Active Galactic Nuclei Feedback Models Research Articles

RABBITS – II. The impact of AGN feedback on coalescing supermassive black holes in disc and elliptical galaxy mergers

ABSTRACT In this study of the ‘Resolving supermAssive Black hole Binaries In galacTic hydrodynamical Simulations’ (RABBITS) series, we investigate the orbital evolution of supermassive black holes (SMBHs) during galaxy mergers. We simulate both disc and elliptical galaxy mergers using the ketju code, which can simultaneously follow galaxy (hydro-)dynamics and small-scale SMBH dynamics with post-Newtonian corrections. With our SMBH binary subgrid model, we show how active galactic nuclei (AGNs) feedback affects galaxy properties and SMBH coalescence. We find that simulations without AGN feedback exhibit excessive star formation, resulting in merger remnants that deviate from observed properties. Kinetic AGN feedback proves more effective than thermal AGN feedback in expelling gas from the centre and quenching star formation. The different central galaxy properties, which are a result of distinct AGN feedback models, lead to varying rates of SMBH orbital decay. In the dynamical friction phase, galaxies with higher star formation and higher SMBH masses possess denser centres, become more resistant to tidal stripping, experience greater dynamical friction, and consequently form SMBH binaries earlier. As AGN feedback reduces gas densities in the centres, dynamical friction by stars dominates over gas. In the SMBH hardening phase, compared to elliptical mergers, disc mergers exhibit higher central densities of newly formed stars, resulting in accelerated SMBH hardening and shorter merger time-scales (i.e. $\lesssim 500$ Myr versus $\gtrsim 1$ Gyr). Our findings highlight the importance of AGN feedback and its numerical implementation in understanding the SMBH coalescing process, a key focus for low-frequency gravitational wave observatories.

Distinguishing Active Galactic Nuclei Feedback Models with the Thermal Sunyaev–Zel’dovich Effect

Current models of galaxy formation require strong feedback from active galactic nuclei (AGN) to explain the observed lack of star formation in massive galaxies since z ≈ 2, but direct evidence of this energy input is limited. We use the SIMBA cosmological galaxy formation simulations to assess the ability of thermal Sunyaev–Zel’dovich (tSZ) measurements to provide such evidence, by mapping the pressure structure of the circumgalactic medium around massive z ≈ 0.2–1.5 galaxies. We undertake a stacking approach to calculate the total tSZ signal and its radial profile in simulations with varying assumptions of AGN feedback, and we assess its observability with current and future telescopes. By convolving our predictions with the 2.′1 beam of the Atacama Cosmology Telescope, we show that current observations at z ≈ 1 are consistent with SIMBA’s fiducial treatment of AGN feedback and inconsistent with SIMBA models without feedback. At z ≈ 0.5, observational signals lie between SIMBA run with and without AGN feedback, suggesting AGN in SIMBA may inject too much energy at late times. By convolving our data with a 9.″5 beam corresponding to the TolTEC camera on the Large Millimeter Telescope Alfonso Serrano, we predict a unique profile for AGN feedback that can be distinguished with future higher-resolution measurements. Finally, we explore a novel approach to quantify the nonspherically symmetric features surrounding our galaxies by plotting radial profiles representing the component of the stack with m-fold symmetry.

FLAMINGO: calibrating large cosmological hydrodynamical simulations with machine learning.

To fully take advantage of the data provided by large-scale structure surveys, we need to quantify the potential impact of baryonic effects, such as feedback from active galactic nuclei (AGN) and star formation, on cosmological observables. In simulations, feedback processes originate on scales that remain unresolved. Therefore, they need to be sourced via subgrid models that contain free parameters. We use machine learning to calibrate the AGN and stellar feedback models for the FLAMINGO (Fullhydro Large-scale structure simulations with All-sky Mapping for the Interpretation of Next Generation Observations) cosmological hydrodynamical simulations. Using Gaussian process emulators trained on Latin hypercubes of 32 smaller volume simulations, we model how the galaxy stellar mass function (SMF) and cluster gas fractions change as a function of the subgrid parameters. The emulators are then fit to observational data, allowing for the inclusion of potential observational biases. We apply our method to the three different FLAMINGO resolutions, spanning a factor of 64 in particle mass, recovering the observed relations within the respective resolved mass ranges. We also use the emulators, which link changes in subgrid parameters to changes in observables, to find models that skirt or exceed the observationally allowed range for cluster gas fractions and the SMF. Our method enables us to define model variations in terms of the data that they are calibrated to rather than the values of specific subgrid parameters. This approach is useful, because subgrid parameters are typically not directly linked to particular observables, and predictions for a specific observable are influenced by multiple subgrid parameters.

Redshift evolution of the baryon and gas fraction in simulated groups and clusters of galaxies

We study the redshift evolution of the baryon budget in a large set of galaxy clusters from the Magneticum suite of smoothed particle hydrodynamical cosmological simulations. At high redshifts (z ≳ 1), we obtain ‘closed-box’ (i.e. baryon mass fraction fbar = Ωbar/Ωtot) systems independently of the mass of the systems on radii greater than 3R500, c, whereas at lower redshifts, only the most massive halos can be considered closed box. Furthermore, in the innermost regions (r < R500, c), the baryon fraction shows a general decrease with redshift, and for less massive objects we observe a much more prominent decrease than for massive halos (fbar × Ωtot/Ωbar = Ybar decreases by ∼4% from z ∼ 2.8 to z ∼ 0.2 for massive systems and by ∼15% for less massive objects in the same redshift range). The gas depletion parameter Ygas = fgas/(Ωbar/Ωtot) shows a steeper and highly scattered radial distribution in the central regions (0.5R500, c ≤ r ≤ 2R500, c) of less massive halos with respect to massive objects at all redshifts, while on larger radii (r ≥ 2R500, c) the gas fraction distributions are independent of the masses or the redshifts. We divide the gas content of halos into the hot and cold phases. The hot, X-ray-observable component of the gas accurately traces the total amount of gas at low redshifts (e.g., for z ∼ 0.2 at R500, c, in the most massive subsample, that is, 4.6 × 1014 ≤ M500, c/M⊙ ≤ 7.5 × 1014 and least massive subsample, that is, 6.0 × 1013 ≤ M500, c/M⊙ ≤ 1.9 × 1014, we obtain Ygas ∼ 0.75 and 0.67, Yhot ∼ 0.73 and 0.64, and Ycold ∼ 0.02 and 0.02, respectively). On the other hand, at higher redshifts, the cold component provides a non-negligible contribution to the total amount of baryons in our simulated systems, especially in less massive objects (e.g., for z ∼ 2.8 at R500, c, in the most massive subsample, that is, 2.5 × 1013 ≤ M500, c/M⊙ ≤ 5.0 × 1013 and least massive subsample, that is, 5.8 × 1012 ≤ M500, c/M⊙ ≤ 9.7 × 1012, we obtain Ygas ∼ 0.63 and 0.64, Yhot ∼ 0.50 and 0.45, and Ycold ∼ 0.13 and 0.18, respectively). Moreover, the behaviour of the baryonic, entire-gas, and hot-gas-phase depletion parameters as functions of radius, mass, and redshift are described by some functional forms for which we provide the best-fit parametrization. The evolution of metallicity and stellar mass in halos suggests that the early (z > 2) enrichment process is dominant, while more recent star-formation processes make negligible contributions to the enrichment of the gas metallicity. In addition, active galactic nuclei (AGN) play an important role in the evolution of the baryon content of galaxy clusters. Therefore, we investigate possible correlations between the time evolution of AGN feedback and the depletion parameters in our numerical simulations. Interestingly, we demonstrate that the energy injected by the AGN activity shows a particularly strong positive correlation with Ybar, Ycold, and Ystar and a negative correlation with Yhot and ZTot. On the other hand, Ygas shows a less prominent level of negative correlation, a result which is highly dependent on the mass of the halos. These trends are consistent with previous theoretical and numerical works, meaning that our results, combined with findings derived from current and future X-ray observations, represent possible proxies with which to test the AGN feedback models used in different suites of numerical simulations.

The SZ flux-mass (Y–M) relation at low-halo masses: improvements with symbolic regression and strong constraints on baryonic feedback

ABSTRACT Feedback from active galactic nuclei (AGNs) and supernovae can affect measurements of integrated Sunyaev–Zeldovich (SZ) flux of haloes (YSZ) from cosmic microwave background (CMB) surveys, and cause its relation with the halo mass (YSZ–M) to deviate from the self-similar power-law prediction of the virial theorem. We perform a comprehensive study of such deviations using CAMELS, a suite of hydrodynamic simulations with extensive variations in feedback prescriptions. We use a combination of two machine learning tools (random forest and symbolic regression) to search for analogues of the Y–M relation which are more robust to feedback processes for low masses ($M\lesssim 10^{14}\, \mathrm{ h}^{-1} \, \mathrm{ M}_\odot$); we find that simply replacing Y → Y(1 + M*/Mgas) in the relation makes it remarkably self-similar. This could serve as a robust multiwavelength mass proxy for low-mass clusters and galaxy groups. Our methodology can also be generally useful to improve the domain of validity of other astrophysical scaling relations. We also forecast that measurements of the Y–M relation could provide per cent level constraints on certain combinations of feedback parameters and/or rule out a major part of the parameter space of supernova and AGN feedback models used in current state-of-the-art hydrodynamic simulations. Our results can be useful for using upcoming SZ surveys (e.g. SO, CMB-S4) and galaxy surveys (e.g. DESI and Rubin) to constrain the nature of baryonic feedback. Finally, we find that the alternative relation, Y–M*, provides complementary information on feedback than Y–M.

Invoking the virial theorem to understand the impact of (dry) mergers on the Mbh–σ relation

ABSTRACT While dry mergers can produce considerable scatter in the (black hole mass, Mbh)-(spheroid stellar mass, M*, sph) and Mbh-(spheroid half-light radius, Re, sph) diagrams, the virial theorem is used here to explain why the scatter about the Mbh–(velocity dispersion, σ) relation remains low in the face of such mergers. Its small scatter has been claimed as evidence of feedback from active galactic nuclei (AGNs). However, it is shown that galaxy mergers also play a significant role. The collision of two lenticular (S0) galaxies is expected to yield three types of merger product (a core-Sérsic S0, an ellicular ES,e or an elliptical E galaxy), depending on the remnant’s orbital angular momentum. It is shown that the major merger of two S0 galaxies with M*, sph ∼ 1011 M⊙ advances the system along a slope of ∼5 in the Mbh-σ diagram, while a major E+E galaxy merger moves a system slightly along a trajectory with a slope of ∼9. Mergers of lower-mass S0 galaxies with M*, sph ∼ 1010 M⊙ move slightly along a trajectory with a slope of ∼3, thereby further contributing to the steeper distribution for the E (and Es,e) galaxies in the Mbh-σ diagram, reported here to have a slope of 7.27 ± 0.91, compared to the S0 galaxies that have a slope of 5.68 ± 0.60. This result forms an important complement to the AGN feedback models like that from Silk & Rees, providing a more complete picture of galaxy/(black hole) co-evolution. It also has important implications for nanohertz gravitational-wave research.

The effects of AGN feedback on the structural and dynamical properties of Milky Way-mass galaxies in cosmological simulations

ABSTRACT Feedback from active galactic nuclei (AGNs) has become established as a fundamental process in the evolution of the most massive galaxies. Its impact on Milky Way (MW)-mass systems, however, remains comparatively unexplored. In this work, we use the auriga simulations to probe the impact of AGN feedback on the dynamical and structural properties of galaxies, focusing on the bar, bulge, and disc. We analyse three galaxies – two strongly and one unbarred/weakly barred – using three setups: (i) the fiducial auriga model, which includes both radio and quasar mode feedback, (ii) a setup with no radio mode, and (iii) one with neither the radio nor the quasar mode. When removing the radio mode, gas in the circumgalactic medium cools more efficiently and subsequently settles in an extended disc, with little effect on the inner disc. Contrary to previous studies, we find that although the removal of the quasar mode results in more massive central components, these are in the form of compact discs, rather than spheroidal bulges. Therefore, galaxies without quasar mode feedback are more baryon-dominated and thus prone to forming stronger and shorter bars, which reveals an anticorrelation between the ejective nature of AGN feedback and bar strength. Hence, we report that the effect of AGN feedback (i.e. ejective or preventive) can significantly alter the dynamical properties of MW-like galaxies. Therefore, the observed dynamical and structural properties of MW-mass galaxies can be used as additional constraints for calibrating the efficiency of AGN feedback models.

Fast radio bursts as probes of feedback from active galactic nuclei

ABSTRACT Fast radio bursts (FRBs) are a promising tool for studying the low-density universe as their dispersion measures (DM) are extremely sensitive probes of electron column density. Active galactic nuclei (AGN) inject energy into the intergalactic medium, affecting the DM and their scatter. To determine the effectiveness of FRBs as a probe of AGN feedback, we analysed three different AGN models from the EAGLE simulation series. We measured the mean DM–redshift relation, and the scatter around it, using 2.56 × 108 sightlines at 131 redshift (z) bins between 0 ≤ z ≤ 3. While the DM–redshift relation itself is highly robust against different AGN feedback models, significant differences are detected in the scatter around the mean: weaker feedback leads to more scatter. We find that ∼104 localized FRBs are needed to discriminate between the scatter in standard feedback and stronger, more intermittent feedback models. The number of FRBs required is dependent on the redshift distribution of the detected population. A lognormal redshift distribution at z = 0.5 requires approximately 50 per cent fewer localized FRBs than a distribution centred at z = 1. With the Square Kilometre Array expected to detect >103 FRBs per day, in the future, FRBs will be able to provide constraints on AGN feedback.

Gas Content Regulates the Life Cycle of Star Formation and Black Hole Accretion in Galaxies

Abstract Feedback from active galactic nuclei (AGNs) is expected to impact the amount of cold gas in galaxies by driving strong galactic winds, by preventing external gas inflows, or by changing the thermodynamical state of the gas. We use estimates of molecular gas mass based on dust absorption (Hα/Hβ) to study gas content of large samples of type 2 AGN host galaxies in comparison with inactive galaxies. Using sparse principal component and clustering analysis, we analyze a suite of stellar and structural parameters of ∼27,100 face-on, central galaxies at redshift z = 0.02–0.15 and with stellar mass M ⋆ ≈ 1010–2 × 1011 M ⊙. We identify four galaxy groups of similar mass and morphology (mass surface density, velocity dispersion, concentration, and Sérsic index) that can be evolutionarily linked through a life cycle wherein gas content mediates their star formation rate (SFR) and level of AGN activity. Galaxies first consume their gas mostly through bursty star formation, then enter into a transition phase of intermediate gas richness in which star formation and AGNs coexist, before settling into retirement as gas-poor, quiescent systems with residual levels of AGN activity (LINERs). Strongly accreting black holes (Seyferts) live in gas-rich, star-forming hosts, but neither their gas reservoir nor their ability to form stars seems to be impacted instantaneously (timescales ≲0.5 Gyr) by AGN feedback. Our results are inconsistent with AGN feedback models that predict that central, bulge-dominated, Seyfert-like AGNs in massive galaxies have significantly lower molecular gas fractions than inactive galaxies of similar mass, morphology, and SFR.

X-raying winds in distant quasars: The first high-redshift wind duty cycle

Aims. Theoretical models of wind-driven feedback from active galactic nuclei (AGN) often identify ultra-fast outflows as being the main agent in the generation of galaxy-sized outflows, which are possibly the main actors in establishing so-called AGN-galaxy co-evolution. Ultra-fast outflows are well characterized in local AGN but much less is known in quasars at the cosmic time when star formation and AGN activity peaked (z ≃ 1–3). It is therefore necessary to search for evidence of ultra-fast outflows in high-z sources to test wind-driven AGN feedback models. Methods. Here we present a study of Q2237+030, the Einstein Cross, a quadruply-imaged radio-quiet lensed quasar located at z = 1.695. We performed a systematic and comprehensive temporally and spatially resolved X-ray spectral analysis of all the available Chandra and XMM-Newton data (as of September 2019). Results. We find clear evidence for spectral variability, possibly due to absorption column density (or covering fraction) variability intrinsic to the source. For the first time in this quasar, we detect a fast X-ray wind outflowing at vout ≃ 0.1c that would be powerful enough (Ėkin ≃ 0.1 Lbol) to significantly affect the evolution of the host galaxy. We report also on the possible presence of an even faster component of the wind (vout ∼ 0.5c). For the first time in a high-z quasar, given the large sample and long time interval spanned by the analyzed X-ray data, we are able to roughly estimate the wind duty cycle as ≃0.46 (0.31) at 90% (95%) confidence level. Finally, we also confirm the presence of a Fe Kα emission line with variable energy, which we discuss in the light of microlensing effects as well as considering our findings on the source.

Spatially Resolved UV Diagnostics of AGN Feedback: Radiation Pressure Dominates in a Prototypical Quasar-driven Superwind

Abstract Galactic-scale winds driven by active galactic nuclei (AGN) are often invoked to suppress star formation in galaxy evolution models, but the mechanisms driving these outflows are hotly debated. Two key AGN feedback models are (1) radiation pressure accelerating cool gas and (2) a hot outflowing wind entraining the interstellar medium (ISM). Highly ionized emission-line diagnostics represent a powerful means of differentiating these scenarios because of their sensitivity to the expected compression of the ISM clouds by the hot wind. Here, we report the first spatially resolved UV emission spectroscopy of a prototypical (radio-quiet) quasar-driven superwind around the obscured quasar SDSS J1356+1026 at z = 0.123. We observe ratios of O vi/C iv, N v/C iv, and C iv/He ii that are remarkably similar for outflowing gas clouds ≲100 pc and ≈10 kpc from the nucleus. Such similarity is expected for clouds with AGN radiation-pressure-dominated dynamics. Comparing the observed line emission to models of clouds in balance with radiation pressure and/or a hot wind, we rule out the presence of a dynamically important hot wind and constrain the ratio of hot gas pressure to radiation pressure to P hot/P rad ≲ 0.25 both at ≲100 pc and ≈10 kpc from the nucleus. Moreover, the predictions of the radiation pressure confined cloud models that best fit observed UV line ratios are consistent with the observed diffuse X-ray spectrum. These results indicate that this AGN superwind is driven by radiation pressure or was driven by a hot wind that has since dissipated despite ongoing AGN activity.

Kinematics of C iv and [O iii] emission in luminous high-redshift quasars

We characterise ionised gas outflows using a large sample of ~330 high-luminosity (45.5 < log(L_bol/erg s^-1) < 49.0), high-redshift (1.5 < z < 4.0) quasars via their [OIII]4960,5008 emission. The median velocity width of the [OIII] emission line is 1540 kms^-1, increasing with increasing quasar luminosity. Broad, blue-shifted wings are seen in the [OIII] profiles of 42 per cent of the sample. Rest-frame ultraviolet spectra with well-characterised CIV 1550 emission line properties are available for more than 210 quasars, allowing an investigation of the relationship between the Broad Line Region (BLR) and Narrow Line Region (NLR) emission properties. The [OIII] blueshift is correlated with CIV blueshift, even when the dependence of both quantities on quasar luminosity has been taken into account. A strong anti-correlation between the [OIII] equivalent width (EW) and CIV blueshift also exists. Furthermore, [OIII] is very weak, with EW<1A, in ~10 per cent of the sample, a factor of 10 higher compared to quasars at lower luminosities and redshifts. If the [OIII] emission originates in an extended NLR, the observations suggest that quasar-driven winds are capable of influencing the host-galaxy environment out to kilo-parsec scales. The mean kinetic power of the ionised gas outflows is then 10^44.7 erg s^-1, which is ~0.15 per cent of the bolometric luminosity of the quasar. These outflow efficiencies are broadly consistent with those invoked in current active galactic nuclei feedback models.

AGN outflows and feedback twenty years on

It is the twentieth anniversary of the publication of the seminal papers by Magorrian et al. and Silk & Rees which, along with other related work, ignited an explosion of publications connecting active galactic nuclei (AGN)-driven outflows to galaxy evolution. With a surge in observations of AGN outflows, studies are attempting to directly test AGN feedback models using the outflow properties. With a focus on outflows traced by optical and CO emission lines, we discuss significant challenges which greatly complicate this task from both an observational and theoretical perspective. We highlight observational uncertainties involved, and the assumptions required, when deriving kinetic coupling efficiencies (i.e., outflow kinetic power as a fraction of AGN luminosity) from typical observations. Based on recent models we demonstrate that extreme caution should taken when comparing observationally-derived kinetic coupling efficiencies to coupling efficiencies from fiducial feedback models.

X-ray Surface Brightness Profiles of Active Galactic Nuclei in the Extended Groth Strip: Implications for AGN Feedback

Using data from the All Wavelength Extended Groth Strip International Survey (AEGIS) we statistically detect the extended X-ray emission in the interstellar medium (ISM)/intracluster medium (ICM) in both active and normal galaxies at 0.3 ≤ z ≤ 1.3. For both active galactic nuclei (AGN) host galaxy and normal galaxy samples that are matched in restframe color, luminosity, and redshift distribution, we tentatively detect excess X-ray emission at scales of 1–10″ at a few σ significance in the surface brightness profiles. The exact significance of this detection is sensitive to the true characterization of Chandra’s point-spread function. The observed excess in the surface brightness profiles is suggestive of lower extended emission in AGN hosts compared to normal galaxies. This is qualitatively similar to theoretical predictions of the X-ray surface brightness profile from AGN feedback models, where feedback from AGN is likely to evacuate the gas from the center of the galaxy/cluster. We propose that AGN that are intrinsically underluminous in X-rays, but have equivalent bolometric luminosities to our sources will be the ideal sample to study more robustly the effect of AGN feedback on diffuse ISM/ICM gas.

AGN feedback models: correlations with star formation and observational implications of time evolution

We examine the correlation between the star formation rate (SFR) and black hole accretion rate (BHAR) across a suite of different AGN feedback models, using the time evolution of a merger simulation. By considering three different stages of evolution, and a distinction between the nuclear and outer regions of star formation, we consider 63 different cases. Despite many of the feedback models fitting the M-\sigma\ relationship well, there are often distinct differences in the SFR-BHAR correlations, with close to linear trends only being present after the merger. Some of the models also show evolution in the SFR-BHAR parameter space that is at times directly across the long-term averaged SFR-BHAR correlation. This suggests that the observational SFR-BHAR correlation found for ensembles of galaxies is an approximate statistical trend, as suggested by Hickox et al. Decomposing the SFR into nuclear and outer components also highlights notable differences between models and there is only modest agreement with observational studies examining this in Seyfert galaxies. For the fraction of the black hole mass growth from the merger event relative to the final black hole mass, we find as much as a factor of three variation among models. This also translates into a similar variation in the post-starburst black hole mass growth. Overall, we find that while qualitative features are often similar amongst models, precise quantitative analysis shows there can be quite distinct differences.

Towards a realistic population of simulated galaxy groups and clusters

We present a new suite of large-volume cosmological hydrodynamical simulations called cosmo-OWLS. They form an extension to the OverWhelmingly Large Simulations (OWLS) project, and have been designed to help improve our understanding of cluster astrophysics and non-linear structure formation, which are now the limiting systematic errors when using clusters as cosmological probes. Starting from identical initial conditions in either the Planck or WMAP7 cosmologies, we systematically vary the most important `sub-grid' physics, including feedback from supernovae and active galactic nuclei (AGN). We compare the properties of the simulated galaxy groups and clusters to a wide range of observational data, such as X-ray luminosity and temperature, gas mass fractions, entropy and density profiles, Sunyaev-Zel'dovich flux, I-band mass-to-light ratio, dominance of the brightest cluster galaxy, and central massive black hole (BH) masses, by producing synthetic observations and mimicking observational analysis techniques. These comparisons demonstrate that some AGN feedback models can produce a realistic population of galaxy groups and clusters, broadly reproducing both the median trend and, for the first time, the scatter in physical properties over approximately two decades in mass ($10^{13} \lesssim M_{500} \lesssim 10^{15}~\textrm{M}_{\odot}$) and 1.5 decades in radius ($0.05 \lesssim r/r_{500} \lesssim 1.5$). However, in other models, the AGN feedback is too violent (even though they reproduce the observed BH scaling relations), implying calibration of the models is required. The production of realistic populations of simulated groups and clusters, as well as models that bracket the observations, opens the door to the creation of synthetic surveys for assisting the astrophysical and cosmological interpretation of cluster surveys, as well as quantifying the impact of selection effects.

Brightest cluster galaxies in cosmological simulations: achievements and limitations of active galactic nuclei feedback models

We analyze the basic properties of Brightest Cluster Galaxies (BCGs) produced by state of the art cosmological zoom-in hydrodynamical simulations. These simulations have been run with different sub-grid physics included. Here we focus on the results obtained with and without the inclusion of the prescriptions for supermassive black hole (SMBH) growth and of the ensuing Active Galactic Nuclei (AGN) feedback. The latter process goes in the right direction of decreasing significantly the overall formation of stars. However, BCGs end up still containing too much stellar mass, a problem that increases with halo mass, and having an unsatisfactory structure. This is in the sense that their effective radii are too large, and that their density profiles feature a flattening on scales much larger than observed. We also find that our model of thermal AGN feedback has very little effect on the stellar velocity dispersions, which turn out to be very large. Taken together, these problems, which to some extent can be recognized also in other numerical studies typically dealing with smaller halo masses, indicate that on one hand present day sub-resolution models of AGN feedback are not effective enough in diminishing the global formation of stars in the most massive galaxies, but on the other hand they are relatively too effective in their centers. It is likely that a form of feedback generating large scale gas outflows from BCGs precursors, and a more widespread effect over the galaxy volume, can alleviate these difficulties.

Theoretical uncertainties due to AGN subgrid models in predictions of galaxy cluster observable properties

Cosmological constraints derived from galaxy clusters rely on accurate predictions of cluster observable properties, in which feedback from active galactic nuclei (AGN) is a critical component. In order to model the physical effects due to supermassive black holes (SMBH) on cosmological scales, subgrid modeling is required, and a variety of implementations have been developed in the literature. However, theoretical uncertainties due to model and parameter variations are not yet well understood, limiting the predictive power of simulations including AGN feedback. By performing a detailed parameter sensitivity study in a single cluster using several commonly-adopted AGN accretion and feedback models with FLASH, we quantify the model uncertainties in predictions of cluster integrated properties. We find that quantities that are more sensitive to gas density have larger uncertainties (~20% for Mgas and a factor of ~2 for Lx at R500), whereas Tx, Ysz, and Yx are more robust (~10-20% at R500). To make predictions beyond this level of accuracy would require more constraints on the most relevant parameters: the accretion model, mechanical heating efficiency, and size of feedback region. By studying the impact of AGN feedback on the scaling relations, we find that an anti-correlation exists between Mgas and Tx, which is another reason why Ysz and Yx are excellent mass proxies. This anti-correlation also implies that AGN feedback is likely to be an important source of intrinsic scatter in the Mgas-Tx and Lx-Tx relations.