Astron. Astrophys. 348, L41-L44 (1999)

4. Discussion

Components A1, A2, and B of HE 0230-2130 have been clearly identified, by multicolour imaging and spectroscopically, as being images of one and the same QSO. The identities of components C and D have not yet been ascertained. From their locations, a reasonable guess might be that C is the expected fourth image and D the lensing galaxy. The colours of D are certainly consistent with this notion, but the case is more complicated with component C. Using only positions as constraints, simple models which take C to be the 4th image give large residuals, whereas models which fit only the positions of images A1, A2 and B are poorly constrained. Moreover, the colours colours of C are very different from the confirmed QSO components. This might indicate either heavy reddening by dust in the fourth image, or the presence of a second galaxy, or even superposition of a galaxy plus fourth image. Without spectroscopic information on C we presently cannot decide between these possibilities.

If C is partly or largely due to a galaxy, it is a priori most likely (but not granted) that C and D are at the same redshifts, forming one common deflector plane. If, on the other hand, C is formed by just the fourth quasar image, this would be one of the few known lenses with strongly differential dust extinction, and the first optically selected one. Furthermore, the inferred amount of differential extinction would be , just intermediate between the very small and highly uncertain values found for most lenses and the few cases where the light path in the optical is strongly obscured (cf. Falco et al. 1999).

HE 0230-2130 is an interesting new target for photometric monitoring. It is moderately symmetric so that the time delay is probably much shorter than a year, avoiding the considerable windowing problem of observations stretching over several years. Yet, it displays enough departure from symmetry that the time delay will not be uncomfortably low. In many respects, it resembles PG 1115+080, the presently cleanest system for determination of (e.g., Schechter et al. 1997). The apparent magnitude and colour of the putative lensing galaxy suggest that its redshift is not too high, .

© European Southern Observatory (ESO) 1999

Online publication: July 26, 1999
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