Determining the location of the tip of the red giant branch in old stellar populations: M33, Andromeda I and II

Abstract

The absolute bolometric luminosity of the point of core helium ignition in old, metal‐poor, red giant stars is of roughly constant magnitude, varying only very slightly with mass or metallicity It can thus be used as a standard candle. Here, we review the main difficulties in measuring this location in any real data set and we develop an empirical approach to optimize it for tip of the red giant branch (TRGB) analysis. We go on to present a new algorithm for the identification of the TRGB in nearby metal‐poor stellar systems. Our method uses a least‐squares fit of a data adaptive slope to the luminosity function in 1‐mag windows. This finds the region of the luminosity function that shows the most significant decline in star counts as we go to brighter magnitudes; the base of this decline is attributed as the location of the tip. This technique then allows for the determination of realistic uncertainties which reflect the quality of the luminosity function used, but which are typically ∼0.02 mag rms +∼0.03 mag systematic, a significant improvement upon previous methods that have used the tip as a standard candle. Finally, we apply our technique to the Local Group spiral galaxy M33 and the dwarf galaxies Andromeda I and II, and derive distance modulii of 24.50 ± 0.06 mag (794 ± 23 kpc), 24.33 ± 0.07 mag (735 ± 23 kpc) and 24.05 ± 0.06 mag (645 ± 19 kpc) respectively. The result for M33 is in excellent agreement with the Cepheid distances to this galaxy, and makes the possibility of a significant amount of reddening in this object unlikely.

The Carnegie-Chicago Hubble Program. II. The Distance to IC 1613: The Tip of the Red Giant Branch and RR Lyrae Period–luminosity Relations*

“…For the AGB population, some studies have assumed the LF slope to be flat (e.g. Durrell et al 2002;McConnachie et al 2004). Modeling observed populations, however, suggests that the AGB LF slope could be in the range 0.3 ± 0.2 dex mag −1 (Makarov et al 2006).…”

Stellar structures in the outer regions of M 33

“…Estimates of the distance to M 33 obtained with different methods vary considerably in the literature, ranging between 800 and 940 kpc Ciardullo et al 2004;McConnachie et al 2004;Tiede et al 2004;Sarajedini et al 2006). Here we derive the distance to M 33 using the tip of the RGB (TRGB) as a standard candle (Lee et al 1993).…”

DEEPHST/ACS PHOTOMETRY OF THE M81 HALO

“…Here, we determine the distance from TRGB in the I band, which has proven to be an excellent distance indicator for metal-poor, old stellar populations (e.g., Lee et al 1993;Sakai et al 1996;Ferrarese et al 2000;Bellazzini et al 2001;McConnachie et al 2004;Mouhcine et al 2005). Visual inspection of our CMD shows a transition in the number counts at m F 814W ∼ 24.0, albeit with a small number of stars.…”