Evidence that most type-1 AGNs are reddened by dust in the host ISM
Journal Article
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· Astrophysical Journal
- School of Physics and Astronomy, Tel-Aviv University, Tel Aviv 69978 (Israel)
- Max Planck Institut für Astronomie, Königstuhl 17, D-69117, Heidelberg (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≈10{sup 45} erg s{sup −1} and redshift z∼0.4 in bins of optical continuum slope α{sub opt} and width of the broad Hβ emission line. We measure the equivalent width (EW) of the NaID absorption feature in each stacked spectrum. We find a linear relation between α{sub opt} and EW(NaID), such that EW(NaID) increases as α{sub 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 EW(NaID)=0, supporting the line of sight dust hypothesis. This conclusion is also supported by the dependence of the Hα/Hβ line ratio on α{sub opt}. The implied relationship between continuum slope and dust reddening is given by E{sub B−V}≈0.2⋅(−0.1−α{sub opt}), and the implied reddening of a typical type-1 AGN with α{sub opt}=−0.5 is E{sub B−V}≈0.08 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 α{sub opt} arises since ISM columns along shielded and non-shielded sightlines are correlated. This scenario is supported by the similarity of the relation between E{sub B−V} and the Na i column implied by our results with the relation in the Milky Way found by previous studies.
- OSTI ID:
- 22868472
- Journal Information:
- Astrophysical Journal, Journal Name: Astrophysical Journal Journal Issue: 1 Vol. 832; ISSN ASJOAB; ISSN 0004-637X
- 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
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Wed Nov 09 19:00:00 EST 2016
· The Astrophysical Journal (Online)
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OSTI ID:1524177