Bibcode
Mason, R. E.; Lopez-Rodriguez, E.; Packham, C.; Alonso-Herrero, A.; Levenson, N. A.; Radomski, J.; Ramos-Almeida, C.; Colina, L.; Elitzur, M.; Aretxaga, I.; Roche, P. F.; Oi, N.
Referencia bibliográfica
The Astronomical Journal, Volume 144, Issue 1, article id. 11 (2012).
Advertised on:
7
2012
Número de citas
65
Número de citas referidas
58
Descripción
We present high-resolution mid-infrared (MIR) imaging, nuclear spectral
energy distributions (SEDs), and archival Spitzer spectra for 22
low-luminosity active galactic nuclei (LLAGNs; L bol <~
1042 erg s-1). Infrared (IR) observations
may advance our understanding of the accretion flows in LLAGNs, the fate
of the obscuring torus at low accretion rates, and, perhaps, the star
formation histories of these objects. However, while comprehensively
studied in higher-luminosity Seyferts and quasars, the nuclear IR
properties of LLAGNs have not yet been well determined. We separate the
present LLAGN sample into three categories depending on their Eddington
ratio and radio emission, finding different IR characteristics for each
class. (1) At the low-luminosity, low-Eddington-ratio (log L
bol/L Edd < -4.6) end of the sample, we
identify "host-dominated" galaxies with strong polycyclic aromatic
hydrocarbon bands that may indicate active (circum-)nuclear star
formation. (2) Some very radio-loud objects are also present at these
low Eddington ratios. The IR emission in these nuclei is dominated by
synchrotron radiation, and some are likely to be unobscured type 2 AGNs
that genuinely lack a broad-line region. (3) At higher Eddington ratios,
strong, compact nuclear sources are visible in the MIR images. The
nuclear SEDs of these galaxies are diverse; some resemble typical
Seyfert nuclei, while others lack a well-defined MIR "dust bump." Strong
silicate emission is present in many of these objects. We speculate that
this, together with high ratios of silicate strength to hydrogen column
density, could suggest optically thin dust and low dust-to-gas ratios,
in accordance with model predictions that LLAGNs do not host a
Seyfert-like obscuring torus. We anticipate that detailed modeling of
the new data and SEDs in terms of accretion disk, jet, radiatively
inefficient accretion flow, and torus components will provide further
insights into the nuclear structures and processes of LLAGNs.
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