Identifying Active Galactic Nuclei at $z\sim3$ from the HETDEX Survey Using Machine Learning

We used data from the Hobby-Eberly Telescope Dark Energy Experiment (HETDEX) to study the incidence of AGN in continuum-selected galaxies at $z\sim3$. From optical and infrared imaging in the 24 deg$^{2}$ Spitzer HETDEX Exploratory Large Area (SHELA) survey, we constructed a sample of photometric-redshift selected $z\sim3$ galaxies. We extracted HETDEX spectra at the position of 716 of these sources and used machine learning methods to identify those which exhibited AGN-like features. The dimensionality of the spectra was reduced using an autoencoder, and the latent space was visualized through t-distributed stochastic neighbor embedding (t-SNE). Gaussian mixture models were employed to cluster the encoded data and a labeled dataset was used to label each cluster as either AGN, stars, high-redshift galaxies, or low-redshift galaxies. Our photometric redshift (photo-z) sample was labeled with an estimated $92\%$ overall accuracy, an AGN accuracy of $83\%$, and an AGN contamination of $5\%$. The number of identified AGN was used to measure an AGN fraction for different magnitude bins. The UV absolute magnitude where the AGN fraction reaches $50\%$ is $M_{UV} = -23.8$. When combined with results in the literature, our measurements of AGN fraction imply that the bright end of the galaxy luminosity function exhibits a power-law rather than exponential decline, with a relatively shallow faint-end slope for the $z\sim3$ AGN luminosity function.