Northern Polar Cap VLBA Survey

Text of the original proposal.


The focus of this proposal is to make a statistical study of the compact emission of all sources down to a relatively low level of flux density. All previous large VLBI surveys selectively observed flat spectrum sources that are nearly all core-dominated. We expect to image an unbiased sample of compact steep spectrum sources (CSS) and extended FRI and FRII sources with radio core.

In order to achieve the proper sample for study, we request three 24 hour sessions for observing with the VLBA at S/X bands of a complete sample of 500 sources in the zone delta > +75° with NVSS flux density > 200 mJy at 1.4 GHz regardless their spectral indexes. The VLBA results will be supplemented by instantaneous broad-band spectra measured in 2005 at RATAN–600 in the 2–22 GHz frequency range including S and X bands. This will allow direct comparison of the parsec scale structure and compactness versus the total flux density continuum spectra. We will propose VLBA follow-up observations for identified CSS at a later date in order to produce and analyze higher dynamic range mas-scale images than the once resulting from this search experiment.

Previous Observations

From the previous 1994–2005 VLBA observations in the framework of the VLBA Calibrator survey (projects BB023, BF071, BP110, BP118, BK124) and 53 RDV observing sessions, positions of 3331 sources were determined and images of more than 2400 objects were produced (Beasley et al. 2002; Fomalont et al. 2003, Petrov et al. 2005a, Petrov et al. 2005b, Kovalev et al, in preparation). Among them, a subset of apprxoimately 2000 objects with dec > -30° forms a homogeneous, reasonably complete, >95%, sample of flat-spectrum sources with correlated flux density greater than 200 mJy at 8 GHz.

Completeness of the sample and uniformity of the data reduction permits robust statistical analysis of this population of bright compact flat spectrum sources, such as: population modeling of the observed core brightness temperature in order to estimate distributions of the intrinsic core brightness temperature, the viewing angle, and Doppler brightening; expanding the cosmological significance of core-jet angular size versus redshift distribution, comparing the compactness of radio structures with IDV properties; correlating the radio core properties with optical class; etc.

However, the selection of candidates for the VLBA Calibrator surveys and the RDV programs was focused on observing the sources with a spectrum flatter than -0.5, which comprise approximately 15% of the entire population of radio sources at centimeter wavelengths. This spectral criterion guarantees that a large proportion of sources will be detected with mas-resolution. This spectral selectiveness is common for almost all previous VLBI surveys and future large VLBA surveys (e.g., VIPS, Taylor et al. 2005, to start in 2006). There are two exceptions, the PR+CJ1 and the Bologna samples.

The PR+CJ1 sample (e.g., Person & Readhead 1988, Xu et al. 1995) includes all sources with flux density > 0.7 Jy at 5 GHz. Unfortunately, only 128 out of 200 were observed with VLBI, so the sample of observed sources is complete only at the 64% level.

Another complete sample of sources, regardless of spectral index value, is the Bologna sample of 95 objects (Giovanni et al. 2004). However this sample, first, limits the objects with redshift z < 0.1, secondly, only a fraction of these objects has been observed with VLBI. Nevertheless, even preliminary results showed that statistics based on flat-spectrum samples may not give a valid role of compact emission in galaxies and quasars since they ``found a dramatically higher fraction of two-sided sources in comparison with that of previous flux-limited VLBI survey.'' We analyzed statistics of 3989 sources observed in 21 VLBA Calibrator survey experiments, 53 VLBA RDV experiments and 3941 Mark3/Mark4 experiments for geodesy and astrometry and compared it with the complete NVSS catalog (Condon et al. 1998). According to Table 1, only 20--30% of sources were observed even among the strong objects with the total flux density > 1 Jy at 1.4 GHz.

Statistics of observed sources in geodesy and astrometry programs

S at 1.4 GHz, Jy # total in NVSS # observed # detected
> 5.0 167 50 44
> 2.0 727 198 179
> 1.0 2206 485 446

Scientific Goals

The scientific goal of the proposed campaign is to perform a statistical study of the complete sample of all 500 objects with NVSS flux density at 1.4 GHz > 200 mJy in the area of 0.21 srad around the northern celestial pole. This sample was chosen because it has been observed with the Russian transit mode radio telescope RATAN--600 in 2005, and simultaneous measurements of total flux density at 2, 5, 8, 11, and 22 GHz are available (Mingaliev et al. 2001, 2005, in preparation).

The proposed observations will identify those sources from this sample which have compact details at the level of 20 mJy or higher at X and/or S bands. We expect that between 100 and 200 sources will be detected (this proportion is really unknown); images and positions at mas level will be obtained. Precise positions will enable performing reliable optical identification and follow-up VLBA studies which are planned for detected sources with mas-scale components.

Analysis of this sample should provide insight into the following questions:

  1. What is the share of CSS in the entire population of AGN? What percentage of compact sources are missed in flat-spectrum surveys? What proportion of the detected sources are radio cores of FRI and FRII sources?
  2. Do these CSS that have an overall steeper spectrum have different morphological properties from their flatter-spectrum cousins?
  3. Do CSS counts and properties change to fainter flux density luminosity level (e.g., a higher portion of compact sources at low flux density level is suggested by the VLA MASIV survey results, Lovell et al. 2003)?
  4. What is the relationship between the total spectrum of CSS from the RATAN supporting observations and the spectrum of compact components? Are differences related to ageing?
  5. What is the probability that a source with a given spectral index and total flux density will have a given compactness?
Finally, results of this campaign will allow us to determine the true distribution of compact emission and its spectral properties for an unbiased complete sample of sources. This knowledge will allow us to assess whether conclusions drawn from VLBI surveys of flat-spectrum sources can be extended to the whole population of extragalactic objects regardless their continuum spectrum.

Proposed Observations

We request three 24 hour observing sessions. Each source will be observed in one scan for 7 minutes. Eight IF channels, four in S-band and four in X-band are proposed, with a total bandwidth of 64 MHz at 1 bit sampling, 128 Mbps. We will reach the 20 mJy detection level and even with the relatively poor u-v coverage, images will be made at both frequencies. We will determine the group delays of the detected sources and obtain better positions of the compact emission. We request the same correlator setup as in the BP118 proposal: 64 spectral channels in each IF and integration time 0.5 sec. This will give us an extra-wide search window needed for fringing sources with poorly known a priori positions. Analysis of BP118 observations showed that the a priori position errors for 20% of detected sources were greater than 1 arcsec and for 8% --- greater than 5 arcsec. The observations scheme will be identical to the recent VLBA Calibrator survey experiments. The calibration and imaging will be made by using standard packages AIPS and difmap. Positions will be determined with Calc/Solve.


The results are published in Popkov, A. V., Y. Y. Kovalev, L. Petrov, and Y. A. Kovalev. "Parsec-scale properties of steep and flat spectrum extragalactic radio sources from a VLBA survey of a complete north polar cap sample", 2021, Astronomical Journal, 161(2), 88 (20pp). doi 10.3847/1538-3881/abd18c.


Team members (in alphabetic order):

List of candidate sources

The master source table.

List of 502 targeted sources in sched key format. The same list in NVSS format.

Pool of 122 sources which are suggested as tropospheric calibrators.

Pool of 10 sources which are suggested as amplitude calibrators.

List of 27 sources scheduled as tropospheric and amplitude calibrators.

Current status

2006.01.06: The Programme Committee has approved it for dynamic scheduling, low priority.

2005.09.30: Proposal was submitted to the Programme Committee on The code assigned: BK130


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This page was prepared by Leonid Petrov ()
Last update: 2021.03.12_22:16:45