We explore deep rest-frame UV to FIR data in the COSMOS field to measure the individual spectral energy distributions (SED) of the ∼ 4000 SC4K (Sobral et al. 2018a) Lyman-α (Lyα) emitters (LAEs) at z ∼ 2 − 6. We find typical stellar masses of 10 9.3±0.6 M and star formation rates (SFR) of SFR SED = 4.5 +8.8 −2.5 M yr −1 and SFR Lyα = 5.9 +6.3 −2.6 M yr −1 , combined with very blue UV slopes of β = −2.0 +0.3 −0.5 , but with significant variations within the population. M UV and β are correlated in a similar way to UV-selected sources, but LAEs are consistently bluer. This suggests that LAEs are the youngest and/or most dust-poor subset of the UV-selected population. We also study the Lyα rest-frame equivalent width (EW 0 ) and find 45 "extreme" LAEs with EW 0 > 240Å (3 σ), implying a low number density of (7±1)×10 −7 Mpc −3 . Overall, we measure little to no evolution of the Lyα EW 0 and scale length parameter (w 0 ) which are consistently high (EW 0 = 140 +280 −70Å , w 0 = 129 +11 −11Å ) from z ∼ 6 to z ∼ 2 and below. However, w 0 is anti-correlated with M UV and stellar mass. Our results imply that sources selected as LAEs have a high Lyα escape fraction (f esc,Lyα ) irrespective of cosmic time, but f esc,Lyα is still higher for UV-fainter and lower mass LAEs. The least massive LAEs (< 10 9.5 M ) are typically located above the star formation "Main Sequence" (MS), but the offset from the MS decreases towards z ∼ 6 and towards 10 10 M . Our results imply a lack of evolution in the properties of LAEs across time and reveals the increasing overlap in properties of LAEs and UV-continuum selected galaxies as typical star-forming galaxies at high redshift effectively become LAEs.

We measure the evolution of the rest-frame UV luminosity function (LF) and the stellar mass function (SMF) of Lyman-α (Ly α) emitters (LAEs) from z ∼ 2 to z ∼ 6 by exploring ∼4000 LAEs from the SC4K sample. We find a correlation between Ly α luminosity (LLy α) and rest-frame UV (MUV), with best fit M$_{\rm UV}=-1.6_{-0.3}^{+0.2}\log _{10} (\rm L_{Ly\,\alpha }/erg\, s^{-1})+47_{-11}^{+12}$ and a shallower relation between LLy α and stellar mass (M⋆), with best fit $\log _{10} (\rm M_\star /{\rm M}_\odot)=0.9_{-0.1}^{+0.1}\log _{10} (\rm L_{Ly\,\alpha }/erg\, s^{-1})-28_{-3.8}^{+4.0}$. An increasing LLy α cut predominantly lowers the number density of faint MUV and low M⋆ LAEs. We estimate a proxy for the full UV LFs and SMFs of LAEs with simple assumptions of the faint end slope. For the UV LF, we find a brightening of the characteristic UV luminosity (M$_{\rm UV}^*$) with increasing redshift and a decrease of the characteristic number density (Φ*). For the SMF, we measure a characteristic stellar mass (${\rm M_\star ^*/{\rm M}_\odot }$) increase with increasing redshift, and a Φ* decline. However, if we apply a uniform luminosity cut of $\log _{10} (\rm L_{Ly\,\alpha }/erg\, s^{-1}) \ge 43.0$, we find much milder to no evolution in the UV and SMF of LAEs. The UV luminosity density (ρUV) of the full sample of LAEs shows moderate evolution and the stellar mass density (ρM) decreases, with both being always lower than the total ρUV and ρM of more typical galaxies but slowly approaching them with increasing redshift. Overall, our results indicate that both ρUV and ρM of LAEs slowly approach the measurements of continuum-selected galaxies at z &gt; 6, which suggests a key role of LAEs in the epoch of reionization.

Context. Molecular line ratios, such as HCN(1-0)/HCO + (1-0) and HCN(4-3)/CS(7-6), are routinely used to identify active galactic nuclei (AGN) activity in galaxies. Such ratios are, however, hard to interpret as they are highly dependent on the physics and energetics of the gas, and hence can seldom be used as a unique, unambiguous diagnostic. Aims. We used the composite galaxy NGC 1068 as a 'laboratory' to investigate whether molecular line ratios between HCN, HCO + , and CS are useful tracers of AGN-dominated gas and determine the origin of the differences in such ratios across different types of gas. Such a determination will enable a more rigorous use of such ratios. Methods. First, we empirically examined the aforementioned ratios at different angular resolutions to quantify correlations. We then used local thermodynamic equilibrium (LTE) and non-LTE analyses coupled with Markov chain Monte Carlo (MCMC) sampling in order to determine the origin of the underlying differences in ratios. Results. We propose that at high spatial resolution (< 50 pc) the HCN(4-3)/CS(2-1) is a reliable tracer of AGN activity. We also find that the variations in ratios are not a consequence of different densities or temperature but of different fractional abundances, yielding to the important result that it is essential to consider the chemical processes at play when drawing conclusions from radiative transfer calculations. Conclusions. From analyses at varying spatial scales, we find that previously proposed molecular line ratios, as well as a new one, have varying levels of consistency. We also determine from an investigation of radiative transfer modelling of our data that it is essential to consider the chemistry of the species when reaching conclusions from radiative transfer calculations.