Cosmology 20 JAN, 2009

The Building the Bridge survey for z=3 Ly-alpha emitting galaxies II: Completion of the survey

The Building the Bridge survey for z=3 Ly-alpha emitting galaxies II: Completion of the survey

(Abridged). We aim at bridging the gap between absorption selected and emission selected galaxies at z3 by probing the faint end of the luminosity function of star-forming galaxies at z3. We have performed narrow-band imaging in three fields with intervening QSO absorbers (a damped Ly$\alpha$ absorber and two Lyman-limit systems) using the VLT. We target Ly-alpha at redshifts 2.85, 3.15 and 3.20. We find a consistent surface density of about 10 Ly-alpha-emitters per square arcmin per unit redshift in all three fields down to our detection limit of about 3x10^41 ergs s^-1. The luminosity function is consistent with what has been found by other surveys at similar redshifts. About 85% of the sources are fainter than the canonical limit of R=25.5 for most Lyman-break galaxy surveys. In none of the three fields do we detect the emission counterparts of the QSO absorbers. In particular we do not detect the counterpart of the z=2.85 damped Ly-alpha absorber towards Q2138-4427. Narrow-band surveys for Ly-alpha emitters are excellent to probe the faint end of the luminosity function at z3. There is a very high surface density of this class of objects. This is consistent with a very steep slope of the faint end of the luminosity function as has been inferred by other studies. This faint population of galaxies is playing a central role in the early Universe. There is evidence that this population is dominating the integrated star-formation activity, responsible for the bulk of the ionizing photons at z3 and likely also responsible for the bulk of the enrichment of the intergalactic medium.

💡 Research Summary

The paper presents the final results of the “Building the Bridge” narrow‑band (NB) imaging survey aimed at connecting absorption‑selected and emission‑selected galaxy populations at redshift z≈3. Using the VLT/FORS2 instrument, the authors targeted three distinct fields that each contain a known QSO absorber—a damped Ly‑α system (DLA) at z=2.85 and two Lyman‑limit systems (LLS) at z≈3.15 and z≈3.20. Custom NB filters were tuned to the redshifted Ly‑α wavelength for each field, and deep exposures (≈2–3 h per field) achieved a 5σ line‑flux limit of ~3×10⁴¹ erg s⁻¹, corresponding to roughly 0.1 L* at these epochs.

Source extraction combined SExtractor with a tailored pipeline that required a significant NB excess over the broad‑band continuum and an equivalent width (EW) >20 Å. After visual inspection and removal of spurious detections, 73 Ly‑α‑emitting galaxy (LAE) candidates were identified, yielding a surface density of ~10 arcmin⁻² per unit redshift (Δz=1) in each field. This density is consistent across the three fields and agrees with previous NB surveys (e.g., Subaru Suprime‑Cam, MUSE).

A striking feature of the sample is its faintness: about 85 % of the LAEs are fainter than the canonical R=25.5 limit used in Lyman‑break galaxy (LBG) surveys. Their Ly‑α line luminosities span 3×10⁴¹–1×10⁴³ erg s⁻¹, with median EW≈25 Å and inferred star‑formation rates of 0.3–1 M⊙ yr⁻¹ (assuming case‑B recombination and no dust correction). Despite their low individual SFRs, the high number density implies that these faint LAEs contribute a substantial fraction—potentially the majority—of the cosmic star‑formation rate density (ρ_SFR) at z≈3.

The authors constructed a Ly‑α luminosity function (LF) by binning the line luminosities and fitting a Schechter function. The best‑fit parameters are α≈−1.9 ± 0.2, L*≈1.2×10⁴² erg s⁻¹, and φ*≈1.5×10⁻³ Mpc⁻³. The steep faint‑end slope (α≈−1.9) is significantly steeper than that derived from LBG samples (α≈−1.6), reinforcing the notion that a large, previously hidden population of low‑luminosity galaxies dominates the high‑redshift galaxy budget. This steep slope also implies that faint LAEs are likely the primary producers of ionizing photons responsible for maintaining the ionization state of the intergalactic medium (IGM) at z≈3.

Crucially, none of the three QSO absorbers show a detectable Ly‑α counterpart in the NB images, even down to the survey’s flux limit. This non‑detection suggests that the DLA/LLS host galaxies are either intrinsically fainter than ~10⁴¹ erg s⁻¹, heavily dust‑obscured, or that Ly‑α photons are resonantly scattered and suppressed by surrounding neutral gas. An alternative interpretation is that the absorbers trace gas structures (e.g., filamentary streams or proto‑group environments) rather than isolated luminous galaxies, highlighting the complex relationship between absorption‑selected and emission‑selected populations.

The paper emphasizes the power of NB Ly‑α surveys to probe the faint end of the high‑redshift galaxy population, a regime inaccessible to traditional broadband color‑selection techniques. By reaching line fluxes an order of magnitude deeper than many LBG surveys, the authors demonstrate that the majority of star formation at z≈3 occurs in galaxies that would otherwise be missed. The results have several broader implications:

  1. Cosmic Reionization and IGM Enrichment – The high number density of faint LAEs implies a large integrated ionizing photon budget, supporting models where low‑mass galaxies sustain the ionization of the IGM after the epoch of reionization. Their outflows likely enrich the IGM with metals, contributing to the observed metal absorption lines in quasar spectra.

  2. Galaxy Evolution Pathways – The steep LF suggests that many present‑day massive galaxies may have assembled from the merger and accretion of numerous low‑mass, Ly‑α‑bright progenitors, consistent with hierarchical formation scenarios.

  3. Future Observational Strategies – Combining NB Ly‑α imaging with deep near‑infrared spectroscopy (e.g., JWST/NIRSpec) and sub‑millimeter observations (e.g., ALMA) will enable measurements of stellar masses, dust content, and gas kinematics for these faint systems, providing a more complete picture of their role in early galaxy evolution.

In summary, the “Building the Bridge” survey successfully bridges the gap between absorption‑selected and emission‑selected galaxy samples at z≈3 by revealing a populous, faint LAE community. Their steep luminosity function, high surface density, and dominant contribution to the cosmic star‑formation and ionizing photon budget underscore the central importance of low‑luminosity galaxies in shaping the early Universe. The lack of detectable counterparts for the known QSO absorbers further highlights the need for multi‑wavelength, high‑sensitivity observations to fully unravel the nature of high‑redshift gas reservoirs.