1. Hard X-ray observations of Extremely Red Objects
Marcella Brusa
Dip. Astronomia, Universita’ di Bologna & INAF- Osservatorio Astr. Di Bologna
In collaboration with:
A. Comastri, M. Mignoli & the HELLAS2XMM team
L. Pozzetti, E. Daddi, A. Cimatti
F. Civano
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GH2003 Workshop

2. Extremely Red Objects
Discovered serendipitously in optical and near-infrared surveys by colour selection (Elston et al. 1988)
Sources with most of the emission in the near-infrared band: R-K>5; I-K>4:
Faint optical magnitude (R>23)
Redshift range: 0.8<z<3
DUSTY STARBURSTS
OBSCURED AGN
OLD ELLIPTICALS
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GH2003 Workshop

3. EROs and hard X-ray sources
Optical follow-up of sources detected in deep and medium deep hard X-ray samples  EROs colors
Why X-ray detected EROs
should be AGN?
X-ray flux = 2x10-16
z=1
 Observed 2-10 keV luminosity: L=1042
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GH2003 Workshop

4. Observational results
How many?
-Fraction of EROs among X-ray sources
-Fraction of X-ray emitters among EROs
2) Optical to X-ray properties
3) Spectroscopic identification
4) Hard X-ray spectral properties
& link with QSO2
5) Non AGN EROs: X-ray constraints
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GH2003 Workshop

5. 1a) Fraction of EROs (R-K>5) among hard X-ray sources
35 ks XMM observation of a complete sample of 350 EROs (K<19.2, Daddi et al. 2000,A&A,361,565)
Flux limit of 5x10-15 erg cm-2 s-1,
46 hard X-ray sources
5 EROs
 12% of X-ray sources are EROs
(Brusa et al. 2003, AN, 324, 113)
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GH2003 Workshop

6. Going toward faint X-ray (and K) fluxes …
Lockman Hole (Mainieri et al. 2002) / %
(flux limit ~1e-15; BUT incomplete K coverage)
CDFS
Roche et al 2003: / % (hard)
Mainieri phD Thesis 2003: / % (full)
(flux limit ~4e-16; GOODS area, K<22)
CDFN
Barger et al. 2003: / % (hard)
(flux limit ~1e-16; complete (?) K cov in all the CDFN area, K=20-22)
Alexander et al 2002: %
(flux limit ~2e-16; complete K cov only in 70.6 arcmin2, K=20)
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GH2003 Workshop

7. 1b) Fraction of X-ray emitters among optically selected EROS (R-K>5)
“Daddi Field”
Brusa et al / %
(flux limit = 5e-15; complete K cov over a large area 450 arcmin2
ELAIS
Roche et al / %
(flux limit ~1.5e-15; K<19.5, small area, 38.7 arcmin2)
CDFS
Roche et al 2003: / %
(flux limit ~4e-16; GOODS area, K<22)
CDFN
Alexander et al 2002: / %
(flux limit ~2e-16; complete K cov only in 70.3 arcmin2, K<20.1)
COMPLETENESS and LARGE AREA are needed
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GH2003 Workshop

8. 2) X-ray to optical properties of AGN powered EROs
AGN EROs populate a well-defined region (high X/O locus) in the R vs. Hard X-ray flux plane
lg(X/O)=1+/-1
1 order of magnitude higher than BL AGN
blue=ID
red= no ID
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GH2003 Workshop

9. A15 in A2390 field: R>23.2, I>22.9 K=18.9, I-K>4 R-K>4.3
Optically obscured or … X-ray obscured?
(an effect of the K-correction)
A15 in A2390 field: R>23.2, I> K=18.9, I-K> R-K>4.3
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GH2003 Workshop

10. 3) Spectroscopic identifications status (hard X-ray selected)
PUBLISHED Spectroscopic Redshifts (15)
8 in CDFN (Barger et al. 2003),
2 in Lockman Hole (Mainieri et al. 2002),
5 additional in other X-ray observations
(1 in ELAIS, 2 in A2390, 1 in K20 survey, 1 in Hellas2XMM)
Photometric Redshifts (29)
25 in CDFN, 3 in Lockman Hole, 1 in ELAIS
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GH2003 Workshop

11. Z-dist for hard X-ray EROs
SHADED:
spectr. (15)
SOLID:
phot.+spec (44)
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GH2003 Workshop

12. 4) Hard X-ray properties of EROs
PKS0312 #127 in the HELLAS2XMM sample
(Brusa et al. 2003)
ERO 524 in the K20 sample
(Brusa et al. 2002)
Lockman #25
(Mainieri et al. 2002)
ELAIS N2_21
(Willott et al. 2002)
A2390 #18
(Cowie et al. 2003)
CDFN #122; #48
(Barger et al. 2003, Vignali et al. 2003)
blue=ID
red= no ID
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GH2003 Workshop

13. PKS0312_127 in the HELLAS2XMM sample
Brusa et al. 2003, A&A,sub
R=23.5, K= R-K=5.1
z=2.251
K band
R band
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GH2003 Workshop

14. PKS0312_127 in the HELLAS2XMM sample
Brusa et al. 2003, A&A,sub
Hard X-ray source (HR=0.37) 
@z=2.251 corresponds to rest frame NH>1023
X-ray spectral analysis
(with Cash statistic): rest frame NH=1.7x1023
Lx=3x1044 erg/s
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GH2003 Workshop

15. The K20 CDF-S EROS sample individual detection: ERO 524
Brusa et al. 2002, ApJ,L89,581
R=23.81, K=18.44, z=1.327
F2-10 = 6.4x10-16 erg cm-2 s-1;
undetected in the soft X-ray band (1 Ms)
Observed X-ray luminosity = 5x1042 erg s-1
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GH2003 Workshop

16. ERO 524: hard X-ray properties
Brusa et al. 2002, ApJ,581,L89
Hardness Ratio: HR>0.76
 intrinsic NH>4x1023 cm-2 (at z=1.327)
X-ray spectral analysis (Cash stat.) NH=2.5x1023 cm-2
(NH>7x1022
Unabsorbed X-ray luminosity > 1043 erg s-1
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GH2003 Workshop

17. #25 in Lockman Hole (Mainieri et al. 2002)
R=25.5, K=19.3
R-K=6.2
z=2.71 (ph.)
Rest frame NH=6.5x1023 cm-2
Lx>1045 erg/s
( see also Stevens et al. 2003)
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GH2003 Workshop

18. #21 in ELAIS_N2 (Willott et al. 2002)
R=25.0, K=18.60
R-K=6.4
z=1.575 from infrared spectrum
Rest frame
NH=5x1021 cm-2
Lx=2x1044 erg s-1
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GH2003 Workshop

19. A2390 #18 (Crawford et al. 2002, Cowie et al. 2001)
R=23.86, K=16.49
R-K=7.37
z=1.467 (from optical spectrum)
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GH2003 Workshop

20. A2390 #18 (Crawford et al. 2002, Gandhi et al. 2002)
Rest frame
NH=2x1023 cm-2
Lx=2x1045 erg s-2
“…genuine X-ray Type II QSO…”
Gandhi, Fabian & Crawford 2002
keV 
rest-frame 6.4 keV line?
too poor statistic….
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GH2003 Workshop

21. #122 in CDFN - Barger et al. 2003 catalog (Alexander et al
#122 in CDFN - Barger et al catalog (Alexander et al. 2002, Vignali et al. 2002, astro-ph/ )
R=25.5, HK=20.3
R-K=5.5
z=1.338 (from optical spectrum)
Rest frame
NH>1022 cm-2
Lx(obs)=1043 erg s-1
Spectra of 4 sources with I-K>4
….no evidences for Fe line…
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GH2003 Workshop

22. #48 in CDFN - Barger et al. 2003 catalog
R=23.7, I=22.2, HK=  R-K=5.2 (I-K=3.7)
NO spec z; phot z=0.86 … but BRIGHT X-ray source
 F(2-10)=1.9x10-14 cgs, ~1000 cts in 2 Ms
z=1.13
EW=255 eV (obs frame)
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GH2003 Workshop

23. #48 in CDFN - Barger et al. 2003 catalog
Rest frame NH=3.2x1023
Observed 2-10 keV luminosity
L(2-10)=1.3x1044
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GH2003 Workshop

24. EROs and QSO2
AGN EROs are highly obscured X-ray sources (6/7 have NH>1022 cm-2)
(>80% including also unid. sources and/or NH from HR)
A significant fraction has LX>1044 erg s-1
IF WE DEFINE AN X-RAY TYPE 2 QSO any object with Lx> erg/s and NH>1022 cm-2
 Bright X-ray EROs are best candidates
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GH2003 Workshop

25. How many QSO2 among EROs? 30% have NH>22 & Lx>44
Search for QSO2:
Select R-K>5 objects
(good optical/NIR data)
 high z
@ bright X-ray fluxes
(high X/O sources)
 high NH
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GH2003 Workshop

26. 4) Hard X-ray properties of EROs
blue=ID
red= no ID
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GH2003 Workshop

27. Non AGN EROs: The K20 CDFS EROS sample
21 EROs in a sub-area of CDFS (K20 survey)
Cimatti et al. 2002, A&A,381,L68
Average spectrum of old passive galaxies: red and smooth continuum (8 objects)
Average spectrum of dusty systems: strong OII emission
(12 objects)
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GH2003 Workshop

28. The K20 CDFS EROS sample X-ray Stacking Results
(Brusa et al. 2002, ApJ,581,L89)
OLD SAMPLE
8 objects, zmean=1.145
Effective 7 Ms exposure
X-ray emission remains undetected in all the adopted energy bands
Upper limit on soft X-ray emission:
L0.5-2<1041 erg s-1
DUSTY SAMPLE
12 objects, zmean=1.053
Effective 10 Ms exposure
Positive detection (>3 sigma level) in the 0.5-8, and 1-5 keV bands
Rest-frame Hard X-ray luminosity L2-10=8.0x1040 erg s-1
Intrinsic X-ray absorption NH<1022 cm -2
Consistent with emission
from ellipticals in the local
Universe (e.g. Pellegrini 1999)
X-ray properties of
Starburst galaxies
(e.g. Ptak et al. 1999)
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GH2003 Workshop
see also Alexander et al. 2002

29. The K20 CDFS EROS sample Star Formation from the X-rays
From the X-rays (Ranalli et al. 2002)
SFR(X)=5-44 Msun/yr
(similar predictions from Bauer et al. 2002, Nandra et al 2002 & Grimm et al relations)
From optical OII flux (Kennicutt 1998)
+ dereddening E(B-V)=
SFR(OII)= Msun/yr
Large uncertainties, but the X-ray emission suggests the lower E(B-V)~ lower SFR
(in agreement with Radio/Fir results; see Smail et al. 2002)
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GH2003 Workshop

30. Conclusions 1 How many AGN are EROs?
10-25% (depending on limiting fluxes)
obscured in both X-ray and optical bands(X/O~10)
high redshift (z=1-3)
30% are QSO2
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GH2003 Workshop

31. 2-15% (depending on limiting fluxes)
Conclusions 2
How many EROs are AGN?
2-15% (depending on limiting fluxes)
Passive EROs remain undetected in the X-ray band
Dusty systems are Hard X-ray emitters at the Starburst level. Column density <1022 cm-2. “Unbiased” SFR estimate.
venerdì 30 novembre 2018
GH2003 Workshop