Institute of Astronomy, University of Cambridge
This guest post was written by Leah Bigwood. Leah is a third year PhD student at the Institute of Astronomy, University of Cambridge. She is interested in understanding AGN feedback and enjoys working at the intersection of astrophysics and cosmology. In particular, she is working towards an understanding of the impact of AGN feedback on our cosmological measurements and whether it could provide a solution to the S8 tension in cosmology.
Paper Title: The hot gas mass fraction in halos. From Milky Way-like groups to massive clusters
Authors: P. Popesso, A. Biviano , I. Marini , K. Dolag , S. Vladutescu-Zopp , B. Csizi , V. Biffi , G. Lamer, A. Robothan, M. Bravo, L. Lovisari, S. Ettori, M. Angelinelli, S. Driver , V. Toptun, A. Dev, D. Mazengo, A. Merloni, J. Comparat, G. Ponti, T. Mroczkowski, E. Bulbul, S. Grandis, and E. Bahar
First Author Institution: European Southern Observatory, Garching bei München, Germany
Status: Submitted to Astronomy & Astrophysics [open access]
Despite lying in the shadow of the better known H0 tension in cosmology, the S8 tension has quietly persisted for the last decade. The S8 tension refers to the 1-3σ discrepancy between measurements of the S8 parameter (a measure of how ‘clumpy’ the matter in the Universe is) made by weak lensing analyses, compared to modelling based on the first light from the Universe, extrapolated to the present day with our best model of the Universe: ΛCDM. Weak gravitational lensing refers to the distortion of the light path when light from distant galaxies passes foreground structure, resulting in galaxies’ shapes appearing elongated to us as observers. By measuring the correlations in the shapes of galaxies as a function of angular scale, we can build up a 3D map of the universe’s matter density, and from that we can extract cosmology. The ΛCDM model assumes the Universe comprises three main components: dark energy in the form of a cosmological constant (Λ), which drives an accelerated expansion of the Universe, cold dark matter (CDM), which interacts only gravitationally, and ordinary baryonic matter. At only 1-3σ, some cosmologists have an issue with it being named a ‘tension’ at all. Yet the discrepancy has persisted consistently, with multiple independent weak lensing surveys finding low S8 values in over ten years of studies. If the S8 tension is indeed real, it would mean that the ‘clumpiness’ of the Universe is different to that expected from our best ΛCDM model, and we may be missing something crucial in how dark matter clusters.
Back in 2022, rival weak lensing and cosmic microwave background research groups decided to stop arguing about whose measurement was correct and teamed up to propose a solution. They argued that the discrepancy may result from how the weak lensers were modelling the smallest scales of the matter distribution, which can actually be as large as the size of an entire galaxy. In addition, weak lensing analyses may have under-estimated how much ‘Active Galactic Nuclei (AGN) feedback’ affected the Universe’s matter distribution.
To understand this, we should recall that Super Massive Black Holes (SMBHs) not only cause the infall of matter, but due to the extreme conditions of this accretion process, they can also end up pushing matter out. AGN feedback describes the heating and outflows of gas close to the black hole’s accretion disk, as well as the jets that can form due to the twisting of magnetic fields near the black hole’s poles. These processes can cause matter to be expelled away from the black hole, potentially ending up outside the host galaxy itself. If feedback was more powerful than we think it is, the redistribution of matter due to AGN feedback would cause the Universe to appear less clumpy to the weak lensing teams and explain the low values of S8 they are measuring. The issue is, none of the observational evidence we have for AGN feedback points towards that conclusion. X-ray measurements of the gas content in galaxy groups (typically containing tens of galaxies) and clusters (typically containing tens to thousands of galaxies) tell us that these objects actually have plenty of gas remaining, meaning AGN feedback hasn’t expelled it all to large distances. In other words, the evidence we have points towards a ‘mild’ AGN feedback scenario, that couldn’t resolve the S8 tension.
That was until last December, when the authors of today’s paper came to a different conclusion. They measured the gas content of galaxy groups using much-awaited and brand-new X-ray observations from eROSITA. They took a different approach to previous X-ray analyses and decided to select the galaxy groups using optical data from the Galaxy and Mass Assembly (GAMA) survey. This differs from the norm, which is to select galaxy groups based on their X-ray emission alone. However, this relies on galaxy groups having sufficient gas to emit X-rays in the first place, meaning previous analyses may have missed the less gas-rich objects, namely those that have experienced stronger feedback which expelled the gas away. The authors stacked the GAMA galaxy group catalogue on top of the eROSITA maps and measured the X-ray surface brightness profile of the underlying group population, including those that may have been missed in previous studies. They then estimated the gas mass profile based on the observed X-ray emission and integrated to estimate the total gas content of the groups.
Surprisingly, the team found that galaxy groups in their sample contain significantly less gas than suggested by previous measurements (see figure 1). They argue that the reason for this is their optical selection; that they have chosen a more realistic population of groups, rather than just those bright in X-rays. As we’ve established, lower gas contents would imply that AGN feedback is more extreme than previously thought, since more gas is ejected, leaving less behind for us to detect. This would mean that weak lensing measurements of S8 that relied on models of AGN feedback from the old X-ray measurements may be underestimating the strength of the feedback process. This would result in low S8 measurements providing a potential explanation for the S8 tension!
One caveat is that these are the first X-ray measurements to hint at a stronger feedback scenario. Therefore, we should not ignore the over twenty-year body of X-ray evidence telling us that galaxy groups have plenty of gas remaining and therefore feedback is mild. Regardless, the evidence for extreme AGN feedback is growing, with several newer observational probes being developed to inform us on feedback strength. This includes measurements of the Kinetic Sunyaev–Zeldovich effect, which uses CMB data to estimate gas density on the outskirts of groups. These new measurements suggested that there was an excess of gas in the outer regions of galaxy groups, hinting at a scenario where AGN jets push matter to greater distances than we thought. Ultimately though, the outlook is the same as it often is in astronomy – we need more data. The good news is that it may not be too far away. It’s a race against time to understand AGN feedback before our brand-new weak lensing experiment, the Rubin Observatory, comes online. Given the unprecedented cosmological constraints this survey promises, we don’t want to risk underestimating feedback and potentially attaining another set of low-S8 biased results! If we can truly understand feedback in time, there is a chance we could put the quieter S8 tension to bed and admit that maybe we did get it right with ΛCDM after all.
Astrobite edited by: Sonja Panjkov
Featured Image Credit: NASA, ESA, S. Baum and C. O’Dea (RIT), R. Perley and W. Cotton (NRAO/AUI/NSF), and the Hubble Heritage Team (STScI/AURA).
