The interstellar medium in Andromeda's dwarf spheroidal galaxies – II. Multiphase gas content and ISM conditions
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abstract
We make an inventory of the interstellar medium material in three
low-metallicity dwarf spheroidal galaxies of the Local Group (NGC147, NGC185
and NGC205). Ancillary HI, CO, Spitzer IRS spectra, H{\alpha} and X-ray
observations are combined to trace the atomic, cold and warm molecular, ionised
and hot gas phases. We present new Nobeyama CO(1-0) observations and Herschel
SPIRE FTS [CI] observations of NGC205 to revise its molecular gas content.
We derive total gas masses of M_gas = 1.9-5.5x10^5 Msun for NGC185 and M_gas
= 8.6-25.0x10^5 Msun for NGC205. Non-detections combine to an upper limit on
the gas mass of M_gas =< 0.3-2.2x10^5 Msun for NGC147. The observed gas
reservoirs are significantly lower compared to the expected gas masses based on
a simple closed-box model that accounts for the gas mass returned by planetary
nebulae and supernovae. The gas-to-dust mass ratios GDR~37-107 and GDR~48-139
are also considerably lower compared to the expected GDR~370 and GDR~520 for
the low metal abundances in NGC 185 (0.36 Zsun) and NGC205 (0.25 Zsun),
respectively.
To simultaneously account for the gas deficiency and low gas-to-dust ratios,
we require an efficient removal of a large gas fraction and a longer dust
survival time (~1.6 Gyr). We believe that efficient galactic winds (combined
with heating of gas to sufficiently high temperatures in order for it to escape
from the galaxy) and/or environmental interactions with neighbouring galaxies
are responsible for the gas removal from NGC147, NGC185 and NGC205.
