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Up: Far-infrared emission from Shakhbazian Groups

1 Introduction

It is widely accepted that collisions or tidal interactions between galaxies may trigger bursts of star formation (e.g., Joseph & Wright 1985; Bushouse 1987; Norman 1988; Laurikainen & Moles 1989). One manifestation is often far-infrared (FIR) radiation which is enhanced in comparison with isolated galaxies of comparable optical luminosity. Indeed, as shown by Sanders et al. 1988, luminous infrared galaxies with $L_{\rm fir}\gt~10^{11}L_{\hbox{$\odot$}}$ are nearly all interacting or merging systems with exceptionally luminous nuclei. Optical spectra of such galaxies indicate a mixture of starbursts and active galactic nuclei (e.g., Veilleux et al. 1995).

In dense galaxy cluster environments, such as systems of compact groups of galaxies, tidal interactions between galaxies are expected to occur frequently. Hence galaxy groups provide a unique environment for testing theories of star formation triggered by tidal interactions, and to search for enhanced FIR dust emission. The most well known compact groups are the Hickson's (1982) Compact Groups (HCGs), which have been studied extensively (e.g., Pildis et al. 1995, and references therein). Hickson et al. (1989) inspected the IRAS Point Source Catalog (PSC) and detected emission at 60 $\mu$m and 100 $\mu$m from 54 galaxies in 45 out of 100 groups. They concluded that the FIR fluxes of galaxies in these groups are enhanced by about a factor of 2 compared to a sample of isolated galaxies. Sulentic & De Mello Rabaça (1993) reconsidered the problem and showed that the conclusion of Hickson et al. on the enhanced FIR fluxes was somewhat overestimated because the large IRAS beam encompassed the emission from more than one galaxy in many groups. In follow-up work, Allam et al. (1996) used the HIRAS software based on the Pyramid Maximum Entropy Method (Bontekoe et al. 1994) to partially resolve the component galaxies. They detected at 60 $\mu$m 87 individual sources in 62 HCGs out of 97 observed. Thus an appreciable number of galaxies in Hickson's compact groups are FIR sources.

Another interesting, but less well known sample is the Shakhbazian Compact Groups of Galaxies (SCGGs) (Shakhbazian 1973; Shakhbazian & Petrosian 1974; Baier et al. 1974; Petrosian 1974, 1978; Baier & Tiersch 1975, 1976a,b, 1978, 1979). These groups are more dense formations than Hickson's groups, with angular sizes typically of the order of 2-3 arcmin. The distances between galaxies in SCGGs are 3-5 times the diameter of a typical member galaxy, and the density of galaxies in the groups approaches 103-104 ${\rm Mpc}^{-3}$. Hence these groups are very compact. An investigation of the axial ratios of the SCGGs (Oleak et al. 1995) showed that these systems are usually highly flattened, with a prolate spheroidal shape ("cigars''). Each SCGG consists typically of 5-15 members. The apparent magnitude of member galaxies ranges approximately from 14$^{\rm m}$ to 19$^{\rm m}$. Almost all galaxies in the groups appeared very red on the POSS prints.

The member galaxies in SCGGs were initially described as being compact ("Compact Groups of Compact Galaxies''). Since they have relatively high surface brightness and usually lack diffuse borders (Shakhbazian 1973) they seemed to be compact on the POSS prints. Later studies showed, however, that the group members are mostly ordinary E or S0 galaxies. The impression of compact galaxies was strenghten due to contamination by foreground stars (Bettoni & Fasano 1993) in some groups in the original lists. However, recent spectral observations showed that contamination by stars is minimal (Tiersch et al., in preparation). Nevertheless, the original "compact'' description of the member galaxies is no longer considered significant.

Being very dense, compact and flattened, the SCGGs are fruitful laboratories to study processes of dynamical friction, tidal interaction, collisions and galaxy merging. The SCGGs have attracted little attention until recently because they are more distant and fainter than HCGs. Indeed, although the redshifts of only a small sample ($\sim\! 50$) of relatively bright SCGGs are known, a comparison of the redshift distributions (Fig. 1) shows that the median distance of SCGGs is at least three times larger than the median distance of HCGs. The redshifts of HCGs are taken from Hickson et al. (1988), and most redshifts for SCGGs are as yet unpublished data (Tiersch et al., in preparation). On the other hand the average flux density of HCGs at 60 $\mu$m is about 1.4 Jy, and the nominal threshold detection of the IRAS Faint Source Survey presented in the Faint Source Catalog (FSC) and Faint Source Catalog Regect (FSCR) is $\sim 0.22$ Jy. Therefore, assuming comparable FIR luminosity distributions of the galaxies in HCGs and SCGGs, FIR emission should be detectable by IRAS from SCGGs with the lowest redshifts or highest luminosities.

In this paper we report the results of a search for FIR emission from SCGGs using the IRAS archives. In Sect. 2 the IRAS data processing methods are reviewed; the results are presented in Sect. 3; and in Sect. 4 we briefly discuss the results and point out directions for future work.

\includegraphics [height=7.6cm,angle=-90]{ds7280f1.eps}\end{figure} Figure 1: Histograms of redshifts of HCGs and of SCGGs

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Up: Far-infrared emission from Shakhbazian Groups

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