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

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4. Clustering analysis

Is there an enhanced density of galaxies around 1138-262? Do the candidates make up a forming cluster?

The number of 50 Ly[FORMULA] emitters in the 35.4 arcmin2 field ([FORMULA] arcmin2 is rendered unusable by bright stars) is equivalent to a surface density of 1.4 per arcmin2. The comoving volume for this field size (8.3 Mpc2) and redshift range ([FORMULA]) is 4758 Mpc3, resulting in a volume density of [FORMULA] emitters per Mpc3. A contour plot of surface density does not show a strong concentration of emitters centered on the radio galaxy or anywhere else in the field.

The angular two point correlation function was determined by the Landy-Szalay estimator (Landy & Szalay 1993). Due to the occurence of close pairs in our sample, the correlation function shows a signal at short distances, but there is no evidence that the distribution of emitters is significantly different from a random distribution on scales larger than 20".

S99 present an overview of luminosity functions of three Ly[FORMULA] emission searches in their Fig. 5. We have converted our Ly[FORMULA] luminosities to star formation rate (SFR) using the relation [FORMULA](1500Å) = 8.0[FORMULA] (SFR / [FORMULA] yr-1) ergs s-1 Hz-1 (Madau et al. 1998) and computed the number of emitters (omitting the radio galaxy halo objects) per SFR bin per Mpc3 assuming [FORMULA] for all emitters. The central wavelength of the B filter is 1366 Å in the restframe, which is close enough to 1500 Å to omit a K-correction carried out with an uncertain spectral index. Fig. 4 shows that the density of the cluster companions of 1138-262 (plus signs with vertical error bars) is comparable to the near-QSO search of Campos et al. (1999) at [FORMULA], which represents a small overdensity relative to the blank field search of Cowie & Hu (1998) at [FORMULA]. The density is, however, lower than the Lyman break galaxy spike of S99 at [FORMULA]. We have assumed that properties of forming clusters and galaxies do not change too fast to allow meaningful comparison between clusters at different epochs. The figure also shows that we detect SFR as low as 1 h-2 [FORMULA] yr-1, while Cowie & Hu (1998) and S99 reach their sensitivity limit at about 8 h-2 [FORMULA] yr-1. This is due, in part, to our sampling of a lower redshift range.

[FIGURE] Fig. 4. The luminosity function of our Ly[FORMULA] emission selected objects at [FORMULA] (+), Cowie & Hu's (1998) blank field narrow band emitters at [FORMULA] ([FORMULA]), Campos et al.'s (1999) near-QSO search at [FORMULA] ([FORMULA]) and S99's overdensity of star forming galaxies at [FORMULA] ([FORMULA]). Note that our bin with the lowest SFR is only a lower limit, due to sensitivity incompleteness. Figure reproduced from S99.

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© European Southern Observatory (ESO) 2000

Online publication: June 26, 2000