Near- to mid-infrared spectroscopy of the heavily obscured AGN LEDA
  1712304 with AKARI/IRC
  Context. Although heavily obscured active galactic nuclei (AGNs) have been
found by many observational studies, the properties of the surrounding dust are
poorly understood. Using AKARI/IRC spectroscopy, we discover a new sample of a
heavily obscured AGN in LEDA 1712304 which shows a deep spectral absorption
feature due to silicate dust. Aims. We study the infrared (IR) spectral
properties of circumnuclear silicate dust in LEDA 1712304. Methods. We perform
IR spectral fitting, considering silicate dust properties such as composition,
porosity, size and crystallinity. Spectral energy distribution (SED) fitting is
also performed to the flux densities in the UV to sub-millimeter range to
investigate the global spectral properties. Results. The best-fit model
indicates 0.1 $\mu$m-sized porous amorphous olivine (${\rm
Mg_{2x}Fe_{2-2x}SiO_4}$; $x=0.4$) with $4\%$ crystalline pyroxene. The optical
depth is $\tau_{\rm sil}{\sim}2.3$, while the total IR luminosity and stellar
mass are estimated to be $L_{\rm IR}=(5\pm1){\times}10^{10}\,L_{\odot}$ and
$M_{\rm star}=(2.7\pm0.8){\times}10^{9}\,M_{\odot}$, respectively. In such low
$L_{\rm IR}$ and $M_{\rm star}$ ranges, there are few galaxies which show that
large ${\tau}_{\rm sil}$. Conclusions. The silicate dust in the AGN torus of
LEDA 1712304 has properties notably similar to those in other AGNs as a whole,
but slightly different in the wing shape of the absorption profile. The
porosity of the silicate dust suggests dust coagulation or processing in the
circumnuclear environments, while the crystallinity suggests that the silicate
dust is relatively fresh.
