Near-infrared properties of asymptotic giant branch stars in nearby dwarf elliptical galaxy NGC 205^⋆,⋆⋆

¹ Department of Astronomy, Yonsei University, 120-749 Seoul, Korea
e-mail: jwko@yonsei.ac.kr
² Yonsei University Observatory, 120-749 Seoul, Korea
³ Korea Astronomy and Space Science Institute, 305-348 Daejeon, Korea
⁴ CEOU, Department of Physics and Astronomy, Seoul National University, 151-742 Seoul, Korea

Received: 14 February 2012
Accepted: 10 May 2012

Abstract

Aims. We investigated the distribution of resolved asymptotic giant branch (AGB) stars over a much larger area than covered by previous near-infrared studies in the nearby dwarf elliptical galaxy NGC 205.

Methods. Using data obtained with the WIRCam near-infrared imager of the CFHT, we selected the AGB stars in the JHK_s color − magnitude diagrams, and separated the C stars from M-giant stars in the JHK_s color − color diagram.

Results. We identified 1,550 C stars in NGC 205 with a mean absolute magnitude of  ⟨ M_Ks ⟩  =  −7.49 ± 0.54, and colors of  ⟨ (J − K_s)₀ ⟩  = 1.81 ± 0.41 and  ⟨ (H − K_s)₀ ⟩  = 0.76 ± 0.24. The ratio of C stars to M-giant stars was estimated to be 0.15 ± 0.01 in NGC 205, and the local C/M ratios for the southern region are somewhat lower than those for the northern region. The (J − K_s) color distributions of AGB stars contain the main peak of the M-giant stars and the red tail of the C stars. A comparison of the theoretical isochrone models with the observed color distribution indicates that most of the bright M-giant stars in NGC 205 were formed at log    (t_yr) ~ 9.0−9.7. The logarithmic slope of the M_Ks luminosity function for M-giant stars was estimated to be 0.84 ± 0.01, which is comparable with dwarf elliptical galaxies NGC 147 and NGC 185. Furthermore, we found that the logarithmic slopes of the M_Ks luminosity function for C and M-giant stars are different to places, implying a different star formation history within NGC 205. The bolometric luminosity function for M-giant stars extends to M_bol =  −6.0 mag, and that for C stars spans  −5.6 < M_bol <  −3.0. The bolometric luminosity function of C stars is unlikely to be a Gaussian distribution and the mean bolometric magnitude of C stars is estimated to be M_bol =  −4.24 ± 0.55, which is consistent with our results for dwarf elliptical galaxies NGC 147 and NGC 185.