|Compilation of distance measurements|
This catalog (table a007 in the database) presents the compilation of published redshift-independent distance mesurements. It can be either original distance measurements, re-calibrations, or even compilations. The data are stored in form of so called a true distance modulus, (m-M)0, which is corrected for both the extinction in the our Galaxy and the absorption in the host galaxy.
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.
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.
calibsys correction, in mag.
The calibsys correction consists in shifting the zero-point of the individual distance scales that we identified in the catalogue to align them with our standerd set. We suceeded to associate most measurements to a calibsys, in the other cases we could not find in the original paper an unambigous description of the distance scale, in some cases because the listed measurements were clearly not bound to an homegeneous system (e. g. in the case of some compilations). No correction is applied in these latter cases. The most populated calibsys is Freedman+2001. For each calibsys intersecting calibset on more than 5 objects, a zero-point correction is computed.
dataset correction, in mag.
The dataset correction, to apply on individual series of measurements is determined by comparing each series to all the rest of the catalogue after applying the calibsys and the previously adopted dataset corrections. Unlike the calibsys correction, the determination of dtc is a supervised process, which is incremental and iterative. For all the dataset having a sufficient intersection with the rest of the catalogue, a correction is suggested, together with the significance level. Then the datasets displaying the most significant offset are carefully examined. If a zero-point correction is adequate, a dataset correction is adopted, and all the calibsys corrections and suggested dataset correction are automatically re-adjusted.