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25 April 2024 |
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The Extended Star Formation History of the Andromeda Spheroid at Twenty One Kiloparsecs on the Minor Axis | | Thomas M. Brown
; Ed Smith
; Henry C. Ferguson
; Puragra Guhathakurta
; Jasonjot S. Kalirai
; R. Michael Rich
; Alvio Renzini
; Allen V. Sweigart
; David Reitzel
; Karoline M. Gilbert
; Marla Geha
; | | Date: |
16 Feb 2007 | | Abstract: | Using the HST ACS, we have obtained deep optical images of a southeast minor-axis field in the Andromeda Galaxy, 21 kpc from the nucleus. In both star counts and metallicity, this field represents a transition zone between the metal-rich, highly-disturbed inner spheroid that dominates within 15 kpc and the metal-poor, diffuse population that dominates beyond 30 kpc. The color-magnitude diagram reaches well below the oldest main-sequence turnoff in the population, allowing a reconstruction of the star formation history in this field. Compared to the spheroid population at 11 kpc, the population at 21 kpc is ~1.3 Gyr older and ~0.2 dex more metal-poor, on average. However, like the population at 11 kpc, the population at 21 kpc exhibits an extended star formation history; one third of the stars are younger than 10 Gyr, although only a few percent are younger than 8 Gyr. The relatively wide range of metallicity and age is inconsistent with a single, rapid star-formation episode, and instead suggests that the spheroid even at 21 kpc is dominated by the debris of earlier merging events likely occurring more than 8 Gyr ago. | | Source: | arXiv, astro-ph/0702448 | | Services: | Forum | Review | PDF | Favorites | |
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