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Contents 1 Chapter 3: Getting Started with GALEX Data 1.1 Ways to Access GALEX Data 1.2 Guide to GALEX Imaging Measurements 1.3 Saturation 1.4 Using GALEX Photometric Flags

Chapter 3: Getting Started with GALEX Data

This chapter serves as a higher level introduction as to how to get started accessing and using GALEX images and catalogs. Please refer to the remaining chapters for more detailed information about many of these topics.

Ways to Access GALEX Data

Public access to GALEX images and source catalogs is provided through the Milkulski Archive for Space Telescopes (MAST). There are several tools available from MAST that can be used to find and download GALEX data.

• GalexView: GalexView is an interactive graphical tool that allows the user to search for any detections in the GALEX catalogs within a specified radius around a position on the sky. A single position can be entered or a list of positions can be uploaded. Alternatively, it is possible to enter the name of a previously cataloged source (e.g. M101) and GalexView will resolve the name using various on-line services. GalexView can also be used to search for all GALEX tiles within a specified radius of a sky position as well. Both the image at that location as well as any cataloged GALEX sources can be overplotted. GalexView can be accessed here: http://galex.stsci.edu/GalexView/. This is the easiest way to search for GALEX data given a relatively small number of sky positions.

• Form-based search: There is also a form-based search that has most of the same capabilities as GalexView but without the nice graphical interface. However, it can allow a bit more flexibility in the columns output and the constraints on the search. The search form can be found here: http://galex.stsci.edu/GR6/?page=mastform.

• SQL Query: It is also possible to search the GALEX database directly using an SQL query here: http://galex.stsci.edu/GR6/?page=sqlform. The most common table to search is the PhotoObjAll table. This contains all GALEX detections in any co-added tile and includes all of the columns present in the GALEX "mcat" table for each tile. (See Appendix A.1 for a full list of the columns in the mcat files). Information about the tiles in which the PhotoObjAll sources were detected can be found in the PhotoExtract table. The two tables can be linked using the PhotoExtractID column in both tables. There are also corresponding tables that contain sources detected in every individual GALEX observation (called a "visit"). The source catalog is saved in the VisitPhotoObjAll table while the lists of GALEX observations are given in the VisitPhotoExtract table. Please see the MAST web site for more details about the data scheme of the database. This is probably the best way to search through the GALEX catalogs for sources matching a given set of selection criteria. However, there are limitations on the time allowed for a query to finish as well as on the number of rows output.

• CasJobs: For larger or more computationally intensive queries, it is better to access that GALEX catalogs using the CasJobs site which can be found here: http://galex.stsci.edu/casjobs/. This service has fewer limits on the length of the resulting output tables as well as the time it takes to complete a query.

• GALEX Source Catalogs: We have generated different versions of the GALEX catalogs that will likely be more useful for most users. These are the GALEX All-Sky Survey Source Catalog (GASC) and GALEX Medium Survey Source Catalog (GMSC) based upon the GR6 data release. These catalogs provide vetted and unique measurements of point and extended sources up to 1 arcminute diameter. The main advantages of using these catalogs over the standard GALEX data products are that they exclude duplicate observations of the same sources, include a S/N cut that reduces the number of spurious sources, and also have accompanying files describing the footprint of the observations. Our intention is that the catalogs serve as the official GALEX reference for cross matching to other surveys such as WISE, SDSS, and 2MASS. The GASC and GMSC are defined by NUV detections, which typically have significantly higher S/N than the FUV. The GASC and GMSC report FUV fluxes within NUV defined apertures for those regions with FUV observations. Covering a total of 26,300 square degrees of sky, the GASC consists of all GALEX observations with exposure times below 800 sec and reaches a depth of NUV 21 (AB mag). The GMSC covers a smaller region of 5000 square degrees with exposure times between 800 and 10,000 sec and reaches a depth of NUV 23 mag. There are a total of 40 million unique sources in the GASC and 22 million in the GMSC. Each catalog is accompanied by exposure time, coverage and flag maps in FITS and Healpix formats. We have also quantified the completeness, reliability, and astrometric uncertainties of both catalogs. Users will be able to access the catalogs through the Multi-mission Archive at the Space Telescope Science Institute (MAST) and eventually through the NASA Extragalactic Database. The images and footprint files will be made available from MAST as well.

Guide to GALEX Imaging Measurements

There are many measurements available for every GALEX source. While the entire list is given in Appendix A, here are some of the columns that we expect will be most useful to users of the data:

• ggoid: A 64-bit integer that is an unique identifier for each GALEX source.

• alpha_j2000, delta_j2000: Right Ascension and Declination for the source in decimal degrees.

• e_bv: Reddening E(B-V) in magnitudes at the location of the source from Schlegel at el. (1998, ApJ, 500, 525).

• fuv_mag, nuv_mag: Total calibrated AB magnitudes measured in an elliptical "Kron" aperture. (See discussion below).

• fuv_magerr, nuv_magerr: One-sigma statistical uncertainty in fuv_mag and nuv_mag.

• fuv_weight, nuv_weight: The effective exposure time at the location of the source in seconds. The effective exposure time is defined as the exposure time multiplied by the flat field.

• fuv_artifact, nuv_artifact: The bitwise "OR" of all of the flags set in the image at the location of the source.

• fuv_skybg, nuv_skybg: The local sky background in units of photons per square arcsec that was subtracted at the location of the source.

• FUV_FWHM_WORLD, NUV_FWHM_WORLD: Full width at half maximum of the source in degrees.

All of the GALEX fluxes are either given in units of micro-Janskys or counts/sec as indicated in Appendix A. Note that 1 micro Jansky is equivalent to 10^-23 ergs/s/cm^2/Hz in cgs units. All of the calibrated GALEX magnitudes are expressed in the AB system as defined by equation (7) of Fukugita et al. (1996, AJ, 111, 1748):

where nu is the photon frequency in Hz, f_nu is the source spectrum in units of ergs/s/cm^2/Hz, and the S_nu is the instrument response as a function of frequency.

The GALEX pipeline makes use of the SExtractor program (Bertin & Arnouts 1996, A&AS, 117, 393) to detect sources and obtain measurements of them. All of the flux and magnitude columns in the mcat that have names in all capital letters are taken directly from the SExtractor output. These are not calibrated. These fluxes are in counts/sec and the magnitudes are instrumental: -2.5 * log( cts/sec). To convert from instrumental to AB magnitudes, simply add 20.08 in the NUV and 18.82 in the FUV. There are a few different choices for the total magnitude:

• The magnitudes listed in the database as fuv_mag and nuv_mag are derived from the "AUTO" measurements made by the SExtractor program. These are most useful for measuring the total flux of a resolved galaxy. These total fluxes are measured within an elliptical aperture scaled to 2.5 times the first moment of an object's radial profile, an algorithm first suggested by Kron (1980, ApJS, 43, 305). The elliptical aperture used in the NUV has semi-major axis in degrees given by NUV_KRON_RADIUS * NUV_A_WORLD, major to minor axis ratio given by NUV_ELONGATION, and position angle in degrees east of north given by NUV_THETA_J2000. There are corresponding quantities for the FUV.

• The magnitudes listed in the as NUV_MAG_APER_X, where X ranges from one to seven, refer to the total NUV flux within a fixed circular aperture. The diameters of these circles are 2, 3, 5, 8, 12, 17, and 23 pixels. (One pixel is 1.5 arcsec). These circular apertures would be most useful for measuring the total flux of an unresolved source. However, it is important to include an "aperture correction" to account for light lost outside of the aperture. A list of aperture corrections as a function of radius can be found in Morrissey et al. (2007, ApJS, 173, 682). There are the same set of measurements for the FUV.

• There are a series of columns beginning with fuv_ncat. These columns are the fluxes measured on the FUV image but using the apertures and detections from the NUV image. There are corresponding columns labeled with nuv_fcat which are the reverse: NUV fluxes measured within the apertures defined from the FUV image. Using these quantities will lead to the most accurate FUV-NUV color because the fluxes in the two bands are measured over the exact same region of sky.

Saturation

Point sources brighter than about NUV=15 tend to have their flux underestimated in the GALEX catalogs. The roll-off in the flux reaches about 10% loss by NUV=14. This saturation not only affects the total flux but also tends to broaden the image of these sources with the very brightest sources having a depression in their center. The saturation is independent of the exposure time but will also vary with position on the detector. The GALEX pipeline does not correct for saturation. See Morrissey et al. (2007, ApJS, 173, 682) for more details.

Using GALEX Photometric Flags

The GALEX catalogs contain a number of flags indicating regions of the images likely contaminated by image artifacts or where image artifacts were corrected. For most of these flagged regions, there is likely nothing wrong with the images or detected sources. For most purposes, we recommend excluding any sources with the following flags set: 2 (NUV bright star window reflection), 4 (dichroic reflection), 32 (detector run proximity), and 512 (bright star ghost). The last flag for the bright star ghost is only relevant for data taken after May 2010 and will not appear in earlier observations. The flags set for a particular source are given by the nuv_artifact and fuv_artifact columns. To determine whether a particular flag was set or not, take the bitwise "and" between the artifact column and the value of the flag. For example, in the python language the expression (nuv_artifact & 4) will return a value of 4 if the dichroic reflection flag is set or 0 if not.