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I Zw 18 Revisited with HST ACS and Cepheids: New Distance and Age
Saha, A.; Tosi, M.; Clementini, G. +9 more
We present new V- and I-band HST ACS photometry of I Zw 18, the most metal-poor blue compact dwarf (BCD) galaxy in the nearby universe. It has been argued in the past that I Zw 18 is a very young system that started forming stars only <~500 Myr ago, but other work has hinted that older (>~1 Gyr) red giant branch (RGB) stars may also exist. O…
Exoplanet HD 209458b: Inflated Hydrogen Atmosphere but No Sign of Evaporation
Ben-Jaffel, Lotfi
Many extrasolar planets orbit closely to their parent star. Their existence raises the fundamental problem of loss and gain in their mass. For exoplanet HD 209458b, reports on an unusually extended hydrogen corona and a hot layer in the lower atmosphere seem to support the scenario of atmospheric inflation by the strong stellar irradiation. Howeve…
Survey of Magnetic Helicity Injection in Regions Producing X-Class Flares
Georgoulis, M. K.; Rust, D. M.; LaBonte, B. J.
Virtually all X-class flares produce a coronal mass ejection (CME), and each CME carries magnetic helicity into the heliosphere. Using magnetograms from the Michelson Doppler Imager on the Solar and Heliospheric Observatory, we surveyed magnetic helicity injection into 48 X-flare-producing active regions recorded by the MDI between 1996 July and 2…
Direct Measurement of the Ratio of Carbon Monoxide to Molecular Hydrogen in the Diffuse Interstellar Medium
France, Kevin; Burgh, Eric B.; McCandliss, Stephan R.
We have used archival far-ultraviolet spectra from observations made by HST STIS and FUSE to determine the column densities and rotational excitation temperatures for carbon monoxide and molecular hydrogen, respectively, along 23 sight lines to Galactic O and B stars. The reddening values range from E(B-V)=0.07 to 0.62, sampling the diffuse to tra…
The Signature of Primordial Grain Growth in the Polarized Light of the AU Microscopii Debris Disk
Matthews, Brenda C.; Graham, James R.; Kalas, Paul G.
We have used the Hubble Space Telescope Advanced Camera for Surveys coronagraph to make the first polarization maps of the AU Microscopii debris disk. The polarization rises from 5% at 20 AU to 40% at 80 AU. The polarization is perpendicular to the disk, indicating that the scattered light originates from micron-sized grains in an optically thin d…
X-Ray Absorption from the Milky Way Halo and the Local Group
Bregman, Joel N.; Lloyd-Davies, Edward J.
Million degree gas is present at near-zero redshift and is due to either a gaseous Galactic halo or a more diffuse but very massive Local Group medium. We can discriminate between these models because the column densities should depend on location in the sky, either relative to the Galaxy bulge or to the M31-Milky Way axis. To search for these sig…
A Narrow-Line Seyfert 1-Blazar Composite Nucleus in 2MASX J0324+3410
Yuan, Weimin; Zhou, Hongyan; Komossa, Stefanie +7 more
We report the identification of 2MASX J032441.19+341045.9 (hereafter 2MASX J0324+3410) with an appealing object that shows the dual properties of both a narrow-line Seyfert 1 galaxy (NLS1) and a blazar. Its optical spectrum, which has a Hβ line width of about 1600 km s-1 (FWHM), an [O III]-to-Hβ line ratio of ~=0.12, and strong Fe II em…
Quasi-periodic Variability in NGC 5408 X-1
Cropper, Mark; Soria, Roberto; Mushotzky, Richard F. +3 more
We report the discovery with XMM-Newton of quasi-periodic variability in the 0.2-10 keV X-ray flux from the ULX NGC 5408 X-1. The average power spectrum of all EPIC-pn data reveals a strong 20 mHz QPO with an average amplitude (rms) of 9% and a coherence Q≡ν0/σ~6. In a 33 ks time interval when the 20 mHz QPO is strongest we also find ev…
The Star Formation Epoch of the Most Massive Early-Type Galaxies
van Dokkum, Pieter G.; van der Marel, Roeland P.
We present new spatially resolved Keck spectroscopy of early-type galaxies in three galaxy clusters at z~0.5. We focus on the fundamental plane (FP) relation and combine the kinematics with structural parameters determined from HST images. The galaxies obey clear FP relations, which are offset from the FP of the nearby Coma Cluster due to passive …
Constraints on Dark Energy from Supernovae, Gamma-Ray Bursts, Acoustic Oscillations, Nucleosynthesis, Large-Scale Structure, and the Hubble Constant
Wright, E. L.
The luminosity distance versus redshift law is now measured using supernovae and γ-ray bursts, and the angular size distance is measured at the surface of last scattering by the CMB and at z=0.35 by baryon acoustic oscillations. In this paper, this data is fit to models for the equation of state with w=-1, w=constant, and w(z)=w0+w…