GCN Circular 9601
D. A. Perley, S. B. Cenko, and J. S. Bloom (UC Berkeley) report: We imaged the field of dark gamma-ray burst GRB 090404 (Ziaeepour et al., GCN 9086) using the Keck I telescope (+LRIS) on the night of 2009-06-25 (UT) for 1670 seconds in g-band and 1560 seconds in I-band under excellent (0.5 arcsecond) seeing and photometric conditions. The object outside the XRT error circle mentioned by Malesani et al. (GCN 9093, GCN 9095) is clearly detected as an extended source, likely a moderate-redshift galaxy (hereafter "G1"). A second, slightly fainter extended source ("G2") is also detected 3.5 arcseconds to its northeast, and the two sources appear to be connected by faint emission, perhaps a tidal bridge. Both sources are marginally detected in Sloan Digital Sky Survey archival images (see also GCN 9096). Additional faint, extended emission is detected just west of the brighter galaxy G1. The emission appears to be spread over several arcminutes in the g-band image, with an obvious "knot" located 1 arcsecond northeast of the millimeter afterglow position reported by Catro-Tirado et al. (GCN 9100). The extended region itself passes directly through the millimeter position. A false-color image of the field, with the millimeter and enhanced XRT positions (http://www.swift.ac.uk/xrt_positions/00348428/image.php) superimposed, is posted to: http://lyra.berkeley.edu/~dperley/090404/090404_gI.png We hypothesize that the extended emission may be a tidal tail extending from G1 as a result of interaction with its northeastern neighbor. Alternatively, the extended region may represent a very faint background host galaxy. Aperture photometry at the location of the millimeter afterglow gives a magnitude of g = 27.3 +/- 0.2. We encourage spectroscopy of the two bright galaxies G1 and G2 to determine their redshifts.