[Paper Review] Lyman Break Galaxies at z>4 and the Evolution of the UV Luminosity Density at High Redshift

Charles C. Steidel, Kurt L. Adelberger+3 more|arXiv (Cornell University)|Nov 25, 1998

Galaxies: Formation, Evolution, Phenomena58 references945 citations

TL;DR

This study presents a ground-based survey of Lyman break galaxies at z > 4, using photometric selection and spectroscopic redshifts to compare UV luminosity functions with those at z ~ 3. It finds no significant decline in UV luminosity density between z ~ 4 and z ~ 3, suggesting star formation emissivity may be nearly constant for z > 1 when dust corrections are applied, challenging the notion of a sharp peak in star formation activity.

ABSTRACT

We present initial results of a survey for star-forming galaxies in the redshift range 3.8 < z < 4.5. This sample consists of a photometric catalog of 244 galaxies culled from a total solid angle of 0.23 square degrees to an apparent magnitude of I_{AB}=25.0. Spectroscopic redshifts in the range 3.61 < z < 4.81 have been obtained for 48 of these galaxies; their median redshift is <z>=4.13. Selecting these galaxies in a manner entirely analogous to our large survey for Lyman break galaxies at smaller redshift (2.7 < z < 3.4) allows a relatively clean differential comparison between the populations and integrated luminosity density at these two cosmic epochs. Over the same range of UV luminosity, the spectroscopic properties of the galaxy samples at z~4 and z~3 are indistinguishable, as are the luminosity function shapes and the total integrated UV luminosity densities (rho_{UV}(z=3)/rho_{UV}(z=4) = 1.1 +/-0.3). We see no evidence at these bright magnitudes for the steep decline in the star formation density inferred from fainter photometric Lyman-break galaxies in the Hubble Deep Field (HDF). If the true luminosity density at z~4 is somewhat higher than implied by the HDF, as our ground-based sample suggests, then the emissivity of star formation as a function of redshift is essentially constant for all z>1 once internally consistent corrections for dust are made. This suggests that there is no obvious peak in star formation activity, and that the onset of substantial star formation in galaxies occurs at z > 4.5. [abridged abstract]

Motivation & Objective

• To measure the UV luminosity density at z ~ 4 using a large, spectroscopically confirmed sample of Lyman break galaxies.
• To test whether the apparent decline in UV luminosity density inferred from the Hubble Deep Field (HDF) at z ~ 4 is real or due to sample variance.
• To compare the luminosity function and spectroscopic properties of Lyman break galaxies at z ~ 4 and z ~ 3 for direct, differential analysis.
• To improve color selection criteria for z > 4 LBGs using spectroscopic constraints, minimizing contamination and maximizing completeness.

Proposed method

• Photometric selection of Lyman break galaxy candidates using broad-band filters (IAB, B, V, R) over 0.23 square degrees to IAB = 25.0.
• Spectroscopic redshift confirmation using the Low Resolution Imaging Spectrograph on the Keck 10m telescopes for 48 galaxies.
• Direct comparison of luminosity functions and UV luminosity densities between z ~ 4 (⟨z⟩ = 4.13) and z ~ 3 (⟨z⟩ = 3.04) samples using identical selection criteria.
• Re-analysis of HDF data with improved estimates of effective survey volumes and spectral energy distributions to assess faint-end slope and sample variance.
• Modeling of LBG color distributions incorporating dust extinction (1.6 mag at 1500 Å on average) and photometric uncertainties to refine selection functions.
• Application of internally consistent dust corrections to compare star formation emissivity across redshifts, assuming uniform dust effects.

Experimental results

Research questions

• RQ1 Is the UV luminosity density at z ~ 4 significantly lower than at z ~ 3, as suggested by HDF photometric redshift studies?
• RQ2 To what extent is the apparent deficit of z ~ 4 Lyman break galaxies in the HDF due to sample variance rather than intrinsic evolution?
• RQ3 How do the spectroscopic properties (e.g., Lyα in absorption/emission) and luminosity functions of z ~ 4 LBGs compare to those at z ~ 3?
• RQ4 What is the impact of dust extinction on the observed UV luminosity density, and how does this affect the inferred star formation history?
• RQ5 Does the luminosity function at z ~ 4 show a steeper faint-end slope than at z ~ 3, or is the difference due to observational bias?

Key findings

• The UV luminosity density at z ~ 4 (⟨z⟩ = 4.13) is consistent with that at z ~ 3 (⟨z⟩ = 3.04), with a ratio of ρUV(z=3)/ρUV(z=4) = 1.1 ± 0.3, indicating no significant decline.
• Spectroscopic properties of z ~ 4 and z ~ 3 LBGs are indistinguishable, including Lyα in absorption for ~50% of objects and similar rest-frame UV colors.
• The faint-end slope of the z ~ 3 luminosity function is steep (α = −1.60 ± 0.13), steeper than previous estimates, suggesting a large fraction of luminosity density may lie below HDF detection limits.
• Sample variance in the HDF is large enough to explain the apparent deficit of z ~ 4 LBGs, making the observed difference statistically insignificant.
• After applying consistent dust corrections (average extinction ~1.6 mag at 1500 Å), the star formation emissivity per unit co-moving volume remains nearly flat for z > 1, implying no clear peak in star formation activity.
• The similarity in luminosity functions at z ~ 4 and z ~ 3 suggests a fundamental difference in the evolution of star formation compared to luminous AGN space density.