Compact Source catalogues

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.

The whole PCCS can be downloaded here [1].

Detection procedure

The Mexican Hat Wavelet 2 (MHW2; Gonzalez-Nuevo et al., 2006) 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 (MTXF; Herranz et al., 2009) and the Bayesian PowellSnake (Carvalho et al. 2009), 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. For each channel, a parent sample is created containing all the sources detected with a signal-to-noise greater than 4. These 9 lists, that contain the positions, native flux densities and errors, are used as inputs in the validation step. The results of the validation process are finally used to decided the final thresholding or removal of spurious sources, defining the final PCCS.

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.

Photometry

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 [reference to the Color Correction description].

Validation process

The parent sample lists constitute only the starting point for the validation process. The results of the validation (both internal and external depending on the kind of information used) determine the final selection of sources to be included in the PCCS.

Internal validation

The PCCS is validated through an internal Monte-Carlo quality assessment (MCQA) process that uses large numbers of source injection and detection loops on realistic simulated maps to constrain detection characteristics.

Source injection consists on the introduction of fake sources in the real maps. The positions of the injected sources are chosen in order to avoid existing detections in the real maps and previously injected sources. By analysing the recovered injected sources we can determine several statistical properties of the detection process: completeness, photometry accuracy, variation of sensitivity along the sky, etc.

On the other hand, in order to study the number of false detections, also known as Purity, we need to perform realistic all-sky simulations. [Description or reference of the simulation used].

External validation

At the lowest frequencies of Planck, it is possible to validate PCCS source identifications using external data sets, particularly large-area radio surveys. This kind of validation allow us also to characterize the detection process, i.e. to determine the Completeness, Purity and positional accuracy. 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.

This statistical characterization of the detection process is used to choose the best signal-to-noise threshold for each channel in order to maximize the Completeness without penalizing the Purity.

Cautionary notes

• Statistical Character : warnings about the statistical analysis of this catalogue... Completeness levels for statistical analysis as number counts, usage of sensitivity maps at fixed completeness level, etc

• 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 (refs?). 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 ~2.5 sky coverages, the current version of the PCCS provides only a single average flux density estimate over all Planck data samples that were included in the all sky maps and does not contain any measure of the variability of the sources.

• Contamination from CO: At infrared/submillimetre frequencies (100GHz and above), the Planck bandpasses straddle energetically significant CO lines (refs?). The effect is the most significant at 100GHz, 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 continuum flux density of the source. See Planck HFI Core Team (2011b) for details.

• Photometry: Each source has multiple measures of photometry APERFLUX, GAUFLUX, PSFFLUX and DETFLUX (or native) 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, GAUSFLUX might be most appropriate although GAUSFLUX appears to overestimate the flux of 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.

• 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 IRAS 100μm surface brightness in MJy sr1 for each of the sources, which is commonly used as a proxy for cirrus, is available through a search of the ERCSC with IRSA. 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. Alternately, the value of CIRRUS in the catalogue can be utilised to flag sources which might be clustered together and thereby associated with ISM structure.

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. A union catalogue is constructed from the detections by all three pipelines. A Galactic dust mask (leaving 85% of the sky), a nearby Galaxy mask and a point source mask are applied a posteriori to remove detections in regions of the sky 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 [2].

The ERCSC is also accessible via the NASA/IPAC Infrared Science Archive [3].

The Bibliography

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1. References

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