Galaxy Zoo: dust and molecular gas in early-type galaxies with prominent
  dust lanes
  We study dust and associated molecular gas in 352 nearby early-type galaxies
(ETGs) with prominent dust lanes. 65% of these `dusty ETGs' (D-ETGs) are
morphologically disturbed, suggesting a merger origin. This is consistent with
the D-ETGs residing in lower density environments compared to the controls
drawn from the general ETG population. 80% of D-ETGs inhabit the field
(compared to 60% of the controls) and <2% inhabit clusters (compared to 10% of
the controls). Compared to the controls, D-ETGs exhibit bluer UV-optical
colours (indicating enhanced star formation) and an AGN fraction that is more
than an order of magnitude greater (indicating higher incidence of nuclear
activity). The clumpy dust mass residing in large-scale features is estimated,
using the SDSS r-band images, to be 10^{4.5}-10^{6.5} MSun. A comparison to the
total (clumpy + diffuse) dust masses- calculated using the far-IR fluxes of 15%
of the D-ETGs that are detected by the IRAS- indicates that only ~20% of the
dust resides in these large-scale features. The dust masses are several times
larger than the maximum value expected from stellar mass loss, ruling out an
internal origin. The dust content shows no correlation with the blue
luminosity, indicating that it is not related to a galactic scale cooling flow.
No correlation is found with the age of the recent starburst, suggesting that
the dust is accreted directly in the merger rather than being produced in situ
by the triggered star formation. Using molecular gas-to-dust ratios of ETGs in
the literature we estimate that the median current and initial molecular gas
fraction are ~1.3% and ~4%, respectively. Recent work suggests that the merger
activity in nearby ETGs largely involves minor mergers (mass ratios between
1:10 and 1:4). If the IRAS-detected D-ETGs form via this channel, then the
original gas fractions of the accreted satellites are 20%-44%. [Abridged]
