Targeting cluster galaxies for the 4MOST CHANCES Low-z sub-survey with photometric redshifts

TL;DR

This paper presents a robust target-selection strategy for the CHANCES Low-z sub-survey, designed to obtain spectra for galaxies in and around ~50 clusters and 2 superclusters at $z<0.07$ using 4MOST. The authors synthesize public photometric redshifts from the Legacy Surveys with their own MDN-based CBPF photo-z estimations, plus S-PLUS/T80S data, to build three sub-samples (bright, faint, and faint-supplementary) aimed at maximizing completeness while controlling contamination. They show quantitative gains in redshift accuracy (e.g., $δz=|z_{spec}-z_{phot}|$, $σ_{ m NMAD}$) and deliver a target catalog for ~320,000 galaxies, with careful treatment of the red sequence and Fornax case. The photometric catalogs are validated by environmental analyses around clusters (e.g., A3376) using DisPerSE, KNN densities, and clustering to demonstrate their utility for tracing large-scale structure and local environments, establishing a solid photometric backbone for CHANCES spectroscopic follow-up and future wide-area surveys.

Abstract

The evolution of galaxies is shaped by both internal processes and their external environments. Galaxy clusters and their surroundings provide ideal laboratories to study these effects, particularly mechanisms such as quenching and morphological transformation. The Chilean Cluster galaxy Evolution Survey (CHANCES) Low-z sub-survey is part of the CHileAN Cluster galaxy Evolution Survey, a 4MOST community survey designed to uncover the relationship between the formation and evolution of galaxies and hierarchical structure formation as it happens, through deep and wide multi-object spectroscopy. We present the target selection strategy followed to select galaxy cluster candidate members for the CHANCES low-z sub-survey, in and around 50 clusters and two superclusters at z<0.07, out to (5XR200) and down to mr= 20.4. Combining public photometric redshift estimates from the DESI Legacy Imaging Survey and T80S/S-PLUS iDR5, with custom photometric redshifts, we identify likely galaxy cluster candidate members whose photometric redshifts are consistent with being at the known redshift of the cluster and measure the average deviations of their photometric redshifts with respect to the spectroscopic redshift measurements σNMAD. We have successfully compiled our CHANCES-low-redshift catalogues, split into three different sub-surveys: low-z bright (mr<18.5), low-z faint (18.5<=mr<20.4) and low-z faint supplementary, by selecting>= 500,000 galaxy cluster candidate members and including confirmed spectroscopic galaxy cluster members, from which we expect to obtain 4MOST low-resolution (R~6500) spectra for ~320,000 galaxies. The CHANCES Low-z target catalogues form a statistically robust sample for spectroscopic follow-up, allowing studies of galaxy evolution and environmental effects in nearby cluster and supercluster environments.

Figures (21)

• Figure 1: Photometric data coverage for the CHANCES Low-z sub-survey. Black circles show the CHANCES-low-z cluster location with their corresponding $\rm 5 \times R_{200}$ regions. The Horologium-Reticulum (HRS) and Shapley (SSC) superclusters regions are shown in magenta dashed boxes. The S-PLUS -DR4 and -iDR5 footprint coverage around our clusters are shown in green, while the additional CHANCES-T80S campaigns are shown in purple. The clusters with available LS-DR9 photometric redshifts are showed with dotted circles, while 49 of 50 clusters (except from Antlia, yellow dash-dotted circle) from our sample are covered by LS-DR10 and LS-DR10-CBPF photometric redshifts. The grey shaded region bounded by black solid lines shows Galactic latitudes $|b|\leqslant20^{\circ}$, while the region below the red curve corresponds to the eROSITA-DE survey Merloni24.
• Figure 2: Comparison between photometric (z$_{phot}$) and spectroscopic (z$_{spec}$) redshifts for each of the photo-z sets used in this work, from left to rigth T80S/S-PLUS, LS-DR9, LS-DR10 and LS-DR10-CBPF. Coloured symbols correspond to the bright ($\rm m_{r}<18.5$) galaxy cluster candidate members belonging to 16 clusters with good spatial and z$_{\mathrm{spec}}$ coverage for all surveys.
• Figure 3: $\sigma_{\rm NMAD}$ curves used to select the most likely galaxy cluster candidate members for the low-z-CHANCES target catalogues for each set of photometric redshifts: T80S/S-PLUS (green), LS-DR9 (light-blue), LS-DR10 (blue) and LS-DR10-CBPF (yellow). The curves were obtained through a polynomial fit on the $\sigma_{\mathrm{NMAD}}$ histogram with equally $\rm log(m_{r})$ sized bins for each of the $\rm z_{phot}$ training sample (see Section \ref{['sec:CMTS']} for details), dashed lines indicate the $\rm m_ {r}$-limit at which a constant value was adopted for selecting the cluster galaxy candidate members. These curves evidence the differences in $\rm m_ {r}$ distributions of the different training samples used to determine $\rm z_{phot}$.
• Figure 4: Photometric redshift as a function of $r$-band magnitude for each set of photometric redshifts, from left to right: T80S/S-PLUS, LS-DR9, LS-DR10 and LS-DR10-CBPF. We include the corresponding parent sample (grey circles), the A0085 galaxy cluster candidate members selected from our photometric method (blue), the spectroscopically confirmed members (yellow circles), and the confirmed spectroscopic cluster members and spectroscopic objects within the cluster redshift range (yellow crosses) that were not selected by our method. The solid black line indicates the cluster redshift obtained from the mean spectroscopic redshift distribution, while the dashed lines indicate the cluster spectroscopic redshift lower and upper limits. The bright/faint boundary at $\rm m_{r}=18.5$ is indicated by a vertical dotted-line. Adopting different $\rm z_{phot}$ with their corresponding $\rm \sigma_ {NMAD}$ curves lead to a different cluster galaxy candidate members selection, specially at fainter magnitudes ($\rm m_ {r}\geqslant18.5$), where S-PLUS selects more candidates as compared with LS-$\rm z_{phot}$.
• Figure 5: Photometric redshift versus spectroscopic redshift for each set of photometric redshifts, from left to right: T80S/S-PLUS, LS-DR9, LS-DR10 and LS-DR10-CBPF. We include the corresponding parent sample (grey circles), the A0085 galaxy cluster candidate members selected from our photometric method (blue), the spectroscopically confirmed members (yellow circles), and the confirmed spectroscopic cluster members and spectroscopic objects within the cluster redshift range (yellow crosses) that were not selected by our method. The solid black vertical-line indicates the cluster redshift obtained from the mean spectroscopic redshift distribution, while the dashed vertical-lines indicate the cluster lower and upper limits.