Résultats de recherche pour « AGN »

Active Galactic Nuclei (AGN) exhibit large luminosity variations over the entire electromagnetic spectrum on timescales ranging from hours to years. The variations in luminosity are devoid of any periodic character and appear stochastic. While complex correlations exist between the variability observed in different parts of the electromagnetic spectrum, no frequency band appears to be completely dominant, suggesting that the physical processes producing the variability are exceedingly rich and complex. In the absence of a clear theoretical explanation of the variability, phenomenological models are used to study AGN variability. The stochastic behavior of AGN variability makes formulating such models difficult and connecting them to the underlying physics exceedingly hard. We study AGN light curves serendipitously observed by the NASA Kepler planet-finding mission. Compared to previous ground-based observations, Kepler offers higher precision and a smaller sampling interval resulting in potentially higher quality light curves. Using structure functions, we demonstrate that (1) the simplest statistical model of AGN variability, the damped random walk (DRW), is insufficient to characterize the observed behavior of AGN light curves; and (2) variability begins to occur in AGN on time-scales as short as hours. Of the 20 light curves studied by us, only 3-8 may be consistent with the DRW. The structure functions of the AGN in our sample exhibit complex behavior with pronounced dips on time-scales of 10-100 d suggesting that AGN variability can be very complex and merits further analysis. We examine the accuracy of the Kepler pipeline-generated light curves and find that the publicly available light curves may require re-processing to reduce contamination from field sources. We show that while the re-processing changes the exact PSD power law slopes inferred by us, it is unlikely to change the conclusion of our structure function study-Kepler AGN light curves indicate that the DRW is insufficient to characterize AGN variability. We provide a new approach to probing accretion physics with variability by decomposing observed light curves into a set of impulses that drive diffusive processes using C-ARMA models. Applying our approach to Kepler data, we demonstrate how the time-scales reported in the literature can be interpreted in the context of the growth and decay time-scales for flux perturbations and tentatively identify the flux perturbation driving process with accretion disk turbulence on length-scales much longer than the characteristic eddy size. Our analysis technique is applicable to (1) studying the connection between AGN sub-type and variability properties; (2) probing the origins of variability by studying the multi-wavelength behavior of AGN; (3) testing numerical simulations of accretion flows with the goal of creating a library of the variability properties of different accretion mechanisms; (4) hunting for changes in the behavior of the accretion flow by block-analyzing observed light curves; and (5) constraining the sampling requirements of future surveys of AGN variability.

In this article, we discuss the state of “AGN feedback” in radio-quiet (RQ) AGN. This study involves heterogeneous samples of nearby Seyfert and LINER galaxies as well as quasi-stellar objects (QSOs) that have been observed at low radio frequencies (few ∼100 MHz) with the upgraded Giant Meterwave Radio Telescope (GMRT) and ∼GHz frequencies with the Karl G. Jansky Very Large Array (VLA) and Very Long Baseline Array (VLBA). These multi-frequency, multi-resolution observations detect a range of arcsecond-scale radio spectral indices that are consistent with the presence of multiple contributors including starburst winds and AGN jets or winds; steep spectrum “relic” emission is observed as well. Polarization-sensitive data from the VLA and GMRT suggest that the radio outflows are stratified (e.g., in IIIZw2, Mrk231); distinct polarization signatures suggest that there could either be a “spine + sheath” structure in the radio outflow, or there could be a “jet + wind” structure. Similar nested biconical outflows can also explain the VLBA and SDSS emission-line data in the KISSR sample of double-peaked emission-line Seyfert and LINER galaxies. Furthermore, the modeling of the emission-lines with plasma modeling codes such as MAPPINGS indicates that parsec-scale jets and winds in these sources can disturb or move the narrow-line region (NLR) gas clouds via the “shock + precursor” mechanism. Apart from the presence of “relic” emission, several Seyfert and LINER galaxies show clear morphological signatures of episodic jet activity. In one such source, NGC2639, at least four distinct episodes of jets are observed, the largest one of which was only detectable at 735 MHz with the GMRT. Additionally, a ∼6 kpc hole in the CO molecular gas along with a dearth of young stars in the center of its host galaxy is observed. Multiple jet episodes on the 10–100 parsec scales and a ∼10 parsec hole in the molecular gas is also observed in the Seyfert galaxy NGC4051. This suggests a link between episodic jet activity in RQ AGN and “AGN feedback” influencing the evolution of their host galaxies. However, a similar simple relationship between radio outflows and molecular gas mass is not observed in the Palomar–Green (PG) QSO sample, indicating that “AGN feedback” is a complex phenomenon in RQ AGN. “AGN feedback” must occur through the local impact of recurring multi-component outflows in RQ AGN. However, global feedback signatures on their host galaxy properties are not always readily evident.

Abstract We compare predictions of how active galactic nuclei (AGNs) populate host galaxies at low redshifts to observations, finding large discrepancies between cosmological simulation predictions and observed patterns. Modern cosmological simulations include AGN feedback models tuned to reproduce the observed galaxy stellar mass function. However, due to a lack of real understanding of the physics of AGN feedback, these models vary significantly across simulations. To distinguish between the models and potentially test the underlying physics, we carry out independent tests of these models. In an earlier study, we found that F AGN —the observed completeness-corrected fraction of galaxies hosting radio AGNs with an Eddington ratio λ > 10 −3 —to be a strong function of host-galaxy stellar mass ( M ⋆ ) but nearly independent of host specific star formation rates (sSFRs) at fixed M ⋆ . In this study, we test the radio mode AGN feedback models of the EAGLE, SIMBA, and TNG100 simulations by comparing their predictions of F AGN M ⋆ to our observational constraint. We find that none of these simulations even qualitatively reproduce the observed dependencies of F AGN on M ⋆ and sSFR. Finally, we find that although the given TNG100 model could be modified in order to better reproduce the observed F AGN trend, this modification would likely also change its prediction for the local stellar mass function and star formation rates—key observations used for calibrating the simulation in the first place. Our findings highlight a pressing need to revisit the AGN feedback prescriptions in EAGLE, SIMBA, TNG100, and other similar models.

Abstract The pharmacokinetics of AGN 190168, a novel synthetic retinoid, and its major metabolite, AGN 190299, in rat blood after intravenous administration was investigated. Approximately 4.4 mg kg −1 (high dose) or 0.49 mg kg −1 (low dose) of AGN 190168 was administered to rats via the femoral vein. Blood was collected from the femoral artery at various time points during an 8 h period. Blood concentrations of AGN 190168 and AGN 190299 were determined by a specific and sensitive high‐pressure liquid chromatographic (HPLC) method. AGN 190168 was rapidly metabolized in rats. The only detectable drug‐related species in the blood was AGN 190299. Therefore, only pharmacokinetics of AGN 190299 were calculated. Elimination of AGN 190299 appeared to be non‐linear after administration of the high dose, and linear after administration of the low dose. The maximum elimination rate ( V max ) and the concentration at half of the V max ( k m ), as estimated by a Michaelis—Menten one‐compartment model, were 7.58 ± 2.42 μg min −1 (mean ± SD) and 6.10 ± 1.58 μg mL −1 , respectively. The value of the area under the blood concentration time curve (AUC) was 9.54 ± 1.68 μg h mL −1 after administration of the high dose and 0.594 ± 0.095 μg h mL −1 after administration of the low dose. The clearance value was 7.79 ± 1.20 mL min −1 kg −1 after the high dose, statistically significantly different from that after the low dose ( p < 0.05), 14.0 ± 2.2 mL min −1 kg −1 . The terminal half‐life ( t 1/2 ) was 1.25 ± 0.74 h for the high‐dose group and 0.95 ± 0.16 h for the low‐dose group. Study results demonstrate rapid systemic metabolism of AGN 190168 to AGN 190299, non‐linear pharmacokinetics of AGN 190299 after the 4.4 mg kg −1 dose, and the lack of difference in disposition profiles between sexes after intravenous administration of AGN 190168 to rats.