arXiv subjects

Thorsten Naab

Publications and source records attributed to Thorsten Naab.

246 records · Page 14Linked to original sources

arXiv2005

The Surprising Anisotropy of Fast Rotating, Disky Elliptical Galaxies

The projected kinematical properties of unequal-mass merger remnants of disk galaxies are analysed and shown to agree well with observations of disky, fast rotating elliptical galaxies. This supports the major merger hypothesis of early-type galaxy formation. However, in contrast to previous claims, the merger remnants are very anisotropic with values of the anisotropy parameter that are similar to equal-mass merger remnants that form boxy, slowly rotating ellipticals. Including gas in the simulations does not change this result although the line-of-sight velocity profile and the intrinsic orbital structure are strongly affected by the presenceof gas. The kinematical difference between boxy and disky ellipticals appears not to be the amount of anisotropy but rather rotation and the shape of the velocity dispersion tensor. The apparent isotropy of observed disky ellipticals is shown to result from inclination effects. Even small inclination angles strongly reduce the measured anisotropy of fast rotating systems, seen in projection. A second problem is the limited amount of information that is available when measuring only the central velocity dispersion and a characteristic rotation and ellipticity. Methods are investigated that allow a better determination of the intrinsic anisotropy of fast rotating early-type galaxies with known inclination angles.

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arXiv2003

Statistical Properties of Collisionless Equal- and Unequal-Mass Merger Remnants of Disk Galaxies

We perform a large parameter survey of 112 collisionless N-body simulations of binary mergers of disk-galaxies with mass ratios of 1:1, 2:1, 3:1, and 4:1 using the special purpose hardware GRAPE. The basic properties of the remnants correlate with the mass ratio of the progenitor disks. We find that about 80% of the equal-mass merger simulations lead to slowly rotating remnants with significant minor-axis rotation. One half of all projected 1:1 remnants shows boxy and the other half shows disky isophotes. Remnants with mass ratios of 3:1 and 4:1 all rotate fast and show a small amount of minor-axis rotation. About 90% of the projected 3:1 and 4:1 remnants show disky isophotes. 2:1 remnants show intermediate properties. Projection effects lead to a large spread in the data in good agreement with observations. They do not change the fundamental kinematical differences between equal-mass and unequal-mass merger remnants. The results of this study weaken the disk merger scenario as the possible formation mechanism of massive boxy giant ellipticals. In general, the properties of equal-mass (and 2:1) merger remnants are consistent with the observed population of giant ellipticals in the intermediate mass regime between low mass fast rotating disky and bright massive boxy giant ellipticals. 3:1 and 4:1 merger remnants, however, are in very good agreement with the class of low luminosity, fast rotating giant elliptical galaxies. Binary mergers of disk galaxies are therefore still very good candidates for being the main formation mechanism for intermediate and low mass giant ellipticals. The homogeneous class of massive boxy ellipticals most likely formed by a different process.

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arXiv2003

Cosmological Simulations of Elliptical Galaxy Formation in LCDM and LWDM Cosmologies

We present the results of a series of gas dynamical cosmological simulations of the formation of individual massive field galaxies in the standard concordance \LCDM and in a \LWDM cosmology. The galaxies form in an initial burst of star formation followed by accretion of small satellites. The simulated galaxies are old ($\approx$ 10 $Gyrs$) hot stellar systems with masses of $\approx 1.7 \times 10^{11} M_{\odot}$. Baryonic matter dominates the mass in the luminous part of the galaxies up to $\approx 5$ effective radii. The \LCDM galaxy is a slowly rotating spheroidal stellar system (E2) with predominantly disky isophotes. The line-of-sight velocity distributions deviate from Gaussian shape and $h_3$ is anticorrelated with $v_{los}$. The corresponding \LWDM galaxy is more elongated (E3 - E4) and rotates faster. The anisotropy parameter $(v/σ)^*$ is close to unity. Within the uncertainties of $M/L$ both galaxies follow the Fundamental Plane. We conclude that the properties of the two galaxies simulated in the \LCDM and \LWDM cosmology are in good agreement with observations of intermediate mass elliptical or S0 galaxies (abstract abbreviated).

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arXiv2003

The Formation of Spheroidal Stellar Systems

We summarize current models of the formation of spheroidal stellar systems. Whereas globular clusters form in an efficient mode of star formation inside turbulent molecular clouds, the origin of galactic spheroids, that is bulges, dwarf ellipticals, and giant ellipticals, is directly coupled with structure formation and merging of structures in the Universe. Disks are the fundamental building blocks of galaxies and the progenitors of galactic spheroids. The origin of the various types of spheroids and their global properties can be understood as a result of disk heating by external perturbations, internal disk instabilities, or minor and major mergers.

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arXiv2003

Major Mergers and the Origin of Elliptical Galaxies

The formation of elliptical galaxies as a result of the merging of spiral galaxies is discussed. We analyse a large set of numerical N-Body merger simulations which show that major mergers can in principle explain the observed isophotal fine structure of ellipticals and its correlation with kinematical properties. Equal-mass mergers lead to boxy, slowly rotating systems, unequal-mass mergers produce fast rotating and disky ellipticals. However, several problems remain. Anisotropic equal mass mergers appear disky under certain projections which is not observed. The intrinsic ellipticities of remnants are often larger than observed. Finally, although unequal-mass mergers produce fast rotating ellipticals, the remnants are in general more anisotropic than expected from observations. Additional processes seem to play an important role which are not included in dissipationless mergers. Resolving these problems might provide interesting new information on the structure and gas content of the progenitors of early-type galaxies.

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arXiv2001

Gas Dynamics and Inflow in gas-rich Galaxy Mergers

We performed N-body/SPH simulations of merging gas-rich disk galaxies with mass ratios of 1:1 and 3:1. A stellar disk and bulge component and a dark halo was realized with collisionless particles, the gas was represented by SPH particles. Since we did not include star formation we focused on the gas dynamics and its influence on the structure of merger remnants. We find that equal-mass mergers are in general more effective in driving gas to the center. Around 50% of the gas resides in the central regions after the merger is complete. The gas in unequal-mass mergers keeps a large amount of its initial angular momentum and settles in a large scale disk while only 20% -30% of the gas is driven to the center. This general result is almost independent of the merger geometry. The gas in the outer regions accumulates in dense knots within tidal tails which could lead to the formation of open clusters or dwarf satellites. Later on, the gas knots loose angular momentum by dynamical friction and successively sink to the center of the remnant thereby increasing the total gas content of the disk. Due to the influence of the gas the simulated merger remnant becomes more oblate than its pure stellar counterpart. The shape of the stellar LOSVD changes and the third-order Gauss-Hermite coefficient $h_3$ is in good agreement with observations.

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arXiv2001

The Formation of Disks in Elliptical Galaxies

We investigate detailed kinematical properties of simulated collisionless merger remnants of disk galaxies with mass ratios of 1:1 and 3:1. The simulations were performed by direct summation using the new special hardware device GRAPE-5. In agreement with observations, the shape of the line-of-sight velocity distribution (LOSVD) is Gaussian with small deviations. For most cases we find that the retrograde wings of the LOSVD are steeper than the prograde ones. This is in contradiction with observations which show broad retrograde and steep prograde wings. This serious problem in the collisionless formation scenario of massive elliptical galaxies can be solved if all rotating ellipticals, even boxy ones, contain an additional stellar disk component with $\propto 15%$ of the total stellar mass and a scale length of order the effective radius of the spheroid. We propose that the progenitor galaxies of massive ellipticals must have contained a significant amount of gas that did not condense into stars during the merger process itself but formed an extended gaseous disk before the star formation epoch. The heating source that prevented the gas from forming stars early and the origin of the large specific angular momentum required for the gas component to form an extended disk are still unsolved problems.

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arXiv2001

Gas dynamics and disk formation in 3:1 mergers

We performed N-body/SPH simulations of merging gas rich disk-galaxies with a mass ratio of 3:1. A stellar disk and bulge component and a dark halo is realized with collisionless particles, the gas is simulated using SPH particles. In this paper we focus on the gas dynamics in mergers and its influence on the structure of the final merger remnant, neglecting star formation. During the merger around $8\times 10^9 M_{\odot}$ of gas accumulate in the central region of the merger remnant (inside 300 pc). Part of the gas falls to the center, channeled by a tidally induced bar in the more massive galaxy. A peak accretion rate of 150 solar masses/year is reached when the two galaxy centers merge. It is likely that the gas will experience a central starburst and/or fuel a central black hole. The gas in the outer regions accumulates in dense knots within tidal tails which could lead to the formation of open clusters or dwarf satellites. Later on, the gas knots loose angular momentum by dynamical friction and also sink to the center of the remnant, thereby increasing the central gas content. In the end, the gas settles in a central power-law disk with surface density $Σ\propto r^{-2}$ surrounded by dense, orbiting gas clumps. The stellar profile is of de Vaucouleurs-type, like in simulations of pure stellar mergers

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arXiv2001

Formation of ellipticals by unequal mass mergers

Collisionless N-body simulations of merging disk-galaxies with mass ratios ($η$) of 1:1, 2:1, 3:1, and 4:1 have been performed using direct summation with the special purpose hardware GRAPE. The simulations are used to examine whether the formation of elliptical galaxies can be explained in the context of the merger scenario. The photometric, kinematical and isophotal properties of the merger remnants are investigated and turn out to be in very good agreement with observations of giant elliptical galaxies. We conclude that equal mass mergers lead to slowly rotating, anisotropic remnants having predominantly boxy isophotes. Mergers with a mass ratio of 3:1 and 4:1, on the other hand, are fast isotropic rotators with disky isophotes. Projection effects can explain the observed scatter in the kinematical and isophotal properties of elliptical galaxies.

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arXiv2001

Evidence for Large Stellar Disks in Elliptical Galaxies

High-resolution numerical simulations of galaxy mergers are analysed. The global structure and isophotal shapes of the merger remnants are in good agreement with the observations. Whereas equal-mass mergers lead to anisotropic, boxy ellipticals, unequal-mass mergers result in disky and isotropic systems. The line-of-sight velocity distributions show small deviations from a Gaussian distribution. In all cases we find that the retrograde wings are steeper and that the prograde wings are broader than a Gaussian distribution. This is in contradiction with the observations which show broader retrograde and steeper prograde wings in all ellipticals. This fundamental difference between observation and theory can be explained if all ellipticals, even anistropic boxy ones, contain extended stellar disk components with luminosities of order 10 to 20 percent the total luminosity and scale radii of order the effective radii of the spheroids.

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arXiv1999

On the Formation of Boxy and Disky Elliptical Galaxies

The origin of boxy and disky elliptical galaxies is investigated. The results of two collisionless N-body simulations of spiral-spiral mergers with mass ratios of 1:1 and 3:1 are discussed and the projected properties of the merger remnants are investigated. It is shown that the equal-mass merger leads to an anisotropic, slowly rotating system with preferentially boxy isophotes and significant minor axis rotation. The unequal-mass merger results in the formation of a rotationally supported elliptical with disky isophotes and small minor axis rotation. The observed scatter in the kinematical and isophotal properties of both classes of elliptical galaxies can be explained by projection effects.

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arXiv1999

Formation of Elliptical and S0 Galaxies by Close Encounters

We study a possible formation mechanism for elliptical/S0 galaxies using N-body simulations with GRAPE. A close galaxy encounter which does not lead to a merger can induce a strong bar in an initially axisymmetric disk. This bar is unstable against bending oscillations and loses angular momentum to the dark halo. After 6 Gyrs the remnant is spheroidal with a de Vaucouleurs-type surface profile and has ellipticities and rotation properties comparable to observed elliptical/S0 galaxies: the system rotates fast (v/sigma \approx 1) and has disky isophotes.

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