[Paper Review] The VIMOS VLT Deep Survey: the redshift distribution N(z) of magnitude-limited samples down to iAB=24.75 and KsAB=22

O. Le Fèvre, P. Cassata+21 more|arXiv (Cornell University)|Jul 24, 2013

Galaxies: Formation, Evolution, Phenomena7 references6 citations

TL;DR

This paper presents the redshift distribution N(z) of magnitude-limited galaxy samples from the VIMOS VLT Deep Survey (VVDS), using spectroscopic redshifts down to iAB = 24.75 and KsAB = 22, covering 0 ≤ z ≤ 5. It demonstrates that magnitude-selected samples identify 1.5–3× more high-redshift galaxies (z > 2) than colour-colour selected samples, and reveals that semi-analytic models on the Millennium simulation under-predict bright, star-forming galaxies at z > 2 and over-predict faint galaxies at z < 1.

ABSTRACT

We measure and analyse the redshift distribution N(z) of magnitude-selected samples using spectroscopic redshift measurement from the magnitude-selected VIMOS VLT Deep Survey (VVDS) with 172. Down to iAB&lt;24.75 the sample has a mean redshift z=1.15 and 17.1% of the galaxies are beyond z=2. The projected sky density is 2.07+/-0.12 gal/arcmin2 at 1.42 than in colour-colour selected samples, and we use the magnitude-selected VVDS to emphasize the large uncertainties associated to other surveys using colour or colour-colour selected samples. Our results further demonstrate that semi-analytical models on dark matter simulations have yet to find the right balance of physical processes and time-scales to properly reproduce a fundamental galaxy population property like the observed N(z).

Motivation & Objective

• To measure the redshift distribution N(z) of galaxies in magnitude-limited samples down to iAB = 24.75 and KsAB = 22, extending to z ≈ 5.
• To compare the completeness and reliability of magnitude-selected versus colour-colour selected galaxy samples in identifying high-redshift galaxies.
• To evaluate the performance of semi-analytic models on the Millennium dark matter simulation against observed N(z) distributions.
• To provide robust, parametric fits to N(z) for use in weak-lensing and cosmological studies.
• To quantify cosmic variance and Poisson noise in projected galaxy number counts across redshift bins.

Proposed method

• The study uses the final VVDS data release from the 0226-04 field, containing 10,765 galaxies with spectroscopic redshifts selected solely on iAB magnitude (17 ≤ iAB ≤ 24.75).
• Redshift distributions N(z) are computed for i-band, J, H, and Ks-band magnitude-limited samples, with parametric fits provided for the global galaxy population.
• Projected galaxy number counts per arcmin² are derived in redshift bins, with errors accounting for both Poisson noise and cosmic variance.
• The observed N(z) is compared to results from other deep surveys and to predictions from semi-analytic models applied to the Millennium simulation.
• Colour-colour selection methods (e.g., BzK, LBG) are tested for their efficiency and contamination levels in selecting galaxies at 1.4 ≤ z ≤ 4.5.
• Statistical comparisons are made between observed N(z) and model predictions to assess discrepancies in galaxy counts at low (z < 1) and high (z > 2) redshifts.

Experimental results

Research questions

• RQ1 How does the redshift distribution N(z) of magnitude-limited galaxy samples compare to that of colour-colour selected samples at z > 2?
• RQ2 What is the true number density of galaxies in key redshift bins (1.4 ≤ z ≤ 2.5, 2.7 ≤ z ≤ 3.4, 3.4 ≤ z ≤ 4.5) for iAB ≤ 24.75 and KsAB ≤ 22?
• RQ3 To what extent are colour-colour selected samples contaminated by galaxies at non-target redshifts?
• RQ4 Do semi-analytic models on the Millennium simulation accurately reproduce the observed N(z) of luminous and faint galaxies across 0 ≤ z ≤ 5?
• RQ5 What is the impact of cosmic variance and Poisson noise on the reliability of projected galaxy number counts in deep surveys?

Key findings

• The mean redshift of a sample with iAB ≤ 24.75 is z̄ = 1.15, with 17.1% of galaxies at z > 2.
• The projected galaxy density is 2.07 ± 0.12 galaxies per arcmin² at 1.4 ≤ z ≤ 2.5 and KsAB ≤ 22.5.
• The projected galaxy density is 1.72 ± 0.15 galaxies per arcmin² at 2.7 ≤ z ≤ 3.4 and 0.59 ± 0.09 galaxies per arcmin² at 3.4 ≤ z ≤ 4.5, all brighter than iAB = 24.75.
• Magnitude-selected samples identify 1.5 to 3 times more galaxies at z ∼ 3 than LBG or BzK colour-colour selected samples, due to varying filter sensitivity to the Lyman break.
• Colour-colour selection methods (e.g., BzK, LBG) miss up to 20–30% of galaxies in target redshift ranges and contaminate samples with galaxies at non-target redshifts.
• Semi-analytic models on the Millennium simulation under-predict the number of bright, star-forming galaxies at z > 2 by a factor of ∼4 and over-predict faint galaxies at z < 1 by ∼40%.