Distant Galaxy Clusters

I developed methods based on solid mathematical and statistical basis to detect distant clusters around radio galaxies (Castignani et al., 2014a,b, 2019) and characterize their galaxy population (Castignani & Benoist, 2016) using photometric redshifts.

Pmem. A new method to assign cluster membership and estimate richness using photometric redshifts

Castignani & Benoist 2016, A&A, 595, 111

The Castignani & Benoist (2016) method (Pmem) estimates both cluster membership and richness using photometric redshift. It is also remarkably implemented in the Euclid pipeline and will be used by the Euclid consortium to perform cosmological studies using cluster counts as well as studies of galaxy evolution in clusters.

Fraction of true members, ROC curves (purity vs completeness), and true vs estimated richness for simulated cluster galaxies and groups (Castignani & Benoist 2016)

The Poisson Probability Method. A new method to search for clusters using photometric redshifts

Castignani, Chiaberge et al. 2014a, ApJ, 792, 113 (PPM cluster finder)

Castignani, Chiaberge et al. 2014b, ApJ, 792, 114 (PPM application to the COSMOS survey)

Castignani, Combes et al. 2019, A&A, 623, 48 (wavelet-based extension of the method: wPPM)

In Castignani et al. (2014a) I developed and tested the Poisson Probability Method (PPM), to search for distant clusters around radio galaxies, using photometric redshifts of galaxies. By applying the PPM to the COSMOS survey, in Castignani et al. (2014b) I demonstrated that distant z ∼ 1 − 2 low-luminosity radio galaxies (LLRGs) are indeed optimal tracers of distant (proto-)clusters. My strategy to search for clusters around LLRGs is unbiased with respect to colors and the star formation history of cluster galaxies and represents a valuable alternative to conventional methods to search for clusters in forthcoming surveys such as LSST and Euclid.

In Castignani et al. (2019) I improved the PPM cluster finder with a wavelet-based extension (wPPM) to evaluate more carefully the miscentering between the radio galaxy and overdensity peak. Thanks also to this improvement a new cluster candidate at z=2.62 around COSMOS-FRI 70 was found. The overdensity detected around COSMOS-FRI 70 is one of the most distant cluster candidates that are found in COSMOS. Target CO(7-6) observations combined with a multiwavelength analysis suggest that COSMOS-FRI 70 is one of the most distant star forming BCGs observed in CO. Please go to this page to know more about this source and other BCGs I observed in CO.

RGB image of the cluster candidate around the COSMOS-FRI 01 radio galaxy at z=0.9 (center), PPM plot with the associated detected overdensity, and ACS image for COSMOS-FRI 01 (center) and a lensed background companion (arclet to the right), from Castignani et al. (2014b)

Properties of the BCG candidate COSMOS-FRI70 and its associated overdensity at z=2.62 (Castignani et al. 2019). • RGB image and IRAM-30m CO(7-6) hint of emisson ; • SED of COSMOS-FRI 70; • color magnitude and color-color plots; • PPM plot and density map of the associated overdensity, with the associated wavelet peak

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