Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: The Extended Star Formation History of the Andromeda Spheroid at Twenty
  One Kiloparsecs on the Minor Axis   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.

21
The categories of this paper are: adap-org
13
The categories of this paper are: quant-ph
24
The categories of this paper are: dg-ga
34
The categories of this paper are: chem-ph
32
The categories of this paper are: atom-ph
10
The categories of this paper are: q-bio
35
The categories of this paper are: mtrl-th
16
The categories of this paper are: stat
30
The categories of this paper are: q-alg
19
The categories of this paper are: eess
4
The categories of this paper are: cond-mat
5
The categories of this paper are: gr-qc
26
The categories of this paper are: bayes-an
22
The categories of this paper are: chao-dyn
36
The categories of this paper are: cmp-lg
14
The categories of this paper are: math-ph