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Detection of Accretion Shelves Out to the Virial Radius of a Low-mass Galaxy with JWST

Conroy, Charlie; Johnson, Benjamin D.; van Dokkum, Pieter; Deason, Alis; Tacchella, Sandro; Belli, Sirio; Bowman, William P.; Naidu, Rohan P.; Park, Minjung; Abraham, Roberto; Emami, Razieh

Detection of Accretion Shelves Out to the Virial Radius of a Low-mass Galaxy with JWST Thumbnail


Charlie Conroy

Benjamin D. Johnson

Pieter van Dokkum

Sandro Tacchella

Sirio Belli

William P. Bowman

Rohan P. Naidu

Minjung Park

Roberto Abraham

Razieh Emami


We report the serendipitous discovery of an extended stellar halo surrounding the low-mass galaxy Ark 227 (M * = 5 × 109 M ⊙; d = 35 Mpc) in deep JWST NIRCam imaging from the Blue Jay Survey. The F200W–F444W color provides robust star–galaxy separation, enabling the identification of stars at very low density. By combining resolved stars at large galactocentric distances with diffuse emission from NIRCam and Dragonfly imaging at smaller distances, we trace the surface-brightness and color profiles of this galaxy over the entire extent of its predicted dark matter halo, from 0.1 to 100 kpc. Controlled N-body simulations have predicted that minor mergers create “accretion shelves” in the surface-brightness profile at large radius. We observe such a feature in Ark 227 at 10–20 kpc, which, according to models, could be caused by a merger with total mass ratio 1:10. The metallicity declines over this radial range, further supporting the minor merger scenario. There is tentative evidence of a second shelf at μ V ≈ 35 mag arcsec−2 extending from 50 to 100 kpc, along with a corresponding drop in metallicity. The stellar mass in this outermost envelope is ≈107 M ⊙. These results suggest that Ark 227 experienced multiple mergers with a spectrum of lower-mass galaxies—a scenario that is broadly consistent with the hierarchical growth of structure in a cold-dark-matter-dominated universe. Finally, we identify an ultra-faint dwarf associated with Ark 227 with M * ≈ 105 M ⊙ and μ V, e = 28.1 mag arcsec−2, demonstrating that JWST is capable of detecting very-low-mass dwarfs to distances of at least ∼30 Mpc.

Journal Article Type Article
Acceptance Date Apr 14, 2024
Online Publication Date Jun 20, 2024
Publication Date Jun 1, 2024
Deposit Date Jul 1, 2024
Publicly Available Date Jul 1, 2024
Journal The Astrophysical Journal
Peer Reviewed Peer Reviewed
Volume 968
Issue 2
Article Number 129
Keywords Galaxy stellar halos
Public URL


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