Science with the Hubble Space Telescope -- II
Book Editors: P. Benvenuti, F. D. Macchetto, and E. J. Schreier
Electronic Editor: H. Payne

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The Encounter of a Gas-Rich Disk Galaxy and a Giant Elliptical: The Fate of the Gas and the Fueling of a QSO

F. Governato, B. Moore, M. Colpi, and L. Maraschi
Department of Physics, University of Washington, Seattle, WA 98195 USA

Department of Physics, Universitá degli Studi di Milano, Milano, Italy
Osservatorio Astronomico di Brera, Milano, Italy
Visiting Scientist at the International School for Advanced Studies, Trieste, Italy

Abstract:

Keywords: Galaxies: Interactions. QSOs activity

Introduction

Recent studies (Bahcall et al. 1994, Disney 1995) of the environment of QSOs with HST have revealed that luminous quasars inhabit galaxies that often show close companions. In one case, i.e., in the bright quasar PKS 2349-014, an apparent ongoing gravitational interaction has been detected (Bahcall et al. 1995).

The consequences of binary encounters between galaxies for the possible fueling of central black holes can be explored combining SPH numerical techniques to N-body codes used to describe the purely gravitational collisionless interaction between the two galaxies (Katz, Hernquist, & Weinberg 1995, Navarro & White 1993). One of the major issues is understanding how large quantities of interstellar gas can accumulate toward the center of one of the galaxies (Hernquist & Mihos 1995).

As luminous QSOs seem to inhabit galaxies having elliptical morphology (Disney 1995), in this paper we explore the encounter between a giant elliptical (primary) and a less massive gas-rich disk galaxy (secondary), a minor merger that can occur during cluster evolution. In fact, a massive galaxy is expected to undergo several encounters of this type in a Hubble time. We intend to address the following questions: (1) What is the fate of the interstellar gas of the secondary ? (2) Is nuclear activity triggered in the satellite gas-rich galaxy during its orbital decay toward the companion ? (3) Is nuclear activity triggered in the elliptical due to infall of gas from the secondary ?

In a previous numerical exploration we considered encounters with Elliptical to Spiral mass ratio 10:1 and with small ratio of pericenter to disk-scale length (Governato et al. 1995). In these mergers we found that shortly after pericentric passage a stream of disk particles (gas and stars) forms which falls almost radially toward the center of the elliptical; the satellite galaxy will eventually accrete onto the giant elliptical in a few dynamical times.

What happens if we change the mass ratio? How sensitive is the process of formation of a gas stream to the pericenter vs disk-scale length ratio? We here report on preliminary results on a new sequence of numerical simulations aimed at describing the details of the merger process in the case in which the E and Sp galaxies have a mass ratio of 100:1.

Results of the Simulations

The initial conditions for our simulations are the following: The elliptical galaxy [E] is modelled as an isothermal sphere with velocity dispersion of 300 which is typical of a galaxy with luminosity of . The spiral [Sp] is a disk galaxy embedded in a spherical dark matter halo (Hernquist 1993); of the disk is gaseous. The gas dynamics is modelled with the SPH technique which includes shocks and radiative cooling (Katz, Hernquist, & Weinberg 1995). The galaxies move initially along parabolic orbits with pericenter of 4 and 8 kpc. The disk of the spiral is tilted by 20 degrees with respect to the orbital plane and is co-rotating. Our aim is to describe the encounter of a gas-rich disk galaxy with circular velocity , comparable to a Magellanic cloud-like system and the elliptical. The simulations used 80,000 particles each.

 
Figure: Encounter with pericenter of 4 kpc. Left panel: the gas and star particles of Sp and E are projected on the orbital plane (one unit scale length is 2 kpc). Right panel: zoom on the gaseous component only around Sp.

We find that at least of the gas has fallen toward the center of the spiral, i.e., within a region of size comparable to the resolution of the numerical code. A close look at the gaseous component around the spiral is shown in the right panel of Figure 1. A central spot of kpc of size containing of the spiral gas is present, but unfortunately not visible in the reduced plot of Figure 1.

In the encounter with pericenter of 4 kpc the satellite suffers a severe deformation. Figure 1 (left panel) shows stars and gas 150 millions years after pericenter passage; the separation between E and Sp is about 145 kpc.

 
Figure: Encounter with pericenter of 8 kpc. Left panel: the gas and star particles of Sp and E are projected on the orbital plane (one unit scale length is 2 kpc). Right panel: zoom on the stars and gas of the Sp.

In the encounter with pericenter of 8 kpc (shown in Figure 2; left panel), the gas-rich disk galaxy suffers tidal deformation and 200 million years after close passage a bar has formed as illustrated in the right panel of Figure 2. The gaseous component shrinks though less severely, since the pericenter to disk-scale length ratio is larger than in the previous case.

Discussion

This sequence of simulations combined with the one presented in Governato et al. (1995) suggests a number of possible outcomes:

(i) In encounters with mass ratio of 10:1 and small pericenter vs disk-scale length ratio, a fraction of the disk particles comprising gas and stars flows toward the center of the elliptical; the disk of the Sp is disrupted after the first close passage.

(ii) In encounters with high mass ratios (100:1), the central part of the disk survives tidal disruption and a fraction of gas streams toward the center of the spiral. In the encounter with pericenter of 4 kpc the gas falls into a region of size comparable to the resolution of the code.

What remains to be explored is the final fate of the perturbed gas: a residual angular momentum may prevent it form falling directly to the center of the galaxy (E or Sp, depending on the mass ratio). A ring of dense clouds could form where starburst activity may originate. This stage may precede or coexist with an efficient fueling of the central black hole triggering QSO activity in either galaxy. In a forthcoming paper (Governato et al. 1996) we plan to explore the parameter space of the initial conditions. We also intend to include the effect of the hot gas component associated with the elliptical on the cold gas of the spiral. We expect that gas infall toward the spiral is relented and/or inhibited in encounters with mass ratio 100:1 and a fraction of the gas is captured by the elliptical.

Acknowledgments:

Simulations were performed at the HPCC of Seattle, WA.

References:

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Governato, F., Colpi, M., & Maraschi, L. 1995 to appear in the proceedings of the conference Interacting Galaxies, Napoli, September 1995.

Governato, F., Moore, B., Katz, N., Colpi, M., & Maraschi, M. 1996, in preparation

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