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)
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)
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
1)
2)
3)
4)
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.
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”
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.
<S20cm> = 0.49 mJy
min: 76 mJy max: 4.0 mJy
0.0
1.0
2.0
S20cm (mJy)
3.0
4.0
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
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
( S24mm > 0.28 mJy )
• S24mm limit from stack of 40 OIRSs: 0.08 mJy
• 9 (17%) of the OFRSs have MIPS counterparts
Radio/midIR Color-Magnitude Diagram
ULIRG: SFR=3000 Mo/yr
z=1
2
3
76
5
• MIPS (red) sources
consistent with z~2-3
ULIRGs.
4
AGN/Seyfert 2
logL1.4= 24-26
z=1
7
65
4
3
2
ULIRG & AGN SEDs From Xu et al. (2001)
3c273 SED from NED
3c273
logL1.4 = 24-26
• S24mm/S20cm (green)
upper-limits imply
AGN dominated
sources at z > 2.
• If QSO-like they can
be at z~5-6.
SCUBA Observations?
Radio/mid-IR/Sub-mm 2-Color Diagram
Sub-mm Flux Predictions
ULIRG
z=1
2
3
4
6
7
• MIPS non-detections
(the vast majority) will
only be detected if
z > 5 for an AGN SED.
AGN
z=1
2
3
3c273
5
•5 MIPS detections will
be easily detected with
SCUBA if z > 0.5.
4
5
6
7
• MIPS non-detections
will not be detected if
SED is 3c273-like at
any redshift.
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?
Optically “Invisible” X-ray Sources:
Starburst
•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
AGN
• 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
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