The galactic center stellar population in the near-infrared.

Abstract

Near infrared photometry is presented for 659 stars within 2.5' of the Galactic Center from three epochs of 2.2μm and one epoch of 1.6μm observations. The data were taken with a Rockwell 64 x 64 HgCdTe IR array at the Steward Observatory 1.6m telescope and are complete to m(K) = 11.0. Additional photometry at 3.4μm for a subsample of 258 stars taken with a 58 x 62 InSb array at CTIO is also included. The sample has an average (H-K)(OBS), which is consistent with a late-type population with (H-K)₀ = 0.3 and A(V) = 31.8 mag, and luminosities several magnitudes brighter than the Baade's Window AGB. A variability analysis shows 59 stars are long period variable candidates at the 3σ level. These stars show correlations in (H-K)₀ vs. (K-L)₀ and (K-L)₀ vs. Δm(K) consistent with Mira variables on the AGB. The LPV luminosities are consistent with a population fewx10⁸ yr old which is older than the Galactic Center central cluster stars and younger than bulge stars in Baade's Window, implying the Galactic Center has repeating episodes of star formation. Observations of the CO(v = 2 ← 0) 2.3μm absorption feature taken at the CTIO 4m with the IR Spectrometer Array of the unresolved stellar emission at 9 positions within 30'' of Sgr A* are presented. It is shown that the CO band strength is highly correlated with r(SgrA*) for radii less than R(CO) ≈ 8.5'' (0.28 pc). The Galactic rotation curve has a radial gradient of -16 km s⁻¹ arcsec⁻¹ inside r(SgrA*) ≈ 10'' (0.33 pc) where it reaches a minimum. Estimates of the enclosed mass vs. radius show there is (1.7 ± 0.3) x 10⁶ M(⊙) inside r(SgrA*) = 5.2'' (0.17 pc) with a mass-to-light ratio on the order of 10. Assuming the CO absorption samples stellar emission at each projected radius, these results are strong evidence for a massive black hole at the Galactic Center.