Is Crater II disrupting?

TL;DR

This study uses deep, multi-band, time-series DECam data to trace the tidal disruption of Crater II by mapping RR Lyrae and Anomalous Cepheid populations in the galaxy's outskirts. By identifying 46 CraII member variables (37 RRLS and 7 AC) and analyzing their spatial distribution and distances, the authors quantify a clear distance gradient along two tidal tails, with $d(kpc)=(-3.70\pm0.21)\, \xi' + (116.2\pm0.32)$ and a distance-modulus gradient $\mu_0(\rm mag)=(-0.072\pm0.006)\,\xi' +(20.296\pm0.018)$. The tails extend to several degrees and radii, with a significant overdensity at $\xi'\approx3.25^\circ$ and a distant member at ~11 $r_h$, indicating substantial mass loss. Contamination from field RRLS is modest, while AC membership remains more uncertain due to potential foreground contamination. Overall, the results corroborate CraII as a disrupting system with extended tidal debris and demonstrate the power of RR Lyrae tracers for mapping distant galactic halos; future LSST data will further enhance such studies.

Abstract

Crater~II (CraII) is a very intriguing, low-surface brightness and extended galaxy in the vicinity of our Milky Way. Motivated by its huge area and the number of RR Lyrae stars (RRLS) detected near to its core, we performed a follow-up campaign to get deep multi-band ($g,i$) and multi-epoch DECam observations in the outskirts of CraII, covering up to 8$r_h$ in the North-East direction and up to 13$r_h$ in the South-West direction (spanning almost 10 degr in the sky across the galaxy). We confirm the existence of tails coming out from CraII. In our survey, we detected a total of 97 periodic variable stars, of which 46 are CraII members [37 RRLS and 7 anomalous Cepheids (AC)]. The RRLS show a strong gradient in distance of 3.7~kpc/deg, which follows the relation: $d (\rm {kpc}) = (-3.70\pm 0.21)\, ξ' + (116.2 \pm 0.32)$ (where $ξ'$ is the planar coordinate rotated to fit the CraII tails). We also found an indication that the gradient in distance in the distant part of the near-side tail (North-East) may be less steep than it is in the inner $5°$. The leading tail also displays a significant over-density of RRLS located $\sim 3.25$ deg from the center of CraII. Despite covering a smaller area than previous works, our deeper photometry has allowed us to unveil more variable stars in the region and better define the tidal tails of CraII.

Figures (10)

• Figure 1: Spatial coverage of our DECam observations of CraII in equatorial coordinates. The gray background, which shows the footprint of our survey, is a density map made with all stars in our catalog. The central region, in blue, shows the single field observed in vivas20a. Black ellipses indicate $1\, r_h$, $4\, r_h$, $8\, r_h$, and $12\, r_h$. The orange star marks the center of CraII.
• Figure 2: Examples of the $g$, $i$ lightcurves of two of the faintest RRLS identified in this work. The solid lines are the best fitted template from the library of baeza25. The full figure set of lightcurves is available in the online version of the journal.
• Figure 3: Distribution of extinction-corrected $g$ magnitudes in the central field of CraII [from][top histogram]vivas20a, and in the outskirts (bottom histogram).
• Figure 4: Location of the RRLS and AC in the CMD of CraII. The left panel shows with colored symbols the variable stars in the central field observed by vivas20a, and the right panel contains the stars identified in the external fields studied in this work. The background is shown as a reference of the CraII CMD and it is the same in both panels. It contains all stars within $1\degr$ from the center of CraII, as measured by walker19.
• Figure 5: Equatorial coordinates of the different type of RRLS and AC stars in CraII vivas20a and its outskirts (this work). Symbols are the same as in the CMDs of Figure \ref{['fig:cmd']}. The gray background is a density map made with all stars in our catalog of outskirts fields. Stars over the blue background are those reported in vivas20a for the central field.