Metal enrichment by radiation pressure in active galactic nucleus outflows - theory and observations

We present a qualitative search for ultra-fast outflows (UFOs) in excess variance spectra of radio-quiet active galactic nuclei (AGNs). We analyse 42 sources from the spectroscopic UFO detection sample, and an additional 22 different sources from the variability sample. A total of 58 sources have sufficient observational data from XMM–Newton EPIC-pn and variability for an excess variance spectrum to be calculated. We examine these spectra for peaks corresponding to variable blue-shifted H- and He-like ion absorption lines from UFOs. We find good evidence for such outflows in 28 per cent of the AGN sample and weak evidence in a further 31 per cent, meaning that ∼30–60 per cent of the AGN sample hosts such UFOs. The mean and median blue-shifted velocity is found to be ∼0.14c and 0.12c, respectively. Current variability methods allow for a fast, model-independent determination of UFOs; however, further work needs to be undertaken to better characterize the statistical significance of the peaks in these spectra by more rigorous modelling. Detecting good evidence for variable UFO lines in a large number of sources also lays the groundwork for detailed analysis of the variability time-scales of the absorbers. This will allow us to probe their densities and hence distances from the central supermassive black hole.

Aim. Ultra-fast outflows (UFO) appear to be common in local active galactic nuclei (AGN) and may be powerful enough (Ėkin ≥ 1% of Lbol) to effectively quench the star formation in their host galaxies. To test feedback models based on AGN outflows, it is mandatory to investigate UFOs near the peak of AGN activity, that is, at high-z where only a few studies are available to date. Methods. UFOs produce Fe resonant absorption lines measured above ≈7 keV. The most critical problem in detecting such features in distant objects is the difficulty in obtaining X-ray data with sufficient signal-to-noise. We therefore selected a distant QSO that gravitational lensing made bright enough for these purposes, the z = 2.64 QSO MG J0414+0534, and observed it with XMM-Newton for ≈78 ks. Results. The X-ray spectrum of MG J0414+0534 is complex and shows signatures of cold absorption (NH ≈ 4 × 1022 cm−2) and of the presence of an iron emission line (E ≈ 6.4 keV, EW = 95 ± 53 eV) consistent with it originating in the cold absorber. Our main result, however, is the robust detection (more than 5σ) of an absorption line at Eint ≈ 9.2 keV (Eobs ≈ 2.5 keV observer frame). If interpreted as due to FeXXVI, it implies gas outflowing at vout ≈ 0.3c. To our knowledge, this is the first detection of an UFO in a radio-loud quasar at z ≥ 1.5. We estimated that the UFO mechanical output is Ėkin ≈ 2.5Lbol with ṗout∕ṗrad ≈ 17 indicating that it is capable of installing significant feedback between the super-massive black hole and the bulge of the host galaxy. We argue that this also suggests a magnetic driving origin of the UFO.

Context. Broad absorption line (BAL) quasars exhibit significant outflows, offering insights into active galactic nucleus (AGN) feedback. While typically associated with high Eddington ratios, BAL quasars also occur in low Eddington ratio regimes, which remain poorly understood. The aim of this study is to compare BAL properties and variability across these regimes. Aims. We investigate the occurrence rates, absorption characteristics, and variability of BAL quasars at low and high Eddington ratios. Methods. Using the SDSS DR16 quasar catalog, we selected a redshift-matched control sample to compare low and high Eddington ratio BAL quasar sources. We first examined the BAL fraction as a function of Eddington ratio. Key absorption parameters (equivalent width, absorption line width, velocity range, and depth) were analyzed, and a multi-epoch variability study was conducted using repeat spectra, followed by a comparison of parameter distributions between the two samples. Results. For the first time, we report an increase in the BAL fraction toward low Eddington ratios, in addition to the previously known trend of high BAL fraction at high Eddington ratios. While high Eddington sources show extreme absorption features, overall distributions are statistically similar except for maximum outflow velocity. No significant variability differences were observed. The correlation between outflow velocity, Eddington ratio, and luminosity supports the role of radiation pressure in driving quasar outflows. For low Eddington ratios, additional mechanisms, such as softer SEDs, larger outflow distances, and thickened accretion disks from radiatively inefficient processes, likely drive outflow formation.

One of the main mechanisms that could drive mass outflows in active galactic nuclei (AGNs) is radiation pressure due to spectral lines. Although straightforward to understand, the actual magnitude of the radiation force is challenging to compute because the force depends on the physical conditions in the gas, as well as the strength, spectral energy distribution (SED), and geometry of the radiation field. We present results from our photoionization and radiation transfer calculations of the force multiplier, M(ξ, t), using the same radiation field to compute the gas photoionization and thermal balance. We assume low gas density (n = 104 cm−3) and column density (N ≤ 1017 cm−2), a Boltzmann distribution for the level populations, and the Sobolev approximation. Here, we describe results for two SEDs corresponding to an unobscured and obscured AGN in NGC 5548. Our main results are the following: (1) although M(ξ, t) starts to decrease with ξ for ξ ≳ 1 as shown by others, this decrease in our calculations is relatively gradual and could be nonmonotonic as M(ξ, t) can increase by a factor of a few for ξ ≈ 10–1000; (2) at these same ξ for which the multiplier is higher than in previous calculations, the gas is thermally unstable by the isobaric criterion; (3) non-local thermodynamic equilibrium effects reduce M(t, ξ) by over two orders of magnitude for ξ ≳ 100. The dynamical consequence of result (1) is that line driving can be important for ξ as high as 1000 when the LTE approximation holds, while result (2) provides a natural cloud formation mechanism that may account for the existence of narrow line regions. Result (3) suggests that line driving may not be important for ξ ≳ 100 in tenuous plasma.

3C 275.1 is a blue quasar at z = 0.55522, hosting powerful outflows and residing in a complex environment. We present a serendipitously detected giant nebula surrounding 3C 275.1, which shows morphological features resembling those of objects known as “jellyfish galaxies,” with extremely long tails of ionized gas extending to 170 kpc in projection. We analyze its optical spectra taken by MUSE on the Very Large Telescope. The brighter part of this giant nebula exceeds 100 kpc, and its rotation curve does not flatten out, making it very different from those of normal spiral galaxies. This system shares some characteristics common to those formed via ram pressure stripping (RPS), yet its long narrow tails and higher ionization are unusual compared to known tails in jellyfish galaxies, not fully consistent with a simple RPS scenario. Our photoionization simulation and the inferred short recombination timescale both suggest that, besides the quasar 3C 275.1, an extra source of ionization is necessary to keep the gas ionized at such a distance from the nucleus, which could be related to RPS, tidal interaction, or active galactic nucleus outflow, providing new evidence of active dynamical interaction of a galaxy with the intracluster medium.

In symbiotic binaries, jets and disk winds may be integral to the physics of accretion onto white dwarfs from cool giants. The persistent outflow from symbiotic star MWC 560 (=V694 Mon) is known to manifest as broad absorption lines (BALs), most prominently at the Balmer transitions. We report the detection of high-ionization BALs from C IV, Si IV, N V, and He II in International Ultraviolet Explorer spectra obtained on 1990 April 29-30, when an optical outburst temporarily erased the obscuring 'iron curtain' of absorption troughs from Fe II and similar ions. The C IV and Si IV BALs reached maximum radial velocities at least 1000 km/s higher than contemporaneous Mg II and He II BALs; the same behaviors occur in the winds of quasars and cataclysmic variables. An iron curtain lifts to unveil high-ionization BALs during the P Cygni phase observed in some novae, suggesting by analogy a temporary switch in MWC 560 from persistent outflow to discrete mass ejection. At least three more symbiotic stars exhibit broad absorption with blue edges faster than 1500 km/s; high-ionization BALs have been reported in AS 304 (=V4018 Sgr), while transient Balmer BALs have been reported in Z And and CH Cyg. These BAL-producing fast outflows can have wider opening angles than has been previously supposed. BAL symbiotics are short-timescale laboratories for their giga-scale analogs, broad absorption line quasars (BALQSOs), which display a similarly wide range of ionization states in their winds.

In a previous comparative study by Weymann et al. of the emission line and continuum properties of radio-quiet QSOs and broad absorption-line (BAL) QSOs, a difference spectrum, in the region of the C IV λ1549 trough, comprising the mean spectrum of 34 BALQSOs and the mean spectrum of 31 non-BALQSOs, surprisingly revealed double troughs separated in velocity space by the N v λ1240-Lyα splitting of ~ 5900 km s^(-1). In this contribution we investigate the reality of these features in a spectroscopic sample of72 BALQSOs consisting of the original 34 BALQSOs and an additional 38 BALQSOs. An atlas of the C IV BALs of72 BALQSOs is presented. We find that only 22% of this sample explicitly exhibits the double trough feature. When present the double troughs are deep. A Monte Carlo simulation of the mean C IV BAL suggests that the double-trough feature is real at only the 95%-98% level (as opposed to being the result of statistical fluctuations of BAL troughs occurring over random outflow velocities in a limited sample). We describe possible scenarios for their formation, involving the line-locking of Lyα-Nv within the BAL region. We show that no simple mechanism involving radiation pressure, alone can create the deep double troughs through the shadowing of N v λ1240 by Lyα. More complicated amplification mechanisms may be necessary. If real, the double-trough signature has the following important implications to our understanding of the BAL phenomenon: (1) The occurrence of broad absorption is somehow tied to an absolute velocity frame which is the same from quasar to quasar. (2) It follows then that the outflowing material within the BAL region is accelerating. (3) If line-locking is the mechanism behind the formation of the double troughs, then radiation pressure plays an important role in the dynamics of the BAL region.

Context. Galaxy-wide outflows driven by star formation and/or an active galactic nucleus (AGN) are thought to play a crucial rule in the evolution of galaxies and the metal enrichment of the inter-galactic medium. Direct measurements of these processes are still scarce and new observations are needed to reveal the nature of outflows in the majority of the galaxy population.Aims. We combine extensive, spatially-resolved, multi-wavelength observations, taken as part of the Close AGN Reference Survey (CARS), for the edge-on disc galaxy HE 1353−1917 in order to characterise the impact of the AGN on its host galaxy via outflows and radiation.Methods. Multi-color broad-band photometry was combined with spatially-resolved optical, near-infrared (NIR) and sub-mm and radio observations taken with the Multi-Unit Spectroscopy Explorer (MUSE), the Near-infrared Integral Field Spectrometer (NIFS), the Atacama Large Millimeter Array (ALMA), and theKarl G. JanskyVery Large Array (VLA) to map the physical properties and kinematics of the multi-phase interstellar medium.Results. We detect a biconical extended narrow-line region ionised by the luminous AGN orientated nearly parallel to the galaxy disc, extending out to at least 25 kpc. The extra-planar gas originates from galactic fountains initiated by star formation processes in the disc, rather than an AGN outflow, as shown by the kinematics and the metallicity of the gas. Nevertheless, a fast, multi-phase, AGN-driven outflow with speeds up to 1000 km s−1is detected close to the nucleus at 1 kpc distance. A radio jet, in connection with the AGN radiation field, is likely responsible for driving the outflow as confirmed by the energetics and the spatial alignment of the jet and multi-phase outflow. Evidence for negative AGN feedback suppressing the star formation rate (SFR) is mild and restricted to the central kpc. But while any SFR suppression must have happened recently, the outflow has the potential to greatly impact the future evolution of the galaxy disc due to its geometrical orientation.Conclusions.. Our observations reveal that low-power radio jets can play a major role in driving fast, multi-phase, galaxy-scale outflows even in radio-quiet AGN. Since the outflow energetics for HE 1353−1917 are consistent with literature, scaling relation of AGN-driven outflows the contribution of radio jets as the driving mechanisms still needs to be systematically explored.

Galaxy-wide outflows driven by active galactic nuclei (AGN) are an important ingredient in galaxy evolution. Analytical calculations suggest that such outflows have significant inertia and can persist long after the AGN itself fades away. We use hydrodynamical simulations of outflows in idealized galaxy bulges to investigate the propagation of these ‘fossil’ AGN outflows. We find that fossil outflows should be common in gas-poor galaxies but form only rarely in gas-rich ones; in general, fossil outflows should outnumber driven ones by a factor of a few in the local Universe, and possibly more at high redshift. When they do form, fossil outflows tend to be lopsided and detached from the nucleus, and colder than their driven counterparts, with a more prominent molecular phase. Spatially resolved and/or multiphase observations can help distinguish fossil AGN outflows from star formation-driven ones, which have similar integrated properties. We discuss a number of spatially-resolved observations of outflows, suggesting that most show evidence of fossil outflow existence, sometimes together with driven outflows on smaller scales.

A fraction of quasars present broad absorption lines, produced by outflowing gas with typical velocities of 3000 - 10,000 km/s. If the outflowing gas fills a significant fraction of the volume where it resides, then it will be highly ionized by the quasar due to its low density, and will not produce the observed UV absorption. The suggestion that the outflow is shielded from the ionizing radiation was excluded by recent observations. The remaining solution is a dense outflow with a filling factor $f<10^{-3}$. What produces such a small $f$? Here we point out that radiation pressure confinement (RPC) inevitably leads to gas compression and the formation of dense thin gas sheets/filaments, with a large gradient in density and ionization along the line of sight. The total column of ionized dustless gas is a few times $10^{22}$ cm$^{-2}$, consistent with the observed X-ray absorption and detectable P V absorption. The predicted maximal columns of various ions show a small dependence on the system parameters, and can be used to test the validity of RPC as a solution for the overionization problem. The ionization structure of the outflow implies that if the outflow is radiatively driven, then broad absorption line quasars should have $L/L_{\rm Edd} \gtrsim 0.1$.

Otherwise acceptable kinematic and dynamic models for emission line regions may be ruled out if the line-emitting clouds are unstable. We discuss the stability of radially-moving clouds to lateral gas flows. Such flows result from internal overpressures arising from the absorption of continuum radiation.

The absorption of continuum radiation from a discrete radio source by interstellar neutral hydrogen (Hagen and McClain 1954; Williams and Davies 1954, 1955; Hagen, Lilley and McClain 1955) permits one to assign relative distance positions to the source and the hydrogen gas along the line of sight. The absorption effect does not of itself yield an absolute distance scale. The ability of the absorption technique to reveal structure~in distance is based on the assumption that the radial velocity along the line of sight is a function of distance. This is generally conceded to be true and to be a function of differential galactic rotation for all galactic longitudes with the exception of the galactic center and anticenter. Here the classical assumption of pure circular motion would preclude the existence of a radial velocity other than zero. An approximate distance scale has been derived in the direction of the galactic center from a study of the hydrogen as a function of radial velocity. If one assumes that the galaxy is a flat disk of thickness 1, an observer inside the disk can relate the angular width ~ of the disk, normal to the plane, to the distance d at which the angle was measured by the relation (p t tan - = - 2 2d The angular extent of the gas was measured in the velocity range -40 to +40 km/sec. By assuming a thickness of 240 pc for the galaxy a relation has been found between random velocity and distance. These measurements indicate that the random velocity of the gas probably increases rather rapidly beyond about 2 kpc from the sun. The absorption effect for source I 7S2A (Sagittarius A) occurs roughly within the velocity range -10 to + 15 km/sec corresponding to a distance of 3 or 4 kpc. A study of the uncertainties involved in the measurements places extreme limits on the distance of from 2 to 6 kpc with the most probable distance being between 3 and 4 kpc. Using an independent method, Williams and Davies (1955) have found a distance of about 3 kpc, in good agreement with the present result. A consideration of the measurements reported herein leads to the conclusion that the radio source responsible for the intense emission peakati = ~~7?8andb =- ~?~isprobablynot associated with the galactic center but rather is superimposed on the broad central galactic background. It is interesting to note that Haddock and McCullough (1955) have suggested that this source is thermal and possibly associated with an HIl region in the group of OB stars studied by Hiltner (1954). Haddock, F. T. and McCullough T. P., 1955, A. J. 6o, i6i. Hagen, J. P. and McClain, E. F. 1954, Ap. J. 120, 364. Hagen, J. P., Lilley, A. E. and McClain, E. F. 1955, Ap. J. 122, 361. Hiltner, W. A. 1954, Ap. J. 120, 41. Williams, D. R. W. and Davies, R. D. 1954, Nature `73, 1182. 1955, Ibid. 175, 1079. Naval Research Laboratory, Washington, D. C.

The excitation of FeII emission lines is reviewed, with special emphasis on photoionized nebulae. The physical conditions, the geometry and the techniques required to calculate FeII models are explained and the following mechanisms discussed: a: Excitation by absorption of continuum radiation. b: Excitation by inelastic collisions, and c: Excitation by line fluorescence. Possibility a. has been over-looked in the past and is probably not important in objects showing strong optical FeII lines. Collisional excitation dominates in nature but cannot, in itself, explain the strong observed lines from energy levels of 7 to 10 eV. Line fluorescence, with Lα, CIV and most important among FeII lines, is probably the key to the understanding of the high energy lines. A recipe for calculating FeII models is given, showing the processes to include and the techniques to use. Major recent findings about the FeII spectrum of AGN are discussed in detail, in particular “the FeII problem” in these objects. Despite big improvements in the understanding of the 2000–4000A emission bump and other FeII features, there are still several open questions like the role of dust, the iron abundance, the geometry and density of the broad line region and the general validity of photoionization models.

The 2024 X-ray/UV observation campaign of NGC 3783, led by XRISM, revealed the launch of an ultrafast outflow (UFO) with a radial velocity of 0.19 c (57 000 km s −1 ). This event is synchronized with the sharp decay, within less than half a day, of a prominent soft X-ray/UV flare. Accounting for the look-elsewhere effect, the XRISM Resolve data alone indicate a low probability of 2 × 10 −5 that this UFO detection is due to random chance. The UFO features narrow H-like and He-like Fe lines with a velocity dispersion of ∼1000 km s −1 , suggesting that it originates from a dense clump. Beyond this primary detection, there are hints of weaker outflow signatures throughout the rise and fall phases of the soft flare. Their velocities increase from 0.05 c to 0.3 c over approximately three days, and they may be associated with a larger stream in which the clump is embedded. The radiation pressure is insufficient to drive the acceleration of this rapidly evolving outflow. The observed evolution of the outflow kinematics instead closely resembles that of solar coronal mass ejections, implying magnetic driving and, conceivably, reconnection near the accretion disk as the likely mechanisms behind both the UFO launch and the associated soft flare.

The detection of blueshifted absorption lines likely associated with ionized iron K-shell transitions in the X-ray spectra of many active galactic nuclei (AGNs) suggests the presence of a highly ionized gas outflowing with mildly relativistic velocities (0.03c–0.6c) named ultra-fast outflow (UFO). Within the SUBWAYS project, we characterized these winds starting from a sample of 22 radio-quiet quasars at an intermediate redshift (0.1 ≤ z ≤ 0.4) and compared the results with similar studies in the literature on samples of local Seyfert galaxies (i.e., 42 radio-quiet AGNs observed with XMM-Newton at z ≤ 0.1) and high redshift radio-quiet quasars (i.e., 14 AGNs observed with XMM-Newton and Chandra at z ≥ 1.4). The scope of our work is a statistical study of UFO parameters and incidence considering the key physical properties of the sources, such as supermassive black hole (SMBH) mass, bolometric luminosity, accretion rates, and spectral energy distribution (SED) with the aim of gaining new insights into the UFO launching mechanisms. We find indications that highly luminous AGNs with a steeper X-ray/UV ratio, αox, are more likely to host UFOs. The presence of UFOs is not significantly related to any other AGN property in our sample. These findings suggest that the UFO phenomenon may be transient. Focusing on AGNs with UFOs, other important findings from this work include: (1) faster UFOs have larger ionization parameters and column densities; (2) X-ray radiation plays a more crucial role in driving highly ionized winds compared to UV; (3) the correlation between outflow velocity and luminosity is significantly flatter than what is expected for radiatively driven winds; (4) more massive black holes experience higher wind mass losses, suppressing the accretion of matter onto the black hole; (5) the UFO launching radius is positively correlated with the Eddington ratio. Furthermore, our analysis suggests the involvement of multiple launching mechanisms, including radiation pressure and magneto-hydrodynamic processes, rather than pointing to a single, universally applicable mechanism.