Probing the gaseous halo of galaxies through non-thermal emission from AGN-driven outflows

2016

Nature Phys

Self Cite

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 ...

mentioning

confidence: 83%

“…The outer forward shock accelerates the swept-up medium, while the inner reverse shock decelerates the wind itself. The equations describing outflow hydrodynamics are given by 17,29 :…”

Section: Methodsmentioning

confidence: 99%

mentioning

confidence: 99%

mentioning

confidence: 99%

See 2 more Smart Citations

Contribution of quasar-driven outflows to the extragalactic gamma-ray background

2016

Nature Phys

Self Cite

“…It includes the detection of multi-phase outflows in nearby ultraluminous infrared galaxies [8,9] and the presence of broad absorption lines in quasars [10,11]. In previous work [12], we derived a detailed hydrodynamical model of quasar outflow's interaction with the ambient interstellar medium (ISM) (See Ref. [12] for details).…”

mentioning

confidence: 99%

Ultrahigh energy cosmic rays from nonrelativistic quasar outflows

2017

Phys. Rev. D

Self Cite

It has been suggested that non-relativistic outflows from quasars can naturally account for the missing component of the extragalactic γ-ray background and explain the cumulative neutrino background through pion decay in collisions between protons accelerated by the outflow shock and interstellar protons. Here we show that the same quasar outflows are capable of accelerating protons to energies of ∼ 10 20 eV during the early phase of their propagation. The overall quasar population is expected to produce a cumulative ultra high energy cosmic ray flux of ∼ 10 −7 GeV cm −2 s −1 sr −1 at ECR 10 18 eV. The spectral shape and amplitude is consistent with recent observations for outflow parameters constrained to fit secondary γ-rays and neutrinos without any additional parameter tuning. This indicates that quasar outflows simultaneously account for all three messengers at their observed levels.

Formation and spatial distribution of hypervelocity stars in AGN outflows

2018

New Astronomy

Self Cite

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.