1. Buried AGNs in nearby ULIRGs Masa Imanishi (National Astronomical Observatory of Japan)

2. Ultraluminous Infrared Galaxies （ ULIRGs ） L(IR)>10^12Lsun Powerful energy source is hidden behind dust Compact cores (<500pc) are dominant Very compact starburst or AGN ? optical IR(12um) Soifer et al. 2000

3. AGNs in ULIRGs are buried AGNs obscured by torus-shaped dust Detectable via optical spectroscopy NLR ULIRGs have a large amount of nuclear gas and dust Buried AGNs are elusive Sy2

4. Infrared 3-4 um spectroscopy IRCS on Subaru 8.2m telescope Optically non-Seyfert ULIRGs 1. Spectral shape (PAH vs dust absorption) 2. Geometry of energy sources and dust Methods:

5. PAH PAHs are excited in starburst PDRs but destroyed near an AGN 1. 3-4 um spectral shape EW(PAH)~100nm starburst 3.3um PAH Buried AGN featureless 3.4um/3.1um EW(PAH)<<100nm composite

6. 3-4 um starburst Subaru Strong PAH Buried AGN AGN/SB composite Abs. featureLow EW(PAH) Ice 3.1um Bare 3.4um Bare 3.4um

7. 2. dust/energy-source geometry (Imanishi & Maloney 2003 ApJ 588 165) Buried AGN Foreground screen dust model strong exp(-τ λ ) starburst Mixed dust model Dust absorption feature: weak Tau(3.1) < 0.3 Tau(3.4) < 0.2 1-exp(-τ λ ) τλτλ

8. 3-4 um Starburst PAH strong (starburst): Dust absorption weak Buried AGNcomposite PAH weak (AGN): Dust absorption strong

9. 5-35 um Starburst Spitzer PAH strong : Silicate abs. weak Buried AGN Composite PAH weak: Silicate abs. strong

10. Strong dust temperature gradient Av(3um) > Av(10um) > Av(20um) Av(3um) =120 mag Av(10um) =50 mag Av(20um) <20 mag

11. 17/27 (63%) LINER ULIRGs 3/13 (23%) HII-region ULIRGs (statistically complete) (complete at RA=10-22hr) Results Buried AGN signatures: Pure buried AGNs show LINER spectra Powerful buried AGNs fraction: LINER > HII (see also Nagar et al. 2003)

12. NLR Our line-of-sight obscuration: Non-Sy >> Sy2 Sy2: Abs weak Non-Sy: strong Amount of nuclear dust: Non-Sy >> Sy2

13. Buried AGNs: both warm/cool FIR colors pure buried AGN F25/F60=0.16(cool) larger dust column cool FIR color NLR warm cool == starburst

14. Summary 1. Buried AGNs fraction: LINER > HII-region 2. Nuclear dust amount: non-Sy ULIRGs > Sy2 ULIRGs Optical Sy (non-)detectability depends on the amount of nuclear dust It is important to understand optically-elusive buried AGNs in ULIRGs. ( Imanishi et al. 2005, submitted ) warm & cool

15. Just for reference After this slide

16. Ice : inside dense molecular gas HCN (= dense gas tracer) is concentrated to nuclei Strong ice absorption is nuclear core origin (not ISM in an edge-on host galaxy)

17. starburst Buried AGN Centrally-concentrated energy source Strong dust temperature gradient Av(3um) > Av(10um) > Av(20um)

18. Weak PAH ULIRGs: Dust temperature gradient Av(3um) > Av(10um) > Av(20um) Av(3um) =120 mag Av(10um) =50 mag Av(20um) <20 mag Strong dust temperature gradient