The global gas and dust budget of the Small Magellanic Cloud

In order to understand the evolution of the interstellar medium (ISM) of a galaxy, we have analysed the gas and dust budget of the Small Magellanic Cloud (SMC). Using the Spitzer Space Telescope, we measured the integrated gas mass-loss rate across asymptotic giant branch (AGB) stars and red supergiants (RSGs) in the SMC, and obtained a rate of 1.4 × 10−3 M yr−1. This is much smaller than the estimated gas ejection rate from Type II supernovae (SNe) (2–4 × 10−2 M yr−1). The SMC underwent an increase in star formation rate in the last 12 Myr, and consequently the galaxy has a relatively high SN rate at present. Thus, SNe are more important gas sources than AGB stars in the SMC. The total gas input from stellar sources into the ISM is 2–4 × 10−2 M yr−1. This is slightly smaller than the ISM gas consumed by star formation (∼8 × 10−2 M yr−1). Star formation in the SMC relies on a gas reservoir in the ISM, but eventually the star formation rate will decline in this galaxy, unless gas infalls into the ISM from an external source. The dust injection rate from AGB and RSG candidates is 1 × 10−5 M yr−1. Dust injection from SNe is in the range 0.2–11 × 10−4 M yr−1, although the SN contribution is rather uncertain. Stellar sources could be important for ISM dust (3 × 105 M ) in the SMC, if the dust lifetime is about 1.4 Gyr. We found that the presence of poly-aromatic hydrocarbons (PAHs) in the ISM cannot be explained entirely by carbon-rich AGB stars. Carbon-rich AGB stars could inject only 7 × 10−9 M yr−1 of PAHs at most, which could contribute up to 100 M of PAHs in the lifetime of a PAH. The estimated PAH mass of 1800 M in the SMC cannot be explained. Additional PAH sources, or ISM reprocessing should be needed.