PHAST. The Panchromatic Hubble Andromeda Southern Treasury. I. Ultraviolet and Optical Photometry of over 90 Million Stars in M31
Open accessAbstract
The Panchromatic Hubble Andromeda Southern Treasury (PHAST) is a large 195-orbit Hubble Space Telescope program imaging ∼0.45 deg 2 of the southern half of M31's star-forming disk at optical and near-ultraviolet (NUV) wavelengths. The PHAST survey area extends the northern coverage of the Panchromatic Hubble Andromeda Treasury (PHAT) down to the southern half of M31, covering out to a radius of ∼13 kpc along the southern major axis and in total ∼two-thirds of M31's star-forming disk. This new legacy imaging yields stellar photometry of over 90 million resolved stars using the Advanced Camera for Surveys in the optical (F475W and F814W), and the Wide Field Camera 3 (WFC3) in the NUV (F275W and F336W). The photometry is derived using all overlapping exposures across all bands, and achieves a 50% completeness-limited depth of F475W ∼ 27.7 in the lowest surface density regions of the outer disk and F475W ∼ 26.0 in the most crowded, high surface brightness regions near M31's bulge. We provide extensive analysis of the data quality, including artificial star tests to quantify completeness, photometric uncertainties, and flux biases, all of which vary due to the background source density and the number of overlapping exposures. We also present seamless population maps of the entire M31 disk, which show relatively well-mixed distributions for stellar populations older than 1–2 Gyr, and highly structured distributions for younger populations. The combined PHAST + PHAT photometry catalog of ∼0.2 billion stars is the largest ever produced for equidistant sources and is available for public download by the community.
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PHAST presents reliable optical photometry down to ~27 magnitude corresponding to an absolute magnitude of ~2.5.
“With over 90 million resolved stars covering 0.45 deg 2 of the star forming disk, PHAST presents reliable optical photometry down to ∼ 27 magnitude corresponding to an absolute magnitude of ∼ 2 . 5.”
From The Panchromatic Hubble Andromeda Southern Treasury (PHAST). II. The Spatially Resolved Recent Star Formation History in M31 (2026) · cites this paper as Chen et al. 2025
Section 3.4.1 (Table 5; page not available) gives an F475W 50% completeness limit near magnitude 27 in some regions, and Section 3.5 gives M31’s distance modulus as 24.47, making an apparent magnitude of 27 roughly absolute magnitude 2.5. But the optical depth varies with crowding and filter: F475W reaches only about 26 in the most crowded regions, while F814W is shallower. A 50% completeness limit also does not establish reliable photometry for all stars at that magnitude. (LLM, conf=0.86)
Experimental — accuracy not yet measuredThe PHAST survey is a large HST program targeting the southern disk of M31 (GO-16778 and 16796-16801; PI: Williams).
“Our analysis is based on imaging from the PHAST survey (Chen et al. 2025), a large HST program targeting the southern disk of M31 (GO-16778 and 16796-16801; PI: Williams).”
From The Panchromatic Hubble Andromeda Southern Treasury (PHAST). II. The Spatially Resolved Recent Star Formation History in M31 (2026) · cites this paper as Chen et al. 2025
The Abstract describes PHAST as a large HST program imaging the southern half of M31’s star-forming disk, and Section 2.1 identifies its observations as GO-16778 and 16796–16801. The passages from our copy do not identify Williams as the program’s principal investigator, so that detail is unsupported. (LLM, conf=0.85)
Experimental — accuracy not yet measuredBecause the PHAST analysis follows the same MATCH-based methodology as the northern PHAT survey, the two data sets can be combined into a single, internally consistent view of recent star formation over roughly two thirds of M31's star-forming disk.
“Because the PHAST analysis follows the same MATCH -based methodology as the northern PHAT survey, we can combine the two data sets into a single, internally consistent view of recent star formation over roughly two thirds of M31's star-forming disk.”
From The Panchromatic Hubble Andromeda Southern Treasury (PHAST). II. The Spatially Resolved Recent Star Formation History in M31 (2026) · cites this paper as Chen et al. 2025
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Gordon, K. D., Bailin, J., Engelbracht, C. W., Rieke, G. H., Misselt, K. A., Latter, W. B., Young, E. T., Ashby, M. L. N., Barmby, P., Gibson, B. K., et al. (2006) · The Astrophysical Journal · doi:10.1086/501046 · Cited by 182 · see citing works
Self-citationLewis, A. R., Dolphin, A. E., Dalcanton, J. J., Weisz, D. R., Williams, B. F., Bell, E. F., Seth, A. C., Simones, J. E., Skillman, E. D., Choi, Y., et al. (2015) · The Astrophysical Journal · doi:10.1088/0004-637x/805/2/183 · Cited by 132 · see citing works
Self-citationLewis, A. R., Simones, J. E., Johnson, B. D., Dalcanton, J. J., Skillman, E. D., Weisz, D. R., Dolphin, A. E., Williams, B. F., Bell, E. F., Fouesneau, M., et al. (2017) · The Astrophysical Journal · doi:10.3847/1538-4357/834/1/70 · Cited by 16 · see citing works
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Wainer, T. M., Williams, B. F., Chen, Z., Lazzarini, M., Dalcanton, J. J., Bell, E. F., Goold, K., Dolphin, A., Durbin, M. J., Dubón, S. L. F., et al. (2026) · The Astrophysical Journal · doi:10.3847/1538-4357/ae6db9
“A standard way to characterize photometric uncertainties, completeness, and bias due to crowding, is to use artificial star tests (ASTs) inserted throughout the survey area (Dol-phin 2002; Dalcanton et al. 2012; Williams et al. 2021; Chen et al. 2025).”
InfluentialSelf-citation
- Authors
- Chen, Z., Williams, B., Lang, D., Dolphin, A., Durbin, M., Dalcanton, J. J., Smercina, A., Girardi, L., Murray, C. E., Bell, E. F., Boyer, M. L., D’Souza, R., Gilbert, K., Gordon, K., Guhathakurta, P., Hammer, F., Johnson, L. C., Lauer, T. R., Lazzarini, M., Murphy, J. W., Patel, E., Quirk, A., Díaz Rodríguez, M., Roman-Duval, J. C., Sanderson, R. E., Seth, A., Wainer, T. M., & Weisz, D. R.
- Affiliations
- University of Washington (US)
- Carnegie Mellon University (US)
- University of Arizona (US)
- Space Telescope Science Institute (US)
- Osservatorio Astronomico di Padova (IT)
- University of Michigan (US)
- Vatican Observatory (IT)
- University of California, Santa Cruz (US)
- Centre National de la Recherche Scientifique (FR)
- Northwestern University (US)
- California State University Los Angeles (US)
- Florida State University (US)
- University of Utah (US)
- Columbia University (US)
- University of Pennsylvania (US)
- University of California, Berkeley (US)
- Corresponding author
- Chen, Zhuo
- ORCID
- Chen, Zhuo
- Williams, Benjamin
- Lang, Dustin
- Dolphin, Andrew
- Durbin, Meredith
- Dalcanton, Julianne J.
- Smercina, Adam
- Girardi, Léo
- Murray, Claire E.
- Bell, Eric F.
- Boyer, Martha L.
- D’Souza, Richard
- Gilbert, Karoline
- Gordon, Karl
- Guhathakurta, Puragra
- Hammer, Francois
- Johnson, L. Clifton
- Lauer, Tod R.
- Lazzarini, Margaret
- Murphy, Jeremiah W.
- Patel, Ekta
- Quirk, Amanda
- Díaz Rodríguez, Mariangelly
- Roman-Duval, Julia Christine
- Sanderson, Robyn E.
- Seth, Anil
- Wainer, Tobin M.
- Weisz, Daniel R.
- Venue
- The Astrophysical Journal
- Publisher
- American Astronomical Society
- Published
- 2025-01-01
- Volume
- 979
- Issue
- 1
- Pages
- 35
- ISSN
- 0004-637X (ISSN-L)
- 1538-4357
- OpenAlex
- W4406439990
- Cited by
- 6
- References
- 3
- Citations by year
- 2026: 1 · 2025: 5
- FWCI
- 2.35
- Type
- journal article
- Language
- en
- Open access
- gold
- doi.org
- Licence
- Locations
- The Astrophysical Journal (publishedVersion, cc-by)
- HAL (submittedVersion, cc-by)
- DOAJ (DOAJ: Directory of Open Access Journals) (submittedVersion, cc-by-sa)
- Funders
- Space Telescope Science Institute (GO-16778, GO-16796, GO-16797, GO-16798, GO-16799, GO-16800, GO-16801)
- Topics
- Physical Sciences › Physics and Astronomy › Astronomy and Astrophysics › Galaxies: Formation, Evolution, Phenomena
- Physical Sciences › Physics and Astronomy › Astronomy and Astrophysics › Stellar, planetary, and galactic studies
- Physical Sciences › Physics and Astronomy › Instrumentation › Astronomy and Astrophysical Research
- Keywords
- M31, PHAST, PHAT, stellar photometry, resolved stellar populations, star-forming disks, near-ultraviolet imaging, photometric completeness, photometric uncertainty, flux bias
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