[Paper Review] Rest-Frame Ultraviolet Spectra of z~3 Lyman Break Galaxies

Alice E. Shapley, Charles C. Steidel+2 more|ArXiv.org|Jan 13, 2003

Galaxies: Formation, Evolution, PhenomenaPhysics and Astronomy69 references797 citations

TL;DR

This study presents a systematic analysis of rest-frame UV spectra from nearly 1,000 z~3 Lyman Break Galaxies (LBGs), using high signal-to-noise composite spectra to reveal correlations between spectral features and galaxy properties. The key finding is that Lyα emission strength, UV slope, and outflow kinematics are strongly linked to covering fraction and velocity structure of outflowing neutral gas, with high-ionization lines and nebular emission indicating metallicity and temperature differences in H II regions.

ABSTRACT

We present the results of a systematic study of the rest-frame UV spectroscopic properties of Lyman Break Galaxies (LBGs). The database of almost 1000 LBG spectra proves useful for constructing high S/N composite spectra. The composite spectrum of the entire sample reveals a wealth of features attributable to hot stars, HII regions, dust, and outflowing neutral and ionized gas. By grouping the database according to galaxy parameters such as Lyman-alpha equivalent width, UV spectral slope, and interstellar kinematics, we isolate some of the major trends in LBG spectra which are least compromised by selection effects. We find that LBGs with stronger Lyman-alpha emission have bluer UV continua, weaker low-ionization interstellar absorption lines, smaller kinematic offsets between Lyman-alpha and the interstellar absorption lines, and lower star-formation rates. There is a decoupling between the dependence of low- and high-ionization outflow features on other spectral properties. Most of the above trends can be explained in terms of the properties of the large-scale outflows seen in LBGs. According to this scenario, the appearance of LBG spectra is determined by a combination of the covering fraction of outflowing neutral gas which contains dust, and the range of velocities over which this gas is absorbing. Higher sensitivity and spectral resolution observations are still required for a full understanding of the covering fraction and velocity dispersion of the outflowing neutral gas in LBGs, and its relationship to the escape fraction of Lyman continuum radiation in galaxies at z~3.

Motivation & Objective

• To understand the physical drivers behind the diversity in rest-frame UV spectra of high-redshift Lyman Break Galaxies (LBGs).
• To isolate spectral trends in LBGs that are minimally affected by observational selection effects.
• To investigate the role of outflows in shaping UV spectral features, particularly in relation to Lyman continuum escape.
• To assess the connection between nebular emission lines and intrinsic properties such as metallicity and H II region temperature.
• To identify the limitations of current data and define critical future observations for resolving gas covering fractions and Lyman continuum escape.

Proposed method

• Construction of high signal-to-noise composite spectra from a sample of 990 spectroscopically confirmed z~3 LBGs.
• Grouping of spectra by key parameters: Lyα equivalent width, UV spectral slope, interstellar absorption kinematics, and Lyα-emission redshift offset.
• Analysis of absorption features (low- and high-ionization lines) and emission lines (nebular and Lyα) to infer gas covering fraction, velocity structure, and dust reddening.
• Use of high-resolution data from Keck Observatory to model velocity profiles and detect saturated absorption components.
• Comparison of composite spectra from different sub-samples to isolate trends independent of selection bias.
• Application of spectral decomposition techniques to infer covering fractions and velocity dispersions of outflowing neutral gas.

Experimental results

Research questions

• RQ1 How do the rest-frame UV spectral properties of z~3 LBGs correlate with Lyα equivalent width and UV continuum slope?
• RQ2 What is the relationship between interstellar absorption kinematics and Lyα emission in LBGs?
• RQ3 How do covering fraction and velocity structure of outflowing neutral gas influence the appearance of UV spectra?
• RQ4 To what extent do high-ionization and nebular emission lines reflect intrinsic H II region properties such as temperature and metallicity?
• RQ5 What are the implications of observed spectral features for the escape fraction of Lyman continuum radiation in high-redshift galaxies?

Key findings

• LBGs with stronger Lyα emission exhibit bluer UV continua, weaker low-ionization interstellar absorption lines, smaller kinematic offsets between Lyα and absorption features, and lower star-formation rates.
• Galaxies with rest-frame Lyα equivalent width ≥20 Å show weaker high-ionization lines and significantly stronger nebular emission lines than the overall sample.
• The dependence of low- and high-ionization outflow features on spectral properties is decoupled, indicating distinct physical mechanisms or gas phases.
• The strength of collisionally excited nebular emission lines is not directly affected by outflow geometry, suggesting they are sensitive to H II region temperature and metallicity.
• The composite spectrum of the Lyman continuum sample is similar to the total LBG sample but shows less dust reddening (⟨E(B-V)⟩ = 0.07 vs. 0.13), indicating potential selection bias.
• In Q0000-D6, the presence of saturated high-ionization lines with zero transmission implies a high covering fraction of neutral gas, suggesting minimal Lyman continuum escape despite apparent Lyman continuum detection.