We present initial results from a Ðeld survey for extremely red objects [EROs, deÐned here as (R[K@) º 6 mag] covering 154 arcmin2 of sky, from the Ðrst of seven deep, wide-Ðeld K@ images obtained as part of the Calar Alto Deep Imaging Survey (CADIS). The 5 p point source detection limits are K@ \ 20.5 mag and R \ 25.0 mag, while extended-source limits are up to 0.50È0.75 mag brighter. We identify a total of eight bright EROs with K@ ¹ 19.0 mag. Six of these bright EROs are resolved and are likely to be galaxies, while the remaining two are unresolved, with colors consistent with their being low-mass galactic stars. We derive a surface density for the six bright, extragalactic EROs of 0.039^0.016 arcmin~2, which is higher by a factor of 4 than previous values. We estimate that the volume density of bright EROs to be as high as that of nearby Seyfert galaxies.

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Optical Transients from gamma-ray burst sources, in addition to offering a distance determination, convey important information on the physics of the emission mechanism, and perhaps also about the underlying energy source. As the gamma-ray phenomenon is extremely diverse, with time scales spanning several orders of magnitude, some diversity in optical counterpart signatures appears plausible.We have studied the Optical Transient, which accompanied the gamma-ray burst of May 8, 1997 (GRB 970508). Observations conducted at the 2.5-m Nordic Optical Telescope (NOT) and the 2.2-m telescope at the German-Spanish Calar Alto observatory (CAHA) cover the time interval starting 3 hours 5 minutes to 96 days after the high energy event. This brackets all other published observations, including radio. When analyzed in conjunction with optical data from other observatories, evidence emerges for a composite light curve. The first interval, from 3 to 8 hours after the event was characterized by a constant, or slowly declining brightness. At a later moment the brightness started increasing rapidly, and reached a maximum approximately 40 hours after the GRB. From that moment the GRB brightness decayed approximately as a power-law of index -1.21. The last observation, after 96 days, m R = 24.28 ± 0.10, is brighter than the extrapolated power-law, and hints that a constant component, m R = 25.50 ± 0.40 is present. The OT is unresolved (FWHM 0.83 ′′ ) at the faintest magnitude level.The brightness of the optical transient, its duration and the general shape of the light curve sets this source apart from the single other optical transient known, that of the February 28, 1997 event.

Abstract. We present K-band number counts for the faint galaxies in the Calar Alto Deep Imaging Survey (CADIS). We covered 4 CADIS fields, a total area of 0.2 deg 2 , in the broad band filters B, R and K. We detect about 4000 galaxies in the K-band images, with a completeness limit of K = 19.75 mag, and derive the K-band galaxy number counts in the range of 14.25 < K < 19.75 mag. This is the largest medium deep K-band survey to date in this magnitude range. The B-and R-band number counts are also derived, down to completeness limits of B = 24.75 mag and R = 23.25 mag. The K-selected galaxies in this magnitude range are of particular interest, since some medium deep near-infrared surveys have identified breaks of both the slope of the K-band number counts and the mean B − K color at K = 17 ∼ 18 mag. There is, however, a significant disagreement in the K-band number counts among the existing surveys. Our large near-infrared selected galaxy sample allows us to establish the presence of a clear break in the slope at K = 17.0 mag from dlog N/dm = 0.64 at brighter magnitudes to dlog N/dm = 0.36 at the fainter end. We construct no-evolution and passive evolution models, and find that the passive evolution model can simultaneously fit the B-, R-and K-band number counts well. The B − K colors show a clear trend to bluer colors for K > 18 mag. We also find that most of the K = 18-20 mag galaxies have a B − K color bluer than the prediction of a no-evolution model for an L * Sbc galaxy, implying either significant evolution, even for massive galaxies, or the existence of an extra population of small galaxies.

An optical transient within the error box of the gamma ray burst GRB 970508 was imaged 4 hours after the event. It displayed a strong ultraviolet excess, and reached maximum brightness 2 days later. The optical spectra did not show any emission lines, and no variations on time scales of minutes were observed for 1 hour during the decline phase. According to the fireball and afterglow models, the intensity should rise monotonically before the observed optical maximum, but the data indicate that another physical mechanism may be responsible for the constant phase seen during the first hours after the burst.