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THE PANCHROMATIC HUBBLE ANDROMEDA TREASURY. XVII. EXAMINING OBSCURED STAR FORMATION WITH SYNTHETIC ULTRAVIOLET FLUX MAPS IN M31*

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Lewis, Alexia R., Simones, Jacob E., Johnson, Benjamin D., Dalcanton, Julianne J., Skillman, Evan D., Weisz, Daniel R., Dolphin, Andrew E., Williams, Benjamin F. +5 more·2017·The Astrophysical Journal
DOI 10.3847/1538-4357/834/1/70
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Abstract

We present synthetic far- and near-ultraviolet ( FUV and NUV ) maps of M31, both with and without dust reddening. These maps were constructed from spatially resolved star formation histories (SFHs) derived from optical Hubble Space Telescope imaging of resolved stars, taken as part of the Panchromatic Hubble Andromeda Treasury program. We use stellar population synthesis modeling to generate synthetic UV maps with a spatial resolution of ∼100 pc (∼24 arcsec), projected. When reddening is included, these maps reproduce all of the main morphological features in the GALEX imaging, including rings and large star-forming complexes. The predicted UV flux also agrees well with the observed flux, with median ratios between the modeled and observed flux of log 10 ( f FUV syn / f FUV obs ) = 0.03 ± 0.24 and log 10 ( f NUV syn / f NUV obs ) = − 0.03 ± 0.16 in the FUV and NUV , respectively. This agreement is particularly impressive given that we used only optical photometry to construct these UV maps. Having verified the synthetic reddened maps, we use the dust-free maps to examine properties of obscured flux and star formation. We compare our dust-free and reddened maps of FUV flux with the observed GALEX FUV flux and FUV + 24 μ m flux to examine the fraction of obscured flux. We find that the maps of synthetic flux require that ∼90% of the FUV flux in M31 is obscured by dust, while the GALEX -based methods suggest that ∼70% of the FUV flux is absorbed by dust. This 30% increase in the estimate of the obscured flux is driven by significant differences between the dust-free synthetic FUV flux and that derived when correcting the observed FUV flux for dust absorption with 24 μ m emission observations. The difference is further illustrated when we compare the SFRs derived from the FUV + 24 μ m flux with the 100 Myr average SFR from the CMD-based SFHs. We find that the 24 μ m corrected FUV flux underestimates the SFR by a factor of 2.3–2.5, depending on the chosen calibration. This discrepancy could be reduced by allowing for variability in the weight applied to the 24 μ m data, as has been recently suggested in the literature.

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  • About 20%-30% of the FUV emission comes from stars older than the adopted 500 Myr reliability threshold.

    “we use the full time resolution of our SFH fits out to log (age / yr) of 10.15, which is required to accurately model the UV flux given that ∼ 20% -30% of the FUV emission comes from stars that are older than our adopted 500 Myr reliability threshold.”

    From The Panchromatic Hubble Andromeda Southern Treasury (PHAST). II. The Spatially Resolved Recent Star Formation History in M31 (2026) · cites this paper as Lewis et al. (2017)

    Section 4.3 (page unavailable) says that older stars can contribute 20%–30% of FUV or NUV emission. The Additional information passage ties that figure to stars older than the usual 100–300 Myr emission timescales, while separately identifying 500 Myr as the SFH reliability limit. It does not establish that 20%–30% of FUV emission comes specifically from stars older than 500 Myr. (LLM, conf=0.89)

    Experimental — accuracy not yet measured

  • One way to test the validity of CMD-based SFH methods is to use the derived SFHs to forward-model a synthetic FUV image and compare it directly to the observed GALEX map, as was done in Lewis et al. (2017).

    “One way to test the validity of the CMD-based SFH methods is to use the derived SFHs to forward-model a synthetic FUV image and compare it directly to the observed GALEX map, as was done with great success in Lewis et al. (2017).”

    From The Panchromatic Hubble Andromeda Southern Treasury (PHAST). II. The Spatially Resolved Recent Star Formation History in M31 (2026) · cites this paper as Lewis et al. (2017)

  • They replicate the methodology of Lewis et al. (2017) to produce a synthetic FUV image and compare it to the GALEX map of Martin et al. (2005).

    “In this section, we replicate the methodology of Lewis et al. (2017) to produce a synthetic FUV image, and compare to the GALEX map of Martin et al. (2005).”

    From The Panchromatic Hubble Andromeda Southern Treasury (PHAST). II. The Spatially Resolved Recent Star Formation History in M31 (2026) · cites this paper as Lewis et al. (2017)

  • FUV-based SFR indicators yield SFRs smaller by factors ranging from 1.3 to 3 compared to those inferred from resolved stellar populations.

    “Several recent studies have highlighted a systematic offset between these approaches, with FUV based SFR indicators yielding results that are smaller by factors ranging from 1.3 to 3 compared to those inferred from resolved stellar populations”

    From The Panchromatic Hubble Andromeda Southern Treasury (PHAST). II. The Spatially Resolved Recent Star Formation History in M31 (2026) · cites this paper as Lewis et al. 2017

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Research works printed but not linked: 1

  1. [41]41. Johnson, L. C., Seth, A. C., Dalcanton, J. J., et al. 2016, arXiv:1606.05349 · arXiv

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Authors
Lewis, 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., Kapala, M., Rosenfield, P., & Schruba, A.
Affiliations
  • University of Washington (US)
  • University of Minnesota (US)
  • Center for Astrophysics Harvard & Smithsonian (US)
  • University of California, Berkeley (US)
  • RTX (United States) (US)
  • University of Michigan (US)
  • Max Planck Institute for Astronomy (DE)
  • University of Cape Town (ZA)
  • Max Planck Institute for Extraterrestrial Physics (DE)
ORCID
Venue
The Astrophysical Journal
Publisher
American Astronomical Society
Published
2017-01-01
Volume
834
Issue
1
Pages
70
ISSN
  • 0004-637X (ISSN-L)
  • 1538-4357
DOI
10.3847/1538-4357/834/1/70
OpenAlex
W2546793261
Cited by
16
References
79
Citations by year
2026: 2 · 2025: 2 · 2024: 1 · 2023: 1 · 2022: 2 · 2021: 2 · 2019: 4 · 2018: 1 · 2017: 1
FWCI
0.90
Type
journal article
Language
en
Open access
Licence
Locations
  • The Astrophysical Journal (publishedVersion)
  • arXiv (Cornell University) (submittedVersion)
  • Max Planck Institute for Plasma Physics (submittedVersion)
  • arXiv (Cornell University)
Funders
  • National Science Foundation (NCC5-626, 1440620, NAS 5-26555)
  • National Aeronautics and Space Administration (NAS 526555, 526555)
  • California Institute of Technology
  • Space Telescope Science Institute (NAS5- 26555, 5-26555, GO-12055, 26555)
  • Jet Propulsion Laboratory
Topics
  • Physical Sciences › Physics and Astronomy › Astronomy and Astrophysics › Galaxies: Formation, Evolution, Phenomena
  • Physical Sciences › Physics and Astronomy › Astronomy and Astrophysics › Astrophysics and Star Formation Studies
  • Physical Sciences › Physics and Astronomy › Astronomy and Astrophysics › Stellar, planetary, and galactic studies
Keywords
M31, obscured star formation, far-ultraviolet emission, near-ultraviolet emission, dust reddening, star formation history, stellar population synthesis, GALEX, 24 micron emission, star-forming complexes

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