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