Galactic Stellar and Substellar Initial Mass Function

We review recent determinations of the present day and initial mass functionsin various components of the Galaxy, disk, spheroid, young and globularclusters. As a general feature, the IMF is well described by a power-law formfor $m\ga 1 \msol$ and a lognormal form below. The extension of the disk IMFinto the brown dwarf (BD) regime is in good agreement with observations andyields a disk BD number-density comparable to the stellar one $\sim 0.1 \pc3$.The IMF of young clusters is found to be consistent with the disk field IMF,providing the same correction for unresolved binaries. The spheroid IMF relieson much less robust grounds. Within all the uncertainties, it is found to besimilar to the one derived for globular clusters, and is well represented alsoby a lognormal form with a characteristic mass slightly larger than for thedisk. The IMF characteristic of early star formation remains undetermined, butdifferent observational constraints suggest that it does not extend below $\sim1 \msol$. These IMFs allow a reasonably robust determination of the Galacticpresent-day and initial stellar and brown dwarf contents. They also haveimportant galactic implications in yielding more accurate mass-to-light ratiodeterminations. The M/L ratios obtained with the disk and the spheroid IMFyield values 1.8 and 1.4 smaller than a Salpeter IMF, respectively. Thisgeneral IMF determination is examined in the context of star formation theory.(shortened)Comment: Published version: PASP, 2003, 115, 763. A few changes. References to recent work adde