Local Volume Late-Type Galaxies with an Extreme Ratio of Hα-to-FUV Star-Formation Rates

Abstract

Abstract Using a currently most representative sample of 477 late-type galaxies within 11 Mpc of the Milky Way with measured star-formation rates (SFRs) from the far ultraviolet (FUV) and Hα emission line fluxes, we select galaxies with the extreme ratios: SFR(Hα)/SFR(FUV) > 2 and SFR(Hα)/SFR(FUV) < 1/20. Each subsample amounts to ∼5% of the total number and consists of dwarf galaxies with the stellar masses M*/M⊙ = (5.5 − 9.5) dex. In spite of a huge difference in their SFR(Hα) activity on a scale of ∼10  Myr, the temporarily “excited” and temporarily “quiescent” galaxies follow one and the same relation between SFR(FUV) and M* on a scale of ∼100 Myr. Their average specific star-formation rate log [SFR(FUV)/M*] = −10.1 ± 0.1 (yr−1) coinsides with the Hubble parameter log (H0) = −10.14 (yr−1). On a scale of t ∼ 10 Myr, variations of SFR have a moderate flash amplitude of less than 1 order above the main-sequence and a fading amplitude to 2 orders below the average level. In general, both temporarily excited and temporarily quiescent galaxies have almost similar gas fractions as normal main-sequence galaxies, being able to maintain the current rate of star-formation on another Hubble time scale. Ranging the galaxies according to the density contrast produced by the nearest massive neighbor exhibits only a low average excess of SFR caused by tidal interactions.