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Hard X-ray observations of Extremely Red Objects

Explore the intriguing properties of Extremely Red Objects (EROs) discovered serendipitously in optical and near-infrared surveys. Learn about dusty starbursts, obscured AGN, and old ellipticals through hard X-ray observations, optical follow-up studies, and spectroscopic identifications. Investigate the X-ray properties of EROs, their linkage with AGN, and the challenges in detecting X-ray emitters among EROs.

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Hard X-ray observations of Extremely Red Objects

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  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 GH2003 Workshop

  2. ExtremelyRedObjects • 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 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 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 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) GH2003 Workshop

  6. Going toward faint X-ray (and K) fluxes … Lockman Hole (Mainieri et al. 2002) 13/53 24% (flux limit ~1e-15; BUT incomplete K coverage) CDFS Roche et al 2003: 14/73 20% (hard) Mainieri phD Thesis 2003: 80/346 23% (full) (flux limit ~4e-16; GOODS area, K<22) CDFN Barger et al. 2003: 70/325 21% (hard) (flux limit ~1e-16; complete (?) K cov in all the CDFN area, K=20-22) Alexander et al 2002: 8% (flux limit ~2e-16; complete K cov only in 70.6 arcmin2, K=20) GH2003 Workshop

  7. 1b) Fraction of X-ray emitters among optically selected EROS (R-K>5) “Daddi Field” Brusa et al. 2002 5/350 2% (flux limit = 5e-15; complete K cov over a large area 450 arcmin2 ELAIS Roche et al. 2002 1/34 3% (flux limit ~1.5e-15; K<19.5, small area, 38.7 arcmin2) CDFS Roche et al 2003: 14/172 8% (flux limit ~4e-16; GOODS area, K<22) CDFN Alexander et al 2002: 4/29 14% (flux limit ~2e-16; complete K cov only in 70.3 arcmin2, K<20.1) COMPLETENESS and LARGE AREA are needed 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 GH2003 Workshop

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

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

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

  13. PKS0312_127 in the HELLAS2XMM sample Brusa et al. 2003, A&A,sub • R=23.5, K=18.4 R-K=5.1 z=2.251 K band R band 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 NH>1022 @90% Lx=3x1044 erg/s 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 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 cm-2 @90%) • Unabsorbed X-ray luminosity > 1043 erg s-1 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) 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 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) GH2003 Workshop

  20. A2390 #18 (Crawford et al. 2002, Gandhi et al. 2002) “…genuine X-ray Type II QSO…” Gandhi, Fabian & Crawford 2002 • Rest frame NH=2x1023 cm-2 • Lx=2x1045 erg s-2 Excess @2.6 keV  rest-frame 6.4 keV line? too poor statistic…. GH2003 Workshop

  21. #122 in CDFN - Barger et al. 2003 catalog(Alexander et al. 2002, Vignali et al. 2002, astro-ph/0209415) • 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… GH2003 Workshop

  22. #48 in CDFN - Barger et al. 2003 catalog • R=23.7, I=22.2, HK=18.8  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 +/-0.05 @90% EW=255 eV (obs frame) GH2003 Workshop

  23. #48 in CDFN - Barger et al. 2003 catalog • Rest frame NH=3.2x1023 (@z=1.13) • Observed 2-10 keV luminosity L(2-10)=1.3x1044 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/orNH from HR) • A significant fraction has LX>1044 erg s-1 IF WE DEFINE AN X-RAY TYPE 2 QSO any object with Lx>1044 erg/s and NH>1022 cm-2  Bright X-ray EROs are best candidates 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 GH2003 Workshop

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

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

  28. DUSTY SAMPLE 12 objects, zmean=1.053 Effective 10 Ms exposure Positive detection (>3 sigma level) in the 0.5-8, 0.5-2 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 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 The K20 CDFS EROS sampleX-rayStacking Results (Brusa et al. 2002, ApJ,581,L89) 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) GH2003 Workshop see also Alexander et al. 2002

  29. The K20 CDFS EROS sampleStar 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. 2002 relations) • From optical OII flux (Kennicutt 1998) + dereddening E(B-V)=0.5-0.8 SFR(OII)=35-110 Msun/yr Large uncertainties, but the X-ray emission suggests the lower E(B-V)~0.5 lower SFR (in agreement with Radio/Fir results; see Smail et al. 2002) 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 GH2003 Workshop

  31. 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. GH2003 Workshop

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