Guide to NED Redshift Search Results

(Latest Revision: 8 May 2009)

This page displays results from searches for redshift data for an individual object. These redshift data are distinct from NED's "Basic Data" because they are not only referenced, but accompanied by detailed qualifiers. Within NED, each measurement (single data point) along with its descriptors is called a "Redshift Data Frame" or a "Redshift Data Point".

The redshifts displayed in the Basic Data page are often NOT included in the Redshift Data page, though they are included in the summary list as No. "0" with their errors and references.

The redshift data returned by your search will be presented as an HTML table (default), HTML preformatted text, ASCII text (tab-separated or bar-separated), or in XML (VOTable) format. The HTML output is formatted for easy reading by eye on your computer screen, while the ASCII and XML output is appropriate for parsing by computer scripts or programs. The ASCII and XML tables are preceded by a header line showing the data contents of each column.

The top section of this page displays a summary table of NED's redshift data frames for a given object. The first line shows the redshift taken from NED's basic data. The second and following lines include

  • A sequential data-point number applicable to this list only.
  • The frequency or wavelength at which the data apply (in common astronomical jargon, e.g. "Optical", "21-cm").
  • The redshift or velocity as published, first in km/s with errors if given; then in dimensionless units z = Δλ/λ0, again with errors if given in the original reference.
  • The errors are expressed as 1-sigma uncertainties (if given in the publication).
  • The reference code for the source of the redshift. This is linked to NED's Reference database so that you may retrieve the full reference or abstract easily.

More extensive data for any measurement in the list may be seen by clicking on the sequential number for that measurement, or by scrolling down the page to the measurement of interest.

The second part of this page displays the data associated with the redshift data frame for the object you selected. Fields which are empty in the database are suppressed in the output. These data include

  • Name in publication: The name, in NED's standard format, of the object to which the data apply.
  • Coordinates: The object's coordinates and equinox, as published.
  • Features targeted: Rest wavelength or frequency of the spectral line(s) used for the observation, if specified, in standard astronomical jargon. Examples include "HI line," "Optical spectrum," "CIV," and "H K G-band".
  • Velocity or redshift: Given first as published, followed by the units of measurement; then as heliocentric redshift.
  • Uncertainty: Given first as published, then as a standard deviation on the heliocentric redshift.
  • Unc(ertainty) Significance: The type of uncertainty reported in the original publication; e.g. "rms uncertainty", "90% confidence", "no unc. reported", etc.
  • Reference code: The literature source for the present redshift data frame, in NED's standard 19-byte code (click on the highlighted reference code to display the full reference).
  • Measurement mode
    • Spectral range: the observed spectral range. Possible values are
      Gamma ray X-ray
      Ultraviolet Optical
      Near IR Infrared
      Far IR Sub-millimeter
      Millimeter Radio
    • Spectrograph: the observing instrumentation, e.g. "Long-slit spectrum," "Heterodyne receiver (e.g. 21-cm)," "Grism," etc.
    • Features: The number and nature of the spectral features measured. For multiple objects (e.g. galaxy clusters), the number of objects measured.
    • Technique: The method of measuring the spectrum, e.g. "Cross-correlation fit to template spectrum," "Mid-point of line profile at 20% level," "Peak of line," etc.
    • Spatial mode: The part of the object measured, e.g. "Systemic redshift," "Average over multiple objects," "Redshift is for individual star," etc.
    • Obs(ervation) Date: Epoch of observation, if given.
  • Reference Frame: Specifies the nature of the redshift or velocity and any corrections applied to it, e.g. "Heliocentric," "Corrected to the 3K microwave background," etc.
  • Apex: the velocity and direction of the apex of the reference frame used to calculate the published redshift, if given. NED adopts heliocentric redshifts or velocities as published whenever possible. If not, NED calculates the heliocentric redshift using the apex as published.
  • Qualifiers: Self-explanatory notes pertaining to
    • The history of the data (e.g. "from new raw data", "transformed from previously published data", etc.).
    • Adjustments to the data (e.g. "zero-point corrected", etc.).
    • Changes introduced by NED (e.g. "obvious typo corrected", "correction published in errata", etc.).
    • Values modified by NED (e.g. "Redshift modified from published value", "Uncertainty modified from published value", etc.).
  • Comments: Other information about the data.

John Huchra at the Harvard-Smithsonian Center for Astrophysics has kindly supplied more information on the definition of extragalactic redshifts.

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