tokens: wavelength. The blue line is a fit to the data. Extinction according to Cardelli et al. (1989) is shown for A v / E ( B − V ) values of 1.5 (green triangles, dashed line) and 3.1 (red diamonds, continuous line), respectively. Dirsch et al. (2005) estimated the maximum and the mean B -band attenuation in a 4′′ diameter area in the patch to be A ′ B ∼10 and 04, respectively. Their maximum attenuation is somewhat higher than the value A ′ B 005 obtained from the GMOS spectrum. This difference is possibly due to the positioning of the GMOS spectrum slit that does not pass exactly through the densest part of the patch, and also to the selected off position. Had the off position been chosen northeast of the patch, the attenuation would be 10% lower, but the wavelength dependence of the attenuation would not change. Our A ′ B /( A ′ B − A ′ R ) ratio (1.6 ± 0.11) is the same as (1.6 ± 0.25) reported in Dirsch et al. (2005) and it is, again, substantially lower than the corresponding value for the mean Milky Way extinction curve for point sources, A B /( A B − A R ) = 2.3. The background surface brightness attenuation in a clumpy foreground screen has been modelled by Natta Panagia (1984) . Clumping causes the reddening curve to flatten, meaning R ′ V and A ′ B /( A ′ B − A ′ R ) increase as compared to a homogeneous screen. Thus, clumping cannot explain the steeper wavelength dependence as observed for the patch. In several cases, dust lanes or patches have been observed in early-type galaxies with R ′ V values as low as 1.9 ( IC 4320 , Warren-Smith Berry 1983 ) and 2.15 ( NGC 5626 , Goudfrooij et al. 1994 ). Such cases have been interpreted as evidence for dust-grain size distributions with mean and maximum grain sizes smaller than those for the mean Milky Way extinction curve (with R V = 3.1). The dust mass for lanes and patches in galaxies has frequently been estimated using the following formula: where ⟨ A ′ B ⟩ is the mean attenuation as measured over the object’s area of Σ ( Sadler Gerhard 1985 ; van Dokkum Franx 1995 ). For the Milky Way’s mean dust properties, the value of the mass absorption coefficient Γ B is Γ = 4 × 10 4 mag cm 2 g −1 ∼ 8 × 10 −6 mag kpc 2 M ⊙ −1 . Adopting as the mean attenuation over the area of Σ = 4′′ × 4′′ of the patch, we obtain the following for its dust mass: M d = 4.8 × 10 4 ( d /50.7 Mpc) 2 M ⊙ . Goudfrooij et al. (1994) presented a method to estimate the value of Γ B for extinction (attenuation) curves with R ′ V values other than 3.1. For NGC 5626 and IC 4320, which have the smallest values of R ′ V in their sample (2.15 and 2.08), they found that the dust masses as obtained from the above formula have to be multiplied by a factor of ∼0.66. In the case of the patch with R ′ V = 1.5, we estimate that a correction factor of ∼0.5 could be appropriate. Thus the dust mass estimate for the patch becomes M d ∼ 2.6 × 10 4 ( d /50.7 Mpc) 2 M ⊙ , which corresponds to a molecular-gas-to-dust ratio of ∼55. If at the
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