Bibcode
Noël, Noelia E. D.; Aparicio, Antonio; Gallart, Carme; Hidalgo, Sebastián L.; Costa, Edgardo; Méndez, René A.
Bibliographical reference
The Astrophysical Journal, Volume 705, Issue 2, pp. 1260-1274 (2009).
Advertised on:
11
2009
Journal
Citations
67
Refereed citations
62
Description
We present a quantitative analysis of the star formation history (SFH)
of 12 fields in the Small Magellanic Cloud (SMC) based on unprecedented
deep [(B - R), R] color-magnitude diagrams (CMDs). Our fields reach down
to the oldest main-sequence turnoff with a high photometric accuracy,
which is vital for obtaining accurate SFHs, particularly at intermediate
and old ages. We use the IAC-pop code to obtain the SFH, using synthetic
CMDs generated with IAC-star. We obtain the SFH as a function ψ(t,
z) of age and metallicity. We also consider several auxiliary functions:
the initial mass function (IMF), phi(m), and a function accounting for
the frequency and relative mass distribution of binary stars, β(f,
q). We find that there are several main periods of enhancement of star
formation: a young one peaked at ~0.2-0.5 Gyr old, only present in the
eastern and in the central-most fields; two at intermediate ages present
in all fields: a conspicuous one peaked at ~4-5 Gyr, and a less
significant one peaked at ~1.5-2.5; and an old one, peaked at ~10 Gyr in
all fields but the western ones. In the western fields, this old
enhancement splits into two, one peaked at ~8 Gyr old and another at ~12
Gyr old. This "two-enhancement" zone is unaffected by our choice of
stellar evolutionary library but more data covering other fields of the
SMC are necessary in order to ascertain its significancy. Correlation
between star formation rate enhancements and SMC-Milky Way encounters is
not clear. Some correlation could exist with encounters taken from the
orbit determination of Kallivayalil et al. But our results would also
fit in a first pericenter passage scenario like the one claimed by Besla
et al. For SMC-Large Magellanic Cloud encounters, we find a correlation
only for the most recent encounter ~0.2 Gyr ago. This coincides with the
youngest ψ(t) enhancement peaked at these ages in our eastern
fields. The population younger than 1 Gyr represents ~7%-12% of the
total ψ(t) in our fields of the wing area. This is not an
exceptional increment as compared with the average ψ(t) but is very
significant in the sense that these eastern fields are the only ones of
this study in which star formation is currently going on. There is also
a strong dichotomy between east/southeast and west in the current
irregular shape of the SMC. We find that this dichotomy is produced by
the youngest population and began ~1.0 Gyr ago or later. The age of the
old population is similar at all radii and at all azimuth, and we
constrain the age of this oldest population to be more than ~12 Gyr. We
do not find yet a region dominated by a true, old, Milky-Way-like, halo
at 4.5 kpc from the SMC center, indicating either that this old stellar
halo does not exist in the SMC or that its contribution to the stellar
populations, at the galactocentric distances of our outermost field, is
negligible. Finally, we derive the age-metallicity relation and find
that, in all fields, the metallicity increased continuously from early
epochs until the present. This is in good agreement with the results
from the Ca II triplet, a completely independent method, constituting an
external consistency proof of IAC-pop in determining the chemical
enrichment law.
Related projects
Milky Way and Nearby Galaxies
The general aim of the project is to research the structure, evolutionary history and formation of galaxies through the study of their resolved stellar populations, both from photometry and spectroscopy. The group research concentrates in the most nearby objects, namely the Local Group galaxies including the Milky Way and M33 under the hypothesis
Martín
López Corredoira