X-ray clues on the nature of sub-mm galaxies
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Transcript of X-ray clues on the nature of sub-mm galaxies
X-ray clues on the nature of sub-mm galaxies
I.Georgantopoulos INAF/OABOA 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
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 detectionsamong ‘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 sourceswith 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 classifiedGalaxies
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
sample No Counts0.5-2 keV
Flux
eCDFS 100 61 (5.6σ) 2.3x10-17
CDFS 21 28 (2.5σ) 2x10-17
Hardness Ratio -0.40 corresponding to Γ~1.4 (or NH~1022 cm-2)
This suggests that most of the undetected SMGs are galaxies
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 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