Scylla is a pure-parallel HST imaging survey that operated alongside the Ultraviolet (UV) Legacy Library of Young Stars as Essential Standards (ULLYSES) survey. Named after the multi-headed monster from the myth of Ulysses, Scylla imaged fields parallel to spectroscopic ULLYSES targets with maximum photometric coverage from the near UV to the near-infrared (IR) with Wide Field Camera 3 (WFC3). Our first data release consists of 96 fields imaged over 342 orbits. In future releases, we will include all survey data (roughly 20 additional fields expected).

In subsequent publications, we will produce a catalog of stellar and dust parameters by fitting the multi-band photometry of each star using the Bayesian Extinction and Stellar Tool (BEAST): a probabilistic method for modeling SEDs for surveys of resolved stellar populations. Our catalogs will describe the age, initial mass, metallicity, distance, dust extinction (Av), average grain size (Rv), and the mixture coefficient between the Milky Way Rv-dependent dust extinction and the SMC Bar dust extinction for ~2 million stars in the MCs.

The primary Scylla science goals are:

• resolve how dust properties vary with interstellar environment

  • probing the total dust column density at ~parsec-scale resolution within the MCs independently of other ISM tracers and comparing our results with Av derived from FIR emission

  • probe variations in the opacity and emissivity of dust throughout the MCs

• constrain the multi-dimensional structure of the ISM of the MCs

• probe their detailed star formation histories (SFH, Cohen et al. 2024a, b)

  • generate maps of the SFH and chemical enrichment history of the MCs with the highest resolution to date

  • resolve how the SFH evolves as a function of position and time