The interstellar medium in Andromeda's dwarf spheroidal galaxies: I.
  Content and origin of the interstellar dust
  Dwarf spheroidal galaxies are among the most numerous galaxy population in
the Universe, but their main formation and evolution channels are still not
well understood. The three dwarf spheroidal satellites (NGC147, NGC185, and
NGC205) of the Andromeda galaxy are characterised by very different
interstellar medium (ISM) properties, which might suggest them being at
different galaxy evolutionary stages. While the dust content of NGC205 has been
studied in detail by De Looze et al. (2012), we present new Herschel dust
continuum observations of NGC147 and NGC185. The non-detection of NGC147 in
Herschel SPIRE maps puts a strong constraint on its dust mass (< 128 Msun). For
NGC185, we derive a total dust mass M_d = 5.1 x 10^3 Msun, which is a factor of
~2-3 higher than that derived from ISO and Spitzer observations and confirms
the need for longer wavelength observations to trace more massive cold dust
reservoirs. We, furthermore, estimate the dust production by asymptotic giant
branch (AGB) stars and supernovae (SNe). For NGC147, the upper limit on the
dust mass is consistent with expectations of the material injected by the
evolved stellar population. In NGC185 and NGC205, the observed dust content is
one order of magnitude higher compared to the estimated dust production by AGBs
and SNe. Efficient grain growth, and potentially longer dust survival times
(3-6 Gyr) are required to account for their current dust content. Our study
confirms the importance of grain growth in the gas phase to account for the
current dust reservoir in galaxies.
