When the contrast between the object seen in the narrowband image as compared with a broadband, continuum image is very high, the emission line sampled by the narrowband filter can be identified as the Lyman alpha line with a high degree of confidence. In a few cases, the selection has turned up active galaxies or quasars.
An important feature of both our spectroscopic and narrowband surveys is the use of target fields which also have deep Hubble Space Telescope images. This allows us to identify both high-redshift galaxies and very faint, low-redshift emission line galaxies, and to separate them by their spectroscopic properties. The faint spiral to the southeast of the z=4.19 emitter's position is at z=0.56. Even such faint galaxies have other emission lines and continuua which allow them to easily be distinguished from the Lyman alpha emitters. LRIS covers a wide field, and in some cases new emission-line objects are found which lie outside the HST images. In these cases we use complementary deep images from groundbased observations.
very deep r band image taken with the SUSI camera on the ESO 3.5m NTT (D'Odorico, Fontana, Giallongo, and Cristiani) showing the position of the z=5.64 emission-line object. The multi-color data for this quasar field is described in
a paper (Giallongo et al.) [astro-ph/9802340] giving combined optical and infrared data.
object is seen in stellar continuum light at these levels.
The position of the z=5.64 emission-line object in a very deep infrared
(K) image taken with the University of Hawaii QUIRC camera. These infrared
bands may sample wavelength regions that are better able to pick out faint
continuum starlight for very distant galaxies.There is possibly a faint
object near the position sampled by the Keck LRIS slit, but again, no convincing
detection in continuum light.