The Bar-enhanced Star-formation Activities in Spiral Galaxies

Abstract

We use the ratio $L_{\rm FIR}/L_{\rm B}$ and the IRAS color index S$_{25}$/S$_{12}$ (both widely used as indices of relative star formation rates in galaxies) to analyse subsets (containing no known AGNs or merging/interacting galaxies) of: (a) the IRAS Bright Galaxy Sample, (b) galaxies from the optically complete RSA sample which have IRAS detections in all four bands, and (c) a volume-limited IR-unselected sample. We confirm that IR-bright barred (SB) galaxies do, on average, have very significantly higher values of the FIR-optical and S$_{25}$/S$_{12}$ ratios (and presumably, higher relative star formation rates, SFR) than that do unbarred ones; the effect is most obvious in the IR colors. We also confirm that these differences are confined to early-type (S0/a - Sbc) spirals and are not evident among late-type systems (Sc - Sdm). {\it Unlike others, we see no enhancement of the SFR in weakly-barred (SAB) galaxies.} We further confirm that the effect of bars on the SFR is associated with the relative IR luminosity and show that it is detectable only in galaxies with $L_{\rm FIR}/L_{\rm B}$ $\proxgreat$ 1/3, suggesting that as soon as they have any effect, bars translate their host galaxies into this relatively IR-luminous group. Conversely, for galaxies with $L_{\rm FIR}/L_{\rm B}$ below$\sim$ 0.1 this luminosity ratio is {\it lower} among barred than unbarred systems, again confirming and quantifying an earlier result. Although there is no simple physical relation between HI content and star formation, a strong correlation of HI content with the presence of bars has been found for early-type spirals with $L_{\rm FIR}/L_{\rm B}$ $\proxgreat$ 1/3. This suggests that the availability of fuel is the factor determining just which galaxies undergo bar-induced starbursts.