zCOSMOSVIMOS spectroscopic survey on the VLT

Stated goal: 25,000 redshifts at 0.2 < z < 1.212,500 redshifts at 1.2 < z < 2.5

~ 600 service mode (guaranteed clear) hours awarded on VLTApril 2005 - ??

zCOSMOS design considerationsCOSMOS contains 2106 galaxies 0.0 < z < 6.5, i.e. some selection requiredPhoto-z for all objects available already

Exploit unique information on environment (from groups up to LSS) Uniform spatial coverage across fieldHigh spatial sampling (~ 70%)Minimize biasses with spectral typeContiguous redshift coverage desirableCompatibility with local surveys such as 2dfGRS etc desirable (i.e. low sv if possible)

Consistent with other goals (X-ray + radio identifications, spectra analysis etc)

VVDS N(z)IAB < 22.5IAB < 24Out to z ~1 a straight I-band selected sample (CFRS, VVDS) approximates a local B-band selected sample (e.g. 2dfGRS). IAB ~ 22.5 ~1hr exposure

2.Beyond z ~ 1 need to implement selection to isolate tail of N(z) at AB ~25. Use well-defined and reproducible colour-colour criteria:Blue UGR colours (Steidel) Cowie flat spectrum 1.5 < z < 2.5Bowl-shape BzK colour (Daddi)(note not Lyman break at z ~3, which remain to be done) AB ~ 25 ~5 hr exposure3.Get X-ray, radio and other identifications for free by inclusion in masks either as compulsory high priority targets, or as a random sample (with high sampling rate)

zCOSMOS bright sample:Decided to use VIMOS medium resolution MR grism to yield velocity accuracies of 100 kms-1 (c.f. ~300 kms-1 than VVDS)

Similar to 2dfGRS at low z, enable to define galaxy groups down to ~ 1012.5 M

Keep exposure time at 1 hr cost of factor of ~2 in multiplexing, but gain of 1.7 in S/N of emission/absorption lines Design goal of bright zCOSMOS is to match 2dfGRS at z ~0.7 (look-back t ~ 50%) over about 1/10 of equivalent volume (actually with better information on individual galaxies)2dfGRS group catalogue, N > 5

Comoving density ~ 0.0015 Mpc-3Comoving density ~ 0.00075 Mpc-3Andrew Benson simulations z ~1, Dz = 0.02Design goal of faint zCOSMOS is to obtain ~50 such slices 1.4 < z < 2.5 zCOSMOS faint sample:

VIMOS: 4 quadrants each ~ 6.5 7.5 arcmin2 separated by ~ 2 arcminUniform survey coverage obtained by stepping field centers by the dimensions of each quadrant results in every point in the sky being potentially observable exactly four times. 4-pass pattern also reduces biasses against nearby neighbours.May be repeated for 8-pass etc. (for longer MR spectra)

Covered area surrounded by 9-10 arcmin margin with reduced but still uniform coverage842Final 90 pointings (bright mosaic)First 15 pointings (P75 plan)26.57.5

COSMOS ACS area = 1.70 deg2

COSMOS ACS area = 1.70 deg2

902 zCOSMOS bright masks cover 1.62 deg2 (95%) with 0.93 deg2 fully covered by 8 passes50 Mpc at z = 0.7Bright: 180 x (1 hr exposure + 0.5 hr overhead) = 270 hr

COSMOS ACS area = 1.70 deg2

42 zCOSMOS bright masks cover 0.84 deg2 (50%) with 0.38 deg2 (22%) fully covered by 4 passes

Simply to limit total time requested anticipated follow-on in original proposal.50 Mpc at z = 2.0Bright: 180 x (1 hr exposure + 0.5 hr overhead) = 270 hr Faint: 45 x (4.5 hr exposure + 1.5 hr overhead) = 270 hr 540 hr + ~50 hr preimage = 590 hr total

Total number of placeable slits and the sampling rate (i.e. % of targets observed) both depend on surface number density of targets

With 4 passes (LR-Blue) or 8 passes (MR-Red) you get ~70% sampling at 20,000 targets degree-2. Each compulsory target included in the masks decreases the number of general survey targets by two Increasing target density so as to increase total targets by n% decreases sampling rate by n/2 % (i.e. not very attractive if it requires increase in exposure time or if extra objects have low success rate)Complicated prioritization schemes do not gain you much (i.e. no spare space in masks).

Limit compulsory targets to several % or less, and aim for input target lists at 20,000 deg-2, yielding ~70% sampling for random survey and for the added X-ray, radio identifications.

BzK selection for all types of galaxies at 1.5 < z < 2.5Main problem is that we are currently limited to KAB < 21.85 (equivalent to K20 survey) few BzK selected objects Spitzer?Also, using VIMOS LR-blue grism, so we set limit of B < 25, implying paucity of red (passive) galaxies in VIMOS target list. [ (B-K) ~ 7 B ~ 29 ]Magellan?19 < KAB < 21.85Faint sample colour selection I

Steidel 3-band UGR selection for flat-spectrum galaxies at 1.5 < z < 2.5COSMOS had 4-band UBVR (now +G) how to select z ~2 galaxies?Used (U-B) vs. (V-R) seemed cleanest. Can check with first set of zCOSMOS data Faint sample colour selection II

Check with Bahram M.s photometric redshifts (May 05) of the (March 05) zCOSMOS target samples encouraging rejection of z < 1 objectsoverall range shifted to slightly lower range 1 < z < 2 with tail to z ~3 vs. the expected 1.5 < z < 2.5 (verify photo-z?)Note paucity of BzKs relative to UBVR-selectedbright catalogueFaint colour-selected catalogue

Projected final zCOSMOS sample of galaxies

We would ideally like to get z ~2 density a bit higher (based on current photo-z) will reassess after success-rate and low-z contamination rate of the P75 observations (~750 spectra) is known Projected comoving number densitybrightfaintProjected final zCOSMOS sample of galaxiesProjected comoving density of galaxies

Catalogue generation (March 2005)Bright sample IAB < 22.5

Ground (CFHT or Subaru) or ACS selection? Blending of images, bright star halo masking, (Subaru) saturation from ground vs. broken up galaxies, junkimages in HST etc ACS selectedMerged Kneib ACS + McCracken CFHT I-band catalogues with visual inspection of all discrepant objectsVisual inspection of blue, red and black categories

Faint sample BAB < 25.0

4x4 arcmin box (1/6 of tile 53 = 0.2% of COSMOS)

Faint sample BAB < 25.0Objects from ip_selected_20050224.cat4x4 arcmin box (1/6 of tile 53 = 0.2% of COSMOS)

Faint sample BAB < 25.0Objects from ip_selected_20050224.catMasked with scheme derived from SDSS g and i magnitudes4x4 arcmin box (1/6 of tile 53 = 0.2% of COSMOS)

4x4 arcmin box (1/6 of tile 53 = 0.2% of COSMOS)Faint sample BAB < 25.0Objects from ip_selected_20050224.catMasked with scheme derived from SDSS g and i magnitudesPlus additional elimination of bright galaxy components (degeneracy in colour-selection and photo-z)

Faint sample BAB < 25.0Objects from ip_selected_20050224.catMasked with scheme derived from SDSS g and i magnitudesPlus additional elimination of bright galaxy components (degeneracy in colour-selection and photo-z) Final selection at B < 25.04x4 arcmin box (1/6 of tile 53 = 0.2% of COSMOS)

Because of need for visual classification (incl. with incomplete ACS) and pressing time constraints, initial spectroscopic target lists were limited in areabrightdeep

Faint masks contain average 250 slits per set of masks 11,250 spectra in total ? Bright masks contain an average of 160 slits per set of masks 29,000 spectra in total ?

Bright catalogueFaint catalogueX-ray ids (excl. those in Magellan lists)152 compulsory +27 random50 compulsory + 12 randomRadio ids48 compulsory + 786 random41 compulsory + 385 randomGALEX ids133 compulsory93 compulsory

We have always known that to be completed within nominal two years zCOSMOS would need observations to be made with the VLT UT3 every minute that the COSMOS field is theoretically visible

Weather alone (transparency and seeing) will inevitably stretch it to three years (67% efficiency)nominal 2 yr durationcompletion!maximum possible rateassuming 0.67 efficiency due to weather alonebrightfaint2005200620072008

Visitor instrument scheduled on UT3We are currently running at about 33% efficiency (due to weather and allocations to other scientific programs) unacceptable 6 years to completion ??

Obvious conclusion: we cannot allow any other users of the telescope!Weather and VISIR nightsAprilMayJune

So far: Only five bright masks 800 spectra may possibly get several more in June dark?So far: only 3-4 faint masks 700 - 900 spectraNot all bad news: With only 3% (bright) and 6% (faint) completed we can continue to check/improve catalogues, selection strategy, and VIMOS service-observing procedures before next season starts in December 2005May possibly get a few more masks in June

Summary

zCOSMOS is underway but with only 5% completed so far (May 2005): We should have approaching 1000 VIMOS spectra from each of bright and faint surveys within a few months but both the MR Red and the 5hr LR Blue grism spectra are relatively unexplored territory (see Alvios talk) plus previous P73 data (J.-P. Kneib)The full zCOSMOS project will be a long haul (and we need to work the VLT time allocation issue)

Issues for COSMOSPresent depth of K-band images severely limits number of BzK objects KAB < 21.85 (=K20!) need deeper near-IR data (Spitzer?)We currently do not have a route towards getting spectra of potentially large numbers of quiescent red galaxies (B >> 25) at z ~2 (Magellan sensitivity issues, etc)We are not doing anything systematic beyond z ~ 2.5 (LBG drop-outs etc)

Fig 5: Two representative spectra (out of several hundred) of faint BZK-selected galaxies observed with the VIMOS/LR-Blue grism for 5hrs (the same setup as proposed here). The galaxies both have BAB ~ 24.5 and have redshifts z = 2.25 and z = 2.36 (left and right respectively). Numerous ultraviolet absorption features are clearly seen (from Daddi et al, in preparation)