A deep optical survey of a small region in Aquarius - I. Stellar photometry and galactic structure

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Abstract
We have identified and classified as accurately as possible all objects with |$B\leqslant20.0$| in 0.31 deg2 centred on the 1950.0 coordinates α = 22h 03m 04s.5, δ = −18°55′03″(l = 36°5, b = −51°1). We determined the B and V magnitudes of 601 normal stars and 2 white dwarfs to this magnitude limit and used these data to investigate galactic structure in this direction. Absolute magnitudes were estimated from the observed colour using the main-sequence relation between MV and B–V on the assumption that close to the sun evolved Population II stars are not a significant component. Within 2 kpc of the sun we found a disc scale height of about 300 pc for |$4\leqslant M_\text V\leqslant6$|⁠, consistent with determinations in other directions. However, at greater distances out to 5 kpc, where the counts are complete for only |$4\leqslant M_\text V\leqslant5$|⁠, the density decreases much less steeply with distance, implying a rather flat spheroid component or a thick disc similar to that which Gilmore & Reid (1983) found in the direction of the south galactic pole. However, our derived parameters for this population are noticeably different from Gilmore & Reid and depend considerably on whether there is a metallicity gradient perpendicular to the plane of the galaxy. We also find important differences with the standard model of Bahcall & Soneira (1981a, b) for our direction. Our conclusions about the spheroid are sensitive to the fraction of evolved stars present in our sample and therefore should be helpful in refining the next series of galactic models.

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