Morphological Properties of Lyman Alpha Emitters at Redshift 4.86 in the COSMOS Field: Clumpy Star Formation or Merger?

TL;DR

This study analyzes the morphological properties of 61 Lyman-alpha emitters at redshift 4.86 in the COSMOS field using HST imaging, revealing diverse sizes, shapes, and potential blending effects, and discusses implications for their star formation and merger status.

Contribution

It provides detailed morphological analysis of high-redshift LAEs, highlighting the effects of PSF broadening and source blending on observed structures, and suggests the need for higher resolution observations.

Findings

Some LAEs are compact, others are extended up to 1.9 kpc.

A positive correlation exists between ellipticity and size, influenced by observational effects.

Many single-component LAEs may actually be multiple blended sources.

Abstract

We investigate morphological properties of 61 Lyman-alpha emitters (LAEs) at z = 4.86 identified in the COSMOS field, based on Hubble Space Telescope Advanced Camera for Surveys (ACS) imaging data in the F814W-band. Out of the 61 LAEs, we find the ACS counterparts for the 54 LAEs. Eight LAEs show double-component structures with a mean projected separation of 0."63 (~ 4.0 kpc at z = 4.86). Considering the faintness of these ACS sources, we carefully evaluate their morphological properties, that is, size and ellipticity. While some of them are compact and indistinguishable from the PSF half-light radius of 0."07 (~ 0.45 kpc), the others are clearly larger than the PSF size and spatially extended up to 0."3 (~ 1.9 kpc). We find that the ACS sources show a positive correlation between ellipticity and size and that the ACS sources with large size and round shape are absent. Our Monte Carlo simulation suggests that the correlation can be explained by (1) the deformation effects via PSF broadening and shot noise or (2) the source blending in which two or more sources with small separation are blended in our ACS image and detected as a single elongated source. Therefore, the 46 single-component LAEs could contain the sources which consist of double (or multiple) components with small spatial separation (i.e., < 0."3 or 1.9 kpc). Further observation with high angular resolution at longer wavelengths (e.g., rest-frame wavelengths of > 4000 A) is inevitable to decipher which interpretation is adequate for our LAE sample.