The interstellar medium of star-forming irregular galaxies: The view with ISO

We present mid-infrared imaging and far-infrared (FIR) spectroscopy of five IBm galaxies observed by ISO as part of our larger study of the interstell ar medium of galaxies. Most of the irregulars in our sample are very active ly forming stars, and one is a starburst system. Thus, most are not typical Im galaxies. The mid-infrared imaging was in a band centered at 6.75 mum t hat is dominated by polycyclic aromatic hydrocarbons (PAHs) and in a band c entered at 15 mum that is dominated by small dust grains. The spectroscopy of three of the galaxies includes [C II] lambda 158 mum and [O I] lambda 63 mum, important coolants of photodissociation regions (PDRs), and [O III] l ambda 88 mum and [N II] lambda 122 mum, which come from ionized gas. [O I] lambda 145 mum and [O III] lambda 52 mum were measured in one galaxy as wel l. These data are combined with PDR and H II region models to deduce proper ties of the interstellar medium of these galaxies. We find a decrease in PA H emission in our irregulars relative to small grain, FIR, and Her emission s for increasing FIR color temperature, which we interpret as an increase i n the radiation field due to star formation resulting in a decrease in PAH emission. The f(15)/f(H alpha) ratio is constant for our irregulars, and we suggest that the 15 mum emission in these irregulars is being generated by the transient heating of small dust grains by single-photon events, possib ly Ly alpha photons trapped in H II regions. The low f(15)/f(H alpha) ratio , as well as the high f([C II])/f(15) ratio, in our irregulars compared to spirals may be due to the lower overall dust content, resulting in fewer du st grains being, on average, near heating sources. We find that, as in spir als, a large fraction of the [C II] emission comes from PDRs. This is partl y a consequence of the high average stellar effective temperatures in these irregulars. However, our irregulars have high [C II] emission relative to FIR, PAH, and small grain emission compared to spirals. If the PAHs that pr oduce the 6.75 mum emission and the PAHs that heat the PDR are the same, th en the much higher f([C II])/f(6.75) ratio in irregulars would require that the PAHs in irregulars produce several times more heat than the PAHs in sp irals. Alternatively, the carrier of the 6.75 mum feature tracks, but contr ibutes only a part of, the PDR heating, that is due mostly to small grains or other PAHs. In that case, our irregulars would have a higher proportion of the PAHs that heat the PDRs compared to the PAHs that produce the 6.75 m um feature. The high f([O III])/f([C II]) ratio may indicate a smaller soli d angle of optically thick PDRs outside the H II regions compared to spiral s. The very high L-[C II]/L-CO ratios among our sample of irregulars could be accounted for by a very thick [C II] shell around a tiny CO core in irre gulars, and PDR models for one galaxy are consistent with this. The average densities of the PDRs and far-ultraviolet stellar radiation fields hitting the PDRs are much higher in two of our irregulars than in most normal spir als; the third irregular has properties like those in typical spirals. We d educe the presence of several molecular clouds in each galaxy with masses m uch larger than typical GMCs.