AGN / Starbursts in the very dusty systems in Bootes

Kate Brand + the Bootes teamNOAO

Lijiang, August 2005

The Importance of ULIRGS• ULIRGs have Lbol>1012Lo 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 24m background?

- What is the nature of the population with extreme 24m to optical ratios?

The Data - The NDWFS Bootes field (9 deg2)

Wavelength # 24m counterparts

Flux limit

24m (Spitzer/MIPS) ~20,000 0.3mJy

8m (Spitzer/IRAC) ~18,000 ~0.005mJy

R band (NDWFS) ~18,000 ~25.5

X-ray (Chandra XBootes) 1200 3E-14 ergscm-2s-1 (5ks)

Optical Spectroscopy (AGES)

5200 I<21.5 (point)I<20.0 (extended)

Optical Spectroscopy (Keck/DEIMOS)

400 0.3mJy

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)~0.5f(24:8)~0.0

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 8m emission.• f(24:8)~ -0.2.

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

The fraction of AGN dominated ULIRGs as a function of f24

24m number counts from Papovich et al. 2004 -> AGN contribute to ~5-13% of the 24 m background.

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 f24=0.3mJy to ~60% at higher f24.

• 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 24m emission still be dominated by an AGN?

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, Lbol~ 10-100 x LArp220 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.

What are the R-[24]>15 sources?

Interacting but optically invisible

Optically extended / interacting

X-ray loud but optically invisible

Optically blank

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

What are the R-[24]>15 sources?• The f24 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%) f24 > 0.75mJy sources (intrinsically more luminous or lower redshift?) have a larger fraction of AGN dominated sources (as found in IRS follow-up observations).

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 f24=0.3mJy to ~60% at higher f24.

• 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 f24 sources are dominated by powerful but heavily enshrouded starbursts whereas the brightest f24 sources have a larger contribution from AGN.

Future Directionsf(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.

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