Sándor Frey Satellite Geodetic Observatory, Institute of Geodesy, Cartography and Remote Sensing (FÖMI) VLBI (Imaging) Surveys 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. 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, S 5GHz > 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, 114 Owsianik & 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, 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 sorces VLBI images at 1.6 and 5 GHz CJ2: 193 sources az 5 GHz CJF: flat-spectrum sample ~300 sources homogeneous 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 GHz multiple 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 , …) 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 × K solid 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: , a supermassive binary black hole system Rodriguez et al. (2006), ApJ 646, 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 snapshots Primary 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 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 VLBA Pre-Launch Survey (VLBApls) Originally to check source compactness on the longest baselines to prepare for the VSOP Survey 5-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 distribution Dashed 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, 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 Survey Bootes 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 VLBA 2cm MOJAVE USNO RRFID rorf.usno.navy.mil/rrfid.shtml VLBA Calibrators VSOP Pre-launch (VLBApls) VSOP Survey VIPS