zCOSMOS Data Release DR1

30 October 2007

zCOSMOS (P.I. Simon Lilly) is a large redshift survey that is being undertaken in the COSMOS field using the VIMOS spectrograph mounted at the Melipal Unit Telescope of the VLT at ESO's Cerro Paranal Observatory, Chile. zCOSMOS (ESO Large Programme LP175.A-0839) has been awarded about 600 hours of Service Mode observing time on the ESO VLT, making it the largest single observing project undertaken so far on that facility.

The zCOSMOS redshift survey has been designed to efficiently utilize VIMOS by splitting the survey into two parts. The first, zCOSMOS-bright, aims to produce a redshift survey of approximately 20,000 I-band selected galaxies at redshifts z < 1.2. Covering the approximately 1.7 deg2 of the COSMOS field (essentially the full ACS-covered area), the transverse dimension at z ~ 1 is 75 Mpc. The second part, zCOSMOS-deep, will observe about 10,000 galaxies selected through well-defined colour selection criteria which mostly lie at 1.5 < z < 3.0. Simply to keep the required amount of telescope time manageable, the field of zCOSMOS-deep is restricted to the central 1 deg2 of the COSMOS field.

This release (DR1) contains the results of the first zCOSMOS-bright spectroscopic observations that were carried out in VLT Service Mode during the period April to June 2005. Seven masks were observed, yielding 1264 spectra. Future releases will be made as the sample builds up.

The full set of extracted and wavelength calibrated 1-dimensional spectra (FITS format), associated 5"x5" image cut-outs for each target (FITS and JPEG format), as well as the set of 2-dimensional wavelength calibrated spectra (FITS format) are being released. Furthermore, a catalogue is being provided where we give for each target the 1-D and 2-D spectra filenames, the I-band magnitude used for the selection, as well as the measured redshift and confidence class. A full description of the survey can be found in the accompanying paper "zCOSMOS: A Large VLT/VIMOS Redshift Survey Covering 0 < z < 3 in the COSMOS field", Lilly et al. 2007, ApJS, 172, 70.


The data were reduced by the zCOSMOS team and prepared for release in collaboration with ESO (Advanced Data Products group in the Virtual Observatory Systems department).

Overview and survey layout

layout_acs650.jpg All observed VIMOS targets in this release are overlaid on the HST/ACS COSMOS F814W band mosaic. Red open circles stand for targets with a redshift and black open circles indicate observed targets without a redshift identification.

Click to get the full image.

Release content

Summary of reduced VIMOS observations

A total of 1264 spectra could be extracted from the VIMOS observations and are presented in this release. Throughout the zCOSMOS-bright survey, 1 arcsec wide slits have been used with a wavelength range of approximately 5500 to 9700 Å sampled at roughly 2.5 Å/pixel.

The primary input catalogue for slit-mask design was generated using SExtractor (Bertin et al. 1996) applied to the COSMOS F814W HST/ACS images sampled at 0.03 arcsec/pixel (Koekemoer et al. 2007, Leauthaud et al. 2007) in a "hot and cold" two-pass process to first identify bright objects. This substantially reduced the tendency of the HST-based catalogue to "over-resolve" extended galaxies into multiple components. This initial SExtractor catalogue was then "cleaned" by carrying out a detailed comparison with one extracted from a stack of i* images obtained with MEGACAM on the 3.6m Canada-France-Hawaii telescope and processed at the TERAPIX data reduction center in Paris. This catalogue was also used to supplement the ACS catalogue for regions where the ACS images were unavailable or unusable. The zCOSMOS-bright target catalogue is intended to be simply defined as having an ACS/HST SExtractor "magauto" brightness in the range 15.00 < IAB(814) < 22.50.

Release Notes

Data reduction method

The VIMOS observations were reduced using the v1.0 of the VIPGI software package (Scodeggio et al., 2005, Pub.Astr.Soc.Pac., 117, 1284). Determination of redshifts is a multi-step process and involves the use of different approaches tailored to the individual spectra. These include first a fully computer-aided determination based on cross-correlation with template spectra coupled to continuum fitting and principal component analysis, using the KBRED software, which are a set of routines implemented in the IDL environment (Scaramella et al., in preparation). The success of the automatic measurements is dependent on the use of a representative set of galaxy templates, which we have drawn from those built for the VVDS (Le Fèvre et al. 2005) and from the current program. This preliminary automated step is followed by a detailed visual examination of the one- and two-dimensional spectrograms of every object to critically assess the validity of the automated redshift. In those cases where the automatic procedure fails, a new redshift is computed based on the wavelengths of recognized features. Two fully independent reductions are carried out of each spectrum, yielding two independent redshift measurements. These are compared and "reconciled" (generally in a face-to-face meeting) to yield a final redshift and Confidence Class.

Redshift accuracy and reliability

From repeat observations, the typical redshift uncertainty in zCOSMOS-bright is estimated to be +/- 110 km/s. Inevitably, zCOSMOS redshifts span a range of reliability from absolutely secure redshifts to a few that are quite unreliable. Each redshift is therefore assigned to a Confidence Class which captures this crucial information. zCOSMOS Confidence Classes have been developed from those adopted in the CFRS (Le Fèvre et al., ApJ, 455, 60) and VVDS (Le Fèvre et al., A&A, 439, 845). It is important to note that they are based on the confidence in the final redshift and not on the quality of the spectrum per se. To summarize, the basic confidence scheme ranges from Class 0 (no redshift obtained) to Class 4 (very secure redshift). In practice there is little real difference between Classes 3 and 4 and they may be safely combined for most purposes. Two additional classes with special meaning are then added. Class 9 signifies a one-line redshift where the line is undoubtedly real, as well as being sufficiently strong and isolated that we can be confident that the line is either H-alpha or [OII] 3727 - yielding two possible redshifts. We use photo-z to discriminate between these two definite possibilities (see below) - finding that we guessed right in over 85% of cases. A Class 8 is also a one line redshift, but for Broad Line AGN. No attempt at photo-z discrimination is made for these. The basic confidence scheme is then modified with possible preceding digit(s) as follows: An additional "1" digit before the Class (i.e. adding "10" to the Class) signifies a broad line AGN: e.g. Class 13 is a very secure BL AGN redshift. A "2" digit before the Class signifies that the object was observed serendipitously because some light from it entered a slit positioned on another target: e.g. Class 24 is a very secure serendipitous object while Class 213 is a secure serendipitous broad line AGN redshift. Note that all Class 0 objects (i.e. redshift failures) have been retained in the catalogue, but that Class 20 objects (redshift failures on serendipitous targets) have not been.

The statistical reliability of the spectroscopic redshifts in the various Confidence Classes is assessed by both the agreement or otherwise of redshifts independently derived from repeat observations of the same galaxies, and by the consistency or otherwise with photometric redshifts derived from the COSMOS multi-band photometric data using the ZEBRA code.

Confidence Class Description Spectroscopic verification ZEBRA photo-z consistency within Δz=0.1(1+z)
Class 4 Very secure redshift >99.5% 97%
Class 3 Secure redshift >99.5% 97%
Class 9 One line either Ha or [OII] 3727 (best guess) 95% 90%
Class 8 Unidentifed one line (best guess)    
Class 2 Probable redshift 92% 93%
Class 1 Insecure redshift 70% 72%

In fact, these two approaches show very good agreement, suggesting that consistency with the photometric redshift can be used to indicate which of the less reliable spectroscopic redshifts are probably right and which are likely wrong. We therefore add a decimal place to the Confidence Class to reflect this additional information. The integer part is based solely on the spectrum itself, followed by a decimal place which contains (a posteriori) information on the consistency of the spectroscopic redshift with the photometric redshift as given in the following table.

Confidence class spectroscopic/photometric consistency
.5 Spectroscopic redshift consistent within 0.1(1+z) of the photometric redshift
.4 No photometric redshift available, includes all spectroscopic AGN and stars
.3 Special case for Class 9: Consistent with photo-z only after the redshift changed to the alternate redshift, a switch which is then applied
.1 Spectroscopic and photometric redshifts are not consistent at the level of 0.1(1+z)

Data format

For each target the following data files are being released: the 1-dimensional spectrum in FITS format, the 2-dimensional spectrum in FITS format, and the corresponding 5"x5" image cut-out from the F814W ACS image (in FITS and JPEG format).

The following naming convention has been adopted for the individual files:

zCOSMOS_BRIGHT_aa_bb.fits 1-d spectrum
zCOSMOS_BRIGHT_aa_bb_pixXX_2D.fits 2-d spectrum
zCOSMOS_BRIGHT_aa_cc_F814W.fits image cut-out (FITS format)
zCOSMOS_BRIGHT_aa_cc_F814W.jpg image cut-out (JPEG format)

aais the object ID; bb indicates the mask ID of the observation (e.g., ZCMRa35_M1), quadrant, slit and object number; pixXX refers to the position of the target from the bottom of the slit during the extraction process; cc are the center coordinates of the image.

The target catalog, containing object ID, target position, redshift, confidence class, target magnitude (F814W), and filenames of 1-d and 2-d spectra, is being released in ASCII table format, FITS table format, VOTable format (XML), and as a DS9 region file.

zCOSMOS_BRIGHT_DR1_TABLE.tab ASCII table 286025 Bytes
zCOSMOS_BRIGHT_DR1_TABLE.fits FITS table 198720 Bytes
zCOSMOS_BRIGHT_DR1_TABLE.xml VOTable (XML) 329619 Bytes
zCOSMOS_BRIGHT_DR1_TABLE.reg DS9 region file 83600 Bytes

Data retrieval

Please note that this release is superseded by data release 2.


When using data products provided in this release, acknowledgement should be given in the text to the zCOSMOS project, referring to this DR2 and the publication. In addition, please also use the following statement in your articles when using these data:

Based on observations made with ESO Telescopes at the La Silla or Paranal Observatories under programme ID 175.A-0839.