Evidence that most Type-1 AGNs are reddened by dust in the host ISM
Journal Article
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· The Astrophysical Journal (Online)
- Tel Aviv Univ., Ramat Aviv (Israel)
- Heidelberg Univ. (Germany)
The typical optical–UV continuum slopes observed in many type-1 active galactic nuclei (AGNs) are redder than expected from thin accretion disk (AD) models. A possible resolution to this conundrum is that many AGNs are reddened by dust along the line of sight. To explore this possibility, we stack 5000 SDSS AGNs with luminosity $$L\approx {10}^{45}\,\mathrm{erg}\,{{\rm{s}}}^{-1}$$ and redshift $$z\sim 0.4$$ in bins of optical continuum slope $${\alpha }_{\mathrm{opt}}$$ and width of the broad Hβ emission line. We calculate the equivalent width (EW) of the NaID absorption feature in each stacked spectrum. We find a linear relation between $${\alpha }_{\mathrm{opt}}$$ and EW(NaID), such that EW(NaID) increases as $${\alpha }_{\mathrm{opt}}$$ becomes redder. In the bin with the smallest Hβ width, objects with the bluest slopes, which are similar to AD predictions, are found to have $$\mathrm{EW}(\mathrm{NaID})=0$$, supporting the line of sight dust hypothesis. This conclusion is also supported by the dependence of the Hα/Hβ line ratio on $${\alpha }_{\mathrm{opt}}$$. The implied relationship between continuum slope and dust reddening is given by $${E}_{{\rm{B}}-{\rm{V}}}\approx 0.2\cdot (-0.1-{\alpha }_{\mathrm{opt}})$$, and the implied reddening of a typical type-1 AGN with $${\alpha }_{\mathrm{opt}}=-0.5$$ is $${E}_{{\rm{B}}-{\rm{V}}}\approx 0.08\,\mathrm{mag}$$. Photoionization calculations show that the line of sight dusty gas responsible for reddening is too ionized to produce the observed sodium features. Therefore, we argue that the sodium absorption arises in regions of the host ISM that are shielded from the AGN radiation along lines of sight to the stars, and the correlation with $${\alpha }_{\mathrm{opt}}$$ arises since ISM columns along shielded and non-shielded sightlines are correlated. This scenario is backed by the similarity of the relation between $${E}_{{\rm{B}}-{\rm{V}}}$$ and the Na i column indicated by our results with the relation in the Milky Way found by previous studies.
- Research Organization:
- Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States)
- Sponsoring Organization:
- USDOE Office of Science (SC)
- Grant/Contract Number:
- AC02-05CH11231
- OSTI ID:
- 1524177
- Alternate ID(s):
- OSTI ID: 22868472
- Journal Information:
- The Astrophysical Journal (Online), Journal Name: The Astrophysical Journal (Online) Journal Issue: 1 Vol. 832; ISSN 1538-4357
- Publisher:
- Institute of Physics (IOP)Copyright Statement
- Country of Publication:
- United States
- Language:
- English
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Evidence that most type-1 AGNs are reddened by dust in the host ISM
Journal Article
·
Sat Nov 19 23:00:00 EST 2016
· Astrophysical Journal
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OSTI ID:22868472