There is increasing evidence for a population of stars in the space between the galaxies in a cluster (Bernstein et al. 1995, Gonzalez et al. 2002, Gonzalez et al. 2005, Gerhard et al. 2002, Gerhard et al. 2005, Gal-Yam et al. 2003, Arnaboldi et al. 2004, Cortese et al. 2004, Feldmeier et al. 2004, Ryan-Weber et al. 2004, Adami et al. 2005, Zibetti et al. 2005, Krick & Bernstein 2007). Depending on the mass of the cluster the fraction of intra-cluster stars (= ratio of number of stars between galaxies to total number of stars) can be as high as 10-50% with the higher fraction being in more massive clusters. This stellar population can originate from stripping of stars from galaxies due to tidal interaction (Cypriano et al. 2006), can be expelled during mergers and the formation of massive galaxies (Murante et al. 2007, Kapferer et al. 2005, Fig. 6) or can have multiple origins (Williams et al. 2007). Simulations show that intra-cluster stars should be ubiquitous in galaxy clusters (Willman et al. 2004) and their numbers should generally increase with time (Rudick et al. 2006). A link between the growth of the brightest cluster galaxy and the intra-cluster light was reported by Zibetti et al. (2005).
When these stars explode as supernovae (mainly type Ia, as it takes a while for the stars to travel away from the galaxies) they can enrich the ICM very efficiently because there is no ISM pressure around them to confine the metals (Domainko et al. 2004, Zaritzsky et al. 2004, Lin & Mohr 2004, Dado et al. 2007).
Considerably more frequent than supernova Ia explosions are their
progenitors - the recurrent novae. With about 10-4
M
outflow per
nova event and typically super-solar abundances (up to ten times Solar,
Gehrz et
al. 1998),
novae could also contribute to the metal enrichment of the
ICM if they have been expelled previously from the galaxies.
A fraction of the AGB stars are also expected to be between the galaxies. These stars have a considerable mass loss with metallicities of about Solar abundances with slightly enhanced abundances of CNO elements (Wheeler et al. 1989, Zijlstra 2006, Van den Hoek & Groenewegen 1997, Busso et al. 2001, Nordström 2003). As the ratio of planetary nebulae (PNe) to AGB stars is well studied in statistical studies of PNe in the ICM (Feldmeier et al. 1998, Theuns & Warren 1997, Arnaboldi et al. 2003) this ratio may be used to estimate the number of AGB stars.
In conclusion the population of stars should also be considered for the enrichment processes in the ICM - even far away from galaxies.