Compact Source catalogues

Planck Catalogue of Compact Sources

The Planck Catalogue of Compact Sources is a set of single frequency lists of sources, both Galactic and extragalactic, extracted from the Planck maps.

The first public version of the PCCS was derived from the nominal mission data acquired by Planck between August 13 2009 and November 26 2010, as described in Planck-2013-XXVIII. It consisted of nine lists of sources, one per channel between 30 and 857 GHz. The second public version of the catalogue (PCCS2) has been produced using the full mission data obtained between August 13 2009 and August 3 2013, as described in 2015 Ref, it consists of eighteen lists of sources, two lists per channel.

The are three main differences between the PCCS and the PCCS2:

1. The amount of data used to build the PCCS (Nominal Mission with 15.5 months) and PCCS2 (Full Mission with 48 months of data).
2. The inclusion of polarization information between 30 and 353 GHz, the seven Planck channels with polarization capabilities.
3. The division of the PCCS2 into two sets of catalogues, PCCS2 and PCCS2E, depending on our ability to validate their contents.

Both the 2013 PCCS and the 2014 PCCS2 can be downloaded from the Planck Legacy Archive.

Detection procedure

The Mexican Hat Wavelet 2 is the base algorithm used to produce the single channel catalogues of the PCCS and the PCCS2. 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) and the Bayesian PowellSnakes . Both of them have been used both in PCCS and PCCS2 for the validation of the results obtained by the MHW2 in total intensity.

In addition, two maximum likelihood methods have been used to do the anlysis in polarization. Both of them can be used to blindly dectect sources in polarization maps. However, the PCCS2 analysis has been performed in a non-blind fashion, looking at the positions of the sources detected in total intensity and providing an estimation of the polarized flux density. As in total intensity, each DPC has its own implementation of this code (IFCAPOL and PwSPOL). The IFCAPOL algorithm is based on the Filter Fusion technique and has been applied to WMAP maps . The PwSPOL algortihm is a modified version of PwS, the code used in the Early Release Compact Source catalogue Planck-Early-VII. In practice, both of them are filtering methods based on matched filters, that filter the Q and U maps before attempting to estimate the flux density at each them.

The detection of the compact sources is done locally on small flat patches to improve the efficiency of the process. The reason for this being that the filters can be optimized taking into accont the statistical properties of the background in the vicinity of the sources. In order to perform this local analysis, 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. Similarly, in polarization a flat patch centered at the position of the source detected in total intensity is obtained from the all-sky Q and U map. Then, a matched filter is computed taking into accoun the beam profile at each frequency and the power spectrum of each of the projected flat patches. In both cases the filters are normalized in such a way that they preserve the amplitude of the sources after filtering, while removing the large scale diffuse emission and the small scale noise fluctuation.

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 were determined, as far as possible, to meet this goal. The reliability of the PCCS 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 was 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.

In the PCCS2 we still have an 80% reliability goal, but a new approach has been followed. There was a demand for the possibility of producing an even higher reliability catalogue from Planck, and a new reliability flag has been added in the catalogues for this purpose.

In this version of the PCCS2 we have splitted the catalogue into two, PCCS2 and PCCS2E, based on our ability to validate each of the sources. For the lower frequencies, between 30 and 70 GHz, we still use a S/N threshold of 4, although some of the unvalidated sources are in the 4-4.5 S/N threshold regime. Moreover, as it will be explained below, we use external catalogues and a multifrequency analysis to validate the sources. For the higher frequency channels, at 100 GHz and above, there is very little external information available to validate the catalogues and the validation has been done statistically and by applying galactic masks and cirrus masks.

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 Planck-2013-XXVIII for a detailed description of these additional photometries). The native flux density estimation is the only one that is obtained directly from the projected filtered maps while for the others the flux density estimates has a local background subtracted. The flux density estimations have not been colour corrected because that would limit the usability of the catalogue. Colour corrections are available in Section 7.4 of the LFI DPC paper REF and Section ?.? of the HFI DPC paper Ref.

Validation process

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.

Internal validation

The catalogues have been validated through an internal Monte-Carlo quality assessment process that uses large numbers of source injection and detection loops to characterize their properties, both in total intensity and polarization. For each channel, we calculate statistical quantities describing the quality of detection, photometry and astrometry of the detection code. The detection in total intensity 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. In polarization, we have also made Monte-Carlo quality assesments injecting polarized sources in the maps and attempting to detect and characterize their properties. In the three lowest frequencies, the sources have been injected in the real Q and U maps, while at 100 Ghz and above, the Full Focal Plane 8 simulations have been used.

External validation

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 (NEWPS, AT20G, CRATES). 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. We have used several external catalogues to validate the data, but one additional excercise has been done. Simulatenous observations of a sample of 92 sources has been carried out in the Very Large Array, the Australia Compact Array and Planck at 30 and 44 GHz. Special Planck maps have been made covering just the observation period to avoid having more than one observation of the same source in the maps, minimizing the variability effects. As a result of this exercise, we have been able to validate our flux densities at the few percent level. Since Planck calibration use the CMB Dipole and it is independent from calibration used by the ground based, we can provide an independent flux density scale between Planck, the VLA and ATCA.

At higher frequencies, surveys as the South-Pole Telescope (SPT), the Atacama Cosmology Telescope (ACT) and H-ATLAS or HERMES form Herschel are 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.

Cautionary notes

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 Planck-2013-IIPlanck-2013-VI. 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 Planck-2013-XIII). 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.

• Bandpass corrections: For many sources in the three lowest Planck frequency channels, the bandpass correction of the Q and U flux densities is not negligible. Even though we have attempted to correct for this effect on a source by source basis and have propagated this uncertainty into the error bars on the polarized flux densities and polarization angles; there is still room for improvement. This can be seen in the residual leakage present at the position of Taurus A in the Stokes U maps. It is anticipated that there will be future updates to the LFI PCCS2 catalogues once the bandpass corrections and errors have been improved.

• Photometry: Each source has multiple estimates of flux den- sity, DETFLUX, APERFLUX, GAUFLUX and PSFFLUX, as defined above. The evaluation of APERFLUX makes the small- est number of assumptions about the data and hence is the most robust, especially in regions of high non-Gaussian back- ground emission, but it may have larger uncertainties than the other methods. For bright resolved sources, GAUFLUX is rec- ommend, with the caveat that it may not be robust for sources close to the Galactic plane due to the strong backgrounds.

• Colour correction: The flux density estimates have not been colour corrected. Colour corrections are described in Planck-2013-II and Planck-2013-VI.

• Cirrus/ISM: The upper bands of HFI, could be contaminated with sources associated with Galactic interstellar medium features (ISM) or cirrus. The values of the parameters, CIRRUS N and SKY BRIGHTNESS in the catalogues may be used as indicators of contamination. CIRRUS N may be used to flag sources that might be clustered together and thereby associated with ISM structure. In order to provide some indications of the range of values of these parameters which could indicate contamination, we compared the properties of the IRAS-identified and non-IRAS-identified sources for both the PCCS2 and the PCCS2E, as outside the galactic plane, at galactic latitudes |b| > 20◦, we can use the RIIFSCz to provide a guide as to the likely nature of sources. We cross match the PCCS2 857 GHz catalogue and the PCCS2E 857 GHz catalogue to the IRAS sources in the RIIFSCz using a 3 arcmin matching radius. Of the 4891 sources in the PCCS2 857 GHz catalogue 3094 have plausible IRAS counterparts while 1797 do not. Examination of histograms of the CIRRUS N and SKY BRIGHTNESS parame- ters in the PCCS2 show that these two classes of objects behave rather differently. The IRAS-identified sources have a peak sky brightness at about 1 MJy.sr−1. The non-IRAS-identified sources have a bimodal distribution with a slight peak at 1 MJy.sr−1 and a second peak at about 2.6 MJy.sr−1 . Both distributions have a long tail, but the non-IRAS-Identified tail is much longer. On this basis sources with SKY BRIGHTNESS > 4 MJy.sr−1 should be treated with caution. In contrast non-IRAS-identified sources with SKY BRIGHTNESS < 1.4 MJy.sr−1 are likely reliable. Examination of their sky distribution, for example, shows that many such sources lie in the IRAS coverage gaps. The CIRRUS N flag tells a rather similar story. Both IRAS-matched and IRAS non-matched sources have a peak CIRRUS N value of 2, but the non-matched sources have a far longer tail. Very few IRAS-matched sources have a value > 8 but many non- matched sources do. These should be treated with caution. The PCCS2E 857 GHz catalogue contains 10470 sources with |b| > 20◦ of which 1235 are matched to IRAS sources in the RIIFSCz and 9235 are not. As with the PCCS2 catalogue the distributions of CIRRUS N and SKY BRIGHTNESS are different, with the differences even more pronounced for these PCCS2E sources. Once again, few IRAS-matched sources have SKY BRIGHTNESS > 4 MJy.sr−1 , but the non-matched sources have brightnesses extending to >55MJy.sr−1. Similarly hardly any of the IRAS-matched sources have CIRRUS N > 8 but nearly half the unmatched sources do. The WHICH ZONE flag in the PCCS2E encodes the region in which the source sits, be it inside the filament mask (WHICH ZONE=1), the Galactic region (WHICH ZONE=2), or both (WHICH ZONE=3). Of the 9235 PCCS2E 857GHz sources that do not match an IRAS source and that lie in the region, |b| > 20◦, 1850 (20%) have WHICH ZONE=1, 2637 (29 %) have WHICH ZONE=2 and 4748 (51 %) have WHICH ZONE=3. The PCCS2E covers 30.36 % of the region, |b| > 20◦ , where 2.47 % is in the filament mask, 23.15 % in the Galactic region and 4.74 % in both. If the 9235 unmatched detections were distributed uniformly over the region, |b| > 20◦, we can predict the number of non-matched sources in each zone and compare this to the values we have. We find that there are 2.5 and 3.3 times more sources than expected in zones 1 and 3, showing that the filament mask is indeed a useful criterion for regarding sources detected within it as sus- picious. It should be noted that the EXTENDED flag could also be used to identify ISM features, but nearby Galactic and extra-galactic sources that are extended at Planck spatial resolution will also meet this criterion.

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 Planck-2013-XXIX. 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 , 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 .