The DECam MAGIC Survey: Uncovering the Tidal Tails of the Crater II Dwarf Galaxy

TL;DR

This study uses CaHK photometry from the DECam MAGIC survey to identify and analyze the tidal tails of the Crater II dwarf galaxy, revealing its extended disruption, metallicity gradient, and implications for dark matter profiles.

Contribution

First detection of tidal tails and metallicity gradient in Crater II using CaHK photometry, providing new insights into its disruption and dark matter profile.

Findings

Identified 37 tidal tail candidates extending at least 95 kpc from Crater II.

Detected a metallicity gradient of -0.34 dex per degree in the galaxy.

Tidal tails suggest Crater II has lost over 25% of its initial stellar mass.

Abstract

Crater II (CraII), a large and low-density dwarf spheroidal galaxy, has unusual observed properties that are difficult to reproduce in cold dark matter simulations. Ongoing tidal disruption may help explain the discrepancies, as evidenced by the recent discovery of tidal tails. Here we present metallicity-sensitive narrowband photometry of the Ca II H and K lines from the Dark Energy Camera, covering $128$ deg$^2$ across the center and identified tidal tails of CraII as part of the Mapping the Ancient Galaxy in CaHK (MAGIC) survey. Our combined photometric metallicity, color-magnitude, proper motion, and parallax selections identify 162 CraII candidates. Of these, 37 candidates are located in the tidal tails which extend at least $7^\circ$ ($\sim 95$ kpc) from the center of CraII, suggesting it has lost $\gtrsim 25$% of its initial stellar mass. We confirm low contamination rates with dedicated control fields and highlight the extremely low surface brightness stellar features that can be uncovered with CaHK data, as faint as $\sim 36$ mag arcsec$^{-2}$. We also make the first detection of a metallicity gradient ($-0.34\pm0.17~{\rm dex}~{\rm deg}^{-1}$) in the center of the galaxy and infer a stream width of $w\sim 0.8^\circ$, roughly 50% larger than the CraII half-light radius. The detection of candidates in the most distant CraII pointings from its center implies that the tidal tails extend beyond our footprint. We compare the CraII stream to $N$-body models with "cored" and "cuspy" dark matter halo progenitors, determining that CraII's density profile is still ambiguous and warrants further modeling.