Galactic Extinction from Colors and Counts of Field Galaxies in WFPC2 Frames: An Application to GRB 970228

We develop the "simulated extinction method" to measure average foreground Galactic extinction from field galaxy number counts and colors. The method comprises simulating extinction in suitable reference fields by changing the isophotal detection limit. This procedure takes into account selection effects; in particular, the change in the isophotal detection limit (and hence in the isophotal magnitude completeness limit) with extinction, and the galaxy color-magnitude relation. We present a first application of the method to the Hubble Space Telescope WFPC2 images of the gamma-ray burster GRB 970228. Four different WFPC2 high-latitude fields, including the Hubble Deep Field, are used as reference to measure the average extinction toward the gamma-ray burst (GRB) in the F606W passband. From the counts, we derive an average extinction of AV=0.5 mag, but the dispersion of 0.4 mag between the estimates from the different reference fields is significantly larger than can be accounted for by Poisson plus clustering uncertainties. Although the counts differ, the average F606W-F814W colors of the field galaxies agree well. The extinction implied by the average color difference between the GRB field and the reference galaxies is AV=0.6 mag, with a dispersion in the estimated extinction from the four reference fields of only 0.1 mag. All of our estimates are in good agreement with the value of 0.81±0.27 mag obtained by Burstein & Heiles, and with the extinction of 0.78±0.12 measured by Schlegel et al. from maps of dust IR emission. However, the discrepancy between the widely varying counts and the very stable colors in these high-latitude fields is worth investigating.