Panchromatic Hubble Andromeda Southern Treasury (PHAST)
Figure 1: PHAST's footprint covering the southern half of M31's star-forming disk.
Overview
The Panchromatic Hubble Andromeda Southern Treasury (PHAST) is a large 195-orbit Hubble Space Telescope program imaging ∼0.45 square degrees of the southern half of M31's star-forming disk at optical and near-ultraviolet (NUV) wavelengths. The southern disk has key structural differences from the northern disk. The southern features suggest that it is more disturbed, and it has been more affected by M31's merger history. M32, a compact dwarf elliptical satellite, is projected just 5 kpc from the center of M31 against the southern disk. M32 has been proposed as the remnant of the progenitor which formed the giant southern stream, a stream spanning ∼150 kpc extending outwards from the southern half of the disk. Some scenarios suggest M32 is the core of the massive merged galaxy, while other models suggest the stream's progenitor was completely disrupted. Consistent, uniform analyses of the stellar populations of M31, M32, and the GSS may allow us to measure relative age differences and velocity signatures in their stellar populations, distinguishing between these models. A first step in such a process is the production of a deep resolved stellar photometry catalog.
The observations for the PHAST program (GO-16778, 16796-16801) were obtained from 2021 December 8 to 2024 January 13 using the Advanced Camera for Surveys (ACS) Wide Field Channel (WFC) and the Wide Field Camera 3 (WFC3) UVIS (ultraviolet) channel. The observations use the UVIS footprint as the survey's fundamental coverage unit. In total, 195 single-orbit visits observed in four filters. At each pointing in the survey, WFC3/UVIS observations in F275W (covering 250-300 nm) and F336W (covering 310-360 nm) filters and ACS/WFC observations in F475W (covering 400-550 nm) and F814W (covering 700-950 nm) were taken in parallel in one orbit. All observations were taken at 2 orientations, separated by 180 degrees, in order to maximize the survey coverage for all cameras and optimize the use of parallel exposures. PHAST extends ∼13 kpc along the southern major axis of M31's disk.
The rectangular "bricks" are areas of ∼12.5'×7.5' that correspond to a 5×3 array of WFC3/UVIS footprints (see Figure 1). In total, the thirteen PHAST bricks are numbered, 24 - 36, following the naming convention used in PHAT which covered the first 1 - 23 bricks, with odd numbers for bricks along the major axis and larger increasing numbers with increasing distance from the center.
Data Release Description
This data release expands on the MAST HLSP release which only has single, combined photometry for each source in each band, along with combined quality metrics in each band for those sources. This release adds the individual exposure measurements for each source, allowing users to check the consistency of measurements across individual detections and to look for variability across sources that appear in multiple exposures. It is important to note that two dithered exposures were taken in each band, which covered the chip gaps in the detectors and allowed many cosmic rays to be removed, but with so few measurements, there may still be many artifacts in the catalogs especially along detector edges and in the detector gaps, which were only covered by single exposures. Thus, sources detected in only one band or one observation should be very carefully verified.
Table Descriptions
| Table Name | Description |
|---|---|
| exposure | Contains the header information for each exposure, such as the start and stop time for time-domain work. By cross-referencing this information with the Measurement Table, it is possible to learn all of the details about the observation that went into each individual measurement. |
| meas | Contains the photometry for each object in each exposure where it was measured. This is the new information, not available on MAST. |
| object | Contains the primary unique objects and their combined photometry, comparable to the data that are in the MAST HLSP |
Catalog Stats
| PHAST DR1 Catalog Stats | |
|---|---|
| Total stars in catalog | 90,885,399 |
| Stars passing all 4 GST flags | 252,254 (0.3%) |
| Stars detected in F275W | 28,207,607 (31.0%) |
| Stars detected in F336W | 39,617,547 (43.6%) |
| Stars detected in F475W | 89,796,000 (98.8%) |
| Stars detected in F814W | 90,625,959 (99.7%) |
| Total sky area | 1410.6 arcmin² = 0.3918 deg² = 73.6 kpc² |
| Mean stellar density | 64,430 stars / arcmin² |
| Catalog size | 3,903 GiB |
| Total exposures | 1,560 |
| Number of bricks | 13 (B24 - B36) |
| Overall time between first and last MJD | MJD 59556.1 - 60322.4 (766.4 d = 2.10 yr; 2021-12-08 - 2024-01-13) |
Exposures per brick
| Brick | Number of Exposures |
|---|---|
| B24 | 120 |
| B25 | 120 |
| B26 | 120 |
| B27 | 120 |
| B28 | 120 |
| B29 | 120 |
| B30 | 120 |
| B31 | 120 |
| B32 | 120 |
| B33 | 120 |
| B34 | 120 |
| B35 | 120 |
| B36 | 120 |
Citation
Chen et al 2025 ApJ 979 35 PHAST. The Panchromatic Hubble Andromeda Southern Treasury. I. Ultraviolet and Optical Photometry of over 90 Million Stars in M31 DOI: 10.3847/1538-4357/ad7e2b
Acknowledgments
Support for this work was provided by NASA through grants #GO-16778 and 16796–16801 from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Incorporated, under NASA contract NAS5-26555.
External Links
a. Mosaic and videos: https://science.nasa.gov/asset/hubble/hubble-m31-phatphast-mosaic/
b. High Level Science Product (HLSP) project page: https://archive.stsci.edu/hlsp/phast
c. Full source data: doi:10.17909/6889-kg02
d. Project Paper:
- Wainer et al 2026: DOI: 10.3847/1538-4357/ae6db9
