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

TL;DR

This study uses advanced spectral modeling to reveal a super-Salpeter initial mass function in the core of NGC 1407, indicating a higher proportion of low-mass stars than previously assumed in massive galaxies.

Contribution

It introduces flexible, non-parametric IMF models to spectral analysis, demonstrating their effectiveness in accurately recovering the IMF shape in galaxy centers.

Findings

The IMF in NGC 1407's center is a steep power-law with index -2.7.

Flexible models can reliably recover IMF shape with high-quality spectra.

Evidence supports the existence of super-Salpeter IMFs in massive galaxy cores.

Abstract

It is now well-established that the stellar initial mass function (IMF) can be determined from the absorption line spectra of old stellar systems, and this has been used to measure the IMF and its variation across the early-type galaxy population. Previous work focused on measuring the slope of the IMF over one or more stellar mass intervals, implicitly assuming that this is a good description of the IMF and that the IMF has a universal low-mass cutoff. In this work we consider more flexible IMFs, including two-component power-laws with a variable low-mass cutoff and a general non-parametric model. We demonstrate with mock spectra that the detailed shape of the IMF can be accurately recovered as long as the data quality are high (S/N$\gtrsim300$) and cover a wide wavelength range (0.4um-1.0um). We apply these flexible IMF models to a high S/N spectrum of the center of the massive elliptical galaxy NGC 1407. Fitting the spectrum with non-parametric IMFs, we find that the IMF in the center shows a continuous rise extending toward the hydrogen-burning limit, with a behavior that is well-approximated by a power-law with an index of -2.7. These results provide strong evidence for the existence of extreme (super-Salpeter) IMFs in the cores of massive galaxies.