Modeling the effects of star formation histories on Hα and ultraviolet fluxes in nearby dwarf galaxies

Daniel R. Weisz, Benjamin D. Johnson, L. Clifton Johnson, Evan D. Skillman, Janice C. Lee, Robert C. Kennicutt, Daniela Calzetti, Liese Van Zee, Matthew S. Bothwell, Julianne J. Dalcanton, Daniel A. Dale, Benjamin F. Williams

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Abstract

We consider the effects of non-constant star formation histories (SFHs) on Hα and GALEX far-ultraviolet (FUV) star formation rate (SFR) indicators. Under the assumption of a fully populated Chabrier initial mass function (IMF), we compare the distribution of Hα-to-FUV flux ratios from ∼1500 simple, periodic model SFHs with observations of 185 galaxies from the Spitzer Local Volume Legacy survey. We find a set of SFH models that are well matched to the data, such that more massive galaxies are best characterized by nearly constant SFHs, while low-mass systems experience burst amplitudes of ∼30 (i.e., an increase in the SFR by a factor of 30 over the SFR during the inter-burst period), burst durations of tens of Myr, and periods of ∼250Myr; these SFHs are broadly consistent with the increased stochastic star formation expected in systems with lower SFRs. We analyze the predicted temporal evolution of galaxy stellar mass, R-band surface brightness, Hα-derived SFR, and blue luminosity, and find that they provide a reasonable match to observed flux distributions. We find that our model SFHs are generally able to reproduce both the observed systematic decline and increased scatter in Hα-to-FUV ratios toward low-mass systems, without invoking other physical mechanisms. We also compare our predictions with those from the Integrated Galactic IMF theory with a constant SFR. We find that while both predict a systematic decline in the observed ratios, only the time variable SFH models are capable of producing the observed population of low-mass galaxies (M * ≲ 10 7 M) with normal Hα-to-FUV ratios. These results demonstrate that a variable IMF alone has difficulty explaining the observed scatter in the Hα-to-FUV ratios. We conclude by considering the limitations of the model SFHs and discuss the use of additional empirical constraints to improve future SFH modeling efforts.

Original languageEnglish (US)
Article number44
JournalAstrophysical Journal
Volume744
Issue number1
DOIs
StatePublished - Jan 1 2012

Keywords

  • galaxies: dwarf
  • galaxies: evolution
  • galaxies: formation
  • galaxies: star formation

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    Weisz, D. R., Johnson, B. D., Johnson, L. C., Skillman, E. D., Lee, J. C., Kennicutt, R. C., Calzetti, D., Van Zee, L., Bothwell, M. S., Dalcanton, J. J., Dale, D. A., & Williams, B. F. (2012). Modeling the effects of star formation histories on Hα and ultraviolet fluxes in nearby dwarf galaxies. Astrophysical Journal, 744(1), [44]. https://doi.org/10.1088/0004-637X/744/1/44