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

In an effort to better understand the UV properties of ultraluminous infrared galaxies (ULIGs), and compare them to the rest-frame UV properties of high redshift sub-mm and Lyman-break galaxies, we have obtained far-and near-UV imaging observations (λ ef f = 1457Å, λ ef f = 2364Å, respectively) of two luminous infrared galaxies (LIGs-VV 114 and IC 883) and five ULIGs (IRAS 08572+3915, Mrk 273, IRAS 15250+3609, Arp 220, and IRAS 19254-7245) using the Hubble Space Telescope. All the galaxies were detected in both channels. UV light, both diffuse and from star clusters, can be traced to within the inner kpc of the dominant near-IR nuclei. However, in general, the brightest UV sources are clearly displaced from the I-band and near-IR peaks by at least hundreds of pc. Further, only 0.07%-7.3% of the total near-UV light is projected within the inner 500 pc radius, even though this is the same region wherein most of the bolometric energy is generated. All nuclei are highly obscured by dust. Even after correction for dust reddening, the global UV emission fails to account for the total bolometric luminosities of these systems by factors of 3-75.

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We present optical and near-infrared photometry and spectroscopy of the type Ia SN 1998bu in the Leo I Group galaxy M96 (NGC 3368). The data set consists of 356 photometric measurements and 29 spectra of SN 1998bu between UT 1998 May 11 and July 15. The well-sampled light curve indicates the supernova reached maximum light in B on UT 1998 May 19.3 (JD 2450952.8 +/- 0.8) with B = 12.22 +/- 0.03 and V = 11.88 +/- 0.02. Application of a revised version of the Multicolor Light Curve Shape (MLCS) method yields an extinction toward the supernova of A_V = 0.94 +/- 0.15 mag, and indicates the supernova was of average luminosity compared to other normal type Ia supernovae. Using the HST Cepheid distance modulus to M96 (Tanvir et al. 1995) and the MLCS fit parameters for the supernova, we derive an extinction-corrected absolute magnitude for SN 1998bu at maximum, M_V = -19.42 +/- 0.22. Our independent results for this supernova are consistent with those of Suntzeff et al. (1999). Combining SN 1998bu with three other well-observed local calibrators and 42 supernovae in the Hubble flow yields a Hubble constant, H_0 = 64^{+8}_{-6} km/s/Mpc, where the error estimate incorporates possible sources of systematic uncertainty including the calibration of the Cepheid period-luminosity relation, the metallicity dependence of the Cepheid distance scale, and the distance to the LMC.Comment: 34 pages, 13 figures, to appear in ApJ

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We present evolutionary synthesis models applied to near-infrared spectral features observed in the spectra of young Magellanic Cloud clusters and starburst galaxies. The temporal evolution of the first and second overtones of CO at 2.29 µm (2-0 bandhead) and 1.62 µm (6-3 bandhead) and of the (U − B), (B − V ) and (J − K) colors are investigated.We find that the current evolutionary tracks of massive stars with sub-solar chemical composition in the red supergiant phase are not reliable for any synthesis of the temporal evolution of infrared stellar features. 1 The high sensitivity of the selected infrared features to the atmospheric parameters of cool stars allows us to place constraints on the temperature and the fraction of time spent in the red part of the Hertzsprung-Russell diagram by massive stars during their core-helium burning phase.We derive a set of empirically calibrated spectrophotometric models by adjusting the red supergiant parameters such that the properties of the observed templates are reproduced.

We have obtained spectra across the K window for the Ðrst large sample of luminous galaxies selected from the IRAS survey. This paper contains the principal analysis of the 43 systems in our sample with luminosities ofThe spectra themselves were presented in a companion paperThe Brc luminosities are proportional to at levels similar to those of star-forming regions. This L IR , strongly suggests that star formation accounts for the bulk of the energy production in these objects, in general agreement with previous studies. Good agreement is found for the continuous star formation models of Leitherer & Heckman with upper mass cuto †s well below 100The models accommodate M _ . a range in starburst ages of D107 to 109 yr. Instantaneous starburst models Ðt the data but imply an unrealistically short range of ages for the entire sample. It is difficult to avoid concluding that the initial mass functions are deÐcient in stars of less than D1 M _ . Strong emission lines from molecular hydrogen are detected. The v \ 1È0 S(1) line luminosities are H 2 proportional to the correlation extends through the ultraluminous infrared galaxies. The emis-L IR ; H 2 sion in the galaxies tends to be more spatially extended than the Brc emission. Measured values and upper limits for the ratios of the various lines visible in our spectra indicate that the seen in H 2 H 2 emission at 2 km is consistent with being shock excited. However, other mechanisms, operating at sufficiently high densities that the energy levels are thermalized by collisions, cannot be excluded. Based H 2 on energy considerations, we suggest that the shocks are due to supernova remnants expanding into the interstellar medium. The frequency of Type II supernovae necessary to account for the line emission H 2 agrees with frequencies deduced from the starburst models and the radio/far-infrared correlation. However, there remain a number of galaxies that cannot be made to Ðt this model. A decade after its discovery, a universal explanation of the strong emission in luminous infrared galaxies continues to H 2 elude us.No previously unrecognized broad-line active nuclei were discovered in our survey ; either they are weak or absent or the true optical depths at 2 km are much higher than indicated by conventional extinction measures. However, there are clear di †erences between the K-band properties of galaxies that contain broad-line active nuclei and those that do not. The di †erences seem to be due to the presence of strong nonstellar continuum emission coming from the active nuclei themselves.With the addition of the 13 ultraluminous galaxies with from Goldader et al. (Paper log (L IR /L _ ) Z 12 I), the number of systems observed in this program totals 56. We have incorporated these ultraluminous galaxies in some parts of the analysis to examine properties across the entire luminosity range of our sample.

We combine existing multiwavelength data (including an HST/GHRS UV spectrum and a ground based optical spectrum) with unpublished HST/WFPC2 images, near-IR photometry and K band spectroscopy. We use these data to constrain the young, the intermediate age and the old stellar populations in the central regions of the starburst galaxy NGC 7714.In a previous paper (González , the stellar features in the HST/GHRS ultraviolet (UV) spectrum and the optical emission lines were used to identify a ∼ 5 Myr old, very little reddened stellar population as the main source of UV light in the central ∼330 pc. The optical data indicated the existence of an older population. The nature of the latter is investigated here. Stellar absorption features in the optical and the near-IR are used to partly break the strong degeneracy between the