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Chapter 4 - The Galaxy Evolution Explorer (GALEX)

Data from the Galaxy Evolution Explorer (GALEX) are archived at MAST. Users interested in retrieving GALEX data should carefully read the available documentation to ensure they understand the subtleties of the data (i.e., non-unique object identifiers, tiles, etc.). MAST provides a standard main search form. An alternate search interface, CASJobs, is available. CASJobs allows the user to create their own SQL for querying the database. Interested users should carefully read the CASJobs web page (

The MAST GALEX website,, should be consulted for the most up to date information. See News in the right hand gutter.

GALEX observations are made on sky regions called tiles, which are usually 1.2 degree diameter circular areas. GALEX datasets, often called tiles, are images of these tiles. Although the nomenclature is "loose," this document will refer to datasets as tiles simply to avoid additional confusion when comparing different documents. GALEX objects, which are identified in the tiles, are presented in source catalogs and consist of metadata (keywords) only. In addition, GALEX produces grism spectra in the near and far UV. MAST houses both tiles (datasets) and extracted spectra for the grism observations.

Observations taken as part of the Guest Investigator (GI) program reside in a separate database. Unless noted on the search page, the GI database must be searched independently for targets of interest.

Table 4.1 lists some useful links. These pages contain a wealth of information, including FAQs, regarding GALEX data and its retrieval.

Table 4.1: Links to GALEX Documentation



CalTech Mission Page
MAST GALEX tutorial
Main Search Form (GR2)
CASJobs Home Page
Guest Investigator site

4.1 Getting Started with GALEX

The GALEX mission is to survey galaxies out to a redshift of z = 2 and to study the evolution of star-forming galaxies. GALEX conducts this work by means of UV sky surveys and deep sky searches in the imaging mode and partial sky surveys in the near and far UV spectroscopic modes. These are, respectively, the Nearby Galaxy Survey, Deep, Medium, and All Sky Surveys (imaging) and a somewhat smaller Spectroscopic (imaging) survey. All observations are made through a beam-splitter allowing light to be recorded by far-UV and near-UV detectors in either direct-imaging or spectroscopic (grism) modes. The wavelength bandpasses of these detectors are approximately 1400-1800 Angstroms and 1800-2800 Angstroms, respectively. Further technical information can be obtained at the GALEX/Caltech website or through a FAQ page periodically updated by Caltech and maintained at MAST. The All Sky Survey (direct imaging) is expected to cover some 3/4 of the sky (generally in single short visits). Be aware that some regions of the Galactic Plane, the Magellanic Clouds, and the area around bright stars (m(AB) ~ 10) will be avoided in order to protect the detectors from saturation and damage.

The Caltech GALEX project delivers its data to MAST as periodic public releases. These have included the Early Release Observations (ERO) in early 2004, the first Public Release (GR1) in early 2005, and the second and third Public Release (GR2/3) in spring and fall of 2006, respectively. At this writing, the direct images for the GR2/3, consisting of the Medium Imaging, Deep Imaging, Nearby Galaxy, and all Sky Surveys (MIS, DIS, NGS, and AIS), have been delivered to MAST and staged for public release. Delivery of GR4 is anticipated in late Fall 2007. As implied by its name, there are no proprietary rights on public release data.

Guest Investigator (GI) data are delivered to MAST on an irregular basis. Pursuant to NASA policy, these data are proprietary to the GI for a nominal period of six months after date of ingest at MAST. The GI office at Goddard Space Flight Center administers this program and coordinates with MAST to communicate release dates as well as instructions to the GIs. At the end of a proprietary period, MAST opens the permissions to a GI dataset and allows them to be accessed by the general users at its public GI Programs entry point.

Novice users may want to familiarize themselves with the site by using the GALEX site tutorial . Users can also navigate to internal sites from any page by clicking on links in the top menu or left gutter of that page. Each of the pages also contains a number of hot links in the form of pop-up windows or tool tips.

4.2 Data Searches

From the MAST GALEX mission page, data selection, browsing, and retrieval proceeds by making queries in the "Search and Retrieval" tab in the left gutter. In general, users have several ways of searching for data from the GR releases. The most effective way of executing a Search is via the SQL search form . This form allows one to find data for one or more objects within a sky region known as a "tile" or metadata for all objects within the tiles satisfying the query. Search queries are written by SQL syntax, and a text box is provided for this purpose. SQL-challenged users may consult a drop-down menu and submit customized requests on objects or sky regions (tiles) by modifying any of the pre-programmed queries. Clicking on "Execute" returns a table embedded in the search results form; the user is free to choose the format of this table as html (default), Comma Separated Value, or VOTable.

A second way to discover General Release data is to use the more familiar Main search form, which searches the object catalog only and returns results. Whenever objects are found, users may click on the "Explore" entry in the search results table. This action takes the user to the Explore page, which provides much information about the object and the observation and also exhibits direct-images cut-outs and, if spectra have been observed, plots of 1- and 2-dimensional spectra. Users may click on the Download button to obtain object products as FITS files.

GR data discovery may be executed in a third manner by clicking on one of the survey links in the Search and Retrieval tab. The user is then free either to enter coordinates for a search or to load a full table of all tiles in the survey -- note this may take several seconds to load. The resulting tile list permits one to browse the far-UV and near-UV jpeg images of a tile of interest and/or to download individual products for the tile. In the latter case, the user may decide whether to download only selected data products (FITS format), a designated set of "minimum recommended files" suggested by the project, all files, or only a catalog of the files. The user should be cautioned that the products downloaded will include those for all objects in the sky tile.

A fourth way to discover data is to search on the public GI program data by clicking on the "Search and Retrieval" tab in the left gutter. Select "Search Tile Catalogs," then "G II." This request mode operates similarly as for those of the GR surveys. Note that as with the surveys, one cannot drill down to the level of individual objects for the released GI program data. One difference in this respect is that in order to download released GI data, one must download a short ftp script and then execute it on one's computer. Note that GI data are expected to be subsumed into the GR release that occurs subsequently to them becoming public.

In November 2007, the GALEX Map (, a sky browsing and data download tool for GALEX data, was made public. The tool, which runs on many browser/platform combinations, presents the distribution of sky "tiles" on the projected sky, with the tiles sorted by survey. The user has the option of displaying tiles from one or more surveys. The user navigates around the projected sky to an area of interest. Switching from "tiles" to "images" brings up the images. Clicking on an image brings up a standard GALEX results page from which the data may be further explored or retrieved. See the on-line help for GALEX Map ( for more information.

4.3 GALEX file naming conventions

GALEX file names contain a prefix, a suffix and an extension (e.g. AISCHV2_153_29114_0002_sv11-xd-mcat.fits or MISDR1_29079_0383_002-xd-int.fits). The prefix contains alphanumeric characters of unspecified length, but contains no hyphens or periods (e.g. “AISCHV2_153_29114_0002_sv11” and "MISDR1_29079_0383_002" above). The first 3 characters of the prefix generally map to the survey (see Table 4.2). Filenames for all-sky survey (AIS) data will have “_sv##” at the end of the prefix, where the ## gives the number of the sub-visit.

The filename suffix contains delimited file type descriptor strings (e.g. "-xd-" in both cases above). The filename suffix may begin with a band/mode substring:

  • -nd- NUV direct (imaging)
  • -fd- FUV direct (imaging)
  • -xd- Merged direct (imaging)
  • -ng- NUV grism
  • -fg- FUV grism
  • -xg- Merged grism
  • -no- NUV opaque
  • -fo- FUV opaque

The extension is usually fits or txt (ASCII), although other extensions are allowed for specialized files.

Table 4.2: GALEX Survey Names



AIS All-sky Imaging Survey
DIS Deep Imaging Survey
DSS Deep Spectroscopy Survey
ETS Engineering ...
GII GI data
MIS Medium Imaging Survey
MSS Medium Spectroscopy Survey
NGS Nearby Galaxy Survey
WSS Wide Spectroscopic Survey

4.4 GALEX Object Nomenclature

The GALEX Project generates a 19-digit GALEX "object identifier," which is not unique for a given object. For example, the object may be named twice if it lies within an overlap region of two sky tiles or has been observed in two visits. Since the GALEX pipeline software does not have a unique name for a given extracted object, it cannot associate it with a stable name over multiple data deliveries. The names MAST denotes are taken from the RA/Dec position from the centroid position in the current data release and are truncated to 0.1 second in RA and to the second in DEC. Thus, there is a finite probability that the coordinates within a given object's "name" will change minutely for a subsequent delivery. Final names will be given to GALEX objects when a final catalog of object positions is generated after the end of the mission. In the meantime, MAST recommends that authors utilize its posted names in publications for which a GALEX name is needed.

4.5 Text descriptions of GALEX file types

The array of files generated by the GALEX pipeline can be baffling to the uninitiated and even experienced users. MAST has ingested all files delivered to it by the GALEX pipeline processing software. Most files have been given "one line" definitions, and occasionally more developed descriptions. Several files have been ingested that have no meaning other than to the pipeline processing staff at Caltech. The complete list of files is given in TABLE 1 of the Documentation/MAST/General tab of the left menu of our GALEX web pages. A subset of these files is listed in Table 4.3. In Table 4.3, n=NUV, f=FUV, d=direct, g=grism and x = merged. The syntax [f/d] means that either character will appear in the filename.

GALEX images are already fully calibrated in c.g.s units and do not need bias/background/flatfield corrections. Only certain raw files are expressed in counts.

Table 4.3: Selected GALEX File Types

Filename Suffix

File Type


-[n/f][d/g]-int.fits FITS image Intensity map (J2000). GALEX FOV image in counts per sec. per pixel, corrected for relative response.
-[n/f]g-gsp.fits FITS binary table Spectral data for each extracted source vs. wavelength; data include FUV and NUV bands.
-[n/f]g-xsp.fits FITS binary table "Raw" spectral data for extracted sources; units are photons/pixel vs offset arcsec, where offset is relative to undispersed image of the source along the dispersion axis.
-[n/f]g-gsax.fits FITS binary table Spectral extraction parameter info for sources; contains parameters used for the spectral extraction and a source table used for the extraction.
-xd-mcat.fits FITS binary table Merged source catalog. Band-merged table of extracted objects. Contains all objects contained in "[n or f]d-cat.fits" matched to the best candidate. The table contains positions, flux, magnitude, and major/minor axes. By definition, it contains all data from the two single-band catalogs.
-[f/n]d-skybg.fits FITS image Sky background image (J2000); photons per pixel per second estimate of the background. This background is subtracted from the intensity image which is then used for source detection.
-[f/n]d-flags.fits FITS image Artifact Flag image (J2000). Identifies predetermined regions which may introduce systematic errors in data extraction. These artifact regions are caused mainly by reflections of nearby bright stars.
-qa.txt ASCII text Manual QA data; includes PASS/FAIL and Coadd status.
-sumstats.txt ASCII text A collection of statistics from various pipeline programs.

Additional (though often obsolete) descriptions can be found on the Caltech GI office at Goddard GR1 Pipeline Data Guide page. In cases of apparent contradiction between these two sources, the MAST site should be preferred as it contains information from the Project team updated for the GR2/GR3 releases. To find the most complete descriptions of GALEX files, consult the SQL query page and type the line

           select * from fileDescriptions

into the SQL text box, and click "Execute." The results page will list the full descriptions for all files.

MAST provides a link from the short descriptions displayed on the Files Available page to these fuller descriptions. In addition, the page contains a menu box, which allows the user to select from a set of pre-defined options for file downloads. These include "Minimum Recommended Products," "Imaging Only," "Catalog Only" and "All Products." There are also a pair of radio buttons, which allow the user to request the data as a tar file or via an ftp script.

4.6 Determining header (keyword) definitions

The columns in the database that the user sees on the Files Available for download page are taken from the FITS header keywords of the datafiles. These keywords are redefined as column names in the Database Info/Tables pages (sometimes with names to make them more intuitive), and often in more than one Table. The definition of these columns can be obtained either by looking in the FITS keyword headers in the datafiles or by entering an appropriate "select" query in the SQL Search Form, and clicking "Execute." This will return a list of Tables containing the column name of interest. For example, to search on Tables containing the column "RA", type the query

 select tablename,name,unit,description from dbcolumns 
     where name like 'ra' 
     order by tablename,name .

The 'ra' field may be wildcarded with a % symbol within the single quotes ('ra%'). To obtain the field's description the user can then note the Table names returned, navigate to the left menu tab under Documentation/Tables, and click on any of the Tables noted.

All comments concerning GALEX data should be sent to MAST ( or the GI office ( and not to the Caltech project office directly. Questions that MAST can not or should not answer will be directed to the Project or GI office, as appropriate. Also, users should be aware that the site has some browser/platform incompatibilities. For example, at the moment one cannot download data in VOTable format using the Internet Explorer browser.

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