1. AGN / Starbursts in the very dusty systems in Bootes Kate Brand + the Bootes team NOAO Lijiang, August 2005

2. The Importance of ULIRGS ULIRGs have L bol >10 12 L o and contribute significantly to the total energy budget of the Universe. ULIRGS are an increasingly significant population at high redshifts. They are difficult to study due to their extreme obscuration at most wavelengths. - What is their dominant power source? AGN or starburst? Crucial in determining the build-up of their galaxy bulges via star formation and SMBH growth via AGN activity. - What is the contribution of AGN to the 24  m background? - What is the nature of the population with extreme 24  m to optical ratios?

3. The Data - The NDWFS Bootes field (9 deg 2 ) Wavelength # 24  m counterparts Flux limit 24  m (Spitzer/MIPS) ~20,0000.3mJy 8  m (Spitzer/IRAC) ~18,000~0.005mJy R band (NDWFS)~18,000~25.5 X-ray (Chandra XBootes)12003E-14 ergscm -2 s -1 (5ks) Optical Spectroscopy (AGES) 5200I<21.5 (point) I<20.0 (extended) Optical Spectroscopy (Keck/DEIMOS) 4000.3mJy

4. Using Log( f (24)/ f (8)) to divide AGN / starburst galaxies Templates copied from Laurent et al. 2000 steep dust continuum + strong PAHs Shallow dust continuum (+ possible silicate abs.) Rest Wavelength (  m) 4 6 8 10 12 14 16 24  m @ z=18  m @ z=1 Starburst galaxies Rest Wavelength (  m) 24  m @ z=18  m @ z=1 4 6 8 10 12 14 16 AGN f(24:8)~0.5 f(24:8)~0.0

5. The low redshift star-forming contaminants Rest Wavelength (  m) 4 6 8 10 12 14 16 8  m @ z=0 8  m @ z=0.6 Normal star-forming galaxies at z < 0.6: enhanced 8  m emission. f(24:8)~ -0.2.

6. Log( f (24)/ f (8)) distributions for different f (24) bins Stars or z=0 galaxies Low z star-forming galaxies AGN High z Starbursts Stars or z=0 galaxies Low z star-forming galaxies AGN High z Starbursts Stars or z=0 galaxies Low z star-forming galaxies AGN High z Starbursts

7. The fraction of AGN dominated ULIRGs as a function of f 24 24  m number counts from Papovich et al. 2004 -> AGN contribute to ~5-13% of the 24  m background.

8. Summary f(24:8) is a good discriminator for distinguishing AGN and starburst dominated ULIRGs. The fraction of AGN dominated ULIRGs increases from ~20% at f 24 =0.3mJy to ~60% at higher f 24. AGN contribute to ~5-13% of the 24  m background. Caveats - Silicate absorption band - lower f(24:8) at z~1-2 Strong PAH emission features - higher f(24:8) Heavily embedded AGN - could the 24  m emission still be dominated by an AGN?

9. The extreme optically obscured ULIRGS Arp 220 - a local starburst dominated ULIRG known to be heavily obscured. R-[24]>15 (Log( f (24)/ f (R)) >1.8) sources - more extreme than Arp 220 out to high redshifts. If at z~2 as IRS follow-up suggest, L bol ~ 10-100 x L Arp220 but fainter than 0.1L * galaxy in the optical. 859 (4% of the 24  m sources) have R-[24]>15 - a large population of extreme dusty obscured sources at high redshifts with no comparable examples in the local Universe.

10. What are the R-[24]>15 sources? Interacting but optically invisible Optically extended / interacting X-ray loud but optically invisible Optically blank

11. Log( f (24)/ f (8)) of R-[24]>15 sources

12. What are the R-[24]>15 sources? The f 24 faint sources (intrinsically less luminous or higher redshift?) tend to be steep spectrum sources dominated by powerful but heavily obscured starbursts. The less numerous (14%) f 24 > 0.75mJy sources (intrinsically more luminous or lower redshift?) have a larger fraction of AGN dominated sources (as found in IRS follow-up observations).

13. Summary f(24:8) is a good discriminator for distinguishing AGN and starburst dominated ULIRGs. The fraction of AGN dominated ULIRGs increases from ~20% at f 24 =0.3mJy to ~60% at higher f 24. AGN contribute to ~5-13% of the 24  m background. There exists a large population of extreme dusty obscured sources at high redshifts with no comparable examples in the local Universe. The faintest f 24 sources are dominated by powerful but heavily enshrouded starbursts whereas the brightest f 24 sources have a larger contribution from AGN.

14. Future Directions f(24:8) diagnostic Redshifts to investigate obscuration as a function of mid-IR luminosity. More IRS spectroscopy to test and confirm f(24:8) diagnostic R-[24]>15 sources Keck spectra of optically brighter sources - z~1-2. IRS spectroscopy of X-ray loud sources to investigate unified schemes of dust and gas distribution.

15. f(24:8) vs. f(24:R)