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3.1. Cold dust surrounding dwarf galaxies

Gas-rich dwarf galaxies and in particular Blue Compact Dwarfs (BCDs) were originally expected to have their FIR emission dominated by dust heated locally in HII regions. Temperatures of 30K or more were anticipated. This was the a priori expectation in particular for the BCDs, and became the standard interpretation for the IRAS results obtained for these systems. Hoffman et al. [44], Helou et al. [37] and Melisse & Israel [57] each found that the 60/100 µm colours of BCDs were clearly warmer than those of spirals.

The IVCD Survey (Tuffs et al. [88], [89]) changed that simple picture of the FIR emission from dwarf galaxies. The IVCDS included measurements at 60, 100, and (for the first time) at 170 µm of 25 optically selected gas-rich dwarf galaxies. The observations at 60 and 100 µm were consistent with the previous IRAS results, though extending knowledge of these systems to lower luminosities. Unexpectedly however, high ratios of 170/100 µm luminosities were found in many of the surveyed systems. Such long-wavelength excesses were found both in relatively high-luminosity dwarfs, such as VCC655, as well as in fainter objects near the limiting sensitivity of the survey. These observations imply the presence of large amounts of cold dust.

As shown by Popescu et al. [73], it seems unlikely that the cold dust resides in the optically thick molecular component associated with star-formation regions, since the implied dust masses would be up to an order of magnitude greater than those typically found in giant spirals. Such large masses could not have been produced through star-formation within the dwarfs over their lifetime. One alternative is that the dust originates and still resides outside the optical extent of these galaxies. In fact, some evidence for this was provided by the ISOPHOT observations themselves, since they were made in the form of scan maps, from which estimates of source sizes could be determined. Even with the relatively coarse beam (1.6' FWHM at 170 µm), the extended nature of the sources could be clearly seen in a few cases for which the FWHM for the 170 µm emission exceeds the optical diameters of the galaxies (to 25.5 Bmag/arcsec2) by factors of between 1.5 and 3.5. It is interesting to note that two of these galaxies (VCC 848 and VCC 81) have also been mapped in HI (Hoffman et al. [45]), revealing neutral hydrogen sizes comparable to the 170 µm extent. This raises the possibility that the cold dust is embedded in the extended HI gas, external to the optical galaxy. This would be analogous to the case of the edge-on spiral NGC 891, where Popescu & Tuffs [68] discovered a cold-dust counterpart to the extended HI disk (see Sect. 2.1.2). In this context the main observational difference between the giant spiral and the dwarfs may be that for the dwarfs the integrated 170 µm emission is dominated by the extended emission component external to the main optical body of the galaxy, whereas for the giant spirals the long-wavelength emission predominantly arises from within the confines of the optical disk of the galaxy.

Apart from the SMC (which is discussed in Sect. 3.2 and is too extended for ISOPHOT to map beyond its optical extent), only three dwarf galaxies were observed by ISOPHOT in the field environment (all Serendipity Survey sources; Stickel et al. [81]). All three sources have comparable flux densities at 100 and 170 µm. But the small statistics mean that it is still an open question to which extent the cold dust emission associated with the extended HI component in dwarf galaxies is a cluster phenomenon or not. Of potential relevance to the origin of the dust seen surrounding Virgo dwarfs is the discovery by ISOPHOT of the source "M 86-FIR" in the halo of M 86 (Stickel et al. [80]) which has no optical counterpart. It may be a "relic" ISM stripped from a spiral galaxy in the Virgo Cluster.

The existence of large quantities of dust surrounding gas-rich dwarf galaxies may have important implications for our understanding of the distant Universe. According to the hierarchical galaxy formation scenarios, gas-rich dwarf galaxies should prevail at the earliest epochs. We would then expect these same galaxies to make a higher contribution to the total FIR output in the early Universe, certainly more than previously expected.

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