Sifting for a Stream: The Morphology of the <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"> <a:mn>300</a:mn> <a:mi>S</a:mi> </a:math> Stellar Stream

Stellar streams are sensitive laboratories for understanding the small-scale structure in our Galaxy’s gravitational field. Here, we analyze the morphology of the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow> <mml:mn>300</mml:mn> <mml:mi>S</mml:mi> </mml:mrow> </mml:math> stellar stream, which has an eccentric, retrograde orbit and thus could be an especially powerful probe of both baryonic and dark substructures within the Milky Way. Due to extensive background contamination from the Sagittarius stream (Sgr), we perform an analysis combining Dark Energy Camera Legacy Survey photometry, <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mtext mathvariant="italic">𝐺𝑎𝑖𝑎</mml:mtext> </mml:math> DR3 proper motions, and spectroscopy from the Southern Stellar Stream Spectroscopic Survey ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:msup> <mml:mtext mathvariant="italic">𝑆</mml:mtext> <mml:mn>5</mml:mn> </mml:msup> </mml:math> ). We redetermine the stream coordinate system and distance gradient, then apply two approaches to describe <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow> <mml:mn>300</mml:mn> <mml:mi>S</mml:mi> </mml:mrow> </mml:math> ’s morphology. In the first, we analyze stars from <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mtext mathvariant="italic">𝐺𝑎𝑖𝑎</mml:mtext> </mml:math> using proper motions to remove Sgr. In the second, we generate a simultaneous model of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow> <mml:mn>300</mml:mn> <mml:mi>S</mml:mi> </mml:mrow> </mml:math> and Sgr based purely on photometric information. Both approaches agree within their respective domains and describe the stream over a region spanning <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:msup> <mml:mn>33</mml:mn> <mml:mo>∘</mml:mo> </mml:msup> </mml:math> . Overall, <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow> <mml:mn>300</mml:mn> <mml:mi>S</mml:mi> </mml:mrow> </mml:math> has three well-defined density peaks and smooth variations in stream width. Furthermore, <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow> <mml:mn>300</mml:mn> <mml:mi>S</mml:mi> </mml:mrow> </mml:math> has a possible gap of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow> <mml:mo>∼</mml:mo> <mml:msup> <mml:mn>4.7</mml:mn> <mml:mo>∘</mml:mo> </mml:msup> </mml:mrow> </mml:math> and a kink. Dynamical modeling of the kink implies that <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow> <mml:mn>300</mml:mn> <mml:mi>S</mml:mi> </mml:mrow> </mml:math> was dramatically influenced by the Large Magellanic Cloud. This is the first model of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow> <mml:mn>300</mml:mn> <mml:mi>S</mml:mi> </mml:mrow> </mml:math> ’s morphology across its entire known footprint, opening the door for deeper analysis to constrain the structures of the Milky Way.