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Register a new userX-ray properties expected from AGN feedback in elliptical galaxies
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Silvia Pellegrini dr.
Publication date
2011
fields
Physics
and research's language is
English
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The ISM evolution of elliptical galaxies experiencing feedback from accretion onto a central black hole was studied recently with high-resolution 1D hydrodynamical simulations including radiative heating and pressure effects, a RIAF-like radiative efficiency, mechanical input from AGN winds, and accretion-driven starbursts. Here we focus on the observational properties of the models in the X-ray band (nuclear luminosity; hot ISM luminosity and temperature; temperature and brightness profiles during quiescence and during outbursts). The nuclear bursts last for ~10^7 yr, with a duty-cycle of a few X (10^-3-10^-2); the present epoch bolometric nuclear emission is very sub-Eddington. The ISM thermal luminosity lx oscillates in phase with the nuclear one; this helps reproduce statistically the observed large lx variation. In quiescence the temperature profile has a negative gradient; thanks to past outbursts, the brightness profile lacks the steep shape typical of inflowing models. Outbursts produce disturbances in these profiles. Most significantly, a hot bubble from shocked hot gas is inflated at the galaxy center; the bubble would be conical in shape, and show radio emission. The ISM resumes a smooth appearance on a time-scale of ~200 Myr; the duty-cycle of perturbances in the ISM is of the order of 5-10%. From the present analysis, additional input physics is important in the ISM-black hole coevolution, to fully account for the properties of real galaxies, as a confining external medium and a jet. The jet will reduce further the mass available for accretion (and then the Eddington ratio $l$), and may help, together with an external pressure, to produce flat or positive temperature gradient profiles (observed in high density environments). Alternatively, $l$ can be reduced if the switch from high to low radiative efficiency takes place at a larger $l$ than routinely assumed.
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Both radiative and mechanical feedback from Active Galactic Nuclei have been found to be important for the evolution of elliptical galaxies. We compute how a shock may be driven from a central black hole into the gaseous envelope of an elliptical galaxy by mechanical as well as radiative feedback (in the form of nuclear winds) using high resolution 1-D hydrodynamical simulations. We calculate the synchrotron emission from the electron cosmic rays accelerated by the shocks (not the jets) in the central part of elliptical galaxies, and we also study the synchrotron spectrums evolution using the standard diffusive shock acceleration mechanism, which is routinely applied to the scaled volume case of supernova remnants. We find good agreement quantitatively between the synchrotron radio emission produced via this mechanism with extant observations of elliptical galaxies which are undergoing outbursts. Additionally, we also find that synchrotron optical and X-ray emission can co-exist inside elliptical galaxies during a certain phase of evolution subsequent to central outbursts. In fact, our calculations predict a synchrotron luminosity of $sim 1.3times 10^6 L_{odot}$ at the frequency 5 GHz (radio band), of $sim 1.1times 10^6 L_{odot}$ at $4.3times10^{14}$ Hz (R band, corresponding to the absolute magnitude -10.4), and of $sim 1.5times 10^{7} L_{odot}$ at $2.4times10^{17}$ Hz (soft X-ray, 0.5 -- 2.0 keV band).
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We find, from high-resolution hydro simulations, that winds from AGN effectively heat the inner parts (~100 pc) of elliptical galaxies, reducing infall to the central SMBH; and radiative (photoionization and X-ray) heating reduces cooling flows at the kpc scale. Including both types of feedback with (peak) efficiencies of 3 10^{-4} < epsilon_mech < 10^{-3} and of epsilon_rad ~10^{-1.3} respectively, produces systems having duty-cycles, central SMBH masses, X-ray luminosities, optical light profiles, and E+A spectra in accord with the broad suite of modern observations of massive elliptical systems. Our main conclusion is that mechanical feedback (including all three of energy, momentum and mass) is necessary but the efficiency, based on several independent arguments must be a factor of 10 lower than is commonly assumed. Bursts are frequent at z>1 and decline in frequency towards the present epoch as energy and metal rich gas are expelled from the galaxies into the surrounding medium. For a representative galaxy of final stellar mass ~3 10^{11} Msun, roughly 3 10^{10} Msun of recycled gas has been added to the ISM since z~2 and, of that, roughly 63% has been expelled from the galaxy, 19% has been converted into new metal rich stars in the central few hundred parsecs, and 2% has been added to the central SMBH, with the remaining 16% in the form hot X-ray emitting ISM. The bursts occupy a total time of ~170 Myr, which is roughly 1.4% of the available time. Of this time, the central SMBH would be seen as an UV or optical source for ~45% and ~71$% of the time, respectively. Restricting to the last 8.5 Gyr, the burst occupy ~44 Myr, corresponding to a fiducial duty-cycle of ~5 10^{-3}.
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X-ray photons, because of their long mean-free paths, can easily escape the galactic environments where they are produced, and interact at long distances with the inter-galactic medium, potentially having a significant contribution to the heating and reionization of the early Universe. The two most important sources of X-ray photons in the Universe are active galactic nuclei (AGN) and X-ray binaries (XRBs). In this Letter we use results from detailed, large scale population synthesis simulations to study the energy feedback of XRBs, from the first galaxies (z~ 20) until today. We estimate that X-ray emission from XRBs dominates over AGN at z>6-8. The shape of the spectral energy distribution of the emission from XRBs shows little change with redshift, in contrast to its normalization which evolves by ~4 orders of magnitude, primarily due to the evolution of the cosmic star-formation rate. However, the metallicity and the mean stellar age of a given XRB population affect significantly its X-ray output. Specifically, the X-ray luminosity from high-mass XRBs per unit of star-formation rate varies an order of magnitude going from solar metallicity to less than 10% solar, and the X-ray luminosity from low-mass XRBs per unit of stellar mass peaks at an age of ~300 Myr and then decreases gradually at later times, showing little variation for mean stellar ages > 3 Gyr. Finally, we provide analytical and tabulated prescriptions for the energy output of XRBs, that can be directly incorporated in cosmological simulations.
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