We present the resolved stellar populations in the inner and outer halo of the nearby lenticular galaxy NGC 3115 .
Using deep HST observations , we analyze stars two magnitudes fainter than the tip of the red giant branch ( TRGB ) .
We study three fields along the minor axis of this galaxy , 19 , 37 and 54 kpc from its center – corresponding to 7 , 14 , 21 effective radii ( r _ { e } ) .
Even at these large galactocentric distances , all of the fields are dominated by a relatively enriched population , with the main peak in the metallicity distribution decreasing with radius from { [ Z / H ] } \sim - 0.5 to -0.65 .
The fraction of metal-poor stars ( { [ Z / H ] } < -0.95 ) increases from 17 \% , at 16 - 37 kpc , to 28 \% , at \sim 54 kpc .
We observe a distinct low metallicity population ( peaked at { [ Z / H ] } \sim - 1.3 and with total mass 2 \times 10 ^ { 10 } M _ { \odot } \sim 14 \% of the galaxy ’ s stellar mass ) and argue that this represents the detection of an underlying low metallicity stellar halo .
Such halos are generally predicted by galaxy formation theories and have been observed in several late type galaxies including the Milky Way and M31 .
The metallicity and spatial distribution of the stellar halo of NGC 3115 are consistent with the galaxy ’ s globular cluster system , which has a similar low metallicity population that becomes dominant at these large radii .
This finding supports the use of globular clusters as bright chemo-dynamical tracers of galaxy halos .
These data also allow us to make a precise measurement of the magnitude of the TRGB , from which we derive a distance modulus of NGC 3115 of 30.05 \pm 0.05 \pm 0.10 _ { sys } ( 10.2 \pm 0.2 \pm 0.5 _ { sys } Mpc ) .