Are Narrow-line Type-1 Quasars Deficient of Hot Dust?

Details Are Narrow-line Type-1 Quasars Deficient of Hot Dust? Are Narrow-line Type-1 Quasars Deficient of Hot Dust?

Mar 2008

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008sptz.prop50681f&link_type=abstract

Spitzer Proposal ID #50681

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Scientific paper

One of the most surprising results from our previous Spitzer observations is the discovery of a class of quasars at z ~ 6 without detectable hot dust emission in the mid-IR. These objects have the narrowest emission lines at z ~ 6 with FWHM ~ 1600 km/s, but share all other characteristics of normal type-1 quasars in their broad-band SEDs from X-ray to radio. Based on virial mass estimates, they have relatively small black hole (BH) masses and high Eddington luminosity ratios. The existence of such objects may indicate strong evolution in dust properties at early epochs; alternatively, it could be an orientation effect and challenge the standard AGN unification model. No such narrow-line type-1 quasars without hot dust emission have been found at low redshift. However, these objects are very rare, counting for only one percent of the quasar population at z < 5, and none of them have been observed at the Spitzer sensitivity. Therefore, we propose to carry out four band Spitzer photometry of a sample of 15 luminous narrow-line type-1 quasars at z = 1 - 5 selected from the SDSS quasar catalog. All objects have excellent optical spectroscopy and BH mass measurements. The Spitzer photometry will be used to directly measure the mid-IR hot dust emission in these objects. Combined with Spitzer observations of broad-line quasars and low-redshift Narrow-line Seyfert 1 galaxis, the new Spitzer data will allow us to answer the following questions: (1) How does the fraction of hot-dust-free quasars evolve with redshift; do hot-dust-free quasars only exist at the earliest epochs, or are they common among mature quasars as well? (2) How is the hot dust emission in narrow-line type-1 quasars related to BH mass, accretion rate, luminosity, line of sight obscuration, and emission line wind? The proposed observations will provide crucial probes to the evolution of dust structures in quasars and its relation to BH growth in the early universe.

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