ELT Stellar Populations Science Near IR photometry and spectroscopy of resolved stars in nearby galaxies provides a way to extract their entire star formation and chemical enrichment histories Crowding is the limiting factor for photometry of bulges, disks, and the main bodies of elliptical galaxies; sensitivity will be limiting factor for regions with lower surface brightness (independent of aperture size) M31 observed with Gemini N+NIRI/Altair (Olsen et al. 2006) K HB ~23.5

Issues addressed through simulations Scientific Issues: Sample size needed Field size needed Filters needed Photometric Issues: Spatial variability of PSF Time variability of PSF Absolute calibration

PSF Simulation AO Error sources included 1.Finite number of guide stars and DMs 2.Finite spatial resolution of wavefront sensors and DMs Sampled on 7x7 20” wide grid in IJHK for 20-m, 30-m, 50-m, and 100-m telescopes Sampled over 12-minute average intervals from hour- long “typical” observation with TMT MASS/DIMM 5 atmospheric profiles  4 filters  49 positions  4 telescopes = 3920 PSFs

30-m J PSF grid, profile 1 Simulating a realistic observation Courtesy of Richard Clare

Simulation procedure Select appropriate population mix Pick stars from stellar isochrones and place in image, making sure to simulate stars well below crowding limit Convolve image with PSFs (49  5 convolutions, combine through weighted average) Add sky background and noise Perform PSF-fitting photometry Correct photometry for Strehl ratio using profile 1 or average of profiles 1 and 5 Derive best-fit population mix

“Raw” photometry With 1 PSF observationWith 2 PSF observations“Perfect” photometry 20-m to 100-m: M31 Bulge K HB ~23.5, K MSTO ~27

20-m to 100-m: M31 Bulge 20-m30-m 50-m100-m

An M31 Survey: 20-m Local Group Survey (Massey et al. 2002) image Name r()  K B/D K lim Time(s) F Bulge F F F Bulge F F F Disk F F Disk hour exposure, S/N=5: J: 28.9 H: 28.0 K: 27.0 K HB ~23.5, K MSTO ~27