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The ACS LCID Project. VI. The star formation history of the Tucana dSph and the relative ages of the isolated dSph galaxies


Monelli, M and Gallart, C and Hidalgo, SL and Aparicio, A and Skillman, ED and Cole, A and Weisz, DR and Mayer, L and Bernard, EJ and Cassisi, S and Dolphin, AE and Drozdovsky, I and Stetson, PB, The ACS LCID Project. VI. The star formation history of the Tucana dSph and the relative ages of the isolated dSph galaxies, Astrophysical Journal, 722, (2) pp. 1864-1878. ISSN 0004-637X (2010) [Refereed Article]

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Copyright © 2010 The American Astronomical Society

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DOI: doi:10.1088/0004-637X/722/2/1864


We present a detailed study of the star formation history (SFH) of the Tucana dwarf spheroidal galaxy. Highquality, deep HST/ACS data, collected in the framework of the LCID project, allowed us to obtain the deepest color–magnitude diagram to date, reaching the old main-sequence turnoff (F814 ∼ 29) with good photometric accuracy. Our analysis, based on three different SFH codes, shows that Tucana is an old and metal-poor stellar system, which experienced a strong initial burst of star formation at a very early epoch (≃13 Gyr ago) which lasted a maximum of 1 Gyr (sigma value). We are not able to unambiguously answer the question of whether most star formation in Tucana occurred before or after the end of the reionization era, and we analyze alternative scenarios that may explain the transformation of Tucana from a gas-rich galaxy into a dSph. Current measurements of its radial velocity do not preclude that Tucana may have crossed the inner regions of the Local Group (LG) once, and so gas stripping by ram pressure and tides due to a close interaction cannot be ruled out. A single pericenter passage would generate insufficient tidal heating to turn an originally disky dwarf into a true dSph; however, this possibility would be consistent with the observed residual rotation in Tucana. On the other hand, the high star formation rate measured at early times may have injected enough energy into the interstellar medium to blow out a significant fraction of the initial gas content. Gas that is heated but not blown out would also be more easily stripped via ram pressure. We compare the SFH inferred for Tucana with that of Cetus, the other isolated LG dSph galaxy in the LCID sample. We show that the formation time of the bulk of star formation in Cetus is clearly delayed with respect to that of Tucana. This reinforces the conclusion of Monelli et al. that Cetus formed the vast majority of its stars after the end of the reionization era implying, therefore, that small dwarf galaxies are not necessarily strongly affected by reionization, in agreement with many state-of-the-art cosmological models.

Item Details

Item Type:Refereed Article
Keywords:galaxies: evolution – galaxies: individual (Tucana dSph) – galaxies: photometry – Galaxy: stellar
Research Division:Physical Sciences
Research Group:Astronomical sciences
Research Field:Cosmology and extragalactic astronomy
Objective Division:Expanding Knowledge
Objective Group:Expanding knowledge
Objective Field:Expanding knowledge in the physical sciences
UTAS Author:Cole, A (Professor Andrew Cole)
ID Code:65931
Year Published:2010
Web of Science® Times Cited:83
Deposited By:Physics
Deposited On:2010-12-09
Last Modified:2015-09-21

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