Radial gradients in initial mass function sensitive absorption features in the Coma brightest cluster galaxies

TL;DR

This study investigates radial variations of IMF-sensitive absorption features in three Coma cluster galaxies using integral field spectroscopy, revealing diverse IMF profiles and challenging the notion of a universally bottom-heavy IMF in massive galaxies.

Contribution

It provides the first detailed radial analysis of IMF-sensitive features in multiple BCGs, employing robust telluric and sky subtraction methods to assess IMF variations.

Findings

NGC4889 shows strong gradients in NaI and CaT but a flat FeH profile, indicating an old, alpha-enhanced population with a Chabrier or bottom-light IMF.

NGC4874 exhibits flat IMF-sensitive feature profiles, consistent with a Chabrier IMF.

NGC4839 displays strong NaI and FeH features, suggesting a potentially heavier IMF, though with large uncertainties.

Abstract

Using the Oxford Short Wavelength Integral Field specTrograph (SWIFT), we trace radial variations of initial mass function (IMF) sensitive absorption features of three galaxies in the Coma cluster. We obtain resolved spectroscopy of the central 5kpc for the two central brightest-cluster galaxies (BCGs) NGC4889, NGC4874, and the BCG in the south-west group NGC4839, as well as unresolved data for NGC4873 as a low-$\sigma_*$ control. We present radial measurements of the IMF-sensitive features sodium NaI$_{\rm{SDSS}}$, calcium triplet CaT and iron-hydride FeH0.99, along with the magnesium MgI0.88 and titanium oxide TiO0.89 features. We employ two separate methods for both telluric correction and sky-subtraction around the faint FeH feature to verify our analysis. Within NGC4889 we find strong gradients of NaI$_{\rm{SDSS}}$ and CaT but a flat FeH profile, which from comparing to stellar population synthesis models, suggests an old, $\alpha$-enhanced population with a Chabrier, or even bottom-light IMF. The age and abundance is in line with previous studies but the normal IMF is in contrast to recent results suggesting an increased IMF slope with increased velocity dispersion. We measure flat NaI$_{\rm{SDSS}}$ and FeH profiles within NGC4874 and determine an old, possibly slightly $\alpha$-enhanced and Chabrier IMF population. We find an $\alpha$-enhanced, Chabrier IMF population in NGC4873. Within NGC4839 we measure both strong NaI$_{\rm{SDSS}}$ and strong FeH, although with a large systematic uncertainty, suggesting a possible heavier IMF. The IMFs we infer for these galaxies are supported by published dynamical modelling. We stress that IMF constraints should be corroborated by further spectral coverage and independent methods on a galaxy-by-galaxy basis.