Double-peaked Narrow Lines Research Articles

Evidence to disfavour dual-core systems leading to double-peaked narrow emission lines

ABSTRACTIn this paper, an interesting method is proposed to test dual-core systems for double-peaked narrow emission lines through a precious dual-core system with double-peaked narrow Balmer lines in the main galaxy but with single-peaked narrow Balmer lines in the companion galaxy. Under a dual-core system, considering narrow Balmer (Hα and Hβ) emissions (fe,α and fe,β) from a companion galaxy that are covered by the SDSS fiber for the main galaxy and narrow Balmer emissions (fc,α and fc,β) from the companion galaxy covered by the SDSS fiber for the companion galaxy, the same flux ratios fe,α/fc,α = fe,β/fc,β can be expected, owing to the totally similar physical conditions of each narrow Balmer emission region. Next, the precious dual-core system in SDSS J2219–0938 is discussed. After subtracting the pPXF code determined stellar lights, double-peaked narrow Balmer emission lines are confirmed in the main galaxy with a confidence level higher than 5σ, but single-peaked narrow Balmer emission lines are confirmed in the companion galaxy. Through measured fluxes of the emission components, fe,α/fc,α is about 0.82, which is different from fe,β/fc,β ∼ 0.52, which disfavours a dual-core system for the double-peaked narrow Balmer emission lines in SDSS J2219–0938.

Torus Constraints in ANEPD-CXO245: A Compton-thick AGN with Double-peaked Narrow Lines

Abstract We report on the torus constraints of the Compton-thick active galactic nucleus (AGN) with double-peaked optical narrow-line region emission lines, ANEPD-CXO245, at z = 0.449 in the AKARI NEP Deep Field. The unique infrared data on this field, including those from the nine-band photometry over 2–24 μm with the AKARI Infrared Camera, and the X-ray spectrum from Chandra allow us to constrain torus parameters such as the torus optical depth, X-ray absorbing column, torus angular width (σ), and viewing angle (i). We analyze the X-ray spectrum as well as the UV–optical–infrared spectral energy distribution (UOI-SED) with clumpy torus models in X-ray (XCLUMPY) and infrared (CLUMPY), respectively. From our current data, the constraints on σ–i from both X-rays and UOI show that the line of sight crosses the torus as expected for a type 2 AGN. We obtain a small X-ray scattering fraction (<0.1%), which suggests narrow torus openings, giving preference to the bipolar outflow picture of the double-peaked profile. Comparing the optical depth of the torus from the UOI-SED and the absorbing column density N H from the X-ray spectrum, we find that the gas-to-dust ratio is ≳4 times larger than the Galactic value.

Intrinsic and observed dual AGN fractions from major mergers

A suite of 432 collisionless simulations of bound pairs of spiral galaxies with mass ratios 1:1 and 3:1, and global properties consistent with the ΛCDM paradigm, is used to test the conjecture that major mergers fuel the dual AGN (DAGN) of the local volume. Our analysis was based on the premise that the essential aspects of this scenario can be captured by replacing the physics of the central black holes with restrictions on their relative separation in phase space. We introduce several estimates of the DAGN fraction and infer predictions for the activity levels and resolution limits usually involved in surveys of these systems, assessing their dependence on the parameters controlling the length of both mergers and nuclear activity. Given a set of constraints, we find that the values adopted for some of the latter factors often condition the outcomes from individual experiments. Still, the results do not, in general, reveal very tight correlations, the clearest effect being the tendency of the frequencies normalized to the merger time to anticorrelate with the orbital circularity. In agreement with other theoretical studies, our simulations predict intrinsic DAGN abundances that range from ∼ a few to 15% depending on the maximum level of nuclear activity achieved, the higher the bolometric luminosity, the lower the fraction. At the same time, we show that these probabilities are reduced by about an order of magnitude when they are filtered with the typical constraints applied by observational studies of the DAGN fraction at low redshift. Seen as a whole, our results prove that consideration of the most common limitations involved in the detection of close active pairs at optical wavelengths is sufficient alone to reconcile the intrinsic frequencies envisaged in a hierarchical universe with the small fractions of double-peaked narrow-line systems which are often reported at kpc-scales.

The Origin of Double-peaked Narrow Lines in Active Galactic Nuclei. IV. Association with Galaxy Mergers

Abstract Double-peaked narrow emission lines in active galactic nucleus (AGN) spectra can be produced by AGN outflows, rotation, or dual AGNs, which are AGN pairs in ongoing galaxy mergers. Consequently, double-peaked narrow AGN emission lines are useful tracers of the coevolution of galaxies and their supermassive black holes, as driven by AGN feedback and AGN fueling. We investigate this concept further with follow-up optical longslit observations of a sample of 95 Sloan Digital Sky Survey (SDSS) galaxies that have double-peaked narrow AGN emission lines. Based on a kinematic analysis of the longslit spectra, we confirm previous work that finds that the majority of double-peaked narrow AGN emission lines are associated with outflows. We also find that eight of the galaxies have companion galaxies with line-of-sight velocity separations <500 km s−1 and physical separations <30 kpc. Since we find evidence of AGNs in both galaxies, all eight of these systems are compelling dual AGN candidates. Galaxies with double-peaked narrow AGN emission lines occur in such galaxy mergers at least twice as often as typical active galaxies. Finally, we conclude that at least 3% of SDSS galaxies with double-peaked narrow AGN emission lines are found in galaxy mergers where both galaxies are resolved in SDSS imaging.

The origin of double-peaked narrow lines in active galactic nuclei – III. Feedback from biconical AGN outflows

We apply an analytic Markov Chain Monte Carlo model to a sample of 18 AGN-driven biconical outflows that we identified from a sample of active galaxies with double-peaked narrow emission lines at z < 0.1 in the Sloan Digital Sky Survey. We find that 8/18 are best described as asymmetric bicones, 8/18 are nested bicones, and 2/18 are symmetric bicones. From the geometry and kinematics of the models, we find that these moderate-luminosity AGN outflows are large and energetic. The biconical outflows axes are randomly oriented with respect to the photometric major axis of the galaxy, implying a randomly oriented and clumpier torus to collimate the outflow, but the torus also allows some radiation to escape equatorially. We find that 16/18 (89%) outflows are energetic enough to drive a two-staged feedback process in their host galaxies. All of these outflows geometrically intersect the photometric major axis of the galaxy, and 23% of outflow host galaxies are significantly redder or have significantly lower specific star formation rates when compared to a matched sample of active galaxies.

A search for double-peaked narrow emission line galaxies and AGNs in the LAMOST DR1

The Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) has released more than two million spectra, which provide the opportunity to search for double-peaked narrow emission line (NEL) galaxies and active galactic nuclei (AGNs). The double-peaked narrow-line profiles can be well modeled by two velocity components, respectively blueshifted and redshifted with respect to the systemic recession velocity. This paper presents 20 double-peaked NEL galaxies and AGNs found from LAMOST DR1 using a search method based on a multi-Gaussian fit of the narrow emission lines. Among them, ten have already been published by other authors, either listed as genuine double-peaked NEL objects or as asymmetric NEL objects, and the remaining ten are original discoveries. We discuss some possible origins for the double-peaked narrow-line features, such as interaction between jet and narrow line regions, interaction with companion galaxies, and black hole binaries. Spatially resolved optical imaging and/or follow-up observations in other spectral bands are needed to further discuss the physical mechanisms at work.

Double-peaked narrow-line signatures of dual supermassive black holes in galaxy merger simulations

We present a first attempt to model the narrow-line (NL) region of active galactic nuclei (AGN) in hydrodynamic simulations of galaxy mergers, using a novel physical prescription. This model is used to determine the origin of double-peaked NL AGN in merging galaxies and their connection to supermassive black hole (SMBH) pairs, motivated by recent observations of such objects. We find that double-peaked NLs induced by the relative motion of dual SMBHs are a generic but short-lived feature of gaseous major mergers. Double-peaked NL AGN should often be observed in late-stage galaxy mergers, during the kiloparsec-scale phase of SMBH inspiral or soon after the nuclear coalescence and subsequent SMBH merger. However, even within the kiloparsec-scale phase, only a minority of double-peaked NLs are directly induced by the relative motion of binary SMBHs; their lifetimes are typically a few Myr. The majority of double-peaked NLs result from gas kinematics near the SMBH, although prior to the SMBH merger up to ∼80 per cent of all double-peaked NL profiles may be influenced by SMBH motion via altered peak ratios or overall velocity offsets. The total lifetimes of double-peaked NL AGN depend strongly on viewing angle and on properties of the merging galaxies; gas-rich, nearly-equal-mass mergers have more NL AGN activity but may also be more obscured. Furthermore, in a typical merger, at least 10–40 per cent of the double-peaked NLs induced by SMBH motion have small projected separations, ∼0.1–1 kpc, making it difficult to clearly identify dual peaks of stellar surface brightness. Diffuse tidal features can indicate a late-stage merger, although they do not distinguish an SMBH pair from a merged SMBH. We demonstrate that double-peaked NL AGN spectra with large peak velocity splittings ( ≳ 500 km s−1) or with discernible overall velocity shifts are often associated with inspiraling SMBH pairs. Our results support the notion that selection of double-peaked NL AGN is a promising method for identifying dual SMBH candidates, but demonstrate the critical importance of high-resolution, multiwavelength follow-up observations, and the use of multiple lines of evidence, for confirming the dual nature of candidate SMBH pairs.

CHANDRAX-RAY ANDHUBBLE SPACE TELESCOPEIMAGING OF OPTICALLY SELECTED KILOPARSEC-SCALE BINARY ACTIVE GALACTIC NUCLEI. I. NATURE OF THE NUCLEAR IONIZING SOURCES

Kiloparsec-scale binary active galactic nuclei (AGNs) signal active supermassive black hole (SMBH) pairs in merging galaxies. Despite their significance, unambiguously confirmed cases remain scarce and most have been discovered serendipitously. In a previous systematic search, we optically identified four kpc-scale binary AGNs from candidates selected with double-peaked narrow emission lines at redshifts of 0.1--0.2. Here we present Chandra and Hubble Space Telescope Wide Field Camera 3 (WFC3) imaging of these four systems. We critically examine and confirm the binary-AGN scenario for two of the four targets, by combining high angular resolution X-ray imaging spectroscopy with Chandra ACIS-S, better nuclear position constraints from WFC3 F105W imaging, and direct starburst estimates from WFC3 F336W imaging; for the other two targets, the existing data are still consistent with the binary-AGN scenario, but we cannot rule out the possibility of only one AGN ionizing gas in both merging galaxies. We find tentative evidence for a systematically smaller X-ray-to-[O III] luminosity ratio and/or higher Compton-thick fraction in optically selected kpc-scale binary AGNs than in single AGNs, possibly caused by a higher nuclear gas column due to mergers and/or a viewing angle bias related to the double-peak narrow line selection. While our result lends some further support to the general approach of optically identifying kpc-scale binary AGNs, it also highlights the challenge and ambiguity of X-ray confirmation.

KILOPARSEC-SCALE SPATIAL OFFSETS IN DOUBLE-PEAKED NARROW-LINE ACTIVE GALACTIC NUCLEI. I. MARKERS FOR SELECTION OF COMPELLING DUAL ACTIVE GALACTIC NUCLEUS CANDIDATES

Merger-remnant galaxies with kpc-scale separation dual active galactic nuclei (AGNs) should be widespread as a consequence of galaxy mergers and triggered gas accretion onto supermassive black holes, yet very few dual AGNs have been observed. Galaxies with double-peaked narrow AGN emission lines in the Sloan Digital Sky Survey are plausible dual AGN candidates, but their double-peaked profiles could also be the result of gas kinematics or AGN-driven outflows and jets on small or large scales. To help distinguish between these scenarios, we have obtained spatial profiles of the AGN emission via follow-up long-slit spectroscopy of 81 double-peaked narrow-line AGNs in SDSS at 0.03 < z < 0.36 using Lick, Palomar, and MMT Observatories. We find that all 81 systems exhibit double AGN emission components with ~kpc projected spatial separations on the sky, which suggests that they are produced by kpc-scale dual AGNs or kpc-scale outflows, jets, or rotating gaseous disks. In addition, we find that the subsample (58%) of the objects with spatially compact emission components may be preferentially produced by dual AGNs, while the subsample (42%) with spatially extended emission components may be preferentially produced by AGN outflows. We also find that for 32% of the sample the two AGN emission components are preferentially aligned with the host galaxy major axis, as expected for dual AGNs orbiting in the host galaxy potential. Our results both narrow the list of possible physical mechanisms producing the double AGN components, and suggest several observational criteria for selecting the most promising dual AGN candidates from the full sample of double-peaked narrow-line AGNs. Using these criteria, we determine the 17 most compelling dual AGN candidates in our sample.

DOUBLE-PEAKED NARROW-LINE ACTIVE GALACTIC NUCLEI. II. THE CASE OF EQUAL PEAKS

Active galactic nuclei (AGNs) with double-peaked narrow lines (DPAGNs) may be caused by kiloparsec-scale binary AGNs, bipolar outflows, or rotating gaseous disks. We examine the class of DPAGNs in which the two narrow-line components have closely similar intensity as being especially likely to involve disks or jets. Two spectroscopic indicators support this likelihood. For DPAGNs from Smith et al., the 'equal-peaked' objects (EPAGNs) have [Ne V]/[O III]ratios lower than for a control sample of non-double-peaked AGNs. This is unexpected for a pair of normal AGNs in a galactic merger, but may be consistent with [O III] emission from a rotating ring with relatively little gas at small radii. Also, [O III]/H{beta} ratios of the redshifted and blueshifted systems in the EPAGN are more similar to each other than in a control sample, suggestive of a single ionizing source and inconsistent with the binary interpretation.

THE LOW FREQUENCY OF DUAL ACTIVE GALACTIC NUCLEI VERSUS THE HIGH MERGER RATE OF GALAXIES: A PHENOMENOLOGICAL MODEL

Dual AGNs are natural byproducts of hierarchical mergers of galaxies in the LambdaCDM cosmogony. Recent observations have shown that only a small fraction (~ 0.1%-1%) of AGNs at redshift z<~ 0.3 are dual with kpc-scale separations, which is rather low compared to the high merger rate of galaxies. Here we construct a phenomenological model to estimate the number density of dual AGNs and its evolution according to the observationally-estimated major merger rates of galaxies and various scaling relations on the properties of galaxies and their central massive black holes. We show that our model reproduces the observed frequency and separation distribution of dual AGNs provided that significant nuclear activities are triggered only in gas-rich progenitor galaxies with central massive black holes and only when the nuclei of these galaxies are roughly within the half-light radii of their companion galaxies. Under these constraints, the observed low dual AGN frequency is consistent with the relatively high merger rate of galaxies and supports the hypothesis that major mergers lead to AGN/QSO activities. We also predict that the number of kpc-scale dual AGNs decreases with increasing redshift and only about 0.02%--0.06% of AGNs are dual AGNs with double-peaked narrow line features at redshifts of z 0.5-1.2. Future observations of high-redshift dual AGNs would provide a solid test for this prediction.

Quasar SDSS J142507.32+323137.4: dual AGNs?

We analyze the optical spectrum of type 1 QSO SDSS J1425+3231. This object is interesting since its narrow emission lines, such as [O III]λλ4959, 5007, are double-peaked, and the line structure can be modeled well by three Gaussian components: two components for the two peaks (we refer to the peaks at low/high redshift as “the blue/red component") and another one for the line wing which has the same line center as that of the blue component, but is ∼ 3 times broader. The separation between the blue and red components is ∼500 km s−1 with the blue component being ∼2 times broader than the red one. The Hβ emission can be separated into four components: two for the double-peaked narrow line and two for the broad line which comes from the broad line regions. The black hole mass estimated from the broad Hβ emission line using the typical reverberation mapping relation is 0.85 × 108 M⊙, which is consistent with that derived from parameters of [O III]λ 5007 of the blue component. We suggest that this QSO might be a dual AGN system; the broad Hβ emission line is mainly contributed by the primary black hole (traced by the blue component) while the broad Hβ component of the secondary black hole (traced by the red component) is hard to be separated out considering a resolution of ∼2 000 for SDSS spectra or it may be totally obscured by the dusty torus.