A Census of Double-Peaked Lyman-alpha Emitters in MAGPI: Classification, Global Characteristics, and Spatially Resolved Properties

TL;DR

This study presents a comprehensive census of 108 double-peaked Lyα emitters from 417 LAEs in the MAGPI/MUSE program spanning $2.9 \le z \le 6.6$, with automated peak classification and spatial verification confirming double peaks and occasional triples. Using forward modeling with double-asymmetric Gaussian profiles convolved by the instrumental LSF, and, where possible, systemic redshifts from C III], the authors derive intrinsic Lyα properties and quantify the blue-to-red flux balance, peak separations, and trough flux. They find that 17% of double-peaked LAEs are blue-dominated and that the red peak narrows with increasing redshift, implying intrinsic ISM/CGM evolution rather than IGM attenuation as the primary driver. Spatially resolved analyses of ten halos reveal increasing blue flux and trough flux with radius, and decreasing peak separation, consistent with radial declines in shell velocity and HI column density; five LAEs are identified as strong LyC leaker candidates, illustrating the diagnostic power of double-peaked Lyα profiles for probing HI distribution and LyC escape pathways in the high-redshift universe.

Abstract

Double-peaked Ly$α$ profiles provide critical insights into gas kinematics and the distribution of neutral hydrogen (HI) from the interstellar to the intergalactic medium (ISM to IGM), and serve as valuable diagnostics of ionising Lyman continuum (LyC) photon escape. We present a study of the global and spatially resolved properties of double-peaked Ly$α$ emitters (LAEs) based on VLT/MUSE data from the MAGPI survey. From a parent sample of 417 LAEs at z = 2.9 - 6.6 in the first 35 fields, we identify 108 double-peaked LAEs using an automated peak classification technique. We measure a double-peak fraction of $\sim37\%$ at $z < 4$, decreasing to $\sim14\%$ at $z > 4$, likely due to enhanced IGM attenuation. Approximately $17\%$ of the double-peaked LAEs are blue-dominated, possibly tracing gas inflows, though backscattering remains a viable alternative for sources without systemic redshift. The blue-to-total flux ratio exhibits a luminosity dependence: fainter lines generally show higher blue flux. We find a narrowing of the red peak at $z > 4$, despite the presence of the blue peak, indicating intrinsic galaxy evolution rather than IGM attenuation. Several LAEs exhibit residual flux in the absorption trough, with normalised trough flux anticorrelating with peak separation, reflecting variations in HI column density. We further investigate spatially resolved properties of ten red-dominated LAEs with extended Ly$α$ halos. Despite azimuthal variations, both the blue-to-total flux ratio and normalised trough flux density increase with radius, while peak separation decreases. The red peak asymmetry shows only minor radial changes. These trends are consistent with variations in shell outflow velocity and HI column density across the halos. Based on peak separation, red peak asymmetry, and residual trough flux, we identify five LAEs as strong LyC-leaker candidates.

Figures (20)

• Figure 1: Ly$\alpha$ luminosity as a function of spectroscopic redshift for 417 LAEs identified in first 35 MAGPI fields, spanning the redshift range $2.9 \lesssim z \lesssim 6.6$.
• Figure 2: Redshift distribution of MAGPI LAEs classified by their Ly$\alpha$ profile as outlined in § \ref{['sec:sample selection']}: single-peaked (light-blue), double-peaked (green), and triple-peaked (red). The gray-shaded region ($z = 3.75$–$3.9$) marks a spectral gap due to the GALACSI laser notch filter, resulting in a lack of detected sources in this range.
• Figure 3: One example of rest-frame spectrum of a bright LAE (ID: 1201250296) with detected C iii] $\lambda 1907$ emission ($\sim 2.75 \, \sigma$) probing the systemic redshift $z_{\mathrm{sys}} = 3.2377$.
• Figure 4: Forward modeling of three representative double-peaked Ly$\alpha$ spectra. The observed spectra are shown in black, with the best-fit forward-modeled profiles overplotted in green after convolution with the instrumental LSF. The intrinsic double asymmetric Gaussian models prior to convolution are shown in red. The corresponding MAGPI IDs are shown in the top left corner of each panel.
• Figure 5: Blue-to-total flux ratio as a function of the total S/N of the Ly$\alpha$ line in our parent sample. Pink triangles are taken from Vitte25. The horizontal black dashed line marks the division between blue- and red-dominated regions.