The origin of the extragalactic γ-ray background permeating throughout the Universe remains a mystery forty years after its discovery 1 . The extrapolated population of blazars can account for only half of the background radiation in the energy range of ∼0.1-10 GeV (refs 2,3). Here we show that quasar-driven outflows generate relativistic protons that produce the missing component of the extragalactic γ-ray background and naturally match its spectral fingerprint, with a generic break above ∼1 GeV. The associated γ-ray sources are too faint to be detected individually, explaining why they had not been identified so far. However, future radio observations may image their shock fronts directly. Our best fit to the Fermi-LAT observations of the extragalactic γ-ray background spectrum provides constraints on the outflow parameters that agree with observations of these outflows 4-7 and theoretical predictions 8,9 . Although our model explains the data, there might be additional contributing sources.The components of the extragalactic γ-ray background (EGB) have been a puzzle since its discovery four decades ago 10 . Recently, the Large Area Telescope (LAT) on Fermi provided a fifty-month measurement of the integrated emission from γ-ray sources, with photon energies extending from 0.1 to 820 GeV (ref. 2). The latest analysis of Fermi-LAT data implies that both resolved and unresolved blazars account for ∼50 +12 −11 % of the EGB in the energy range of 0.1-10 GeV, leaving the origin of the remaining component in question 3 .Active galactic nuclei (AGN) are observed to exhibit strong outflows, with velocities of ∼0.1c, as manifested by broad absorption lines 5,7 . The ratio between the input kinetic luminosity of the outflows L in and the bolometric luminosity of quasars L bol , f kin , is observationally inferred to be f kin ∼ 1-5% (refs 4-7). The shock wave produced by the interaction of a quasar-driven outflow with the surrounding interstellar medium is expected to accelerate protons to relativistic energies, similarly to the shocks surrounding supernova (SN) remnants, where observations of γ-ray emission due to decay of neutral pion (π 0 ) indicate relativistic proton-proton (pp) collisionsHere, we calculate the analogous γ-ray emission from quasar-driven outflows. The energy distribution of accelerated protons per unit volume can be written as, where E p is the proton energy, N 0 is a normalization constant, and the power-law index Γ p ∼ 2-3, based on theoretical models 12 and observations of shocks around SN remnants 11,13 . We adopt a fiducial value of Γ p ∼ 2.7 and show that our results are not very sensitive to variations around this value (see Supplementary Fig. 1 and we have verified that our results are not sensitive to variations around this value. Here, n p,0 = (n p /1 cm −3 ) is the proton number density, σ pp,−26 = (σ pp /10 −26 cm 2 ) is the inelastic cross section of pp collision, and R s,kpc = (R s /1 kpc) and v s,3 = (v s /10 3 km s −1 ) are the radius and velocity of the outflowing shell, which can ...

We present deep 3 GHz VLA observations of the potentially recoiling black hole CID-42 in the COSMOS field. This galaxy shows two optical nuclei in the HST/ACS image and a large velocity offset of ≈ 1300 km s −1 between the broad and narrow Hβ emission line although the spectrum is not spacially resolved (Civano et al. 2010). The new 3 GHz VLA data has a bandwidth of 2 GHz and to correctly interpret the flux densities imaging was done with two different methods: multi-scale multi-frequency synthesis and spectral windows stacking. The final resolutions and sensitivities of these maps are 0.7 ′′ with rms = 4.6 µJy beam −1 and 0.9 ′′ with rms = 4.8 µJy beam −1 respectively. With a 7σ detection we find that the entire observed 3 GHz radio emission can be associated with the South-Eastern component of CID-42, coincident with the detected X-ray emission. We use our 3 GHz data combined with other radio data from the literature ranging from 320 MHz to 9 GHz, which include the VLA, VLBA and GMRT data, to construct a radio synchrotron spectrum of CID-42. The radio spectrum suggests a type I unobscured radio-quiet flat-spectrum AGN in the South-Eastern component which may be surrounded by a more extended region of old synchrotron electron population or shocks generated by the outflow from the supermassive black hole. Our data are consistent with the recoiling black hole picture but cannot rule out the presence of an obscured and radio-quiet SMBH in the North-Western component.

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 ∼ 10 3 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 ∼ 8 ′′ from galaxies out to redshift z ∼ 5. Millimeter observations by The Atacama Large Millimeter/submillimeter Array (ALMA) are sensitive to non-thermal emission of angular size ∼ 18′′ from galaxies at redshift z 1, while X-ray observations by Chandra, XMM-Newton and The Advanced Telescope for High Energy Astrophysics (ATHENA) is limited to local galaxies (z 0.1) with an emission angular size of ∼ 2 ′ . Overall, the extended non-thermal emission provides a new way of probing the gaseous halos of galaxies at high redshifts.

Quasar-driven outflows naturally account for the missing component of the extragalactic γ-ray background through neutral pion production in interactions between protons accelerated by the forward outflow shock and interstellar protons. We study the simultaneous neutrino emission by the same protons. We adopt outflow parameters that best fit the extragalactic γ-ray background data and derive a cumulative neutrino background of ∼ 10 −7 GeV cm −2 s −1 sr −1 at neutrino energies Eν 10 TeV, which naturally explains the most recent IceCube data without tuning any free parameters. The link between the γ-ray and neutrino emission from quasar outflows can be used to constrain the high-energy physics of strong shocks at cosmological distances.Introduction.-There is currently strong observational evidence for the existence of large-scale outflows driven by the active galactic nuclei (AGN), including the presence of broad absorption lines in quasars [3,4] and multiphase outflows in nearby ultraluminous infrared galaxies (ULIRGs) [1,2]. Semi-relativistic winds with a speed of ∼ 0.1 c are typically produced by quasars in the surrounding interstellar medium, driving a forward shock that accelerates a swept-up shell accompanied by a reverse shock that decelerates the wind itself [5,6].In a previous paper, we derived a detailed hydrodynamical model for the quasar outflow's interaction with the ambient medium [7] (see Supplemental Material), including a disk and a halo components for the host galaxy gas. The gas density profile was self-consistently determined by the halo mass and redshift. The continuous energy injection was assumed to be a fraction of the quasar's bolometric luminosity f kin L bol during the quasar's lifetime, which is of order the Salpeter time t sal ∼ 4 × 10 7 yrs for a radiative efficiency of 0.1 [8]. In the upper panel of Fig.2, we show the forward shock velocity v s as a function of radius R s for the outflow in a dark matter halo of mass ∼ 10 12 M ⊙ at a redshift of z ∼ 0.1. We find that v s 10 3 km s −1 within the galactic disk with a decline to few hundreds km s −1 when the outflow reaches the edge of the halo. In analogy with supernova (SN) remnants [9,10], protons should be accelerated via Fermi acceleration to relativistic energies in the forward outflow shock.The resulting proton number density per unit volume per unit energy can be expressed as a power-law with an exponential high-energy cutoff:

We study star formation within outflows driven by active galactic nuclei (AGN) as a new source of hypervelocity stars (HVSs). Recent observations revealed active star formation inside a galactic outflow at a rate of ∼ 15 M ⊙ yr −1 . We verify that the shells swept up by an AGN outflow are capable of cooling and fragmentation into cold clumps embedded in a hot tenuous gas via thermal instabilities. We show that cold clumps of ∼ 10 3 M ⊙ are formed within ∼ 10 5 yrs. As a result, stars are produced along outflow's path, endowed with the outflow speed at their formation site. These HVSs travel through the galactic halo and eventually escape into the intergalactic medium. The expected instantaneous rate of star formation inside the outflow is ∼ 4 − 5 orders of magnitude greater than the average rate associated with previously proposed mechanisms for producing HVSs, such as the Hills mechanism and three-body interaction between a star and a black hole binary. We predict the spatial distribution of HVSs formed in AGN outflows for future observational probe.