We present the recently derived Wolf et al .
( 2009 ) mass estimator , which is applicable for spherical pressure-supported stellar systems spanning over ten orders of magnitude in luminosity , as a tool to test galaxy formation theories .
We show that all of the Milky Way dwarf spheroidal galaxies ( MW dSphs ) are consistent with having formed within a halo of mass approximately 3 \times 10 ^ { 9 } \ > { M } _ { \odot } in \Lambda CDM cosmology .
The faintest MW dSphs seem to have formed in dark matter halos that are at least as massive as those of the brightest MW dSphs , despite the almost five orders of magnitude spread in luminosity .
We expand our analysis to the full range of observed pressure-supported stellar systems and examine their I-band mass-to-light ratios \Upsilon ^ { I } _ { { } _ { 1 / 2 } } .
The \Upsilon ^ { I } _ { { } _ { 1 / 2 } } vs. half-light mass { M } _ { { } _ { 1 / 2 } } relation for pressure-supported galaxies follows a U-shape , with a broad minimum near \Upsilon ^ { I } _ { { } _ { 1 / 2 } } \simeq 3 that spans dwarf elliptical galaxies to normal ellipticals , a steep rise to \Upsilon ^ { I } _ { { } _ { 1 / 2 } } \simeq 3 , 200 for ultra-faint dSphs , and a more shallow rise to \Upsilon ^ { I } _ { { } _ { 1 / 2 } } \simeq 800 for galaxy cluster spheroids .