Post on 11-Jan-2016
Sándor Freyfrey@sgo.fomi.hu
Satellite Geodetic Observatory, Institute of Geodesy, Cartography and
Remote Sensing (FÖMI)
VLBI (Imaging) Surveys
8th European VLBI Network SymposiumSeptember 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 samplingetc.
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.
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.).
Preston et al. (1985), AJ 90, 1599
Morabito et al. (1986), AJ 91, 1038
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
Pearson & Readhead (1988),
ApJ 328, 114
”Asymmetric II”
Lobanov et al. (2006), PASJ 58, 253
The core-jet structure of a luminous quasar at z=2.17
Example #2
”Compact S double”
Pearson & Readhead (1988),
ApJ 328, 114Owsianik & Conway (1998), A&A 337, 69
A very young CSO at z=0.52
Eventually, new classes of objects may be identified among the survey targets, triggering astronomy type
Example: Compact Symmetric Objects (CSOs)
Taylor et al. (2000), ApJ 541, 11215-GHz images
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
Polatidis et al. (1995), ApJS 98, 1
Thakkar et al. (1995), ApJS 98, 33
Xu et al. (1995), ApJS 99, 279
Taylor et al. (1994), ApJS 95, 345
Henstock et al. (1995), ApJS 100, 1
Taylor et al. (1996), ApJS 107, 37
CJ1: lowered the flux density limit of the PR sample to 0.7 mJy+135 sorcesVLBI images at 1.6 and 5 GHz
CJ2: 193 sources az 5 GHz
CJF: flat-spectrum sample~300 sourceshomogeneous integration of the survey data available that time, essentially before the VLBA era
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
good resolution at 15 GHzmultiple epochs from 1994~200 sources
Kellermann et al. (1998), AJ 115, 1295
Zensus et al. (2002), AJ 124, 662
Kellermann et al. (2004), ApJ 609, 539
Kovalev et al. (2005), AJ 130, 2473
Plus detailed studies of jet kinematics in a number of individual sources(NGC 1058, 3C 279, PKS 1345+125, …)
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…
The –z diagram for 110 sources The solid line is the maximum value of app for
=25
Apparent velocity vs. variability Doppler factor for the fastest component found in 49 sources with an intrinsic brightness temperature 2 × 1010 Ksolid line: the expected locus of points for Lorentz factor values of 25
Kellermann et al. (2004),
ApJ 609, 539
Monitoring of Jets in Active galactic nuclei with VLBA Experiments (MOJAVE)
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
Lister & Homan (2005), AJ 130, 1418
Homan & Lister (2006), AJ 131, 1262
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
Rodriguez et al. (2006), ApJ 646, 49
7.3 pc
Maness et al. (2004), ApJ 602, 123
Surveys for specific goals
VLBA Calibrator Survey (VCS 1…5)Over 3000 images, S and X band snapshotsPrimary goal: to provide phase-reference calibrators in sufficient sky density
Beasley et al. (2002), ApJS 141, 13, etc.
Fomalont et al. (2000), ApJS 131, 95
USNO Radio Reference Frame Image Databasereference frame source images to study the influence of their structure on the astrometric qualityOriginally at 2 and 8 GHz, now many epochs and (for some sources) additional frequencies (24 and 43 GHz) are available
VLBA Pre-Launch Survey (VLBApls)Originally to check source compactness on the longest baselines to prepare for the VSOP Survey5-GHz snapshots for ~370 sources
Fey et al. (1996), ApJS 105, 299, etc.
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)
Horiuchi et al. (2004), ApJ 616, 110
Relative visibility distributionDashed line: 3-component model
Brightness temperature distribution
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
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
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
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 SurveyBootes Field
VLBA + GBT wide-field VLBI with in-beam phase-referencing at 1.4 GHz
Garrett et al. (2005), ApJ 619, 105
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 inverse-Compton 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/