Is Crater II Disrupting?

Abstract Crater II (CraII) is a very intriguing low surface brightness and extended galaxy in the vicinity of our Galaxy. Motivated by its huge area and the number of RR Lyrae stars (RRLSs) detected near to its core, we performed a follow-up campaign to get deep multiband ( g , i ) and multiepoch DECam observations in the outskirts of CraII, covering up to 8 r h in the northeast direction and up to 13 r h in the southwest direction (spanning almost 10° 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 44 are CraII members (37 RRLSs and 7 anomalous Cepheids). The RRLSs show a strong gradient in distance of 3.7 kpc deg −1 , which follows the relation <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>d</mml:mi> <mml:mo stretchy="false">(</mml:mo> <mml:mi mathvariant="normal">kpc</mml:mi> <mml:mo stretchy="false">)</mml:mo> <mml:mo>=</mml:mo> <mml:mo stretchy="false">(</mml:mo> <mml:mo>−</mml:mo> <mml:mn>3.70</mml:mn> <mml:mspace width="0.25em"/> <mml:mo>±</mml:mo> <mml:mspace width="0.25em"/> <mml:mn>0.21</mml:mn> <mml:mo stretchy="false">)</mml:mo> <mml:mspace width="0.25em"/> <mml:msup> <mml:mrow> <mml:mi>ξ</mml:mi> </mml:mrow> <mml:mrow> <mml:mo accent="true">′</mml:mo> </mml:mrow> </mml:msup> <mml:mo>+</mml:mo> <mml:mo stretchy="false">(</mml:mo> <mml:mn>116.2</mml:mn> <mml:mspace width="0.25em"/> <mml:mo>±</mml:mo> <mml:mspace width="0.25em"/> <mml:mn>0.32</mml:mn> <mml:mo stretchy="false">)</mml:mo> </mml:math> (where <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msup> <mml:mrow> <mml:mi>ξ</mml:mi> </mml:mrow> <mml:mrow> <mml:mo accent="true">′</mml:mo> </mml:mrow> </mml:msup> </mml:math> 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 (northeast) may be less steep than it is in the inner 5°. The leading tail also displays a significant overdensity of RRLSs located ∼3 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mover accent="true"> <mml:mrow> <mml:mi>.</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>°</mml:mi> </mml:mrow> </mml:mover> </mml:math> 25 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.