TY - JOUR
T1 - Unobscured type 2 active galactic nuclei
AU - Shi, Yong
AU - Rieke, George H.
AU - Smith, Paul
AU - Rigby, Jane
AU - Hines, Dean
AU - Donley, Jennifer
AU - Schmidt, Gary
AU - Diamond-Stanic, Aleksandar M.
PY - 2010
Y1 - 2010
N2 - Type 2 active galactic nuclei (AGNs) with intrinsically weak broad emission lines (BELs) would be exceptions to the unified model. After examining a number of proposed candidates critically, we find that the sample is contaminated significantly by objects with BELs of strengths indicating that they actually contain intermediate-type AGNs, plus a few Compton-thick sources as revealed by extremely low ratios of X-ray to nuclear IR luminosities. We develop quantitative metrics that show two (NGC3147 and NGC4594) of the remaining candidates to have BELs 2-3 orders of magnitude weaker than those of typical type1 AGNs. Several more galaxies remain as candidates to have anomalously weak BELs, but this status cannot be confirmed with the existing information. Although the parent sample is poorly defined, the two confirmed objects are well under 1% of its total number of members, showing that the absence of a BEL is possible, but very uncommon in AGN. We evaluate these two objects in detail using multi-wavelength measurements including new IR data obtained with Spitzer and ground-based optical spectropolarimeteric observations. They have little X-ray extinction with N H < ∼ 1021cm-2. Their IR spectra show strong silicate emission (NGC4594) or weak aromatic features on a generally power-law continuum with a suggestion of silicates in emission (NGC3147). No polarized BEL is detected in NGC3147. These results indicate that the two unobscured type2 objects have circumnuclear tori that are approximately face-on. Combined with their X-ray and optical/UV properties, this behavior implies that we have an unobscured view of the nuclei and thus that they have intrinsically weak BELs. We compare their properties with those of the other less-extreme candidates. We then compare the distributions of bolometric luminosities and accretion rates of these objects with theoretical models that predict weak BELs.
AB - Type 2 active galactic nuclei (AGNs) with intrinsically weak broad emission lines (BELs) would be exceptions to the unified model. After examining a number of proposed candidates critically, we find that the sample is contaminated significantly by objects with BELs of strengths indicating that they actually contain intermediate-type AGNs, plus a few Compton-thick sources as revealed by extremely low ratios of X-ray to nuclear IR luminosities. We develop quantitative metrics that show two (NGC3147 and NGC4594) of the remaining candidates to have BELs 2-3 orders of magnitude weaker than those of typical type1 AGNs. Several more galaxies remain as candidates to have anomalously weak BELs, but this status cannot be confirmed with the existing information. Although the parent sample is poorly defined, the two confirmed objects are well under 1% of its total number of members, showing that the absence of a BEL is possible, but very uncommon in AGN. We evaluate these two objects in detail using multi-wavelength measurements including new IR data obtained with Spitzer and ground-based optical spectropolarimeteric observations. They have little X-ray extinction with N H < ∼ 1021cm-2. Their IR spectra show strong silicate emission (NGC4594) or weak aromatic features on a generally power-law continuum with a suggestion of silicates in emission (NGC3147). No polarized BEL is detected in NGC3147. These results indicate that the two unobscured type2 objects have circumnuclear tori that are approximately face-on. Combined with their X-ray and optical/UV properties, this behavior implies that we have an unobscured view of the nuclei and thus that they have intrinsically weak BELs. We compare their properties with those of the other less-extreme candidates. We then compare the distributions of bolometric luminosities and accretion rates of these objects with theoretical models that predict weak BELs.
KW - Galaxies: Seyfert
KW - Galaxies: nuclei
UR - http://www.scopus.com/inward/record.url?scp=77951130788&partnerID=8YFLogxK
UR - http://www.scopus.com/inward/citedby.url?scp=77951130788&partnerID=8YFLogxK
U2 - 10.1088/0004-637X/714/1/115
DO - 10.1088/0004-637X/714/1/115
M3 - Article
AN - SCOPUS:77951130788
SN - 0004-637X
VL - 714
SP - 115
EP - 129
JO - Astrophysical Journal
JF - Astrophysical Journal
IS - 1
ER -