In Publications of the Astronomical Society of Australia, Volume 32, 27, 2015.
http://arxiv.org/abs/1505.07483

For a PDF version of the article, click here.

LYMAN ALPHA EMITTING GALAXIES IN THE NEARBY UNIVERSE

Matthew Hayes


Department of Astronomy, The Oskar Klein Centre for Cosmoparticle Physics, Stockholm University, AlbaNova University Centre, SE-106 91 Stockholm, Sweden


Abstract: The Lyman alpha emission line (Lyα) of neutral hydrogen (HI) is intrinsically the brightest emission feature in the spectrum of astrophysical nebulae, making it a very attractive observational feature with which to survey galaxies. Moreover as an ultraviolet (UV) resonance line, Lyα possesses several unique characteristics that make it useful to study the properties of the interstellar medium (ISM) and ionizing stellar population at all cosmic epochs. In this review I present a summary of Lyα observations of galaxies in the nearby universe. By UV continuum selection, at the magnitudes reachable with current facilities, only ≈ 5% of the local galaxy population shows a Lyα equivalent width (WLyα) that exceeds 20 Å. This fraction increases dramatically at higher redshifts, but only in the local universe can we study galaxies in detail and assemble unprecedented multi-wavelength datasets. I discuss many local Lyα observations, showing that when galaxies show net Lyα emission, they ubiquitously also produce large-scale halos of scattered Lyα, that dominate the integrated luminosity. Concerning global measurements, we discuss how WLyα and the Lyα escape fraction (fescLyα) are higher (WLyα ≳ 20Å and fescLyα ≳ 10%) in galaxies that represent the less massive and younger end of the distribution for local objects. This is connected with various properties, such that Lyα-emitting galaxies have lower metal abundances (median value of 12 + log(O/H) ∼ 8.1) and dust reddening. However, the presence of galactic outflows/winds is also vital to Doppler shift the Lyα line out of resonance with the atomic gas, as high WLyα is found only among galaxies with winds faster than ∼ 50 km s−1. The empirical evidence is then assembled into a coherent picture, and the requirement for star formation driven feedback is discussed in the context of an evolutionary sequence where the ISM is accelerated and/or subject to hydrodynamical instabilities, which reduce the scattering of Lyα. Concluding remarks take the form of perspectives upon future developments, and the most pressing questions that can be answered by observation.


Keywords : Galaxies: evolution – Galaxies: starburst – Physical data and processes: radiative transfer — Ultraviolet: galaxies — Galaxies: individual: Haro 11; IZw 18; SBS 0335–052; Haro 2; Mrk 702; IRAS 08208+2816; ESO 338-IG04


Table of Contents

INTRODUCTION
Key Applications of Lyman alpha
Lyman alpha Observations of the Nearby Universe - This Review

LYMAN ALPHA OBSERVABLES
Flux, Luminosity, and Equivalent Width
Lyα/Balmer Ratios and Escape Fraction

THE FIRST LOW REDSHIFT LYMAN ALPHA OBSERVATIONS
The First Lyα Spectra of Active Galactic Nuclei
Star-forming Galaxies

HIGH RESOLUTION SPECTROSCOPY WITH THE HUBBLE SPACE TELESCOPE
HST Finds Deep Lyα Absorption
Galaxy Winds and Atomic Gas Kinematics
Dissecting the Neutral Medium in Detail

FROM SMALL TO LARGE SCALES: COMPLETING THE QUANTITATIVE PICTURE WITH HST IMAGING
Resolved Analyses: A Case Study of Haro 11
Extended Halos

LYMAN ALPHA SURVEYS AT LOW REDSHIFT
Evolution of Lyman alpha Galaxies into the Nearby Universe
The Properties of Nearby Lyman alpha Galaxies
Stellar Populations
Galaxy Morphology
Interstellar Dust and Metals
Neutral Gas

SYNTHESIZING THE OBSERVATIONAL DATA: WHAT MAKES A LYMAN ALPHA EMITTER?

PERSPECTIVES AND FUTURE DEVELOPMENTS: THE MOST PRESSING QUESTIONS
What is the Atomic Gas Distribution in Star-Forming Galaxies?
How is the Lyα Spectral Profile Built?
Ionization State of the Interstellar Medium
Can we Predict Lyα Observables from Other Information?

REFERENCES

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