[Paper Review] The Lyman alpha reference sample VII. Spatially resolved H alpha kinematics

E. C. Herenz, Pieter Gruyters+20 more|arXiv (Cornell University)|Jan 1, 2016

Astronomy and Astrophysical ResearchPhysics and Astronomy24 citations

TL;DR

This study presents spatially resolved Hα kinematics from integral field spectroscopy of 14 galaxies in the z ~ 0.1 Lyman Alpha Reference Sample using the Potsdam Multi-Aperture Spectrograph. It reveals that dispersion-dominated systems (vshear/σ₀ < 1) exhibit higher Lyα equivalent widths and escape fractions, indicating turbulence in the interstellar medium enhances Lyα radiation escape, with kinematic statistics serving as key indicators of Lyα radiative transfer conditions in star-forming galaxies.

ABSTRACT

We present integral field spectroscopic observations with the Potsdam Multi-Aperture Spectrophotometer of all 14 galaxies in the z similar to 0.1 Lyman Alpha Reference Sample (LARS). We produce 2D line-of-sight velocity maps and velocity dispersion maps from the Balmer alpha (H alpha) emission in our data cubes. These maps trace the spectral and spatial properties of the LARS galaxies' intrinsic Ly alpha radiation field. We show our kinematic maps that are spatially registered onto the Hubble Space Telescope H alpha and Lyman alpha (Ly alpha) images. We can conjecture a causal connection between spatially resolved H alpha kinematics and Ly alpha photometry for individual galaxies, however, no general trend can be established for the whole sample. Furthermore, we compute the intrinsic velocity dispersion sigma(0), the shearing velocity v(shear), and the v(shear)/sigma(0) ratio from our kinematic maps. In general LARS galaxies are characterised by high intrinsic velocity dispersions (54 km s(-1) median) and low shearing velocities (65 km s(-1) median). The v(shear/sigma 0) values range from 0.5 to 3.2 with an average of 1.5. It is noteworthy that five galaxies of the sample are dispersion-dominated systems with v(shear)/sigma(0) &lt; 1, and are thus kinematically similar to turbulent star-forming galaxies seen at high redshift. When linking our kinematical statistics to the global LARS Ly alpha properties, we find that dispersion-dominated systems show higher Ly alpha equivalent widths and higher Ly alpha escape fractions than systems with v(shear)/sigma(0) &gt; 1. Our result indicates that turbulence in actively star-forming systems is causally connected to interstellar medium conditions that favour an escape of Ly alpha radiation.

Motivation & Objective

• To investigate the spatially resolved kinematics of Hα emission in local starburst galaxies to understand their intrinsic Lyα radiation fields.
• To determine whether correlations exist between Hα kinematics and Lyα photometry across the full LARS sample.
• To quantify global kinematical statistics (σ₀, vshear, vshear/σ₀) and link them to Lyα properties to assess their role in radiative transfer.
• To assess the impact of turbulence and shear on Lyα escape fractions and equivalent widths in high-redshift analogs.

Proposed method

• Acquired integral field spectroscopy (IFS) data with the Potsdam Multi-Aperture Spectrograph (PMAS) for all 14 galaxies in the Lyman Alpha Reference Sample (LARS).
• Produced 2D maps of line-of-sight velocity (vLOS) and intrinsic velocity dispersion (σ₀) from spatially resolved Hα emission line profiles.
• Used Gaussian fitting to model Hα profiles in spatial bins (spaxels), with Voronoi binning to improve signal-to-noise ratio.
• Computed non-parametric global kinematical statistics: intrinsic velocity dispersion (σ₀), shearing velocity (vshear), and the ratio vshear/σ₀.
• Spatially registered kinematic maps with Hubble Space Telescope (HST) Hα and Lyα images for direct morphological comparison.
• Correlated kinematical statistics with global Lyα properties (equivalent width and escape fraction) to identify physical drivers of Lyα radiative transfer.

Experimental results

Research questions

• RQ1 Is there a general correlation between spatially resolved Hα kinematics and Lyα photometry across the full LARS sample?
• RQ2 How do global kinematical statistics (σ₀, vshear, vshear/σ₀) vary across the LARS galaxies, and what do they reveal about internal dynamics?
• RQ3 What is the relationship between kinematic dominance (dispersion vs. shear) and Lyα escape properties (equivalent width and escape fraction)?
• RQ4 To what extent does turbulence in the interstellar medium influence the observed Lyα radiation field in star-forming galaxies?

Key findings

• Five galaxies in the LARS sample are dispersion dominated, with vshear/σ₀ < 1, indicating kinematic similarity to high-redshift turbulent star-forming galaxies.
• The median intrinsic velocity dispersion (σ₀) across the sample is 54 km s⁻¹, and the median shearing velocity (vshear) is 65 km s⁻¹.
• The average vshear/σ₀ ratio is 1.5, with values ranging from 0.5 to 3.2 across the sample.
• Dispersion-dominated systems (vshear/σ₀ < 1) show significantly higher Lyα equivalent widths and higher Lyα escape fractions compared to shear-dominated systems.
• The Hα kinematic maps show complex velocity gradients and elevated dispersions, particularly in interacting systems like LARS 13, where multiple kinematic components are resolved.
• In LARS 14, broad Hα wings indicative of outflows are observed, consistent with its classification as a green pea galaxy and Lyman continuum leaker, with vshear/σ₀ = 0.6 ± 0.1.