Database
Q: Are the redshifts reported in NED spectroscopic or photometric? And what do the codes following the redshift mean?
A: Redshifts in NED are integrated from the published literature or survey catalogs, and can be spectroscopic or photometric depending on how they are acquired. The codes following the redshift is a flag to indicate the technique (1 character) and method (2 characters) used to acquire the redshifts, and the quality (1 character) of the values if available. For example, if a redshift is determined through spectroscopy (S) using multiple spectral lines (LS), the corresponding redshift flag would be "SLS " if it's a secured measurement; "SLS?" if it's a questionable measurement.
The current list of acceptable techniques, methods and quality flags is provided below.
| Code | Abbreviation | Description |
| Technique | S | SPEC | a spectroscopic redshift measurement |
| P | PHOT | a photometric redshift estimate |
| M | MOD | a modelled value |
| L | MULT | determined using multiple techniques |
| I | INFD | an inferred value |
| U | UNK | unknown or not clearly stated in the publication |
| Method | 1L | 1LIN | spectroscopic redshift estimated from a single spectral line, assuming the line is known |
| CO | CONT | continuum, based on Balmer/4000A break (e.g., 2008ApJ…677..219K) |
| FO | FoF | friends of friends |
| LE | LENS | estimated using a lensing model |
| LS | LINS | spectroscopic redshift utilizing multiple spectral lines |
| LU | LUM | estimated from assumed luminosity for a brightest cluster galaxy (e.g, 2001ApJ...563..629N) |
| MF | MFA | a value from a matched filter algorithm (e.g., 1996AJ....111..615P) |
| ML | ML | Machine Learning |
| MU | MULT | estimated using multiple methods |
| PA | PAH | determined from PAH features |
| SE | SED | estimated from a spectral energy distribution |
| ST | STAT | statistically acquired value, e.g., average based on N measurements |
| SN | SN | host galaxy redshift is determined from the expansion velocity of a supernova |
| TO | TOMO | a tomographic redshift for a lensing object (e.g., 2005ApJ...624...59H) |
| UN | UNK | unknown or not clearly stated in the publication |
| Quality | | | blank, indicating a secure value |
| ? | ? | an uncertain (questionable, tentative, noisy) value as indicated by the authors |
Q: How is a fiducial (preferred) redshift assigned for an object that has one or more redshifts available?
A: The NED preferred redshifts have historically been chosen by human experts during the data ingestion process. However, due to the dramatic increase in redshift measurements appearing in sky survey catalogs and in the astrophysical literature, in 2021 an automated decision tree algorithm was implemented. A summary of the process follows:
For objects with a single redshift measurement, it is automatically taken as the fiducial value. For objects with multiple measurements, the fiducial redshift is selected using both the published uncertainties and the technique used to measure the redshift. The objects with multiple redshift measurements fall into two main categories: 1) At least one redshift uncertainty exists, in which case the redshift with the lowest uncertainty value is chosen as fiducial. 2) No redshift uncertainties were published, so the fiducial redshift is chosen according to the priority of the technique. In order of highest to lowest priority (general reliability), these are: spectroscopic (SPEC), photometric (PHOT), modeled (MOD), multiple methods (MULT), inferred (INFD), and unknown (UNK). In cases where measurements have the same lowest uncertainty value, the fiducial redshift is chosen based on the priority-ordered technique listed above. When there is a tie between two or more priority-ordered techniques, the most recently published value is chosen. Results of scientific vetting shows that this methodology works very well in general, resulting in consistent assignments for tens of millions of objects. However, refinements to the programmed decision tree may be necessary in special cases, such as BL Lac objects; this work is in progress. Further details about the algorithm will be provided on the NED website. Please keep in mind that in addition to the fiducial redshifts selected by NED’s algorithm, all individual redshift measurements for an object are also available in the user interface (or via API) for users to make their own selections.
Q: What kind of magnitude is listed in the Basic Data?
A: These are usually optical magnitudes taken from
the astronomical literature, and should be understood as being indicative
only. We are adding letters after the magnitudes indicating the band pass
to which the magnitude applies. For example, we use "U", "B", "V", "R", and
"I" for the standard Johnson and/or Cousins magnitudes in the optical; "p" for
photographic magnitudes from e.g. IIa-O or 103a-O plates, "g" for Gunn g-band
magnitudes, "j" for magnitudes from III-aJ plates, "J" for 2MASS near-IR
magnitudes, and so on.
We will eventually have all of NED's Basic Data magnitudes flagged with
the band passes. In the meantime, the magnitude may have already been
included in NED's table of of referenced
Photometric Data.
Q: How do I read a 19-digit REFCODE?
A: NED reference codes are 19-character strings of the form:
YYYYPUBLNVVVVMPPPPA
Unused characters are padded with dots ".".
The fields within the string are as follows:
| YYYY | The four digits of the year of
publication |
| PUBLN | The journal code, left-justified
within the five-digit field |
| | The codes for those journals regularly entered into
NED are: |
| A&A.. | Astronomy and Astrophysics |
| A&AS. | Astronomy and Astrophysics Supplement Series |
| AJ... | Astronomical Journal |
| ApJ.. | Astrophysical Journal |
| ApJS. | Astrophysical Journal Supplement Series |
| ARep. | Astronomy Reports (formerly Soviet Astronomy) |
| AstL. | Astronomy Letters (formerly Soviet Astronomy
Letters) |
| Ast.. | Astronomy |
| IAUC. | IAU Circulars |
| MNRAS | Monthly Notices of the Royal Astronomical Society
|
| Natur | Nature |
| PASP. | Publications of the Astronomical Society of the
Pacific |
| PASJ. | Publications of the Astronomical Society of Japan
|
| VVVV | Volume number of the journal,
right-justified within the four-digit field |
| M | Tie-breaker code. Where ambiguity is
possible (e.g. between the main journal section, |
| and the "letters" section of a journal), the following
characters or digits in this field break the ambiguity: |
| L | Letters sections in various journals |
| p | Pink pages in MNRAS. (changed to "L" in 1993)
|
| 1, 2, ..., 9, | Issue numbers 1 through 9, then "a"
through "o", within the same volume of a journal |
| 0, a, b, c, | (e.g. Sky and Telescope, Physics
Today). |
| ..., o |
| A, B, ... | Issue designations used by the
publisher within the same volume, where each issue starts with page
one. |
| OR |
| Two or more articles appearing on the same
page within a single issue of a journal
|
| (e.g. Nature, IAU Circulars) are lettered successively
beginning with A - Z. |
| PPPP | Starting page number of the article,
right-justified |
| A | First letter of the first author's last
name, or a ":" when no authors are specified for a reference |
More information on reference codes is available in
NED and SIMBAD Conventions for Bibliographic Reference Coding.
Q: What do notations such as "NED01", ID", and "NOTES02" mean?
A: These are suffixes which we add to a root name to distinguish physically separate objects which do not otherwise carry separate names. For example, UGC 01562 is a double galaxy, but has only one entry in UGC. In order to include both galaxies in NED with unique names, we add the notation "NED01" and "NED02" to "UGC 01562". Similarly, we add "NOTES01" to UGC 01562 to refer to a galaxy included only in the UGC Notes for UGC 01562.
We typically use the "ID" suffix to indicate that a non-optical source (e.g. an infrared source) has been associated with an otherwise unnamed physical object, usually a galaxy. An example is "IRAS 00182+1130". This has been identified as a galaxy, but not given a separate name. The "ID" notation makes it clear that the object has been reclassified from its original type.
Other suffixes that we use include "COM" for a nearby companion, not necessarily physically connected to the main object (e.g. "3C 227 COM01"), "ABS" for an absorption line system seen along the line of sight to a background object (e.g. "[HB89] 1727+502 ABS01"), and "ARC" for gravitationally-lensed arcs usually found in rich clusters (e.g. "ABELL 0164 ARC01").
Q: How often is NED updated?
A: Usually three or four times a year. A backup working copy of the database is continually updated and checked for problems and errors before being released.
Q: How complete is NED?
A: We have tried to get all data published since 1988 into the database, but we invariably miss some and probably have errors as well. We are also constantly changing and refining the Basic Data so you may not always see the same data from month to month (although if the change is large we will generally include a note to that effect). Also, we scan the refereed literature only in the larger professional journals. Thus, if data are published only in an observatory monograph, only in preprint, only in a conference proceeding, or only in a thesis, we may well not see it. In these cases, we are happy to add the data to NED when you bring it to our attention.
Q: What is the source of the morphological types in the NED database? Is there a way to find out where the classifications for each galaxy came from?
A: Many of the morphological types in NED come from the Third Reference Catalog of Bright Galaxies (RC3). This is essentially the same system as described in the RC2. Both of these texts should be available from your library. We have also added physical classifications from spectroscopy when available (e.g. "Sy1", "BLLAC", "HII", and so forth). These and other classifications come from the literature, but are not yet specifically referenced.
Q: The "Morphology" line of NED's Basic Data sometimes contains abbreviations such as "BCD" and "Sbrst". What exactly do these mean?
A: These are usually "standard" abbreviations, taken from the literature, for various sorts of nuclear activity in galaxies. Occasionally, they are simply descriptive abbreviations relating to the morphology of the galaxy. Here is a list:
| AGN | active galactic nucleus |
| BAL | broad absorption line |
| BEL | broad emission line |
| BBG | Balmer-break galaxy |
| BCDG | blue compact dwarf galaxy |
| BH | black hole |
| BLAGN | broad-line active galactic nucleus |
| BLLAC | BL Lacertae-type object |
| BLAZAR | Strongly variable and optically-polarized QSO or
BL Lac object, often with strong gamma- and X-ray
emission |
| BlueCG | blue compact galaxy (may be called BCG in
the literature) |
| BLR | broad line region |
| BLRG | broad line radio galaxy |
| BrClG | brightest cluster galaxy (may be called BCG in
the literature) |
| cD | supergiant galaxy with an extensive envelope in a
cluster |
| CNELG | compact narrow emission line galaxy |
| DANS | dwarf amorphous nuclear starburst |
| DLA | damped Lyman-alpha |
| DLyA | damped Lyman-alpha |
| DOG | dust-obscured galaxy |
| ELG | emission line galaxy |
| ERO | extremely red object |
| FSRQ | flat spectrum radio QSO |
| HEG | high-excitation narrow-line radio galaxy |
| HEX | high excitation line galaxy |
| HII | HII-type object |
| HPQ | high polarization QSO |
| IFRS | infrared faint radio source |
| LAE | Lyman-alpha emitter |
| LBG | Lyman-break galaxy |
| LCBG | luminous compact blue galaxy |
| LCG | luminous compact galaxy |
| LEG | low-excitation narrow-line radio galaxy |
| LERG | low-excitation radio galaxy (not to be confused
with LIRG) |
| LEX | low excitation line galaxy |
| LINER | low-ionization nuclear emission-line region |
| LIRG | luminous infrared galaxy (not to be confused with
LERG) |
| LPQ | low-polarization QSO |
| LSB | low surface brightness |
| NELG | narrow-emission-line galaxy |
| NLAGN | narrow-line active galactic nucleus |
| NLRG | narrow-line radio galaxy |
| NLSy1 | narrow-line Seyfert 1 |
| OFRG | Optically-faint radio galaxy |
| PAS | passive nucleus |
| PEG | passive elliptical galaxy |
| QSO | Quasi-stellar object |
| RET | retired nucleus |
| RLG | radio-loud galaxy |
| RLQ | radio-loud quasar |
| RQQ | radio-quiet quasar |
| Sbrst | starburst object |
| SFRG | Submillimeter-faint, star-forming radio galaxy |
| Sy1 | Seyfert 1 |
| Sy1.2 | Seyfert 1.2 |
| Sy1.5 | Seyfert 1.5 |
| Sy1.8 | Seyfert 1.8 |
| Sy1.9 | Seyfert 1.9 |
| Sy2 | Seyfert 2 |
| Sy3 | Seyfert 3 |
| ULIRG | ultra-luminous infrared galaxy |
| XBONG | X-ray bright/optically normal galaxy |
| XMPG | eXtremely Metal-Poor Galaxy |
Q: Is there any information which explains the jargon used in the photometry of objects in NED?
A: We encourage our users to go back to the original papers to fully understand the magnitudes adopted by NED. Here are a few examples of magnitudes currently found in NED's detailed photometric data:
- u, g, r, i, or z usually refers to the five-band SDSS photometric
system
- B is a B-band magnitude on the Johnson system
- BT is a total magnitude in the B-band
- BT0 is a total magnitude in the B-band corrected
to "face-on" (i.e. inclination = 0 degrees)
- bj is approximately a B magnitude derived from photometry
on a IIIa-J plate
- R25 is an R magnitude at the 25th mag arcsec-2
isophote level
- J, H, or Ks usually refers to the three-band 2MASS photometic
system
Q: The fields of view from the 2MASS Large Galaxy Atlas (LGA) do not agree between the FITS and GIF versions of the images. Which is correct?
A: Both; each is correctly labeled. The FITS images cover the entire image from the LGA, while the GIF images typically cover only the inner parts of the FITS images. The GIFs are used for the quick-look thumbnails in the image list; while the FITS images always cover the entire galaxy, and are suitable for scientific use.
Q: What are the object type codes that NED uses?
A: Here is the current list of object types used in NED, listed alphabetically (objects within the Milky Way Galaxy have their types preceded by an exclamation point "!"):
| * | Star or Point Source |
| ** | Double star |
| *Ass | Stellar association |
| *Cl | Star cluster |
| AbLS | Absorption line system |
| Blue* | Blue star |
| C* | Carbon star |
| EmLS | Emission line source |
| EmObj | Emission object |
| exG* | Extragalactic star (not a member of an identified galaxy) |
| Flare* | Flare star |
| G | Galaxy |
| GammaS | Gamma ray source |
| GClstr | Cluster of galaxies |
| GGroup | Group of galaxies |
| GPair | Galaxy pair |
| GTrpl | Galaxy triple |
| G_Lens | Lensed image of a galaxy |
| HII | HII region |
| IrS | Infrared source |
| MCld | Molecular cloud |
| Neb | Nebula |
| Nova | Nova |
| Other | Other classification (e.g. comet; plate defect) |
| PN | Planetary nebula |
| PofG | Part of galaxy |
| Psr | Pulsar |
| QGroup | Group of QSOs |
| QSO | Quasi-stellar object |
| Q_Lens | Lensed image of a QSO |
| RadioS | Radio source |
| Red* | Red star |
| RfN | Reflection nebula |
| SN | Supernova |
| SNR | Supernova remnant |
| UvES | Ultraviolet excess source |
| UvS | Ultraviolet source |
| V* | Variable star |
| VisS | Visual source |
| WD* | White dwarf |
| WR* | Wolf-Rayet star |
| XrayS | X-ray source |
| !* | Galactic star |
| !** | Galactic double star |
| !*Ass | Galactic star association |
| !*Cl | Galactic Star cluster |
| !Blue* | Galactic blue star |
| !C* | Galactic carbon star |
| !EmObj | Galactic emission line object |
| !Flar* | Galactic flare star |
| !HII | Galactic HII region |
| !MCld | Galactic molecular cloud |
| !Neb | Galactic nebula |
| !Nova | Galactic nova |
| !PN | Galactic planetary nebula |
| !Psr | Galactic pulsar |
| !RfN | Galactic reflection nebula |
| !Red* | Galactic red star |
| !SN | Galactic supernova |
| !SNR | Galactic supernova remnant |
| !V* | Galactic variable star |
| !WD* | Galactic white dwarf |
| !WR* | Galactic Wolf-Rayet star |
Q: Why doesn't NED have any information about the Sun, the planets, or the stars?
A: NED is an extragalactic database. Data and references for Galactic objects may be retrieved from SIMBAD,
maintained by the Centre de Donnees astronomiques de Strasbourg, France. Solar system and planetary data may be retrieved from NASA's Planetary Data System at JPL.
Q: What images are included in NED?
A: Most of the images NED holds on line are optical, infrared, and radio images of galaxies, though we do have some UV and X-ray images as well. We also collect HI spectra, isophote maps, and other graphical representation of extragalactic data. NED can also display an image of a given object from the Digitized Sky Survey using one of the Object Search options, or the Image Search option, all available on NED's Home Page.
Q: How may I contribute my data (catalogs, FITS images or spectra) to NED?
A: Please leave a comment with your name and email address so that we may contact you.
Q: Why is it that not all of the IRAS catalog entries are in NED?
A: When we loaded the IRAS catalogs, we used various filters to improve the chances of a source being extragalactic.
FSC sources in NED were chosen using the following "color" and quality constraints:
1) F(25)/F(60) < 2.0 AND
2) Q(60) ≥ 3
PSC sources used the same flux and color filters, and added three additional filters based on the IRAS "cirrus" flags:
1) cirr1 < 9 AND
2) cirr2 < 7 AND
3) cirr3 < 75
In addition, all PSC sources in the areas of the LMC, SMC,
M31, and M33 were loaded into NED and flagged as being within
the boundaries of those galaxies.
Q: There are only 1500 entries in the Markarian catalog. Yet NED has some Markarian numbers over 1500. Where do these extra numbers come from?
A: In 1986, V. A. Lipovetsky and J. A. Stepanian collected all the Markarian objects from the fifteen published lists into a complete "First Byurakan Survey." This list, never published but privately circulated, included an additional 32 objects given provisional numbers from 9001 to 9032. A few of these provisional numbers have appeared in the
literature, so we have put all of them into NED. Then, in 1989, Markarian, Lipovetsky, Stepanian, L. K. Erastova, and A. I. Shapovalova published their complete Markarian catalog in Communications of the Special Astrophysical Observatory, No. 62 as "The First Byurakan Survey. A Catalogue of Galaxies with UV-continuum." This catalog has an additonal 15 objects numbered from 1501 to 1515, 14 from the 32 new objects in the 1986 list, and one new. We have also put these 15 numbers into NED, so the 14 new objects in common to the 1986 and 1989 lists carry two Markarian numbers.
Q: There is a gap in the Fairall numbers between 1185 and 1201. Do these objects exist?
A: No. Fairall's original lists (see Fairall, MNRAS 233, 691, 1988 and references therein) contained data for 1185 compact and bright-nucleus galaxies, while the latest paper (Fairall and Woudt, MNRAS 366, 267, 2006) has data for 336 galaxies found in large-scale structures across the sky near the South Celestial Pole. The two samples are thus independent. Therefore, Fairall started his numbers for his latest large-scale structure sample at 1201 to avoid confusion between it and his earlier sample of compact galaxies.
Q: The RC3 data in NED sometimes do not agree with the RC3 printed version. Why?
A: NED's version of RC3 (the Third Reference Catalogue of Bright Galaxies) includes the many corrections detailed in Corwin et al. AJ 108, 2128, 1994 as well as a few others made since that paper was published.
Q: DSS images of some galaxies near the celestial poles are rotated with respect to 2MASS images. Which image is correct?
A: The 2MASS images are correctly oriented. The direction of north on the DSS images is dependent on several factors, including their position on the original 6.4 x 6.4-degree sky survey plate, the epoch of the original plate, and the software used for scanning and image extraction. The 2MASS images come directly from the sky at an epoch very close 2000.0, so were processed with equinox J2000.0 coordinates.
