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

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

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. 1)Obscured Starbursts z~1-3 2)Radio loud AGN z = 3-6 3)Low luminosity AGN z<1 * 4)Over-resolved one-sided radio jets Possible Origin of this Population * - Option 3) not likely – hosts are >L* and should have I < 24.

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

Statistics of OIRSs 371 point-like radio sources in survey field ( A ~ 0.6 deg 2 ) 45 (12%) have no optical counterpart in B w, R, I maps. 52 (14%) have faint (e.g., I ~ 24) optical counterparts S 20cm (mJy) = 0.49 mJy min: 76  Jy max: 4.0 mJy

20cm/24  m 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

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

Statistics of OIRSs 371 point-like radio sources in survey field ( A ~ 0.6 deg 2 ) 45 (12%) have no optical counterpart in B w,R,I maps. 52 (14%) have faint (e.g., I ~ 24) optical counterparts (OFRSs). 5 (11%) of the 45 OIRSs have MIPS counterparts ( S 24  m > 0.28 mJy ) S 24  m limit from stack of 40 OIRSs: 0.08 mJy 9 (17%) of the OFRSs have MIPS counterparts

6 5 ULIRG: SFR=3000 Mo/yr AGN/Seyfert 2 logL 1.4 = c273 logL 1.4 = z = Radio/midIR Color-Magnitude Diagram ULIRG & AGN SEDs From Xu et al. (2001) 3c273 SED from NED MIPS (red) sources consistent with z~2-3 ULIRGs. S 24  m /S 20cm (green) upper-limits imply AGN dominated sources at z > 2. If QSO-like they can be at z~5-6.

z = ULIRG AGN 3c273 SCUBA Observations? Radio/mid-IR/Sub-mm 2-Color Diagram 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. Sub-mm Flux Predictions

X-ray Emission From OIRSs 10 OIRSs within Chandra/LALA field (Wang et al. 2004) 2 are detected at 24  m 0 are detected in X-rays (F keV < 1.5 x 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 (N H > 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?

Optically “Invisible” X-ray Sources: Starburst AGN 19 Optically Invisible X-ray in Chandra/LALA field (Wang et al. 2004) 8 Detected at 24  m 0 Detected at 20 cm 4/19 : S 24  m /F.5-2 keV > 0.2 Starburst dominated 15/19: S 24  m /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

Summary 12% of 20cm point-sources have no optical counterpart to > 26 AB in B w, R, & I maps. Most are not detected by MIPS at 24  m (S 24 < 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 (F keV < 1.5 x 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  m?) Investigate properties of the OFRSs