Spitzer Space Telescope - General Observer Proposal #40356


Evolution of Hot Dust Emission in z>6 Quasars

Principal Investigator: Xiaohui Fan
           Institution: University of Arizona

     Technical Contact: Xiaohui Fan, University of Arizona

Co-Investigators: 
Linhua Jiang, University of Arizona                                   
Niel Brandt, Penn State                                               
Chris Carilli, NRAO                                                   
Dean Hines, Space Science Institute                                   
Klaus Meisenheimer, MPIA                                              
Gordon Richards, Drexel University                                    
Michael Strauss, Princeton                                            
Fabian Walter, MPIA                                                   
Ran Wang, NRAO                                                        

Science Category: AGN/quasars/radio galaxies 
 Observing Modes: IracMap IrsPeakupImage MipsPhot 
  Hours Approved: 14.9 

Abstract:
Strong hot dust emission from dust torus surrounding the central engine is an
ubiquitous feature among Type-1 quasars at low redshift. In our previous GTO and
GO-1 Spitzer programs, we observed a sample of quasars at 4<z<6.4 to probe the
evolution of hot dust at the highest redshift. While most quasars in this sample
also show prominent hot dust emission at 24 micron, two objects at z~6 have
extremely weak IR emission with SEDs consistent with power-law or pure disk
emission without hot dust contribution. These two quasars also have the smallest
black hole (BH) masses and highest Eddington ratios among z~6 quasars,
indicative of short BH growth timescale. These observations suggest possible
evolution in either dust torus structure or dust properties of quasars at the
earliest epoch, constrained by both BH/AGN growth and timescale of dust
formation in AGB envelops. In Cycle 4 we propose to carry out IRAC, IRS PUI 16
micron and MIPS 24 micron imaging of a sample of 9 new quasars at z>6 discovered
in the last three years. The new observations will more than double the Spitzer
sample size at z>6. All objects have excellent multiwavelength data, with BH
mass estimates based on broad emission line width, and are GTO targets for
Herschel at far-IR. With the combined Cycle 1/4 sample, we will (a) measure the
fraction of quasars without hot dust emission at early epoch; (b) test the
relation between hot dust properties and emission line properties, BH masses and
accretion rates in quasars; (c) improve quasar IR SED measurements at z>6, and
(d) probe the relation between hot and warm/cool dust traced by Herschel and
sub/mm observations. In particular, three quasars in our new sample have similar
emission line and BH properties to the two IR-weak quasars we found previously.
We predict that they are likely candidates of new dust-deficient quasars at z>6.