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Astron. Astrophys. 358, L1-L4 (2000)

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3. Photometry and selection of Ly[FORMULA] emitters

Detection and photometry of objects in the field around 1138-262 was carried out using SExtractor (Bertin & Arnouts, 1996). We have taken advantage of all observing time by normalizing and adding the narrow and broad band images to obtain maximum signal to noise for the majority of objects, which resulted in the extraction of 1727 sources. All detections have at least 9 connected pixels with a value equal or larger than 2.5 times the rms sky noise. During this first application of SExtractor, aperture sizes and shapes were determined for each object separately. We gave the choice of aperture careful consideration by comparing the signal-to-noise for fixed circular apertures with a range of sizes and for elliptical apertures with a range of scale factors. The half lengths of the principal axes of the elliptical apertures are [FORMULA] and [FORMULA], where [FORMULA] is the ellipticity determined by the isophots of the object, [FORMULA] the first moment of the radial (non-elliptical) light distribution and k a scale factor. The highest average signal-to-noise for faint objects was obtained with elliptical apertures using [FORMULA]. The apertures determined in this way were used to carry out photometry on the narrow and broad band images.

Objects displaying twice as much flux density in the narrow band as in broad band have EW 65 Å. This equals to a rest frame EW of 20.5 Å, very close to the 20 Å used as selection criterium by Steidel et al. (1999, S99). There are 60 objects with EW larger than 65 Å and narrow band flux density of at least 2 [FORMULA] erg s-1 cm-2 Å-1. The signal-to-noise of these objects is at least 10 in narrow band, making the EW computation meaningful. We consider these to be candidate Ly[FORMULA] emitting galaxies. We subdivide the candidates in very promising ones with EW three times this value (13) and promising ones with EW two times this value (11), as shown in Fig. 1. Ten candidates coincide with peaks in the extended ([FORMULA] 160 kpc 2) halo of ionized hydrogen around the radio galaxy. So in total, there are 50 previously unknown emitters with a range of narrow band fluxes from 0.1 to 5[FORMULA] ergs s-1 cm-2. The positions of the emitters are shown in Fig. 2. Images of three candidates are shown in Fig. 3.

[FIGURE] Fig. 1. Flux densities of extracted objects are shown as dots. Plus signs represent 55 objects selected as narrow band emitters, diamonds are part of the Ly[FORMULA] halo of 1138-262 (5 are not shown since they have flux densities up to 8 [FORMULA] erg s-1 cm-2 Å-1, exceeding the range of this plot) and triangles are the examples shown in Fig. 3. Straight lines of EW 195, 130 and 65 Å divide the region of candidates in strong (13), average(11) and weak emitters (36). Also shown are lines of EW -65 Å, narrow band flux density 2 [FORMULA] erg s-1 cm-2 Å-1 and the median NB - B flux density of all extracted objects.

[FIGURE] Fig. 2. Relative positions in arcseconds of the 60 narrow band emitters (including sources in the halo of 1138-262) in the final registered image. The center of the image is at RA [FORMULA], dec [FORMULA] (J2000). Diamonds, plus signs and crosses are emitters with EW respectively higher than 195, 130 and 65 Å. The Ly[FORMULA] halo of the radio galaxy is visible as a concentration of symbols in the center of the image.

[FIGURE] Fig. 3. Three probable cluster companions shown at the top in narrow band (Ly[FORMULA]) and at the bottom in broad band (B).

We believe that there will be few or no low redshift interlopers, since [OII ][FORMULA]3727 at redshift 0.02 is the only strong line which falls in the narrow band filter, apart from Ly[FORMULA] at redshift 2.14. Galaxies at redshift 0.02 would easily be identified by their large angular size. Since the narrow band is positioned on the blue side of the broad band, our sample will not be contaminated by objects with abnormal colors as Extremely Red Objects or red M dwarfs or by objects with a break in their spectrum around 3900 Å as LBGs at [FORMULA]. Contamination by bright quasars will be small, since their surface density (0.04 arcmin-2; Thompson et al. 1999) is low enough that not more than 2 quasars will appear in our field. Although there still is a possibility that we have selected some rare, very blue objects with U - B [FORMULA] -1.3, we shall assume for the remainder of this paper that all objects with EW [FORMULA] Å are Ly[FORMULA] emitting galaxies.

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© European Southern Observatory (ESO) 2000

Online publication: June 26, 2000