Guide to NED Photometric Data Search Results

(Latest Revision: 15 November 2011)

This page displays results from searches for photometric data for an individual object. These photometric data are distinct from NED's "Basic Data" because they are referenced and accompanied by detailed qualifiers. Each measurement (single data point) along with its descriptors is called a "Photometric Data Frame" or a "Photometric Data Point".

If your search output options included an HTML table (the default) or HTML preformatted text, your output page will begin with a spectral energy distribution (SED) plot. Otherwise, the photometric data will be presented as 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.

Spectral Energy Distribution Plot

All of the photometric data points for a single object are shown in a standard log [flux density] (in Janskys) vs. log [frequency] (in Hertz) plot. Line fluxes are shown as the word "LINE" at the lower edge of the plot at their correct wavelength or frequency.

Any part of the plot may be enlarged by clicking on it. The default enlargement is 4X, but you may change this with the "Select zoom factor" button. The enlarged portion of the plot appears in a new window.

You may change the appearance of the plot with the option buttons to the right of the plot. Error bars and point labels may be turned off, the units of either axis may be changed, and you may choose autoscaling (for the frequency/wavelength axis) if data points are outside the default frequency range. Once you have made your selections, click the "Plot Again" button to replot the data.

You may retrieve a PostScript version of the SED, or any enlarged portion of it, by clicking on the link just below the plot.

The photometric data stored and displayed by NED represent the originally published data as closely as possible. For this reason, the apparent agreement between data points nominally on the same system at the same wavelength may not appear to be good, particularly when displayed on the spectral energy distributions. There are several causes for this:

1. Aperture differences. Because galaxies are extended objects, the percentage of the galaxy included in the observing aperture has a strong influence on the final flux displayed. This fact alone probably accounts for most of the scatter seen on these plots in the flux densities at a single frequency.
2. Variability. Many QSOs and some galaxy nuclei are variable at some or all of the wavelengths at which they are observed.
3. Photometric system differences. Differences in filter functions, observation techniques (including fixed sky-chopping offsets), reduction procedures, and absolute calibrations among different observers can lead to significant differences in published data nominally on the "same" photometric system, and even at the same aperture.
4. Identification errors. NED depends on the accuracy of the published identifications and positions to match data for the same object from different sources. While we correct misidentifications whenever we find them, we are sure that many remain undetected. This, too, will lead to scatter in the SED plots.

NED has adopted effective wavelengths, band widths, and absolute calibrations for the different photometric systems from the papers containing the data, or from the papers originally defining the photometric systems, whenever possible. When this information is not given in these papers, we have assumed the values in this table taken from various papers, often -- for optical bandpasses -- from Table 9 in "Galaxy Colors in Various Photometric Band Systems" by M. Fukugita, K. Shimasaku, and T. Ichikawa, published in PASP, 107, 945, 1995.

Summary Information

The first part of this page displays a one-line summary of the photometric data frame including

  • A sequential data-point number applicable to this list only,
  • The frequency or wavelength at which the data apply (in standard astronomical jargon, e.g. "V" for the Johnson V band, "IRAS 12 microns" for the IRAS 12 micron band, etc.).
  • The photometric data and uncertainty (if given) along with the units of measurement, both as published and in absolute units as calculated by NED.
  • The absolute ("homogenized") unit section also includes the mode of observation ("Line" and "Broad-band" are the most common modes), the aperture (if given) at which the observation was made, and other comments. The "SDSS Qualifiers" comment links to the "Detailed Photometric Data" section (described below) where the SDSS flags appropriate to the data point are displayed.
  • The NED 19-digit reference code. The code is linked to the full title or abstract in NED's bibliographic database.
  • The sequential number at the beginning of the line, repeated.

Optionally, you may choose to display only the data as-published, or only as homogenized by NED, and in wavelength or frequency units, and in flux or flux-density units using Janskys or SI units.

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

Detailed Photometric Data

Every photometric data frame contains information that will help you to interpret the data. This information, some repeated from the summary table above, is included in the final section of this page. You may scroll down the page to it, or you may click on the sequence number in the summary table at the top of the page. NED stores the following information about each data point.

  • Reference code: The literature source for the present photometric data point.
  • Freq(uency) targeted: The frequency, wavelength, or photometric passband, in standard astronomical jargon (e.g. "IRAS 12 microns", "B (B_T)", "HI (21 cm line)", etc.).
  • Measurement: The photometric data, given first in the published units, along with those units; followed by the absolute flux or flux density (and its units), as calculated by NED.
  • Uncertainty: The uncertainty in the data, if given in the original publication, and its units; followed by the uncertainty in the absolute flux or flux density, as calculated by NED.
  • Significance: The type of uncertainty reported in the original publication; e.g. "rms uncertainty", "90% confidence", "no unc. reported", etc.
  • Freq(uency) or Vel(ocity): The frequency or wavelength of observation as given in the original publication, or the standard value assumed by NED if not given; the units; the reference frame in which the frequency or wavelength is defined (OBS for observatory reference frame, and REST for object or source rest frame); and the frequency in Hz, as translated by NED; OR the velocity or redshift to which line observations were tuned.
  • Frequency mode: The method(s) by which the data were collected, reduced, or derived, in relation to the frequency or wavelength; e.g. "broad-band measurement", "flux integrated over line", "absorption lines measured", "color-corrected", etc.
  • Coord(inate) targeted: The position at which the measurements were taken, followed by the coordinate reference frame (e.g. "B1950", "J2000", etc.).
  • Spatial mode: The method(s) by which the data were collected, reduced, or derived, in relation to the spatially integrated flux; e.g. "flux in fixed aperture", "from multi-aperture data", "integrated from scans", "estimated by eye", etc.
  • Notes: 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. "extinction corrected for Milky Way", "corrected for contaminating foreground sources", "K-correction applied", etc.),
    • changes introduced by NED (e.g. "obvious typo corrected", "correction published in errata", etc.),
    • values modified by NED (e.g. "Coord1 modified from published value", "Frequency modified from published value", etc.), and
    • assumptions made by NED about the precise parameters of the data (e.g. "Standard Johnson UBVRI filters assumed", etc.).
  • Qualifiers: Usually SDSS Flags for galaxies with SDSS photometry.
  • Comments: Other information from the published source or from NED that may be helpful in interpreting the data.

2MASS Data

Detailed information about photometric data taken from the 2MASS Extended Source Catalog (XSC) is in the 2MASS Explanatory Supplement. NED lists the following 2MASS XSC magnitudes and their errors when available:

NED Label2MASS Variable NameQuantity
J_K20 (2MASS) or J_20 (2MASS LGA) j_m_k20fe J magnitude within K = 20.0 mag/arcsec^2^ isophote
H_K20 (2MASS) or H_20 (2MASS LGA) h_m_k20fe H magnitude within K = 20.0 mag/arcsec^2^ isophote
K_K20 (2MASS) or K_20 (2MASS LGA) k_m_k20fe K magnitude within K = 20.0 mag/arcsec^2^ isophote
J_total (2MASS) or J_tot (2MASS LGA) j_m_ext Extrapolated ("Total") J magnitude
H_total (2MASS) or H_tot (2MASS LGA) h_m_ext Extrapolated ("Total") H magnitude
K_total (2MASS) or K_tot (2MASS LGA) k_m_ext Extrapolated ("Total") K magnitude
J_14arcsec (2MASS) j_m_7 14.0 arcsec circular aperture J magnitude
H_14arcsec (2MASS) h_m_7 14.0 arcsec circular aperture H magnitude
K_14arcsec (2MASS) k_m_7 14.0 arcsec circular aperture K magnitude
J_Kron (2MASS LGA) j_m_fe J Kron elliptical aperture magnitude
H_Kron (2MASS LGA) h_m_fe H Kron elliptical aperture magnitude
K_Kron (2MASS LGA) k_m_fe K Kron elliptical aperture magnitude

Photometry taken from the The 2MASS Large Galaxy Atlas (LGA) is included in XSC for 550+ large galaxies. NED uses the LGA data when possible; these are distinguished by their reference code (2003AJ....125..525J) and by their labels noted above. 2MASS apertures and integration areas are given in the Comments for each observation.


Photometry in NED from the Sloan Digital Sky Survey, Data Release 6 includes

We include data from all five of the SDSS bands (u, g, r, i, and z) if it is given in the original SDSS data files (some data are missing, typically because the object is too faint to be detected in a particular band). In addition, we decode and display the flags for each band in the "Qualifiers" section of the Detailed Information block for every object. These flags are fully explained in a document prepared by the SDSS project, kindly displayed here with their permission.

We also give the r-band "effective radius" as a "Comment" in the r-band model magnitude. This is the effective radius adopted for reduction of the model magnitudes in the other bands, insuring that colors determined from the model magnitudes will be meaningful. The effective radii are often close to or identical to zero (to within the errors) for QSOs. For these objects, the model magnitudes will usually be identical (again, to within the errors) to the PSF magnitudes.

All SDSS magnitudes are asinh magnitudes. These essentially identical to logarithmic Pogson magnitudes at high signal-to-noise ratios, and may more accurately reflect the true flux at low signal-to-noise ratios where Pogson magnitudes are likely to fail.

All SDSS magnitudes are on or close to the AB system of Oke and Gunn (see e.g. 1983ApJ...266..713O and references therein). We have followed SDSS in adopting the flux density conversion S = 3631 Jy * f/f0 (with small zero-point corrections in the u and z bands applied before the conversion).

Finally, the use of asinh magnitudes by SDSS can lead to negative flux densities if the quoted asinh magnitude is larger than the "zero-flux" magnitude. These zero-flux magnitudes are dependent on the SDSS band as follows:

  • u -- 24.63
  • g -- 25.11
  • r -- 24.80
  • i -- 24.36
  • z -- 22.83
We have chosen to display the errors of the SDSS photometry so affected as upper limits in the "homogenized" data section and on the SEDs, though the "as-published" section displays the asinh magnitudes and errors as present in the SDSS data files.

Known Issue (2008-2012 March): SDSS flux densities in NED are incorrect at magnitudes within 2.51 mag of the sky noise limit. The incorrect softening parameters and zero-point corrections were used to compute these flux densities. This will be fixed in the planned March 2012 NED release. Click here for further information and here for the proper procedure to compute flux densities from SDSS asinh magnitudes.

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