Stellar Stream and Halo Structure in the Andromeda Galaxy From a Subaru/Hyper Suprime-Cam Survey

Abstract

We present wide and deep photometry of the northwest part of the halo of the Andromeda galaxy (M31) using Hyper Suprime-Cam on the Subaru Telescope. The survey covers 9.2 deg${}^2$ field in the $g$, $i$, and $NB515$ bands and shows a clear red giant branch (RGB) of M31's halo stars and a pronounced red clump (RC) feature. The spatial distribution of RC stars shows a prominent stream feature, the North Western (NW) Stream, and a diffuse substructure in the south part of our survey field. We estimate the distances based on the RC method and obtain $(m-M)$ = 24.63$\pm 0.191$(random)$\pm 0.057$(systematic) and 24.29$\pm 0.211$(random)$\pm 0.057$(systematic) mag for the NW stream and diffuse substructure, respectively, implying that the NW Stream is located behind M31, whereas the diffuse substructure is located in front. We also estimate line-of-sight distances along the NW Stream and find that the south part of the stream is $\sim$20 kpc closer to us relative to the north part. The distance to the NW Stream inferred from the isochrone fitting to the color-magnitude diagram favors the RC-based distance, but the TRGB-based distance estimated for $NB515$-selected RGB stars does not agree with it. The surface number density distribution of RC stars across the NW Stream is found to be approximately Gaussian with a FWHM of $\sim$25 arcmin (5.7 kpc), with a slight skew to the south-west side. That along the NW Stream shows a complicated structure including variations in number density and a significant gap in the stream.

24 pages, 18 figures, accepted for publication in ApJ

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• Active gas features in three HSC-SSP CAMIRA clusters revealed by high angular resolution analysis of MUSTANG-2 SZE and XXL X-ray observations
• A Tale of a Tail: A Tidally-Disrupting Ultra-Diffuse Galaxy in the M81 Group
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• Metallicities of Outer Halo M31 Globular Clusters from Integrated Light Calcium-II Triplet Spectroscopy
• A Dwarf Disrupting -- Andromeda XXVII and the North West Stream
• Scaling relations of X-ray Luminous Clusters in the Hyper Suprime-Cam Subaru Strategic Program Field
• A New Polar Ring Galaxy Discovered in the COSMOS Field
• One Stream or Two -- Exploring Andromeda's North West Stream
• Analysing Ultra High Energy Cosmic Rays' Anisotropy in $\mathbf{f}(\mathbf{R},\mathbf{T})$ Gravity Theory
• HSC-XXL : Baryon budget of the 136 XXL Groups and Clusters
• Properties of the Lower Segment of M31's North West Stream