VLT/VIMOS Observations of an Occulting Galaxy Pair: Redshifts and Effective Extinction Curve

B. W. Holwerda; T. Boker; J. J. Dalcanton; W. C. Keel; R. S. de Jong

doi:10.1093/mnras/stt669

paper

VLT/VIMOS Observations of an Occulting Galaxy Pair: Redshifts and Effective Extinction Curve

arXiv:1304.3615 · doi:10.1093/mnras/stt669

Abstract

We present VLT/VIMOS IFU observations of an occulting galaxy pair previously discovered in HST observations. The foreground galaxy is a low-inclination spiral disk, which causes clear attenuation features seen against the bright bulge and disk of the background galaxy. We find redshifts of $z = 0.064 \pm 0.003$ and z=0.065 for the foreground and background galaxy respectively. This relatively small difference does not rule out gravitational interaction between the two galaxies. Emission line ratios point to a star-forming, not AGN-dominated foreground galaxy. We fit the Cardelli, Clayton & Mathis (CCM) extinction law to the spectra of individual fibres to derive slope ( $R_{V}$ ) and normalization ( $A_{V}$ ). The normalization agrees with the HST attenuation map and the slope is lower than the Milky Way relation ( $R_{V} < 3.1$ ), which is likely linked to the spatial sampling of the disk. We speculate that the values of $R_{V}$ point to either coherent ISM structures in the disk larger than usual ( $\sim 9$ kpc) or higher starting values of $R_{V}$ , indicative of recent processing of the dust. The foreground galaxy is a low stellar mass spiral ( $M_{*} \sim 3 \times 1 0^{9} M_{⊙}$ ) with a high dust content ( $M_{dust} \sim 0.5 \times 1 0^{6} M_{⊙}$ ). The dust disk geometry visible in the HST image would explain the observed SED properties of smaller galaxies: a lower mean dust temperature, a high dust-to-stellar mass ratio but relatively little optical attenuation. Ongoing efforts to find occulting pairs with a small foreground galaxies will show how common this geometry is.

16 pages, 3 tables, 13 figures, accepted for publication in MNRAS

VLT/VIMOS Observations of an Occulting Galaxy Pair: Redshifts and Effective Extinction Curve

Abstract

References in corpus (11)

Cited by in corpus (14)