ALMA detection of the dusty object silhouetted against the S0 galaxy NGC
  3269 in the Antlia cluster
  An intriguing silhouette of a small dust patch can be seen against the disk
of the S0 galaxy NGC 3269 in the Antlia cluster in optical images. The images
do not provide any clue as to whether the patch is a local Jupiter mass-scale
cloudlet or a large extragalactic dust complex. We aim to resolve the nature of
this object: is it a small Galactic cloudlet or an extragalactic dust complex?
ALMA and APEX spectroscopy and Gemini GMOS long-slit spectroscopy were used to
measure the velocity of the patch and the NGC 3269 disk radial velocity curve.
A weak 16$\pm$2.5 km/s wide $^{12}$CO (2-1) T$_{MB}$ 19$\pm$2.5 mK line in a
2.0" by 2.12" beam associated with the object was detected with ALMA. The
observed heliocentric velocity, V$_r$,hel = 3878$\pm$5.0km/s, immediately
establishes the extragalactic nature of the object. The patch velocity is
consistent with the velocity of the nucleus of NGC 3269, but not with the
radial velocity of the NGC 3269 disk of the galaxy at its position. The
$\sim$4" angular size of the patch corresponds to a linear size of $\sim$1 kpc
at the galaxy's Hubble distance of (d/50.7 Mpc)$^2$ Msun, while the attenuation
derived from the optical spectrum implies a dust mass of $\sim$2.6x10$^4$
(d/50.7 Mpc)$^2$ Msun. The derived attenuation ratio A'$_B$/(A'$_B$-A'$_R$) of
1.6$\pm$0.11 is substantially lower than the corresponding value for the mean
Milky Way extinction curve for point sources (2.3). We established the
extragalactic nature of the patch, but its origin remains elusive. One
possibility is that the dust patch is left over from the removal of
interstellar matter in NGC 3269 through the interaction with its neighbour, NGC
3268.
