The Lack of Torus Emission from BL Lacertae Objects: An Infrared View of Unification with WISE

📝 Abstract

We use data from the Wide-Field Infrared Survey Explorer (WISE) to perform a statistical study on the mid-infrared (IR) properties of a large number ( $\sim10^2 $) of BL Lac objects — low-luminosity Active Galactic Nuclei (AGN) with a jet beamed toward the Earth. As expected, many BL Lac objects are so highly beamed that their jet synchrotron emission dominates their IR spectral energy distributions. In other BL Lac objects, however, the jet is not strong enough to completely dilute the rest of the AGN emission. We do not see observational signatures of the dusty torus from these weakly beamed BL Lac objects. The lack of observable torus emission is consistent with suggestions that BL Lac objects are fed by radiatively inefficient accretion disks. Implications for the “nature vs. nurture” debate for FR I and FR II radio galaxies are briefly discussed. Our study supports the notion that, beyond orientation, accretion rate plays an important role in AGN unification.

💡 Analysis

We use data from the Wide-Field Infrared Survey Explorer (WISE) to perform a statistical study on the mid-infrared (IR) properties of a large number ( $\sim10^2 $) of BL Lac objects — low-luminosity Active Galactic Nuclei (AGN) with a jet beamed toward the Earth. As expected, many BL Lac objects are so highly beamed that their jet synchrotron emission dominates their IR spectral energy distributions. In other BL Lac objects, however, the jet is not strong enough to completely dilute the rest of the AGN emission. We do not see observational signatures of the dusty torus from these weakly beamed BL Lac objects. The lack of observable torus emission is consistent with suggestions that BL Lac objects are fed by radiatively inefficient accretion disks. Implications for the “nature vs. nurture” debate for FR I and FR II radio galaxies are briefly discussed. Our study supports the notion that, beyond orientation, accretion rate plays an important role in AGN unification.

📄 Content

arXiv:1112.5162v1 [astro-ph.CO] 21 Dec 2011 Accepted for Publication in ApJ Letters Preprint typeset using LATEX style emulateapj v. 5/2/11 THE LACK OF TORUS EMISSION FROM BL LACERTAE OBJECTS: AN INFRARED VIEW OF UNIFICATION WITH WISE Richard. M. Plotkin,1 Scott F. Anderson,2 W. N. Brandt,3,4 Sera Markoff,1 Ohad Shemmer,5 Jianfeng Wu3,4 Accepted for Publication in ApJ Letters ABSTRACT We use data from the Wide-Field Infrared Survey Explorer (WISE) to perform a statistical study on the mid-infrared (IR) properties of a large number (∼102) of BL Lac objects — low-luminosity Active Galactic Nuclei (AGN) with a jet beamed toward the Earth. As expected, many BL Lac objects are so highly beamed that their jet synchrotron emission dominates their IR spectral energy distributions. In other BL Lac objects, however, the jet is not strong enough to completely dilute the rest of the AGN emission. We do not see observational signatures of the dusty torus from these weakly beamed BL Lac objects. The lack of observable torus emission is consistent with suggestions that BL Lac objects are fed by radiatively inefficient accretion disks. Implications for the “nature vs. nurture” debate for FR I and FR II radio galaxies are briefly discussed. Our study supports the notion that, beyond orientation, accretion rate plays an important role in AGN unification. Subject headings: accretion, accretion disks — BL Lacertae objects: general — infrared: galaxies

1. INTRODUCTION The standard orientation-based unification paradigm posits that every Active Galactic Nucleus (AGN) is com- prised of the same basic components: an accretion disk around a supermassive black hole, a broad emission line region (BELR), an obscuring torus, and a narrow line region; radio-loud AGN also launch large-scale relativis- tic jets (e.g., Antonucci 1993; Urry & Padovani 1995). The torus is a key component in this unified model, as it can block the BELR along certain lines of sight, and its dust also reprocesses UV/X-ray radiation into the in- frared (IR). In simple unification, the only other way to hide the BELR is also via a geometric argument: emis- sion lines are outshown by a relativistic jet beamed to- ward Earth. In addition to orientation, intrinsic differences among AGN also play a role in AGN unification. For example, Urry & Padovani (1995) conclude their seminal review with 10 outstanding questions, including one regarding Fanaroff-Riley galaxies (FR; Fanaroff& Riley 1974): “do FR Is have broad emission line regions?” The significance of this question is that if FR I galaxies lack BELRs, then intrinsic properties might play a prominent role in driving the so-called FR I/II dichotomy (in which the more powerful FR II galaxies have edge-brightened radio lobes). Indications so far are that emission line lumi- nosities from FR I galaxies are 5–30 times weaker than for FR II galaxies, and FR I galaxies may also have weaker tori (Zirbel & Baum 1995; Chiaberge et al. 1999; 1 Astronomical Institute ‘Anton Pannekoek’, University of Amsterdam, Science Park 904, 1098 XH, Amsterdam, the Netherlands; r.m.plotkin@uva.nl 2 Department of Astronomy, University of Washington, Box 351580, Seattle, WA 98195, USA 3 Department of Astronomy and Astrophysics, Pennsylvania Sate University, 525 Davey Laboratory, University Park, PA 16802, USA 4 Institute for Gravitation and the Cosmos, Pennsylvania State University, University Park, PA 16802, USA 5 Department of Physics, University of North Texas, Denton, TX 76203, USA Donato et al. 2004; although also see Cao & Rawlings 2004; Leipski et al. 2009). However, the requisite obser- vations and their interpretation are technically challeng- ing (Capetti et al. 2000; Evans et al. 2006), and there are still many ongoing “nature vs. nurture” debates. We can turn to beamed versions of FR I and FR II galaxies for additional guidance — i.e., BL Lac objects and flat spectrum radio quasars (FSRQs), respectively, which are collectively called blazars. Broad emission lines and sometimes dusty tori are routinely detected in the spectral energy distributions (SEDs) of FSRQs, but less often from BL Lac objects (e.g. Abdo et al. 2010; Giommi et al. 2011). However, it is generally still not observationally clear if BL Lac BELRs and tori appear weak solely due to dilution by jet emis- sion (e.g., Chen & Shan 2011; Malmrose et al. 2011), or rather if they really are intrinsically weak (e.g., Ghisellini et al. 2011; Sbarrato et al. 2011). Constraints on blazar BELRs and tori are furthermore important, as both components are potential sources of seed photons for Comptonized gamma-ray emission, especially for FS- RQs and low synchrotron peaked (LSP) BL Lac objects. Note, gamma-ray emission from high synchrotron peaked (HSP) BL Lac objects is generally consistent with syn- chrotron self Compton (e.g. Abdo et al. 2010). The preliminary data release of the Wide-field Infrared Survey Explorer (WISE; Wright et al. 2010) recently opened a new multiwavelength window

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