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 deg2

• 4.5-hours/pointing: 15 Jy/beam

The VLA 20cm Pilot Survey

white circles: VLAred squares: NDWF fieldsyellow 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 24m / 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~1-32) Radio loud AGN z = 3-63) 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) Speak S/N > 5Optical criteria: (1) no optical counterpart within R < 3”

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.

0.0 1.0 2.0 3.0 4.0 S20cm (mJy)

<S20cm> = 0.49 mJy

min: 76 Jy max: 4.0 mJy

20cm/24 m Detection Limits For Starbursts

Any high-z starburst detectedin the radio should be seen by MIPS.

Starburst SED templates from Xu et al. (2001)

Red: MIPS 24 mm sensitivityGreen: 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 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 ( S24m > 0.28 mJy )

• S24m limit from stack of 40 OIRSs: 0.08 mJy

• 9 (17%) of the OFRSs have MIPS counterparts

65

ULIRG: SFR=3000 Mo/yr

AGN/Seyfert 2logL1.4= 24-26

3c273logL1.4 = 24-26z = 1

35

7

7 4

3

2z = 1

6

24

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.

• S24m/S20cm (green) upper-limits imply AGN dominated sources at z > 2.

• If QSO-like they can be at z~5-6.

z = 12

3

56

7

z = 12

3 4 56

7

4

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 24m• 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?

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 : S24m/F.5-2 keV > 0.2 Starburst dominated

• 15/19: S24m/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 Bw, R, & I maps.

• Most are not detected by MIPS at 24 m (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 m?)• Investigate properties of the OFRSs