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

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

• I.Georgantopoulos INAF/OABO
• A Comastri INAF/OABO
• E. Rovilos MPE

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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).

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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 ?

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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
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SMGs an Introduction

• It is expected many SMG are at high redshift zgt2
• 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, Blain02)
• Chapman05 did a breakthrough observing radio
  positions of SMGs and then observing counterparts
  with Keck z2-3
• (altough Daddi09, Capak08 find counterparts
  even at z4-5)

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SMG Introduction
The median luminosity is 1013 Lsolar Spitzer
IRS spectra show star-forming spectra (Pope08)
Smail98
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X-ray observations CDFN

• Alexander05 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 ?
• Laird09 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)

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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

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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
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X-ray properties spectra and Lx
Galaxies
Galaxies are sources with low luminosity and
soft spectra eg Georgakakis06
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Yet another check Lx vs LFIR
Ranalli03 Lx-Lfir relation Star-forming
Galaxies
Open circles denote the X-ray classified Galaxies

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AGN fraction

• eCDFSCDFS 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

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Non-X-ray detected sources stacking analysis
sample No Counts 0.5-2 keV Flux
eCDFS 100 61 (5.6s) 2.3x10-17
CDFS 21 28 (2.5s) 2x10-17
Hardness Ratio -0.40 corresponding to G1.4 (or
NH1022 cm-2) This suggests that most of the
undetected SMGs are galaxies
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AGN absorption

• Significant absorption among the AGN

Hash upper limits
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What is the power mechanism ?
Hopkins07 Bolometric correction
Star-formation is the dominant power mechanism
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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
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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 Laird09 in the CDFN
• Stacking analysis for the undetected SMGs gives a
  signal in the soft band 2x10-17 cgs or Lx5x1041
  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