The cycle of metals in the infalling elliptical galaxy NGC 1404

TL;DR

This study uses deep X-ray observations of NGC 1404 to explore metal abundances, revealing a new bias in measurements, and suggests early enrichment processes across galaxy mass scales.

Contribution

It introduces the 'double Fe bias' in X-ray spectral fitting and provides new insights into metal content and distribution in a lower mass galaxy, contrasting with cluster observations.

Findings

Identification of the 'double Fe bias' affecting abundance estimates.

Fe and Mg masses are significantly below what stellar processes could produce.

Solar abundance ratios in the galaxy's halo differ from stellar populations.

Abstract

Hot atmospheres pervading galaxy clusters, groups, and early-type galaxies are rich in metals, produced during epochs and diffused via processes that are still to be determined. While this enrichment has been routinely investigated in clusters, metals in lower mass systems are more challenging to probe with standard X-ray exposures and spectroscopy. In this paper, we focus on very deep XMM-Newton ($\sim$350 ks) observations of NGC 1404, a massive elliptical galaxy experiencing ram-pressure stripping of its hot atmosphere while infalling toward the centre of the Fornax cluster, with the aim to derive abundances through its hot gas extent. Importantly, we report the existence of a new fitting bias - the "double Fe bias" - leading to an underestimate of the Fe abundance when two thermal components cannot realistically model the complex temperature structure present in the outer atmosphere of the galaxy. Contrasting with the ''metal conundrum'' seen in clusters, the Fe and Mg masses of NGC 1404 are measured 1-2 orders of magnitude below what stars and supernovae could have reasonably produced and released. In addition, we note the remarkable Solar abundance ratios of the galaxy's halo, different from its stellar counterpart but similar to the chemical composition of the ICM of rich clusters. Completing the clusters regime, all these findings provide additional support toward a scenario of early enrichment, at play over two orders of magnitude in mass. A few peculiar and intriguing features, such as a possible double metal peak as well as an apparent ring of enhanced Si near the galaxy core, are also discussed.