4. Optical properties of ISO counterparts
The photometric properties of the identified optical counterparts are illustrated in Figs. 7, 8 and 9. Fig. 7 shows that the less secure counterparts are fainter in on average than the secure counterparts. In addition to the 17% of ISOCAM LW2 sources that have no obvious optical counterparts, the counterparts for another 18% are fainter than the limit ( = 22.5) of CFRS spectroscopy, and hence some of these may be at z1. Fig. 8 compares the distribution of all the secure and less secure extragalactic identifications (lower panel) with the same distribution for all CFRS galaxies () (upper panel). This diagram shows that ISOCAM LW2 and CFRS sources have the same median 1), which corresponds to the color of an Sbc in the redshift range from z = 0.3 to 1. Fig. 9 shows that the median of the histogram of the ISOCAM sources identified with known galaxies is 0.5 mag redder than that of CFRS galaxies, which is mag. Five of the sources (114, 331, 379, 461, 477) are ultra-red (with ). One of these is at z = 0.7, but the other four are possible high redshift objects.
Spectra for 22 of the 45 optical counterparts of our ISOCAM LW2 sources are available in the CFRS database (including 7 stars, generally M or K stars). Several methods were used to classify the objects through their spectra and other properties: diagnostic diagrams (using , , , , ratios), radio properties (Hammer et al. 1995), star formation rate ( equivalent widths) and the Balmer index D(3550 - 3850) defined as (3750-3950)/(3450-3650) at rest (see Hammer et al. 1997) The latter is an indicator of recent star formation, and is very well correlated to the equivalent width. Large values of D(3550 - 3850) ( 0.18) indicate a significant contribution of the A star population (such objects are classified as ''), suggesting a major burst of star formation some 0.5 Gyr ago. Altogether, we find: eight galaxies (Balmer index, D(3550 - 3850) 0.18 and/or detection of , absorption), one starburst (diagnostic diagram), four QSO and Seyfert 1 galaxies (from their continuum flux and broad lines) and two Seyfert 2 galaxies (based on the diagnostic diagrams).
The spectra, divided according to the above classifications, are shown in Fig. 10 and relevant data for these sources are listed in Table 3. The optical counterparts of the eight less secure (catalogues 4 & 5) ISOCAM sources include a large fraction (6 of 8) of galaxies. Our confidence in the reliability of the identifications is strengthened by the fact that five of them are also detected either at radio wavelength or at 15µm with S/N 4 (Flores et al. 1998, submitted).
Table 3. ISOCAM LW2 galaxies with spectra.
As shown in Fig. 11, the median redshift of the 15 extragalactic sources is z 0.60, close to the average CFRS value (this is not too surprising since our spectroscopic sample of ISOCAM sources has the same optical flux limit as the CFRS). The two most distant objects are QSOs at z 1 and 1.6 (Schade et al. 1995). The latter has the highest redshift in this CFRS field. Most of the strongest sources with secure identifications (catalogues 1 & 2) are AGNs (QSO and Seyfert).
© European Southern Observatory (ESO) 1999
Online publication: March 1, 1999