Two Extreme IR-weak Quasars at z~6: Probing Dust Evolution

Details Two Extreme IR-weak Quasars at z~6: Probing Dust Evolution Two Extreme IR-weak Quasars at z~6: Probing Dust Evolution

May 2006

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006sptz.prop30402f&link_type=abstract

Spitzer Proposal ID #30402

Computer Science

Scientific paper

We propose to carry out deep Spitzer observations of two z~6 quasars with unusual IR properties to constrain the properties of dust in their environments. Most of the high-redshift (z>4.5) quasars we have observed in our GO-1 and GTO programs have rest-frame near to mid-IR spectral energy distributions similar to those of low-redshift quasars, suggesting very similar dust properties, and indicating that most dust tori form in short timescales at high redshift. However, two objects stand out as highly unusual: SDSS J1411+1217,(z=5.93) has an IR SED consistent with a pure power-law, with no IR excess due to hot dust (T > 1000 K). SDSS J0005-0006 (z=5.85) is completely undetected in our GO-1 24 micron MIPS observation, indicating an IR deficit; its SED is consistent with a pure disk model without a dust torus. Such dust-deficient AGN have not been found at either high or low redshift; the IR/optical flux ratios of these two sources are lower than those of all known type I objects at low redshift with Spitzer photometry. Their IR properties could be due to a complete lack of a dust torus at early epochs, or moderate obscuration in the mid-IR. We will obtain deep Spitzer photometry at 8, 16 and 24 micron for J0005-0006, aiming to detect or put stringent upper limit on its hot dust emission. We will carry out deep photometry at 16 micron and low-resolution IRS spectroscopy for the brighter J1411+1217 in order to constrain dust models. These new observations will shed light on the possible evolution of dust properties in the earliest luminous quasars.

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