Disentangling microlensing and differential extinction in the double QSO HE 0512–3329

¹ Institut für Physik, Universität Potsdam, Am Neuen Palais, 14469 Potsdam, Germany
² Astrophysikalisches Institut Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany
³ Departamento de Astronomía, Universidad de Chile, Casilla 36-D, Santiago, Chile
⁴ Institute of Geophysics and Planetary Physics, Lawrence Livermore National Laboratory, 7000 East Ave, 413 Livermore, CA 94551-9900, USA

Corresponding author: O. Wucknitz, olaf@astro.physik.uni-potsdam.de

Received: 28 January 2003
Accepted: 14 April 2003

Abstract

We present the first separate spectra of both components of the small-separation double QSO HE 0512–3329 obtained with HST/STIS in the optical and near UV. The similarities especially of the emission line profiles and redshifts strongly suggest that this system really consists of two lensed images of one and the same source.
The emission line flux ratios are assumed to be unaffected by microlensing and are used to study the differential extinction effects caused by the lensing galaxy. Fits of empirical laws show that the extinction properties seem to be different on both lines of sight. With our new results, HE 0512–3329 becomes one of the few extragalactic systems which show the 2175 Å absorption feature, although the detection is only marginal.
We then correct the continuum flux ratio for extinction to obtain the differential microlensing signal. Since this may still be significantly affected by variability and time-delay effects, no detailled analysis of the microlensing is possible at the moment.
This is the first time that differential extinction and microlensing could be separated unambiguously. We show that, at least in HE 0512–3329, both effects contribute significantly to the spectral differences and one cannot be analysed without taking into account the other. For lens modelling purposes, the flux ratios can only be used after correcting for both effects.