Starburst99 is a comprehensive set of model predictions for spectrophotometric and related properties of galaxies with active star formation. The models are an improved and extended version of the data set previously published by Leitherer & Heckman (1995). We have upgraded our code by implementing the latest set of stellar evolution models of the Geneva group and the model atmosphere grid compiled by Lejeune et al. (1997). Several predictions which were not included in the previous publication are shown here for the first time. The models are presented in a homogeneous way for five metallicities between Z = 0.040 and 0.001 and three choices of the initial mass function. The age coverage is 10^6 to 10^9 yr. We also show the spectral energy distributions which are used to compute colors and other quantities. The full data set is available for retrieval at http://www.stsci.edu/science/starburst99/. This website allows users to run specific models with non-standard parameters as well. We also make the source code available to the community.Comment: 32 pages, LaTeX. All the Figures and the summary Table are located at http://www.stsci.edu/science/starburst99/, ApJ accepte

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With the goal of investigating the degree to which the mid-infrared emission traces the star formation rate (SFR), we analyze Spitzer 8 µm and 24 µm

The SIRTF Nearby Galaxy Survey is a comprehensive infrared imaging and spectroscopic survey of 75 nearby galaxies. Its primary goal is to characterize the infrared emission of galaxies and their principal infrared-emitting components, across a broad range of galaxy properties and star formation environments. SINGS will provide new insights into the physical processes connecting star formation to the interstellar medium properties of galaxies, and provide a vital foundation for understanding infrared observations of the distant universe and ultraluminous and active galaxies. The galaxy sample and observing strategy have been designed to maximize the scientific and archival value of the data set for the SIRTF user community at large. The SIRTF images and spectra will be supplemented by a comprehensive multiwavelength library of ancillary and complementary observations, including radio continuum, HI, CO, submillimeter, BV RIJHK, Hα, Paschen-α, ultraviolet, and X-ray data. This paper describes the main astrophysical issues to be addressed by SINGS, the galaxy sample and the observing strategy, and the SIRTF and other ancillary data products.

The Wide Field Planetary Camera 2 of the Hubble Space T elescope has been used to obtain highresolution images of NGC 4038/4039 that go roughly 3 mag deeper in V than previous observations made during cycle 2. These new images allow us to measure the luminosity functions (LFs) of clusters and stars over a range of 8 mag To Ðrst order, the LF is a power law, with expo-. nent a \ [2.12^0.04. However, using a variety of di †erent techniques to decouple the cluster and stellar LFs, which overlap in the range we Ðnd an apparent bend in the young cluster 9^0.1, and is dominated by young red and blue supergiants. The star clusters of the Antennae appear slightly resolved, with median e †ective radii of 4^1 pc, similar to or perhaps slightly larger than those of globular clusters in our Galaxy. However, the radial extents of some of the very young clusters (ages less than 10 Myr) are much larger than those of old globular clusters (e.g., the outer radius of knot S exceeds 450 pc). This may indicate that the tidal forces from the galaxies have not had time to remove some of the outer stars from the young clusters. A combination of the UBV I colors, Ha morphology, and Goddard High Resolution Spectrograph (GHRS) spectra enables us to age date the clusters in different regions of the Antennae. Star clusters around the edge of the dust overlap region appear to be the youngest, with ages Myr, while clusters in the western loop appear to be 5È10 Myr old. Many star [5 clusters in the northeastern star formation region appear to be D100 Myr old, with an LF in V that has shifted faintward by D1.0 mag relative to the younger (0È20 Myr) clusters that dominate over most of the rest of the galaxy. A third cluster population consists of intermediate-age clusters (D500 Myr) that probably formed during the initial encounter responsible for ejecting the tails. A handful of old globular clusters from the progenitor galaxies are also identiÐed. Most of these lie around NGC 4039, where the lower background facilitates their detection. Age estimates derived from GHRS spectroscopy yield 3^1 Myr for knot K (just south of the nucleus of NGC 4038) and 7^1 Myr for knot S in the western loop, in good agreement with ages derived from the UBV I colors. E †ective gas outÑow velocities from knots S and K are estimated to be about 25È30 km s~1, based on the above cluster ages and the sizes of the surrounding Ha bubbles. However, the measured widths of the interstellar absorption lines suggest dispersion velocities of D400 km s~1 along the lines of sight to knots S and K.