The ROSAT X-ray observations were carried out at three different epochs between 1992 and 1996. Two PSPC (called "A" and "B") and one HRI ("H") observations were scheduled on NGC 2451. (3 data files).

In this paper we discuss the reason for the widely found discrepancy between Sérsic brightness law fitted along major and minor axes of elliptical galaxies. We show that this difference is a natural consequence of the isophotes eccentricity law. We present a mathematical transformation that allows to obtain the minor axis Sérsic's law from that of

In the framework of our narrow-band survey of the Local Group galaxies, we present the results of the search for planetary nebulae (PNe) in the dwarf irregular galaxies IC 10, Leo A and Sextans A. Using the standard on-band/off-band technique, sixteen new candidate PNe have been discovered in the closest starburst galaxy, IC 10. The optical size of

This paper describes a new determination of the primordial helium abundance (YP), based on the abundance analysis of five metal-poor extragalactic H II regions. For three regions of the sample (SBS 0335-052, I Zw 18, and H29) we present tailored photoionization models based on improved calculations with respect to previous models. In particular, we

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The SIRTF Wide-Area Infrared Extragalactic Survey (SWIRE), the largest SIRTF Legacy program, is a wide-area imaging survey to trace the evolution of dusty, star-forming galaxies, evolved stellar populations, and active galactic nuclei (AGNs) as a function of environment, from redshifts z~3 to the current epoch. SWIRE will survey seven high-latitude