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

Abstract

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 isophotal detection limit (and hence in isophotal magnitude completeness limit) with extinction, and the galaxy color--magnitude relation. We present a first application of the method to the HST WFPC2 images of the gamma-ray burster GRB 970228. Four different WFPC2 high-latitude fields, including the HDF, are used as reference to measure the average extinction towards the GRB in the F606W passband. From the counts, we derive an average extinction of A_V = 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 by Poisson plus clustering uncertainties. Although the counts differ, the average colors of the field galaxies agree well. The extinction implied by the average color difference between the GRB field and the reference galaxies is A_V = 0.6 mag, with a dispersion in the estimated extinction from the four reference fields of only 0.1 mag. All our estimates are in good agreement with the value of 0.81\pm0.27 mag obtained by Burstein & Heiles, and with the extinction of 0.78\pm0.12 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.

14 pages, 2 figures; submitted to the Astrophysical Journal

References in corpus (3)

• Maps of Dust IR Emission for Use in Estimation of Reddening and CMBR Foregrounds
• A novel image reconstruction method applied to deep Hubble Space Telescope Images
• The Opacity of Nearby Galaxies from Colors and Counts of Background Galaxies: I. The Synthetic Field Method and its Application to NGC 4536 and NGC 3664

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• Thermal X-Ray Iron Line Emission from the Galactic Center Black Hole Sagittarius A*
• A Photometric Investigation of the GRB970228 Afterglow and the Associated Nebulosity
• The Opacity of Nearby Galaxies from Counts of Background Galaxies: II. Limits of the Synthetic Field Method
• The Extinction Towards the GRB970228 Field
• The Galactic extinction and reddening from the South Galactic Cap U-band Sky Survey: u band galaxy number counts and $u-r$ color distribution