Context. The high-resolution imaging of active galactic nuclei (AGNs) on subkiloparsec scales offers an important avenue for investigating their fueling. Observations in the near-infrared (NIR) are especially valuable as they minimize the absorption by dust. However, the inferred nuclear morphology may depend on the method used to analyze the images. Aims. The aim of this study is to assess whether ground-based adaptive-optics observations of the cores of Seyfert 2 galaxies in the Ks band provide advantages over HST observations at shorter wavelengths (i.e., in the V and H bands). We also investigate whether a dedicated 2D analysis is preferable to a 1D approach. Methods. A sample of 18 Seyfert 2 galaxies was observed with the adaptive optics (AO) system in the Ks band at the Large Binocular Telescope (LBT) and compared to archival HST V- or H-band images. The analysis included a 2D modeling procedure via GALFIT as well as an unsharp masking technique. Results. The results obtained in different filters are mutually consistent, indicating no clear advantage when using a redder filter. Using both GALFIT and unsharp-masking in tandem is preferable, as the two methods provide complementary strengths. We identified nuclear stellar rings in 8 of the 18 galaxies in our sample (44 +/- 12%). This fraction is significantly higher than reported in previous studies and about a factor of 2 higher than what was reported in the most complete atlas of nuclear rings. The radii, size distribution, and inferred masses of the detected nuclear rings are similar to those observed in non-active galaxies. Low-luminosity AGNs seem to have little (if any) impact on the formation and evolution of nuclear rings. Conclusions. The high incidence of nuclear stellar rings in our study is unexpected and warrants further investigation. This could be tested further using the large number of suitable archival HST images available. If confirmed, it would imply that nuclear stellar rings are considerably more common than previously recognized.

We present the first direct comparison between accretion-disk (AD) and broad-line region (BLR) sizes in quasars at z > 2, combining continuum reverberation mapping with interferometric BLR constraints from GRAVITY and GRAVITY+. Using medium-band photometric monitoring with the MPG/ESO 2.2 m telescope, we measure inter-band continuum lags in SDSS J092034.17+065718.0 at z = 2.33 (J0920) and SMSS J052915.80-435152.0 at z = 3.96 (J0529), among the most luminous quasars known (Lbol 10^48 erg s^-1). These are currently the only two quasars at z > 2 with spatially resolved BLRs and dynamical black-hole masses from interferometry. We detect significant continuum lags in both quasars, increasing monotonically with wavelength. The inferred UV disk sizes are RAD = 4.35 +0.78/-0.91 light-days for J0529 and RAD = 3.15 +0.50/-0.48 light-days for J0920. For J0920, accreting at lambda_Edd 7-20, the disk size is consistent with standard thin-disk expectations despite its super-Eddington regime. For J0529, the disk size agrees with thin-disk predictions using the single-epoch black-hole mass, but implies disk inflation by a factor of a few if the GRAVITY+ dynamical mass, an order of magnitude lower, is adopted. UV continuum disk sizes therefore provide an independent physical scale constraining black-hole mass and accretion-rate models, particularly where BLR kinematics are dominated by outflows. The interferometric BLR sizes reveal pronounced radial hierarchies, with RBLR/RAD 270 for J0529 (Hbeta) and 115 for J0920 (Halpha). The successful lag detections at Lbol 10^48 erg s^-1 show that continuum reverberation mapping remains feasible for the most luminous systems, opening a path to larger samples with surveys such as the Vera C. Rubin Observatory's LSST.

We report adaptive-optics Ks-band imaging of the composite Seyfert 2 and Wolf-Rayet galaxy NGC 6764 (D = 32 Mpc), obtained with LUCI/SOUL at the Large Binocular Telescope. The nucleus resolves into three compact sources with pairwise projected separations of 0.07-0.20 arcsec, corresponding to 11.0-30.9 pc. All three have indistinguishable H, Ks, H2, and Br-gamma colours, so photometry alone cannot separate accretion from star formation. The system is consistent with a Seyfert nucleus and two star-forming regions, but a dual or triple AGN cannot be excluded. In the latter case, the separations would be two orders of magnitude smaller than in any confirmed or candidate AGN triplet reported to date. Diffraction-limited near-infrared integral-field spectroscopy is required to establish the nature of each component.

Eddington ratio (lambda Edd) is a paramount parameter governing the accretion history and life cycles of Active Galactic Nuclei (AGNs). This short review presents a multi-faceted view of the importance of the Eddington ratio spanning varied AGN studies. We find that lambda Edd is crucial for standardizing the Radius-Luminosity (R-L) relation-a necessary step for employing quasars (QSOs) as standardizable cosmological probes to help clarify the standing of the Hubble tension. In this data-driven era, we consolidated disparate aspects by developing novel relations borne out of large datasets, such as the robust, nearly universal anti-correlation between fractional variability (Fear) and AEdd derived from Zwicky Transient Facility (ZTF) data, which is vital for interpreting forthcoming high-cadence surveys like Rubin Observatory's LSST. Addressing the conundrum where JWST results suggest an overabundance of massive high-redshift black holes, we demonstrate that local AGNs offer clarification: Changing-Look AGNs (CLAGNs), driven by rapid AEdd shifts, cluster in the low-accretion regime (AEdd similar to 0.01), a rate independently confirmed by our integral field spectroscopy and photoionization modeling of a well-known Seyfert 2 galaxy, rich in high-ionization, forbidden, coronal lines. Conversely, for the high-redshift, high-luminosity population where traditional reverberation mapping (RM) is highly impractical, photometric reverberation mapping (PRM) offers a rapid alternative to constrain accretion disk sizes, enabling efficient estimates of black hole masses (MBH) and AEdd. Finally, we developed tailored semi-empirical spectral energy distributions (SEDs) for extremely high-accretion quasars, successfully validating their characteristic extreme physical conditions.

Reverberation mapping accurately determines virial black hole masses only for redshifts z < 0.2 by utilizing the relationship between the H β broad-line region (BLR) size and the 5100 Å continuum luminosity established with ∼200 active galactic nuclei. For quasars at z ∼ 2–3 determining the BLR size is time-consuming and limited by seasonal gaps, requiring, e.g., ∼20 yr of monitoring of the C iv emission lines. In this work, we demonstrate that an efficient alternative is to use a continuum size–luminosity relation, which can be obtained up to 150 times faster than BLR sizes using photometric reverberation mapping (PRM). We outline the method and its feasibility based on simulations and propose an observational strategy that can be carried out with meter-class telescopes. In particular, we focus on the ESO La Silla 2.2 m telescope as it is suitable for an efficient PRM campaign. These observations will provide the scaling factor between the accretion disk and the BLR size (for C iv -1350 Å), which is crucial for estimating the masses of black holes at higher redshifts ( z ≳ 2–3).

Surveys are versatile tools for astronomers working in the near-infrared. They are useful for scientific applications, for calibration purposes as well as for the technical preparation and execution of their own observations from ground or space-based. In this chapter, we give an overview of the currently available and planned surveys in the infrared from 1 μm to 1 mm wavelengths. These include DENIS, 2MASS, UKIDSS, VHS, IRAS, AKARI, WISE, Herschel, Planck, EUCLID, SPHEREx, and NGRST. At the end of the chapter, a table summarizing the properties of the surveys (field of view, bands covered, limiting fluxes, number of sources detected) is provided. This will be accompanied by a compilation of links to the surveys as well as to the corresponding data products.

This book serves as introduction to basic concepts and strategies of preparing and performing near-infrared observations with maximized scientific return.

Following on the discussion of the properties of the near-infrared sky, we now describe the hardware suite required to successfully carry out observations in this wavelength range. This includes a brief illustration of the standard telescope concepts, a discussion on the various flavors of active and adaptive optics systems, an outline of how imagers, spectrographs, integral-field units and interferometers work, a description of the filter systems in the near-infrared, and a discussion of the detector technologies currently in use. The understanding of all these is imperative to properly prepare, execute, and reduce near-infrared observations. Finally, we comment on the upcoming 30m class telescopes and their instrumentation.

Two new flagship missions have already gone or will go into space in this decade. These are the James Webb Space Telescope and the Nancy Grace Roman Space Telescope. Both work at near-infrared wavelengths. Their location at the Lagrange point L2 about 930,000 miles = 1.5 million kilometers from the Earth poses several challenges for the preparation and execution of scientific programs. It may also affect data reduction strategies of the resulting observations used for ground-based data. In this chapter, we will describe these two missions, their telescopes, and instrumentation and will work out the differences and similarities when doing astrophysics from the ground and space.

The near-infrared sky is a very dynamic entity. It varies across all timescales spatially, temporarily, and thermally. Unavoidably, the near-infrared sky dictates how instruments need to be built to optimize the scientific return, dictates how astronomers need to prepare their observations, and dominates the various strategies they require to reduce the resulting data. In this chapter first some fundamental quantities such as airmass, atmospheric refraction, and extinction are introduced. This is followed by a discussion on turbulence and its consequences like seeing, Airy pattern, and Strehl ratio. In the second part of this chapter, the sky brightness and variability are described and the emission and absorption spectrum of the near-infrared sky be introduced. The chapter ends with a brief description of how observations from space are affected by the near-infrared sky and contributions off the atmosphere.

We present the first on-sky results of the microlens ring tip-tilt sensor. This sensor uses a 3D printed microlens ring feeding six multimode fibers to sense misaligned light, allowing centroid reconstruction. A tip-tilt mirror allows the beam to be corrected, increasing the amount of light coupled into a centrally positioned single-mode (science) fiber. The sensor was tested with the iLocater acquisition camera at the Large Binocular Telescope in Tucson, Arizona, in November 2019. The limit on the maximum achieved rms reconstruction accuracy was found to be 0.19 λ / D in both tip and tilt, of which approximately 50% of the power originates at frequencies below 10 Hz. We show the reconstruction accuracy is highly dependent on the estimated Strehl ratio and simulations support the assumption that residual adaptive optics aberrations are the main limit to the reconstruction accuracy. We conclude that this sensor is ideally suited to remove post-adaptive optics noncommon path tip-tilt residuals. We discuss the next steps for concept development, including optimization of the lens and the fiber, tuning of the correction algorithm, and selection of optimal science cases.

The LUCI instruments are a pair of NIR imagers and multi-object spectrographs located at the front bent Gregorian foci of the Large Binocular Telescope (LBT). One of their special features is their diffraction-limited imaging and long-slit spectroscopic capability in combination with the LBT adaptive secondary mirrors. This allows to achieve a spatial resolution down to 60mas and a spectral resolution of up to 25000. Switching from seeing-limited to diffraction-limited observations changes several operational aspects due to features such as the non-common path aberration or the flexure of the instruments. They all require novel techniques to optimize the image quality and to maximize the scientific return. Non-common path aberration can be corrected via look-up tables. For active flexure compensation the night-sky emission is used. The commissioning of the instruments in diffraction-limited mode on sky is largely finished and the instruments have been handed over to the LBT in April 2018.

LUCI1 and LUCI2 are a pair multi-mode, fully cryogenic near-infrared instruments installed at the Large Binoc- ular Telescope (LBT). The instruments provide imaging, long-slit and multi-object spectroscopy over a 4/ FoV in seeing-limited mode. Ground-layer AO (GLAO) correction for imaging and spectroscopy over the 4/ FoV is available using the ARGOS laser system, as well as diffraction-limited AO over a 30// FoV using the LBT first light AO (FLAO) system with natural guide stars. Internal flexure of the instrument is taken care of by passive and active flexure compensation. Image shifts in seeing-limited modes are compensated by a passive flexure con- trol algorithm using pre-defined look-up tables. For AO observations, passive compensation is replaced by active control. In the following, we present the details of the newly developed active flexure compensation algorithm for the LUCI instruments. We also describe some hardware modifications to the instruments and the results obtained with active flexure compensation.

A detailed analysis of the optical polarimetric variability of the TeV blazar 1ES 1959+650 from 2007 October 18 to 2011 May 5 is presented. The source showed maximum and minimum brightness states in the R band of 14.08 +/- 0.03 mag and 15.20 +/- 0.03 mag, respectively, with a maximum variation of 1.12 mag, and a maximum polarization degree of P = (12.2 +/- 0.7)%, with a maximum variation of 10.7%. From 2009 August to November, a correlation between the optical R-band flux and the degree of linear polarization was found with a correlation coefficient r(pol) = 0.984 +/- 0.025. The source presented a preferential position angle of optical polarization of similar to 153 degrees, with variations of 10 degrees-50 degrees, which is in agreement with the projected position angle of the parsec-scale jet found at 43 GHz. From the Stokes parameters we infer the existence of two optically thin synchrotron components that contribute to the polarized flux. One of them is stable with a constant polarization degree of 4%. Assuming a stationary shock for the variable component, we estimated some parameters associated with the physics of the relativistic jet: the magnetic field, B similar to 0.06 G, the Doppler factor, delta(0) similar to 23, the viewing angle, Phi similar to 2 degrees.4, and the size of the emission region r(b) similar to 5.6 x 10(17) cm. Our study is consistent with the spine-sheath model of explaining the polarimetric variability displayed by this source during our monitoring.

We present observations of a major outburst at centimeter, millimeter, optical, X-ray, and gamma-ray wavelengths of the BL Lacertae object AO 0235+164 in 2008. We analyze the timing of multi-waveband variations in the flux and linear polarization, as well as changes in Very Long Baseline Array (VLBA) images at 7mm with ~0.15 milliarcsecond resolution. The association of the events at different wavebands is confirmed at high statistical significance by probability arguments and Monte-Carlo simulations. A series of sharp peaks in optical linear polarization, as well as a pronounced maximum in the 7mm polarization of a superluminal jet knot, indicate rapid fluctuations in the degree of ordering of the magnetic field. These results lead us to conclude that the outburst occurred in the jet both in the quasi-stationary core and in the superluminal knot, both at >12 parsecs downstream of the supermassive black hole. We interpret the outburst as a consequence of the propagation of a disturbance, elongated along the line of sight by light-travel time delays, that passes through a standing recollimation shock in the core and propagates down the jet to create the superluminal knot. The multi-wavelength light curves vary together on long time-scales (months/years), but the correspondence is poorer on shorter time-scales. This, as well as the variability of the polarization and the dual location of the outburst, agrees with the expectations of a multi-zone emission model in which turbulence plays a major role in modulating the synchrotron and inverse Compton fluxes.

We introduce MAPCAT, a long-term observing program for "Monitoring of AGN with Polarimetry at the Calar Alto Telescopes". Multi-spectral-range studies are critical to understand some of the most relevant current problems of high energy astrophysics of blazars such as their high energy emission mechanisms and the location of their γ-ray emission region through event associations across the spectrum. Adding multi-spectral-range polarimetry allows for even more reliable identification of polarized flares across the spectrum in these kind of objects, as well as for more accurate modeling of their magnetic field. As part of a major international effort to study the long term multi-spectral range polarimetric behavior of blazars, MAPCAT uses -since mid 2007- CAFOS on the 2.2m Telescope at the Calar Alto Observatory (Almería, Spain) to obtain monthly optical (R-band) photo-polarimetric measurements of a sample of 34 of the brightest γ-ray, optical, and radio-millimeter blazars accessible from the northern hemisphere.

We locate the gamma-ray and lower frequency emission in flares of the BL Lac object AO 0235+164 at >12pc in the jet of the source from the central engine. We employ time-dependent multi-spectral-range flux and linear polarization monitoring observations, as well as ultra-high resolution ( 0.15 milliarcsecond) imaging of the jet structure at lambda=7mm. The time coincidence in the end of 2008 of the propagation of the brightest superluminal feature detected in AO 0235+164 (Qs) with an extreme multi-spectral-range (gamma-ray to radio) outburst, and an extremely high optical and 7mm (for Qs) polarization degree provides strong evidence supporting that all these events are related. This is confirmed at high significance by probability arguments and Monte-Carlo simulations. These simulations show the unambiguous correlation of the gamma-ray flaring state in the end of 2008 with those in the optical, millimeter, and radio regime, as well as the connection of a prominent X-ray flare in October 2008, and of a series of optical linear polarization peaks, with the set of events in the end of 2008. The observations are interpreted as the propagation of an extended moving perturbation through a re-collimation structure at the end of the jet's acceleration and collimation zone.