L. Bottinelli

The extragalactic distance scale. II: The unbiased distance to the Virgo cluster from the B-band Tully-Fisher relation

The behavior of the B-band Tully-Fisher (TF) relation with respect to the observational biases and parameter uncertainties is studied from an almost complete sample of spiral galaxies belonging to the Virgo Cluster. The strong influence of the limiting apparent magnitude when using the direct TF relation is confirmed. A distance modulus of 31.4 + or - 0.2 is found along with a corresponding H(0) = 68 + or - 8 km/s/Mpc, assuming a cosmological velocity of the cluster V = 1300 + or - 100 km/s. The Virgo S and S-prime clouds are shown to lie at significantly different distances. Different distance moduli found by other authors are explained. 127 refs.

The Malmquist bias in the extragalactic distance scale: controversies and misconceptions

Les auteurs examinent quelques critiques sur un travail anterieur concernant le biais de Malmquist dans la relation de Tully-Fisher et demontrent que ces critiques sont injustifiees et reposent sur une mauvaise interpretation de ce biais

LEDA: The Lyon-Meudon Extragalactic Database

In 1981 our scientific team was involved in the management of large amount of HI-data. The goal, at this time, was only our own scientific research. In a collaboration with G. and A. de Vaucouleurs we started to add optical information to our first HI-catalog. It was our first contact with the difficult problem of cross-identifying two catalogs and merging them. To face this problem, one of us (GP) decided to rationalize the management by creating a database. It was created in 1983 at Lyon Observatory. LEDA is thus the oldest Extragalactic Database. Since this time, the database has been continuously updated and improved.

Challenges of Using Planetary Nebulae for Extragalactic Distances

The planetary nebulae (hereafter PN) distance criterion (Ciardullo et al. 1989a) relies on the shape adopted for the luminosity function (hereafter LF) and its adequate fit to the observed data. Crucial hypothesis concerning the LF are its universal shape with an universal value of the absolute magnitude cut-off and concerning the sample, its completeness and the selection of the rejected data. Because the completeness is achieved only in the bright part of the LF, the brighest PN play a crucial role in the distance determination and it is well known that the brightest objects (with intrinsic scatter) in a giant galaxy are brighter than in a dwarf galaxy. Thus more luminous galaxies are given smaller distances. Because giant galaxies are rare they are not seen at small distances; on the contrary, they are typically observed at large distances. This effect induces a progressive compression of the distance scale.

Corrections for Velocity Dispersion in the Tully-Fisher Relation

The 21-cm line width is a good indication of the maximum rotational velocity in a galaxy. However, the turbulent motions affect the observed line width and must be corrected. Not taking them into account introduces both accidental and systematic errors. A first example of this effect is the dependence of the slope a of the Tully-Fisher relation: on the level at which the line width ΔV obs . is measured. In their 1977 paper, Tully and Fisher find the following values: A second example is the departure from linearity in the H-band Tully-Fisher relation obtained by Aaronson et al. (1982) (their figure 2): the narrow lines are more strongly affected by these turbulent motions.

The Extragalactic Distance Scale and the Hubble Constant

It is not easy to measure distance to galaxies. That we even try is as musch a measure of our audacity as it is an indication of our ingenuity. But we do try, and we feel that we must try, because so many of our grandest questions and most basic problems about the universe depend on our knowledge of its size and scale.