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abstract: 'It has been often considered that the dissipative collapse caused by a merger of two gas-rich galaxies is responsible for the intense nuclear starbursts or the nonthermal quasar activity in ultraluminous infrared galaxies due to the efficient fueling induced by it. It is also widely known that Ultraluminous Infrared Galaxies (ULIGs) are often found in merging systems. Some ULIGs, such as Arp 220, show two compact starburst regions which are considered to be associated with two galactic nuclei in the process of merging. However, since a merger between two galaxies may make only one compact starburst region, we suggest the possibility that double-nucleus ULIGs are composed of two merging nuclei, each of which contains a couple of galactic nuclei.'
author:
- 'Yoshiaki Taniguchi, and Yasuhiro Shioya'
title: A MULTIPLE MERGER MODEL FOR THE ORIGIN OF ULTRALUMINOUS INFRARED GALAXIES
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INTRODUCTION
============

Ultraluminous Infrared Galaxies (hereafter ULIGs) have attracted much attention since their discovery by [*IRAS*]{} in 1984 (Soifer et al. 1984; Wright, Joseph, & Meikle 1984; see for a review Sanders & Mirabel 1996). The multiple morphological studies published to date have shown that these objects tend to be found in galaxy mergers or in strongly interacting galaxies (Sanders et al. 1988a; Lawrence et al. 1991; Leech et al. 1994). Although the origin of their huge infrared luminosities is still not fully understood, it is considered to come from intense starbursts, central active galactic nuclei or a combination of both (Joseph & Wright 1985; Sanders et al. 1988a, 1988b; Solomon & Sage 1988; Scoville et al. 1991; Condon et al. 1991; Majewski et al. 1993; Lonsdale et al. 1994; Skinner et al. 1997). As numerical simulations have shown (see for a review Shlosman, Begelman, & Frank 1990; Barnes & Hernquist 1992), galaxy mergers cause efficient gas fueling toward the nuclear regions of the merging systems that ultimately can trigger and maintain any of the central activities mentioned above, either as a result of the piling of gas (Negroponte & White 1983; Barnes 1988; Barnes & Hernquist 1991; Olson & Kwan 1990a, 1990b; Noguchi 1991; Bekki & Noguchi 1994; Mihos & Hernquist 1994a, 1994b; Hernquist & Mihos 1995) or by the dynamical effect of supermassive binaries (Taniguchi & Wada 1996; Taniguchi 1997; Taniguchi, Wada, & Murayama 1997).

Current models for the origin of ULIGs only consider the merging of [*two*]{} gas-rich galaxies (Sanders et al. 1988a; Kormendy & Sanders 1992). However, taking into account that there are a large number of nearby compact galaxy groups (e.g., Hickson 1982), formation of ULIGs due to a multiple merger cannot be ruled out. In fact, the presence of three OH maser components in the archetypical ULIG Arp 220 suggests the possibility that this ULIG originates from a multiple merger (Diamond et al. 1989). In this [*Letter*]{}, we discuss the multiple merger scenario for the formation of ULIGs based on the observational properties of Arp 220.

THE COMPACT STARBURSTS IN ARP 220
=================================

Arp 220, being the nearest double-nucleus ULIG known, has been subject to observations in a wide range of wavelengths during the last decade. A brief summary of the observational properties of the nuclear starbursts in this object[^1] is given below.

Radio continuum images show the presence of two compact starburst regions (sizes $\leq$ 100 pc)(Norris 1988; Condon et al. 1991) located in the nuclear region of this object with a projected separation of 0.95 arcsec (350 pc). They are surrounded by a circumnuclear post-starburst region with a spatial extent of 1.4 kpc $\times$ 0.7 kpc in which stars with ages of $\sim 10^8$ years are dominant (Shaya et al. 1994; Larkin et al. 1995; Armus et al. 1995).Taking into account that typical starburst nuclei have ionized regions with sizes of $\sim$ 500 pc to 1 kpc (e.g., Sugai & Taniguchi 1992), it is clear that the nuclear starbursts in this object are quite outstanding in the sense that they are able to produce huge luminosities from quite more compact regions ($\simeq$ 100 pc).

It has been known for a while that starburst activity is associated to high-density gas components (traced by high-density tracer molecules like HCN or HCO$^+$). We therefore expect that the nuclear starbursts in this object also are related to dense gaseous systems formed during the course of the merger that created it. In order to demonstrate the presence of high-density gas around the two starburst regions in Arp 220, we show the profiles of the HCN and HCO$^+$ emission lines (Solomon, Downes, & Radford 1992) in Figure 1. Although the signal-to-noise ratios of these observations are not so high, both clearly show double-peaked profiles. The line peaks lie at 5300 km s$^{-1}$ and 5600 km s$^{-1}$, which roughly correspond to the recession velocities of the eastern and the western nuclei, respectively (Larkin et al. 1995), as shown in the bottom panels of Figure 1. These two peaks can also be seen at similar velocities in the CO($J$=2-1) map by Scoville, Yun, & Bryant (1997). Although there seems to be a velocity difference in the western component between the molecular gas and Pa$\alpha$ emission (5400 km s$^{-1}$ in Larkin et al. 1995), it should be noted that the large extinction toward the central region of Arp 220 may be also affecting the latter and that a velocity difference of $\sim$ 100 km s$^{-1}$ does not affect qualitatively our discussion below. We therefore conclude that the two compact starbursts occurred in the dense gas media surrounding the two nuclei.

The fact that these nuclear starbursts are associated to high-density regions can naturally explain their compactness. As is widely known, given the ionizing flux and the local density of a region, the radius of the equivalent fully ionized zone can be derived (e.g., Osterbrock 1989). The number of ionizing photons in the nuclear region of Arp 220 is estimated from the de-reddened Br$\gamma$ emission-line flux as $2\times 10^{-14}$ erg s$^{-1}$ cm$^{-2}$ (Shier et al. 1994). Since on average 77 ionizing photons produce one Br$\gamma$ photon, we obtain an ionizing photon rate of $Q({\rm H}^0) \simeq 1.1 \times 10^{54}$ photons s$^{-1}$. Therefore, the radius of the ionized nebula will be $r_{\rm neb} = [(3/4\pi) n_{\rm H}^{-2} 
\alpha_{\rm B}^{-1}  Q({\rm H}^0)]^{1/3} \sim 50 (n_{\rm H}/ 500~ {\rm cm}^{-3})^{-2/3}$ pc, where $n_{\rm H}$ is the number density of hydrogen in the nebula and $\alpha_{\rm B}$ is the effective recombination coefficient for hydrogen ($\alpha_{\rm B} = 2.59\times 10^{-13}$ cm$^3$ s$^{-1}$ for an electron temperature of $T_{\rm e} = 10^4$ K, Osterbrock 1989). Since ULIGs tend to have larger amounts of high-density gas (e.g., $10^{4{\rm -}5}$ cm$^{-3}$) than normal galaxies (Solomon, Downes, & Radford 1992), it is expected that their average molecular cloud gas density in their nuclear regions will exceed $\sim 10^2$ cm$^{-3}$ significantly. In fact, Scoville et al. (1997) estimated an average molecular hydrogen density of $n_{\rm H_2} \simeq 2\times 10^4$ cm$^{-3}$ in the central 130 pc region of Arp 220. The filling factor-corrected number density of hydrogen of this high-density component can be estimated from the fact that typical starburst nuclei have a few $10^8 M_\odot$ of molecular gas within their central $\sim$1 kpc regions (Devereux et al. 1994), giving an average hydrogen number density of at most $n_{\rm H} \sim$ 50 cm$^{-3}$ (an order of magnitude larger than the large scale average for a typical disk of a normal galaxy) and therefore a radius for the ionized nebulae of $r_{\rm neb} \simeq 230$ pc. We may thus conclude that the compactness of the nuclear starbursts in the ULIGs may be due to the higher gas density in their nuclear regions.

DISCUSSION
==========

Origin of the Nuclear Starbursts in ULIGs
-----------------------------------------

Numerical simulations of galaxy mergers between two gas-rich disk galaxies have shown that they induce efficient gas fueling into the central a few 100 pc region of the merging systems (e.g., Mihos & Hernquist 1994b and references therein). However, it is known that the properties of the merger-driven starbursts are sensitive to the structure of the progenitor galaxies (Mihos & Hernquist 1994b). If the progenitor galaxies are bulgeless (i.e., late-type spirals), the successive close encounters during the merger strongly affect their respective gas disks. As the merger proceeds, gas clouds in each galaxy are channeled to each nuclear region and if a dynamical instability occurs in the central region of each member, intense star formation would occur there (e.g., Noguchi 1991; Shlosman & Noguchi 1994). Since the gas contained in the progenitors would be used up while the merger is in progress, no intense starbursts would occur when the merger is complete. The well-known merger, NGC 7252, may be a good example for this case because only moderate star-forming regions can be seen in the central part of this galaxy (Whitmore et al. 1993). On the other hand, if the galactic bulge of each progenitor is massive enough to stabilize their nuclear gas disks, it will prevent bulge strong gas inflows until the galaxies merge, giving rise to a single intense starburst in the central part of the merger remnant (see also Bekki & Noguchi 1993). Since in the case of Arp 220, the projected separation between the two compact starburst regions is 350 pc, it is suggested that at least if double-nucleus ULIGs come from mergers between two galaxies, their progenitors could be gas-rich galaxies without prominent bulges.

The double-nucleus (traced by radio continuum and NIR emission) nature of Arp 220 has often used as an example of this two-galaxy merger scenario (Baan & Haschick 1987; Norris 1988; Sanders et al. 1988a; Graham et al. 1990; Scoville et al. 1998) However, there is some observational evidences that Arp 220 may come in fact from the merger of more than two spirals. VLBI mapping of OH megamaser emission in Arp 220 has revealed the presence of at least three bright OH megamaser spots in its nuclear region (Diamond et al. 1989), which would imply the coexistence of three active galactic nuclei in the region. Therefore, given the fact that there exist a fairly large amount of nearby compact galaxy groups that could end up merging since their merging timescales are generally shorter than the Hubble time (Barnes 1989; Weil & Hernquist 1996), we cannot rule out the multiple merger scenario for the formation of ULIGs.

For simplicity, we consider the case of ULIG coming from a merger of four comparably nucleated disk galaxies. At an early stage of the merger it is expected that parings would occur; i.e., two pairs of galaxies merge first. Then the merging remnants would merge again into one final object (cf. Barnes 1989; Weil & Hernquist 1996). It is known that a merger between two [*nucleated*]{} galaxies can form a single dense gaseous system in the merger remnant because of the dynamical disturbance of the binary potential to the gas clouds (Bekki & Noguchi 1994; Taniguchi & Wada 1996). If double-nucleus ULIGs are on the way to the final merger, we can explain why they have two dense gaseous systems each of which can be associated with a merger remnant between two galaxy nuclei.

A Multiple Merger Model for Arp 220
-----------------------------------

We now consider more carefully the possibility that Arp 220 comes from a merger of four comparably nucleated disk galaxies. In this scenario, the double nucleus of this object should correspond to the final stages of the merging of two pairs of nuclei. As mentioned above, there are at least three bright OH megamaser spots in the nuclear region of Arp 220 (Diamond et al. 1989). The eastern nucleus contains two bright OH maser spots with a projected separation of 47.6 pc while the western shows only one component. Recent VLBI measurements by Lonsdale et al. (1994) have shown that the western (i.e., the brightest) component of the OH maser originates from a very compact region whose size is less than 1 pc. This measurement suggests strongly that the western component is produced by the pumping by far-infrared continuum emitted by a dusty torus around an active galactic nucleus rather than by the luminous nuclear starbursts. If this is also the case for the two eastern OH maser components, Arp 220 would contain at least three active galactic nuclei (i.e., three supermassive black holes) as suggested by Diamond et al. (1989). It is of course possible that the single western OH megamaser component actually represents the accidental alignment along our line of sight of two nuclei, which would be resolved if observed in other conditions.

Another support for the multiple merger scenario comes from the observational fact that the two OH maser spots in the eastern nucleus show a velocity gradient which is almost perpendicular to the eastern-western nucleus axis, implying that the eastern nucleus is dynamically different from the western nucleus (Diamond et al. 1989). Furthermore, H$_2$CO maser observations of this object (Baan & Haschick 1995) show velocity gradients in maser emission associated with both the eastern and western nuclei that are more compatible with rotational motion around the individual nuclei rather than with the global rotation around the two nuclei (see Baan & Haschick 1995).

In our scenario it should also be expected that each pair of nuclei has a rotating gas disk settled roughly in the orbital plane of each of the two nuclei in each pair because nuclear gas will settle in a relatively short timescale there. In fact, the double-peaked nature of the HCN and HCO$^+$ (Solomon et al. 1992) and CO($J$=1-0) emissions (Scoville et al. 1997) suggest that the dense gaseous systems are associated with each pair of merging nuclei. At larger scales, a circumnuclear gas disk with a radius of $\sim$ 300 pc surrounds the two pairs of nuclei (Scoville et al. 1997).

In summary, the multiple merger scenario that we proposed here explains almost all the observational properties of Arp 220 consistently. A schematic illustration of the nuclear region of Arp 220 is shown in Fig. 2. The rotation of the eastern black hole binary was determined from the velocity difference between the two OH megamaser components, IIa and IIb (Diamond et al. 1989) while that of the eastern one is from the rotation of the H$_2$CO masing gas (Baan & Haschick 1995). The global rotation of the E and W nuclei is from the NIR spectroscopy (Larkin et al. 1995). Although any current observational facilities may not be able to verify this model, we hope that the multiple merger scenario will be taken into account in the future study on the origin of ULIGs.

We would like to thank Dave Sanders, Baltasar Vila-Vilaro, Sumio Ishizuki, Seiichi Sakamoto, and Neil Trentham for useful discussion and suggestions. We also thank John Hibbard and Dave Sanders for providing us with their CCD image of Arp 220. This work was financially supported in part by Grant-in-Aids for the Scientific Research (No. 0704405) of the Japanese Ministry of Education, Culture, Sports and Science.

**Figure captions**

[**Fig. 1:**]{} $^{12}$CO($J$=1-0), HCN, and HCO$^+$ emission profiles taken from Solomon et al. (1992) are shown in the top, the second and the third panel, respectively. The rotation curve of the nuclear region (Larkin et al. 1995) and the double radio components which show the two compact starburst regions are also shown in the bottom left and right panels, respectively. The double peaks of the HCN, and HCO$^+$ emission appear to correspond to the two starburst regions kinematically.

[**Fig. 2:**]{} A schematic picture of the multiple merger model of Arp 220. The direct $R$-band CCD image shown in the left panel was kindly supplied by J. Hibbard and D. Sanders. The two compact starbursts traced by the the radio continuum imaging at 8.44 GHz (Condon et al. 1991) are shown in the lower middle panel. The two right panels show the tangential and the line-of-sight view of Arp 220 based on the multiple merger model.

[^1]: We use a distance of 74 Mpc (1 arcsec = 359 pc) derived from its recession velocity to the Galactic Standard of Rest of $V_{\rm GSR}$ = 5,531 km s$^{-1}$ (de Vaucouleurs et al. 1991) and a Hubble constant of $H_0$ = 75 km s$^{-1}$ Mpc$^{-1}$.
