Indicators of Intrinsic Active Galactic Nucleus Luminosity: A Multi-Wavelength Approach

Abstract

Active Galactic Nuclei (AGN) consist of an accretion disk around a supermassive black hole which is in turn surrounded by an obscuring torus of dust and gas. As the resulting geometry of this system affects the observable properties, quantifying isotropic indicators of intrinsic AGN luminosity is important in selecting unbiased samples of AGN. In this paper we consider five such proxies: the luminosities of the [OIII]λ5007 line, the [OIV]25.89µm line, the mid-infrared (MIR) continuum emission by the torus, and the radio and hard X-ray (E > 10keV) continuum emission. We compare these different proxies using two complete samples of low-redshift type 2 AGN selected in a homogeneous way based on different indicators: an optically selected [OIII] sample and a mid-infrared selected 12µm sample. To assess the relative merits of these proxies, we have undertaken two analyses. First, we examine the correlations between all five different proxies, and find better agreement for the [OIV], MIR, and [OIII] luminosities than for the hard X-ray and radio luminosities. Next, we compare the ratios of the fluxes of the different proxies to their values in unobscured Type 1 AGN. The agreement is best for the ratio of the [OIV] and MIR fluxes, while the ratios of the hard X-ray to [OIII], [OIV], and MIR fluxes are systematically low by about an order-of-magnitude in the Type 2 AGN, indicating that hard X-ray selected samples do not represent the full Type 2 AGN population. In a similar spirit, we compare different optical and MIR diagnostics of the relative energetic contributions of AGN and star formation processes in our samples of Type 2 AGN. We find good agreement between the various diagnostic parameters, such as the equivalent width of the MIR polycyclic aromatic hydrocarbon features, the ratio of the MIR [OIV]/[NeII] emission-lines, the spectral index of the MIR continuum, and the commonly used optical emission-line ratios. Finally, 2. The Data 2.

Exploring the Connection Between Star Formation and Active Galactic Nucleus Activity in the Local Universe

“…Also, as it is formed in the narrow-line (Heckman et al 2005;Meléndez et al 2008;Rigby et al 2009;Diamond-Stanic et al 2009;LaMassa et al 2010). However, in a majority of our spectra, this feature was not significant above the noise.…”

“…As discussed in LaMassa et al (2009) Spinoglio & Malkan 1989;Heckman et al 2005;LaMassa et al 2010), these Sy2 samples should to first order represent unbiased views of the AGN population. The [O iii] sample has optical spectroscopy from SDSS, whereas the optical data for the majority of the 12 μm sample were based on literature values.…”

“…Only 4 of the 12 μm selected sources have SDSS spectra (3 Sy2s and 1 composite). Optical data for the remainder of these sources were culled from the literature, as noted in LaMassa et al (2010). Hence, these 24 Sy2s and 1 composite are omitted from the SDSS SFR analysis in Section 4.…”

“…As discussed in LaMassa et al (2010), the [O iv]26 μm/ [Ne ii]12.81 μm flux ratio is a reliable tracer of the incident radiation field hardness (Genzel et al 1998;Pereira-Santaella et al 2010;Veilleux et al 2009) since [O iv] is primarily ionized by AGN activity Diamond-Stanic et al 2009;Meléndez et al 2008;LaMassa et al 2010), whereas [Ne ii]12.81 μm is excited by star formation (Ho & Keto 2007 values compared with star-forming and composite galaxies, which can be seen by comparing the best-fit trend to the Sy2s (red dashed line) with the star-forming galaxies (blue dot-dashed line). Compared with Figure 1, a clear offset is discernable between the best-fit relations between Sy2s and quiescent galaxies, suggesting the AGN enhances [Ne iii] emission.…”

“…Disentangling the contribution from the AGN versus star formation then becomes a necessary intermediate step in using continuum IR emission as a tracer of star formation in active galaxies, and such a process can be plagued with uncertainties. Previous studies have indicated that emission from PAHs tends to be suppressed in galaxies hosting an active nucleus, which is sometimes attributed to the harsh radiation field produced by the AGN accretion disk destroying PAHs (e.g., Voit 1992;O'Dowd et al 2009;Treyer et al 2010;LaMassa et al 2010;.…”