A near-infrared study of AGB and red giant stars in the Leo I dSph galaxy

Abstract

A near-infrared imaging study of the evolved stellar populations in the dwarf spheroidal galaxy Leo I is presented. Based on JHK observations obtained with the WFCAM wide-field array at the United Kingdom Infrared Telescope, we build a near-infrared photometric catalogue of red giant branch (RGB) and asymptotic giant branch (AGB) stars in Leo I over a 13.5 arcmin2 area. The V - K colours of RGB stars, obtained by combining the new data with existing optical observations, allow us to derive a distribution of global metallicity [M/H] with average [M/H] = -1.51 (uncorrected) or [M/H] =-1.24 +/- 0.05(int) +/- 0.15(syst) after correction for the mean age of Leo I stars. This is consistent with the results from spectroscopy once stellar ages are taken into account. Using a near-infrared two-colour diagram, we discriminate between carbon- and oxygen-rich AGB stars and obtain a clean separation from Milky Way foreground stars. We reveal a concentration of C-type AGB stars relative to the red giant stars in the inner region of the galaxy, which implies a radial gradient in the intermediate-age (1-3 Gyr) stellar populations. The numbers and luminosities of the observed carbon- and oxygen-rich AGB stars are compared with those predicted by evolutionary models including the thermally pulsing AGB phase, to provide new constraints to the models for low-metallicity stars. We find an excess in the predicted number of C stars fainter than the RGB tip, associated to a paucity of brighter ones. The number of O-rich AGB stars is roughly consistent with the models, yet their predicted luminosity function is extended to brighter luminosity. Although these discrepancies can be partly ascribed to significant uncertainties in the Leo I star formation history and incompleteness of the spectroscopic samples of C stars fainter than the RGB tip, it appears more likely that the adopted evolutionary models overestimate the C-star lifetime and underestimate their K-band luminosity

Asymptotic giant branch stars in the Leo I dwarf spheroidal galaxy

“…However, Mateo et al (2008) find that the AGB stars are almost exclusively located within a radius of 400 arcsec, whereas giant branch (GB) stars have a more extended distribution. The analysis of infrared photometry by Held et al (2010) also indicates a strong radial gradient in the intermediate‐age populations and supports the findings by Mateo et al (2008).…”

The Acs Nearby Galaxy Survey Treasury. Ix. Constraining Asymptotic Giant Branch Evolution With Old Metal-Poor Galaxies

“…It is also evident that the simulated AGB stars reach much brighter magnitudes than the observed ones. The excess of AGB stars is probably related to what has been already detected by Gullieuszik et al (2008), Held et al (2010), and Melbourne et al (2010).…”

“…Figure 3) for the low-mass, low-metallicity AGB stars (M 1 M , [Fe/H] −1.2) which are typically being sampled. The need for such a reduction in TP-AGB lifetimes, with respect to those in the MG07 models, has already been indicated by Gullieuszik et al (2008) in their study of the old metal-poor Leo II dSph, and to a lesser extent also by Held et al (2010) and Melbourne et al (2010) while studying metalpoor galaxies with somewhat younger SFHs. On the other hand, it is quite reassuring that the final core masses of our best-fitting AGB models are between 0.51 and 0.55 M (again for low-mass stars, with say M 1 M ), which is in good agreement with the observed masses of white dwarfs in globular clusters of the MW (of 0.53 ± 0.01 M ; cf.…”

“…Indeed, Gullieuszik et al (2008) and Held et al (2010) are able to derive important indications from ground-based data of the Leo II and Leo I dSphs (see also Lagadec et al 2008 for Fornax and Sagittarius). The main uncertainties in these works derive from the small numbers of AGB stars, from the high MW foreground contamination, and the uncertainties in the dSphs star formation histories (SFHs).…”

The Star Formation History of Isolated Dwarf Ugc 4879

“…In order to test this assumption with UGC 4879, we generate a series of test SFHs using the methods described below, and a variety of metallicities ranging [Fe/H] = −2.0 to −0.5, and find that they consistently include large numbers of stars with ages greater than 10 Gyr. Moreover, a star's position on the RGB is more sensitive to metallicity than age, for example, an age difference of several Gyr is needed for two stars to occupy the same space on the RGB when they have a metallicity difference of order 0.1 dex (e.g., Held et al 2010).…”