Local Volume Late-Type Galaxies with an Extreme Ratio of $H\alpha$-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 ($SFR$s) from the far ultraviolet ($FUV$) and H$\alpha$ emission line fluxes, we select galaxies with the extreme ratios: $SFR(H\alpha)/SFR(FUV) > 2$ and $SFR(H\alpha)/SFR(FUV) < 1/20$. Each subsample amounts to $\sim5$\% of the total number and consists of dwarf galaxies with the stellar masses $M^*/M_{\odot} = (5.5 - 9.5)$~dex. In spite of a huge difference in their $SFR(H\alpha)$ activity on a scale of $\sim10$~ Myr, the temporarily "excited" and temporarily "quiescent" galaxies follow one and the same relation between $SFR(FUV)$ and $M^*$ on a scale of $\sim100$~Myr. Their average specific star-formation rate $\log[SFR(FUV)/M^*] = -10.1\pm0.1$ (yr$^{-1}$) coinsides with the Hubble parameter $\log(H_0)= -10.14$ (yr$^{-1}$). On a scale of $t \sim10$~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.