Melbourne, J. and Williams, B. and Dalcanton, J. and Ammons, S. M. and Max, C. and Koo, D. C. and Girardi, Leo and Dolphin, A. (2010) The Asymptotic Giant Branch and the Tip of the Red Giant Branch as Probes of Star Formation History: The Nearby Dwarf Irregular Galaxy KKH 98. Astrophysical Journal, 712 (1). pp. 469-483. ISSN 0004-637X http://resolver.caltech.edu/CaltechAUTHORS:20100409-120508055
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We investigate the utility of the asymptotic giant branch (AGB) and the red giant branch (RGB) as probes of the star formation history (SFH) of the nearby (D = 2.5 Mpc) dwarf irregular galaxy, KKH 98. Near-infrared (near-IR) Keck Laser Guide Star Adaptive Optics (AO) images resolve 592 IR-bright stars reaching over 1 mag below the tip of the RGB. Significantly deeper optical (F475W and F814W) Hubble Space Telescope images of the same field contain over 2500 stars, reaching to the red clump and the main-sequence turnoff for 0.5 Gyr old populations. Compared to the optical color-magnitude diagram (CMD), the near-IR CMD shows significantly tighter AGB sequences, providing a good probe of the intermediate-age (0.5-5 Gyr) populations. We match observed CMDs with stellar evolution models to recover the SFH of KKH 98. On average, the galaxy has experienced relatively constant low-level star formation (5 × 10^(–4) M_☉ yr^(–1)) for much of cosmic time. Except for the youngest main-sequence populations (age <0.1 Gyr), which are typically fainter than the AO data flux limit, the SFH estimated from the 592 IR-bright stars is a reasonable match to that derived from the much larger optical data set. Differences between the optical- and IR-derived SFHs for 0.1-1 Gyr populations suggest that current stellar evolution models may be overproducing the AGB by as much as a factor of 3 in this galaxy. At the depth of the AO data, the IR-luminous stars are not crowded. Therefore, these techniques can potentially be used to determine the stellar populations of galaxies at significantly further distances.
|Additional Information:||© 2010 American Astronomical Society. Received 2009 October 22; accepted 2010 February 2; published 2010 March 3. The adaptive optics data presented hereinwere obtained at the Keck Observatory, which is operated as a scientific partnership among Caltech, UC, and NASA. The authors recognize and acknowledge the very significant cultural role and reverence that the summit of Mauna Kea has always had within the indigenous Hawaiian community. The laser guide star adaptive optics system was funded by the W. M. Keck Foundation. This work has been supported in part by the NSF Science and Technology Center for Adaptive Optics, managed by the University of California (UC) at Santa Cruz under the cooperative agreement AST-9876783. S.M.A. acknowledges support from the Bachmann instrumentation program through UCO/Lick. L. Girardi was partially supported by contract ASI-INAF I/016/07/0.|
|Subject Keywords:||galaxies: irregular; galaxies: stellar content; Hertzsprung-Russell and C-M diagrams; instrumentation: adaptive optics; stars: AGB and post-AGB|
|Classification Code:||PACS: 98.52.Wz; 98.62.Lv; 98.62.Ai; 97.10.Cv; 97.20.Li|
|Official Citation:||J. Melbourne et al 2010 ApJ 712 469 doi: 10.1088/0004-637X/712/1/469|
|Usage Policy:||No commercial reproduction, distribution, display or performance rights in this work are provided.|
|Deposited By:||Jason Perez|
|Deposited On:||12 Apr 2010 17:19|
|Last Modified:||26 Dec 2012 11:57|
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