18 February 2008

As part of the Great Observatories Origins Deep Survey (GOODS), multi-object spectroscopy of faint galaxies in the Chandra Deep Field South (CDF-S) has been carried out, using the VIMOS instrument mounted at the Melipal Unit Telescope of the VLT at ESO's Cerro Paranal Observatory, Chile.
This data release contains the results of the first half of the GOODS/VIMOS spectroscopic campaign of the ESO/GOODS large programme 171.A-3045 (P.I. C. Cesarsky) which have been obtained in service mode observations using two different grisms (VIMOS 'Low resolution Blue', 3500-6900 Å, and 'Medium resolution Orange', 4000-10000 Å) between fall 2004 and fall 2006. 3312 fully reduced and calibrated spectra of 3121 unique sources down to i₇₇₅=25 AB, along with a catalog of 2000 redshifts of which 985 are classified "high quality" (A) are being released. The 1-sigma redshift accuracy is ~300 km/s and ~200 km/s for the low and the medium resolution observations, respectively. A full description of the survey can be found in the accompanying publication "The Great Observatories Origins Deep Survey: VLT/VIMOS Spectroscopy in the GOODS-South Field" by Popesso et al. 2009.

• Overview and survey layout
• Release content
• Release notes
• Data format
• Data retrieval
• Acknowledgements

Overview and survey layout

VIMOS field coverage of the GOODS area in the CDF-S. Click to view full-size image… — Size: 458.3 kB.

The GOODS/VIMOS survey has been designed to complement the GOODS/FORS2 spectroscopic survey (Vanzella et al. 2005, 2006) in terms of completeness and sky coverage. Therefore, targets which had been already observed within GOODS/FORS2 or another redshift survey in the CDF-S region (K20 survey of Cimatti et al. 2002, the survey of X-ray sources by Szokoly et al. 2004, the VIMOS VLT Deep Survey, Le Fevre et al. 2005) have been avoided as far as possible when designing the masks.
The target selection is based on the multiwavelength photometry available for the GOODS-S field. Different criteria have been used for the low resolution LR-Blue grism and the medium resolution MR Orange grism. According to the usable wavelength ranges targets with estimated redshifts between 1.8 and 3.8 have been selected for observations with the VIMOS LR-Blue grism, targets with redshift z < 1 or 2.8 < z < 4.8 have been selected for observations with the VIMOS MR Orange grism. A magnitude cut R < 25 AB has been applied to the LR-Blue targets. Please refer to Popesso et al. 2009 for a detailed description of the target selection procedure.

Release content

Summary of reduced VIMOS observations

In total 6 masks have been observed in the LR-Blue grism and 6 with the MR Orange grism where a slit width of 1 arcsec has been used for both yielding a dispersion of 5.7 Å/pixel and 2.55 Å/pixel, respectively. This release contains 3312 extracted, fully calibrated spectra of which 1996 redshifts of unique sources have been measured using the cross-correlation technique (xcsao). The following table shows the breakdown by redshift quality class.

Redshift quality flags are defined following the GOODS/FORS2 survey as:

• flag A: high quality, values of the xcsao correlation coefficient R > 5; emission lines and strong absorption features are well identified.
• flag B: intermediate quality, values of the xcsao correlation coefficient 3 < R < 5; one emission line plus few absorption features are well identified.
• flag C: low quality, values of the xcsao correlation coefficient R < 3, features of the continuum not well identified.
• flag X: no redshift estimated, no features identified.

The internal redshift accuracy can be estimated from a sample of galaxies which have been observed twice in independent VIMOS mask sets. We find 39 of such objects in the LR-Blue masks and 40 in the MR masks. ~45% of this objects have been observed as serendipitous objects. The mean of the LR_Blue and the MR Δz distributions is close to zero in both cases. The redshift dispersion is σ_z=0.001 (~300 km/s) for the LR-Blue objects and σ_z=0.0007 (~200 km/s) for the MR redshifts

Release Notes

Data reduction method

The pipeline processing of the VIMOS-GOODS data is performed using the VIMOS Interactive Pipeline Graphical Interface (VIPGI, see Scodeggio et al. 2005 for a full description). The data reduction is performed in several interactive steps: the spectra location in the individual spectroscopic frames, the wavelength calibration, sky subtraction and fringing correction, combination of the 2D spectra of dithered observations, extraction of the 1D spectra and flux calibration. The location of the slits is known from the mask design process, hence, knowing the grism zero deviation wavelength and the dispersion curve, the spectra location is known a priori on the detectors. However, small shifts from predicted positions are possible due to the complete manufacturing and observation process. From the predicted position, the location of the spectra are identified accurately on the 4 detectors and an extraction window is defined for each slit. The wavelength calibration is secured by the observation of night arc-lamps through the observed mask. Wavelength calibration spectra are extracted at the same location as the object spectra and calibration lines are identified to derive the pixel to wavelength mapping for each slit. The wavelength to detector pixel transformation is fit using a third order polynomial, resulting in a mean deviation ~0.7 Å r.m.s. across the wavelength range in the LR-Blue masks and ~ 0.36 Å r.m.s. in the MR masks. A low order polynomial (second order) is fit along the slit, modeling the sky background contribution at each wavelength position, and subtracted from the 2D spectrum. In the case of the LR-Blue campaign the fringing is not present and all 10 exposures of a sequence are directly combined by shifting the 2D spectra following the offset pattern to register the object at the same position. The individual frames are combined with a median, sigma-clipping algorithm to produce the final summed, sky subtracted 2D spectrum. In the case of the Medium resolution spectra, the fringing is significant at λ > 7000 Å and needs to be removed. Therefore, a fringing correction is applied before combining the dithered frames. As the object is moved to different positions along the slit following the effect pattern, the median of the 2D sky subtracted spectra produces a frame from which the object is eliminated, but that includes all residuals not corrected by sky subtraction, in particular the fringing pattern varying with position across the slit and wavelength. This sky/fringing residuals is, then, subtracted from each individual 2D sky subtracted frame. The fringing corrected frame are, then, combined as in the case of the LR-Blue spectra.
The last step done automatically by VIPGI is to extract a 1D spectrum from the summed 2D spectrum, using an optimal extraction following the slit profile measured in each slit (Horne, 1986). The 1D spectrum is flux calibrated using the ADU to absolute flux transformation computed from the observations of spectrophotometric standard stars (LTT-3864).
A final check of the 1D calibrated spectra is performed and the most discrepant features are removed manually, cleaning each spectrum of zero order contamination, sky lines residuals and negative non physical features.

Note that the VIMOS LR-Blue wiggles are not corrected for the reasons outlined in Popesso et al. 2009, Sect. 3.3.

Target coordinates

Due to the rotation angle of the VIMOS GOODS pointings (-20 deg), which is different from the default values accepted by VIPGI (0 and 90 deg), VIPGI does not provide the astrometry of the extracted spectra. The only information provided by VIPGI are the coordinates in mm on the focal plane stored in the so called VIPGI object table. To overcome this problem, we transform the focal plane coordinates of each object into CCD coordinates trough the appropriate distortion map stored in the header of our VIMOS observations. Slits which contain only one object (only one spectrum extracted) are used to calculate the transformation matrix from VIMOS to the GOODS R-band WFI CCD coordinates through the IRAF routines geomap and geoxytran. As a last step the WFI X_CCD and Y_CCD assigned to each extracted spectrum are converted to α, δ on the basis of the WFI-GOODS astrometry. These 'reconstructed' coordinates are, then, matched to the original GOODS VIMOS target catalog to identify the primary targets and the serendipitous objects. After the identification, the original WFI target coordinates are assigned to the primary targets while the secondary objects keep the reconstructed coordinates.

It is worth to mention that due to a bug, VIPGI assigns wrong focal plane coordinates to a small sample of objects in slits with more than 2 spectra (VIPGI assigns zero coordinates to several objects in the same mask). We identify 82 cases in the LR-Blue campaign, of which 80% has no redshift determination, and 34 in the MR campaign, of which 50% has no redshift determination. On the basis of the reconstructed WFI coordinates these objects are located completely out of the slit where they should be. Accordingly, there are no valid target coordinates for those objects and the corresponding fields in the redshift tables have been set to -999. For the respective spectra's FITS headers the target coordinates given by RA_TARG, DEC_TARG have been set to the slit center position and the positional error (ERR_TARG) to 5.0 arcsec.

Data format

For each spectrum the following data products are being released: the 1-dimensional spectrum in FITS format and the corresponding plot of the spectrum in postscript format. The following naming convention has been adopted for the individual files:

where <bb> indicates the mask ID of the observation, quadrant, slit and object number. For convenience the data products of this release are packed in two tarballs containing the LR-Blue spectra and the MR spectra, respectively.

The redshift catalogs, containing spectrum filename, target position (ACS GOODS astrometry), slit position, redshift, quality flag, spectral features, and a label for primary or secondary (i.e. serendipitous) object are being released in ASCII table format. The first two catalogs, labelled GOODS_VIMOS_Release1.0_*_table.tsv, are cleaned for duplicate observations of the same object in which case the best spectrum is kept, while the other two, labelled GOODS_VIMOS_Release1.0_*_fulltable.tsv, contain the complete set of observations including duplicates.

Please request your copy of the data from the ESO Science Archive using the data request submission form.

Data retrieval

Please make your selection on the page linked to below.

Acknowledgements

When using data products provided in this release, we request acknowledgement of the ESO/GOODS project and referring to the related publication "The Great Observatories Origins Deep Survey - VLT/VIMOS Spectroscopy in the GOODS-South Field: Part II" by Balestra et al. (2010), A&A 512, A12. Please also use the following statement in your articles when using these data: