Compilation of radial velocity measurements

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This catalogue (table a110 in the database) presents the compilation of redshift of galaxies. The measurements are given as Heliocentric cz in km/s (so, for distant galaxies the values can be greater than the speed of light).

The relativistic expression relating frequencies or wavelengths to the radial velocities are:

v/c = (f₀² - f²) / (f₀² + f²) v/c = (l₀^-2 - l^-2) / (l₀^-2 + l^-2)

Where v and c are respectively the radial velocity and the speed of light (299792.458 km/s) in km/s, f and f₀ the observed and rest frequencies and l and l₀ the observed and rest wavelengths.

Astronomers commonly use two approximations of these formula, the first one is called the "radio" convention:

v_radio/c = 1 - f/f₀ = 1 - l₀/l

The second one is the "optical" convention:

v_optical/c = f₀/f - 1 = l/l₀ - 1

The radio definition has the advantage that points sampled at equal increments in frequency translate to equal increments in velocity. However the radio definition is now deprecated by the IAU, but this does not stop it being commonly used.

The redshift, z, more commonly used for distant galaxies, is defined as:

z = v_optical/c

HyperLeda uses the optical convention , which allows a simple and rapid conversion to the redshift (note that naturally for distant galaxies the values may be larger than c.

The resframe used in this compilation is the barycenter of the solar system, or heliocentric restframe. After homogenization and averaging of the different measurements of cz, the velocity is converted to various restframes.

Use this calculator from CSIRO to convert velocities.

The five main references for this compilation are (March 2006):

Ref. number Reference Number of cz

27127 SDSS - DR2 254507

24401 2DF 236202

27139 SDSS - DR4 136637

27128 SDSS - DR3 114701

27124 SDSS (Stoughton 2002) 85221

Ref. number	Reference	Number of cz
27127	SDSS - DR2	254507
24401	2DF	236202
27139	SDSS - DR4	136637
27128	SDSS - DR3	114701
27124	SDSS (Stoughton 2002)	85221

The three main references from radio measurements are (March 2006):

23289 Theureau et al. 1998 2143

4008 de Vaucouleurs et al. RC3 1487

24372 Haynes et al. 1999 1201

See also the article on redshifts of galaxies from the NED knowledge database (Level 5).

Description of the fields

Below is a description of the fields of the table that can be accessed through the interfaces.

Field: vel

Heliocentric radial velocity (CZ) in km/s with its uncertainty and quality flag in parentheses (in html displays). The quality flag has the following meaning:

description of quality flag

value signification

0 regular measurement

1 uncertain measurement, its weight is decreased by a factor 2 when computing the average value

3 preliminary measurement, probably superseeded in a more recent publication, it is ignored unless no other measurement is available

4 compilation, it is ignored unless no other measurement is available

9 rejected measurement

description of quality flag
value	signification
0	regular measurement
1	uncertain measurement, its weight is decreased by a factor 2 when computing the average value
3	preliminary measurement, probably superseeded in a more recent publication, it is ignored unless no other measurement is available
4	compilation, it is ignored unless no other measurement is available
9	rejected measurement

Field: method

Method of Measurement

code	definition
0	unspecified method
1	optical measurement
2	radio measurement

Field: res

Resolution in km/s when the velocity comes from radio measurement

Field: line

Line Code: code of the spectral line used for making a measurement

code	definition
0	unspecified
1	emission line (like 21-cm line or H_alpha line)
2	absorption line

HyperLeda

Questions: prugniel@obs.univ-lyon1.fr

23289	Theureau et al. 1998	2143
4008	de Vaucouleurs et al. RC3	1487
24372	Haynes et al. 1999	1201