GD-1 Stellar Stream and Cocoon in the DESI Early Data Release
Monica Valluri, Parker Fagrelius, S. E. Koposov, Ting S. Li, Oleg Y. Gnedin, Eric F. Bell, R. G. Carlberg, Andrew P. Cooper, J. Aguilar, S. P. Ahlen, Carlos Allende Prieto, Vasily Belokurov, Leandro Beraldo e Silva, D. Brooks, Amanda Byström, T. Claybaugh, K. Dawson, Arjun Dey, P. Doel, J. E. Forero-Romero, E. Gaztañaga, Satya Gontcho A Gontcho, J. Han, K. Honscheid, Theodore Kisner, Anthony Kremin, Andrew Lambert, Martin Landriau, L. Le Guillou, M. E. Levi, Axel de la Macorra, Marc Manera, J. Ereza, G. E. Medina, Aaron Meisner, R. Miquel, John Moustakas, Adam D. Myers, Joan Najita, Claire Poppett, Francisco Prada, Mehdi Rezaie, G. Rossi, A. H. Riley, E. Sánchez, David J. Schlegel, M. Schubnell, David Sprayberry, G. Tarlé, Guillaume F. Thomas, B. A. Weaver, Risa H. Wechsler, Rongpu Zhou, Hu Zou
Abstract
Abstract We present ∼115 new spectroscopically identified members of the GD-1 tidal stream observed with the 5000-fiber Dark Energy Spectroscopic Instrument (DESI). We confirm the existence of a “cocoon,” which is a broad (FWHM ∼ 2 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mover> <mml:mrow> <mml:mo>.</mml:mo> </mml:mrow> <mml:mrow> <mml:mtext>°</mml:mtext> </mml:mrow> </mml:mover> </mml:math> 932 ∼ 460 pc) and kinematically hot (velocity dispersion, σ ∼ 5–8 km s −1 ) component that surrounds a narrower (FWHM ∼ 0 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mover> <mml:mrow> <mml:mo>.</mml:mo> </mml:mrow> <mml:mrow> <mml:mtext>°</mml:mtext> </mml:mrow> </mml:mover> </mml:math> 353 ∼ 55 pc) and colder [( σ = 3.09 ± 0.76 km s −1 )] thin stream component (based on a median per star velocity precision of 2.7 km s −1 ). The cocoon extends over at least a 30 ∘ segment of the stream observed by DESI. The thin and cocoon components have similar mean values of [Fe/H]: −2.54 ± 0.04 dex and −2.47 ± 0.06 dex, suggestive of a common origin. The data are consistent with the following scenarios for the origin of the cocoon. The progenitor of the GD-1 stream was an accreted globular cluster (GC) and: (a) the cocoon was produced by pre-accretion tidal stripping of the GC while it was still inside its parent dwarf galaxy; (b) the cocoon comprises debris from the parent dwarf galaxy; (c) an initially thin GC tidal stream was heated by impacts from dark subhalos in the Milky Way; (d) an initially thin GC stream was heated by a massive Sagittarius dwarf galaxy; or a combination of some of these. Future DESI spectroscopy and detailed modeling may enable us to distinguish between these possible origins.