4. MERGER SEQUENCE

Q2: Do ULIGs follow a merger sequence from colliding disk galaxies with large bulges to ellipticals ?

Figure 2. A subsample of R-band images (Mazzarella et al. 1999) of luminous infrared galaxies from the IRAS RBGS (Sanders et al. 1999) that illustrate the strong interactions/mergers that are characteristic of nearly all objects with Lir > 1011.3 L. The scale bar represents 10 kpc, tick marks are at 20" intervals, and the infrared luminosity (log Lir / L) is indicated in the lower left corner of each panel.

Ground-based optical and near-infrared imaging of complete samples of the brightest infrared galaxies clearly show that a substantial fraction of LIGs are strongly interacting or merging spirals, and that the higher the luminosity the more advanced is the merger (e.g. Joseph & Wright 1985; Sanders, Surace, & Ishida 1999; Mazzarella et al. 1999). Millimeterwave observations of have shown these spirals to be rich in molecular gas - M(H₂) ~ 10⁹-3 x 10¹⁰ M (e.g. Sanders et al. 1988a; Mirabel et al. 1990; Sanders, Scoville, & Soifer 1991) - and that there is an increasing central concentration of this gas with increasing infrared luminosity (Scoville et al. 1991; Downes & Solomon 1998). There is no clear evidence in favor of early versus late-type spirals, only that they both typically appear to be large (i.e. 0.5-2 L^*) and molecular gas-rich. The three LIGs shown in Figure 2 provide a coarse illustration of early, mid, and late type mergers commonly represented in the complete samples of LIGs. Comparison of these images with numerical simulations (e.g. Barnes & Hernquist 1992; Mihos & Hernquist 1994; C. Mihos, these proceedings) aids in allowing these objects to be placed in a rough time sequence.

Nearly all ULIGs appear to be late-stage mergers (e.g. Sanders et al 1988a, b; Melnick & Mirabel 1990; Kim 1995; Murphy et al. 1996; Clements et al. 1996). The large-scale ground-based images shown in the left panel of Figure 1 illustrate the largely overlapping disks that are seen in a complete sample of the nearest and best-studied ULIGs. Greater detail in the inner disks of these ULIGs is better revealed in the higher resolution ground-based images and HST images shown in the center and rightmost panels of Figure 1. The mean lifetime for the ULIG phase, estimated from the observed mean separation and relative velocity of the merger nuclei, is ~ 1-2 x 10⁸ yrs.

Figure 3. K-band radial surface brightness profiles (Wright et al. 1990) for two of the luminous infrared galaxies shown in Fig. 2 - VV 79 (NGC 2623), and Arp 220. The straight line represents a normalized r1/4-law (deVaucouleurs) profile characteristic of elliptical galaxies. The inner gap corresponds to the lack of information at radii smaller than 1" set by seeing. More recent higher resolution K-band imaging of these galaxies by NICMOS shows that a r1/4-law continues to be a good representation of the K-band radial profile over the range 0.1-5 kpc (Scoville et al. 1999).

There is now substantial evidence that ULIGs are indeed elliptical galaxies forming by merger-induced dissipative collapse (e.g. summary by Kormendy & Sanders 1992), including r^1/4-law brightness profiles [e.g. Schweizer 1982; Joseph & Wright 1985; Wright et al. 1990 (see Figure 3); Kim 1995; Zheng et al. 1999), newly-formed globular clusters (e.g. Surace et al. 1998), central gas densities that are as high as stellar mass densities in the cores of giant ellipticals (e.g. 10² M pc^-3 at r 0.5-1 kpc: Scoville et al. 1991; Downes & Solomon 1998), and powerful ``superwinds" that will likely leave behind a largely dust free core (Heckman, Armus, & Miley 1987; Armus, Heckman, & Miley 1989).

A2: YES, ULIGs appear to represent an advanced stage in the merger of two large spirals, where the merger remnant already has assumed a r1/4-law profile, but there is no evidence that only early-type spirals are involved, only that both progenitors be molecular gas-rich.