Astron. Astrophys. 317, 694-700 (1997)

2. Previous Searches

Searches for intra-cluster matter fall into two groups, those that search for gas, and those that search for associated dust. A comprehensive list of searches is given by Lynch & Rossano (1990; hereafter LR90) and these are summarized here; a full list of references may be found in LR90.

2.1. Gas

Searches for gas have been conducted in a number of wavelength bands:

(i) X-ray. Einstein observations show possible evidence for intracluster gas interacting with a hot Galactic corona;

(ii) Optical. H searches have been negative (e.g. a limit of 0.7 for HII by Hesser & Shawl 1977);

(iii) Near-IR. 2...10 µm work has produced upper limits or conflicting results;

(iv) Millimetre. GCs have been surveyed for CO molecular gas by its line emission (see also Smith et al. 1995);

(v) Radio. 21 cm searches have been negative, as have searches for free-free emission in the 2...10 GHz ranges (e.g. a limit of 0.8 for neutral hydrogen by Conklin & Kimble 1976).

The best upper limits from these searches have been in the range 0.1...1.0 of gas.

2.2. Dust

Searches for dust have also been carried out in a number of wavelength bands; a full list of references may again be found in LR90.

(i) Optical. Searches for 'dark patches' through star counts, polarimetry and colour gradients have generally given negative, ambiguous or doubtful results. The most recent study has been by Forte et al. (1992), who find a number of such patches in NGC 6624 and estimate a total dust mass of 0.07...0.8 in two regions, one some and one some from the cluster centre. This value could be consistent with the lack of detection by the IRAS survey (LR90), if the dust temperature at those distances were less than 20 K.

(ii) Mid IR. A general search of GCs in the IRAS Catalogs by LR90, and later of the IRAS Faint Source Catalog (Moshir et al. 1992) by Knapp, Gunn & Connolly (1995; hereafter KGC95), gave only upper limits for the intracluster dust, in the 0.001 to 0.01 range. This value depended on assuming a dust temperature at the cluster cores of 50 K, using the calculations of Angeletti et al. (1982). LR90 found that there are many IRAS point sources near GCs, with over two thirds of clusters having such sources. But these sources were in the field and unrelated to the GCs, except for M 22 and NGC 2298. In the former case the planetary nebula in the cluster was detected (Gillett et al. 1986), with a temperature of 100 K and a mass of ; in the latter case a very extended source exists. However neither of these detections relate to the problem of the general level of dust in the GCs. IRAS pointed observations of the cluster 47 Tuc (Gillett et al. 1988), with some 4000 seconds of observing, did find an excess of radiation at 100 µm, over that expected from the long-wavelength tail of the radiation from GC stars. The measured integrated excess of Jy leads to a dust mass of 3 10^-4 .

2.3. Gas versus dust

Searches for gas have resulted in upper limits at levels usually above the searches for dust. The 0.001...0.01 limit for dust may be translated into corresponding limits on the mass of gas if a dust-to-gas ratio is assumed. The dust-to-gas ratio in the outflows of evolved field stars, both O- and C-rich, is in the range [see e.g. Whittet (1992) and references therein]. If we assume this value for GC stars we get upper limits on the mass of gas in the range 0.1...2.5 . However the metallicity of GCs is considerably lower than it is for field stars and so this upper limit is a strong one.

© European Southern Observatory (ESO) 1997

Online publication: July 8, 1998
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