X-ray clues on the nature of sub-mm galaxies I.Georgantopoulos INAF/OABO A Comastri INAF/OABO E. Rovilos MPE

Outline What are the sub-mm galaxies SMGs ? What can the X-ray observations tell us about SMGs? (Alexander et al claim that sub-mm galaxies present a phase of co-eval growth of black holes and star-formation).

Questions to be addressed 1. Fraction of AGN among sub-mm galaxies 2. Level of Obscuration 3. What is the major power-mechanism that produces the Bolometric luminosity ?

SMG: an Introduction SCUBA detector (850micron) on JCMT gave a boost to sub-mm Cosmology The negative K-correction plays a major role making high- redshift galaxies easier to detect 850 μm

SMGs: an Introduction It is expected many SMG are at high redshift z>2 Of course the identification is very very difficult because of Large PSF and associated uncertainties in source position 8 arcsec (see reviews by Maiolino 2008, Blain+02) Chapman+05 did a breakthrough observing radio positions of SMGs and then observing counterparts with Keck z=2-3 (altough Daddi+09, Capak+08 find counterparts even at z=4-5)

SMG: Introduction The median luminosity is ~10 13 L solar Spitzer IRS spectra show star-forming spectra (Pope+08) Smail+98

X-ray observations CDFN Alexander+05 find that a large fraction 70% of SMGs are X-ray sources and thus AGN. Many of these appear to be highly obscured Co-evolution of black hole growth and star-formation under a veil of dust ? Laird+09 criticise the sample selection (a mixture of SMGs with radio positions and Radio sources with sub-mm detection) Laird et al find instead a lower fraction of X-ray detections among ‘true’ SMGs while they claim that many of the X-ray sources are normal galaxies (not AGN)

The CDFS LABOCA observations Here we attempt to address anew these issues in the CDFS Which is the field with the deepest X-ray observations ever. (2 MS Chandra + another 2Ms starting in June) (4 Chandra 200ksec observations around the CDFS) (3 Ms of XMM observations) Spitzer IRAC and MIPS observations Sub-mm observations at 870 μm LABOCA camera APEX telescope. 128 sources detected in the area of the eCDFS

X-ray/sub-mm associations Background: R Contours: MIPS Circle: sub-mm Blue: X-ray Maximum-Likelihood Method for the association between MIPS and LABOCA 13 associations

X-ray properties: spectra and Lx Galaxies Galaxies are sources with low luminosity and soft spectra eg Georgakakis+06

Yet another check: Lx vs L FIR Ranalli+03 Lx-Lfir relation Star-forming Galaxies Open circles denote the X-ray classified Galaxies

AGN fraction eCDFS+CDFS AGN fraction : 9/128 (7%) Considering only the CDFS: 7/38 (18+/-7 %) very close to the estimates of Laird et al. IMPLYING THAT THE MAJORITY OF SMGs ARE STARFOMING

Non-X-ray detected sources: stacking analysis sampleNoCounts keV Flux eCDFS10061 (5.6σ)2.3x CDFS2128 (2.5σ)2x Hardness Ratio corresponding to Γ~1.4 (or N H ~10 22 cm -2 ) This suggests that most of the undetected SMGs are galaxies

AGN absorption Significant absorption among the AGN Hash= upper limits

What is the power mechanism ? Hopkins+07 Bolometric correction Star-formation is the dominant power mechanism

Example SED A torus is needed in most cases at 24 μm but the IR luminosity is dominated by the SFR component Torus Arp220 Total

Summary 13 X-ray detections out of 128 LABOCA SMGs in the eCDFS and CDFS In the CDFS the AGN fraction is only 18+/-7 % in agreement with Laird+09 in the CDFN Stacking analysis for the undetected SMGs gives a signal in the soft band 2x cgs or Lx=5x10 41 cgs Suggesting that most are normal galaxies. Even among the AGN SMGs, the contribution of the AGN is small suggesting that the major contributor to their luminosities (all but one are ULIRGs) is star-formation