The follow-up of objects discovered by Planck mission


Introduction

When at the end of working day of January 11, 2011 we heard a breaking news of Planck compact source catalogues early release, we rushed to see what kind of new high frequency sources have been discovered.

This low-resolution catalogue is complete at 0.5 and 0.2 Jy level at 30 GHz and 217 GHz respectively. A fundamental questions arises: what is the nature of emission of these sources? How compact these sources? What are the properties of the population of high-frequency sources as a whole? We examined three catalogues: 30 GHz, 143 GHz, and 217 GHz, and checked every object against the cumulative database of more than 6000 compact extragalactic sources detected in the absolute astronomy mode with VLBI.

Among 682 objects in both 30 GHz, 147 GHz, and 217 GHz Planck catalogues with |b| >3°, δ > -30°, 612 sources have been observed and detected with VLBI. We scrutinized the 30 GHz and 147 GHz lists further and found that 15 objects are identified with planetary nebulae, 4 with ultra-compact HII regions and 7 with supernova remnants. The nature of 54 remaining sources (6%) is mysterious. We would like to solve this mystery.

Scientific goal

The target objects are either very bright steep spectrum sources, f.e. 3C161, that were not included in VLBI surveys because of their spectrum, or objects with raising spectra that were not included in VLBI surveys because they are too weak at low frequencies. Objects of the first class are interesting because a natural question arises: what is the origin of the emission at high frequencies? Objects of the second class dubbed by Murphy et al. (2010) as ultra inverted spectrum radio sources are also interesting and they belong to rather a narrow class. The first goal of our project is to understand the nature of 54 objects missing in VLBI surveys by determining their images at 2.2 and 8.4 GHz and deriving their position with sub-mas accuracy. Do they have emission at mas-scales? Are they AGNs? What are their associations in the optical range?

The beauty of the Planck compact source catalogues is that they are complete flux limited catalogues without any pre-selection bias. This means that statistics derived from analysis of this sample can be generalized to the entire population of a given class of objects. Proposed VLBI observations will help to match the completeness of the Planck catalogue with completeness of VLBI image database. We will be able to say that every Planck object at the 3/4 of the celestial sphere is either a galactic object, such as a planetary nebulae, compact HII region or the SNR, or it is a radio loud AGN. The second goal of our project is to perform population analysis of Planck/VLBI surveys using prior VLBI observations and results of proposed observations. We are going to compare the distribution of brightness temperatures, spectral indices of mas-scale components, compactness of the population of high-frequency AGNs detected with Planck with the general population of AGNs. The key scientific question is whether the population of of high frequency sources has distinguished features with respect to the general population of AGNs.

The target objects are expected to be useful as high frequency phase calibrators. The number of calibrators at frequencies above 22 GHz is limited and every object is on premium. We will determine coordinates of detected objects with accuracies better than 0.5 mas — a six order of magnitude improvement with respect to the position accuracy of the Planck 30 GHz catalogue.

People

Team members (in alphabetic order):

Sang-Sung Lee KASI, South Corea
Leonid Petrov Astrogeo Center, USA
Alexandr Pushkarev Crimean Astrophysical Observatgory, Ukraine
Alexandr Volvach Crimean Astrophysical Observatory, Ukraine

Source list

The list of 54 objects that will be observed in a single-dish mode at the RT-22 (also known as CRIMEA or SIMEIZ) radiotelescope at 36 GHz.

Observations

First, target sources will be observed in a singe-dish mode with the 22 meter radiotelescope RT-22 (also known as CRIMEA and Simeiz) at 36 GHz. The goal of these observations is 1) to get single-dish integral flux density at high frequency; 2) to improve position of Planck source to arcsec level. Since the position error of 30 GHz Planck catalogue reach 9', VLBI observations will be risky without improvement in source coordinates; 3) to learn whether sources has structure at scales tens arcseconds. The detection limit of RT-22 observations will be 200 mJy — deeper than Planck.

Those sources that will be detected with RT-22 observations will be observed with VLBI in order 1) to get their maps at 2.2 and 8.6 GHz, 2) learn spectral indexes of compact components, and 3) determine their positions with sub-mas accuracies. Since the target objects are uniformly distributed over right ascensions, we request a 24 hour observing session at X/S at the full EVN network. Ef-Yb-Mc-Nt-On-Hh-Sv-Zc-Ba-Ur-Sh-Wz. Each source will be observed in 6 scans of 3 minutes each. Observations will be made with 1Gbs bit rate. We are going to use the wide frequency setup over 720 MHz at the X-band and over 140 MHz at the S-band that is routinely used for IVS geodetic observations. We request the data be correlated at Bonn DiFX correlator for two reasons: 1) we need an extra-wide field that covers the entire beam; 2) we need derive accurate absolute source positions. We received consent from Bonn to correlate this experiment. Our prior EVN observations in a similar mode processed with the DiFX at Bonn (EP066) were successful and resulted in a publication.

For 18 minutes of total on-source integration time, we expect to reach the thermal noise 0.2 mJy/beam and to detect all sources brighter 3 mJy. Non-detections, if there will be at least one, will be also an interesting result. This will mean an identification of a peculiar object(s). Our astrometric analysis of 112 IVS experiments under EURO program allows us to predict that the position accuracy of detected sources will be in a range of 0.2&ndahs;0.5 mas, depending on source flux density. We are well aware of problems of imaging sources with declinations [-30°, -10°] and understand the image quality will be compromised, but the requirement of completeness overweights an expected degradation of the image quality.

Current status

Preliminary results

Data

References

  1. Ade P.A.R. et al., Planck Early Results: The Early Release Compact Source Catalog, 2011, A&A, 536, 7

  2. Ade P.A.R. et al., Planck Early Results: Statistical properties of extragalactic radio sources in the Planck Early Release Compact Source Catalogue, 2011, A&A, 536, 13

  3. Murphy T. et al., The Australia Telescope 20 GHz Survey: the source catalogue 2010, MNRAS, 420, 2403

  4. Petrov, L., Kovalev, Y.Y., Fomalont, E., Gordon, D., The sixth VLBA Calibrator Survey — VCS6, 2008, AJ, 136, 580

  5. Petrov, L., Phillips, C., Bertarini, A., Murphy T., & Sadler E. M. The LBA Calibrator Survey of southern compact extragalactic sources — LCS1, 2011a, MNRAS, 414(3), 2528

  6. Petrov L., The EVN Galactic Plane Survey — EGaPS, 2012, MNRAS, 419, 1097

  7. Pushkarev, A.; Kovalev, Y., Probing parsec scale jets in AGN with geodetic VLBI, in in Proc. of the 9th European Probing parsec scale jets in AGN with geodetic VLBI Network Symp. 2008, 86

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This web page was prepared by Leonid Petrov ()
Last update: 2012.11.22_14:21:15