The near-infrared broad emission line region of active galactic nuclei - II. The 1-μm continuum

Abstract

We use quasi‐simultaneous near‐infrared (near‐IR) and optical spectroscopy from four observing runs to study the continuum around 1 μm in 23 well‐known broad emission line active galactic nuclei (AGN). We show that, after correcting the optical spectra for host galaxy light, the AGN continuum around this wavelength can be approximated by the sum of mainly two emission components, a hot dust blackbody and an accretion disc. The accretion disc spectrum appears to dominate the flux at ∼ 1 μm, which allows us to derive a relation for estimating AGN black hole masses based on the near‐IR virial product. This result also means that a near‐IR reverberation programme can determine the AGN state independent of simultaneous optical spectroscopy. On average we derive hot dust blackbody temperatures of ∼1400 K, a value close to the sublimation temperature of silicate dust grains, and relatively low hot dust covering factors of ∼7 per cent. Our preliminary variability studies indicate that in most sources, the hot dust emission responds to changes in the accretion disc flux with the expected time lag; however, a few sources show a behaviour that can be attributed to dust destruction.

Reaching the peak of the quasar spectral energy distribution – II. Exploring the accretion disc, dusty torus and host galaxy

“…This may suggest that the grains were formed in an oxygen-rich environment. We do see evidence for a spread in T hot values that is likely to be a feature of the limited quality of our data, although Landt et al (2011) note that in NGC 5548 there is some evidence for higher dust temperatures than other objects in their sample. Other studies finding similar results for T hot include Kobayashi et al (1993), who use a similar approach to ours, and Suganuma et al (2006).…”

“…Landt et al (2011) obtain values of T hot = 1365 ± 18 and C hot = 7 ± 2 per cent. The Landt et al (2011) sample had NIR spectra from the NASA Infrared Telescope Facility's SpeX spectrograph, and as such the data were of significantly higher quality than was available for this work, in which the torus components were only constrained by WISE photometry. Their sample was also at a lower redshift (z ≤ 0.3) and lower average luminosity than our sample.…”

Strong variability of the coronal line region in NGC 5548

“…As we have shown in Landt et al (2011), the former is dominated by the accretion disc flux, which is believed to be the main source of ionising radiation in AGN and so the driver of the observed variability, whereas the latter is emitted from the hot dust component of the obscuring torus. Furthermore, we have derived the hot dust temperature from blackbody fits to the near-IR spectral continuum as described in Landt et al (2011) and list it also in Table 3. In Section 4, we will compare these values to the optical coronal line ratios [Fe VII] λ6087/λ3759 and [Fe VII] λ5159/λ6087, which are suitable indicators of the gas temperature and density (Nussbaumer et al 1982;Keenan & Norrington 1987), respectively (listed in Table 3).…”

“…All data were obtained through a slit of 0.8 × 15 ′′ giving an average spectral resolution of full width at half maximum (FWHM) ∼ 400 km s −1 . The four epochs spanning the years 2004 − 2007 are our own data and were discussed and presented in Landt et al (2008) and Landt et al (2011). The near-IR spectrum from 2002 was discussed by Riffel et al (2006).…”

“…However, the latter spectrum was observed at a very low air mass (sec z ∼ 1.01). The optical data were discussed and presented in Landt et al (2008) and Landt et al (2011), with the exception of the May 2004 spectrum, which was retrieved from the FAST archive.…”