We use the large homogeneous sample of late M dwarfs , M7 to M9.5 , of \citet Ahmed19 matched to Gaia DR2 , to measure the relation between absolute magnitude and spectral type , and to infer the multiplicity fraction of the population , and the distribution of mass ratios in the binary systems .
Binaries are identified photometrically as overluminous sources .
In order to define a sample that is unbiased with respect to multiplicity we use distance limits that are a function of G - J colour to define a volume-complete sample of 2706 systems .
The G - J colours are very precise , with random errors all less than 0.02 .
We measure absolute magnitudes M _ { J } that are on average 0.5 mag .
brighter than previous determinations .
We find evidence that the discrepancies arise from differences in spectral types in different samples .
The measured binary fraction is 16.5 \pm 0.8 \% , of which 98 \% are unresolved : both values are consistent with results of previous studies .
The distribution of excess flux in the binaries , compared to the singles , is used to infer the mass ratio distribution f ( q ) \propto q ^ { \gamma } , where q = M _ { s } / M _ { p } .
We infer a very steep distribution over this spectral range , with \gamma > 10 ( 99 \% probability ) .
This says that unresolved ultracool M dwarf binaries reside almost exclusively in equal mass systems , and implies that the spectral types of the unresolved binaries match to with 0.5 spectral subtypes .
The intrinsic scatter in absolute magnitude M _ { J } for ultracool M dwarfs at fixed G - J colour is measured to be 0.21 mag .