Search Publications
Heating and Acceleration of Protons and Minor Ions by Fast Shocks in the Solar Corona
Lee, L. C.; Wu, B. H.
We propose a new mechanism of coronal heating, in which protons and minor ions are heated and accelerated by fast shocks. The interaction between network magnetic fields and emerging intranetwork fields may lead to the formation of thin current sheets, which triggers magnetic reconnection and microflares. The disruption of current sheet leads to f…
How Young are Early-type Cluster Galaxies? Quantifying the Young Stellar Component in a Rich Cluster at z=0.41
Ferreras, Ignacio; Silk, Joseph
We present a new method of quantifying the mass fraction of young stars in galaxies by analyzing near-ultraviolet (NUV)-optical colors. We focus our attention on early-type cluster galaxies, whose star formation history is at present undetermined. Rest-frame NUV (F300W) and optical (F702W) images of cluster Abell 851 (z=0.41) using Hubble Space Te…
The cometary blue compact dwarf galaxies Mkn 59 and Mkn 71. A clue to dwarf galaxy evolution?
Thuan, T. X.; Izotov, Y. I.; Papaderos, P. +3 more
``Cometary'' Blue Compact Dwarf Galaxies (iI,C BCDs) are characterized by an off-center starburst close to the end of their elongated stellar bodies. This rare phenomenon may carry some clues on how collective star formation ignites and propagates in gas-rich low-mass stellar systems. This off-center burst may be a fortuitous enhancement of the ot…
Hubble Space Telescope WFPC2 Photometry of M33: Properties of the Halo Star Clusters and Surrounding Fields
Sarajedini, Ata; Schommer, Robert; Geisler, Doug +1 more
We present CCD photometry for 10 fields centered on halo star clusters in the nearby spiral galaxy M33. The data are based on Hubble Space Telescope Wide-Field Planetary Camera 2 observations in the F555W (~V) and F814W (~I) filters. The resultant color-magnitude diagrams (CMDs) extend ~1.5 mag below the horizontal branch (HB). From these diagrams…
Intensive HST, RXTE, and ASCA Monitoring of NGC 3516: Evidence against Thermal Reprocessing
Nandra, Kirpal; Peterson, Bradley M.; Maoz, Dan +10 more
During 1998 April 13-16, the bright, strongly variable Seyfert 1 galaxy NGC 3516 was monitored almost continuously with HST for 10.3 hr at ultraviolet wavelengths and 2.8 days at optical wavelengths, and simultaneous RXTE and ASCA monitoring covered the same period. The X-ray fluxes were strongly variable with the soft (0.5-2 keV) X-rays showing s…
The evolution of the color gradients of early-type cluster galaxies
Greggio, L.; Saglia, R. P.; Bender, R. +2 more
We investigate the origin of color gradients in cluster early-type galaxies to probe whether pure age or pure metallicity gradients can explain the observed data in local and distant (z ≈ 0.4) samples. We measure the surface brightness profiles of the 20 brightest early-type galaxies of CL0949+44 (hereafter CL0949) at redshift z=0.35-0.38 from HST…
The Dwarf Spheroidal Companions to M31: WFPC2 Observations of Andromeda II
Da Costa, G. S.; Armandroff, T. E.; Caldwell, Nelson +1 more
The Hubble Space Telescope WFPC2 camera has been used to image Andromeda II, a dwarf spheroidal (dSph) companion to M31. The resulting color-magnitude (c-m) diagrams reveal the morphology of the horizontal branch (HB) in this dwarf galaxy. We find that like Andromeda I, and like most of the Galactic dSph companions, the HB morphology of And II is …
The elusive active nucleus of NGC 4945
Maiolino, R.; Marconi, A.; Macchetto, F. +4 more
We present new HST NICMOS observations of NGC 4945, a starburst galaxy hosting a highly obscured active nucleus that is one of the brightest extragalactic sources at 100 keV. The HST data are complemented with ground based Fe II line and mid-IR observations. A 100pc-scale starburst ring is detected in Paα, while H2 traces the walls of a…
The cluster environments of the z~1 3CR radio galaxies
Best, P. N.
An analysis of the environments around a sample of 28 3CR radio galaxies with redshifts 0.6<z<1.8 is presented, based primarily upon K-band images down to K~20 taken using the UK Infrared Telescope (UKIRT). A net overdensity of K-band galaxies is found in the fields of the radio galaxies, with the mean excess counts being comparable to that …
Resolved Spectroscopy of the Narrow-Line Region in NGC 1068. II. Physical Conditions near the NGC 1068 ``Hot Spot''
Crenshaw, D. Michael; Kraemer, Steven B.
The physical conditions near the optical continuum peak (``hot spot'') in the inner narrow-line region of the Seyfert 2 galaxy NGC 1068 are examined using ultraviolet and optical spectra and photoionization models. The spectra were taken with the Hubble Space Telescope/Space Telescope Imaging Spectrograph (STIS) through the 0.1"x52.0" slit, coveri…