Simulating AGN wind feedback with variable feedback efficiencies in idealised disc galaxies

Abstract

Active Galactic Nucleus (AGN) feedback plays a critical role in galaxy formation and evolution. AGN-driven winds can significantly influence their host galaxies, although the details of their impact remain unclear. In this study, we investigate the feedback effects of AGN winds on idealized disc galaxies using the SWIFT hydrodynamical code with COLIBRE subgrid physics. We implement a new thermal AGN feedback model in which the energy injection coupling efficiency has a power-law dependence on the Eddington ratio of the black hole (BH) accretion rate, motivated by scaling relations for AGN winds from numerical models and observations. We simulate idealised Milky Way-mass galaxies, incorporating a BH, cold gas disc, stellar disc, and hot circumgalactic medium, within a static dark matter halo. We vary the BH mass and the slope and normalisation of the new coupling efficiency model. For a fixed BH mass, we find that while systematic trends with coupling efficiency exist, most galaxy and BH properties show only modest variations. This likely reflects BH self-regulation in the COLIBRE model, which modulates the effects of changes in the feedback efficiency, provided the BH mass is sufficiently high. Key exceptions are the BH accretion rate and mass growth history, and outflow behaviour, where lower coupling efficiencies lead to faster BH growth and weaker outflows, potentially helping to explain the presence of overmassive BHs at high redshifts. Varying the BH mass, however, has a much larger impact, confirming that BH mass remains the primary factor shaping galaxy and BH evolution in our simulations.

Figures (22)

• Figure 1: Initial temperature (left panel), density (middle panel), and pressure (right panel) profiles of the CGM in this work, using the initialisation method from Nobels22 modified for the inclusion of a stellar disc (see text). The black dotted lines indicate the initial profiles, while the vertical black dashed lines show the radius $R_{\rm 200}$. Coloured solid lines represent the profile in the hydro- and gravity-only simulation at different times, as indicated in the panels, with darker colours corresponding to progressively later times. Equilibrium within $2R_{200}$ is reached already after 0.3 Gyr, whereas the gas profiles at radii beyond $2R_{200}$ continue to evolve at later times.
• Figure 2: The coupling efficiency $\eta$ as function of the Eddington ratio $\dot{m}$, using our variable coupling efficiency model ($\eta=N_\eta\dot{m}^{\alpha_\eta}$) where different colours correspond to different combinations of the model parameters. We show the variations of $\alpha_\eta$ (varying $N_\eta$ as $39.81\times 10^{\alpha_\eta-2.6}$) in the upper panel while the variations of $N_\eta$ (keeping $\alpha_\eta$ fixed) are shown in the lower panel. Dark colors correspond to larger values of the parameter being varied. The parameters for the fiducial variable model are $N_\eta=39.81$ and $\alpha_\eta=2.6$, included as black lines in both panels. The gray shaded area represents the ceiling set on the coupling efficiency. All $\eta$ in this region will be set to unity.
• Figure 3: Cold gas surface density, gas temperature and gas radial velocity in different runs. From left to right, the columns represent the SN-only run, the run with $M_{\rm BH} = 10^8$ M$_\odot$ using the fiducial constant coupling efficiency model, and the run with $M_{\rm BH} = 10^8$ M$_\odot$ using the fiducial variable coupling efficiency model, respectively. Top row: Face-on cool ($T\leq 8000$ K) gas surface density maps, each 30 kpc wide including all particles in projection, at $t=3.04$ Gyr. A 5 kpc scale bar and a colour bar indicating cold gas surface density are shown in the left and middle panels respectively. The white 'X' in the second and third panels shows the position of the central BH. Middle row: Edge-on mass-weighted, projection-averaged temperature maps, each 100 kpc wide including all particles in depth, at $t=0.36$ Gyr. A 20 kpc scale bar and a colour bar indicating gas temperature are shown in the left and middle panels respectively. Bottom row: Edge-on mass-weighted, projection-averaged radial velocity maps, each 100 kpc wide including all particles in depth, at $t=0.36$ Gyr. A 20 kpc scale bar and a colour bar indicating gas radial velocity are shown in the left and middle panels respectively.
• Figure 4: Effects of parameter variations in the variable coupling efficiency model on AGN properties. We show the coupling efficiencies $\eta$ (first row), Eddington-normalised accretion rates $\dot{m}$ (second row), and cumulative AGN energy injection $E_{\rm AGN}$ (third row), for different times in runs with $M_{\rm BH}=10^8$ M$_\odot$ using the variable coupling efficiency model with varying normalisations (first column) and varying slopes (second column). Variations in normalisation (slope) are represented by blue (green) colours, with darker shades corresponding to larger parameter values. The arrows or wedges in the right of the panels indicate the mean values averaged over the final 1.5 Gyr. The values plotted are all averaged over 0.09 Gyr time intervals. The fiducial variable coupling efficiency model (solid lines) and fiducial constant coupling efficiency model (dashed lines) are shown by gray lines. The vertical dotted lines represent the time at which the BH is placed into the simulations. On average, higher values of $\eta$ lead to lower $\dot{m}$ and higher $E_{\rm AGN}$.
• Figure 5: Effects of parameter variations in the variable coupling efficiency model on galaxy properties. We show the evolution of the galaxy SFR (first row) and the mass outflow rate $\dot{M}_{\rm out, 50~kpc}$ measured at a radius of 50 kpc (second row). The mass outflow rate is calculated for gas that is moving outwards with radial velocity $v_r > 40~{\rm km~s^{-1}}$. The black horizontal dot-dashed lines in the first row mark the quenching threshold of sSFR = 0.01 Gyr$^{-1}$ (or $\mathrm{SFR} \approx 0.49~\mathrm{M_\odot} ~\text{yr}^{-1}$, given that the stellar masses in these simulations change by less than 3 per cent). The values plotted are all smoothed over 0.09 Gyr time intervals. Colours and linestyles remain the same as in Fig. \ref{['fig:AGNprop_vary']}. On average, larger values of $\eta$ result in lower SFRs and higher mass outflow rates.