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

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.

Figures (14)

• Figure 1: Twenty-one observed DECam pointings (gray) and eleven candidate control fields (blue-gray) relative to the center of CraII. The direction of the stream track as predicted by the $N$-body model in blue (see Section \ref{['sec:$N$-body']}) is shown as a solid black line. Pointings within the dark teal outlines were observed under the best weather conditions and dithered, providing the highest quality data of the sample. The 2 fields outside the outline but directly next to the CraII core were dithered in poor conditions and the remaining fields were not dithered. The 25 dithered pointings are 1 magnitude deeper than these 7. Concentric 1, 3, 5, and 10 half-light radii Torrealba16 are drawn as dotted black circles. RGB stream candidates from Coppi24 are marked as orange circles and known spectroscopic members Ji2021Limberg25 are marked as orange Xs. CraII stream RRL from Coppi24 and Vivas25 are shown as green triangles.
• Figure 2: Comparison of 114 CraII (orange) stellar metallicities with CaHK photometry and high-resolution calcium triplet spectroscopic metallicities from the AAOmega spectrograph on the 3.9 m Anglo-Australian Telescope Ji2021. We also show 1969 previously measured metallicity validation results from Barbosa25 for spectroscopic measurements Reyes22Tolstoy23 of stars in the Sculptor (light orange) dwarf galaxy. The upper panel gives both MAGIC CaHK and spectroscopic metallicities and the bottom panel shows normalized difference histograms between estimates. The dashed brown line traces where the metallicities are equal and the gray curve illustrates a Gaussian function with mean = 0 and standard deviation = 1 for comparison. We note that this offset implies our CaHK metallicities should all shift by a small correction factor that we do not apply in this study.
• Figure 3: Color-magnitude and color-color diagrams of MAGIC data after each selection. All quality cut MAGIC data surrounding CraII (as seen in Figure \ref{['fig:CraII_pointing']}) is shown in gray. The remaining data after sequential color-magnitude (purple), membership score (light purple), and metallicity (dark purple) selections are further shown. The color-magnitude cut selects stars around the isochrone in purple from the Dartmouth Stellar Evolution Database Dotter08. Known spectroscopic members Ji2021Limberg25 are marked as orange Xs. Left: Color-magnitude diagram with DECam $g_0$, $r_0$, $i_0$ photometry. Center: Color-color diagram in $r_0-i_0$ vs $g_0-r_0$. Right: Metallicity-sensitive color-color space. The lowest metallicities are located at the top and become more metal-rich as they move down the y-axis. The dashed-dotted curves in black are lines of fixed [Fe/H] at $\log{\rm g}=2$.
• Figure 4: Candidates in the central CraII pointing before and after applying proper motion and parallax criteria from the membership score Tolstoy23. The membership score limit, ${\rm Mem}_{Gaia} = 14.2$, is determined by $3\sigma$ for a 1D normal distribution and is drawn as a dashed black line. The remaining data after quality cuts is sequentially selected by color-magnitude and membership score selections, shown in purple and light purple, respectively. Known spectroscopic members Ji2021Limberg25 are marked as orange Xs. Left: Histogram of $\log_{10}({\rm Mem}_{Gaia})$ for CraII candidates and known members. The distribution of $\log_{10}({\rm Mem}_{Gaia})$ is shown as a function of Center: proper motion in right ascension and Right: proper motion in declination.
• Figure 5: Comparison of MAGIC CaHK photometric metallicity distribution for different selection criteria across all candidates and in the isolated candidate control fields. Left: Metallicity distribution function of color-magnitude and membership score selected CraII candidates overall/in the background before (light purple/gray) and after (dark purple/gray) metallicity cuts of [Fe/H] < $-2$ (dashed line), [Fe/H] < $-1.75$ (solid line), and [Fe/H] < $-1.5$ (dotted line) compared to the CaHK metallicities of known spectroscopic members Ji2021Limberg25 in orange. Center: Metallicity across the length of the stream for all CraII candidates remaining before and after the metallicity selection. Right: Metallicity across the length of the stream for CraII candidates in the 11 candidate control fields remaining before and after the metallicity selection.