HLA ACS/WFC G800L data

ACS/WFC G800L spectra

For the sample release, we have selected two fields (UDFNICP1 and UDFNICP2) in the GOODS-South area, and Figure 1 shows their location. The very deep grism (up to 18 ksec) and direct imaging data (up to 60 ksec, distributed over several filters) had been taken parallel to the NICMOS UDF observations.

The reduction of the data follows the methods and procedures developed for the HLA NICMOS G141 project, which are described in an accompanying data paper. The sources are identified on the combined undispersed filter images. These direct images are used to identify the locations of the objects in order to extract spectra from the dispersed images, to derive their relevant morphological and photometric parameters and to identify and asses possible contamination from neighbouring objects. Spectra with problems or deficiencies were rejected (via eyballing), thus not all obvious sources in the field may have a spectrum in the release.

Figure 2 shows the previews of some spectra (late-type stars, emission line objects etc.) that are part of the sample release. The previews contain a stamp image of both the grism and the direct image cutout. In the direct image cutout, the dispersion direction and the orientation and length of the extraction are indicated with the blue arrow and line, respectively. A detailed paper describing the ACS/WFC G800L data reduction is in preparation.

The astrometry of the original HST frames has been improved by cross-correlating the direct images with the GOODS-South catalogue as an external reference. Unfortunately, because of a bug in the astrometric correction algorithm, the released coordinates are still affected by significant offsets (of the order of 1 arcsec). These offsets will be corrected in the final release; however, in the meanwhile, users can apply the following correction to the published coordinates:
Field Correction R.A.Correction Dec.
UDFNICP1 +1.05 arcsec = +0.070 sec -0.42 arcsec
UDFNICP2 -0.26 arcsec = -0.017 sec -0.57 arcsec

Note that the values reported in the table above must be added to the published coordinates to obtain corrected locations. For example, the object HAG_J033233.61-275549.7_UDFNICP1 has published coordinates

R.A. = 03h 32m 33.606s  and  Dec. = -27° 55' 49.69"

Since it belongs to the field UDFNICP1, the new, corrected coordinates will be

R.A. = 03h 32m 33.676s  and  Dec. = -27° 55' 50.11"

Figure 3 shows the residual offset between our source positions and the corresponding positions in the GOODS catalogue. The final astrometric accuracy, after application of the offsets, is approximately σ=0.03 arcsec.

The accuracy of the absolute flux calibration of the spectra is better than 8%. The flux calibration was independently verified by comparing the total integrated flux within the F775W wavelength window with the magnitude as derived from the undispersed images in the same filter. The results obtained are shown in Figure 4.

The ST-ECF HLA archive returns for each target a cut-out of the undispersed image, a 2-dimensional spectrum (position versus wavelength), and a 1-dimensional integrated spectrum of the target. These products are provided as VO-compatible FITS files and as static preview images. The FITS files contain metadata in the form of FITS keywords. A description of the keywords for the 1d spectra is listed in Table 1.

Figure 1: The location of the two UDF NICMOS parallels field selected for the sample release in the GOODS-South area

[Location of the UDF NICMOS parallels fields]

Figure 2: The previews of some spectra that are part of the sample release

Figure 3: The residual offset between the HLA and the GOODS-ACS South catalogue positions.

[Astrometric accuracy]

Figure 4: The "spectroscopic" F775W magnitude, i.e. the magnitude obtained from the integration of the spectral flux within the total F775W filter throughput, in comparison with the direct image F775W magnitude (SExtractor mag_auto). Overall we find good agreement. The targets which appear to be too bright in the spectroscopic magnitude (for mag_auto F775W > 21) are contaminated spectra where we judged the spectrum to be still good enough to be included in this sampler release. For the group of bright targets (mag_auto F775W < 20) which are below the one-to-one line the direct image magnitude is affected by saturation while the spectral magnitude is judged to be of good quality. For the group of targets which are above the one-to-one line (mag_auto F775W ~ 23) a variety of causes explains the deviation, however, mostly the direct magnitude is judged to be too bright while the spectroscopic magnitudes seem reasonable.

[Photometric accuracy]

Table 1:The metadata of the 1-dimensional integrated fits spectra

Keyword Description
TFIELDS number of table fields
TTYPE1 column 1 name
TFORM1 column 1 data format
TUNIT1 column 1 unit
TUCD1 column 1 UCD
TUTYP1 column 1 UTYPE
TTYPE2 column 2 name
TFORM2 column 2 data format
TUNIT2 column 2 unit
TUCD2 column 2 UCD
TTYPE3 column 3 name
TFORM3 column 3 data format
TUNIT3 column 3 unit
TUCD3 column 3 UCD
TTYPE4 column 4 name
TFORM4 column 4 data format
TUNIT4 column 4 unit
TUCD4 column 4 UCD
TUTYP4 column 4 UTYPE
TTYPE5 column 5 name
TFORM5 column 5 data format
TUNIT5 column 5 unit
TUCD5 column 5 UCD
TUTYP5 column 5 UTYPE
TTYPE6 column 6 name
TFORM6 column 6 data format
TUNIT6 column 6 unit
TUCD6 column 6 UCD
VOCLASS data model name and version
VOSEGT dataset or segment type
VOPUB the VO publisher
TITLE dataset title
CREATOR VO creator ID
VOLOGO URL for creator logo
VERSION data release version
RADECSYS astrometric reference system
REFCAT astrometric reference catalogue
TIMESYS timescale
MJDREF zeropoint of timescale in MJD
SPECSYS spectral frame origin
DATE file creation date (YYYY-MM-DDThh:mm:ss UT)
TELESCOP the observing facility
INSTRUME the instrument used
PHOTMODE sequence of optical elements in the light path
FILTER the main optical element
TIMESDIM SI factor and dimensions
SPECSDIM SI factor and dimensions
FLUXSDIM SI factor and dimensions
FLUX_UCD flux axis ucd
SPEC_UCD spectral axis ucd
SPEC_RES spectral resolution
TIMEUNIT unit for time
CR_IDENT software version used to generate data
SPEC_FIL sampling filling factor
SPECBAND bandpass description
SKY_RES the pixel scale of the instrument [arcsec/pix]
CRETYPE dataset xcreation type
ASN_ID name of HST association
APERNUM internal spectrum identification
OBJECT the target name
RA_TARG the target right ascension [deg]
DEC_TARG the target declination [deg]
RA location of spectral axis RA [deg]
DEC location of spectral axis DEC [deg]
EQUINOX equinox
GAL_LONG target galactic longitude [deg]
GAL_LAT target galactic latitude [deg]
ECL_LONG target ecliptic longitude [deg]
ECL_LAT target ecliptic latitude [deg]
OBJSHAPA major axis object extend [arcsec]
OBJSHAPB minor axis object extend [arcsec]
OBJPA orientation of major axis in NESW [deg]
APERLEN the slit length in ["]
APERWID the "slit width" in [arcsec]
OBJTYPE object treated as poinlike (1/0 = y/n)
MAG SExtractor AB-magnitude in direct image
EMAG SExtractor AB-magnitude error in direct image
DER_SNR derived SNR
SPEC_BW width of the spectral axis [AA]
SPEC_VAL mean of the spectral axis [AA]
TSTART start of first exposure [MJD]
TSTOP end of last exposure [MJD]
TMID date of observation
DATALEN number of spectral points
WCSOFFRA RA offset applied to the original WCS ["]
WCSOFFDE DEC offset applied to the original WCS ["]
WCSNOBJ Number of sources used for offset determination
WCSNOBJ1 Number of UCAC2 sources used for offset determination
WCSNOBJ2 Number of 2MASS-PSC sources used for offset determination
WCSNOBJ3 Number of SDSS-DR5 sources used for offset determination
WCSNOBJ4 Number of USNO-B1.0 sources used for offset determination
WCSNOBJ5 Number of GSC2.3.2 sources used for offset determination
WCSNOBJ6 Number of GOODS sources used for offset determination
MAGBAND1 filter name for spectral magnitude
SMAG1 spectral AB magnitude value in filter 1
SMAGERR1 spectral AB magnitude error in filter 1
MAGBAND2 (optional) filter name for spectral magnitude 2
SMAG2 (optional) spectral AB magnitude value in filter 2
SMAGERR2 (optional) spectral AB magnitude error in filter 2
MAGBAND3 (optional) filter name for spectral magnitude 3
SMAG3 (optional) spectral AB magnitude value in filter 3
SMAGERR3 (optional) spectral AB magnitude error in filter 3
APERRA1 RA of first slit corner
APERDE1 DEC of first slit corner
APERRA2 RA of second slit corner
APERDE2 DEC of second slit corner
APERRA3 RA of third slit corner
APERDE3 DEC of third slit corner
APERRA4 RA of fourth slit corner
APERDE4 DEC of fourth slit corner