The Hubble Legacy Archive ACS Grism Data

TL;DR

This paper presents a comprehensive public release of ACS/WFC G800L grism slitless spectra from 153 archival fields, including data processing, quality control, and initial scientific analysis, enabling broad community access to high-quality spectral data.

Contribution

The paper introduces an automated pipeline for extracting, calibrating, and classifying a large set of slitless spectra from archival Hubble data, providing a valuable resource for the community.

Findings

Nearly 48,000 high-quality spectra for over 32,000 objects were released.

The spectra cover a redshift range of 0.2 to 4.6, including emission-line galaxies.

Comparison with ground-based spectra validated the sensitivity and accuracy of the data.

Abstract

A public release of slitless spectra, obtained with ACS/WFC and the G800L grism, is presented. Spectra were automatically extracted in a uniform way from 153 archival fields (or "associations") distributed across the two Galactic caps, covering all observations to 2008. The ACS G800L grism provides a wavelength range of 0.55-1.00 \mu$m, with a dispersion of $40 \ \AA / pixel$ and a resolution of $\sim 80\ \AA$ for point-like sources. The ACS G800L images and matched direct images were reduced with an automatic pipeline that handles all steps from archive retrieval, alignment and astrometric calibration, direct image combination, catalogue generation, spectral extraction and collection of metadata. The large number of extracted spectra (73,581) demanded automatic methods for quality control and an automated classification algorithm was trained on the visual inspection of several thousand spectra. The final sample of quality controlled spectra includes 47,919 datasets (65% of the total number of extracted spectra) for $32,149$ unique objects, with a median $i_{\rm AB}$-band magnitude of 23.7, reaching 26.5 AB for the faintest objects. Each released dataset contains science-ready 1D and 2D spectra, as well as multi-band image cutouts of corresponding sources and a useful preview page summarising the direct and slitless data, astrometric and photometric parameters. In order to characterize the slitless spectra, emission-line flux and equivalent width sensitivity of the ACS data were compared with public ground-based spectra in the GOODS-South field. An example list of emission line galaxies with two or more identified lines is also included, covering the redshift range $0.2-4.6$.