|Compilation of distance measurements|
To date, a number of distance determination methods have been invented. They vary in the class of objects used (Cepheids, Globular Clusters, Spiral Galaxies an so on) and in the physical background (e.g. period-luminosity relation, typical linear size, inner kinematics-luminosity relation etc.). See also the article on the cosmic distance ladder from Wikipedia.
Direct methods determine distances straight from the measurement data and do not depend on external calibrations. They are the basis for construction of the cosmic distance ladder. The most important distance estimates use trigonometric parallax of individual stars. The methods of statistical parallax and moving cluster parallax allow us to derive distances for groups of stars. It is very useful for the calibration of methods based on luminosity of Cepheids and RR Lyrae. Unfortunately, these methods are usually restricted to our Galaxy or to its nearby satellites. A notable exception is NGC 4258 whose precise Maser distance of 7.6 Mpc is precious to calibrate the other methods.
A wide number of methods uses individual objects or stellar populations in the galaxies for distance determination. This class contains some of the most precise and important distance indicators for extragalactic astronomy: the Cepheids and RR Lyrae variable stars, the tip of the red giant branch (TRGB) and the horizontal branch (HB) stars. These distance indicators can be calibrated using direct methods. Except SN Ia, all these methods are effective only for the nearby Universe on scale from several to few dozen Mpc.
Methods, based on scaling relations, are empirical relationships between the intrinsic luminosity of a galaxy and its properties such as kinematics, surface brightness, and so on. The most important ones are the Tully-Fisher (TF) relation for spirals and the fundamental plane (FP) for early-type galaxies. Because the methods use the total luminosity of a galaxy as a standard candle, they can be applied on scales up to hundreds Mpc. These methods provide low precisions for individual measurements, but they give good results in a statistical sense with huge sets of data. This is especially true for the Tully-Fisher relation, where obtaining observational data is relatively inexpensive. The TF and FP methods allow us to investigate the cosmic flows in the Universe on scale of hundred Mpc.
Statistics of the compilation of distances
The first ten sources of distances
The measurements collected in this catalogue form an inhomogeneous set where individual publications were each (i) calibrated onto a specific distance scale, and (ii) affected by their own systematics. The goal of the homogenization is to bring all the individual measurements to a common distance scale, after correcting the systematics.
The strategy is the following: First we define a set of calibrators that define our distance scale. Then we apply two zero-point corrections, the first one, that we call the calibsys correction, accounts for the shift between a given calibration system and our adopted dis- tance scale. The second, called dataset correction, compensates systematics of individual series of measurements. These steps are described in details in a paper to appear in A&A.
The set of standard distances (calibset) consists in 211 galaxies with precise Cepheid, TRGB or Maser observations.
The homogenization procedure determines two parameters, clc, the calibsys correction, and dtc, the dataset correction, that are stored in the dataset description table, and used to compute the standardized modulus, mdstd. mdstd is in turn used to determine the mean homogenized distance modulus, mod0, for each galaxy, distributed in the Leda catalogue.