Kiloparsec-scale Spatial Offsets in Double-peaked Narrow-line Active Galactic Nuclei. I. Markers for Selection of Compelling Dual Active Galactic Nucleus Candidates

The Astrophysical Journal (Impact Factor: 5.99). 11/2011; 753(1). DOI: 10.1088/0004-637X/753/1/42
Source: arXiv


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.

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    ABSTRACT: 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 (dNL) AGN in merging galaxies and their connection to supermassive black hole (SMBH) pairs, motivated by recent observations of such objects. We find that dNL AGN induced by the relative motion of SMBH pairs are a generic but short-lived feature of gaseous major mergers. dNL AGN should often be observed in late-stage mergers, during the kpc-scale phase of SMBH inspiral or soon after the SMBH merger. However, even within the kpc-scale phase, only a minority of dNL AGN are directly induced by SMBH motion; their lifetimes are typically a few Myr. Most double peaks arise from gas kinematics near the SMBH, although prior to the SMBH merger up to 80% of all dNL profiles may be influenced by SMBH motion via altered peak ratios or velocity offsets. The total lifetimes of dNL AGN depend strongly on viewing angle and on properties of the merging galaxies. Also, in a typical merger, at least 10-40% of the double peaks induced by SMBH motion have small projected separations, 0.1-1 kpc, such that dual peaks of stellar surface brightness are not easily resolved. Diffuse tidal features can indicate late-stage galaxy mergers, although they do not distinguish SMBH pairs from merged SMBHs. We show that dNL profiles with peak velocity splittings > 500 km s^-1 or with measurable overall velocity shifts are often associated with SMBH pairs. Our results support the notion that selection of dNL AGN is a promising method for identifying dual SMBH candidates, but demonstrate the critical importance of high-resolution, multi-wavelength follow-up observations, and the use of multiple lines of evidence, for confirming the dual nature of candidate SMBH pairs. (Abridged)
    Monthly Notices of the Royal Astronomical Society 01/2012; 429(3). DOI:10.1093/mnras/sts533 · 5.11 Impact Factor
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    ABSTRACT: The galaxy CXOC J100043.1+020637, also known as CID-42, is a highly unusual object. An apparent galaxy merger remnant, it displays signatures of both an inspiraling, kiloparsec-scale active galactic nucleus (AGN) pair and of a recoiling AGN with a kick velocity > 1300 km s^-1. Among recoiling AGN candidates, CID-42 alone has both spatial offsets (in optical and X-ray bands) and spectroscopic offsets. In order to constrain the relative likelihood of both scenarios, we develop models using hydrodynamic galaxy merger simulations coupled with radiative transfer calculations. Our gas-rich, major merger models are generally well matched to the galactic morphology and to the inferred stellar mass and star formation rate. We show that a recoiling supermassive black hole (SMBH) in CID-42 should be observable as an AGN at the time of observation. However, in order for the recoiling AGN to produce narrow-line emission, it must be observed shortly after the kick while it still inhabits a dense gaseous region, implying a large total kick velocity (v_k > 2000 km s^-1). For the dual AGN scenario, an unusually large broad-line offset is required, and the best match to the observed morphology requires a galaxy that is less luminous than CID-42. Further, the lack of X-ray emission from one of the two optical nuclei is not easily attributed to an intrinsically quiescent SMBH or to a Compton-thick galactic environment. While the current data do not allow either the recoiling or the dual AGN scenario for CID-42 to be excluded, our models highlight the most relevant parameters for distinguishing these possibilities with future observations. In particular, high-quality, spatially-resolved spectra that can pinpoint the origin of the broad and narrow line features will be critical for determining the nature of this unique source.
    Monthly Notices of the Royal Astronomical Society 05/2012; 428(2). DOI:10.1093/mnras/sts114 · 5.11 Impact Factor
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    ABSTRACT: We calculate the observable signature of a black hole accretion disk with a gap or hole created by a secondary black hole embedded in the disk. We find that for an interesting range of parameters of black hole masses (~10^6 to 10^9 solar masses), orbital separation (~1 AU to ~0.1 pc), and gap width (10 to 190 disk scale heights), the missing thermal emission from a gap manifests itself in an observable decrement in the spectral energy distribution. We present observational diagnostics in terms of power-law forms that can be fit to line-free regions in AGN spectra or in fluxes from sequences of broad filters. Most interestingly, the change in slope in the broken power-law is almost entirely dependent on the width of gap in the accretion disk, which in turn is uniquely determined by mass ratio of the black holes, such that it scales roughly as q^(5/12). Thus one can use spectral observations of the continuum of bright active galactic nuclei to infer not only the presence of a closely separated black hole binary but also the mass ratio. When the black hole merger opens an entire hole (or cavity) in the inner disk, the broad band SED of the AGN or quasar may serve as a diagnostic. Such sources should be especially luminous in optical bands but intrinsically faint in X-rays (i.e., not merely obscured). We briefly note that viable candidates may have already been identified, though extant detailed modeling of those with high quality data have not yet revealed an inner cavity.
    The Astrophysical Journal 07/2012; 761(2). DOI:10.1088/0004-637X/761/2/90 · 5.99 Impact Factor
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