Transcript 1. dia - Toruń

VLBI (Imaging) Surveys
Sándor Frey
[email protected]
Satellite Geodetic Observatory,
Institute of Geodesy, Cartography and
Remote Sensing (FÖMI)
8th European VLBI Network Symposium
September 25-30, Toruń, Poland
”Types” of
astronomers /
astronomies
Conclusion (!)
Surveys provide (often essential) ”raw material” for all types of astronomies

DO SURVEYS!
… or if they are done already, use them!
”Few of us dedicate much time to searching for new objects. We
are mainly content to let the others do it for us.”
S. Beckwith (1993), ASPC 43, 303
What is a survey?
A systematic study of the sky (or often only a part of it) to explore
an unknown region in the observing parameter space.
Such parameters can be the
• observing wavelength / band
• instrument used
• sensitivity
• angular resolution
• spectral resolution
• polarization
• sky coverage
• temporal sampling
etc.
The discovery potential of a survey essentially depends on how ”large” the
new part of the parameter space is.
VLBI surveys: history
Caveats:
I will mainly cover imaging surveys in this talk!
There will be giant leaps in the history!
I do know that the list is by far not complete!
If there is a genealogical tree of the VLBI imaging surveys, the 5-GHz
Pearson-Readhead (PR) survey (1988) is located at the roots.
Pearson & Readhead (1988), ApJ 328, 114
Note that it took about two decades for VLBI as a new instrument to have
such an imaging survey completed. (The work itself took ~10 years.)
Non-imaging surveys were done earlier, giving valuable information on
source compactness using large (~1000) samples.
Preston et al. (1985), AJ 90, 1599
Morabito et al. (1986), AJ 91, 1038
What makes a survey different from studying a set of individual objects?
The systematics and the statistical completeness in some sense (e.g. all
sources above a given flux density limit, in a well-defined spectral range,
covering a certain area of the sky, etc.).
The PR sample ( > 35º, |b| > 10º, S5GHz > 1.3 Jy) contained 65 sources,
45 of which had detectable mas-scale emission that time.
The basis of the AGN classification in terms of VLBI morphology was laid
down by this work.
As of today, the PR survey has nearly 300 citations in the ADS.
Since these are the brightest sources, the survey
work gave impetus for the type of research
concentrating on selected individual objects…
...which we denote with
Example #1
The core-jet structure of a luminous quasar at z=2.17
”Asymmetric II”
Pearson & Readhead (1988),
ApJ 328, 114
Lobanov et al. (2006), PASJ 58, 253
Example #2
A very young CSO at z=0.52
”Compact S double”
Pearson & Readhead (1988),
ApJ 328, 114
Owsianik & Conway (1998), A&A 337, 69
Eventually, new classes of objects may be identified among the survey
targets, triggering astronomy type
Example:
Compact Symmetric Objects (CSOs)
15-GHz images
Taylor et al. (2000), ApJ 541, 112
New, more massive surveys emerge as it becomes possible to extend
the range of one or more observing parameters
A family of surveys (1)
Caltech-Jodrell Bank (CJ) surveys
CJ1: lowered the flux density limit
of the PR sample to 0.7 mJy
+135 sorces
VLBI images at 1.6 and 5 GHz
Polatidis et al. (1995), ApJS 98, 1
Thakkar et al. (1995), ApJS 98, 33
Xu et al. (1995), ApJS 99, 279
CJ2: 193 sources az 5 GHz
Taylor et al. (1994), ApJS 95, 345
Henstock et al. (1995), ApJS 100, 1
CJF: flat-spectrum sample
~300 sources
homogeneous integration of the
survey data available that time,
essentially before the VLBA era
Taylor et al. (1996), ApJS 107, 37
A sample of this size is already promising for statistical studies.
(The follow-up work of the CJ surveys is still going on…)
An example: the angular size – redshift relation as a cosmological test
Wilkinson et al. (1998), ApSS Lib. 226, 221
New instrument – new survey(s)
The VLBA is the first (and only) ”survey machine” in VLBI
The survey machine
The VLBA is well suited for conducting surveys of large samples,
primarily because of
• the dedicated telescope array (sufficient operations time,
continuous observations) and
• the possibility of highly automated data processing
The regular monitoring capability opens up the posibility to study source
evolution as well.
A family of surveys (2)
VLBA 2-cm survey
Kellermann et al. (1998), AJ 115, 1295
Zensus et al. (2002), AJ 124, 662
good resolution at 15 GHz
multiple epochs from 1994
~200 sources
Kellermann et al. (2004), ApJ 609, 539
Kovalev et al. (2005), AJ 130, 2473
goal: to bulid up a sample for studying the bulk relativistic motion in AGN jets
Apparent superluminal motion statistics with a sample of sources that were
not seletced based on the earlier detection of superluminal motion itself…
Plus detailed studies of jet kinematics in a
number of individual sources
(NGC 1058, 3C 279, PKS 1345+125, …)
Apparent velocity vs. variability
Doppler factor for the fastest
component found in 49 sources with
an intrinsic brightness temperature
2 × 1010 K
solid line: the expected locus of points
for Lorentz factor values of 25

Kellermann et al. (2004),
ApJ 609, 539
The –z diagram for 110 sources The
solid line is the maximum value of app
for =25

Monitoring of Jets in Active galactic nuclei with VLBA Experiments (MOJAVE)
Lister & Homan (2005), AJ 130, 1418
Homan & Lister (2006), AJ 131, 1262
A ”continuation” of the VLBA 2-cm survey, with more monitoring epochs, full
linear & circular polarization images, contemporaneous single-dish radio
data, and an extended sample of objects
VLBA Imaging and Polarimetry Survey (VIPS)
Taylor et al. (2005), ApJS 159, 27
>1100 sources, S > 85 mJy (at 8.4 GHz from CLASS)
located in the SDSS northern cap
 multi-wavelength science
First-epoch VLBA observations completed this year (and available on the web!)
Deep Extragalactic VLBI-Optical Survey (DEVOS)
Mosoni et al. (2006), A&A 445, 413
(in the pilot stage; global/EVN)
Matches the sky coverage with SDSS, but targets much weaker sources with
phase-referencing
VIPS spin-off: 0402+379, a supermassive binary black hole system
7.3 pc
Maness et al. (2004), ApJ 602, 123
Rodriguez et al. (2006), ApJ 646, 49
Surveys for specific goals
VLBA Calibrator Survey
Beasley et al. (2002), ApJS 141, 13, etc.
(VCS 1…5)
Over 3000 images, S and X band snapshots
Primary goal: to provide phase-reference calibrators in sufficient sky density
USNO Radio Reference Frame Image Database
reference frame source images to study the
influence of their structure on the astrometric quality
Originally at 2 and 8 GHz, now many epochs and (for some sources)
additional frequencies (24 and 43 GHz) are available
Fey et al. (1996), ApJS 105, 299, etc.
VLBA Pre-Launch Survey (VLBApls)
Originally to check source compactness on the longest baselines to prepare
for the VSOP Survey
Fomalont et al. (2000), ApJS 131, 95
5-GHz snapshots for ~370 sources
New instrument – new survey
The unique angular resoultion of Space VLBI made the
VSOP 5-GHz AGN Survey possible
(done in part time, and typically with limited ground resources)
Brightness temperature distribution
Horiuchi et al. (2004), ApJ 616, 110
Relative visibility distribution
Dashed line: 3-component
model
PR Survey from Space:
a sub-sample of 27 sources
Lister et al. (2001), ApJ 554, 948
Tingay et al. (2001), ApJ 554, 964
2200+420
at 5 GHz
ground-only

SVLBI

correlation analysis (morphology, pc/kpc-scale jet misalignment, IDV, core
dominance, optical polarization, emission line equivalent width) support to the
beaming model
Surveys with the EVN?
The session-based observing and thus the limited time makes (large all-sky)
surveys incompetitive at the EVN
However, recent advances in the data rate could somewhat compensate for
the time!
Deep surveys, concentrating on a small region of the sky, could utilise the
high sensitivity offered by the EVN
Wide-field VLBI imaging technique could be employed
e-VLBI?
Future: multi-band approach
Astronomical surveys (whether deep narrow-field surveys, or all-sky ones)
are not at all exlusive in radio or VLBI!
Nowadays surveys are still ”fashionable” either becasue
• new powerful (often space-based) instruments are available for
conducting them, or
• future instruments need targets and/or calibrator objects
There is an extra advantage to combine surveys at multiple wavebands: we
could figure out a lot more about the physics that drives the objects!
A few examples of what we could gain – from a VLBI point of view:
• Redshifts from optical spectroscopy (for doing cosmology, or calculating
physical parameters in the source rest frame)
• Broad-band spectral energy distribution
• Morphology
Recent VLBI surveys of deep fields
Hubble Deep Field (HDF)
sub-mJy objects for which
the radio emission is
dominated by the AGN
process could be identified
Garrett et al. (2001),
A&A 366, L5
Recent VLBI surveys of deep fields
NOAO Deep Wide-Field Survey
Bootes Field
Garrett et al. (2005), ApJ 619, 105
VLBA + GBT wide-field VLBI with in-beam phase-referencing at 1.4 GHz
Wrobel et al. (2005), AJ 130, 923
Targeting > 10 mJy FIRST
radio sources with the VLBA
at 5 GHz
~1/3 detected (above 2 mJy)
Obscured AGN at high
redshift?
z=6.12 QSO
A few things we learned from VLBI surveys
• Many AGN brightness temperatures exceed the equipartition and inverseCompton limits
• There exist components that remain unresolved even with the longest
baselines
• On average, IDV sources / -ray loud AGNs / BL Lacs tend to be more
compact
• High-speed apparent superluminal motion is coupled with high brightness
temperatures
• The parent population of radio jets is not dominated by highly relativistic
flows
• Jet motions cannot be described with a simple ballistic model
Appendix: VLBI survey resources on the web
PR and CJ surveys
www.astro.caltech.edu/~tjp/cj/
VLBA 2cm
www.cv.nrao.edu/2cmsurvey/
MOJAVE
www.physics.purdue.edu/astro/MOJAVE/
USNO RRFID
rorf.usno.navy.mil/rrfid.shtml
VLBA Calibrators
www.vlba.nrao.edu/astro/calib/
VSOP Pre-launch (VLBApls)
www.vlba.nrao.edu/astro/obsprep/sourcelist/6cm/
VSOP Survey
www.vsop.isas.jaxa.jp/survey/
VIPS
www.phys.unm.edu/~gbtaylor/VIPS/