Nearby Seyfert Galaxies Research Articles

Caltech–NRAO Stripe 82 Survey (CNSS). III. The First Radio-discovered Tidal Disruption Event, CNSS J0019+00

Abstract We present the discovery of a nuclear transient with the Caltech–NRAO Stripe 82 Survey (CNSS), a dedicated radio transient survey carried out with the Karl G. Jansky Very Large Array (VLA). This transient, CNSS J001947.3+003527, exhibited a turn-on over a timescale of ≲1 yr, increasing in flux density at 3 GHz from <0.14 mJy in 2014 February to 4.4 ± 0.1 mJy in 2015 March, reaching a peak luminosity of around 2015 October. The association of CNSS J0019+00 with the nucleus (Gaia and our very-long baseline interferometry positions are consistent to within 1 pc) of a nearby S0 Seyfert galaxy at 77 Mpc, together with the radio spectral evolution, implies that this transient is most likely a tidal disruption event (TDE). Our equipartition analysis indicates the presence of a ∼15,000 km s−1 outflow, having energy ∼1049 erg. We derive the radial density profile for the circumnuclear material in the host galaxy to be proportional to R −2.5. All of these properties suggest resemblance with radio-detected thermal TDEs like ASASSN-14li and XMMSL1 J0740-85. No significant X-ray or optical emission is detected from CNSS J0019+00, although this may simply be due to the thermal emission being weak during our late-time follow-up observations. From the CNSS survey we have obtained the first unbiased measurement of the rate of radio TDEs, R(>500μJy) of about 2 × 10−3 deg−2, or equivalently a volumetric rate of about 10 Gpc−3 yr−1. This rate implies that all-sky radio surveys such as the VLA Sky Survey and those planned with ASKAP, will find many tens of radio TDEs over the next few years.

Searching for molecular gas inflows and outflows in the nuclear regions of five Seyfert galaxies

Active galactic nucleus (AGN) driven outflows are believed to play an important role in regulating the growth of galaxies, mostly via negative feedback. However, their effects on their hosts are far from clear, especially for low- and moderate-luminosity Seyferts. To investigate this issue, we obtained cold molecular gas observations, traced by the CO(2-1) transition, using the NOEMA interferometer of five nearby (distances between 19 and 58 Mpc) Seyfert galaxies. The resolution of ∼0.3–0.8 (∼30–100 pc) and field of view of NOEMA allowed us to study the CO(2-1) morphology and kinematics in the nuclear regions (∼100 pc) and up to radial distances of ∼900 pc. We detected CO(2-1) emission in all five galaxies with disky or circumnuclear ring-like morphologies. We derived cold molecular gas masses on nuclear (∼100 pc) and circumnuclear (∼650 pc) scales in the range from 106 to 107 M⊙ and from 107 to 108 M⊙, respectively. In all of our galaxies, the bulk of this gas is rotating in the plane of the galaxy. However, noncircular motions are also present. In NGC 4253, NGC 4388, and NGC 7465, we can ascribe the streaming motions to the presence of a large-scale bar. In Mrk 1066 and NGC 4388, the noncircular motions in the nuclear regions are explained as outflowing material due to the interaction of the AGN wind with molecular gas in the galaxy disk. We conclude that for an unambiguous and precise interpretation of the kinematics of the cold molecular gas, we need detailed knowledge of the host galaxy (i.e., presence of bars, interactions, etc.), and also of the ionized gas kinematics and ionization cone geometry.

HST/COS Observations of Quasar Outflows in the 500–1050 Å Rest Frame. VII. Distances and Energetics for 11 Outflows in Five Quasars*

Abstract From Hubble Space Telescope/Cosmic Origins Spectrograph spectra of five quasars, 16 outflows are detected. For 11 outflows, we are able to constrain their distances to the central source (R) and their energetics. In instances of multiple electron number density determinations (used in the calculation of R) for the same outflow, the values are consistent within errors. For the 11 outflows, eight have measurements for R (between 10 and 1000 pc), one has a lower limit, another has an upper limit, and the last has a range in R. There are two outflows that have enough kinetic luminosity to be major contributors to active galactic nucleus feedback. The outflowing mass is found primarily in a very high-ionization phase, which is probed using troughs from, e.g., Ne viii, Na ix, Mg x, and Si xii. Such ions connect the physical conditions of these ultraviolet outflows to the X-ray warm absorber outflows seen in nearby Seyfert galaxies. The ion Cl vii and several new transitions from Ne v have been detected for the first time.

Cold molecular gas and PAH emission in the nuclear and circumnuclear regions of Seyfert galaxies

We investigate the relation between the detection of the 11.3 μm polycyclic aromatic hydrocarbon (PAH) feature in the nuclear (∼24−230 pc) regions of 22 nearby Seyfert galaxies and the properties of the cold molecular gas. For the former we use ground-based (0.3−0.6″ resolution) mid-infrared (mid-IR) spectroscopy. The cold molecular gas is traced by ALMA and NOEMA high (0.2−1.1″) angular resolution observations of the CO(2–1) transition. Galaxies with a nuclear detection of the 11.3 μm PAH feature contain more cold molecular gas (median 1.6 × 107 M⊙) and have higher column densities (N(H2) = 2 × 1023 cm−2) over the regions sampled by the mid-IR slits than those without a detection. This suggests that molecular gas plays a role in shielding the PAH molecules in the harsh environments of Seyfert nuclei. Choosing the PAH molecule naphthalene as an illustration, we compute its half-life in the nuclear regions of our sample when exposed to 2.5 keV hard X-ray photons. We estimate shorter half-lives for naphthalene in nuclei without a 11.3 μm PAH detection than in those with a detection. TheSpitzer/IRS PAH ratios on circumnuclear scales (∼4″ ∼ 0.25−1.3 kpc) are in between model predictions for neutral and partly ionized PAHs. However, Seyfert galaxies in our sample with the highest nuclear H2column densities are not generally closer to the neutral PAH tracks. This is because in the majority of our sample galaxies, the CO(2–1) emission in the inner ∼4″ is not centrally peaked and in some galaxies traces circumnuclear sites of strong star formation activity. Spatially resolved observations with the MIRI medium-resolution spectrograph on theJames WebbSpace Telescope will be able to distinguish the effects of an active galactic nucleus (AGN) and star formation on the PAH emission in nearby AGN.

Hidden Cores of Active Galactic Nuclei as the Origin of Medium-Energy Neutrinos: Critical Tests with the MeV Gamma-Ray Connection

Mysteries about the origin of high-energy cosmic neutrinos have deepened by the recent IceCube measurement of a large diffuse flux in the 10-100TeV range. Based on the standard disk-corona picture of active galactic nuclei (AGN), we present a phenomenological model enabling us to systematically calculate the spectral sequence of multimessenger emission from the AGN coronae. We show that protons in the coronal plasma can be stochastically accelerated up to PeV energies by plasma turbulence, and find that the model explains the large diffuse flux of medium-energy neutrinos if the cosmic rays carry only a few percent of the thermal energy. We find that the Bethe-Heitler process plays a crucial role in connecting these neutrinos and cascaded MeV gamma rays, and point out that the gamma-ray flux can even be enhanced by the reacceleration of secondary pairs. Critical tests of the model are given by its prediction that a significant fraction of the MeV gamma-ray background correlates with ∼10 TeV neutrinos, and nearby Seyfert galaxies including NGC 1068 are promising targets for IceCube, KM3Net, IceCube-Gen2, and future MeV gamma-ray telescopes.

HST/COS Observations of Quasar Outflows in the 500–1050 Å Rest Frame. III. Four Similar Outflows in 2MASS J1051+1247 with Enough Energy to Be Major Contributors to AGN Feedback*

Abstract We detect four very energetic outflows in the Hubble Space Telescope/Cosmic Origins Spectrograph spectra of quasar 2MASS J1051+1247 with a combined kinetic luminosity ( ) of 1046 erg s−1. Remarkable similarities are seen in these outflows: velocity centroids between 4900 and 5700 km s−1, distances from the central source (R) of a few hundred parsecs that are all consistent within the errors, and an within a factor of two for all outflows. Hence, a common origin for the outflows is probable. Most of the outflowing mass resides in a very high-ionization phase evident by troughs from Ne viii, Na ix, Mg x, and Si xii, which connect the physical conditions of these ultraviolet outflows to the X-ray warm absorber outflows seen in nearby Seyfert galaxies. Three of the outflows have two or three independent diagnostics for the electron number density, yielding consistent values for each outflow, which increase the robustness of the R determinations. Troughs from never-before-seen ionic transitions of Ar vi, O iv*, Ne vi*, and Ne v* are identified. With a combined that is 7.0 % of the quasar’s Eddington luminosity, these outflows are prime candidates to be major agents for various active galactic nuclei feedback effects.

On the Origin of High-energy Neutrinos from NGC 1068: The Role of Nonthermal Coronal Activity

Abstract NGC 1068, a nearby type-2 Seyfert galaxy, is reported as the hottest neutrino spot in the 10 yr survey data of IceCube. Although there are several different possibilities for the generation of high-energy neutrinos in astrophysical sources, feasible scenarios allowing such emission in NGC 1068 have not yet been firmly defined. We show that the flux level of GeV and neutrino emission observed from NGC 1068 implies that the neutrino emission can be produced only in the vicinity of the supermassive black hole in the center of the galaxy. The coronal parameters, such as magnetic field strength and corona size, that make this emission possible, are consistent with the spectral excess registered in the millimeter range. The suggested model and relevant physical parameters are similar to those revealed for several nearby Seyferts. Due to the internal gamma-ray attenuation, the suggested scenario cannot be verified by observations of NGC 1068 in the GeV and TeV gamma-ray energy bands. However, the optical depth is expected to become negligible for MeV gamma-rays, thus future observations in this band will be able to validate our model.

X-ray, UV, and optical time delays in the bright Seyfert galaxy Ark 120 with co-ordinated Swift and ground-based observations

ABSTRACT We report on the results of a multiwavelength monitoring campaign of the bright, nearby Seyfert galaxy Ark 120, using a ∼50-d observing programme with Swift and a ∼4-month co-ordinated ground-based observing campaign, pre-dominantly using the Skynet Robotic Telescope Network. We find Ark 120 to be variable at all optical, UV, and X-ray wavelengths, with the variability observed to be well correlated between wavelength bands on short time-scales. We perform cross-correlation analysis across all available wavelength bands, detecting time delays between emission in the X-ray band and the Swift V, B, and UVW1 bands. In each case, we find that the longer wavelength emission is delayed with respect to the shorter wavelength emission. Within our measurement uncertainties, the time delays are consistent with the τ ∼ λ4/3 relation, as predicted by a disc reprocessing scenario. The measured lag centroids are τcent = 11.90 ± 7.33, 10.80 ± 4.08, and 10.60 ± 2.87 d between the X-ray and V, B, and UVW1 bands, respectively. These time delays are longer than those expected from standard accretion theory and, as such, Ark 120 may be another example of an active galaxy whose accretion disc appears to exist on a larger scale than predicted by the standard thin-disc model. Additionally, we detect further inter-band time delays: most notably between the ground-based I and B bands (τcent = 3.46 ± 0.86 d), and between both the Swift XRT and UVW1 bands and the I band (τcent = 12.34 ± 4.83 and 2.69 ± 2.05 d, respectively), highlighting the importance of co-ordinated ground-based optical observations.

Multiphase Gas Flows in the Nearby Seyfert Galaxy ESO428–G014. Paper I

Abstract We present ALMA 230 GHz continuum and CO(2–1) observations of the nearby Compton-thick Seyfert galaxy ESO428–G14, with angular resolution 0.″7 (78 pc). CO(2–1) is distributed in clumpy spiral arms, a lopsided circumnuclear ring (CNR) with ∼200 pc radius, and a transverse gas lane with size <100 pc, which crosses the nucleus and connects the two portions of the CNR. The main CO velocity gradient is consistent with a rotating disk with dynamical mass M dyn = 5 × 109 M ⊙ within ∼1 kpc. We detect off-plane gas motions with respect to the main disk plane which likely trace a molecular outflow with rate , along a biconical structure with radius 700 pc. The CO outflow smoothly joins the warm molecular outflow detected in SINFONI/Very Large Telescope data in the central 170 pc, suggesting that the outflow may cool with increasing distance. Our dynamical modeling of the inner 100 pc region suggests a warped disk or bar, and of fast gas streams which may trace an inflow toward the AGN. The inner warped disk overlaps with the most obscured, CT region seen in X-rays. There, we derive a column density , suggesting that molecular gas may contribute significantly to the AGN obscuration. Most of the hard X-ray emitting nuclear region is deprived of cold molecular gas and shows a CO-cavity. The CO-cavity is filled with warm molecular gas traced by H2, confirming that the 3–6 keV continuum and Fe Kα emission are due to scattering from dense ISM clouds.

Reverberation Measurements of the Inner Radii of the Dust Tori in Quasars

Abstract We present the results of a dust-reverberation survey of quasars at redshifts z < 0.6. We found a delayed response of the K-band flux variation after the optical flux variation in 25 out of 31 targets, and obtained the lag time between them for 22 targets. Combined with the results for nearby Seyfert galaxies, we provide the largest homogeneous collection of K-band dust-reverberation data for 36 type 1 active galactic nuclei (AGNs). This doubles the sample and includes the most distant AGN and the largest lag so far measured. We estimated the optical luminosity of the AGN component of each target using three different methods: spectral decomposition, the flux-variation-gradient method, and image decomposition. We found a strong correlation between the reverberation radius for the innermost dust torus and the optical luminosity over a range of approximately four orders of magnitude in luminosity, as is already known for Seyfert galaxies. We estimated the luminosity distances of the AGNs based on their dust-reverberation lags, and found that the data in the redshift–distance diagram are consistent with the current standard estimates of the cosmological parameters. We also present the radius–luminosity relations for isotropic luminosity indicators such as the hard X-ray (14–195 keV), [O IV] 25.89 μm, and mid-infrared (12 μm) continuum luminosities, which are applicable to obscured AGNs.

A Cepheid-based Distance to the Seyfert Galaxy NGC 6814

Abstract We present a Cepheid-based distance to the nearby Seyfert galaxy NGC 6814 from Hubble Space Telescope observations. We obtained F555W and F814W imaging over the course of 12 visits with logarithmic time spacing in 2013 August−October. We detected and made photometric measurements for 16,469 unique sources across all images in both filters, from which we identify 90 excellent Cepheid candidates spanning a range of periods of 13–84 days. We find evidence for incompleteness in the detection of candidates at periods <21 days. Based on the analysis of Cepheid candidates above the incompleteness limit, we determine a distance modulus for NGC 6814 relative to the LMC of mag. Adopting the recent constraint of the distance modulus to the LMC determined by Pietrzyński et al., we find which gives a distance of 21.65 ± 0.41 Mpc to NGC 6814.

On High-energy Particles in Accretion Disk Coronae of Supermassive Black Holes: Implications for MeV Gamma-rays and High-energy Neutrinos from AGN Cores

Abstract Recent observations with ALMA have revealed evidence for nonthermal synchrotron emission from the core regions of two nearby Seyfert galaxies. This suggests that the coronae of accretion disks in active galactic nuclei (AGNs) can be conducive to the acceleration of nonthermal electrons, in addition to the hot, thermal electrons responsible for their X-ray emission through thermal Comptonization. Here, we investigate the mechanism of such particle acceleration, based on observationally inferred parameters for AGN disk coronae. One possibility to account for the observed nonthermal electrons is diffusive shock acceleration, as long as the gyrofactor η g does not exceed ∼106. These nonthermal electrons can generate gamma-rays via inverse Compton scattering of disk photons, which can appear in the MeV band, while those with energies above ∼100 MeV would be attenuated via internal γγ pair production. The integrated emission from all AGNs with thermal and nonthermal Comptonization can reproduce the observed cosmic background radiation in X-rays as well as gamma-rays up to ∼10 MeV. Furthermore, if protons are accelerated in the same conditions as electrons and η g ∼ 30, our observationally motivated model is also able to account for the diffuse neutrino flux at energies below 100–300 TeV. The next generation of MeV gamma-ray and neutrino facilities can test these expectations by searching for signals from bright, nearby Seyfert galaxies such as NGC 4151 and IC 4329A.

Nuclear kinematics in nearby AGN – I. An ALMA perspective on the morphology and kinematics of the molecular CO(2–1) emission

We present the molecular gas morphology and kinematics of seven nearby Seyfert galaxies obtained from our 230~GHz ALMA observations. The CO J=2-1 kinematics within the inner $\sim30$" ($\lesssim9$~kpc) reveals rotation patterns that have been explored using the Bertola rotation model and a modified version of the Kinemetry package. The latter algorithm reveals various deviations from pure circular rotation in the inner kiloparsec of all seven galaxies, including kinematic twists, decoupled and counter-rotating cores. A comparison of the global molecular gas and stellar kinematics show overall agreement in the position angle of the major axis and the systemic velocity, but larger discrepancies in the disc inclination. The residual maps obtained with both the methods shows the presence of non-circular motions in most of the galaxies. Despite its importance, a detailed interpretation of the physics responsible for non-circular motions will be discussed in a forthcoming work.

An Accreting Supermassive Black Hole Irradiating Molecular Gas in NGC 2110

Abstract The impact of active galactic nuclei (AGNs) on star formation has implications for our understanding of the relationships between supermassive black holes and their galaxies, as well as for the growth of galaxies over the history of the universe. We report on a high-resolution multiphase study of the nuclear environment in the nearby Seyfert galaxy NGC 2110 using the Atacama Large Millimeter/submillimeter Array, Hubble and Spitzer Space Telescopes, and the Very Large Telescope/SINFONI. We identify a region that is markedly weak in low-excitation CO emission from cold molecular gas, but appears to be filled with ionized and warm molecular gas, which indicates that the AGN is directly influencing the properties of the molecular material. Using multiple molecular gas tracers, we demonstrate that, despite the lack of CO line emission, the surface densities and kinematics of molecular gas vary smoothly across the region. Our results demonstrate that the influence of an AGN on star-forming gas can be quite localized. In contrast to widely held theoretical expectations, we find that molecular gas remains resilient to the glare of energetic AGN feedback.

Unveiling the origin of the gamma-ray emission in NGC 1068 with the Cherenkov Telescope Array

Several observations are revealing the widespread occurrence of mildly relativistic wide-angle AGN winds strongly interacting with the gas of their host galaxy. Such winds are potential cosmic-ray accelerators, as supported by gamma-ray observations of the nearby Seyfert galaxy NGC 1068 with the Fermi gamma-ray space telescope. The non-thermal emission produced by relativistic particles accelerated by the AGN-driven wind observed in the circum-nuclear molecular disk of such galaxy is invoked to produce the gamma-ray spectrum. The AGN wind model predicts a hard spectrum that extend in the very high energy band which differs significantly from those corresponding to other models discussed in the literature, like starburst or AGN jet. We present dedicated simulations of observations through the Cherenkov Telescope Array (CTA), the next-generation ground based gamma-ray observatory, of the very high energy spectrum of the Seyfert galaxy NGC 1068 assuming the AGN wind and the AGN jet models. We demonstrate that, considering 50 hours of observations, CTA can be effectively used to constrain the two different emission models, providing important insight into the physics governing the acceleration of particles in non-relativistic AGN-driven outflows. This analysis strongly motivates observations of Seyfert and starburst galaxies with CTA in order to test source population models of the extragalactic gamma-ray and neutrino backgrounds.

Molecular and ionised gas kinematics in a sample of nearby active galaxies

AbstractWe used Gemini NIFS integral field spectroscopy to analyse the molecular and ionised gas kinematics of six nearby (z ⩽ 0.015) Seyfert galaxies with a spatial coverage of 0.1 – 0.6 kpc2. By fitting the emission-line profiles using multiple Gaussian components we determined that the ionised and hot molecular gas kinematics are dominated by gas outflows and rotation, respectively, even though three objects also present molecular outflows.

Outflow signatures in Gemini GMOS-IFU observations of 5 nearby Seyfert 2 galaxies

AbstractWe use Gemini Multi-Object Spectrograph (GMOS) Integral Field Unit (IFU) observations of a sample of 5 bright nearby Seyfert galaxies to map their emission-line flux distributions and kinematics at a spatial resolution ranging from 110 to 280 pc. For all galaxies, the gas kinematics show two components: a rotation and an outflow component.

Interstellar medium properties and feedback in local AGN with the MAGNUM survey

AbstractWe investigated the interstellar medium (ISM) properties in the central regions of nearby Seyfert galaxies characterised by prominent conical or bi-conical outflows belonging to the MAGNUM survey by exploiting the unprecedented sensitivity, spatial and spectral coverage of the integral field spectrograph MUSE at the Very Large Telescope. We developed a novel approach based on the gas and stars kinematics to disentangle high-velocity gas in the outflow from gas in the disc to spatially track the differences in their ISM properties. This allowed us to reveal the presence of an ionisation structure within the extended outflows that can be interpreted with different photoionisation and shock conditions, and to trace tentative evidence of outflow-induced star formation (“positive” feedback) in a galaxy of the sample, Centaurus A.

The MAGNUM survey: different gas properties in the outflowing and disc components in nearby active galaxies with MUSE

We investigated the interstellar medium (ISM) properties of the disc and outflowing gas in the central regions of nine nearby Seyfert galaxies, all characterised by prominent conical or biconical outflows. These objects are part of the Measuring Active Galactic Nuclei Under MUSE Microscope (MAGNUM) survey, which aims to probe their physical conditions and ionisation mechanism by exploiting the unprecedented sensitivity of the Multi Unit Spectroscopic Explorer (MUSE), combined with its spatial and spectral coverage. Specifically, we studied the different properties of the gas in the disc and in the outflow with spatially and kinematically resolved maps by dividing the strongest emission lines in velocity bins. We associated the core of the lines with the disc, consistent with the stellar velocity, and the redshifted and the blueshifted wings with the outflow. We measured the reddening, density, ionisation parameter, and dominant ionisation source of the emitting gas for both components in each galaxy. We find that the outflowing gas is characterised by higher values of density and ionisation parameter than the disc, which presents a higher dust extinction. Moreover, we distinguish high- and low-ionisation regions across the portion of spatially resolved narrow-line region (NLR) traced by the outflowing gas. The high-ionisation regions characterised by the lowest [N II]/Hα and [S II]/Hα line ratios generally trace the innermost parts along the axis of the emitting cones where the [S III]/[S II] line ratio is enhanced, while the low-ionisation regions follow the cone edges and/or the regions perpendicular to the axis of the outflows, also characterised by a higher [O III] velocity dispersion. A possible scenario to explain these features relies on the presence of two distinct populations of line emitting clouds: one is optically thin to the radiation and is characterised by the highest excitation, while the other is optically thick and is impinged by a filtered, and thus harder, radiation field which generates strong low-excitation lines. The highest values of [N II]/Hα and [S II]/Hα line ratios may be due to shocks and/or a hard filtered radiation field from the active galactic nucleus.

Near-infrared observations of star formation and gas flows in the NUGA galaxy NGC 1365

In the framework of understanding the gas and stellar kinematics and their relations to AGNs and galaxy evolution scenarios, we present spatially resolved distributions and kinematics of the stars and gas in the central ∼800 pc radius of the nearby Seyfert galaxy NGC 1365. We obtained H + K- and K-band near-infrared (NIR) integral-field observations from VLT/SINFONI. Our results reveal strong broad and narrow emission-line components of ionized gas (hydrogen recombination lines Paα and Brγ) in the nuclear region, as well as hot dust with a temperature of ∼1300 K, both typical for type-1 AGNs. From MBH − σ* and the broad components of hydrogen recombination lines, we find a black-hole mass of (5 − 10)×106 M⊙. In the central ∼800 pc, we find a hot molecular gas mass of ∼615 M⊙, which corresponds to a cold molecular gas reservoir of (2 − 8)×108 M⊙. However, there is a molecular gas deficiency in the nuclear region. The gas and stellar-velocity maps both show rotation patterns consistent with the large-scale rotation of the galaxy. However, the gaseous and stellar kinematics show deviations from pure disk rotation, which suggest streaming motions in the central < 200 pc and a velocity twist at the location of the ring which indicates deviations in disk and ring rotation velocities in accordance with published CO kinematics. We detect a blueshifted emission line split in Paα, associated with the nuclear region only. We investigate the star-formation properties of the hot spots in the circumnuclear ring which have starburst ages of ≲10 Myr and find indications for an age gradient on the western side of the ring. In addition, our high-resolution data reveal further substructure within this ring which also shows enhanced star forming activity close to the nucleus.