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Astron. Astrophys. 341, 641-652 (1999)

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5. The multicolor selection for high redshift galaxies

Steidel & Hamilton (1993) used a combination of three filters U, G and [FORMULA] to select star forming galaxies at 2.8[FORMULA] 3.4. At these redshifts, due to the shift of the Lyman continuum break in the UV band, the U flux is attenuated and the [FORMULA] color is strongly reddened while the [FORMULA] color still reflects a flat spectrum.

To select galaxies at higher redshifts, a different set of filters is required. At [FORMULA]4, the combination of Lyman [FORMULA] clouds absorption and the Lyman continuum reduces significantly the observed flux also in the B filter. Giallongo et al. (1998) have shown that an optimized set of filters can be found to detect star forming galaxies at [FORMULA] and to reduce the confusion with low redshift galaxies of similar colors. This set is based on the B,V,r and I bands, and corresponds to the one adopted in the present work. An optimized detection of star forming galaxies in the redshift range 3.8[FORMULA]4.4 can be obtained by using the following criteria: [FORMULA]2, [FORMULA]0.5 and [FORMULA]0.1. In Fig. 9, we have applied the criteria to our catalogue. Two approaches have been followed to select galaxy candidates up to [FORMULA].

  • A: Galaxies with measured colors (or upper limits) located inside the area defined by the above criteria (8 galaxies).

  • B: The above sample and galaxies with [FORMULA] upper limits falling less than [FORMULA] outside the selection criterion (15 galaxies).

[FIGURE] Fig. 9. Color criteria to select star forming galaxies at redshift 3.8 [FORMULA] 4.4. Triangles show objects with [FORMULA] upper limits distant less than 1[FORMULA] from the color selection criterion, circles show objects with upper limits distant between 1 and 2[FORMULA] from the selection criterion. Both are considered as upper-limits (defined with arrows). The galaxy with B and V upper-limit and filled triangle (# 596 in the catalogue) is spectroscopically confirmed at [FORMULA]. Stars (with stellarity index [FORMULA]0.9) are shown with star symbols. For clarity, error-bars are put only for galaxies matching the color criteria.

The photometric redshifts for the galaxies of this field will be estimated in a forthcoming paper (Fontana et al., 1998) by comparing the colors (including J and K) with a library of synthetic galaxy spectra as described by Giallongo et al. (1998). Due to their faint magnitudes, the spectroscopic confirmation of our candidates requires an 8-m class telescope. Only one of the galaxies (object # 596 in our catalog) is included in the spectroscopic sample by Hu et al. (1998) and Hu (1998,private communication) and has a spectroscopic redshift of [FORMULA]. This galaxy has upper limits in the B and V bands and a magnitude [FORMULA].

The number of galaxy candidates in different magnitude intervals are given in Table 3 and Table 4 for both classes A and B described above. The surface density at [FORMULA]25, is in the range 0.7-0.9 arcmin-2 (case A and B respectively). At [FORMULA]26, the density increases to 1.4-2.7 arcmin-2.


Table 3. Number distribution of galaxies in redshift range [FORMULA] located inside the color selection criteria (case A with 8 objects).


Table 4. Same as Table 3 for case B (with 15 galaxies).

Assuming a uniform redshift distribution of the sample in the range 3.8[FORMULA]4.4, the comoving galaxy number density at [FORMULA] for [FORMULA] is estimated to be [FORMULA] (q0=0.5) and [FORMULA] (q0=0.5) for r[FORMULA].

The star formation rate history at [FORMULA] 4.1 can be derived from the luminosity in the I band corresponding to the restframe wavelength 1500 Å. We adopt the conversion factor (from the UV luminosity at 1500 Å to the SFR) of Madau et al. (1996). For a Salpeter IMF (0.1[FORMULA]) with constant SFR, solar metallicity and age range of 0.1-1 Gyr, a galaxy with [FORMULA] produces [FORMULA].

The brightest galaxy in our sample has a magnitude of [FORMULA] corresponding to a [FORMULA] for [FORMULA] ([FORMULA]), and the faintest one has a magnitude of [FORMULA] corresponding to a [FORMULA] for [FORMULA] ([FORMULA]) (the uncertainties are linked to the redshift range [FORMULA]).

Assuming that the redshift range is uniformly probed, the star formation rate per unit comoving volume at [FORMULA] derived from objects with [FORMULA] varies (for cases A and B, respectively) between [FORMULA] with [FORMULA] ([FORMULA]). For [FORMULA], the estimates increase to [FORMULA] with [FORMULA] ([FORMULA]).

This latter estimate is close to the value of [FORMULA][FORMULA] computed by Madau (1997) for the HDF in the same redshift interval down to [FORMULA]

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© European Southern Observatory (ESO) 1999

Online publication: December 16, 1998