Here we use the term "early-type" to embrace both elliptical and lenticular galaxies. Their IR emission, when detected, has been interpreted very differently from the IR emission from spiral galaxies. Indeed, early-type galaxies belong to a subset of the Hubble sequence where star-formation activity is at its minimum, or has stopped, and the ISM is almost exhausted. Given what we have seen above, one does not expect these objects to be IR bright. This is indeed the case, judging from the small fraction of early-type objects in the IRAS catalogues (see e.g. Jura et al. 1987), and the large uncertainties associated with their fluxes Bregman et al. 1998. For those few galaxies where IR emission is detected, it is commonly attributed either to the Rayleigh-Jeans tail of the stellar emission, or to dust in mass-losing stars. These interpretations however rest on the 4 broad-band low spatial resolution data of IRAS. ISO, with its much improved sensitivity to both spatial and spectral details, has shone a sharper light on the subject.
The FIR observations of early-type galaxies selected from the ISO archive have been analysed by Temi et al. (2004). They found that the FIR SED requires emission from dust with at least two different temperatures, with mean temperatures for the warm and cold components of 43 and 20K, respectively. This result is quite similar to the results obtained for quiescent spiral galaxies, in particular to the result obtained by Popescu et al. (2002) for the Virgo Cluster spiral galaxies. Since ellipticals are known to be ISM-poor and have reduced star-formation activity, the similarity between their FIR properties and those of spirals is surprising. One explanation could be the fact that the galaxies included in the ISO archive are not representative of typical ellipticals. As pointed out by Temi et al., many galaxies selected for ISO observations were chosen because they were known to have large IRAS fluxes or large masses of cold gas, and in this sense they have properties closer to those of spirals. For the same reason Temi et al. considered that these IR-selected galaxies are likely to have experienced unusual dust-rich mergers, and that the dust in these galaxies could have an external origin. The lack of a correlation found between the FIR luminosity (or dust mass) and the luminosity of the B band emission was taken as evidence for this scenario. Another scenario proposed is one in which most of the FIR is emitted by central dust clouds, where the clouds could be disturbed at irregular intervals by low-level AGN activity in the galactic cores, creating the stochastic variation in the FIR luminosity. It is difficult to distinguish between these two scenarios, also because the sensitivity of ISO was not sufficient to detect FIR emission from many optically luminous elliptical galaxies. In order to distinguish between an external and an internal origin for the dust, deeper FIR observations of an optically selected sample of elliptical galaxies are needed. It is even possible that such surveys may occasionally detect bright FIR emission from galaxies traditionally classified as ellipticals, but which in fact have hidden starburst activity in their central regions. This is the case for the IRAS source F15080+7259, which was detected in the ISOPHOT serendipity survey, and was shown by Krause et al. (2003) to harbour large quantities of gas and cold dust.
Another elliptical detected by ISOPHOT is the Virgo Cluster galaxy M 86 (NGC 4406). The deep FIR imaging data obtained with ISOPHOT at 60, 90, 150 and 180 µm revealed a complex FIR morphology for this galaxy (Stickel et al. 2003). It was found that the FIR emission originates from both the centre of M 86 and from the optically discovered dust streamers, and has a dust temperature of 18K. Again, this elliptical is not representative for this class of objects, since it was considered either to be experiencing ram-pressure stripping or to be gravitationally interacting with its neighbouring galaxies. In particular the ISOPHOT observations were interpreted by Stickel et al. as being consistent with M 86 having a gravitational interaction with its nearest spiral galaxy NGC 4402. In this case the detected dust would again have (at least in part) an external origin. The importance of gravitational interaction as a driver for FIR emission in early-type galaxies was demonstrated by Domingue et al. (2003), who showed these systems to have a 50% detection rate with ISOPHOT when they are paired with spirals.
In the MIR ISOCAM was used to observe both elliptical and lenticular galaxies, though the objects selected differ from those observed by ISOPHOT. An unbiased sample of 16 IRAS-detected lenticular galaxies was observed using low resolution MIR spectroscopy by Sauvage et al. 2004. The vast majority of the detected galaxies present MIR SEDs extremely similar to those of later-type spiral galaxies (i.e. PAH bands). Thus, from their MIR spectral properties, lenticular galaxies appear to belong to the spiral galaxy group, although with smaller luminosities.
Elliptical galaxies have drawn much more interest from ISOCAM observers, even though they are ISM-poorer than the lenticulars. The main result from these studies is that elliptical galaxies are really a mixed bag, as far as their MIR properties are concerned. Quillen et al. 1999 mapped with ISOCAM a number of E+A galaxies 3 in the Coma cluster, but detected only those which showed emission lines characteristic of an on-going star-formation episode. This might be thought to imply that star-formation is required for ellipticals to emit in the MIR, but in fact more likely reflects the sensitivity reached by Quillen et al. 1999. With deep ISOCAM observations of closer "normal" elliptical galaxies, Athey et al. 2002 showed that their MIR emission is dominated by the emission from K and M stars, with the addition of a spectral feature near 10 µm that they attribute to silicates in the circumstellar envelopes of AGB stars. Xilouris et al. 2004 brought balance to the star/ISM debate for the origin of MIR emission in early-type galaxies by showing that the complete range of phenomena occur in this class of galaxies. Systems vary from those that are dominated up to ~ 20 µm by the emission from their old stellar populations to those in which a small star-forming episode, possibly fueled by recent accretion, dominates the MIR luminosities. In between one also finds galaxies where only the long-wavelength part of the MIR spectrum deviates from the extrapolation of the stellar light, which indicates the presence of small amounts of diffuse dust much hotter than what is commonly found in spirals. This is most likely to be due to the higher interstellar radiation field generated by the denser stellar populations.
Although ISO has had some success in detecting and measuring the IR SED from early-type galaxies, a critical look at the observations done reveals that we still do not know the IR properties of typical elliptical galaxies. To achieve this, systematic deep observations of optically selected samples are required.
3 Early-type galaxies showing evidence of a post-starburst population. Back.