IRAS Galaxies with the Coldest Far-Infrared Colors
Session 77 -- Spirals II
Display presentation, Wednesday, 11, 1995, 9:20am - 6:30pm

## [77.07] IRAS Galaxies with the Coldest Far-Infrared Colors

Nanyao Y. Lu, G. Helou and L. Hermans (IPAC/Caltech)

While IRAS data have triggered many studies on IR bright galaxies with warm far-infrared (FIR) colors of $r(60,100) \equiv f(60\mu{\rm m})/ f(100\mu{\rm m}) > 0.3$, little attention has gone to galaxies with cold FIR colors, say, $r(60,100) < 0.25$ (roughly $T_{\rm dust} < 26$K), except for one or two very nearby galaxies (e.g., M31 with $r[60,100] \approx 0.18$). Spiral galaxies with very cold global dust temperatures may be unique cases of normal galaxies caught in an extremely quiescent phase of their evolution, and are just as critical as starburst galaxies in gaining an unbiased insight into galaxy evolution processes.

We have selected a sample of 147 galaxies from among over 60,000 galaxies in IRAS FSC using $r(60,100) < 0.25$ based on FSC flux densities. Co-added IRAS fluxes were obtained for each sample source in order to correct for IRAS beam effect, optical and radio data were collected from the literature for comparisons with FIR-warm galaxies, and H$\alpha$ imaging photometry was measured on a subset of 13 galaxies. Based on these data, we have the following statistical results: (1) The luminosity-weighted spatial extent of the FIR emission in these FIR-cold galaxies is much larger than that of FIR-warm galaxies. As a result, the FSC underestimates $r(60,100)$ by $0.3$ on average and only 47 sample galaxies actually have ($0.1 <$) $r(60,100) < 0.25$. (2) Most FIR-cold galaxies are spirals of types Sab to Scd with fairly normal optical luminosities and HI masses ($10^9$ to $10^{10.5}M_{\odot}$), suggesting that they must have experienced a more vigorous star-forming period in the past and that they may not lack the raw material for active star formation. (3) As $r(60,100)$ decreases from $0.35$ to $\sim 0.15$, the apparent distribution of H$\alpha$ emission tends to transfer from a regular spiral pattern to a less regular or ring-like structure. Below $r(60,100) \sim 0.25$, the observed FIR flux tends to exceed what the H$\alpha$ flux would predict from a correlation between these two fluxes for FIR-warm galaxies. This suggests not only a very low current star-forming rate in these galaxies, but also a heating source additional to that associated with the current star formation.