<i>α</i> -MC: Self-consistent <i>α</i> -enhanced Stellar Population Models Covering a Wide Range of Age, Metallicity, and Wavelength

Abstract We present new stellar population models, α -MC, self-consistently taking into account nonsolar [ α /Fe] abundances for both isochrones and stellar spectra. The α -MC models are based on α -enhanced MIST isochrones and C3K spectral libraries, which are publicly available in FSPS. Our new models cover a wide range of ages ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>log</mml:mi> <mml:mo stretchy="false">(</mml:mo> <mml:mi>age</mml:mi> <mml:mo>/</mml:mo> <mml:mi>yr</mml:mi> <mml:mo stretchy="false">)</mml:mo> <mml:mo>=</mml:mo> <mml:mn>5.0</mml:mn> <mml:mo>-</mml:mo> <mml:mspace width="0.1em"/> <mml:mn>10.3</mml:mn> </mml:math> ), metallicities ([Fe/H] = [−2.5, +0.5] in steps of 0.25, [ α /Fe] = −0.2, +0.0, +0.2, +0.4, +0.6), and wavelengths (0.1–2.5 μ m). We investigate the separate and combined effects of α -enhanced isochrones and stellar spectral libraries on simple stellar populations, including their broadband colors, spectral indices, and full spectra. We find that the primary effect of α -enhancement in isochrones is to lower the overall continuum levels and redden the continuum shapes, while α -enhancement in stellar spectra mainly affects individual spectral lines. At constant [Fe/H], α -enhancement has significant impacts on the broadband colors by ∼0.1–0.4 mag across all ages (0.01–10 Gyr). The effects of α -enhancement on colors at fixed [Z/H] are smaller, by ∼0.1–0.2 mag. The spectral indices involving α -elements, Ca4227 and Mg b , increase with [ α /Fe] (both at fixed [Fe/H] and fixed [Z/H]) due to enhanced α -abundances. At constant [Fe/H], α -enhancement weakens most Fe-sensitive and hydrogen Balmer lines. Our new self-consistent α -enhanced models will be essential in deriving accurate physical properties of high-redshift galaxies, where α -enhancement is expected to be common.