We compute the rate of supernovae (SNe) of different types along the Hubble sequence normalized to the near-infrared luminosity and to the stellar mass of the parent galaxies. This is made possible by the new complete catalog of near-infrared galaxy magnitudes obtained by 2MASS. We find that the rates of all SN types, including Ia, Ib/c and II, show a sharp dependence on both the morphology and the (B-K) colors of the parent galaxies and, therefore, on the star formation activity. In particular we find, with a high statistical significance, that the type Ia rate in late type galaxies is a factor ∼20 higher than in E/S0. Similarly, the type Ia rate in the galaxies bluer than B-K=2.6 is about a factor of 30 larger than in galaxies with B-K>4.1. These findings can be explained by assuming that a significant fraction of Ia events in late Spirals/Irregulars originates in a relatively young stellar component.

We report the first results from a deep Lyα imaging program of local starburst galaxies with the Advanced Camera for Surveys (ACS) of the Hubble Space Telescope. The two observed galaxies ESO 350-IG038 and SBS 0335-052 have luminosities similar to those of the Magellanic Clouds but differ in their chemical composition. ESO 350-IG038 has an oxygen abundance of 1/8 solar, whereas SBS 0335-052 is known to have one of the lowest abundances among blue galaxies (∼1/30). The ACS imaging reveals a complex Lyα morphology, with sometimes strong offsets between the emission of Lyα and the location of stellar light, ionized gas traced by Hα, and the neutral gas. Overall, more Lyα photons escape from the more metal-and dust-rich galaxy ESO 350-IG038. The absence of clear SBS 0335-052 Lyα emission over all observed knots, whatever their dust content or/and color indices, contradicts model expectations of a lower escape fraction from dust-rich gas due to destruction of Lyα photons by dust grains. Rather, the results are in qualitative agreement with models suggesting the kinematic properties of the gas as the dominant Lyα escape regulator. If the properties of the two observed galaxies are representative for starburst galaxies in general, Lyα will be difficult to interpret as a star-formation indicator, in particular if based on Lyα imaging at low spatial resolution.

Abstract. ESO 338-IG04 (Tololo 1924-416) is a well-known, luminous (M V = −19.3) Blue Compact Galaxy in the local universe. Its complex morphology indicates a recent merger and/or close interaction, and it contains a central young starburst with compact star clusters of ages 40 Myr. The galaxy was imaged using the Advanced Camera for Surveys onboard the Hubble Space Telescope (HST) in the Lyman α line and continuum. Using the Starburst99 synthetic spectra and other imaging data from the Wide Field and Planetary Camera 2, we developed a technique that allows us to make the first photometrically valid subtraction of continuum from the Lyα line. The method allows us to disentangle the degenerate effects of age and reddening by careful sampling of the UV continuum slope and 4000 Å discontinuity. Our results are in qualitative agreement with the models of Lyα escape being regulated by kinematical properties of the interstellar medium. The line-only image shows Lyα in both emission and absorption. Most notably, Lyα emission is seen from central bright young clusters and is in spatial agreement with features present in a longslit spectrum taken with the Space Telescope Imaging Spectrograph. Lyα is also seen in diffuse emission regions surrounding the central starburst where photons escape after one or more resonant scatterings in H. Quantitative photometry reveals a total flux in the Lyα line of f Lyα,TOT = 194 × 10 −14 erg s −1 cm −2 . The Lyα flux in a 10 × 20 elliptical aperture centred on the brightest central star cluster measures f Lyα,IUE = 134 × 10 −14 erg s −1 cm −2 with an equivalent width of 22.6 Å. This is in close agreement with previous studies made using spectra from the IUE satellite to which our aperture was created to match. Thus we demonstrate that we have software in place to create line-only Lyα maps of nearby galaxies. Analysis of parameter dependencies show our technique to be largely parameter independent, producing Lyα maps indistinguishable from one another by eye and with Lyα fluxes consistent with one another to better than 50%. We see large amounts of diffuse Lyα emission that dominates the total Lyα output which are interpreted as centrally produced Lyα photons scattered by neutral hydrogen. By comparison of Lyα fluxes with Hα fluxes of a previous study, we estimate that each observed Lyα photon has undergone 2 additional scatterings. We see that Lyα line kinematics closely correlate with other kinematic tracers but, within these data, find no evidence for Lyα emission or absorption from star clusters being a function of age.