Abstract
Feedback from outflows driven by active galactic nuclei (AGN) can affect the distribution and properties of the gaseous halos of galaxies. We study the hydrodynamics and non-thermal emission from the forward outflow shock produced by an AGN-driven outflow. We consider a few possible profiles for the halo gas density, self-consistently constrained by the halo mass, redshift and the disk baryonic concentration of the galaxy. We show that the outflow velocity levels off at $\sim 10^3\,\rm km\, s^{-1}$ within the scale of the galaxy disk. Typically, the outflow can reach the virial radius around the time when the AGN shuts off. We show that the outflows are energy-driven, consistently with observations and recent theoretical findings. The outflow shock lights up the halos of massive galaxies across a broad wavelength range. For Milky Way (MW) mass halos, radio observations by The Jansky Very Large Array (JVLA) and The Square Kilometer Array (SKA) and infrared/optical observations by The James Webb Space Telescope (JWST) and Hubble Space Telescope (HST) can detect the emission signal of angular size $\sim 8"$ from galaxies out to redshift $z\sim5$. Millimeter observations by The Atacama Large Millimeter/submillimeter Array (ALMA) are sensitive to non-thermal emission of angular size $\sim 18"$ from galaxies at redshift $z\lesssim1$, while X-ray observations by Chandra, XMM-Newton and The Advanced Telescope for High Energy Astrophysics (ATHENA) is limited to local galaxies ($z\lesssim 0.1$) with an emission angular size of $\sim2'$. Overall, the extended non-thermal emission provides a new way of probing the gaseous halos of galaxies at high redshifts.
Highlights
Outflows from active galactic nuclei (AGN) regulate black hole (BH) growth (Silk & Rees 1998; Di Matteo et al 2005) and may quench star formation (Springel et al 2005; Hopkins et al 2008) in galaxies
As the forward shock plows through the ambient medium supersonically, a broken power-law distribution of non-thermal electrons N (γ) dγ ∝ γ−p (1 + γ/γb)−1 is generated via Fermi acceleration in the shock to produce non-thermal emission, where p is the power-law index. γb is the break Lorentz factor, which is obtained by equating the dynamical timescale ∼ Rs/vs and the cooling timescale 3mec/4(UB + UAGN + UCMB)σTγ
Since the gas distribution in the intergalactic medium (IGM) is uncertain, we restrict our calculation to halo scale within Rvir
Summary
Outflows from active galactic nuclei (AGN) regulate black hole (BH) growth (Silk & Rees 1998; Di Matteo et al 2005) and may quench star formation (Springel et al 2005; Hopkins et al 2008) in galaxies. The velocity of AGN-driven outflows can reach ∼ 103 km s−1 on galaxy scale, indicating that the outflows are likely to propagate into the halos of galaxies while the AGN is active. We propose to use AGN-driven outflows as a probe of the halo gas in galaxies. We explore different gas density profiles in galaxy halos and examine the non-thermal emission from the forward shock plowing into the ambient medium in details. We predict the multiwavelength spectrum and detectability of the non-thermal emission and discuss how the outflow shock and halo gas affect each other. We propose a new way to probe the gaseous halo using the non-thermal emission from the outflow shocks as they travel through the ambient medium in the galaxy and halo.
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