Observed UV Continuum Slopes (β) of Galaxies at z = 0.40-0.75 in the GOODS-North Field

We estimate the UV continuum slope (β) of 465 galaxies (with luminosities of 0.028-3.3 L z = 0.5 * ) in the Great Observatories Origins Survey Northern field in the redshift range z = 0.40-0.75. We use two AstroSat/UVIT (N242W, N245M) bands, two Hubble Space Telescope (F275W, F336W) bands, and a KPNO (U) band to sample the UV continuum slope of selected galaxies between 1215 and 2600 Å. The mean (median) and 1σ scatter in the observed β are found to be −1.33 ± 0.07 ( − 1.32) and 0.60 within the considered redshift range. We do not find any significant evolution in the mean β within our redshift window. Our measurements add new data points to the global β-z relation in the least-explored redshift regime, further reinforcing the gradual reddening of galaxy UV continuum with cosmic time. We notice no strong consistent trend between β and M ₁₅₀₀ for the entire luminosity range −21 <M ₁₅₀₀ < − 15 mag. However, the majority of the most-luminous galaxies (M ₁₅₀₀ < − 19 mag) are found to have relatively redder slopes. Using UVIT, we detect galaxies as faint as M ₁₅₀₀ = − 15.6 mag (i.e., 0.028 L z = 0.5 * ). The faintest galaxies (M ₁₅₀₀ > − 16 mag) tend to be redder, which indicates they were less actively forming stars during this cosmic time interval. Our study highlights the unique capability of UVIT near-UV imaging to characterize the rest-frame far-UV properties of galaxies at redshift z ∼ 0.5.