Near-infrared photometric properties of asymptotic giant branch stars in the dwarf irregular galaxy IC 1613^⋆

¹ Yonsei University Observatory, 120-749 Seoul, Republic of Korea
e-mail: sohnyj@yonsei.ac.kr, shchun@galaxy.yonsei.ac.kr
² Choong Ang High School, 110-800 Seoul, Republic of Korea
³ Department of Astronomy, Yonsei University, 120-749 Seoul, Republic of Korea
⁴ Center for the Exploration of the Origin of the Universe (CEOU), Department of Physics and Astronomy, Seoul, National University, 151-742 Seoul, Republic of Korea

Received: 9 February 2015
Accepted: 21 April 2015

Abstract

Aims. We investigated the photometric properties of asymptotic giant branch (AGB) stars of the dwarf irregular galaxy IC 1613 and its metallicity.

Methods. We used near-infrared JHK_s photometric data obtained with the WIRCam array on Canada-France-Hawaii Telescope (CFHT). The optical gi imaging data of a previous study were cross-matched with ours to distinguish the stellar populations in the galaxy and to select the AGB stars. We also separated the C stars from the M-giant stars in the JHK_s color−color diagram.

Results. We identified 140 C stars and 306 M-giant stars in IC 1613. The (J − K_s) color histogram of AGB stars shows the main peak of M-giant stars and the red tail of C stars. The broad color distribution of M-giant stars and the isochrones in the color magnitude diagram indicate that IC 1613 has a wide range of intermediate-age stars. We also know that most C stars were formed over the past 2−8 Gyr according the M_bol-age relation. The C/M ratio of 0.46 ± 0.05 was estimated and [Fe/H] abundance of −1.23 ± 0.06 was derived from the C/M ratio. Radial distributions of C/M ratio and metallicity show the shallow trend of the negative metallicity gradient from the center. The local C/M ratio also indicates that the HI cavity has higher C/M values than the bright HI spot and HI gas region. However, the low [Fe/H] value near the central region of the galaxy is likely to be caused by the change in age of the dominant stellar population according to the radial distance, not the real metallicity variation. It might also be that relatively younger and metal-rich C stars are concentrated in the center of the galaxy, while the M-giant stars were homogeneously distributed from the center to the borders of the galaxy.