Mixing between AGN winds and ISM clouds produces luminous X-ray emission

TL;DR

Active galactic nuclei drive energy-driven winds that shock and mix with a clumpy ISM; a newly identified mixing phase at $T \sim 10^{6-7}$ K radiatively cools and fuels a long-lived cool outflow. The paper uses ACDC simulations with Arepo to quantify X-ray emission from this wind-ISM mixing via Bremsstrahlung and full line spectra, including metal-line cooling, across various ISM clump sizes and AGN luminosities. The results show that mixing-generated X-rays peak in equatorial regions, extend to $D \simeq 3-4$ kpc, scale roughly linearly with AGN luminosity, and are enhanced in the soft band by metal lines, potentially detectable in local quasars with Chandra, AXIS, or Lynx. These findings implicate wind-ISM mixing as a dominant source of extended X-ray emission and a tracer of cold-cloud survival in AGN feedback.

Abstract

Active galactic nuclei (AGN) drive powerful, multiphase outflows that are thought to play a key role in galaxy evolution. The hot, shocked phase of these outflows ($T \gtrsim 10^{6} \rm{\ K}$) is expected to dominate the energy content, but is challenging to observe due to its long cooling time and low emissivity. The cool phase ($T \lesssim 10^{4} \rm{\ K}$) is easier to detect observationally, but it traces a less energetic outflow component. In prior simulations of the interaction between an energy-driven AGN outflow and a clumpy ISM, we found that mixing between hot wind and cool ISM clouds produces a new, highly radiative, phase at $T \approx 10^{6-7} \rm{\ K}$ which fuels the formation of a long-lived ($\geq 5\ \rm{Myr}$) cool outflow. We investigate the X-ray emission generated by thermal Bremsstrahlung and high-ionisation metal line emission in this mixing phase, finding that it could contribute significantly to the X-ray output of the outflow. This mixing-induced X-ray emission is strongest in the part of the outflow propagating equatorially through the disc, and is extended on scales of $D\simeq 3-4\ \rm{kpc}$. For quasar luminosities of $L_{\rm{AGN}}\simeq 10^{45-46}\rm{\ erg\ s^{-1}}$, the resulting X-ray luminosity is equivalent to that expected from star formation rates $\rm{SFR}\simeq 10-200\ \rm{M_\odot\ yr^{-1}}$, showing that it could be an important source of soft X-rays in AGN host galaxies. Our results suggest that this extended emission could be resolvable in local quasars ($z\lesssim 0.11$) using high spatial-resolution X-ray observatories such as Chandra, or proposed missions such as AXIS and Lynx.

Figures (14)

• Figure 1: An overview of our simulations, showing the resulting X-ray luminosity from Bremsstrahlung emission at $t{=}{1}{\rm{\ Myr}}$. The top row shows a $(5\ \rm{kpc})^3$ projection of the galaxy disc side-on (left) and top-down (right). Across all the panels, the X-ray luminosity has been integrated along the line of sight and the other quantities are density-weighted averages (temperature, electron density and wind tracer density). The bottom panels show a zoom-in of an initially dense gas cloud and the tail formed from stripped and mixed gas behind it. We find that the strongest X-ray emission due to the quasar-driven outflow comes from gas mixing with the wind in the tail behind the dense clump, forming a luminous chimney, on a scale of a few hundred parsecs.
• Figure 2: Histogram of X-ray luminosity as a function of the wind tracer ($\mathcal{P}$). Each bin shows the sum of the X-ray emission within it. We split the wind into pure, mixed and shocked ISM. The pink and yellow lines show the results from an initially clumpy setup, with $\lambda_{\rm{max}}{=}{170}{\ \rm{pc}}$ and $\lambda_{\rm{max}}{=}{40}{\ \rm{pc}}$ respectively. The blue line shows a smooth setup. The triangle points show the wind tracer value at which 50% of the emission is produced. We can see that the smooth case is dominated by low-$\mathcal{P}$ values (shocked ISM), whereas the clumpy setup has a larger relative contribution from the mixed phase.
• Figure 3: Cumulative total Bremsstrahlung emission as a function of outflow velocity, split into the wind phases as presented in Figure \ref{['fig:tracer']} with the mixed-ISM boundary set at $\mathcal{P}_{\rm{mix}}=10^{-3}$. The horizontal dashed line shows the emission over the whole velocity space (i.e., including the static background). We show (from left to right) the results from our medium clumps (fiducial), small clumps and smooth simulations. In the two clumpy cases, the pure wind dominates at the fastest velocities $v_{\rm{r}}{\gtrsim}{3000}{\rm{\ km\ s^{-1}}}$ (i.e., in the centre near injection and high-velocity vents) but this is negligible overall. Gas that has significantly mixed with the wind dominates from $v_{\rm{r}}{\approx}{100-3000}{\rm{\ km\ s^{-1}}}$. Overall, the shocked ISM and the mixed wind have roughly equal contributions to the total X-ray luminosity for the clumpy case. However, in the smooth case the emission from shocked, unmixed ISM gas ($\mathcal{P}_{\rm{mix}}\lesssim 10^{-3}$) dominates the other sources.
• Figure 4: The radial evolution of the X-ray producing outflow. We show the X-ray luminosity in increasing radial shells, normalised by the volume of the shell. The peak of the outflow decreases and broadens with time, but still shows strong emission at radii $R<2\ \rm{kpc}$. There is some larger-scale emission in the halo ($R>2\ \rm{kpc}$), but this is much fainter than the outflow within the disc, although we not this may be due to our simplistic CGM model.
• Figure 5: The time evolution of the X-ray luminosity from Bremsstrahlung emission. The contribution from the static background ($2.5\times 10^{40}\ \rm{erg\ s^{-1}}$) has been subtracted to reveal the underlying trends. The pink lines show the disc with medium-sized clumps at a range of AGN luminosities (as shown by variable line-style). The dark blue lines show an initially smooth disc at $L_{\rm{AGN}}{=} 10^{45}\rm{\ erg\ s^{-1}}$, and the light blue/orange lines show initially large/small clump sizes.