Difference between revisions of "Compact Source catalogues"
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[[Category:HFI/LFI joint data processing|001]]
[[Category:HFI/LFI joint data processing|001]]
Revision as of 13:56, 18 July 2014
Planck Catalogue of Compact Sources
The Planck Catalogue of Compact Sources (PCCS) is a sample of reliable sources, both Galactic and extragalactic, extracted directly from the Planck nominal maps. The first public version of the PCCS is derived from the data acquired by Planck between August 13 2009 and November 26 2010. The PCCS consists of nine lists of sources, extracted independently from each of Planck's nine frequency channels. It is fully described in planck2013-p05 Planck-2013-XXVIII .
The whole PCCS can be downloaded from the Planck Legacy Archive.
The Mexican Hat Wavelet 2 #nuevo2006 (MHW2) is the base algorithm used to produce the single channel catalogues of the PCCS. Although each DPC has is own implementation of this algorithm (IFCAMEX and HFI-MHW), the results are compatible at least at the statistical uncertainty level. Additional algorithms are also implemented, like the multi-frequency Matrix Multi-filters #herranz2009 (MTXF) and the Bayesian PowellSnake #carvalho2009, but for the current version of the PCCS they are used just for the validation of the results obtained by the MHW2.
The full-sky maps are divided into a sufficient number of overlapping flat patches in such a way that 100% of the sky is covered. Each patch is then filtered by the MHW2 with a scale that is optimised to provide the maximum signal-to-noise ratio in the filtered maps. A sub-catalogue of objects is produced for each patch and then, at the end of the process, all the sub-catalogues are merged together, removing repetitions.
The driving goal of the ERCSC was reliability greater than 90%. In order to increase completeness and explore possibly interesting new sources at fainter flux density levels, however, the initial overall reliability goal of the PCCS was reduced to 80%. The S/N thresholds applied to each frequency channel have been determined, as far as possible, to meet this goal. The reliability of the catalogues has been assessed using the internal and external validation described below.
At 30, 44, and 70 GHz, the reliability goal alone would permit S/N thresholds below 4. A secondary goal of minimizing the upward bias on flux densities led to the imposition of an S/N threshold of 4.
At higher frequencies, where the confusion caused by the Galactic emission starts to become an issue, the sky has been divided into two zones, one Galactic (52% of the sky) and one extragalactic (48% of the sky). At 100, 143, and 217 GHz, the S/N threshold needed to achieve the target reliability is determined in the extragalactic zone, but applied uniformly on sky. At 353, 545, and 857 GHz, the need to control confusion from Galactic cirrus emission led to the adoption of different S/N thresholds in the two zones. The extragalactic zone has a lower threshold than the Galactic zone. The S/N thresholds are given in Table 1.
Bandﬁlling is the process by which ﬂux density estimates at speciﬁc bands are generated based on source positions deﬁned in another band. For the current PCCS release we compute the ﬂux density at 217, 353, and 545 GHz at the positions of each source detected at 857 GHz, using aperture photometry. Bandﬁlling is not attempted at other frequencies due to the variation in spatial resolution across the bands, which makes multifrequency associations challenging, especially in crowded regions such as the Galactic Plane.
In addition of the native flux density estimation provided by the detection algorithm, three additional measurements are obtained for each of the source in the parent samples. These additional flux density estimations are based on aperture photometry, PSF fitting and Gaussian fitting (see #planck2013-p05 for a detailed description of these additional photometries). The native flux density estimation is the only one that is obtained directly from the filtered maps while for the others the flux density estimates has a local background subtracted. The flux density estimations have not been colour corrected. Colour corrections are available in Table 15 of LFI Appendix and HFI spectral response pages.
The PCCS, its sources and the four different estimates of the flux density, have undergone an extensive internal and external validation process to ensure the quality of the catalogues. The validation of the non-thermal radio sources can be done with a large number of existing catalogues, whereas the validation of thermal sources is mostly done with simulations. These two approaches will be discussed below. Detections identified with known sources have been appropriately flagged in the catalogues.
The catalogues for the HFI channels have primarily been validated through an internal Monte-Carlo quality assessment process that uses large numbers of source injection and detection loops to characterize their properties. For each channel, we calculate statistical quantities describing the quality of detection, photometry and astrometry of the detection code. The detection is described by the completeness and reliability of the catalogue: completeness is a function of intrinsic flux, the selection threshold applied to detection (S/N) and location, while reliability is a function only of the detection S/N. The quality of photometry and astrometry is assessed through direct comparison of detected position and flux density parameters with the known inputs of matched sources. An input source is considered to be detected if a detection is made within one beam FWHM of the injected position.
At the three lowest frequencies of Planck, it is possible to validate the PCCS source identifications, completeness, reliability, positional accuracy and flux density accuracy using external data sets, particularly large-area radio surveys. Moreover, the external validation offers the opportunity for an absolute validation of the different photometries, directly related with the calibration and the knowledge of the beams.
At higher frequencies, surveys as the South-Pole Telescope (SPT), the Atacama Cosmology Telescope (ACT) and H-ATLAS or HERMES form Herschel will also be very important, although only for limited regions of the sky. In particular, the Herschel synergy is crucial to study the possible contamination of the catalogues caused by the Galactic cirrus at high frequencies.
We list here some cautionary notes for users of the PCCS.
- Variability: At radio frequencies, many of the extragalactic sources are highly variable. A small fraction of them vary even on time scales of a few hours based on the brightness of the same source as it passes through the different Planck horns #planck2013-p02,#planck2013-p03. Follow-up observations of these sources might show significant differences in flux density compared to the values in the data products. Although the maps used for the PCCS are based on 2.6 sky coverages, the PCCS provides only a single average flux density estimate over all Planck data samples that were included in the maps and does not contain any measure of the variability of the sources from survey to survey.
- Contamination from CO: At infrared/submillimetre frequencies (100 GHz and above), the Planck bandpasses straddle energetically significant CO lines (see #planck2013-p03a). The effect is the most significant at 100 GHz, where the line might contribute more than 50% of the measured flux density. Follow-up observations of these sources, especially those associated with Galactic star-forming regions, at a similar frequency but different bandpass, should correct for the potential contribution of line emission to the measured continuum flux density of the source.
- Photometry: Each source has multiple estimates of flux density, DETFLUX, APERFLUX, GAUFLUX and PSFFLUX, as defined above. The appropriate photometry to be used depends on the nature of the source. For sources which are unresolved at the spatial resolution of Planck, APERFLUX and DETFLUX are most appropriate. Even in this regime, PSF fits of faint sources fail and consequently these have a PSFFLUX value of NaN (‘Not a Number’). For bright resolved sources, GAUFLUX might be most appropriate although GAUFLUX appears to overestimate the flux density of the sources close to the Galactic plane due to an inability to fit for the contribution of the Galactic background at the spatial resolution of the data. For the 353–857 GHz channels, the complex native of the diffuse emission and the relative undersampling of the beam produces a bias in DETFLUX, so we recommend that APERFLUX is used instead.
- Colour correction: The flux density estimates have not been colour corrected. Colour corrections are described in #planck2013-p02 and #planck2013-p03.
- Cirrus/ISM: A significant fraction of the sources detected in the upper HFI bands could be associated with Galactic interstellar medium features or cirrus. The 857 GHz brightness proxy described in Sect. 3.4), can be used as indicator of cirrus contamination. Alternately, the value of CIRRUS N in the catalogue can be used to flag sources which might be clustered together and thereby associated with ISM structure. Candidate ISM features can also be selected by choosing objects with EXTENDED = 1 although nearby Galactic and extragalactic sources which are extended at Planck spatial resolution will meet this criterion too.
Planck Sunyaev-Zeldovich catalogue
The Planck SZ catalogue is a nearly full-sky list of SZ detections obtained from the Planck data. It is fully described in #planck2013-p05a. The catalogue is derived from the HFI frequency channel maps after masking and filling the bright point sources (SNR >= 10) from the PCCS catalogues in those channels. Three detection pipelines were used to construct the catalogue, two implementations of the matched multi-filter (MMF) algorithm and PowellSnakes (PwS), a Bayesian algorithm. All three pipelines use a circularly symmetric pressure profile, the non-standard universal profile from #arnaud2010, in the detection.
- MMF1 and MMF3 are full-sky implementations of the MMF algorithm. The matched filter optimizes the cluster detection using a linear combination of maps, which requires an estimate of the statistics of the contamination. It uses spatial filtering to suppress both foregrounds and noise, making use of the prior knowledge of the cluster pressure profile and thermal SZ spectrum.
- PwS differs from the MMF methods. It is a fast Bayesian multi-frequency detection algorithm designed to identify and characterize compact objects in a diffuse background. The detection process is based on a statistical model comparison test. Detections may be accepted or rejected based on a generalized likelihood ratio test or in full Bayesian mode. These two modes allow quantities measured by PwS to be consistently compared with those of the MMF algorithms.
A union catalogue is constructed from the detections by all three pipelines. A mask to remove Galactic dust, nearby galaxies and point sources (leaving 83.7% of the sky) is applied a posteriori to avoid detections in areas where foregrounds are likely to cause spurious detections.
Early Release Compact Source Catalogue
The ERCSC is a list of high reliability (>90%) sources, both Galactic and extragalactic, derived from the data acquired by Planck between August 13 2009 and June 6 2010. The ERCSC consists of:
- nine lists of sources, extracted independently from each of Planck's nine frequency channels
- two lists extracted using multi-channel criteria: the Early Cold Cores catalogue (ECC), consisting of Galactic dense and cold cores, selected mainly on the basis of their temperature ; and the Early Sunyaev-Zeldovich catalogue (ESZ), consisting of galaxy clusters selected by the spectral signature of the Sunyaev-Zeldovich effect.
The whole ERCSC can be downloaded here .
The ERCSC is also accessible via the NASA/IPAC Infrared Science Archive .
Data Processing Center
(Planck) High Frequency Instrument
Early Release Compact Source Catalog