ORBITAL STRUCTURE OF MERGER REMNANTS. I. EFFECT OF GAS FRACTION IN PURE DISK MERGERS

Journal Article · Mon Nov 01 04:00:00 EDT 2010 · Astrophysical Journal

DOI:https://doi.org/10.1088/0004-637X/723/1/818· OSTI ID:21467149

Hoffman, Loren ^[1]; Cox, Thomas J; Dutta, Suvendra; Hernquist, Lars ^[2]

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1. Department of Physics and Astronomy, Northwestern University, Dearborn Observatory, 2131 Tech Drive, Evanston, IL 60208 (United States)
2. Department of Astronomy, Harvard University, 60 Garden Street, Cambridge, MA 02138 (United States)

Since the violent relaxation in hierarchical merging is incomplete, elliptical galaxies retain a wealth of information about their formation pathways in their present-day orbital structure. Recent advances in integral field spectroscopy, multi-slit infrared spectroscopy, and triaxial dynamical modeling techniques have greatly improved our ability to harvest this information. A variety of observational and theoretical evidence indicates that gas-rich major mergers play an important role in the formation of elliptical galaxies. We simulate 1:1 disk mergers at seven different initial gas fractions (f{sub gas}) ranging from 0% to 40%, using a version of the TreeSPH code Gadget-2 that includes radiative heating and cooling, star formation, and feedback from supernovae and active galactic nuclei. We classify the stellar orbits in each remnant and construct radial profiles of the orbital content, intrinsic shape, and orientation. The dissipationless remnants are typically prolate-triaxial, dominated by box orbits within r{sub c} {approx} 1.5 R{sub e} , and by tube orbits in their outer parts. As f{sub gas} increases, the box orbits within r{sub c} are increasingly replaced by a population of short-axis tubes (z-tubes) with near zero net rotation, and the remnants become progressively more oblate and round. The long-axis tube (x-tube) orbits are highly streaming and relatively insensitive to f{sub gas}, implying that their angular momentum is retained from the dynamically cold initial conditions. Outside r{sub c} , the orbital structure is essentially unchanged by the gas. For f{sub gas} {approx}> 15%, gas that retains its angular momentum during the merger re-forms a disk that appears in the remnants as a highly streaming z-tube population superimposed on the hot z-tube distribution formed by the old stars. In the 15%-20% gas remnants, this population appears as a kinematically distinct core (KDC) within a system that is slowly rotating or dominated by minor-axis rotation. These remnants show an interesting resemblance, in both their velocity maps and intrinsic orbital structure, to the KDC galaxy NGC 4365. At 30%-40% gas, the remnants are rapidly rotating, with sharp embedded disks on {approx}1 R{sub e} scales. We predict a characteristic, physically intuitive orbital structure for 1:1 disk merger remnants, with a distinct transition between 1 and 3 R{sub e} that will be readily observable with combined data from the two-dimensional kinematics surveys SAURON and SMEAGOL. Our results illustrate the power of direct comparisons between N-body simulations and dynamical models of observed systems to constrain theories of galaxy formation.

• OSTI ID: 21467149
• Journal Information: Astrophysical Journal, Journal Name: Astrophysical Journal Journal Issue: 1 Vol. 723; ISSN ASJOAB; ISSN 0004-637X
• Country of Publication: United States
• Language: English

Related Subjects

79 ASTRONOMY AND ASTROPHYSICS
ABSORPTION SPECTROSCOPY
ANGULAR MOMENTUM
BINARY STARS
ERUPTIVE VARIABLE STARS
GALAXIES
GALAXY NUCLEI
INFRARED SPECTRA
SIMULATION
SPECTRA
SPECTROSCOPY
STARS
SUPERNOVAE
VARIABLE STARS