ESO Public Spectroscopic Surveys

1

Phase 1 proposal

Title: VANDELSA deep VIMOS survey of the CANDELS UDS and CDFS fields
Co-PIs: R. McLure, University of Edinburgh, UK, L. Pentericci, INAF-Rome, Italy;
CoIs: J. Dunlop, University of Edinburgh, UK; A. Fontana, INAF-Rome, Italy; A. Cimatti, University of Bologna, Italy; D. Elbaz, CEA, France; K. Nandra, MPE, Germany; O. Almaini, Universityof Nottingham, UK; R. Amorin, INAF-Rome, Italy; S. de Barros, UC Riverside, USA; I. Balestra,
INAF-Trieste, Italy; E. Bell, Michigan, USA; M. Bolzonella, INAF-Bologna, Italy; A. Bongiorno,
INAF-Rome, Italy; N. Bourne, University of Edinburgh, UK; R. Bowler, University of Edinburgh,
UK; V. Bruce, University of Edinburgh, UK; M. Brusa, University of Bologna, Italy; F. Buitrago,
University of Edinburgh, UK; K. Caputi, Groningen, Netherlands; P. Cassata, LAM, France; M.
Castellano, INAF-Rome, Italy; S. Charlot, IAP, France; M. Cirasuolo, University of Edinburgh,
UK; G. Cresci, INAF-Arcetri, Italy; S. Cristiani, INAF-Trieste, Italy; O. Cucciati, University of
Bologna, Italy; F. Cullen, University of Edinburgh, UK; E. Curtis-Lake, University of Edinburgh, UK;
M. Dickinson, NOAO, USA; S. Faber, University of California, USA; G. Fazio, Harvard, USA; H.
Ferguson, STScI, USA; S. Finkelstein, Texas, USA; F. Fiore, INAF-Rome, Italy; F. Fontanot,
INAF-Trieste, Italy; P. Franzetti, INAF-Milan, Italy; J. Fynbo, DARK, Denmark; A. Galametz,
INAF-Rome, Italy; B. Garilli, INAF-Milan, Italy; A. Georgakakis, MPE, Germany; M. Giavalisco,
UMASS, USA; A. Grazian, INAF-Rome, Italy; L. Guaita, INAF-Rome, Italy; W. Hartley, ETH,
Switzerland; A. Iovino, INAF-Brera, Italy; M. Jarvis, University of Oxford, UK; S. Juneau, CEA,France; W. Karman, Groningen, Netherlands; S. Kim, PUC, Chile; A. Koekemoer, STScI, USA;
O. Le Fèvre, LAM, France; J. Lotz, STScI, USA; F. Mannuci, INAF-Arcetri, Italy; E. MarmolQueralto, University of Edinburgh, UK; D. McLeod, University of Edinburgh, UK; J. Medez-Abreu,
University of St Andrews, UK; M. Mignoli, INAF-Bologna, Italy; M. Moresco, University of Bologna,
Italy; A. Mortlock, University of Edinburgh, UK; M. Nonino, INAF-Trieste, Italy; M. Pannella,
CEA, France; C. Papovich, Texas, USA; P. Popesso, MPE, Germany; L. Pozzetti, INAF-Bologna,
Italy; D. Rosario, MPE, Germany; P. Rosati, University of Ferrara, Italy; M. Salvato, MPE, Germany; C. Schreiber, CEA, France; M. Scodeggio, INAF-Milan, Italy; V. Sommariva, Universityof Bologna, Italy; D. Stark, Arizona, USA; M. Talia, University of Bologna, Italy; L. Tasca, LAM,
France; T. Treu, UCSB, USA; E. Vanzella, INAF-Bologna, Italy; V. Wild, University of St Andrews,
UK; C. Williams, UMASS, USA; G. Zamorani, INAF-Bologna, Italy

1.1

Abstract:(10 lines)

We propose a uniquely deep VLT spectroscopic survey of high-redshift galaxies, carefully designed to exploit
the multi-wavelength imaging and near-IR grism spectroscopy available in the CANDELS UDS and CDFS
fields. Our aim is to move beyond redshift acquisition, obtaining spectra with high enough signal-to-noise
to derive metallicities and velocity offsets from absorption and emission lines, allowing a detailed investigation of the physics of galaxies in the early Universe. Using integration times set to obtain a constant S/N(20 < tint < 80 hours), we will target: a) 2.5 < z < 5.5 star-forming galaxies with HAB ≤ 24 (IAB ≤ 25), b)
HAB < 22.5 passive galaxies at 1.5 < z < 2.5; c) fainter (HAB ≤ 27) star-forming galaxies at 3.0 < z < 7.0 andd) X-ray/radio selected AGN. Our strategy will deliver ≥ 2500 high signal-to-noise spectra within an area of' 0.2 deg2 . Combining the proposed VIMOS spectroscopy with the best optical+nearIR+Spitzer imaging willproduce a unique legacy dataset, capable of unveiling the physics underpinning high-redshift galaxy evolution.

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ESO Public Spectroscopic Surveys

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Description of the survey: (Text: 3 pages, Figures: 2 pages)

2.1

Scientific rationale:

Phase 1 proposal

We propose to undertake a major ESO Public Spectroscopic Survey to perform the deepest ever spectroscopic
study of the high-redshift Universe. Our proposed investment of ESO time is only merited, and our key scientificgoals can only be realised, if this survey targets the fields with the very best multi-wavelength photometry andHubble Space Telescope (HST) infrared+optical imaging. The CANDELS survey1 (Co-PIs Faber & Ferguson)
has recently completed its allocation of 902 HST orbits, delivering 0.25 deg2 of deep WFC3 and ACS imaging over
GOODS N+S, UDS, COSMOS & EGS (Grogin et al. 2011). Our proposed spectroscopic survey (VANDELS)
aims to provide fundamental new insights into the processes of high-redshift galaxy evolution by obtaining
repeated VIMOS observations of the CANDELS UDS and CDFS fields (see Fig. 1).
In addition to the quality of the available optical-nearIR data, our chosen survey fields are covered by ultra-deep
Spitzer imaging (50 hrs per filter) from S-CANDELS (PI Fazio), providing the deepest available 3.6+4.5µm
imaging on these angular scales (mAB ' 26.5 (5σ) at 3.6µm). Moreover, each field also features deep WFC3/IR
grism spectroscopy from the public 3D-HST programme (Brammer et al. 2012). When combined with the
deepest available Y + K imaging provided by our large HAWK-I programme (Fontana et al. 2014), it is clearthat the UDS/CDFS fields (and COSMOS, see Section 9.3) are now the key legacy fields for studying the highredshift Universe. This proposal aims to fully capitalise on ESO’s and ESA’s existing investment by providing
the ESO community with an unprecedented spectroscopic dataset for high-redshift galaxy evolution studies.
Our fundamental science goal is to move beyond simple redshift acquisition and obtain a spectroscopic dataset
deep enough to study the astrophysics of high-redshift galaxy evolution. Consequently, we propose to target alarge sample (' 900) of HAB ≤ 24 star-forming galaxies in the redshift range 2.5 < z < 5.5 using photometricredshift pre-selection. Our adopted HAB ≤ 24 magnitude limit (S/N ≥ 50 in the CANDELS H−band imaging),
which corresponds closely to a stellar-mass completeness limit of M? ≥ 5 × 109 M , is both pragmatic andscientifically motivated. Pragmatically, ≥ 90% of HAB ≤ 24 galaxies also have IAB ≤ 25, meaning that wecan provide robust continuum detections (S/N ≥ 10 per pixel) in 20 hour integrations for the brightest objects(IAB ≤ 23.5), increasing to 80 hours for the faintest IAB ' 25 galaxies. Scientifically, at this magnitude limit
the existing 3D-HST (1.1µm < λ < 1.7µm) grism spectroscopy provides continuum detections with S/N ≥ 5
per resolution element. Moreover, HAB = 24 is also the practical limit for extracting reliable morphologicalinformation from the available CANDELS HST imaging (e.g. Grogin et al. 2011; Bruce et al. 2012).
To study the descendants of our high-redshift star-forming galaxies, we also propose to target a complementary
sample of massive (HAB ≤ 22.5) passive galaxies at 1.5 < z < 2.5, for which age/metallicity information fromVIMOS can be combined with robust Balmer break measurements from 3D-HST grism spectroscopy. Finally,
we also propose to extend our spectroscopic study to fainter magnitudes and higher redshifts by targeting faint(HAB ≤ 27) Lyman-break galaxies (LBGs) and active galactic nuclei (AGN) at 3 < z < 7. Our adopted strategy
of repeated observations of each pointing, with on-source integrations of between 20 and 80 hours, will delivera minimum of ' 2560 high signal-to-noise spectra over a total survey area of ' 0.2 deg2 . In Fig. 2 we show
the expected VANDELS redshift distribution, estimated using the high-quality photometric redshifts available
in UDS+CDFS, within the context of previous high-redshift spectroscopic surveys. In the next section weconcentrate on describing the four science cases which are the main drivers of our survey design, before finishingwith a brief discussion of the immense legacy value offered by VANDELS.

2.2

Objectives: Understanding the physics of high-redshift galaxy evolution

The luminosity function of star-forming galaxies at 2.5 < z < 8.0 has now been determined to high accuracy,
over two orders of magnitude in luminosity (e.g. McLure et al. 2013a; Bowler et al. 2014; Bouwens et al. 2014;
Reddy & Steidel 2009). Consequently, the focus is now moving beyond basic demographics to understanding
the physics of high-redshift galaxy evolution. Within this context, it is clear that the observational priorities
1 http://candels.ucolick.org/

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ESO Public Spectroscopic Surveys

Phase 1 proposal

are accurate determinations of the form and evolution of the stellar mass function, star-formation rate density(SFRD), specific star-formation rate (sSFR) and the mass-metallicity relation at z ≥ 2.5. However, all current
studies are fundamentally undermined by interrelated and insidious uncertainties in stellar mass, metallicity,
star-formation rate and dust attenuation. The main goal of this proposal is to circumvent many of these issues
by providing, for the first time, spectra with high enough signal-to-noise and resolution to subject high-redshift
galaxies to detailed absorption line studies.
Constraining the metallicity, dust and star-formation rate of 2.5 < z < 5.5 galaxies
It has become increasingly clear that metallicity provides a powerful constraint on galaxy evolution because ofits direct link to past star formation and sensitivity to interaction (inflow/outflow) with the IGM (e.g. Davé etal. 2011). Knowledge of the metallicity of stellar populations is also fundamental to achieve accurate estimatesof both star-formation rates (and hence global SFRD) and dust extinction (e.g. Castellano et al. 2014). Indeed,
the slope of the UV continuum, currently used as a proxy for dust extinction, is known to be a strong functionof both metallicity and age (Rogers et al. 2014).
Our recent work has demonstrated that, given deep enough spectroscopy, this problem can be overcome using
photospheric UV absorption lines (1370-1900Å) whose equivalent width is sensitive to metallicity and independent of other stellar parameters (Sommariva et al. 2012). By providing ' 900 spectra at 2.5 < z < 5.5 withwhich to measure stellar metallicities, both individually and via stacking, the proposed observations offer theprospect of transforming our understanding of metallicity at high redshift (c.f. Halliday et al. 2008; Fig. 3).
By combining absorption-line metallities with accurate stellar masses based on WFC3/IR+IRAC photometry,it will be possible, for the first time, to study the evolution of the stellar mass − stellar metallicity relation out
to z ' 5 (c.f. Mannucci et al. 2010; Cullen et al. 2014).
The stellar mass function and star-formation histories at z ≥ 2.5
Any viable model of high-redshift galaxy evolution must be able to simultaneously reproduce the evolution ofboth the SFRD and the stellar mass function. The key to addressing these issues is accurate spectral energy
distribution (SED) fitting, including both the near-IR and mid-IR bands. This was the original motivationof the CANDELS survey and the resulting HST imaging dataset is currently unrivalled. However, current
uncertainties in the estimates of stellar masses, SFRs and SFHs caused by the lack of spectroscopic redshifts, andthe metallicity/dust degeneracy discussed above, make it impossible to exploit the full potential of CANDELS.
Moreover, progress is further limited by the uncertain impact of nebular line emission (e.g. de Barros, Schaerer& Stark 2014), compounded by recent results which show that the equivalent widths of key nebular lines increase
rapidly with redshift (Shim et al. 2011; Stark et al. 2013; Smit et al. 2014).
Our VIMOS programme will deliver a unique sample of ' 2200 spectroscopically-confirmed star-forming galaxies
at 2.5 < z < 7.0, with ' 900 bright enough to use for metallicity measurements. The spectroscopic redshifts
alone will substantially improve the accuracy of stellar-mass estimates (e.g. Santini et al. 2012) and largely
circumvent the nebular emission problem by allowing detailed SED fitting (in extreme cases excluding thephotometric bands affected by strong nebular emission lines). Metallicity-constrained SED fitting will also
remove degeneracies between metallicity and age, allowing us to derive the improved age measurements necessary
to constrain evolutionary models. Our proposal will deliver a spectroscopic determination of the bright end of
the evolving stellar mass function and SFRD with unprecedented accuracy, providing the benchmark dataset
for future high-redshift galaxy evolution studies.
Massive galaxy assembly at 1.5 < z < 2.5
A key component of this proposal is obtaining deep spectroscopy of ' 300 massive (M? ≥ 1010.5 M ) passive
galaxies at 1.5 < z < 2.5. This population holds the key to understanding the quenching mechanisms responsible
for producing the strong colour bi-modality observed at z ≤ 1, together with the significant evolution in thenumber density, morphology and size of passive galaxies observed between z = 2 and the present day (e.g.
Bruce et al. 2012; McLure et al. 2013b). For passive z ' 2 galaxies, VIMOS spectroscopy will provide thecrucial rest-frame UV absorption-line information (e.g. MgUV, 2640Å/2900Å breaks, see Fig. 4) needed tocomplement Balmer break measurements from 3D-HST. Using the VIMOS+3D-HST spectra, it will be possible
to break age/dust/metallicity degeneracies and deliver accurate stellar mass, dynamical mass, star-formation
rate, metallicity and age measurements via full spectrophotometric SED fitting (e.g. McLure et al. 2013b).

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Phase 1 proposal

Measuring galactic outflows at 1.5 < z < 4.5
An additional goal of our proposal is to study outflows of interstellar gas. Recent work suggests that highvelocity outflows may be ubiquitous for star forming galaxies at z > 1, with mass outflow rates comparable
to the rates of star formation (e.g. Weiner et al. 2009; Bradshaw et al. 2013). There is also evidence thatvery compact starbursts can produce extreme outflows with velocities in excess of 1000 km s−1 , yielding windsthat were previously only thought possible from AGN activity (Diamond-Stanic et al. 2012). Such outflows
may be playing a major role in the termination of star formation at high redshift and the build-up of the
mass-metallicity relation.
The high signal-to-noise, rest-frame UV, spectra delivered by VANDELS for both individual and small-number
stacks of star-forming galaxies at 2.5 < z < 4.5 will allow accurate measurements of outflowing ISM velocities
from high and low-ionization UV (1200Å-1700Å) interstellar absorption features (e.g. Shapley et al. 2003).
At lower redshifts, 1.5 < z < 2.5, it will be possible to exploit the strong Mg ii (2800Å) interstellar aborption
feature (e.g. Weiner et al. 2009; Bradshaw et al. 2013). Our key aim is to measure the outflow rate as afunction of stellar mass, SFR, and galaxy morphology, to understand the impact of galactic outflows on starformation at z ≥ 2. Measuring the balance of inflow, outflow and star formation will enable us to test models
for the evolving gas reservoir (e.g. Dayal, Ferrara & Dunlop 2012) and address the origins of the Fundamental
Metallicity Relation (Mannucci et al. 2010). Finally, comparing the outflow velocities of star-forming galaxies
with and without hidden AGN (as identified from X-ray emission) will allow the role of AGN feedback inquenching star formation and the build-up of the red sequence to be investigated (e.g. Cimatti et al. 2013).
Summary and legacy value
The immediate objective of the VANDELS survey is to obtain uniquely-deep (20-80 hour integrations), moderateresolution, optical spectra (0.48µm < λ < 1.0µm) of ≥ 2500 galaxies in the CANDELS UDS and CDFS fields. In
combination with the unparalleled multi-wavelength photometry available in these fields, the high signal-to-noise(10 − 20 per pixel) VANDELS spectra are primarily designed to yield accurate stellar metallicity, stellar age, gas
outflow and stellar mass measurements for large, statistical, samples of star-forming galaxies at 2.5 < z < 7.0.
The fundamental goal of VANDELS is to provide spectra with sufficient signal-to-noise to investigate the astrophysics of high-redshift star-formation via the first detailed absorption line studies of well-defined samples ofhigh-redshift galaxies.
The immense legacy value of the VANDELS survey is compelling: simply by providing spectra of relatively fainttargets with unprecedentedly high signal-to-noise, VANDELS is guaranteed to open up new parameter spacefor investigating the physical properties of high-redshift galaxies. For example, VANDELS will fundamentally
improve our knowledge of the statistics of Lyα emission in star-forming galaxies approaching the reionization
epoch (e.g. Pentericci et al. 2011; Stark et al. 2011; Curtis-Lake et al. 2012; Cassata et al. 2014) andfacilitate the identification of the progenitors of compact galaxies amongst star-forming galaxies at z ≥ 2.5 (e.g.
Williams et al. 2013). Finally, thanks to the large number of targets that can be allocated on each VIMOS
mask, VANDELS will also facilitate the study of rarer bright systems such as Herschel detected galaxies andAGN. For these bright systems, the deep VANDELS spectroscopy will make it possible to assess their physicalconditions (e.g. metallicities, ionizing fluxes and outflows signatures) and compare them with those of less active
systems at the same redshifts.
The following table provides a summary of the different VANDELS sub-samples and the expected number ofobjects to be targeted for 20, 40 and 80 hour integrations:
TARGET TYPE
Star-forming 2.5 < z < 5.5
Passive 1.5 < z < 2.5
Faint LBGs 3.0 < z < 7.0
Faint AGN 3.0 < z < 5.0
TOTAL

OPO, ESO (opo@eso.org)

Selection
H < 24
H < 22.5
H < 27
X-ray/Radio

Science
Metallicity/dust/outflows
SF quenching
Redshifts, LFs, Lyα fraction
Outflows, feedback

N80
335
210
95
−
640

N40
350
60
820
50
1280

N20
195
30
365
50
640

TOTAL
880
300
1280
100
2560

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Phase 1 proposal

Figure 1: Layout of the proposed VANDELS pointings (4 in UDS and 4 in CDFS). In each figure the VIMOS
quadrants of a given pointing are shown as a different colour, overlaid on a greyscale image showing the available
HST H−band imaging (centre) and wider-area, ground-based, near-IR imaging. The total area covered by the 8
VIMOS pointings is ' 0.2 square degrees. The quality of the multi-wavelength data available over the wider UDS
and E-CDFS fields is now excellent, and more than sufficient to select H ≤ 24 targets over the full VANDELS
footprint. The pointings are placed to ensure that each VIMOS quadrant always intersects the CANDELS HST
imaging which, due to the high surface density of faint z ≥ 3 galaxies, allows us to maximize the number of slits
in the central HST area. The spectroscopic slits will be placed E-W on the sky, as recommended to minimize
slit losses during long integrations on fields at these declinations (Sánchez-Janssen et al. 2014).

Figure 2: LHS: the redshift distribution of the VIMOS Ultra Deep Survey (VUDS) within the context of othernotable spectroscopic surveys (Le Fèvre et al. 2014). It is clear that even VUDS has a redshift distribution
which is strongly peaked at z = 2.5 − 3, with only a small fraction of galaxies at z ≥ 4 and only 26 galaxies intotal at z ≥ 5. RHS: the expected redshift distribution of VANDELS, based on photometric redshifts derived
from the multi-wavelength data available in the UDS and CDFS fields. The VANDELS selection function should
ensure that ' 20% of the final sample are at z ≥ 4, with ' 200 galaxies at z ≥ 5.
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ESO Public Spectroscopic Surveys

1300

1400

Phase 1 proposal

1500

1600

1700

1800

1900

2000

2100

Figure 3: Stacked UV spectrum of 75 star-forming galaxies at z ' 2 observed as part of the GMASS survey(Halliday et al. 2008). This stacked spectrum has S/N' 100 and illustrates the wealth of information available
from interstellar (dotted) and photospheric (dashed) absorption features. In red we have highlighted the socalled 1978-index (Rix et al. 2004), used by Halliday et al. to determine the stellar metallicity from the stack.
The final VANDELS sample will contain ' 900 spectra covering the rest-frame UV of star-forming galaxies atz ≥ 2.5, each with S/N ' 10 − 20 per pixel. Consequently, using stacks of small numbers of objects, it will bepossible to determine the average stellar metallicity as a function of redshift, stellar mass, SFR and age.

Figure 4: Stacked spectrum of 13 early-typegalaxies at hzi = 1.6 from GMASS (Cimatti etal. 2008), showing the crucial age-dependent information provided by deep spectroscopy of the
MgUV region. The stacked GMASS spectrum has
an equivalent exposure time of 480 hours and thesame spectral resolution as the proposed VANDELS spectroscopy. The red and blue curves
show 1 Gyr old model fits with solar metallicity
BC03 and Maraston05 models respectively. VANDELS will provide stacked spectra of this quality
for small (i.e. ' 6 object) sub-samples.

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ESO Public Spectroscopic Surveys

3

Phase 1 proposal

Are there similar ongoing or planned surveys? (1 page)

ESO has a long standing tradition of allocating many hundreds of hours of service time observations to large
spectroscopic surveys, many of which have made extensive use of the multi-plex capabilities of VIMOS: VVDS,GOODS, zCOSMOS, VUDS & VIPERS. The spectroscopic redshift distributions produced by these projects
are shown in Fig. 2. Most of these previous VIMOS spectroscopic surveys are either long-completed (e.g.
VVDS, GOODS & zCOSMOS), or primarily low-redshift cosmology experiments (i.e. VIPERS). However, it
is clearly of interest to discuss our current VANDELS proposal within the context of the recently-completed
VIMOS Ultra Deep Survey (VUDS; Le Fèvre et al. 2014).
The primary objective of VUDS was to obtain deep VIMOS spectroscopy for 10,000 (IAB ≤ 25) galaxies overan area of 1 square degree in three survey fields: COSMOS (8 pointings), VVDS-02 (5 pointings) & E-CDFS(3 pointings). The survey used a combination of photometric and colour pre-selection techniques to target
galaxies in the redshift interval 2 < z ∼ 6, typically obtaining 14 hours of integration on each target with the
low resolution red and blue grisms (i.e. LR RED+LR BLUE). The VUDS spectra therefore provide a total
wavelength coverage of 3600Å< λ < 9350Å at a spectral resolution of 180 < R < 210 (Le Fèvre et al. 2014).
In terms of selection function, depth and spectral resolution it is clear that VANDELS is both complementary to
VUDS and surpasses it. Although VUDS targeted objects in the redshift range 2 < z < 6, it is clear from Fig. 2that the majority of the spectroscopic redshifts are in the range 2 < z < 3. Moreover, Fig. 2 also demonstrates
that only a very small fraction of VUDS galaxies are at z ≥ 4, with virtually no galaxies at z ≥ 5 (26 in total,
Le Fèvre et al. 2014). In contrast, the pre-selection we will employ for VANDLES is based on photometricredshift probability functions derived from high-quality multi-wavelength catalogues, which are already in place(Galametz et al. 2013; Guo et al. 2013), and will ensure that ' 80% of VANDELS targets are in the range
2.5 < z < 7.0. Crucially, our pre-selection will allocate ' 20% of VANDELS slits to galaxies at z ≥ 4 and will
target ' 200 galaxies at 5.0 < z < 7.0. It is therefore clear that VANDELS promises to open up new parameter
space for detailed galaxy evolution studies at z ' 4 and beyond.
The minimum VANDELS exposure time is 20 hours of on-source integration, with 75% of the sample being
allocated 40-80 hours of integration (c.f. 14 hours in VUDS). Moreover, our choice of the MR grism with the
GG475 order sorting filter provides wavelength coverage out to 1.0µm (c.f. 0.935µm in VUDS), fully exploiting
the red sensitivity of the upgraded VIMOS detectors and pushing our Lyα redshift limit to z ' 7.1. In addition
to improved signal-to-noise, the factor of three improvement in spectral resolution provided by the MR grism
is crucial for extracting accurate stellar metallicities, ages and outflow velocities from absorption features in
the rest-frame UV. Moreover, the improved spectral resolution will lead to significant improvements in sky
subtraction at λ > 7500Å, which is vital for reliably measuring faint absorption/emission features at z ≥ 5.
The final area in which VANDELS holds a significant advantage over previous spectroscopic surveys is in
the quality of the multi-wavalength data available in the UDS+CDFS fields. The key advantages lie in theunparalleled CANDELS WFC3/IR imaging and the ultra-deep Spitzer data from the S-CANDELS survey. In
addition to providing a significant improvement in the initial selection function, the availability of these data
ensure that the majority of VANDELS spectra will be accompanied by detailed HST morphological information
and accurate stellar masses.

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Phase 1 proposal

Observing strategy: (1 page)

We propose to target the CDFS and UDS with four overlapping VIMOS pointings per field, designed to provide
full coverage of the WFC3/IR imaging provided by CANDELS (see Fig. 1). We intend to design four VIMOS
masks per pointing, observing each mask for 20 hours of on-source integration. The instrument set-up for allobservations will be identical: MR grism+GG475 order sorting filter, 100 slit widths and a minimum slit lengthof 600 . We intend to employ individual integrations of 1200s, using a three-point off-set pattern along the slit
(i.e. +0.7500 , 0.000 , −0.7500 ). Using this instrumental set-up, repeated simulations with VMMPS indicates that
we can typically allocate 180 slits per mask (45 per VIMOS quadrant). The pointings (Fig. 1) are placed toensure that each VIMOS quadrant always intersects the CANDELS HST imaging which, due to the high surface
density of faint z ≥ 3 galaxies, allows us to maximize the number of slits in the central HST area. To ensurethat our slits can be placed with maximum accuracy we will require short R−band pre-imaging observations of
all eight pointings to properly account for VIMOS focal plane distortions. Moreover, due to the extended redcoverage provided by the MR grism, we will also require additional spectrophotometric standard observations
designed to provide good signal-to-noise out to λ ' 1µm.
To accommodate the range of exposure times needed to meet our science goals we will pursue a nested slit allocation strategy, whereby objects requiring 80 hours of integration are retained on all four masks, those requiring
40 hours are included on two masks and those only requiring 20 hours integration appear on a single mask.
The following table illustrates our slit-allocation for a single VIMOS quadrant over a series of four masks. The
columns indicate the number of objects that would be observed for 80 (N80), 40 (N40) and 20 hours (N20):
Mask
1
2
3
4
no. of spectra

N80
20
20
20
20
20

N40
20
20
20
20
40

N20
5
5
5
5
20

Total

80

Using this nested strategy, each quadrant provides 20 objects which receive 80 hours of integration (retained on all
4 masks), 40 objects which receive 40 hours of integration(two sub-samples of 20) and 20 objects which receive 20
hours of integration (four sub-samples of 5).

Over our 8 VIMOS pointings we will therefore obtain a total of 2560 spectra, comprised of 640 (N80), 1280
(N40) and 640 (N20) targets. Uniquely, 75% of our targets will receive 40+ hours, with 25% receiving 80 hours.
Feasibility and scheduling
Here we evaluate the feasibility of scheduling VANDELS within a nominal ∼ 3 year survey period in visitor
mode. We explicitly assume that we require dark/grey time, photometric/clear sky conditions and ≤ 100 FWHM
seeing. In addition to accounting for the frequency of poor seeing and non-photometric/clear conditions, we also
assume that we will not observe either of our two fields when they are above an airmass of secz > 1.4 and that
visitor mode is impractical unless we can observe for 4+ hours per night. Moreover, in liaison with the ESO
survey team, we have accounted for telescope downtime due to weather (10% in Oct, 5% in Sep, Nov & Dec)
and technical faults (2.5%; Hammersley et al. 2013). Finally, we have used the P2PP3 software to calculate aconservative overhead estimate of 25% (including calibration).
Under these restrictions, we calculate that there are ' 277 hours of actual on-source science observations
available for VANDELS between 1st September and 21st December each year, with the majority ' 166 hours
available during Oct-Nov. These figures should be compared against our requirement for 640 hours of on-source
science observations (8x80 hours). This leads to two potential scenarios for completing VANDELS within a ' 3
year survey period. In both cases, the total number of visitor nights refers to the actual number of nights wewould require a visiting astronomer at the telescope.
Scenario 1: VANDELS is allocated a flat distribution of the dark/grey time during Sep-Dec. In this scenario
the survey could be completed in a 3-year period using a total of ' 195 visitor nights (including half nights).
This requires VANDELS to be allocated ' 77% of the available dark/grey time on UT3 over periods P94-P99.
Scenario 2: VANDELS is allocated '100% of the dark/grey time available in Oct-Nov, when the UDS+CDFS
fields are at their most observable from Paranal. In this scenario VANDELS could be completed over a ' 3-year
period (P94-P100) using a total of ' 180 visitor nights.
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Phase 1 proposal

Instrumental setup & pre-imaging needs

The complete VANDELS survey consists of 8 overlapping VIMOS pointings (4 in UDS and 4 in CDFS), with
4 different masks being observed at each pointing. In total we therefore require 32 VIMOS masks and preimaging at 8 different pointings. Our pre-imaging requirements are independent of the survey schedule so, as anillustration, in the table we have assumed an observing schedule where all VANDELS observations are confinedto Oct+Nov. In this scenario we would begin observing the VANDELS fields at the start of each even period(1st Oct), which means that the pre-imaging would have to be obtained in service mode at the end of theprevious odd period (see Table in Section 6).
Periods (94-104)
P94
P96
P98
P100

6

grism/filter
MR/GG475
MR/GG475
MR/GG475
MR/GG475

total no. of masks
88
8
8

no. pre-images required2222

pre-imaging time (n)
0.2n
0.2n
0.2n
0.2n

Estimated observing time:

In the table below we have calculated our likely observing time requirement. In the first scenario we haveassumed VANDELS is allocated ' 77% of the dark/grey time on UT3 during Sep-Dec for 3 years (P94-P99).
In the second scenario we have assumed VANDELS is allocated 100% of the dark/grey time on UT3 during
Oct-Nov during periods P94, P96, P98 & P100. In both cases the nights listed in column 2 are the actual
number of nights we expect to need an observer at the telescope in visitor mode (including half/part nights),
and the conversion between nights and hours assumes 1 night = 8 hours. We have inserted an extra column toseparate traditional observing overheads (estimated to be ' 25%) from unavoidable time losses in visitor mode
which would not be charged to service mode observations (i.e. fields visible for part nights, bad weather, poor
seeing, cloud and technical downtime). Scenario one is less efficient than scenario two simply because of thegreater number of half/part nights in Sep and Dec.
Scenario 1: Sep-Dec observing
Periods(94-104)
P93
P94
P95
P96
P97
P98
P99

Total Timeinc. all overheads (n)
0.2n
51n
14.2n
51n
14.2n
51n
14n

Pre-imaging
time (n)
0.1n
0.0n
0.1n
0.0n
0.1n
0.0n
0.0n

Science exp.
time (n)
0.0n
21.0n
5.7n
21.0n
5.7n
21.0n
5.7n

Overheadstime(n)
0.1n
5.3n
1.5n
5.3n
1.5n
5.3n
1.4n

Vistor mode loss
time (n)
0.0n
24.7n
6.9n
24.7n
6.9n
24.7n
6.9n

Mean RA
–
03h
03h
03h
03h
03h
03h
03h

Moon
–
grey
grey
grey
grey
grey
grey
grey

Pre-imaging
time (n)
0.1n
0.0n
0.1n
0.0n
0.1n
0.0n
0.1n
0.0n

Science exp.
time (n)
0.0n
20.0n
0.0n
20.0n
0.0n
20.0n
0.0n
20.0n

Overheadstime(n)
0.1n
5.0n
0.1n
5.0n
0.1n
5.0n
0.1n
5.0n

Vistor mode loss
time (n)
0.0n
20.0n
0.0n
20.0n
0.0n
20.0n
0.0n
20.0n

Mean RA
–
03h
03h
03h
03h
03h
03h
03h
03h

Moon
–
grey
grey
grey
grey
grey
grey
grey
grey

Scenario 2: Oct-Nov observing
Periods(94-104)
P93
P94
P95
P96
P97
P98
P99
P100

Total Timeinc. all overheads (n)
0.2n
45n
0.2n
45n
0.2n
45n
0.2n
45n

OPO, ESO (opo@eso.org)

page 9 of 18

ESO Public Spectroscopic Surveys
6.1

Phase 1 proposal

Time justification: (1 page)

To allow detailed (rest-frame UV) absorption-line studies of faint star-forming galaxies at 2.5 < z < 5.5
we require robust continuum detections of individual objects at IAB ≤ 24.5 and in stacks of fainter objects.
Specifically, as shown in Sommariva et al. (2012), we will require a continuum S/N ' 10−20 per pixel to reliably
measure the most robust stellar metallicity indicators at λrest = 1370 − 1501Å. Moreover, with reference to
Cimatti et al. (2008), for our passive galaxy sample at 1.5 < z < 2.5 we require a high enough S/N to robustly
detect the prominent UV breaks at λrest = 2640Å/2900Å and the other metal absorption lines (e.g. MgI
λ2852, MgII λ2800, FeI and FeII) which together comprise the MgUV feature (Dunlop et al. 1996; Daddi et al.
2005). Crucially, it is the presence of the MgUV feature which confirms the lack of significant star formation
over the last ' 0.5 Gyr. Finally, for the faint LBG component of our sample we require to reach S/N = 3 inthe continuum or detect emission-line fluxes of ' 1 − 2 × 10−18 erg s−1 cm−2 at S/N > 5.
The instrument set-up for all our VIMOS MOS observations is identical: MR grism with the GG475 order sorting
filter, providing wavelength coverage over the wavelength range 480-1000nm at a mean spectral resolution of
R ' 580 (100 wide slits). Based on this observational set-up, we have used the on-line VIMOS ETC to calculate thecontinuum and line-flux sensitivity of our proposed observations. All calculations are based on a flat spectrum
point-source input, and reflect the average observing conditions we expect in visitor mode (see Section 4):• Sky conditions of 7 days from new moon (i.e. grey)• Airmass=1.15• Seeing 0.900 FWHM
In the following table we report the signal-to-noise ratio we expect in the final extracted 1D spectra. Column
one lists the input continuum apparent magnitude, while columns 2-4 list the final signal-to-noise ratio per pixel
expected for 80, 40 and 20 hours of on-source integration. It should be noted that these numbers include a
2x2 binning of the final spectra in the spectral direction, which can be achieved without degrading the spectral
resolution because of the oversampling of a projected 100 wide slit by the VIMOS detectors.
Continuum
IAB = 25.0
IAB = 24.5
IAB = 24.0
IAB = 23.5

S/N (80)
10.8
17.1
−
−

S/N (40)
7.6
12.1
19.1
−

S/N (20)
5.4
8.5
13.5
21.2

It can be seen that the desired constant SNR of ' 20
per pixel can be achieved in 20, 40 and 80 hours ofintegration for objects with IAB = 23.5, 24.0 & 24.5
respectively. Objects with IAB ' 25 will need to be
stacked in small sub-samples to reach the same SNR.

In addition to absorption-line studies, a key element of VANDELS is the ability to detect faint Lyα emission atz ≥ 3. The following table reports the expected Lyα line-flux limits (/erg s−1 cm−2 ) for the full integration timeof 80 hours (both 10σ and 5σ limits). In the table, the relationship between line-flux limit and redshift reflects
the varying VIMOS throughput and sky background as a function of wavelength (column 2). The line-flux
limits assume a line with FWHM=10Å and are integrated over the line profile.
Redshift
z=3
z=4
z=5
z=6
z=7

Lyα Wavelength
4864Å
6080Å
7296Å
8512Å
9728Å

OPO, ESO (opo@eso.org)

10σ
4.4E-18
2.0E-18
2.3E-18
4.8E-18
9.5E-185σ
2.2E-18
1.0E-18
1.2E-18
2.4E-18
4.8E-18

It should be noted that these line-flux limits correspond to spectral regions between strong sky lines.
Limits within bad sky-line regions are expected to bea factor of two higher.

page 10 of 18

ESO Public Spectroscopic Surveys

7

Phase 1 proposal

Data management plan: (3 pages)

Survey monitoring, managing, and data reduction will be centralised within the data reduction and management
centre at INAF−IASF Milano. The full management of these operations will be achieved by adapting the
Easylife system (Garilli et al. 2012, PASP, 124, 1232) to VANDELS needs.
A dedicated Survey Web Site will allow the whole community to follow the survey progress in terms of observations, data reduction and data products availability. The web site will also allow team members to retrieve
raw or reduced data, and to upload further data products into the survey database. All survey related data,from the input photometric catalogues, to all spectroscopic data, will be made available through a dedicateddatabase, with an available Web interface accessed from the Survey Web Site, along the lines of the ongoing
VIPERS survey (see http://vipers.inaf.it).
The Easylife administrative layer, accessible via the Survey Web Site, will allow the survey status to be bothmonitored and managed, handling all survey management steps from the pre-imaging phase to the distributionof data reduction and redshift measurement tasks. A data presentation layer will be used to generate the live
public Web pages of the VANDELS survey. Data ingestion, organisation and reduction tasks will be carried out
via dedicated tools running on a dedicated machine. Finally, the results database (based on MySQL) will beaccessed through a dedicated GUI. Access to these different layers is controlled via an administrative SQL-based
database, which will keep track of the global status of the survey, the composition of the survey team, and of
the data access privileges for the public and for the survey team. It will be structured to have different accesslevels: public and team restricted. Through the public level, the whole community will have access to the survey
description and goals, and can inspect how the survey is advancing in terms of observations, reduction and data
availability. The public level will also allow the community to connect to the database system, which will provide
the reduced spectra together with catalogues of scientific information extracted from them. Through the teamrestricted level, each member of the team will be able to retrieve data products prior to public distribution for
quality checks and analysis and upload their corresponding scientific measurements.
REFERENCES
Bouwens R.J., et al., 2014, ApJ arXiv:1403.4295• Bowler R., et al., 2014, MNRAS, in press, arXiv:1312.5643•
Bradshaw E., et al., 2013, MNRAS, 433, 194• Brammer G., et al., 2012, ApJS, 200, 13• Bruce V.A., et al., 2012,
MNRAS, 427, 1666• Cassata P., et al., 2014, A&A, arXiv:1403.3693• Castellano M., et al., 2014, A&A, in press,
arXiv:1403.0743 • Cimatti et al., 2008, A&A, 482, 21• Cimatti et al., 2013, ApJ, 779, L13• Cullen F., et al.,2014, in press, arXiv:1310.0816• Curtis-Lake E., et al., 2012, MNRAS, 422, 1425• Daddi E., et al., 2005, ApJ,
626, 680• Davé et al., 2011, MNRAS, 416, 1354• Dayal P., et al., 2013, MRAS, 430, 2891 • Diamond-Stanic A.,
2012, ApJ, 755, L26 • Dunlop J.S. et al., 1996, Nature, 381, 581 • Fontana A., et al., 2014, A&A, submitted•
Grogin N.A., et al., 2011, ApJS, 197, 35• Halliday C., et al., 2008, A&A, 479, 417• Hammersley P., et al., 2013,
Msngr, 151, 2• Le Fèvre O., et al., 2014, A&A, arXiv:1403.3938• Mannucci et al., 2010, MNRAS, 408, 2115•
McLure R.J., et al., 2013a, MNRAS, 432, 2696• McLure R.J., et al., 2013b, MNRAS, 428, 1088• Pentericci L.,
et al. 2011, ApJ, 743, 132• Rogers A., et al., 2014, MNRAS, in press, arXiv:1312.4975 • Reddy N., Steidel C.,
2009, ApJ, 692, 778• Rix S., et al., 2004, ApJ, 615, 98 • Sánchez-Janssen et al., 2014, A&A, arXiv:1402.5970•
Santini P., et al., 2012, A&A, 538, 33• Schaerer D., de Barros S., 2009, A&A, 502, 423• Shapley A., et al., 2003,
ApJ, 588, 65• Shim H., et al., 2011, ApJ, 738, 69• Sommariva V., et al. 2012, A&A, 539, 136• Stark D., et al.,2011, ApJ, 728, L2• Stark D., et al., 2013, ApJ, 763, 129• Smit R., et al., 2014, ApJ, 784, 58• Vanzella E., etal., 2009, ApJ, 695, 1163• Weiner B., et al., 2009, ApJ, 692, 187

OPO, ESO (opo@eso.org)

page 11 of 18

ESO Public Spectroscopic Surveys
7.1

Phase 1 proposal

Team members:

R. McLure
L. Pentericci
J. Dunlop
A. Fontana
K. Nandra
D. Elbaz
A. Cimatti
S. Finkelstein
A. Grazian
R. Bowler
M. Cirasuolo
A. Iovino
M. Nonino
O. Cucciati
A. Bongiorno
E. Curtis-Lake
M. Castellano
J. Mendez-Abreu
B. Garilli
M. Scodeggio
P. Franzetti
E. Vanzella
E. Marmol-Queralto
F. Cullen
I. Balestra
L. Guaita
P. Popesso
K. Caputi
W. Hartley
V. Sommariva
G. Cresci
A. Mortlock
N. Bourne
R. Amorin
S. Charlot
M. Mignoli

OPO, ESO (opo@eso.org)

Co-PI
Co-PI
Management GroupManagement GroupManagement GroupManagement GroupManagement Group
Target selection
Target selection
Target selection
Target selection
Mask production
Mask production
Mask production
Mask production
Observation supervision
Observation supervision
Observation supervision
Data reduction pipeline
Data reduction pipeline
Survey handling web site
Data quality + redshift determination
Data quality + redshift determination
Data quality + redshift determination
Data quality + redshift determination
Data quality + redshift determination
Data quality + redshift determination
Data quality + redshift determination
Data quality + redshift determination
Enhanced data products
Enhanced data products
Enhanced data products
Enhanced data products
Enhanced data products
Enhanced data products
Enhanced data products

University of Edinburgh
INAF, Rome
University of Edinburgh
INAF, Rome
MPE
CEA
University of Bologna
University of Texas
INAF, Rome
University of Edinburgh
University of Edinburgh
INAF, Brera
INAF, Trieste
INAF, Bologna
INAF, Rome
University of Edinburgh
INAF, Rome
University of St. Andrews
IASF-Milano
IASF-Milano
IASF-Milano
INAF, Bologna
University of Edinburgh
University of Edinburgh
INAF, Trieste
University of Stockholm
MPE
Groningen
ETH
University of Bologna
INAF, Arcetri
University of Edinburgh
University of Edinburgh
INAF, Rome
IAP
INAF, Bologna

UK
Italy
UK
Italy
Germany
France
Italy
USA
Italy
UK
UK
Italy
Italy
Italy
Italy
UK
Italy
UK
Italy
Italy
Italy
Italy
UK
UK
Italy
SwedenGermany
Netherlands
Switzerland
Italy
Italy
UK
UK
Italy
FranceItalypage 12 of 18

ESO Public Spectroscopic Surveys
7.2

Phase 1 proposal

Detailed responsibilities of the team: (1 page)The overall responsibility for both the successful execution of the survey and the distribution of the final data
products to the community will be shared by McLure and Pentericci as VANDELS Co-PIs. Importantly, both
McLure and Pentericci can guarantee ≥ 80% research time over the full duration of the proposed VANDELS
programme. The Co-PIs will be supported by the members of the VANDELS management group (Dunlop,
Fontana, Nandra, Elbaz & Cimatti) who have been chosen on the basis of their multi-wavelength expertise andtheir previous experience in managing large ESO projects (e.g. GOODS, Ultra-VISTA, K20, GMASS & UDSz).
In order to ensure that the various tasks needed for the successful planning, execution, reduction and distributionof the VANDELS survey and dataset are carried out efficiently, we have constructed small teams to focus on
specific tasks:
1. Target selection teamMembers: S. Finkelstein • A. Grazian • R. Bowler • M. Cirasuolo
The members of this team will be responsible for producing robust target catalogues from the available multiwavelength imaging in the UDS and CDFS fields to be used in VIMOS slit allocation. Together they willproduce consolidated catalogues of potential targets based on independent measurements of the photometricredshift probability functions.
2. VIMOS mask production teamMembers: A. Iovino • M. Nonino • O. Cucciati • A. Bongiorno
The members of this team will be responsible for the necessary VANDELS pre-imaging and using VMMPS to
optimally allocate slits based on the information supplied by the target selection team.
3. Observing supervision teamMembers: E. Curtis-Lake • M. Castellano • J. Mendez-Abreu
The members of this team will be responsible for constructing the OBs for VANDELS and the organisation andsupervision of the visitor observing programme.
4. Data Reduction teamMembers: B. Garilli • M. Scodeggio • P. Franzetti
The members of this team will be responsible for the full data-reduction process, taking the raw data andproducing fully wavelength-calibrated and flux-calibrated 1D and 2D spectra using the VIPGI and Easylife
software developed in IASF-Milano.
5. Data quality and redshift measurement teamMembers: E. Vanzella • E. Marmol-Queralto • F. Cullen • I. Balestra • L. Guaita • P. Popesso• K. Caputi • W. Hartley
The members of this team will be responsible for monitoring the quality of the spectra as they are obtained. Subsequently, this team will be responsible for redshift determination using a combination of automated algorithms
and detailed visual inspection.
6. Enhanced data products
Members: V. Sommariva • G. Cresci • A. Mortlock • N. Bourne • R. Amorin • S. Charlot• M. Mignoli
The members of this team will be responsible for the production and dissemination of value-added data products,
in addition to the basic 1D+2D spectra and associated redshifts. These data products will include line flux andEW measurements, together with basic stellar-population parameters derived from SED fitting.

OPO, ESO (opo@eso.org)

page 13 of 18

ESO Public Spectroscopic Surveys
7.3

Phase 1 proposal

Data reduction plan: (1 page)

VANDELS data reduction will be performed with a fully-automated pipeline, starting from the raw data andflowing down to the wavelength- and flux-calibrated spectra. The pipeline is an updated version of the algorithms
and dataflow from the original VIPGI system, fully described in Scodeggio et al. 2005, PASP, 117, 1284. The
original VIPGI system has been used to reduce all the spectra from the VVDS (Le Fèvre et al. 2005, A&A, 439,
845 and Garilli et al. 2008, A&A, 486, 683), zCosmos (Lilly et al. 2007, ApJS, 172, 70), and VUDS surveys(Le Fèvre et al. 2014), while the updated system, called Easylife, has been used to reduce all the spectra fromthe ongoing VIPERS survey (Garilli 2014, A&A, 486, 683).
As a first step, in each raw frame the 2D dispersed spectrum for each slit is located and traced. Each spectrum
is collapsed along the dispersion direction, and the object location computed. An inverse dispersion solution
is computed for each slit making use of an arc calibration lamp, and further checked on the wavelength ofprominent sky lines (if needed, a rigid offset is applied to the dispersion solution in order to bring the sky lines
to their correct wavelengths). The algorithm used leads to a wavelength calibration uncertainty below 1/5 of a
pixel for 97% of the spectra (see Garilli et al. 2014). Finally, a first sky subtraction is performed row by row,
avoiding the region identified as the object.
The sky-subtracted 2D spectra are then resampled to a linear wavelength scale making use of the inverse
dispersion solution, and packed together in a convenient data format. The different scientific exposures of eachslit are registered and co-added, and a second background subtraction is performed repeating the procedure
carried out before. Finally, 1D spectra are extracted applying the Horne extraction algorithm, and spectra arecorrected for the instrument sensitivity function, as derived from spectrophotometric standard star observations.
To obtain an absolute flux calibration, correcting for slit losses and varying observing conditions, we will
normalize observed spectral fluxes to the available photometry, convolving each spectrum with a photometricfilter response function (normally the i filter) and deriving a normalisation factor between the spectrum andthe photometric magnitude. Given a 2D spectrum, sky-subtraction residuals are computed and combined withthe poissonian statistical error to build an error spectrum.
Formally the above recipes, in their original form, always end successfully, but this does not automatically meanthat the results meet the degree of accuracy required to achieve the specific scientific goals. Consequently, inthe transition from VIPGI to Easylife, we have added additional quality checks on spectra location, wavelength
calibration, target detection rate, and general data quality, based on the mean PSF as measured from thereduced image, the measured sky brightness, and the object centring in the slit (see Garilli et al. 2012, PASP,
124, 1232, for more details). These additional checks are designed to ensure that the final reduction results are
scientifically exploitable.

OPO, ESO (opo@eso.org)

page 14 of 18

ESO Public Spectroscopic Surveys
7.4

Phase 1 proposal

Expected data products: (2 pages)

In the following description we have split the VANDELS data products into basic and enhanced categories. The
basic data products are those which could be regarded as the minimum required for a spectroscopic survey of thisnature. The enhanced data products are those which the team is committed to delivering as an additional service
to the community, and are designed to maximize the scientific productivity and legacy value of VANDELS. Alldata products listed here will be made available to the community in VO-compliant formats.

Basic Data Products• Final extracted one-dimensional (1D) spectra
The final wavelength-calibrated and flux-calibrated 1D spectra for all objects targeted by VANDELS. In addition
to flux calibration using spectrophotometric standards, the final flux calibration will be normalized to the multiwavelength photometry available for each target.
• Noise and sky spectra (1D)
In addition to the final extracted spectra, 1D noise and sky spectra will be made available for each target.
• Final stacked two-dimensional (2D) spectra
The final wavelength-calibrated 2D spectrum for each target will be made available. The 2D spectra will beessential for confirming the reality of faint emission features and identifying potential emission-line velocity
gradients.
• Final spectroscopic redshift catalogue
A final catalogue will be made available which contains the essential information for each VANDELS target:
ID, RA, Dec, I-magnitude, H-magnitude, spectroscopic redshift and redshift quality flag. The redshift quality
flag will be defined on the same system previously employed by the VVDS and zCOSMOS surveys.

Enhanced Data Products• VANDELS selection function
We intend to make available all of the relevant information necessary to compute the VANDELS selection
function. This information will allow the community to accurately calculate the completeness of the survey for
their own scientific purposes.
• Final combined spectroscopic redshift+photometric catalogue
In addition to the final spectroscopic redshift information, this catalogue will feature aperture-matched optical+nearIR photometry for each target. Based on this information the community should be able to directly
proceed to SED fitting using their preferred choice of software and templates.
• Basic stellar population parameters
Based on the final redshift+photometric catalogue we will release our own catalogue of basic stellar population
parameters (e.g. stellar mass, age & reddening) derived by fitting standard templates (e.g. Bruzual & Charlot
2003 + Chabrier IMF) using publicly available SED-fitting codes (e.g. Le Phare).
• Final catalogue of spectroscopic measurements
In addition to the final spectroscopic redshift determinations, a catalogue will be made available to the community which contains line flux and equivalent width measurements (together with error estimates and data
quality flags) for robustly detected emission/absorption features in each VANDELS spectrum.

OPO, ESO (opo@eso.org)

page 15 of 18

ESO Public Spectroscopic Surveys
7.5

Phase 1 proposal

General schedule of the project: (1 page)

Here we discuss the schedule of the observations, the data reduction process and the release of the basic andenhanced data products.
Observations
In Section 6 we presented two alternative scenarios for the scheduling of VANDELS in visitor mode, both ofwhich assume that spectroscopic observations will commence on 1st Oct 2014 (start of P94).
The first scenario assumed that VANDELS was allocated ' 77% of the available dark/grey time on UT3 in SepDec each year. According to this schedule we would require pre-imaging observations for the first VANDELS
pointings in mid-late Aug 2014 (P93), with the first spectroscopic observations starting on 1st Oct 2014. The
last spectroscopic observations would be taken in Sep 2017 (P99).
The second scenario assumed that VANDELS was allocated ' 100% of the available dark/grey on UT3 in
Oct-Nov each year. As before, spectroscopic observations would begin on 1st Oct 2014 (start of P94), with arequirement for some pre-imaging at the end of P93. However, in the case, the final spectroscopic observations
would be obtained in Nov 2017 (P100).
Data reduction and basic data products
Due to the RAs of our two target fields there will be a natural structure to the VANDELS programme, with
spectroscopic observations finishing at the end of a given calendar year, before beginning again ' 9 months
later the following Sep/Oct. Consequently, we propose a data release schedule whereby the basic data products(1D+2D reduced spectra and spectroscopic redshift catalogues) are made available to the community at theend of September each year, 9 months after the previous season’s observations have been completed.
Based on their extensive experience of previous VIMOS surveys, the data reduction team in Milan are confident
they can reduce the spectra for a given VANDELS observing season ' 1 month after the end of observations.
Consequently, to meet our proposed data release schedule, the VANDELS team would face the manageable taskof determining robust spectroscopic redshifts and quality flags for ' 750 spectra over an eight-month period.
Enhanced data products
The enhanced data products consist of catalogues providing the final spectroscopic redshift information, line
flux/EW measurements, full aperture-matched optical-nearIR photometry and basic stellar population parameters derived from SED fitting. The VANDELS team would commit to releasing this information to the community one year after the final spectroscopic observations have been completed.
Summary• Initial pre-imaging observations mid-late August 2014.
• Start of spectroscopic observations 1st October 2014.
• Annual release of basic data products, starting September 2015.
• End of spectroscopic observations Sep/Nov 2017 depending on schedule.
• Final release of enhanced data products, one year after final observations are completed.

OPO, ESO (opo@eso.org)

page 16 of 18

ESO Public Spectroscopic Surveys

8

Phase 1 proposal

Envisaged follow-up: (1 page)The fields targeted by VANDELS are already two of the most complete, multi-wavelength survey fields inthe sky. As a consequence, these fields already feature unique, largely publicly available, multi-wavelength
datasets spanning the X-ray, UV, optical, nearIR, midIR, sub-mm and radio regimes. However, despite this, it
is clear that the unique VANDELS spectroscopic dataset will immediately motivate a variety of further follow-up
observations:• WFC3/IR imaging: one of the key motivations of VANDELS is the prospect of obtaining ultra-deep
spectroscopy for z ≥ 2 galaxies for which deep optical-nearIR HST imaging is already available. However,
it is anticipated that ' 30% of our spectroscopic slits will target galaxies lying outside the main CANDELS
imaging footprint in UDS and CDFS. Consequently, in order to maximize the potential for morphologicalstudies based on the VANDELS dataset, we would envisage applying for an additional ' 100 orbits of
HST WFC3/IR imaging in the H160 filter to fully cover the VANDELS footprint in UDS and CDFS.
• IFU spectroscopy: the large number of ultra-deep optical spectra provided by VANDELS will clearly
act as a major resource for follow-up IFU spectroscopy. The two obvious ESO instruments of choicefor follow-up IFU observations are the KMOS near-IR spectrograph and the MUSE optical spectrograph(currently being commissioned). For KMOS observations, the VANDELS spectra can be used to identify
spectroscopically confirmed galaxies at z ≥ 2 which place luminous emission lines (i.e. Hα, O[iii], O[ii])within suitable near-IR observing windows. Opclauraerating over a very similar wavelength range to the
VANDELS spectra, obvious targets for IFU follow-up with MUSE (i.e. those with strong emission-line
velocity gradients) can be selected from the VANDELS 2D spectra.
• ALMA: the southern position of both UDS and CDFS make them ideal survey fields for sub-mm andmm follow-up observations with ALMA. One of the key scientific questions that VANDELS will help toaddress is the evolution of star-formation and metallicity in galaxies at z ≥ 2. However, in order to derivea complete picture it will be necessary to obtain dust mass and star-formation rate measurements atlong wavelength, which can be now be provided by short, targeted, continuum observations with ALMA.
Alternatively, a contiguous 1.2mm ALMA continuum mosaic of the HUDF has already been approved(PI Dunlop), and when ALMA is capable of making larger-area mosaics, the central CANDELS imaging
areas of UDS+CDFS covered by VANDELS would be an obvious choice of location.
• JWST: if VANDELS is approved, it is likely that the VIMOS spectroscopic observations will be completed
in 2017/18, shortly before the currently-scheduled launch date for JWST. High-redshift galaxies with
spectroscopic redshifts provided by VANDELS would provide an excellent source of candidates for followup imaging and spectroscopy with JWST. As an example, high signal-to-noise (S/N≥ 10) spectra of theBalmer break region of typical z ' 5 galaxies targeted by VANDELS could be obtained with NIRSPEC
in 1hr integrations. Moreover, longer-wavelength λ ≥ 6µm imaging with MIRI could provide much moreaccurate measurements of the M/L ratio of high-redshift stellar populations than currently available from
Spitzer IRAC imaging.
• E-ELT: given their location in two of the very best multi-wavelength survey fields observable from Paranal,z ≥ 3 galaxies with high signal-to-noise ratio VANDELS spectra would be prime targets for initial highredshift galaxy evolution studies with E-ELT. The two first-light instruments for E-ELT are an imager and
IFU, both AO-fed and both operating in the near-IR. The photon gathering power and spatial resolution
provided by E-ELT will facilitate studies of the rest-frame optical structure and dynamics of z ≥ 3 galaxies
well beyond the reach of current ground-based instrumentation.

OPO, ESO (opo@eso.org)

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ESO Public Spectroscopic Surveys

9

Other remarks, if any: (1 page)

9.1

Strategic importancePhase 1 proposal

We note that this proposal can be regarded as the culmination of a highly successful tradition of targeting
deep ESO spectroscopy on the extragalactic survey fields which have received the largest investments of deep,high-resolution, HST imaging. This approach, so well demonstrated by GOODS, and then maintained via
zCOSMOS, has proved highly successful in enabling the ESO community to maximise its scientific exploitationof ESA’s investment in NASA’s great observatories. It would be a mistake to regard this as simply the VLT
following a strategy established by HST. On the contrary, many leading ESO astronomers, including key CoIs on this proposal, have been instrumental in the development of the HST programmes and understand theenormous added value offered by VLT spectroscopy. This is especially true of CANDELS, where much of theimaging strategy has been driven by the key ESO Co-Is on this VLT proposal.

9.2

Visitor mode versus service mode

Starting with the initial call for letters of intent (LOI), it has been made clear that any potential ESO public
spectroscopy programme will be observed in visitor mode. As a consequence, we have set out in detail the likely
visitor mode requirements to complete VANDELS within a nominal ' 3 year survey period. However, it is clear
from the calculations presented in Sections 4 & 6 that the decision to observe a survey of this nature in visitor
mode is highly inefficient. This is simply unavoidable given that we have been instructed to focus on two fields
at very similar RAs that, even at peak visibility, are not observable for full nights. In addition, in visitor mode,
it is necessary to account for time losses due to bad weather, poor seeing, non photometric/clear sky conditions
and technical downtime. Although we would be happy to execute the survey in visitor mode, it is clear that
service mode would be more efficient in terms of both telescope and observer time.
However, if VANDELS is approved in visitor mode we are confident that we have sufficient numbers of potential
observers within the VANDELS team to successfully staff the survey. The VANDELS survey consists of 79
Co-Is, of which 66 are from ESO member institutes. Realistically this means that we have a pool of ≥ 40
observers with which to staff the survey.

9.3

The COSMOS field

In the original VANDELS LOI, we proposed targeting the CANDELS fields in UDS, CDFS and COSMOS. Given
that COSMOS features the same level of deep, multi-wavelength, HST and ground-based data as UDS+CDFS,
it was an obvious choice as the third VANDELS field. However, following instructions from the PSS panelthat the COSMOS field is reserved for a scientifically distinct survey, we have removed it from the survey plansubmitted here.
The two Co-PIs of VANDELS have been in contact with A. van der Wel, the PI of the PSS proposal which wasallocated the COSMOS field (but proposed COSMOS+CDFS at the LOI stage). Together we have agreed thatshould both surveys be approved, it would be beneficial to share half nights in Dec/Jan when both COSMOS
and CDFS are observable. However, if only one survey can be approved it would seem sensible for ESO tolift the current RA restrictions to allow our team to observe COSMOS (or the other PSS team to observe
CDFS). Observations in the COSMOS field would proceed in an identical fashion to those proposed here for
UDS+CDFS, and would require a < 50% increase in time allocation due to increased observing efficiency.

9.4

Back-up programme

In visitor mode it is inevitable that perfectly useable telescope time will be lost because conditions do not meet
our adopted seeing limit of ≤ 100 FWHM. In this situation we would observe masks targeting bright iAB ≤ 23.5
galaxies as a back-up programme. Observations of this nature would clearly have important legacy value in the
UDS, where spectroscopic coverage is comparatively poor. However, even in CDFS there are ≥ 1000 galaxies
with iAB ≤ 23.5 without spectroscopic redshifts.
OPO, ESO (opo@eso.org)

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