Near-infrared properties of asymptotic giant branch stars in nearby dwarf elliptical Galaxy NGC 205

TL;DR

This study analyzes the near-infrared properties of asymptotic giant branch stars in NGC 205, revealing their distribution, types, and implications for the galaxy's star formation history using CFHT data.

Contribution

It provides the first extensive near-infrared analysis of AGB stars in NGC 205, including C/M ratios, luminosity functions, and star formation insights, expanding previous studies.

Findings

Identified 1,550 C stars with specific magnitudes and colors.

Estimated the C/M ratio to be 0.15, varying across regions.

Found differences in luminosity function slopes indicating diverse star formation histories.

Abstract

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. Using data obtained with the WIRCam near-infrared imager of the CFHT, we selected the AGB stars in the JHKs color-magnitude diagrams, and separated the C stars from M-giant stars in the JHKs color-color diagram. We identified 1,550 C stars in NGC 205 with a mean absolute magnitude of M_Ks = -7.49 \pm 0.54, and colors of (J - Ks) = 1.81 \pm 0.41 and (H - Ks) = 0.76 \pm 0.24. The ratio of C stars to M-giant stars was estimated to be 0.15 \pm 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 - Ks) 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) \sim 9.0-9.7. The logarithmic slope of the M_Ks luminosity function for M-giant stars was estimated to be 0.84 \pm 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 \pm 0.55, which is consistent with our results for dwarf elliptical galaxies NGC 147 and NGC 185.