X-ray clues on the nature of sub-mm galaxies

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 • 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 850μm

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 ~1013 Lsolar 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 LFIR 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 Hardness Ratio -0.40 corresponding to Γ~1.4 (or NH~1022 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 Total A torus is needed in most cases at 24 μm but the IR luminosity is dominated by the SFR component Torus Arp220

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 2x10-17 cgs or Lx=5x1041 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

X-ray clues on the nature of sub-mm galaxies