Galaxy formation and evolution

Abstract:

We present the results of a new study investigating the relationship between observed Ly α equivalent width (Wλ(Ly α)) and the metallicity of the ionizing stellar population (Z★) for a sample of 768 star-forming galaxies at 3 ≤ z ≤ 5 drawn from the VANDELS survey. Dividing our sample into quartiles of rest-frame Wλ(Ly α) across the range −58 Å ≾ Wλ(Ly α) ≾ 110 Å, we determine Z★ from full spectral fitting of composite far-ultraviolet spectra and find a clear anticorrelation between Wλ(Ly α) and Z★. Our results indicate that Z★ decreases by a factor ≳ 3 between the lowest Wλ(Ly α) quartile (≺Wλ(Ly α)≻ = −18 Å) and the highest Wλ(Ly α) quartile (≺Wλ(Ly α)≻ = 24 Å). Similarly, galaxies typically defined as Lyman alpha emitters (LAEs; Wλ(Ly α) > 20 Å) are, on average, metal poor with respect to the non-LAE galaxy population (Wλ(Ly α) ≤ 20 Å) with Z★non-LAE ≳ 2 × Z★LAE. Finally, based on the best-fitting stellar models, we estimate that the increasing strength of the stellar ionizing spectrum towards lower Z★ is responsible for ≈15−25 per cent of the observed variation in Wλ(Ly α) across our sample, with the remaining contribution (≈75−85 per cent) being due to a decrease in the H I/dust covering fractions in low- Z★ galaxies.

Abstract:

We present an overview and description of the e-MERGE Survey (e-MERLIN Galaxy Evolution Survey) Data Release 1 (DR1), a large program of high-resolution 1.5-GHz radio observations of the GOODS-N field comprising ∼140 h of observations with enhanced-Multi-Element Remotely Linked Interferometer Network (e-MERLIN) and ∼40 h with the Very Large Array (VLA). We combine the long baselines of e-MERLIN (providing high angular resolution) with the relatively closely packed antennas of the VLA (providing excellent surface brightness sensitivity) to produce a deep 1.5-GHz radio survey with the sensitivity (⁠∼1.5μ Jy beam−1), angular resolution (0.2–0.7 arcsec) and field-of-view (∼15 × 15 arcmin2) to detect and spatially resolve star-forming galaxies and active galactic nucleus (AGN) at z ≳ 1. The goal of e-MERGE is to provide new constraints on the deep, sub-arcsecond radio sky which will be surveyed by SKA1-mid. In this initial publication, we discuss our data analysis techniques, including steps taken to model in-beam source variability over an ∼20-yr baseline and the development of new point spread function/primary beam models to seamlessly merge e-MERLIN and VLA data in the uv plane. We present early science results, including measurements of the luminosities and/or linear sizes of ∼500 galaxies selected at 1.5 GHz. In combination with deep Hubble Space Telescope observations, we measure a mean radio-to-optical size ratio of re-MERGE/rHST ∼ 1.02 ± 0.03, suggesting that in most high-redshift galaxies, the ∼GHz continuum emission traces the stellar light seen in optical imaging. This is the first in a series of papers that will explore the ∼kpc-scale radio properties of star-forming galaxies and AGN in the GOODS-N field observed by e-MERGE DR1.