zCOSMOS VIMOS spectroscopic survey on the VLT Stated goal: 25,000 redshifts at 0.2 < z < 1.2 12,500 redshifts at 1.2 < z < 2.5 ~ 600 service mode (guaranteed clear) hours awarded on VLT April 2005 - ??
zCOSMOS design considerations COSMOS contains 2106 galaxies 0.0 < z < 6.5, i.e. some selection required Photo-z for all objects available already Exploit unique information on environment (from groups up to LSS) Uniform spatial coverage across field High spatial sampling (~ 70%) Minimize biasses with spectral type Contiguous redshift coverage desirable Compatibility 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)
Out 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 < 2.5 Bowl-shape BzK colour (Daddi) (note not Lyman break at z ~3, which remain to be done) AB ~ 25 ~5 hr exposure 3. 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) VVDS N(z) IAB < 24 IAB < 22.5
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 2dfGRS group catalogue, N > 5 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)
zCOSMOS “faint” sample: Andrew Benson simulations z ~1, Dz = 0.02 Comoving density ~ 0.00075 Mpc-3 Comoving density ~ 0.0015 Mpc-3 Design goal of “faint” zCOSMOS is to obtain ~50 such slices 1.4 < z < 2.5
VIMOS: 4 quadrants each ~ 6.5 7.5 arcmin2 separated by ~ 2 arcmin 2’ Uniform 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 coverage 6.5’ 7.5’ Final 90 pointings (bright mosaic) First 15 pointings (P75 plan) 4 2 8
COSMOS ACS area = 1.70 deg2
Bright: 180 x (1 hr exposure + 0.5 hr overhead) = 270 hr COSMOS ACS area = 1.70 deg2 902 zCOSMOS “bright masks” cover 1.62 deg2 (95%) with 0.93 deg2 fully covered by 8 passes 50 Mpc at z = 0.7 Bright: 180 x (1 hr exposure + 0.5 hr overhead) = 270 hr
Bright: 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 = 2.0 Bright: 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 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.
Faint sample colour selection I BzK selection for all types of galaxies at 1.5 < z < 2.5 Main 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 < 21.85
Faint sample colour selection II “Steidel” 3-band UGR selection for “flat-spectrum” galaxies at 1.5 < z < 2.5 COSMOS 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 colour-selected catalogue Check with Bahram M.’s photometric redshifts (May ‘05) of the (March ‘05) zCOSMOS target samples bright catalogue Faint colour-selected catalogue encouraging rejection of z < 1 objects overall 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 BzK’s relative to UBVR-selected
Projected final zCOSMOS sample of galaxies
Projected final zCOSMOS sample of galaxies Projected comoving density of galaxies Projected comoving number density bright faint 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
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, “junk”images in HST etc ACS selected Merged Kneib ACS + McCracken CFHT I-band catalogues with visual inspection of all discrepant objects Visual 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 < 25.0 Objects from ip_selected_20050224.cat 4x4 arcmin box (1/6 of tile 53 = 0.2% of COSMOS)
Faint sample BAB < 25.0 Objects from ip_selected_20050224.cat Masked with scheme derived from SDSS g and i magnitudes 4x4 arcmin box (1/6 of tile 53 = 0.2% of COSMOS)
Faint sample BAB < 25.0 Objects from ip_selected_20050224.cat Masked with scheme derived from SDSS g and i magnitudes Plus additional elimination of bright galaxy components (degeneracy in colour-selection and photo-z) 4x4 arcmin box (1/6 of tile 53 = 0.2% of COSMOS)
Faint sample BAB < 25.0 Objects from ip_selected_20050224.cat Masked with scheme derived from SDSS g and i magnitudes Plus additional elimination of bright galaxy components (degeneracy in colour-selection and photo-z) Final selection at B < 25.0 4x4 arcmin box (1/6 of tile 53 = 0.2% of COSMOS)
bright deep Because of need for visual classification (incl. with incomplete ACS) and pressing time constraints, initial spectroscopic target lists were limited in area
X-ray ids (excl. those in Magellan lists) Bright masks contain an average of 160 slits per set of masks 29,000 spectra in total ? Faint masks contain average 250 slits per set of masks 11,250 spectra in total ? Bright catalogue Faint catalogue X-ray ids (excl. those in Magellan lists) 152 compulsory + 27 random 50 compulsory + 12 random Radio ids 48 compulsory + 786 random 41 compulsory + 385 random GALEX ids 133 compulsory 93 compulsory
assuming 0.67 efficiency due to weather alone 2005 2006 2007 2008 nominal 2 yr duration bright faint 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) maximum possible rate completion! assuming 0.67 efficiency due to weather alone
unacceptable 6 years to completion ?? April May June We 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 nights Visitor instrument scheduled on UT3
May possibly get a few more masks in June 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 spectra May possibly get a few more masks in June Not 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 2005
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 Alvio’s 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 COSMOS Present 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)