Population Structure of the Small Magellanic Cloud

Photographic photometry using prime-focus AAT plates has been carried out in the western regions of the Small Magellanic Cloud. To calibrate the photographic emulsions a new electronographic standard sequence was established. However this sequence disagreed in scale with the existing electronographic sequence in K3 due to Walker and the conflict was resolved by photoelectric photometry using the twin photometer on the AAT, which showed that Walker's sequence was correct. A combination of the new photoelectric photometry, Walker's K3 photometry and some photoelectric photometry by Gascoigne was then used to calibrate the photography.The colour-magnitude diagrams which resulted showed the existence of three separate populations, all well defined in age. A group of age ≤10⁸ years was found near the optical centre which decayed very rapidly in surface density away from this centre. A second group of age 8 x 10⁸ years was found to be more widespread and to be the dominant stellar population at a distance of 1 Kpc from the centre of the SMC. This latter population includes the cluster NGC152. The third group, which included the cluster K3, was found to dominate further out, at 2 Kpc from the optical centre, and to be of age 4 x 10⁹ years.Thus the picture envisaged is of concentric stellar groups whose radius varies directly with age. A model to account for this structure is outlined, involving slow contraction of the HI in the SMC disc, which is stabilised by rotation, and at certain well defined instants widespread star formation is initiated, after which the HI unprocessed into stars continues its contraction. As mechanism for the initiation of star formation the perigalactic passage of the Magellanic Clouds in their orbit about the Galaxy is invoked and it is shown that the last two perigalactica coincide in time with the formation of the two oldest stellar groups discovered in the photometry