Search and Retrieval
Q: How to retrieve all of the objects mentioned in a given paper?
Any of the search options will return a list of papers. Click on the reference code for the paper in question. This will open a new screen with the title and abstract text (if
available), and a link to Retrieve NNN NED Objects ("NNN" is the number of NED objects included in the reference). This will give you a list of all NED objects mentioned
in the selected paper.
Q: How to find all of the papers written by a given author?
A: If the author is among the first 20 or so listed for an extragalactic paper, select the Author Name option in the "Literature" menu. This option will open the Search for Abstracts by Author Name screen. Submit the query using the author's last name as input parameter.
Only the first 160 ASCII characters of the author list and article title can be searched or displayed on the results page. Long author lists and long titles are not completely
displayed, and author searches will not return papers when the name is not included in the first 160 characters of the author list. Note, however, that NED's abstracts include the full title and author list, so you may use a Text Search to find authors in long author lists.
Q: How can I find QSO 2251+113 without doing a position search?
A: You may use the IAU Format option from the "Objects" menu. This option will open the Search for Object for Given IAU Name screen. You may then submit the query using an IAU-style name as the input parameter.
Q: In the Advanced All-Sky Search page, is it possible to add a filter for the V magnitude of the target?
A: Yes. In the "Photometric Constraints" section of the page, you may choose many different passbands, including Johnson V. These constraints use NED's "Photometry Data Frames". If an object does not yet have photometry available through the Photometry option in the "Data" menu on the home page, it will not be included in a search depending on photometric constraints.
Q: Is there any way to retrieve data concerning velocity dispersions within galaxies, so I can get, for example, the velocity distributions of stars as a function of galaxy morphology?
A: NED does not yet list velocity dispersions for its galaxies. However, you may use the Text Search option to scan NED's titles and abstracts, and LEVEL 5, for papers that contain velocity dispersions.
Q: How to get measured fluxes for my objects using NED?
A: For single objects, you may search NED's photometric database using the Photometry option in the "Data" menu on the home page.
Q: Is there a way to perform a NED search for objects based only on spectroscopic redshifts, and not on redshifts from colors or magnitudes?
A: We adopt spectroscopic redshifts whenever they are available. Sometimes, however, only a photometric redshift is available for an object, or the source of a redshift is not clearly stated in the published paper. In those cases, we flag the redshifts if we know that they are not from spectroscopy. It is still possible, however, for photometric
redshifts to slip in unflagged. In cases of doubt, we urge you to go back to the published paper to check the source of the redshifts.
Q: When I use the search for References from the "Literature" menu on NED's home page, why do I sometimes get results referring to papers that are not related to my intended object?
A: The default for this type of search is to return all "related" objects, that is, all objects with the same root name. For example, if you search for NGC 1614, you will also get references for NGC 1614A, NGC 1614:SN 1996D, etc. If you want just the references for a specific object, set the Related object name search? option to No.
Q: Why does a search for M51 return the wrong position?
A: "M51" refers to the pair of galaxies, NGC 5194 and NGC 5195, not just to the larger spiral NGC 5194. The position that NED adopts for M51 is therefore the mean position of the two galaxies. We treat other pairs, triplets, groups, and clusters listed in NED the same way: we adopt mean positions for all the galaxies rather than for just the brightest or for the most centrally-located. Many pairs and triplets in NED have position errors that -- rather than representing the 95% confidence ellipse -- represent half the separation of the objects. The "error ellipse" therefore encloses both/all positions for the member galaxies.
Q: Why doesn't NED provide simple text files containing basic data such as preferred coordinates and redshifts for all objects with available redshifts?
A: The database is updated frequently, thus any such files would be quickly out-of-date, and would no longer represent the current state of knowledge available in NED.
In addition to new objects and data that are continuously being integrated into the database, the cross-identifications of incoming sources with existing objects in NED, as well as the selection of the most reliable ("preferred") positions, redshifts and other quantities that appear by default in object queries when there are multiple measurements available, are constantly being refined with improving algorithms and scientific vetting procedures. Also, the NED team is working on database restructuring and a new search engine that will enable users to select what fields are contained in query output tables, for example particular photometric bands from large surveys and derived quantities. (This will be an extension of NED functionality currently available in the service to search for objects by classifications/attributes.) Stay tuned!
Q: Why does NED contain so many stars in the Milky Way and unclassified objects, especially at low Galactic latitudes?
A: Most astronomical sky survey catalogs are produced by automated pipelines that typically do not accurately classify Galactic versus extragalactic objects. "Star" versus "Galaxy" classifications published in many catalogs are often actually discriminators between point sources and extended sources, such as in the SDSS photometric catalogs. Statistical techniques that use regions of parameter space to classify objects are also error prone, or limited to the information measured within a particular survey. The true nature of an astronomical source often cannot be determined or confirmed until its properties are compared with other measurements. Therefore, when integrating data from large sky survey catalogs into NED,
all sources are processed in order to:
- accelerate data ingest and cross-matching by avoiding the pitfalls of attempting to classify catalog
sources before their attributes are merged with other measurements in NED;
- ensure that NED is as complete as possible by not omitting actual extragalactic objects, which would
occur if statistical selection criteria such as color-magnitude or color-color diagrams, or point source
versus extended source classifications, were used to filter incoming catalogs;
- minimize incorrect cross-identifications that would result if stars or suspected stars were omitted from
previous catalogs processed by NED (i.e., Galactic stars projected very close to distant galaxies or quasars);
- assist with characterizing the source confusion that occurs when sources from low resolution
surveys contain emission from multiple sources detected in higher resolution observations;
- facilitate the discovery of new galaxies and AGNs among the many unclassified sources in modern sky surveys by using the photometry and other measurements that are fused within NED across spectral regions;
- avoid confusion among NED users about what sources were or were not included in the database from a catalog. (Such confusion did arise in the early days of NED when catalog sources were omitted for various reasons).
Note that when searching for objects Near Name or Near Position (Cone Search) or In a Refcode, in the user interface you can select Object Type Constraints in the Search Options to include (or exclude) Classified Extragalactic, Components of Galaxies, or Unclassified Objects (Source Types). The current default settings include all object types in the query results.
Q: Why are some magnitude conversions resulting in negative flux densities?
A: SDSS magnitudes are inverse hyperbolic sine magnitudes (Lupton, Gunn, &
m=-2.5/ln(10) * asinh((f/f0)/(2b))+ln(b)
This magnitude system accounts for the fact that background subtracted flux measurements may be zero or even negative, which can not be represented by a logarithmic
SDSS sources with magnitudes larger than the zero flux magnitude (see table below) formally have negative flux in the corresponding band. This is possible because such sources
are detected in one or more of the SDSS bands, but not all of them. For example, SDSS J154914.44+023647.2 is detected in the g, r, i, and z bands, but not the u band.
The u-band PSF magnitude for this source is 25.277, which is fainter than the u-band zero-flux magnitude (24.63), hence the flux is negative (-0.66 mJy).
| No. || Observed Passband || Photometry Measurement || Uncertainty || Units || Frequency (Hz) || Flux Density (Jy)
||Flux Density Uncertainty (Jy) || Frequency Mode || Qualifiers || Refcode |
| 1 || u (SDSS PSF) AB || 25.277 || +/-1.206 || asinh mag || 8.36E+14 || -6.61E-07
|| +/-1.38E-06 || Broad-band || SDSS Qualifiers || 2007SDSS6.C...0000: |
More information about the SDSS magnitude system is presented here:
SDSS Asinh Magnitudes
Magnitudes within the SDSS are expressed as inverse hyperbolic sine (or “asinh”) magnitudes, described in detail by Lupton, Gunn, & Szalay 1999. They are sometimes referred to informally as /luptitudes/. The transformation from linear flux measurements to asinh magnitudes is designed to be virtually identical to the standard astronomical magnitude at high signal-to-noise ratio, but to behave reasonably at low signal-to-noise ratio and even at
negative values of flux, where the logarithm in the Pogson magnitude fails. This allows
us to report a magnitude even in the absence of a formal detection; we quote no upper limits in our photometry.
The asinh magnitudes are characterized by a softening parameter b, the typical 1-sigma noise of the sky in a PSF aperture in 1′′ seeing. The
relation between detected flux f and asinh magnitude m is:
m = -2.5/ln(10) * [asinh((f/f0)/(2b)) + ln(b)].
Here, f0 is given by the classical zero point of the magnitude scale, i.e., f0 is the flux of an object with conventional magnitude
of zero. The quantity b is measured relative to f0, and thus is in maggies; it is given in the table of asinh softening parameters below (Table 21 in the EDR paper), along with the asinh magnitude associated with a zero flux object. The table also lists the flux corresponding to 10f0, above which the asinh magnitude and the traditional logarithmic magnitude differ by less than 1% in flux. For details on converting asinh magnitudes to other flux measures, see converting counts to magnitudes.
Asinh Softening Parameters
||Zero-flux Magnitude [m(f/f0 = 0)]
||m(f/f0 = 10b)
Q: What is the difference between Strict and Liberal in the IAU-format search?
A: The IAU names are in the form HHMMSS.d+DDMMd where
H = hours of RA
D = degrees of DEC
M = minutes of RA or DEC
S = seconds of RA or DEC
+ = + or - declination
d = decimal numbers.
2214+3307 would represent: 22h14m +33deg 07'
2214+337 would represent: 22h14m +33.7 deg
The Strict convention understands the input as a truncated
version of the coordinates and adds a half decimal to the last digit
given in RA and Dec, then searches for all objects within a radius
equal to the larger of the half decimal suffixes.
→ 22h 14m +33deg 7'
Since the range in position is up to
22h 15m +33deg 8'
the center position is determined
by adding half a decimal to the input position
22h 14.5m +33deg 7.5'
22h 14m 30s +33deg 7' 30"
This position becomes the center of the search circle.
Since the center is within 30 sec and 30 arcsec from the
input position, the radius of the search should cover the
largest distance, in this case 30 seconds of time.
The Liberal convention assumes that the last digit could be either truncated or
rounded, and considers the resulting possibilities. NED then searches
for all objects within the largest circle containing all the possibilities.
Input: 22142+332 could be:
221420+3320 → 221429+3329
22h14m20s +33deg 20' → 22h14m29s +33deg 29'
221412+3312 → 221418+3318
22h14m12s +33deg 12' → 22h14m18s +33deg 18'
So, the search is between the extremes of these positions:
22h14m12s +33deg 12' → 22h14m29s +33deg 29'
which would make the center at:
22h14m20.5s +33deg 20' 30"
The search radius which encompasses all possible interpretations
of the IAU name is then:
10.7' = 10' 42"