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Description
The space density of late M dwarfs, sub-types M7 to M9.5, is not well determined. We have applied the photo-type method of Skrzypek et al. to iz photometry from SDSS and YJHK photometry from UKIDSS, over an effective area of 3070deg^2^, to produce a new, bright J(Vega)<17.5, homogeneous sample of 33665 M7 to M9.5 dwarfs. The typical S/N of each source summed over the 6 bands is >100. Classifications are provided to the nearest half spectral sub-type. Through comparison with the classifications in the BUD spectroscopic sample of Schmidt et al. (2010, Cat. J/AJ/139/1808), the typing is shown to be accurately calibrated to the BUD classifications, with a precision better than 0.5 sub-types rms, i.e. is as precise as good spectroscopic classification. Sources with large chisq>20 include several catalogued late-type subdwarfs. The new sample of late M dwarfs is highly complete, but there is a bias in the classification of rare peculiar blue or red objects. For example L subdwarfs are misclassified towards earlier types by approximately two spectral sub-types. We estimate that this bias affects only ~1% of sources. Therefore the sample is well suited for measuring the luminosity function, as well as investigating the softening towards the Galactic plane of the exponential variation of density with height.
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