[Paper Review] Lyman Alpha Imaging of a Proto-Cluster Region at =3.09

Charles C. Steidel, Kurt L. Adelberger+4 more|ArXiv.org|Oct 7, 1999

Astronomy and Astrophysical ResearchPhysics and Astronomy351 citations

TL;DR

This study uses deep narrow-band imaging to detect Lyman-alpha emitting galaxies in a high-redshift proto-cluster at ⟨z⟩=3.09, revealing a surface density of emission-line candidates six times higher than previous blind surveys. It finds that only 20–25% of UV-bright galaxies are detectable via narrow-band excess due to weak Lyman-alpha lines, and discovers two massive, diffuse Lyman-alpha 'blobs' without associated radio sources, suggesting alternative excitation mechanisms or obscured ionizing sources.

ABSTRACT

We present very deep imaging observations, through a narrow-band filter tuned to Lyman alpha at , of a volume containing a significant over-density of galaxies at this redshift previously discovered in our survey for continuum-selected Lyman break galaxies (LBGs). The new observations are used in conjunction with our spectroscopic results on LBGs to compare the effectiveness of continuum and emission line searches for star forming galaxies at high redshift, and to extend the search for members of the structure at = 3.09 to much fainter continuum luminosities. We find that only 20-25 percent of all galaxies at a given UV continuum luminosity would be flagged as narrow-band excess objects subject to the typical limits W_Lya > 80 Angstroms in the observed frame. The density enhancement of strong Lyman alpha emitters in this field is consistent with that inferred from the analysis of the spectroscopic Lyman break galaxy sample in the same region (6+/-1), but extends to continuum luminosities up to 2 magnitudes fainter. There is no evidence for a significantly higher fraction of large Lyman alpha line equivalent widths at faint continuum luminosities. By combining the 24 spectroscopic members of the z=3.09 ``spike'' with the narrow-band candidates, we are able to produce a sample of 162 objects which are either known or likely members of this large structure. We have also discovered two extremely large and diffuse Lyman alpha emitting ``blobs'', which have physical extents >100/h kpc and Lyman alpha line fluxes 20-40 times larger than the typical line emitters at the same redshifts in the field. The possible nature of the ``blobs'' is discussed. (abstract abridged)

Motivation & Objective

• To compare the effectiveness of continuum versus emission-line selection for identifying high-redshift star-forming galaxies.
• To extend the search for proto-cluster members to fainter UV continuum luminosities using narrow-band imaging.
• To investigate the nature of two extremely bright, diffuse Lyman-alpha nebulae (‘blobs’) found in the proto-cluster region.
• To map the large-scale structure of the proto-cluster using emission-line and spectroscopic data.
• To assess the utility of narrow-band imaging for probing galaxy over-densities at high redshift despite selection biases.

Proposed method

• Conducted very deep narrow-band imaging over an 8.7′ × 8.9′ field centered on a known proto-cluster at ⟨z⟩=3.09.
• Combined narrow-band excess detections with spectroscopic redshifts from prior Lyman break galaxy (LBG) surveys to identify cluster members.
• Used a combination of NB-excess, NB-deficit, and spectroscopic catalogs to build a comprehensive sample of 162 candidate or confirmed members.
• Constructed a smoothed surface density map to identify regions of local over-density on 1 h⁻¹ Mpc scales.
• Analyzed the spatial correlation of high-density peaks with known QSOs and Lyman-alpha blobs to assess environmental associations.
• Evaluated the selection efficiency of narrow-band imaging by comparing detected emission-line objects to UV-bright LBGs at fixed continuum luminosity.

Experimental results

Research questions

• RQ1 How effective is narrow-band imaging at detecting high-redshift star-forming galaxies compared to continuum-based Lyman break selection?
• RQ2 What is the surface density of Lyman-alpha emission-line candidates in a known proto-cluster region at z ≈ 3.09?
• RQ3 What causes the existence of two extremely luminous, diffuse Lyman-alpha nebulae without associated radio emission or bright UV sources?
• RQ4 Are the Lyman-alpha blobs and QSOs preferentially located in the densest regions of the proto-cluster?
• RQ5 To what extent does narrow-band selection miss fainter UV-bright galaxies due to weak Lyman-alpha equivalent widths?

Key findings

• The narrow-band survey detected a Lyman-alpha emission-line candidate surface density approximately six times higher than in previous blind narrow-band surveys at similar depth and redshift.
• Only about 20–25% of galaxies at a given far-UV continuum luminosity are detectable via narrow-band excess, due to low rest-frame equivalent widths (<20 Å) in the majority.
• The two giant Lyman-alpha 'blobs' have physical extents of at least 100 h⁻¹ kpc and Lyman-alpha line fluxes of ~10⁻¹⁵ erg s⁻¹ cm⁻², both ~20–40 times higher than typical Lyman-alpha emitters at z ≈ 3.09.
• The blobs are not associated with radio sources at the 100 μJy level and lack obvious UV-bright ionizing sources, suggesting possible excitation by obscured sources or cooling-flow-like processes.
• One of the three highest local density peaks in the smoothed surface density map is centered on a z = 3.083 QSO, and another on one of the Lyman-alpha blobs, suggesting environmental clustering of rare objects.
• The combined sample of 162 objects (spectroscopic, NB-excess, and NB-deficit) provides the most complete high-redshift 'snapshot' of a proto-cluster structure to date, with strong evidence for large-scale over-density at z ≈ 3.09.