The Stellar Initial Mass Function and Population Properties of M89 from Optical and NIR Spectroscopy: Addressing Biases in Spectral Index Analysis

Abstract

The complexity of constraining the stellar initial mass function (IMF) in early-type galaxies cannot be overstated, given the necessity of both very high signal-to-noise (S/N) data and the difficulty of breaking the strong degeneracies that occur among several stellar population parameters including age, metallicity and elemental abundances. With this paper, the second in a series, we present a detailed analysis of the biases that can occur when retrieving the IMF shape by exploiting both optical and NIR IMF sensitive spectral indices. As a test case, here we analyze data for the nearby galaxy M89, for which we have high S/N spectroscopic data that cover the 3500-9000Å spectral region and allow us to study the radial variation of the stellar population properties out to 1 R_e. Carrying out parallel simulations that mimic the retrieval of all the explored stellar parameters from a known input model, we quantify the amount of bias at each step of our analysis. From more general simulations we conclude that to accurately retrieve the IMF, it is necessary not only to retrieve accurate estimates of the age and metallicity, but also of all the elemental abundances that the spectral index fits are sensitive to. With our analysis technique applied to M89, we find consistency with a bottom-heavy IMF with a negative gradient from the center to half R_e when using the Conroy et al. 2018 as well as Vazdekis et al. 2016 EMILES stellar population models. We find agreement both with a parallel full spectral fitting of the same data and with literature results.

35 pages, 21 figures. Accepted for publication in ApJ

References in corpus (15)

• Improving the full spectrum fitting method: accurate convolution with Gauss-Hermite functions
• Evolutionary Stellar Population Synthesis with MILES. Part I: The Base Models and a New Line Index System
• Molecfit: A general tool for telluric absorption correction. I. Method and application to ESO instruments
• A systematic variation of the stellar initial mass function in early-type galaxies
• UV-extended E-MILES stellar population models: young components in massive early-type galaxies
• Molecfit: A general tool for telluric absorption correction II. Quantitative evaluation on ESO-VLT X-Shooter spectra
• Evolutionary stellar population synthesis with MILES - II. Scaled-solar and α-enhanced models
• Radial variations in the stellar initial mass function of early-type galaxies
• Variations of the stellar initial mass function in the progenitors of massive early-type galaxies and in extreme starburst environments
• IMF and [Na/Fe] abundance ratios from optical and NIR Spectral Features in Early-type Galaxies
• The Stellar Initial Mass Function in Early-Type Galaxies From Absorption Line Spectroscopy. IV. A Super-Salpeter IMF in the center of NGC 1407 from Non-Parametric Models
• Radial gradients in initial mass function sensitive absorption features in the Coma brightest cluster galaxies
• The initial mass functions of M31 and M32 through far red stellar absorption features
• What does (not) drive the variation of the low-mass end of the stellar initial mass function of early-type galaxies
• Comparing IMF-sensitive indices of intermediate-mass quiescent galaxies in various environments