Comparison of H-alpha and UV Star Formation Rates in the Local Volume: Systematic Discrepancies for Dwarf Galaxies

Abstract: (abridged) Using a complete sample of ~300 star-forming galaxies within 11 Mpc, we evaluate the consistency between star formation rates (SFRs) inferred from the far ultraviolet (FUV) non-ionizing continuum and H-alpha nebular emission, assuming standard conversion recipes in which the SFR scales linearly with luminosity at a given wavelength. Our analysis probes SFRs over 5 orders of magnitude, down to ultra-low activities on the order of ~0.0001 M_sun/yr. The data are drawn from the 11 Mpc H-alpha and Ultraviolet Galaxy Survey (11HUGS), which has obtained H-alpha fluxes from ground-based narrowband imaging, and UV fluxes from imaging with GALEX. For normal spiral galaxies (SFR~1 M_sun/yr), our results are consistent with previous work which has shown that FUV SFRs tend to be lower than H-alpha SFRs before accounting for internal dust attenuation, but that there is relative consistency between the two tracers after proper corrections are applied. However, a puzzle is encountered at the faint end of the luminosity function. As lower luminosity dwarf galaxies, roughly less active than the Small Magellanic Cloud, are examined, H-alpha tends to increasingly under-predict the SFR relative to the FUV. Although past studies have suggested similar trends, this is the first time this effect is probed with a statistical sample for galaxies with SFR~<0.1 M_sun/yr. A range of standard explanations does not appear to be able to account for the magnitude of the systematic. Some recent work has argued for an IMF which is deficient in high mass stars in dwarf and low surface brightness galaxies, and we also consider this scenario.

• The paper demonstrates that Hα-derived SFRs underpredict FUV-based rates in dwarf galaxies, especially below 0.1 M⊙/yr.
• The study uses data from 11HUGS and GALEX to confirm these discrepancies even after accounting for dust effects.
• The findings suggest that bursty star formation histories or IMF variations in dwarf galaxies necessitate recalibration of traditional SFR indicators.

Analysis of Discrepancies in Star Formation Rate Indicators in Dwarf Galaxies

The paper by Lee et al. investigates inconsistencies in star formation rates (SFRs) deduced from Hα nebular line emission versus the far ultraviolet (FUV) continuum in star-forming galaxies within 11 Mpc of the Milky Way. This analysis is particularly concentrated on dwarf galaxies, which display significant discrepancies between these SFR indicators, contrary to the more consistent results typically seen in larger spiral galaxies. The study employs data from the 11Mpc Hα and Ultraviolet Galaxy Survey (11HUGS), leveraging Hα fluxes obtained from ground-based narrowband imaging and UV fluxes from GALEX imaging.

Key Findings

1. Systematic Discrepancies at Low Luminosities: The study finds that SFRs derived from Hα tend to underpredict those inferred from FUV as luminosity decreases. This deviation is particularly marked in dwarf galaxies with SFRs less than 0.1 M⊙/yr, indicating an increasing discrepancy at lower star formation activities.
2. Robustness to Dust Attenuation: The discrepancies are identified convincingly even before corrections for internal dust attenuation. While dust more significantly affects FUV than Hα, the conclusion remains robust post corrections for such attenuation effects.
3. Statistical Examination: This research represents the first time such a discrepancy has been statistically verified for galaxies with SFRs below 0.1 M⊙/yr, down to ultra-low activities of ~0.0001 M⊙/yr. The significance of this expanded dataset underlines the systematic nature of the issue at low star-forming activities.

Implications and Hypotheses

The contrasting SFRs between Hα and FUV raise important questions about the underlying physics in dwarf galaxies:

• Star Formation History (SFH) Effects: The findings may suggest bursty or fluctuating star formation histories typical of dwarf galaxies. After a burst, ionizing stars fade out before lower-mass stars, altering the expected Hα-to-FUV ratio.
• Initial Mass Function (IMF) Variations: Another hypothesis considered is a non-universal IMF deficient in high-mass stars in dwarf galaxies. Some recent studies have suggested the IMF varies in dwarf galaxies, potentially leading to fewer O and early-type B stars, which would disproportionately affect the Hα SFRs.

Impact on SFR Calibration

If the FUV indicator is more reliable in these low-SFR regimes, this necessitates a recalibration of the Hα indicator under these conditions. The study proposes adjusting the SFR conversion for dwarf galaxies when only Hα data is available, thus aiding consistent interpretation across different galaxy types and sizes.

Future Considerations

The research prompts several areas for further investigation:

• Stellar Evolution Models: Better constraints on massive star evolution, especially at low metallicity, are essential to refine SFR indicators.
• Role of Environment: Understanding the environmental impact on star formation and its sustainability in dwarf galaxies will clarify Hα and FUV discrepancies.
• Expanded Survey: Extending such studies beyond the 11 Mpc volume would test the universal applicability of current SFR conversions and help refine models in increasingly low-metallicity and low-luminosity conditions.

In conclusion, the work of Lee et al. highlights critical issues in star formation metrics in dwarf galaxies. By revealing and addressing systematic discrepancies between cornerstone SFR indicators, the paper advances our understanding of small galaxies' evolutionary processes and their star-forming capabilities.