek, Patricia M.; Saha, A.; Freeman, Kenneth C. +6 more
We present Hubble Space Telescope/WFPC2 observations of the dwarf galaxy HIPASS J1321-31. This unusual galaxy lies in the direction of the Centaurus A group of galaxies and has a color-magnitude diagram with a distinctive red plume of luminous stars. This feature could arise from (1) a red giant branch if the galaxy were much nearer than previousl…
Acoustic Wave Propagation in the Sun: Implications for Wave Field and Time-Distance Helioseismology
Thompson, M. J.; Rajaguru, S. P.; Tong, C. H. +2 more
We present results of the numerical simulation of acoustic wave propagation in the Sun's subphotospheric layers. A finite-difference code is used to calculate the pressure perturbation in the frequency domain. We show that the oscillatory seismic signals are closely associated with the solar density and sound speed structures. Owing to the acousti…
Circumnuclear Structures in the Interacting Seyfert Galaxy NGC 1241: Kinematics and Optical/Infrared Morphology
Díaz, Rubén J.; Dottori, Horacio; Vera-Villamizar, Nelson +1 more
We studied the spiral pattern in the inner 6.5" (1''=257 pc) central regions of the interacting active nucleus galaxy NGC 1241 using Gemini North Telescope high-resolution Ks- and J-band images and Hubble Space Telescope (HST) Paα and H- and (V+R)-band images with high resolution in the range from ~0.1" to ~0.3" along with in…
Cosmic Gravitational Shear from the Hubble Space Telescope Medium Deep Survey
Casertano, Stefano; Griffiths, Richard E.; Ratnatunga, Kavan U.
We present a measurement of cosmic shear on scales ranging from 10" to 2' in 347 Wide Field Planetary Camera 2 (WFPC2) images of random fields. Our result is based on shapes measured via image fitting and on a simple statistical technique; careful calibration of each step allows us to quantify our systematic uncertainties and to measure the cosmic…
Free-Free Emission in the Far-Ultraviolet Spectral Range: A Resource for Diagnosing Solar and Stellar Flare Plasmas
Curdt, W.; Landi, E.; Doschek, G. A. +2 more
We report the detection of free-free (bremsstrahlung) emission near 1200 Å from a flare at the solar limb observed with the Solar Ultraviolet Measurements of Emitted Radiation (SUMER) spectrometer on the Solar and Heliospheric Observatory (SOHO) spacecraft. The observations consist of a time series of slit spectra at a fixed pointing that lasted a…
The Nature of the Ultraviolet/X-Ray Absorber in PG 2302+029
Hamann, Fred; Shields, Joseph C.; Jannuzi, Buell T. +2 more
We present Chandra X-ray observations of the radio-quiet QSO PG 2302+029. This quasar has a rare system of ultra-high-velocity (-56,000 km s-1) UV absorption lines that form in an outflow from the active nucleus. The Chandra data indicate that soft X-ray absorption is also present. We perform a joint UV and X-ray analysis, using photoio…
Technique for Detecting Warm-Hot Intergalactic Gas in Quasar Ultraviolet Spectra
Binette, Luc; Rodríguez-Martínez, Mario; Haro-Corzo, Sinhue +1 more
The ionizing spectral energy distribution of quasars exhibits a steepening of the distribution shortward of ~1200 Å. The change of the power-law index from approximately -1 (near-UV) to -2 (far-UV) has so far been interpreted as being intrinsic to quasars. We consider the possibility that the steepening may result from a tenuous absorption compone…
Comparison of Total Solar Irradiance with NASA/National Solar Observatory Spectromagnetograph Data in Solar Cycles 22 and 23
Jones, Patricia B.; Jones, Harrison P.; Branston, Detrick D. +1 more
NASA/National Solar Observatory Spectromagnetograph (SPM) data are compared with spacecraft measurements of total solar irradiance (TSI) variations for 8 yr beginning with the declining phase of solar cycle 22 and extending into the maximum of cycle 23. Previously reported conclusions based on a similar comparison for a shorter time period appear …
Hubble Space Telescope Observations of Three Very Young Star Clusters in the Small Magellanic Cloud
Mutchler, Max; Villaver, Eva; Shaw, Richard A. +1 more
We present Space Telescope Imaging Spectrograph (STIS) broadband imagery and optical slitless spectroscopy of three young star clusters in the Small Magellanic Cloud (SMC). MA 1796 and MG 2 were previously known as planetary nebulae and were observed as such in our Hubble Space Telescope (HST) survey. With the HST spatial resolution, we show that …