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The Nature Of Optically “Invisible” Radio Sources In The NOAO Deep Wide Field In Bootes

This study investigates the radio sources that have no or very faint optical counterparts in the NOAO Deep Wide Field in Bootes. The goal is to detect high-z starburst/AGN objects and improve positional accuracy for follow-ups. The study also aims to find AGN-dominated objects using 24mm/20cm data.

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The Nature Of Optically “Invisible” Radio Sources In The NOAO Deep Wide Field In Bootes

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  1. The Nature Of Optically “Invisible” Radio Sources In The NOAO Deep Wide Field In Bootes J. L. Higdon, S. J. U. Higdon, D. Weedman, J. Houck (Cornell) B. T. Soifer (Caltech), B. Jannuzi, A. Dey, M. Brown (NOAO) E. Le Floc’h, & M. Rieke (Arizona)

  2. The VLA 20cm Pilot Survey “A pilot study is better than no study at all. ” - Bob Becker • Observations • ( July 2003 ) • A-configuration: 1.4” fwhm • 3-array pointings: ~0.6 deg2 • 4.5-hours/pointing: 15 mJy/beam Goals 1) Detect 20cm continuum from MIPS high-z starburst/AGN. 2) Improved positional accuracy for IRS followups. 3) Use 24mm / 20cm to find AGN dominated objects. white circles: VLA red squares: NDWF fields yellow circle: Chandra LALA survey (Wang et al. 2004)

  3. Optically Faint Radio Sources (OFRSs) Deep radio images routinely find a sizable fraction of point-sources that have very faint optical counterparts … or none at al. e.g., Richards et al. (1999) & Fomalont et al. (2002) ~20% have no optical counterpart to I > 25. Possible Origin of this Population • Obscured Starbursts z~1-3 • Radio loud AGN z = 3-6 • Low luminosity AGN z<1 * • Over-resolved one-sided radio jets * - Option 3) not likely – hosts are >L* and should have I < 24.

  4. Optically Invisible Radio Sources (OIRSs) in Bootes Radio criteria: (1) point/v.compact (2) Speak S/N > 5 Optical criteria: (1) no optical counterpart within R < 3”

  5. <S20cm> = 0.49 mJy min: 76 mJy max: 4.0 mJy 0.0 1.0 2.0 3.0 4.0 S20cm (mJy) Statistics of OIRSs • 371 point-like radio sources in survey field ( A ~ 0.6 deg2 ) • 45 (12%) have no optical counterpart in Bw, R, I maps. • 52 (14%) have faint (e.g., I ~ 24) optical counterparts.

  6. 20cm/24 mm Detection Limits For Starbursts Any high-z starburst detected in the radio should be seen by MIPS. Starburst SED templates from Xu et al. (2001) Red: MIPS 24 mm sensitivity Green: VLA 20cm sensitivity

  7. Mid-IR Counterparts of OIRSs Criteria: MIPS/VLA match if R < 4”

  8. Statistics of OIRSs • 371 point-like radio sources in survey field ( A ~ 0.6 deg2 ) • 45 (12%) have no optical counterpart in Bw,R,I maps. • 52 (14%) have faint (e.g., I ~ 24) optical counterparts (OFRSs). • 5 (11%) of the 45 OIRSs have MIPS counterparts • ( S24mm > 0.28 mJy ) • S24mm limit from stack of 40 OIRSs: 0.08 mJy • 9 (17%) of the OFRSs have MIPS counterparts

  9. ULIRG: SFR=3000 Mo/yr z = 1 2 3 7 6 4 5 AGN/Seyfert 2 logL1.4= 24-26 3c273 logL1.4 = 24-26 z = 1 2 3 4 5 6 7 Radio/midIR Color-Magnitude Diagram • MIPS (red) sources • consistent with z~2-3 • ULIRGs. • S24mm/S20cm (green) • upper-limits imply • AGN dominated • sources at z > 2. • If QSO-like they can • be at z~5-6. ULIRG & AGN SEDs From Xu et al. (2001) 3c273 SED from NED

  10. ULIRG z = 1 2 3 7 4 6 5 AGN z = 1 2 7 3 6 4 5 3c273 SCUBA Observations?Radio/mid-IR/Sub-mm 2-Color Diagram Sub-mm Flux Predictions • 5 MIPS detections will • be easily detected with • SCUBA if z > 0.5. • MIPS non-detections • (the vast majority) will • only be detected if • z > 5 for an AGN SED. • MIPS non-detections • will not be detected if • SED is 3c273-like at • any redshift.

  11. X-ray Emission From OIRSs • 10 OIRSs within Chandra/LALA field (Wang et al. 2004) • 2 are detected at 24mm • 0 are detected in X-rays (F0.5-2 keV < 1.5 x 10-16 erg s-1 cm-2) Slightly surprising: - radio loud AGN are luminous X-ray sources (cf., Cappi et al. 1997) - X-rays can be blocked by high gas columns (NH > 1024 cm-2), but energy re-emitted in the IR by dust. Possible Solutions: - We are not using the right SED! Maybe IR under luminous, not radio over luminous. ( I.e., z > 5 dust poor starbursts.) - X-ray quenching in dust poor ISM?

  12. Starburst AGN Optically “Invisible” X-ray Sources: • 19 Optically Invisible X-ray • in Chandra/LALA field • (Wang et al. 2004) • 8 Detected at 24 mm • 0 Detected at 20 cm • 4/19 : S24mm/F.5-2 keV > 0.2 • Starburst dominated • 15/19: S24mm/F.5-2 keV < 0.2 • AGN dominated • (Weedman et al. 2004) Appear to be two “Optically Invisible” populations: - Radio & mid-IR “quiet” X-ray sources - X-ray & mid-IR “quiet” Radio sources

  13. Summary • 12% of 20cm point-sources have no optical counterpart to > 26 AB in Bw, R, & I maps. • Most are not detected by MIPS at 24 mm (S24 < 0.08 mJy) • Some appear to be z~2-3 ULIRG • Most (~75%) appear to be z~2-6 AGN • None of the 10 OIRSs in Chandra/LALA field are detected in soft X-rays (F0.5-2 keV < 1.5 x 10-16 erg s-1 cm-2) Future Work • Explore dust-poor (e.g., BCD?) SEDs • VLA 6cm mapping • Sub-mm imaging • Near-IR (IRAC 3.6 – 8.0 mm?) • Investigate properties of the OFRSs

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