Title: A non-uniform metal distribution in the ram-pressure stripped M86 group of galaxies
Authors: Sinancan Kara, François Mernier, Norbert Werner, E. Nihal Ercan
First Author’s Institution: Department of Physics, Boğaziçi University, Bebek, 34342 Istanbul, Turkey
Status: Available on ArXiv
When we look at the night sky, the starlight we see is only part of the story—the universe is also filled with gas, which makes up a significant portion of its baryonic (ordinary matter) content. In the early universe, this gas was almost entirely hydrogen and helium. Heavier elements like magnesium and iron came later, forged in the fiery explosions of supernovae. While stars create these elements, observations show that their distribution in galaxy clusters—vast systems that are millions of times larger than typical star-forming regions—is surprisingly uniform. Additionally, the abundance ratios of these elements in the centers of clusters also closely match those found in our Solar System. This smooth metallicity pattern makes galaxy clusters excellent laboratories for studying the formation of their member galaxies. Any deviation from this solar-like composition may hint at unusual or active events in a cluster’s past.
Meet M86, our protagonist for today: a giant elliptical galaxy plunging into the Virgo Cluster. Next to M86 is a halo of hot, X-ray-emitting gas which shows intricate structures, including features astronomers have nicknamed the “plume” and the “arm” (see Figure 1 below). Interestingly, M86 is only a weak radio source, which means that active galactic nucleus (AGN) activity cannot be the only source of these structures. So what’s stirring up the gas? Astronomers propose two main explanations:
- Ram-pressure stripping: As M86 barrels through the intracluster medium (ICM) of the Virgo Cluster, the “wind” (or, ram pressure) that it feels can strip hot gas from the galaxy’s core. This process might also trigger star formation within the stripped material, possibly giving rise to the observed emission features.
- Galaxy-galaxy interactions: Observations suggest that M86 has interacted with its close neighbor, NGC 4438. In this scenario, the X-ray-emitting structures may have formed from hot gas in the ICM that was disturbed by the collision, rather than from gas originally stripped from M86’s own core.
Figure 1. Left: An image of the Virgo Cluster taken by eROSITA, with the white dashed circle indicating the field of view of the XMM-Newton/EPIC observation. Right: XMM-Newton/EPIC X-ray image of M86 in the 0.3–2.0 keV energy band. The overlaid contours represent regions of equal X-ray surface brightness, highlighting detailed structures within the hot gas. (Adapted from Figure 1 in the original paper.)
Using observations from XMM-Newton, the authors measured the metallicity, which is the abundance of elements heavier than hydrogen and helium, in the core of M86. As shown in Figure 2, they found that the abundance of these elements relative to the abundance of iron closely matches solar values and is similar to those seen in other hot gas environments across the Universe. However, these ratios are lower than those typically observed in isolated elliptical galaxies, which usually show enhanced levels of these heavy elements because they only enrich their own halo. This finding suggests that in M86, ram-pressure stripping is not efficiently clearing out the older, primordial gas from the galaxy’s core. Because this old gas remains, it dilutes the chemical enrichment from newly formed supernova ejecta, making it harder to detect the fresh heavy elements expected from recent stellar activity. This chemical signature is similar to that of NGC 1404, a ram-pressure-stripped galaxy without AGN activity. In contrast, M89, which also undergoes ram-pressure stripping but hosts an AGN, shows evidence that AGN-driven outflows help remove central gas, allowing newly synthesized elements from supernovae to dominate the observed abundance ratios.
Figure 2. Abundance ratios of various elements relative to iron in the core of M86, shown as blue circles and purple squares. A ratio of 1 indicates Solar abundance, and gray bars represent the measurement uncertainties. For comparison, yellow bars show the average abundance ratios in the surrounding ICM based on 44 nearby systems. The elemental ratios in M86’s core are close to Solar and significantly lower than those found in typical isolated elliptical galaxies, suggesting that stellar winds from stars in M86 are not the dominant source of the hot halo’s chemical composition. (Adapted from Figure 9 in the original paper.)
In contrast to the galaxy core, the plume and tail structures in M86 show a significantly higher magnesium-to-iron (Mg/Fe) ratio. As shown in Figure 3, the Mg/Fe ratio in the core is similar to that of typical hot gas environments, while the plume shows an enhanced Mg/Fe ratio, matching the levels found in the Virgo ICM (indicated by the dashed blue band).
Further evidence comes from the temperature structure of these features. Despite spanning large distances, both the plume and the tail are nearly isothermal, with temperature variations about four times smaller than those observed in the core. As achieving such uniform temperatures across a large region takes time, these structures would not have had enough time to reach such an equilibrium state if they were formed recently through ram-pressure stripping of the galaxy’s dense core.
Taken together, the chemical and thermal signatures suggest that the plume and tail did not originate from stripped core gas. Instead, the authors propose that these structures formed from gas within M86’s extended halo, which has had time to mix thoroughly with the surrounding Virgo ICM as M86 moves through the cluster.
Figure 3. Left: Magnesium-to-iron (Mg/Fe) ratios measured in different regions of M86. Each data point corresponds to a numbered region shown in the right panel. Notably, Region 3, which surrounds but excludes the galaxy core, is plotted twice to highlight comparisons. The dashed blue band indicates the typical Mg/Fe ratio of the Virgo ICM. Right: The spatial map of M86 showing the defined extraction regions used for the measurements. (Adapted from Figures 2 and 6 in the original paper.)
This study of M86 provides valuable insight into how galaxies evolve as they fall into dense cluster environments. Using XMM-Newton observations, the authors show that ram-pressure stripping alone is insufficient to fully remove gas from the galaxy’s core. Their analysis of the chemical composition and temperature structure further suggests that the plume and tail features did not originate from recently stripped core gas, but rather from group-scale halo gas that has gradually mixed with the surrounding Virgo ICM. This work underscores the power of chemical fingerprints in uncovering a galaxy’s dynamical history. With the advent of next-generation X-ray missions, such studies will continue to deepen our understanding of galaxy transformation in cluster environments.
Astrobite edited by Chloe Klare.
Featured image credit: Figure 1 in Kara et al. (2025)
