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Photoionization models for extreme Ly <i>α λ</i> 1216 and Hell <i>λ</i> 1640 line ratios in quasar halos, and PopIII vs. AGN diagnostics
摘要： Aims. We explore potential mechanisms to produce extremely high Lyα/HeII ?ux ratios, or to enhance the observed number of Lyα photons per incident ionizing photon, in extended active galactic nucleus (AGN) photoionized nebulae at high-redshift. Methods. We computed models to simulate, in the low density regime, photoionization of interstellar gas by the radiation ?eld of a luminous AGN. We have explored the impact of ionization parameter, gas metallicity, ionizing spectrum, electron energy distribution, and cloud viewing angle on the relative ?uxes of Lyα, HeII and other lines, and on the observed number of Lyα photons per incident ionizing photon. We have compared our model results with recent observations of quasar Lyα halos at z ～ 3.5. Results. Low ionization parameter, a relatively soft or ?ltered ionizing spectrum, low gas metallicity, κ-distributed electron energies, or re?ection of Lyα photons by neutral hydrogen can all result in signi?cantly enhanced Lyα relative to other lines (≥10%), with log Lyα/HeII reaching values of up to 4.6. In the cases of low gas metallicity, re?ection by HI, or a hard or ?ltered ionizing spectrum, the observed number of Lyα photons per incident ionizing photon is itself signi?cantly enhanced above the nominal Case B value of 0.66 due to collisional excitation, reaching values as high as 5.3 in an “extreme case” model which combines several of these e?ects. We ?nd that at low gas metallicity (e.g. Z/Z(cid:12) = 0.1) the production of Lyα photons is predominantly via collisional excitation rather than by recombination. In addition, we ?nd that the collisional excitation of Lyα becomes much more e?cient if the ionizing continuum spectrum has been pre-?ltered through an optically thin screen of gas closer to the AGN (e.g. by a wide-angle, feedback-driven out?ow). We also show that the Lyα and HeII emission line ratios of a sample of previously studied quasars at z ～ 3.5 are consistent with AGN-photoionization of gas with moderate to low metallicity and/or low ionization parameter, without requiring exotic ionization or excitation mechanisms such as strong line-transfer e?ects. In addition, we present a set of UV-optical diagnostic diagrams to distinguish between photoionization by Pop III stars and photoionization by an AGN.
关键词： quasars: emission lines,galaxies: active,galaxies: ISM,galaxies: abundances,galaxies: high-redshift
更新于2025-09-23 15:23:52
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Optical polarisation variability of radio-loud narrow-line Seyfert 1 galaxies
摘要： Context. Narrow-line Seyfert 1 galaxies (NLSy1s) constitute the active galactic nuclei subclass associated with systematically lower black hole masses. A few radio-loud NLSy1s have been detected in MeV–GeV energy bands by Fermi, and evidence that blazar-like jets are operating also in radio-loud NLSy1s, has been accumulated. Aims. We wish to quantify the temporal behaviour of the optical polarisation, fraction, and angle for a selected sample of radio-loud NLSy1s. We also search for rotations of the polarisation plane similar to those commonly observed in blazars. Methods. We have conducted R-band optical linear polarisation monitoring of a sample of ten radio-loud NLSy1 galaxies; five of them have previously been detected by Fermi. The dataset obtained with our pivoting instrument, the RoboPol polarimeter of the Skinakas observatory, has been complemented with observations from the KANATA, Perkins, and Steward observatories. When evidence for long rotations of the polarisation plane was found (at least three consecutive measurements covering at least 90?), we carried out numerical simulations to assess the probability that they are caused by intrinsically evolving electric vector position angles (EVPAs) instead of observational noise. Results. Even our moderately sampled sources show clear indications of variability in both polarisation fraction and angle. For the four best-sampled objects in our sample we find multiple periods of significant polarisation angle variability. Several of these events qualify as long rotations. In the two best-sampled cases, namely J1505+0326 and J0324+3410, we find indications for three long rotations of the polarisation angle. We show that although noise can induce the observed behaviour, it is much more likely that the apparent rotation is indeed caused by intrinsic evolution of the EVPA. To our knowledge, this is the very first detection of such events in this class of sources. In the largest dataset (J0324+3410), we find that the EVPA concentrates around a direction that is at 49.3? to the 15 GHz radio jet, implying a projected magnetic field at an angle of 40.7? to that axis. Conclusions. We assess the probability that pure measurement uncertainties are the reason behind the observed long rotations of the polarisation plane. We conclude that although this is not improbable, it is much more likely that intrinsic rotations are responsible for the observed phenomenology. We conclude, however, that much better sampled and larger datasets of larger source samples are necessary to constrain the physical mechanism(s) that generate long EVPA rotations in NLSy1s.
关键词： galaxies: active,methods: statistical,galaxies: Seyfert,methods: numerical,polarization,techniques: polarimetric
更新于2025-09-23 15:23:52
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Polarimetric and spectroscopic study of radio-quiet weak emission line quasars
摘要： A small subset of optically selected radio-quiet quasars showing weak or no emission lines may turn out to be the elusive radio-quiet BL Lac objects, or simply be radio-quiet QSOs with a still-forming/shielded broad line region (BLR). High polarization (p > 3–4 per cent), a hallmark of BL Lacs, can be used to test whether some optically selected ‘radio-quiet weak emission line quasars’ (RQWLQs) show a fractional polarization high enough to qualify as radio-quiet analogues of BL Lac objects. Out of the observed six RQWLQs candidates showing an insignificant proper motion, only two are found to have p > 1 per cent. For these two RQWLQs, namely J142505.59 + 035336.2 and J154515.77+003235.2, we found polarization of 1.03 ± 0.36 per cent and 1.59 ± 0.53 per cent, respectively, which again is too modest to justify a (radio-quiet) BL Lac classification. We also present here a statistical comparison of the optical spectral index, for a set of 40 RQWLQs with redshift-luminosity matched control sample of 800 QSOs and an equivalent sample of 120 blazars. The spectral index distribution of RQWLQs is found to differ, at a high significance level, from that of blazars and is consistent with that of the ordinary QSOs. Likewise, a structure–function analysis of photometric light curves presented here suggests that the mechanism driving optical variability in RQWLQs is similar to that operating in QSOs and different from that of blazars. These findings are consistent with the common view that the central engine in RQWLQs, as a population, is akin to that operating in normal QSOs and the primary differences between them might be related to differences in the BLR.
关键词： BL Lacertae objects: general,quasars: general,galaxies: active,galaxies: jet
更新于2025-09-23 15:23:52
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The near-to-mid infrared spectrum of quasars
摘要： We analyse a sample of 85 luminous (log (νLν(3 μm)/erg s?1) > 45.5) quasars with rest frame ～2–11 μm spectroscopy from AKARI and Spitzer. Their high luminosity allows a direct determination of the near-infrared quasar spectrum free from host galaxy emission. A semi-empirical model consisting of a single template for the accretion disc and two blackbodies for the dust emission successfully reproduces the 0.1–10 μm spectral energy distributions (SEDs). Excess emission at 1–2 μm over the best-fitting model suggests that hotter dust is necessary in addition to the ～1200 K blackbody and the disc to reproduce the entire near-infrared spectrum. Variation in the extinction affecting the disc and in the relative strength of the disc and dust components accounts for the diversity of individual SEDs. Quasars with higher dust-to-disc luminosity ratios show slightly redder infrared continua and less prominent silicate emission. We find no luminosity dependence in the shape of the average infrared quasar spectrum. We generate a new quasar template that covers the rest-frame range 0.1–11 μm, and separate templates for the disc and dust components. Comparison with other infrared quasar composites suggests that previous ones are less reliable in the 2–4 μm range. Our template is the first one to provide a detailed view of the infrared emission on both sides of the 4 μm bump.
关键词： infrared: galaxies,galaxies: active,quasars: emission lines,quasars: general
更新于2025-09-23 15:23:52
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Long-term optical color behavior of a sample of blazars
摘要： We report on the optical color variability of a sample of 24 blazars, consisting of nine flat spectrum radio quasars (FSRQs) and 15 BL Lacertae objects (BL Lacs), on timescales of years using simultaneous V- and R-band observations observed by the Kanata telescope at Higashi-Hiroshima Observatory. The correlations between color indices V ? R and magnitudes reveal that 11 BL Lacs and one FSRQ exhibited significant (i.e., r > 0.2 and P < 0.01) bluer-when-brighter (BWB) trend and two FSRQs followed the redder-when-brighter (RWB) tend, indicating a possibility that the BWB chromatic trend is dominant for BL Lacs and the RWB trend is especially found in FSRQs, which has been presented occasionally in different samples of blazars. The superpositions of the red emission component from the Doppler-boosted relativistic jet and the blue component arising from the accretion disk might be a possible interpretation for the long-term color behaviors.
关键词： BL Lacertae objects: general,methods: statistical,quasars: general,galaxies: active
更新于2025-09-23 15:23:52
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ALMA-backed NIR high resolution integral field spectroscopy of the NUGA galaxy NGC 1433
摘要： Aims. We present the results of near-infrared (NIR) H- and K-band European Southern Observatory SINFONI integral field spectroscopy (IFS) of the Seyfert 2 galaxy NGC 1433. We investigate the central 500 pc of this nearby galaxy, concentrating on excitation conditions, morphology, and stellar content. NGC 1433 was selected from our extended NUGA(-south) sample, which was additionally observed with the Atacama Large Millimeter/submillimeter Array (ALMA). NGC 1433 is a ringed, spiral galaxy with a main stellar bar in roughly east–west direction (PA 94?) and a secondary bar in the nuclear region (PA 31?). Several dusty filaments are detected in the nuclear region with the Hubble Space Telescope. ALMA detects molecular CO emission coinciding with these filaments. The active galactic nucleus is not strong and the galaxy is also classified as a low-ionization emission-line region (LINER). Methods. The NIR is less affected by dust extinction than optical light and is sensitive to the mass-dominating stellar populations. SINFONI integral field spectroscopy combines NIR imaging and spectroscopy, allowing us to analyse several emission and absorption lines to investigate the stellar populations and ionization mechanisms over the 10'' × 10'' field of view (FOV). Results. We present emission and absorption line measurements in the central kpc of NGC 1433. We detect a narrow Balmer line and several H2 lines. We find that the stellar continuum peaks in the optical and NIR in the same position, indicating that there is no covering of the center by a nuclear dust lane. A strong velocity gradient is detected in all emission lines at that position. The position angle of this gradient is at 155? whereas the galactic rotation is at a position angle of 201?. Our measures of the molecular hydrogen lines, hydrogen recombination lines, and [Fe II] indicate that the excitation at the nucleus is caused by thermal excitation, i.e., shocks that can be associated with active galactic nuclei emission, supernovae, or outflows. The line ratios [Fe II]/Paβ and H2/Brγ show a Seyfert to LINER identification of the nucleus. We do not detect high star formation rates in our FOV. The stellar continuum is dominated by spectral signatures of red-giant M stars. The stellar line-of-sight velocity follows the galactic field whereas the light continuum follows the nuclear bar. Conclusions. The dynamical center of NGC 1433 coincides with the optical and NIR center of the galaxy and the black hole position. Within the central arcsecond, the molecular hydrogen and the 12CO(3?2) emissions – observed in the NIR and in the submillimeter with SINFONI and ALMA, respectively – are indicative for a nuclear outflow originating from the galaxy’s SMBH. A small circumnuclear disk cannot be fully excluded. Derived gravitational torques show that the nuclear bar is able to drive gas inward to scales where viscosity torques and dynamical friction become important. The black hole mass, derived using stellar velocity dispersion, is ～107 M☉.
关键词： galaxies: nuclei,galaxies: individual: NGC 1433,galaxies: active,infrared: galaxies,galaxies: ISM,galaxies: kinematics and dynamics
更新于2025-09-23 15:22:29
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High cadence, linear, and circular polarization monitoring of OJ 287
摘要： Aims. We present a multifrequency, dense radio monitoring program of the blazar OJ 287 using the 100-m Effelsberg radio telescope. The program aims to test different binary supermassive black hole (SMBH) scenarios and studying the physical conditions in the central region of this bright blazar. Here, we analyze the evolution in total flux density, linear and circular polarization as a means to study the OJ 287 jet structure and its magnetic field geometry. Methods. We used a recently developed, high-precision data analysis methodology to recover all four Stokes parameters. We measured the total flux density of OJ 287 at nine bands from 2.64 GHz to 43 GHz, the linear polarization parameters at four bands between 2.64 GHz and 10.45 GHz, and the circular polarization at two bands, 4.85 GHz and 8.35 GHz. The mean cadence of our measurements is ten days. Results. Between December 2015 and January 2017 (MJD 57370–57785), OJ 287 showed flaring activity and complex linear and circular polarization behavior. The radio electric vector position angle (EVPA) showed a large clockwise (CW) rotation by ～340? with a mean rate of ?1.04?/day. Based on concurrent very long baseline interferometric (VLBI) polarization data at 15 GHz and 43 GHz, the rotation seems to originate within the jet core at 43 GHz (projected angular size ≤0.15 mas or 0.67 pc at the redshift of the source). Moreover, optical polarization data show a similar monotonic CW rotation with a rate of about ?1.1?/day which is superposed with shorter and faster rotations that exhibit rates of about 7.8?/day, mainly in the CW sense. Conclusions. The flux density and polarization variability of the single dish, VLBI and optical data is consistent with a polarized emission component propagating on a helical trajectory within a bent jet. We constrained the helix arc length to 0.26 pc and radius to ≤0.04 pc as well as the jet bending arc length projected on the plane of the sky to ≤1.9–7.6 pc. A similar bending has been observed also in high angular resolution VLBI images of the OJ 287 jet at its innermost regions. The helical trajectory covers only a part of the jet width, possibly its spine. In addition, our results indicate the presence of a stable polarized emission component. Its EVPA (?10?) is oriented perpendicular to the large scale jet, suggesting dominance of the poloidal magnetic field component. Finally, the EVPA rotation begins simultaneously with an optical flare and hence the two might be physically connected. That optical flare has been suggested to be linked to the interaction of a secondary SMBH with the inner accretion disk or originating in the jet of the primary.
关键词： BL Lacertae objects: individual: OJ 287,galaxies: active,quasars: supermassive black holes,magnetic fields,galaxies: jets,polarization
更新于2025-09-23 15:22:29
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Numerical problems in coupling photon momentum (radiation pressure) to gas
摘要： Radiation pressure (RP; or photon momentum absorbed by gas) is important in a tremendous range of astrophysical systems. But we show the usual method for assigning absorbed photon momentum to gas in numerical radiation-hydrodynamics simulations (integrating over cell volumes or evaluating at cell centres) can severely underestimate the RP force in the immediate vicinity around unresolved (point/discrete) sources (and subsequently underestimate its effects on bulk gas properties), unless photon mean free paths are highly resolved in the fluid grid. The existence of this error is independent of the numerical radiation transfer (RT) method (even in exact ray-tracing/Monte Carlo methods), because it depends on how the RT solution is interpolated back onto fluid elements. Brute-force convergence (resolving mean free paths) is impossible in many cases (especially where UV/ionizing photons are involved). Instead, we show a 'face-integrated' method – integrating and applying the momentum fluxes at interfaces between fluid elements – better approximates the correct solution at all resolution levels. The 'fix' is simple and we provide example implementations for ray-tracing, Monte Carlo, and moments RT methods in both grid and mesh-free fluid schemes. We consider an example of star formation in a molecular cloud with UV/ionizing RP. At state-of-the-art resolution, cell-integrated methods underestimate the net effects of RP by an order of magnitude, leading (incorrectly) to the conclusion that RP is unimportant, while face-integrated methods predict strong self-regulation of star formation and cloud destruction via RP.
关键词： galaxies: active,galaxies: formation,hydrodynamics,methods: numerical,stars: formation
更新于2025-09-23 15:22:29
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X-Ray, UV, and Radio Timing Observations of the Radio Galaxy 3C 120
摘要： We report the results of monitoring of the radio galaxy 3C?120 with the Neil Gehrels Swift Observatory, Very Long Baseline Array, and Mets?hovi Radio Observatory. The UV-optical continuum spectrum and R-band polarization can be explained by a superposition of an inverted-spectrum source with a synchrotron component containing a disordered magnetic ?eld. The UV-optical and X-ray light curves include dips and ?ares, while several superluminal knots appear in the parsec-scale jet. The recovery time of the second dip was longer at UV-optical wavelengths, in con?ict with a model in which the inner accretion disk (AD) is disrupted during a dip and then re?lled from outer to inner radii. We favor an alternative scenario in which occasional polar alignments of the magnetic ?eld in the disk and corona cause the ?ux dips and formation of shocks in the jet. Similar to observations of Seyfert galaxies, intra-band time lags of ?ux variations are longer than predicted by the standard AD model. This suggests that scattering or some other reprocessing occurs. The 37 GHz light curve is well-correlated with the optical-UV variations, with a ～20 day delay. A radio ?are in the jet occurred in a superluminal knot 0.14 milliarcseconds downstream of the 43 GHz “core,” which places the site of the preceding X-ray/UV/optical ?are within the core 0.5–1.3?pc from the black hole. The inverted UV-optical ?are spectrum can be explained by a nearly monoenergetic electron distribution with energy similar to the minimum energy inferred in the TeV γ-ray emitting regions of some BL?Lacertae objects.
关键词： ultraviolet: galaxies,accretion,galaxies: individual (3C 120),galaxies: active,accretion disks,X-rays: galaxies,radio continuum: galaxies
更新于2025-09-23 15:21:21
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The power of infrared AGN selection in mergers: a theoretical study
摘要： The role of galaxy mergers in fuelling active galactic nuclei (AGNs) is still debated, owing partly to selection effects inherent to studies of the merger/AGN connection. In particular, luminous AGNs are often obscured in late-stage mergers. Mid-infrared (IR) colour selection of dust-enshrouded AGN with, e.g. the Wide-field Infrared Survey Explorer (WISE) has uncovered large new populations of obscured AGN. However, this method is sensitive mainly to AGN that dominate emission from the host. To understand how selection biases affect mid-IR studies of the merger/AGN connection, we simulate the evolution of AGN throughout galaxy mergers. Although mid-IR colours closely trace luminous, obscured AGN, we show that nearly half of merger-triggered AGNs are missed with common mid-IR selection criteria, even in late-stage, gas-rich major mergers. At z (cid:2) 0.5, where merger signatures and dual nuclei can most easily be detected, we find that a more lenient W1 ? W2 > 0.5 cut greatly improves completeness without significantly decreasing reliability. Extreme nuclear starbursts are briefly able to mimic this AGN signature, but this is largely irrelevant in mergers, where such starbursts are accompanied by AGN. We propose a two-colour cut that yields high completeness and reliability even in starbursting systems. Further, we show that mid-IR colour selection very effectively identifies dual AGN hosts, with the highest fraction at the smallest separations (<3 kpc). Thus, many merger hosts of mid-IR AGN should contain unresolved dual AGN; these are ideal targets for high-resolution follow-up, particularly with the James Webb Space Telescope.
关键词： galaxies: interactions,infrared: galaxies,accretion,accretion discs,galaxies: active,black hole physics
更新于2025-09-23 15:21:21