M dwarfs from Hubble Space Telescope star counts .3. The Groth strip

We analyze the disk M dwarfs found in 31 new fields observed with the Wide Field Camera 2 (WFC2) on the Hubble Space Telescope, together with the sample previously analyzed from 22 WFC2 fields and 162 prerepair Planetary Camera 1 fields. The new observations, which include the 28 high-latitude fields comprising the Large Area Multi-Color Survey (Groth Strip), increase the total sample to 337 stars, and more than double the number of late M dwarfs (M-v > 13.5) from 23 to 47. The mass function changes slope at M similar to 0.6 M., from a near-Salpeter power-law index of alpha = -1.21 to alpha = 0.44. In both regimes, the mass function at the Galactic plane is given by (DN)-N-2/d log M dV = 8.1 x 10(-2) pc(-3)(M/0.59 M.)(alpha) The correction for secondaries in binaries changes the low-mass index from alpha = 0.44 to alpha similar to 0.1. If the Salpeter slope continued to the hydrogen-burning limit, we would expect 500 stars in the last four bins (14.5 < M-v < 18.5), instead of the 25 actually detected. The explanation of the observed microlensing rate toward the Galactic bulge requires either a substantial population of bulge brown dwarfs or that the disk and bulge mass functions are very different for stars with M less than or similar to 0.5 M..