Galaxy Evolution Explorer
+
Galaxy And Mass Assembly

PEOPLE:

The individuals directly involved with the acquisition of GALEX data and/or construction of the GALEX-GAMA catalogs:

NAME INSTITUTION CONTACT
Richard Tuffs (PI) Max-Planck-Institut für Kernphysik Richard.Tuffs@mpi-hd.mpg.de
Cristina Popescu (PI) University of Central Lancashire cpopescu@uclan.ac.uk
Ellen Andrae Max-Planck-Institut für Kernphysik Ellen.Andrae@mpi-hd.mpg.de
Mark Seibert Carnegie Institution for Science mseibert@obs.carnegiescience.edu
Meiert Grootes Max-Planck-Institut für Kernphysik Meiert.Grootes@mpi-hd.mpg.de
Barry Madore Carnegie Institution for Science
Dinuka Wijesinghe University of Sydney
Simon Driver University of St. Andrews
Jochen Liske European Southern Observatory
Ivan Baldry Liverpool John Moores University
Andrew Hopkins University of Sydney
Steve Eales Cardiff University
Loretta Dunne University of Nottingham
Jon Loveday University of Sussex
Aaron Robotham University of St. Andrews

If you have questions or comments please direct them to Ellen Andrae or Mark Seibert.

GENERAL DESCRIPTION:

This web page details the construction of version 2 of the GALEX ultraviolet source catalogs and auxiliary products overlapping the GAMA survey footprint. The GAMA footprint definitions used for this project are defined as three primary regions (GAMA-1) and possible future extensions to the survey (GAMA-2).

GAMA-1*
FIELD
id
RA
min
RA
max
DEC
min
DEC
max
Sqr.
Deg.
G09129.0141.0-1.0+3.048.0
G12174.0186.0-2.0+2.048.0
G15211.5223.5-2.0+2.048.0
*The primary regions were originally only 36 deg2
  (3o x 12o) each.
   
GAMA-2 Extended
FIELD
id
RA
min
RA
max
DEC
min
DEC
max
Sqr.
Deg.
G09129.0141.0-3.03.072.0
G12174.0186.0-3.03.072.0
G15211.5223.5-3.03.072.0
G0229.85 44.15 -36.0-30.0 85.8
G23331.9 346.2 -36.0 -30.085.8

The GALEX ultraviolet data is a combination of archival data and pointed observations from the GALEX guest investigator program GALEX-GAMA: UV/Optical/Near-IR/Far-IR/Radio observations of ~100k galaxies (GI5-048; PIs: R. Tuffs, C. Popescu, US-PI: M. Seibert). Note that the GI program was designed and proposed to cover only the original GAMA survey area (i.e. three regions of 36 square degrees each) as shown in figure 2 of the GALEX proposal.

Archival data has been used to extend the ultraviolet coverage to the current GAMA-1 and future GAMA-2 footprint definitions as much as possible.

GALEX DATA:

All GI and archival data overlapping the GAMA-2 footprint and available as of March 30, 2010 were compiled for the project. This consists of 347 GALEX tiles (circular fields 1 degree wide). If interested one may view a list of the GALEX tile names, centers, and exposure times used as input for the G09, G12, G15, G02, and G23 fields.

All GALEX data have been reprocessed at Caltech with a new version of the pipeline (v7ops). There are many improvements and changes with v7ops that will cause differences with version 1 of the GALEX-GAMA catalogs. The most significant are:

  • Matched Apertures: The GALEX-GAMA catalogs are now NUV centric catalogs. The S/N of the NUV detector is significantly higher than the FUV detector. We have taken advantage of this by reporting FUV fluxes within apertures matched to the NUV detections. This has the advantage of improving the reliability and completeness of the FUV detections and providing meaningful UV colors.

  • Calibration: The major changes to the GALEX calibration include updates to the flat fields, flux response trend with time, and a magnitude zero-point correction (+0.043 magnitudes fainter in NUV and +0.033 magnitudes fainter in FUV). Further information can be found on the GALEX web page or detailed in GI_Doc_Ops7.pdf. Note that due to the time trend and flat field changes one can not simply apply fixed zero-point corrections to v6 pipeline (GALEX-GAMA version 1) data. Examples of the calibration difference between the v6 and v7 pipelines for the G09, G12, and G15 fields are shown below with delta magnitude versus magnitude plots.


  • Deblending/Shredding: The deblending threshold parameter has been modified to improve the shredding of extended sources. Prior to v7 the pipeline was tuned for point sources. With v7 a better compromise has been reached that reduces the shredding for extended objects less than 1 arc-minute in diameter.

  • Astrometry: Not a new issue but useful information. The GALEX PSF is ~5 arc-seconds and the astrometry is only accurate to within ~0.3 arc-seconds. Below is a density histogram of the offsets between the GAMA positions and and GALEX positions after cross-matching the Blind GALEX and Master GAMA catalogs. The offsets are not systematic and vary from GALEX tile to tile. The principle cause of the astrometric offsets is the limited precision of astrometric catalogs used to derive plate solutions within the GALEX pipeline.

A final general note on GALEX data. Version 2 of the GALEX-GAMA catalogs are created using the methods and techniques of the GALEX CATalog Team (GCATT). This is a large project underway to produce a definitive GALEX source catalog. This means that many of the column names and units change in version 2. This will be detailed further.

UNIQUE AREA (Primary vs Secondary):

GALEX tiles overlap one another. This, of course, is inevitable when trying to cover a contiguous area with circular fields of view. Furthermore, because we include archival survey data which is observed on a sky grid system and pointed observations (which are not on the sky grid system) the overlap is often very hap-hazard. There are several ways to handle this such that we include only unique detections and allow a precise footprint computation. We use a method that optimizes the areal coverage and weights tiles with FUV coverage highest.

For each GAMA region the GALEX tiles are rank-sorted by FUV exposure time and secondary rank-sorted by NUV exposure time. This ensures that any tile with FUV coverage takes precedence over any tile with only NUV coverage (note that the FUV detector on GALEX is non-operational at this time).

Once sorted, the footprint of each tile is derived from the relative response map (effective exposure time map) by comparing it to all neighboring tiles with a higher rank-order. In this way a "primary map" is made for each tile indicating which parts of the field of view are to be considered "primary" and which parts are to be considered "secondary".

The figure below displays an example FUV (top left) and NUV (top right) primary map for a single tile. Red represents the primary area for the tile. The lower two panels are the corresponding relative response (effective exposure time) for each band.

MAPS:

Five types of mosaiced maps have been produced for each of the GAMA fields using HEALPix to aid in the analysis of the GALEX-GAMA data set. Specifically, these are:

  • Effective Exposure Time: Relative response (seconds). Source = TILE_[f/n]d-rrhr images.

  • Coverage Fraction: Indicates what fraction of the area of each HEALPix pixel has GALEX coverage.

  • Background: Count rate subtracted from intensity maps prior to photometry. Source = TILE_[f/n]d-skybg images.

  • Flags: Pipeline artifact flags (description). Source = TILE_[f/n]d-flags images.

  • MANFLAGS:manual flagging of artifacts.
  • Milky Way Foreground Reddening: Schlegel, Finkbeiner & Davis color excess E(B-V).

Because GALEX coverage is neither uniform in exposure time or fully contiguous over the GAMA fields, it is important to be able to derive an accurate footprint for any sample selections driven by the above quantities.

Each UV based map is built by ingesting the primary designated region of each tile's corresponding map into HEALPix format. The HEALPix maps are built at ~13 arc-second resolution (nested, nside = 214 = 16384). These maps allow the calculation of footprint area to better than 0.1%.

Additionally, we have used HEALPix format maps of the Schlegel, Finkbeiner & Davis (1998) Milky Way foreground reddening maps from the LAMBDA project archive to extract color excess (E(B-V)) maps for each GAMA field rebinned at the same pixel scale as the other UV maps (~13 arc-sec pixels, the resolution of course remains only ~6 arc-min).

For those who do not wish to use HEALPix format maps, Gnomonic projection standard fits images (produced from the HEALPix maps) have also been produced. The standard fits images are slightly lower resolution (15 arc-sec).

Images of the maps for each GAMA field are displayed below. Click on the images to view them at high resolution. The GAMA-2 footprint boundaries are indicated by dashed lines and the H-Atlas boundaries are indicated by dotted lines.

FILTERING:

The following filters are applied to the GALEX pipeline produced MCAT (band merged catalog) file for each tile.

  • Signal to Noise: The catalogs are cut at a S/N of 2.5 in NUV. The FUV flux (within the NUV aperture) is always carried along regardless of the S/N. Note that FUV only sources are not possible in this catalog as a NUV detection is required. As these catalogs will be matched to the optical priors from GAMA, the low S/N sources have a slightly higher probability of being true detections. End users may make a higher S/N cuts if desired.

  • Primary: Source centroid positions must fall with the "primary" designated area for a given tile. These sources are for the "primary" catalog(s).

  • Secondary: If a source lies outside the "primary" area of a given tile it is deemed "secondary" and added to the "secondary" catalog(s). Note that the secondary catalog sources may include many overlaps (tertiary etc.) of the same source.

BLIND CATALOG COLUMNS:

The catalog column names, units, as well as a description of how they were derived from the standard GALEX MCAT files can be viewed as an html table, text or csv file.

SIMPLE MATCHING:

Simple nearest neighbor matching within 4 arc-seconds has been performed between the GALEX blind catalogs and the GAMA Master Catalog (catmast_v2.fits). The resulting catalog (glx_pricat_v2c-gama_catmast_v2.fits.gz) corresponds as one-to-one with the the catmast_v2 catalog.

The subset of this master catalog that forms the the galaxies catalog (catgama_v4) have been extracted and the resulting catalog (glx_pricat_v2c-gama_catgama_v4.fits.gz) corresponds as one-to-one with the catgama_v4 catalog.

In addition to all the columns found in the blind UV catalogs a number of additional matching related columns have been added.

  • NUVCVG = NUV effective exposure time at source position (for all entries)
  • FUVCVG = FUV effective exposure time at source position (for all entries)
  • NUVMATCH = 1 if NUV source with S/N >=2.5 within 4 arc-sec (0 otherwise)
  • FUVMATCH = 1 if FUV source with S/N >=2.5 within 4 arc-sec (0 otherwise)
  • NMATCH4 = number of matches found within 4 arc-seconds.
  • MANY2ONE = number of GAMA sources an object is matched to.
  • GNEIGHBOR_DIST = array of distances to all GALEX sources within 20 arc-seconds.
  • GNEIGHBOR_NAME = array of names of all GALEX sources within 20 arc-seconds.

ADVANCED MATCHING:

In order to match the UV blind catalog to the optical GAMA catalogues, we employ an advanced matching routine that takes into account multiple matches for each UV and optical source and seeks to reconstruct the original UV flux of a given optical source.

We illustrate the routine for an example optical target (e.g. a GAMA galaxy, see figure). Firstly, shape information of the target in the optical (i.e. ellipticity, position angle, and effective radius along the semimajor axis) is extracted from the GAMA master catalogue, and used to define a target area within which UV counterparts will be deemed to be associated with the optical source. To take into account scatter in apparent positions from real positions in the UV and possibly unavailable optical shape information, the minimal search region, i.e. optical target area, is a circle of radius 4" (the matching radius used in the simple matching procedure, see above). The question that is then to be answered (see flowchart) is which UV sources in the blind catalogue have their central coordinates within this optically defined search region. The number, the UV ID and the total flux of all such UV sources are extracted from the blind UV catalogue and written (in case needed in user applications), to the final matched catalogue.

Since it is possible that a UV source may itself be extended, or have more than one potential close-lying optical counterpart in the GAMA master catalogue, we also need to take into account the possibility that one or more of the UV sources in the search region around the optical target may themselves be related to further optical sources. This eventuality is catered for in the second step of the advanced matching procedure (see flowchart). In this step, a UV area is constructed for each UV source within the optical search region, using the position angle, ellipticity and semimajor axis as given in the UV blind catalogue (Sextractor's Kron-radius). The GAMA MasterCatalogue (which includes both galaxies and stars) in then searched for optical sources within the UV area of each of the UV sources. In the case there is only one UV source in the optical search region which has only one optical source within its UV radius, and this optical source is the target, the whole flux of the UV source is associated with the target. This is by far the most common case. If, however, there are more than one potential optical counterparts to one or more of the UV sources in the optical search region, the UV flux of each UV source is split among all potential optical counterparts for that source, weighted inversely by angular offset. In recognition that this may not be the true flux contribution, and users may for specific applications like to apply other methods (eg using prior astrophysical information) we have included a string listing the IDs of all optical and all UV sources involved in the advanced matching process for each target. This redistribution of the flux is done for all UV counterparts of the target. The match is considered as unambiguous if there is only one UV counterpart which has only the target as its optical counterpart. In all other cases the reported UV flux is to be considered as an estimate of the true flux distribution, as the true flux distribution of the optical counterparts is not known. This estimate is the most likely one considered over a population of galaxies but is necessarily imprecise for targets considered individually.

Catalogue entries (details see tag description below):

  • NUV/FUV "flux best" of the target -our recommendation of the best flux. For some highly ambiguous matches, and for all large galaxies (r-band Sersic derived effective radius > 10" ) a curve-of-growth (COG; see below) flux is used for the "flux best". The COG integrations are effected over elliptical rings constrained to optically determined values of position angle and axis ratio, and performed to an automatically determined asymptote from the GALEX images, after masking unrelated sources like foreground stars. The use of COG fluxes is advisable for highly resolved galaxies, whose typical flocculent appearence in the UV can lead to flux shredding by automatic shape fitting routines.
  • total NUV/FUV flux of inside the optical search region
  • redistributed NUV/FUV flux associated with the target
  • integer number of UV matches and number of optical sources involved in the matching for the target
  • string-list of each of the following sets of IDs: GALEX pipeline IDs of the associated UV sources and GAMA master catalogue IDs of all optical sources potentially related to these UV sources (i.e. Unambiguous = only one ID in each list)
  • The full UV blind catalogue entry will be given for unambiguous matches. In cases where there is more than one UV source within the target area, the full UV blind catalogue entry for the UV source closest to the target is given.

    COMPARING SIMPLE VS ADVANCED MATCHING

    These color magnitude diagrams (NUV-r vs r, contour level = [0.02,0.04,0.08,0.16,0.32] per cent of the according subsample) for GAMA-galaxies (as defined in TilingCatv20) illustrate the advantage of the redistributed flux of the advanced match method compared to the simple match flux. The color magnitude diagram for unique matches (i.e. no redistribution of the flux necessary since it is a one-on-one match) is shown on the left. This subset is considered to represent the true color magnitude distribution, as there are no potentially contaminating sources involved in the matching process. For ambiguous matches (i.e. non one-on-one match), the central image shows the color magnitude diagram using the NUV simple match flux, while the right image uses the NUV advanced match flux. The outer contour of the unique match color magnitude diagram is overlaid in red for comparison. It is easily seen that the simple match color magnitude diagram is biased towards the blue by about ~0.2mag while the advanced match color magnitude distribution is roughly identical to the one for unique matches.

    CURVE-OF-GROWTH:

    The right image illustrates how the curve-of-growths method derives the flux of a galaxy for the example of the GAMA-galaxy CATAID 601395. The map shows a contour plot of the masked data within the UV radius of the galaxy and the unmasked data outside of the galaxy (see next paragraph for detail). The actual curve of growth plots the sum of the enclosed flux as a function of radius. The leftt image (obtained with the "SDSS DR7 Finding Chart Tool" on the SDSS web pages) shows the optical SDSS data for the same region of sky (roughly same scaling) for comparison.

    For a given galaxy (listed in the galaxy catalogue v4, displayed as green circle in the right image), the curve-of-growths algorithm imports prior knowledge from the r-band about the optical position, and if available about position angle and ellipticity (as listed in the galaxy catalogue v4). Furthermore, the positions of nearby objects are extracted from the MasterCatv2 (displayed as blue stars for nearby stars, and as red triangle for nearby galaxies). These nearby objects are masked with a circular aperture with radius=1.1*FWHM_of_NUV_beam=1.1*5.3arcsec (masked area shown as dashed-dotted "circles" on the right image). In order to measure the flux of this given galaxy, the COG algorithm sums up the flux within elliptical annuli of 3arcsec width centred of the r-band position of the galaxy, interpolating for masked regions. The UV size of the galaxy (right image: dashed circle in the COG map and right vertical line on the COG plot) is defined by the annulus with mean pixel flux less equal the background flux (estimated from unmasked pixel of the region around the galaxy). Since the annuli are binned and since this definition of the radius is very sensitive to minor flux in the outer regions, we also report the half-light radius (right image: left vertical line on the COG plot). (Note: The star below and to the right of the galaxy is just on the edge of the UV size of the galaxy. The part inside the galaxy is masked and the interpolation is shown on the map, while the original data is shown for the part outside of the galaxy - demonstrating nicely the effect and necessity of the masking.)

    This algorithm is done separately for the NUV and FUV data, using the same mask for confusion objects. The error of the flux measurement is the propagated error resulting from the background noise per pixel. This error ignores the possibility of a signal dependent noise component in the measurement.

    Very bright neighouring sources (e.g. bright stars) may still have a significant UV flux outside of the masked area, leading to a bias in the flux and size estimate. As stars are generally less bright in the FUV than in the NUV, chances are that the masking is sufficent in the FUV even though it might not be in the NUV. Therefore, comparing the galaxy size estimates of NUV and FUV, it is possible to identify (the majority of) cases where a bright star has been misinterpreted as part of the galaxy in the NUV, leading to a much larger galaxy radius in the NUV than in the FUV (criterion: 30 arcsec difference). Since the flux measurement is most likely contaminated in such a case, the default value -99 for "no measurement" is reported for all COG values in the catalogue.

    The background noise is a poisson noise with verly low mean, resulting in many pixel with zero photon counts. As the current version of the background estimate does not import UV coverage information, it does not distinguish between "not observed" and "observed, zero photon counts". In order to garantee a stabe background estimate, we limit the allowed number of pixel with value zero (due to eigher of the two reasons) to a maximum of 40% in the NUV and 88% in the FUV, otherwise the default value -99 for "no measurement" is reported for all COG values in the catalogue. This is a very conservative aproach, as it may reject observed galaxies on the edge of the UV tiles. In the future, this will be avoided by incorporating exposure time maps.

    WHEN TO USE COG:

    The advantage of the COG method for crowded regions of the sky is obvious as it masks potentially contaminating neighbouring objects (rather than redistributing the UV flux among several optical, potentially non-UV sources as done by the advanced matching). For isolated, unresolved objects, the flux estimates of the GALEX pipeline using beam convolved shape fits are expected to be more precise. The plots below motivate the the criteria (r-band effective radius larther than 10 arcsec or non one-on-one matches) we defined for when to use COG flux rather than redistributed flux.

    The left plot shows the ratio of COG flux and redistributed flux as a function of the r-band effective radius, with the error bars indicating the inner quartile for each bin. Color coding: one-on-one matches: black, one optical and two UV sources or two optical and one UV source involved in the advanced matching("low multiplicity matches": red, many sources involved in the advanced matching ("high multiplicity matches"): blue. The COG flux is slightly fainter than the redistributed flux for one-on-one matches. For low multiplicity matches, the dominating case is two optical sources and one UV sorce involved in the match. As not all optical sources are actually UV bright, the redistributed flux of galaxies (on average larger structures that get flux taken away) is slightly biased low - counterbalacing the slight underestimate of the COG flux. For large sources (r-band effective radius larger than 10 arcsec), the redistributed flux of the low multiplicity matches is increasingly biased low; clearly indicating the superiority of the COG estimate for large sources. The high multiplicity matches are affected even stronger by this bias.
    As the redistributed flux is identical with the simple match GALEX pipeline flux for one-on-one matches, the advanced matching method is to be favored for

    The left plot shows the ratio of COG flux and redistributed flux as a function of the NUV flux (same color coding).

    COMPARING SIMPLE MATCHING VS COG:

    OBSERVING CAMPAIGN GAMA-II/H-ATLAS/DINGO FIELDS:

    The latest definitions of fields for GAMA phase I and phase II, and for H-ATLAS: equatorial fields, sgp, and ngp

    Mark's visualisation:

    An overview (i.e. pictures) over the GALEX-GAMA products for each of the three survey depth here: gsc, gmc, and gdc.

    DOWNLOADING THE DATA:

    All catalogs and maps are password protected at this time. Please contact Ellen Andrae if you need the username and password.

    Newest products are available for each of the three survey depth here: gsc, gmc, and gdc.

    The older version, i.e. GALEX-GAMA products untill 2012 are available here.