Dust physics in the nucleus of NGC 4151

Abstract

The unified model of active galactic nuclei (AGNs) presumes the existence of a so-called dusty torus around the outer edge of the broad-line region. Despite the solid observational evidence for the existence of dust around AGNs and our growing information on the characteristic scales of these obscuring tori, the origin of this dust and its morphology are not yet well understood. Using dust reverberation mapping, we monitor the effects of AGN continuum variability to determine the temperature and covering factor of the circumnuclear dust, in order to constrain the physical conditions for dust survival and formation in the radiation field of the AGN. Multi-band photometry observations in the z, Y, J, H, and K bands were carried out on the nucleus of the prototypical Seyfert 1 galaxy NGC 4151 over six epochs from 2010 January to June, supported by spectroscopic observations, in order to investigate the response of the hot dust to varying accretion disk emission. Our data confirm that most of the hot dust reacts to increased radiation from the central source with a delayed brightening of ∼50 days. In accretion disk brightening, we see no signatures of dust destruction in our data. The innermost dust appears to increase in temperature rather than sublimate, suggesting that it is cooler than sublimation temperature and located beyond the current sublimation radius. We characterize the dust geometry by interpreting the wavelength-dependent reverberation response with a simplified torus model, pointing to a static radially extended distribution of the central (∼0.1 pc) hot dust.

Reverberation Measurements of the Inner Radii of the Dust Tori in Quasars

“…However, much smaller differences in the dust-reverberation lag at different wavelengths have been reported for multiwavelength dust reverberation. Significant, but small differences were found in some AGNs (Tomita 2005;Tomita et al 2006;Lira et al 2011;Jun et al 2015;Vazquez et al 2015;Lyu et al 2019), 3 and no significant difference was found in some other AGNs (Lira et al 2015;Pozo Nuñez et al 2015;Schnülle et al 2015;Ramolla et al 2018;Oknyansky et al 2019). In some studies, small differences between lag times for different near-infrared bands were found, although it may be difficult to attribute the observed differences in lag times conclusively to the wavelength dependence of the dust-reverberation lag, because the accretion-disk component had not been subtracted from the near-infrared fluxes (Clavel et al 1989;Sitko et al 1993;Glass 2004;Soldi et al 2008;Pozo Nuñez et al 2014;Oknyansky et al 2015Oknyansky et al , 2017Mandal et al 2018).…”

A Test of the Formation Mechanism of the Broad Line Region in Active Galactic Nuclei

“…In general, silicates evaporate in the temperature as low as ∼ 800 K, while amorphous grain, and particularly graphite, can exists at much higher temperatures. The maximum temperature of the dust in the solar system spans the range 1370 -1770 K (Posch et al 2007), and the measured dust temperature in the case of the active galaxy NGC 4151 varied with the nuclear flux, reaching up to 1500 K (Schnuelle et al 2013). In Czerny & Hryniewicz (2011) we used 1000 K as a reference, but Galianni & Horne (2013) in his study of NGC 5548 advocated 1670 K, with the difference due to different assumptions about the geometry.…”

Variability of the coronal line region in NGC 4151

“…The four epochs spanning the years 2004−2007 are our own data and were presented in Landt et al (2008) and Landt et al (2011). The near-IR spectra from 2002 and 2010 were discussed by Riffel et al (2006) and Schnülle et al (2013), respectively. The optical spectra were obtained with the FAST spectrograph (Fabricant et al 1998) at the Tillinghast 1.5 m telescope on Mt.…”