On the star formation history of IZw 18

It has been suggested that a continuous low star formation rate has been the dominant regime in IZw 18 and in dwarf galaxies for the lifetime of these objects (Legrand et al. 1999). Here, we discuss and model various star-forming histories for IZw 18. Particularly, we show that if the metallicity observed in IZw 18 results from starburst events only, the observed colors constrain the fraction of the metals ejected from the galaxy to be less than 50-70 %. We demonstrate that the continuous star formation scenario reproduces the observed parameters of IZw 18. A continuous star formation rate (SFR) of about 10E-4 Msol/yr during 14 Gyr reproduces precisely the observed abundances. This SFR is comparable with the lowest SFR observed in low surface brightness galaxies (van Zee et al. 1997). Generalized to all galaxies, the low continuous SFR scenario accounts for various facts: the presence of star formation in quiescent dwarfs and LSBG, the metallicity increase with time in the most underabundant DLA systems, and the metal content extrapolations to the outskirts of spiral galaxies. Also the apparent absence of galaxies with a metallicity lower than IZw 18, the apparent absence of HI clouds without optical counterparts, and the homogeneity of abundances in dwarfs galaxies are natural outcomes of the scenario. This implies that, even if starbursts are strong and important events in the life of galaxies, their more subdued but continuous star formation regime cannot be ignored when accounting for their chemical evolution.Comment: 10 pages, 5 postscript figures. Accepted for publication in Astronomy and Astrophysics main journa

Slowly cooking galaxies

Recent spectroscopic observations of IZw~18 have revealed homogeneous abundance throughout the galaxy and several observations of other starburst galaxies have shown no significant gradient or discontinuity in the abundance distributions within the HII regions. I thus concur with Tenorio-Tagle (1996) and Devost et al. (1997) that these observed abundance homogeneities cannot be produced by the material ejected from the stars formed in the current burst and result from a previous star formation episode. Metals ejected in the current burst of star formation remain most probably hidden in a hot phase and are undetectable using optical spectroscopy. Combining various observational facts, for instance the faint star formation rate observed in low surface brightness galaxies (van Zee et al., 1997), I propose that a low and continuous star formation rate occurring during quiescent phases between bursts is a non negligible source of new elements in the interstellar medium. Using a spectrophotometric and chemical evolution model for galaxies, I investigated the star formation history IZw~18. I demonstrate that the continuous star formation scenario reproduces all the observed parameters of IZw~18. I discuss the consequences of such a quiet star formation regime.Comment: Proceedings of the JENAM Conference (Toulouse, September 1999). To be published in New Astronomy Reviews, Editors Daniel Schaerer and Rosa Gonzalez Delgado. 8 pages, 3 figure

Empowering the Democratic Resistance in Syria

When the peaceful uprising in Syria started in spring 2011 turned into an armed resistance after a few months in the face of savage repression by the Assad regime. Since then, the activists who picked up arms became dependent on support in money and arms to be able to continue. Few other than the Assad regime question this narrative. Yet the consequences of this dependence are often overlooked. The sources of funding for the rebels and the strings attached to them have since shaped the landscape of the armed rebellion, not the other way round. What we have in Syria is not an Islamist revolution but a popular uprising that received funding primarily from Islamist sources. Acknowledging this is essential and has far-reaching implications for defining an effective policy in the Syrian conflict. As the United States, France and regional powers of the Middle East prepare for what appears to be an inevitable military strike on the Syrian regime of Bashar el Assad, questions are posed more urgently than ever: how to work with the armed opposition? who are the reliable forces? what are their capabilities? which groups can be part of the plan to replace Assad and how can the extremists be contained?This paper examines the circumstances and conditions that shaped the Syrian armed opposition and surveys the groups that remain committed to a democratic political system and a pluralistic society in Syria

Annotated directory of US Government information system projects of potential interest to NASA/SSPO

The purpose of this research activity was to develop a list for NASA of major U.S. government information systems contacts who are able to cooperate with NASA on technical interchange. The list contains the names of appropriate managers involved in major information system projects, U.S. government office officials, and their hierarchy up to the highest officials whose major responsibilities include government information systems development