It is well-established that many spiral galaxies, such as the Milky Way and M31, host metal-poor stellar halos. The properties of these halos place important constraints on the hierarchical formation and assembly histories of these galaxies. Observations of old globular clusters in early-type galaxies find ubiquitous evidence for bimodal color distributions (blue and red), suggesting the presence of two underlying populations of field stars: metal-poor and metal-rich. However, no direct evidence has been found for bimodal stellar populations, nor specifically for a distinct metal-poor stellar halo, in any massive early-type galaxy. We propose a study of red giant branch (RGB) stellar metallicities in the halo of NGC 3115, a nearby (9 Mpc) lenticular that is among the closest normal massive early-type galaxies, and which hosts a particularly well-characterized bimodal color distribution of globular clusters. Our proposed observations, across three fields extending out to the distant outer halo of the galaxy, will provide the first unambiguous test for the presence of a metal-poor stellar halo in a massive early-type galaxy, and in addition will measure the stellar halo density and metallicity gradients with galactocentric radius. Our goals are to detect a bimodal metallicity distribution in the stellar halo of NGC 3115, and to measure the gradients with radius of the amplitudes and mean metallicities of the bimodal peaks. We will observe three fields near the minor axis: one in the "near" halo, a middle field where we expect the contribution from metal-poor and metal-rich stars to be approximately equal, and a distant field where very few metal-rich stars should be present. These pointings are located at projected radii of 6, 12, and 18 arcmin (~16, 32, 47 kpc). We observe along the minor axis because the metal-poor globular clusters (GCs) have a rounder spatial distribution than the metal-rich GC system (which is elongated along the major axis), so the metal-poor red giants are expected to "emerge" sooner along the minor than major axis. To do this as efficiently as possible, we will observe simultaneously with ACS (F606W) and WFC3 (F110W), doing half of the observations with two visits, then when HST has rolled to the opposite side, doing the other half of the observations in two more visits. The middle field gets twice the exposure time of the other two fields, as is necessary to distinguish the two metallicity peaks of nearly equal amplitude that are expected in this field. The choice of filters is motivated by the requirements of a wide wavelength baseline (to separate out the metallicity peaks as much as possible), and matching between instrumental efficiency and red giant branch spectral energy distributions. Previous work on this topic has focused on the F606W−F814W combination (similar to V−I), but the advent of the F110W filter with its redder wavelength and wide bandpass allows for significant improvements in measuring metallicity distributions. The limiting factor in the observations is the exposure time in F606W, which needs to go deep enough to reach even the reddest red giant branch stars. Our goal for the long, middle pointing is to obtain the complete metallicity distribution function in this region, allowing for metallicities up to [Fe/H] = −0.2, which we detect with S/N=5 per star. For the other two, shallower pointings, the primary objective is to characterize the metal-poor peak, which does not require reaching the most metal-rich end of the distribution. At the distance and foreground reddening of NGC 3115 the brightest of these metal-rich giants will have F606W~29.2, which by the ACS ETC and previous observations requires 31500 sec of exposure time to reach. RESOLVED STELLAR POPULATIONS 12 false 0.0 0.0 false false false false SUPPORTED GALAXY HALO LENTICULAR This object was generated by the targetselector and retrieved from the SIMBAD database. J2000 J2000 GALAXY HALO LENTICULAR This object was generated by the targetselector and retrieved from the SIMBAD database. J2000 J2000 GALAXY HALO LENTICULAR This object was generated by the targetselector and retrieved from the SIMBAD database. J2000 J2000 ACS/WFC Imaging No No F606W WFC3/IR Imaging No No F110W ACS/WFC Imaging No No F606W WFC3/IR Imaging No No F110W ACS/WFC Imaging No No F606W WFC3/IR Imaging No No F110W ACS/WFC Imaging No No F606W WFC3/IR Imaging No No F110W POS-TARG DITHER DITHER 3 3.011 ACS-WFC-DITHER-LINE false false 0.170 2 85.28 POS-TARG ACS-WFC-DITHER-LINE 47.23 2 true POS-TARG DITHER DITHER 4 3.011 ACS-WFC-DITHER-BOX false 69.05 false 0.187 0.265 2 85.28 POS-TARG ACS-WFC-DITHER-LINE 20.67 3 true The scheduling data was generated by CASM 20.2. CASM was run in Phase II mode. 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CASM was run in Phase II mode. 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CASM was run in Phase II mode. 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CASM was run in Phase II mode. 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CASM was run in Phase II mode. 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CASM was run in Phase II mode. (Tic-Visit :proposal-id 13048 :visit-id "06" :proposal-version -1 :trans-version "20120604" :date "03-Jul-2012 21:29:04.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 3 :saa-model "30" :si-bright-earth-limb-angle 20.0 :data-volume 3272128 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 13048 :visit-id "06" :obset-id "06" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 13048 :visit-id "06" :orbit-number 1 :pcs-time 333 :si-vis-utilization 2900 :al-overhead-at-end 0 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3233 :pcs-gap 0 ) (Tic-Orbit :proposal-id 13048 :visit-id "06" :orbit-number 2 :pcs-time 222 :si-vis-utilization 3004 :al-overhead-at-end 0 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3233 :pcs-gap 0 ) (Tic-Orbit :proposal-id 13048 :visit-id "06" :orbit-number 3 :pcs-time 222 :si-vis-utilization 3004 :al-overhead-at-end 0 :internal-dur-at-end 619 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3233 :pcs-gap 0 ) true 3 0 The scheduling data was generated by CASM 20.2. 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CASM was run in Phase II mode. 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