To critically test the possibility of systematic differences between the SFR estimators we have compiled from the literature a sample of all the nearby well studied star forming galaxies for which good data is available in H, H, [OII]3727, UV continuum and FIR. We call it the "reference" sample of galaxies.
The resulting sample consists of galaxies classified either as HII galaxy, Starburst (SB) or Blue Compact (BCG) and although it covers a range of galaxian properties, a large fraction of them is of low luminosity and low metal content. Work by Koo and collaborators (Koo et al. 1996, Lowenthal et al. 1997) has shown deep similarities between the faint blue galaxies found typically at z ~ 0.5 and the type of galaxies in our sample. This could also be the case for Lyman limit galaxies [Giavalisco, Steidel & Macchetto 1996].
Spectroscopic data for 14 of the galaxies come from McQuade, Calzetti and Kinney (1995) where they combined spectral data in the UV from the IUE [Kinney et al. 1993] with optical observations. The optical spectra were obtained with the KPNO 0.9m telescope covering a spectral range from 3500 Å to 8000 Å with 10 Å resolution. The circular aperture used by McQuade et al. (1995), 13.5" in diameter, matches the 10" × 20" IUE aperture. Therefore no area renormalization was necessary for these galaxies when comparing UV with optical data.
Optical data for other 12 galaxies were obtained by Storchi-Bergmann, Kinney and Challis (1995) and combined with the [Kinney et al. 1993] IUE ultraviolet atlas. The optical observations were made with the 1m and 1.5m CTIO telescopes. They used a long slit of 10 arcsecond width which was vignetted to equal the 10" × 20" IUE aperture. The 1m telescope covered a range from 3200 to 6400 Å with a resolution of 5.5 Å and the 1.5m telescope covered from 6400 to 10000 Å with a resolution of 8 Å. In general their spectra flux levels agree within 20% with the spectra from the IUE satellite. In the cases where the differences were larger (observations made under non-photometric conditions) they assumed that the flux given by the IUE observations is the correct one.
The optical spectra of CAM0840, TOL1247 and CAM1543 were observed by Terlevich et al. (1991). TOL1247 was observed under photometric conditions with the 3.6m ESO telescope and an 8 × 8 arcsec aperture. The spectra of CAM0840 and CAM1543 were obtained at Las Campanas Observatory with the 2.5m telescope. An aperture of 2 × 4 arcsec and a resolution of about 5 Å were used. The ultraviolet spectra of these galaxies are from IUE satellite observations, Terlevich et al. (1993) (CAM0840 and TOL1247) and Meier & Terlevich (1981) (CAM1543).
The optical data of MRK309 is from the UCM objective-prism survey [Gallego et al. 1996]. The UV data comes from the observations made with IUE and reported by McQuade et al. (1995).
Table 1 shows the complete sample of star forming galaxies used in this study. All the objects are located at large galactic latitudes (|b| > 25). The apparent blue magnitudes are from De Vaucouleurs et al. RC3 and are corrected for galactic extinction, internal extinction and aperture differences. Radial velocities were corrected by the Local Group motion using the NED velocity correction software (Table 1).
The 60 µm IRAS data for all the galaxies come from Moshir et al. (1990).
We computed the equivalent width of H using the easily deduced expression,
where L(H) and Lc(4861 Å) are the H and adjacent continuum luminosities respectively. The field of view used to estimate MB is larger than the apertures for the spectroscopic observations, therefore the estimated EW(H) represents a lower limit. Where MB was not available, as for CAM0840, CAM1543 and TOL1247, we used EW(H) from the spectrophotometry of Terlevich et al. (1991).
The extinction correction to the observed fluxes both on the continuum and on the emission lines was estimated following standard procedures. Two different extinction curves were used: the Milky Way extinction law (MW) given by Seaton (1979) and Howarth (1983) and the Large Magellanic Cloud one (LMC) given by Howarth (1983). A detailed description of the procedure can be found in Appendix 1.
|Name||Type||mB||( km s-1)||(*)||(60 µm)||Ref.|
NASA/IPAC Extragalactic Database. Back.