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

Abstract

Using a currently most representative sample of 477 late-type galaxies within 11 Mpc of the Milky Way with measured star-formation rates (SF Rs) from the far ultraviolet (F U V ) and Hα emission line fluxes, we select galaxies with the extreme ratios: SF R(Hα)/SF R(F U V ) > 2 and SF R(Hα)/SF R(F U V ) < 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 SF R(Hα) activity on a scale of ∼ 10 Myr, the temporarily "excited" and temporarily "quiescent" galaxies follow one and the same relation between SF R(F U V ) and M * on a scale of ∼ 100 Myr. Their average specific star-formation rate log[SF R(F U V )/M * ] = −10.1 ± 0.1 (yr −1 ) coinsides with the Hubble parameter log(H 0 ) = −10.14 (yr −1 ). On a scale of t ∼ 10 Myr, variations of SF R 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 SF R caused by tidal interactions.