Old-Aged Stellar Population Distance Indicators

TL;DR

This paper reviews old-aged stellar distance indicators like RR Lyrae, T2C, and TRGB, highlighting their historical development, theoretical basis, observational methods, and future prospects for cosmic distance measurement.

Contribution

It provides a comprehensive overview of Population II standard candles, including their theoretical foundations, observational techniques, and future potential with upcoming observational facilities.

Findings

Detailed description of the methods to establish absolute scales for each standard candle.

Discussion of observational systematics affecting distance measurements.

Outlook on future advancements with new telescopes and theoretical models.

Abstract

Old-aged stellar distance indicators are present in all Galactic structures (halo, bulge, disk) and in galaxies of all Hubble types and, thus, are immensely powerful tools for understanding our Universe. Here we present a comprehensive review for three primary standard candles from Population II: (i) RR Lyrae type variables (RRL), (ii) type II Cepheid variables (T2C), and (iii) the tip of the red giant branch (TRGB). The discovery and use of these distance indicators is placed in historical context before describing their theoretical foundations and demonstrating their observational applications across multiple wavelengths. The methods used to establish the absolute scale for each standard candle is described with a discussion of the observational systematics. We conclude by looking forward to the suite of new observational facilities anticipated over the next decade; these have both a broader wavelength coverage and larger apertures than current facilities. We anticipate future advancements in our theoretical understanding and observational application of these stellar populations as they apply to the Galactic and extragalactic distance scale.