The Next Generation Virgo Cluster Survey-Infrared (NGVS-IR). I. A New Near-Ultraviolet, Optical, and Near-Infrared Globular Cluster Selection Tool

摘要

The NGVS-IR project (Next Generation Virgo Cluster Survey-Infrared) is a contiguous, near-infrared imaging survey of the Virgo cluster of galaxies. It complements the optical wide-field survey of Virgo (NGVS). In its current state, NGVS-IR consists of K_s -band imaging of 4 deg^2 centered on M87 and J- and K_s -band imaging of ~16 deg^2 covering the region between M49 and M87. We present observations of the central 4 deg^2 centered on Virgo's core region. The data were acquired with WIRCam on the Canada-France-Hawaii Telescope, and the total integration time was 41 hr distributed over 34 contiguous tiles. A survey-specific strategy was designed to account for extended galaxies while still measuring accurate sky brightness within the survey area. The average 5σ limiting magnitude is K_s = 24.4 AB mag, and the 50% completeness limit is K_s = 23.75 AB mag for point-source detections, when using only images with better than 0."7 seeing (median seeing 0."54). Star clusters are marginally resolved in these image stacks, and Virgo galaxies with µ_K_s ≃ 24.4 AB mag arcsec^(–2) are detected. Combining the K_s data with optical and ultraviolet data, we build the uiK_s color-color diagram, which allows a very clean color-based selection of globular clusters in Virgo. This diagnostic plot will provide reliable globular cluster candidates for spectroscopic follow-up campaigns, needed to continue the exploration of Virgo's photometric and kinematic substructures, and will help the design of future searches for globular clusters in extragalactic systems. We show that the new uiK_s diagram displays significantly clearer substructure in the distribution of stars, globular clusters, and galaxies than the gzK_s diagram—the NGVS + NGVS-IR equivalent of the BzK diagram that is widely used in cosmological surveys. Equipped with this powerful new tool, future NGVS-IR investigations based on the uiK_s diagram will address the mapping and analysis of extended structures and compact stellar systems in and around Virgo galaxies.