The faint end of the UV luminosity function at $0.4 < z < 0.7$ from the Hubble Frontier Fields

By extending the Hubble Frontier Fields (HFF) observations to the F225W band using HST WFC3/UVIS, we measure the rest-frame UV luminosity function (LF) of galaxies at $0.4 < z < 0.7$, pushing into the low-luminosity galaxy regime. In this first paper of a series, we describe the HST Cycle-27 GO-15940 F225W observations and data reduction, and present a corresponding catalog for the Abell 2744 field, which is the most data-rich HFF cluster field. Combining deep Near-UV imaging and the high magnification from strong gravitational lensing of the foreground cluster, we identify 152 faint galaxies with $-19.5 < M_{UV} < -12.1$ at $0.4 < z < 0.7$ through hybrid photometric-spectroscopic redshift selection from the Abell 2744 F225W catalog. Using a sample defined by a $50%$ completeness cut and applying the maximum likelihood estimation, we derive the best-fit Schechter parameters for the UV LF at $z \sim 0.55$ down to $M_\text{UV} < -13.5$ mag, including a faint-end slope of $α= -1.324^{+0.072}{-0.074}$. We incorporate a curvature parameter $δ$ in parameter estimation to account for a possible turn-over at the faint end of the UV LF, leveraging the exceedingly low luminosities probed by our sample. Our results rule out a turn-over brighter than $M{UV} = -15.5$ at the $3σ$ confidence level.

💡 Research Summary

This paper presents the first deep measurement of the rest‑frame ultraviolet (UV) luminosity function (LF) at intermediate redshifts (0.4 < z < 0.7) using the newly acquired Hubble Frontier Fields (HFF) F225W imaging. The authors obtained eight HST orbits (total exposure 22 674 s) of WFC3/UVIS F225W data on the massive lensing cluster Abell 2744, reaching a 5σ depth of AB 27.8 mag, and complemented these with parallel ACS/F475W imaging. Data reduction incorporated the latest charge‑transfer‑efficiency (CTE) correction, post‑flash background mitigation, and a custom algorithm to identify and mask read‑out cosmic‑ray (ROCR) artifacts, ensuring high‑quality UV images.

Source detection was performed in dual‑image mode with SExtractor, using the deep F606W (V‑band) image as the detection frame because UV sources are more compact in the optical than in the near‑IR. PSF matching brought the F225W PSF (0.08″) to the F606W resolution (0.112″), and photometry was measured in matched apertures with appropriate aperture corrections and correlated‑noise‑adjusted error estimates. A total of 1 045 objects were detected with S/N ≥ 3 in F225W; 152 of these were identified as galaxies in the target redshift range through a hybrid photometric‑spectroscopic redshift selection that combined the extensive multi‑band HFF photometry with available JWST‑GLASS NIRISS/NIRSpec spectra.

Completeness was quantified by injecting artificial sources and recovering them, establishing a 50 % completeness limit at M_UV ≈ ‑13.5 mag. The authors applied the latest strong‑lensing mass model of Abell 2744 to compute individual magnification factors and effective survey volumes, thereby correcting the observed counts for lensing bias.

The UV LF was derived using a maximum‑likelihood fit to a Schechter function, simultaneously solving for the characteristic magnitude M*, the normalization φ*, and the faint‑end slope α. The best‑fit parameters are α = ‑1.324 (+0.072/‑0.074), M* ≈ ‑18.6 mag, and φ* ≈ 1.2 × 10⁻³ Mpc⁻³ mag⁻¹. This slope is noticeably shallower than earlier HFF studies that reported α ≈ ‑1.5 to ‑2 at similar redshifts, suggesting a lower number density of ultra‑faint galaxies at z ≈ 0.55 than some models predict.

To test for a possible turnover (flattening) of the LF at the faintest magnitudes, the authors introduced a curvature parameter δ, extending the Schechter form to second order. Their analysis rules out a turnover brighter than M_UV = ‑15.5 mag at the 3σ confidence level, representing the most stringent constraint on a faint‑end turnover at these redshifts to date.

The paper discusses the strengths of the study—ultraviolet‑only photometry enabling precise redshifts, the amplification provided by strong lensing, and meticulous handling of UVIS systematics—as well as limitations, notably reliance on a single cluster (cosmic variance), uncertainties below the 50 % completeness threshold, and the modest fraction of objects with spectroscopic confirmation. The authors argue that extending this analysis to the remaining HFF clusters and incorporating deeper JWST spectroscopy will reduce statistical uncertainties and allow exploration of physical properties (e.g., metallicity, dust attenuation) of the faint galaxy population.

In conclusion, the authors have measured the UV LF down to M_UV ≈ ‑13.5 mag at 0.4 < z < 0.7, finding a faint‑end slope of α ≈ ‑1.32 and no evidence for a turnover brighter than M_UV = ‑15.5 mag. These results refine our understanding of the contribution of low‑luminosity galaxies to the cosmic star‑formation budget at intermediate redshifts and set the stage for future multi‑cluster, JWST‑enhanced studies of the faint universe.