We present Herschel observations of 62 early-type galaxies ( ETGs ) , including 39 galaxies morphologically classified as S0+S0a and 23 galaxies classified as ellipticals using SPIRE at 250 , 350 and 500 \mu m as part of the volume-limited Herschel Reference Survey ( HRS ) .
We detect dust emission in 24 % of the ellipticals and 62 % of the S0s .
The mean temperature of the dust is \langle T _ { d } \rangle = 23.9 \pm 0.8 K , warmer than that found for late-type galaxies in the Virgo Cluster .
The mean dust mass for the entire detected early-type sample is { log } M _ { d } = 6.1 \pm 0.1 M _ { \odot } with mean dust-to-stellar mass ratio of { log } ( M _ { d } / M _ { * } ) = -4.3 \pm 0.1 .
Including the non-detections , these parameters are { log } M _ { d } = 5.6 \pm 0.1 and { log } ( M _ { d } / M _ { * } ) = -5.1 \pm 0.1 respectively .
The average dust-to-stellar mass ratio for the early-type sample is fifty times lower , with larger dispersion , than the spiral galaxies observed as part of the HRS , and there is an order of magnitude decline in M _ { d } / M _ { * } between the S0s and ellipticals .
We use UV and optical photometry to show that virtually all the galaxies lie close to the red sequence yet the large number of detections of cool dust , the gas-to-dust ratios and the ratios of far-infrared to radio emission all suggest that many ETGs contain a cool interstellar medium similar to that in late-type galaxies .
We show that the sizes of the dust sources in S0s are much smaller than those in early-type spirals and the decrease in the dust-to-stellar mass ratio from early-type spirals to S0s can not simply be explained by an increase in the bulge-to-disk ratio .
These results suggest that the disks in S0s contain much less dust ( and presumably gas ) than the disks of early-type spirals and this can not be explained simply by current environmental effects , such as ram-pressure stripping .
The wide range in the dust-to-stellar mass ratio for ETGs and the lack of a correlation between dust mass and optical luminosity suggest that much of the dust in the ETGs detected by Herschel has been acquired as the result of interactions , although we show these are unlikely to have had a major effect on the stellar masses of the ETGs .
The Herschel observations tentatively suggest that in the most massive systems , the mass of interstellar medium is unconnected to the evolution of the stellar populations in these galaxies .