[Paper Review] The MUSE Hubble Ultra Deep Field Survey VIII : Extended Lyman-alpha haloes around high-redshift star-forming galaxies

Floriane Leclercq, Roland Bacon+18 more|arXiv (Cornell University)|Oct 27, 2017

Galaxies: Formation, Evolution, PhenomenaPhysics and Astronomy87 references70 citations

TL;DR

This study detects and characterizes extended Lyman-alpha haloes around 145 high-redshift star-forming galaxies in the MUSE UDF, using a two-component exponential model to quantify halo sizes and flux contributions.

ABSTRACT

We report the detection of extended Lyman-alpha (Lya) haloes around 145 individual star-forming galaxies at redshifts 3 M_{UV}> -22) Lya emitters (LAEs). Using a 2D, two-component decomposition of Lya emission assuming circular exponential distributions, we measure scale lengths and luminosities of Lya haloes. We find that 80% of our objects having reliable Lya halo measurements show Lya emission that is significantly more extended than the UV continuum detected by HST (by a factor ~4 to >20). The median exponential scale length of the Lya haloes in our sample is ~4.5 kpc. By comparing the maximal detected extent of the Lya emission with the predicted dark matter halo virial radii of simulated galaxies, we show that the detected Lya emission of our selected sample of LAEs probes a significant portion of the cold circum-galactic medium (CGM) of these galaxies (>50% in average). This result shows that there must be significant HI reservoirs in the CGM and reinforces the idea that Lya haloes are ubiquitous around high-redshift Lya emitting galaxies. Our characterization of the Lya haloes indicates that the majority of the Lya flux comes from the halo (~65%) and that their scale lengths seem to be linked to the UV properties of the galaxies. We do not observe a significant Lya halo size evolution with redshift. We also find that the Lya lines cover a large range of full width at half maximum (FWHM) from 118 to 512 km/s. While the FWHM does not seem to be correlated to the Lya scale length, most compact Lya haloes and those that are not detected with high significance tend to have narrower Lya profiles. Finally, we investigate the origin of the extended Lya emission but we conclude that our data do not allow us to disentangle the possible processes.

Motivation & Objective

• Motivate understanding of the circum-galactic medium (CGM) around high-redshift star-forming galaxies.
• Quantify the spatial extent and flux contribution of Lyα haloes relative to UV continua.
• Investigate how Lyα halo properties relate to host UV properties and redshift.
• Assess the line-profile diversity and possible mechanisms for extended Lyα emission.

Proposed method

• Construct Lyα narrowband images from MUSE data cubes and UV continuum images from HST counterparts.
• Perform a two-dimensional, two-component exponential fit to Lyα emission, with the halo scale length fixed to the UV continuum scale length in the core component.
• Convolve models with the MUSE PSF and fit to the Lyα image, extracting halo scale length and core/halo fluxes.
• Estimate uncertainties via noise realizations and empty-region tests to derive errors on halo and core scales.
• Define detection limits via S/N simulations and PSF-based size limits to assess reliable halo measurements.

Experimental results

Research questions

• RQ1 What is the prevalence of extended Lyα emission around high-redshift star-forming galaxies in the MUSE UDF?
• RQ2 How do Lyα halo sizes and flux distributions compare to the UV continuum and Lyα luminosity?
• RQ3 What is the range of Lyα line profiles and do they correlate with halo properties?
• RQ4 Can the extended Lyα emission be attributed to scattering, fluorescence, cooling radiation, or satellite contributions?

Key findings

• Approximately 145 galaxies show statistically significant Lyα haloes, representing about 80% of the reliable sample.
• The median Lyα halo exponential scale length is ~4.5 kpc, with some haloes exceeding 10 kpc.
• About 65% of the total Lyα flux comes from the halo component, not the central core.
• Lyα haloes are typically more extended than the UV continuum and their sizes correlate with UV properties of the host galaxies.
• Lyα line widths span from ~118 to ~512 km s−1, with narrower profiles associated with compact or less-significant haloes.
• No strong evidence for evolution of halo size with redshift within the sampled range.