Home | Browser | Access | FreeSQL

Frequently asked questions, bugs and mistakes

The stellar masses in the VESPA database are too large - what is going on?

From DR6 onwards, the flux calibration is done using the PSF magnitudes of standard stars, as opposed to fibre magnitudes. This yields a calibration that is less sensitive on seeing, and has a better wavelength response. However, this also results in an offset between the fibre magnitude (i.e., the magnitude within a 3-arcsec fibre) and the spectro magnitude (i.e., the magnitude synthesised from the spectra) for extended objects such as galaxies - see Fig. 4 of Adelman-McCarthy et al. 2008. This offset is around 0.35 magnitudes, or a factor of 0.72 (0.14 dex) in flux. Therefore, to convert to a photometric scale (in which fibre and spectro magnitudes match for galaxies) when using DR6 and DR7 spectra, one must revert back to the old flux scale -- which was fainter by a factor of 0.72.

This factor was NOT applied to the fluxes before they were processed by VESPA, and must be applied to the masses and star-formation histories in the database. In Tojeiro et al. 2009 we quoted this factor as being 0.26 dex. This estimate was based on a comparison between stellar masses in DR5 and DR7. Please note however, that this is wrong. The correct factor due to the change in the flux scale is 0.14 dex only. It remains true that the DR5 and DR7 masses are offset by more than 0.14 dex. The remainder is mostly explained by a wavelength dependence in the change of the spectrophotometric calibrations, which is larger for galaxies than what is shown in Fig. 5 of Adelman-McCarthy et al. 2008 for standard stars. We are currently actively investigating this issue.

The fibre corrections returned by the SQL query in Section 5.1 are smaller than one. Shouldn't they be larger than one?