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Astron. Astrophys. 342, 313-336 (1999)

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7. Discussion

The number counts derived from the ISO-HDF observations implying strong evolution at 15 µm and no evolution at 6.75µm, may appear surprising at first sight. This apparent discrepancy is in fact easy to interpret, considering that we are not observing the same physical processes at these two wavelengths.

As shown in Fig. 2, the LW2 filter tends to select the old stellar population of elliptical galaxies; taking advantage of the K correction, the detection of such systems are favored at z [FORMULA] 0.5. For gaseous systems, dominated by dust and UIB emission, as the redshift rises, the LW2 filter crosses a gap between the UIB features above 6.2 µm and stellar emission below 4µm in the rest frame, leading to a small number of detections, even though the sensitivity is higher than with the LW3 filter.

The LW3 filter picks up hot dust in nearby systems as well as redshifted UIB emission at z as high a 1.5. Studies of nearby galaxies show that this emission is enhanced in the regions surrounding a starburst, so that the detection of starbursts, often in mergers, is favoured. This is borne out in the results. Indeed, a close examination of the morphology of ISO sources shows that early type galaxies are numerous among the few LW2 detections (6 out 11 detections), while spirals, irregulars, or mergers dominate in the LW3 sample.

The z distribution of our sample of LW3 sources (with signal in wavespace 7 times above the threshold) is displayed in Fig. 22, and compared to that of sources with B [FORMULA] 25 and K [FORMULA] 22., taken from the list by Cowie et al. (1997). The redshifts in our sample range from z = 0.078 to z=1.242, with a median value of z=0.585; most of ISOCAM LW3 sources have z [FORMULA] 0.4. The distribution in redshift of our two samples is very similar, implying that the LW3 filter does not select a particular range in z.

[FIGURE] Fig. 22. Histogram of redshifts distribution in two samples Solid: ISOCAM LW3 sources. Dashed: Cowie et al. (1997) sample on HDF with [FORMULA] and [FORMULA].

A forthcoming paper (Elbaz et al., in prep. ) will present a detailed study of each ISO-HDF sources. This study will allow to determine if the evolution observed at 15 µm is a general trend in galaxies or is due to certain types in the galaxy population, and put limits on the evolution of the early type systems at z [FORMULA]1.5.

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© European Southern Observatory (ESO) 1999

Online publication: February 22, 1999