Astron. Astrophys. 347, 434-441 (1999)

1. Introduction

MG J0414+0534 is one of the gravitational lens systems found in the MIT-Green Bank radio survey (Bennett et al., 1986). Its high radio flux allowed a detailed mapping with the Very Large Array (Hewitt et al., 1988, Hewitt et al., 1992, Katz & Hewitt, 1993), revealing a trapeziform configuration (almost identical to that of PG 1115+080) with 4 bright images labelled A1, A2, B and C.

This system is remarkable because, in the optical domain, the counterparts of the radio images are extremely red and faint, and their spectrum is almost featureless. The source appears to be a very obscured quasar; its redshift (z_S = 2.639) could only be obtained from infrared spectra (Lawrence et al. 1995a). The strong and broad emission line, as well as other fainter Balmer and Fe II lines, indicate a classical active nucleus spectrum after correction for the reddening. The most prominent spectral feature in the optical may be identified with an absorption complex of Fe II lines at redshifts close to the emission redshift of the quasar (Lawrence et al. 1995b). The lensing galaxy is easily detected on sub-arcsecond images (Shechter & Moore, 1993, Angonin-Willaime et al., 1994) but its redshift, z_L = 0.958, remained unknown until recently (Tonry and Kochanek 1998).

The exact value of the reddening in MG J0414+0534 depends on its origin, but is high in any case. The flux-ratio as well as the upper limit for (Lawrence et al. 1995a) suggest an extinction A_v 6 mag for the brightest QSO pair, assuming it occurs in the lensing galaxy. The fact that the flux ratios between the different QSO images change with wavelength can be interpreted in terms of differential reddening along different line of sight, through the lensing galaxy. Lawrence et al. (1995a) proposed that most of the extinction occurs in the lens, rather than close to the source (galaxy hosting the quasar, associated cluster...). The present paper compares optical and infrared data in order to test this interpretation. The ground-based optical and near-IR data are presented and discussed in Sect. 2. Sect. 3 presents a re-analysis of archival HST data and shows how the presence of a gravitational arc may affect the photometry derived from lower resolution images. Finally, the origin of the measured extinction is discussed in Sect. 4.

© European Southern Observatory (ESO) 1999

Online publication: June 30, 1999
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