A NEW DIAGNOSTIC OF ACTIVE GALACTIC NUCLEI: REVEALING HIGHLY ABSORBED SYSTEMS AT REDSHIFT >0.3

By S. Juneau, Mark Dickinson, D. M. Alexander, S. Salim. 2011

146 citations recorded by the source corpus.

We introduce the Mass-Excitation (MEx) diagnostic to identify active galactic nuclei (AGNs) in galaxies at intermediate redshift. In the absence of near-infrared spectroscopy, necessary for using traditional nebular line diagrams at z > 0.4, we demonstrate that combining [O iii] λ5007/Hβ and stellar mass successfully distinguishes between star formation and AGN emission. The MEx classification scheme relies on a novel probabilistic approach splitting galaxies into sub-categories with more confidence than alternative high-z diagnostic diagrams. It recognizes that galaxies near empirical boundaries on traditional diagrams have an uncertain classification and thus a non-zero probability of belonging to more than one category. An outcome of this work is a system of statistical weights that can be used to compute global properties of galaxy samples. We apply the MEx diagram to 2812 galaxies at 0.3 < z < 1 in the Great Observatories Origins Deep Survey North and Extended Groth Strip fields, and compare it to an independent X-ray classification scheme. We identify Compton-thick AGN candidates with large X-ray absorption, which we infer from the luminosity ratio between hard X-ray emission and [O iii] λ5007, a nearly isotropic tracer of AGNs. X-ray stacking of sources that were not detected individually supports the validity of the MEx diagram and yields a very flat spectral slope for the Compton-thick candidates (Γ ≈ 0.4; unambiguously indicating absorbed AGNs). We present evidence that composite galaxies, which are difficult to identify with alternative high-redshift diagrams, host the majority of the highly absorbed AGNs. Our findings suggest that the interstellar medium of the host galaxy provides significant absorption in addition to the torus invoked in AGN unified models.

Subfields: Astronomy and Astrophysics, Instrumentation

View Paper: https://www.semanticscholar.org/paper/70547b64c8b968f196401f8b9778f443fd5a243d

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Citations in the corpus

Tracing the cosmic growth of supermassive black holes to z ∼ 3 with Herschel (2014, background): “However, these techniques have still to be tested on high redshift (z > 1) sizeable samples (Juneau et al. 2011; Trump et al. 2013) and are sensitive to comparatively large AGN-to-host flux ratios in the MIR domain (Barmby et al. 2006; Brusa et al. 2010; Donley et al. 2012).”
THE MOSFIRE DEEP EVOLUTION FIELD (MOSDEF) SURVEY: REST-FRAME OPTICAL SPECTROSCOPY FOR ∼1500 H-SELECTED GALAXIES AT 1.37 ≤ z ≤ 3.8 ?> (2014, methodology): “In a first paper (Coil et al. 2014) we test various optical AGN classification diagnostics at z ∼ 2, including the BPT, MEx (Juneau et al. 2011), and CEx diagrams (Yan et al. 2011).”
The DEEP3 Galaxy Redshift Survey: the impact of environment on the size evolution of massive early-type galaxies at intermediate redshift (2011, background): “…likely to inhabit slightly more overdense regions even relative to galaxies of like colour and luminosity (i.e. stellar mass; Yan et al. 2006, 2011; Montero-Dorta et al. 2009; Juneau et al. 2011) – though, recent work suggests that LINERs may not be the product of AGN activity (Yan & Blanton 2011).”
BLACK HOLE GROWTH AND ACTIVE GALACTIC NUCLEI OBSCURATION BY INSTABILITY-DRIVEN INFLOWS IN HIGH-REDSHIFT DISK GALAXIES FED BY COLD STREAMS (2011, background): “Many AGNs are highly absorbed, individually undetectable in X-rays (e.g., Daddi et al. 2007), and many of these obscured AGNs have moderate intrinsic luminosities and lie in star-forming galaxies (Juneau et al. 2011).”
Nebular Emission Line Ratios in z ≃ 2–3 Star-forming Galaxies with KBSS-MOSFIRE: Exploring the Impact of Ionization, Excitation, and Nitrogen-to-Oxygen Ratio (2016, unknown): “These results are largely consistent with those reported by Coil et al. (2015), who explicitly tested the success of the MEx in identifying z ∼ 2 AGN using observations from the MOSDEF survey.”
Dusty Star Forming Galaxies at High Redshift (2014, methodology): “…spectral line diagnostics can provide a valuable probe to AGN activity in galaxies via comparison of emission line ratios (e.g. Kewley et al., 2006; Juneau et al., 2011), DSFGs, even if they have optical spectral observations, often lack the high-quality, high-S/N spectra necessary for…”
Galaxy Interactions Trigger Rapid Black Hole Growth: an unprecedented view from the Hyper Suprime-Cam Survey (2017, background): “Indeed, many studies have now shown that no one wave-band can identify all AGN (Alexander et al. 2008; Donley et al. 2008; Hickox et al. 2009; Juneau et al. 2011; Mendez et al. 2013; Goulding et al. 2014; Trump et al. 2015; Azadi et al. 2017).”
What drives the growth of black holes (2011, methodology): “Optical spectroscopy is also effective at identifying distant obscured AGNs (e.g., Steidel et al., 2002; Polletta et al., 2006; Alexander et al., 2008b; Juneau et al., 2011; Yan et al., 2011).”
An extreme [O III] emitter at z = 3.2: a low metallicity Lyman continuum source (2015, methodology): “Juneau et al. (2011) proposed an alternative to the BPT diagram, with the so-called MEx diagram based on the comparison between stellar mass and [O iii]λ5007/Hβ ratio.”
Near infrared spectroscopy and star-formation histories of 3 ≤ z ≤ 4 quiescent galaxies (2018, methodology): “Using the stellar masses derived in the next section (or the ones initially derived at z = zphot), the mass-excitation diagram (Juneau et al. 2011) classifies all the [O iii]-detected galaxies as “AGN”, and Article number, page 15 of 41 this remains true even if we use the stricter criterion…”
Morphologies of z ∼ 0.7 AGN host galaxies in CANDELS: no trend of merger incidence with AGN luminosity (2014, methodology): “However, these systems are expected to be rare (see e.g. Juneau et al. 2011, and references therein).”
GOODS-Herschel: the far-infrared view of star formation in active galactic nucleus host galaxies since z ≈ 3 (2011, methodology): “Whilst we acknowledge that these fractions are only crude es- 15 It was recently suggested that stellar mass, combined with emission lines, can be used as an effective AGN diagnostic out toz∼ 1 (in the “MassExcitation” diagram; Juneau et al. 2011).”