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

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 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