GALEX Release 1 (GR1) basic data description

 

1.         Overview.. 1

2.         Early Release Description. 1

3.         Early Release Download Data. 3

4.         Filenaming Convention. 3

5.         Quick Look (Basic) Imaging File Description. 4

6.         Quick Look (Basic) Spectroscopy File Description. 5

 

 

1.    Overview

The GALEX data release 1 (GR1) contains images, spectra, and catalogs generated by the GALEX data pipeline in Nov-Dec 2004. These data products make up the first major public data release to the astronomical community and are representative of what will be provided to Guest Investigators. This document describes the release data at a basic level. Other documents on this site provide additional information regarding data collection modes, performance parameters, pipeline performance, and detailed data descriptions.

 

2.    GALEX Release 1 (GR1) Description

The GALEX surveys are on-going. As of December 2004, the GALEX data analysis pipeline had reduced observations covering about 8000 square degrees of sky with an average exposure time of about 800 sec. The typical exposure time varies greatly from 125 sec for the All-sky Imaging Survey (AIS) to 20 ksec for the Deep Imaging Survey (DIS).

 

Table 1) Survey coverages and depths for the data released in GR1

Survey

abbreviation

Survey name

Coverage

(sq. degrees)

Average depth

(sec)

NGS

Nearby Galaxy Survey

48

2235

AIS

All-sky Imaging Survey

2792

121

MIS

Medium Imaging Survey

129

1883

DIS

Deep Imaging Survey

18

15371

WSS

Wide Spectroscopic Survey

6.0

13064

MSS

Medium Spectroscopic Survey

2.5

20831

 

           

Figure 1 below shows the location of the GR1 targets. Targets observed at different Galactic latitudes with varying infrared cirrus levels (100 mm) have a wide range of extinction values and UV background levels (see Figure 2). The median cirrus level over the whole sky is ~ 3.0 MJy/sr. GALEX deep imaging and spectroscopy fields are generally selected to have I100 < 1.0 MJy/sr. Currently the all-sky survey is limited to fields with cirrus levels below 3.0 MJy/sr.

 

Figure 1) Aitoff projection maps (Galactic coordinates) of sky coverage for the GALEX survey data released in GR1. The first panel displays all surveys. Subsequent panels display NGS, AIS, MIS, DIS, and spectroscopic (WSS and MSS), respectively.

 

 

 

 

Figure 2) Early Release Observation target locations superimposed upon Schlegel, Finkbeiner, Davis dust maps, for comparison.

 

3.    GR1 Data

GR1 may be thought of a containing two categories of release files. A “basic” set provides the user with a minimal source extraction catalog and images which can be used for an initial “quick look.” We also provide an “advanced” set which contains the full complement of pipeline outputs for each of these observations. In general the “advanced” data products may be thought of as the results of intermediate steps in the pipeline that are required analyses along the path leading to the “basic” data products. These advanced outputs are beyond the scope of this document and are described in more detail in the GR1 Pipeline_and_advanced_data_description. Nevertheless, many users–particularly potential Guest Investigators–may find it useful to explore the full set of pipeline outputs.

                              

The most significant files for direct imaging are the combined-visit files ending in -xd-mcat.fits, -nd-int.fits, and -fd-int.fits which provide the bandmerged source catalog and intensity images for NUV and FUV in photons per second per pixel (each is pixel is 1.5" on a side). For grism data, the most significant files are the combined-visit files ending in -xg-gsp.fits, and (to a lesser degree) the -ng-prc.fits and -fg-prc.fits files, which contain the flux calibrated (band combined) spectra and 2-dimensional spectral image strips in the NUV and FUV bands. All other files (including similarly-named files for the individual visits) are probably only useful for detailed analyses and quality checking.

 

4.    Filenaming Convention

A filename contains a basename (or prefix), a suffix and an extension (e.g. AISCHV2_153_29114_0002_sv11-xd-mcat.fits).

 

The filename basename contains alphanumeric characters (of unspecified length, but containing no hyphens or periods, e.g. “AISCHV2_153_29114_0002_sv11” above). Typically this basename is related to the observed target. Files in directories containing multi-leg observations (e.g. all-sky survey) will have “_sv##” appended to the prefix with ## indicating the number of the sub-visit of that particular leg. No other requirements have been placed on the filename prefix. 

 

The filename suffix contains delimited file type descriptor strings. In GR1 Pipeline_and_advanced_data_description, Table 3.1– Pipeline Products Summary shows the full set of files currently being generated by orbpipe. The rows in bold indicate those files that will be most useful for a first look at the GALEX data. 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

 

Pipeline files which are generated for both NUV and FUV are indicated with “[n or f]” in the pipeline products table.

 

5.    Quick Look (Basic) Imaging File Description

-[f,n]d-int.fits:

 

These files are fits images from fuv [f] or nuv [n] channel in the imaging (direct d) mode. Images are intensity maps [int] which are created from count [cnt] images [in detected counts=photons], divided by exposure [exp] images [in seconds]. Images are 3840 by 3840 pixels, pixels are 1.5”x1.5” arcseconds. 1 count in a pixel corresponds roughly to a flat-fielded 1 ct/s. These images are not background subtracted. Image is in pixels with J2000 world coordinate system (WCS). Image can be immediately viewed in sky coordinates using ds9, gaia, or other image viewing routines.

 

-mcat.fits:

 

This file is a fits binary table of matched (FUV+NUV) and unmatched (FUV-only and NUV-only) sources detected from a single image.

 

Table 2) Selected, significant columns from the bandmerged -xd-mcat.fits catalog:

Parameter

Column number

Format

Units

Description

band

11

1J

number

Band number (1=nuv,2=fuv,3=both)

id

12

1J

number

Merged ID.

alpha_j2000

15

1D

degrees

Merged Right Ascension(degrees).

delta_j2000

16

1D

degrees

Merged Declination (degrees).

E_bv

19

1E

number

E(B-V) Galactic Reddening.

nuv_flux

33

1E

flux

NUV calibrated flux (micro Jansky)

nuv_fluxerr

34

1E

flux

NUV calibrated flux error

nuv_mag

35

1E

mag

NUV calibrated magnitude

nuv_magerr

36

1E

mag

NUV calibrated magnitude error

fuv_flux

37

1E

flux

FUV calibrated flux (micro Jansky)

fuv_fluxerr

38

1E

flux

FUV calibrated flux error

fuv_mag

39

1E

mag

FUV calibrated magnitude

fuv_magerr

40

1E

mag

FUV calibrated magnitude error

nuv_artifact

71

1J

number

NUV artifact flag (logical OR near source)

fuv_artifact

72

1J

number

FUV artifact flag (logical OR near source)

 

6.    Quick Look (Basic) Spectroscopy File Description

-xg-gsp.fits file

 

The xg-gsp.fits file is the final product from the spectroscopy pipeline. From the spectral image strip files (-[n,f]g-pri.fits), uncalibrated spectra are extracted and stored in extracted spectra files (-[n,f]g-xsp.fits). These files are then combined to create the *-xg-gsp.fits file which contains the band-combined flux-calibrated extracted spectra data, in units of photons per second per centimeter squared per Angstrom vs. Angstrom, on a linear dispersion scale. The range is roughly 1300 to 3000 Angstroms. The two bands are joined at approximately 1820 Angstroms with a gap region containing no data between 1810 and 1830 Angstroms.

 

After a combined strip is built up from individual grism rotation image strips, a final merged, flux calibrated spectrum is extracted and stored in the -xg-gsp.fits file. In this example from the ELAISS1_00 WSS data, GALEX has detected a known quasar and also an X-ray source (Alexander et al (2001) object #11) at  z= 0.398. OVI, Lyman Alpha, C IV, CIII lines are seen at measured wavelengths (1445, 1700, 2165, and 2660, respectively).

 

 

 

 

The following table contains a basic description of the band-combined -xg-gsp file columns. For a more detailed description see Column_descriptions_for_-xg-gsp.fits .

 

Table 3) Extracted columns in text matched catalog:

Parameter

Col #

For-mat

Units

Description

ggoid

1

2J

number

Global ID of spectral source

ggoid_D

2

2J

number

Global ID of direct image source

band

3

1J

number

1=nuv only, 2=fuv only, 3=both

id

4

1J

number

Spectral source ID

alpha_j2000

5

1D

degrees

Right Ascension J2000 (deg)

delta_j2000

6

1D

degrees

Declination J2000 (deg)

numpt

7

1J

number

Number of points in spectra

objwidth

8

1E

arcsec

Object extraction width (arcsec)

bckwidth

9

1E

arcsec

Background extraction width (arcsec)

objbcksep

10

1E

arcsec

Object-Background separation (arcsec)

bckcolrad

11

1J

pixel

Radius for averaging background columns (pix)

truthid

12

1J

number

Truth catalog ID (matched)

objtype

13

1J

number

Object Type(0=galaxy,1=star)

temperature

14

1E

degreesKelvin

Estimated effective temper.(K)

redshift

15

1E

number

Estimated redshift

slope

16

1E

number

Estimated spectral slope

qa

17

1J

number

Quality Analysis value

flag

18

1J

number

Miscellaneous flag (not used)

weight

19

2E

seconds

Average weight (each band)

background

20

2E

flux

Average background (each band)

median_s2n

21

2E

number

Median Signal/Noise (each band)

centroid

22

2E

arcseconds

Relative position(spatial)(each)

specfwhm

23

2E

arcseconds

Estim. Spatial FWHM (each band)

effexp

24

2E

seconds

Effective exposure time in spectral image strip

effmsk

25

2E

fraction

Effective masking fraction in spectral image strip

nuv_numpro

26

1J

number

Number of points in NUV profile

fuv_numpro

27

1J

number

Number of points in FUV profile

nuv_profile

28

198E

flux

NUV profile (spatial)

fuv_profile

29

198E

flux

FUV profile (spatial)

nuv

30

1E

flux

NUV direct image flux (pho/sec)

nuve

31

1E

fluxerror

Error in direct image flux rate

fuv

32

1E

flux

FUV direct image flux (pho/sec)

fuve

33

1E

fluxerror

Error in direct image flux rate

nuvs

34

1E

flux

Spectral flux scaled to direct (pho/sec)

nuvse

35

1E

fluxerror

Error in nuvs

fuvs

36

1E

flux

Spectral flux scaled to direct (pho/sec)

fuvse

37

1E

fluxerror

Error in fuvs

nuv0s

38

1E

flux

NUV Zeroth order flux rate

nuv0se

39

1E

fluxerror

Error in NUV Zeroth order flux rate

nuv1s

40

1E

flux

NUV First order flux rate

nuv1se

41

1E

fluxerror

Error in NUV First order flux rate

nuv2s

42

1E

flux

NUV Second order flux rate

nuv2se

43

1E

fluxerror

Error in NUV Second order flux rate

fuv1s

44

1E

flux

FUV First order flux rate

fuv1se

45

1E

fluxerror

Error in FUV First order flux rate

fuv2s

46

1E

flux

FUV Second order flux rate

fuv2se

47

1E

fluxerror

Error in FUV Second order flux rate

fuv3s

48

1E

flux

FUV Third order flux rate

fuv3se

49

1E

fluxerror

Error in FUV Third order flux rate

zero

50

1E

Angstroms

Wavelength of first pixel (Ang.)

disp

51

1E

Angstromsperpixel

Angstroms per pixel

obj

52

552E

fluxperAngstrom

Flux (pho/sec/cm^2/Ang) (standard)

objerr

53

552E

fluxperAngstrom

Error on obj

opx

54

552E

fluxperAngstrom

Flux (pho/sec/cm^2/Ang) (optimal)

opxerr

55

552E

fluxperAngstrom

Error on opx

objs

56

552E

fluxperAngstrom

Flux (pho/sec/cm^2/Ang) (FUV 3rd + NUV 2nd)

objserr

57

552E

fluxperAngstrom

Error on objs

objmdn

58

552E

fluxperAngstrom

Flux spectrum using median image strip.