Interstellar Grains in Elliptical Galaxies: Infrared Emission

We calculate infrared luminosities and surface brightness profiles from dust in three bright model elliptical galaxies lying on the fundamental plane. The emission is assumed to arise from dust grains that are being continuously formed in dusty outflows from evolving red giant stars and continuously destroyed by sputtering in the hot interstellar medium. The ratios of the mass of this "distributed" dust to that of the hot interstellar gas are typically several hundred times smaller than those in the interstellar medium of our own Galaxy. Equilibrium grain temperatures are presented as functions of galactic radius for several assumed grain compositions. In computing the infrared emission, we include the effects of stochastic heating of the smallest grains. Although the infrared emission from this distributed dust makes a nontrivial contribution to the infrared luminosities detected by the IRAS satellite, most elliptical galaxies have 60 or 100 micron luminosities that exceed that of the distributed dust by factors of 10-100. In addition, the ratio of 100 micron flux to 60 micron flux in observed galaxies is higher for most ellipticals than for those expected from distributed dust. We conclude that elliptical galaxies typically contain an "extra" component of dust which is cooler than the average dust grains in the distributed component. The cool extra dust component could be optically thin to galactic starlight provided it is located in the outer parts of ellipticals, at or beyond the effective radius, where heating by starlight is less efficient. We describe a procedure for estimating the total mass of this dust and its characteristic location in the galaxy. The estimated dust mass is sensitive to the assumed grain composition and is only a lower limit, since additional cold dust could be present that emits at wavelengths not detectable by IRAS. Alternatively, the extra dust could reside within the dark, dusty clouds often observed in ellipticals, shielded from direct irradiation by galactic starlight. Although cooler than distributed dust grains, these shielded grains must be heated by some mechanism. Heating by galactic X-rays seems inadequate, but the energy provided by stars embedded in the clouds is energetically promising.