The stellar population and initial mass function of NGC 1399 with MUSE

TL;DR

This study uses MUSE data to measure the spatial variation of the stellar initial mass function in NGC 1399, revealing a heavier IMF in the center that becomes more Milky Way-like outward, and reports a central ionised gas filament likely ionised by the galaxy's AGN.

Contribution

It provides the first spatially resolved measurements of the IMF and elemental abundances in NGC 1399 using advanced spectral fitting techniques.

Findings

IMF is heavier than Milky Way in the center

IMF slope decreases with radius, approaching Milky Way values

Detection of a narrow ionised gas filament likely ionised by the AGN

Abstract

We present spatially resolved measurements of the stellar initial mass function (IMF) in NGC 1399, the largest elliptical galaxy in the Fornax Cluster. Using data from the Multi Unit Spectroscopic Explorer (MUSE) and updated state-of-the-art stellar population synthesis models from Conroy et. al 2018, we use full spectral fitting to measure the low-mass IMF, as well as a number of individual elemental abundances, as a function of radius in this object. We find that the IMF in NGC 1399 is heavier than the Milky Way in its centre and remains radially constant at a super-salpeter slope out to 0.7 R$_{e}$. At radii larger than this, the IMF slope decreases to become marginally consistent with a Milky Way IMF just beyond R$_{e}$. The inferred central V-band M/L ratio is in excellent agreement with the previously reported dynamical M/L measurement from Houghton et. al 2006. The measured radial form of the M/L ratio may be evidence for a two-phase formation in this object, with the central regions forming differently to the outskirts. We also report measurements of a spatially resolved filament of ionised gas extending 4 arcseconds (404 pc at $D_{\mathrm{L}}=21.1$ Mpc) from the centre of NGC 1399, with very narrow equivalent width and low velocity dispersion ($65\pm14$ \kms). The location of the emission, combined with an analysis of the emission line ratios, leads us to conclude that NGC 1399's AGN is the source of ionising radiation.