MODELING GALAXIES AND THEIR ENVIRONMENT: ON-GOING EFFORTS A. C. Gonzalez-Garcia Instituto de Astrofisica de Canarias M. Balcells Instituto de Astrofisica de Canarias 1. L.O.S.V.D. of Elliptical Galaxies Formed by Mergers Improved observational techniques and the large telescopes available today have made possible to study the variation with radius of a number of quantities in elliptical galaxies. Bender, Saglia & Gerhard (1994) already measured the line-of-sight velocity distributions (LOSVDs) of a number of elliptical galaxies up to 1 effective radius. Bender et al. fit the LOSVD to a Gaussian plus a combination of GaussHermite polynomials (van der Marel & Franx 1993). They find an anti-correlation between the h3 parameter, which gives the amplitude of the 3rd-Hermite polynomial, and V /σ. h3 describes the asymmetry or skewness of the LOSVD from gaussianity. The correlation appears to be more prominent for disky elliptical galaxies. This has been claimed as a signature of the presence of disks inside disky ellipticals (Bender et al. 1994, Naab & Burkert 2001). In the hierarchical merging scenario, elliptical galaxies can be formed by a major merger between two spiral galaxies (Toomre & Toomre 1972). It has been argued that 1:1 mergers between disk galaxies give as a result a body that resembles a boxy elliptical galaxy, while mergers with mass ratios of the order 3:1 give disky ellipticals (Bendo & Barnes 2000, Naab & Burkert 2001, Gonzalez-Garcia & Balcells 2004). It is therefore quite straightforward to ask whether the correlation found by Bender et al. (1994) can be found by means of mergers. Naab & Bukert (2001) study this question by collisionless simulations of mergers of disks. They use models with galaxy mass ratios 1:1 and 3:1 with a disk:bulge ratio 3:1. They find that for 1:1 mergers their systems end up being boxy ellipticals not showing the correlation found by Bender et al. 3:1 mergers although they show disky deviations, do also not reproduce Bender’s 2 correlation. They claim that this is a proof of the dissipational origin of these structures. We present here selected results from a new set of simulations with increased resolution and with slightly different initial conditions. A full account on this issue will be given in a forthcoming paper (Gonzalez-Garcia & Balcells 2005). Our simulations make use of initial systems with a larger disk:bulge ratio than previous studies (2:1), which has proven to be important in keeping the signature of the initial large disk into the final remnant (Gonzalez-Garcia & Balcells 2004). We have also tested if this feature in the LOSVD can appear from mergers of elliptical galaxies. Model details We build the initial disk models following Kuijken & Dubinski (1995). Our initial models have a bulge a disk and a halo. The masses of the different components in our initial disk model are in the ratios: 19:2:1 (halo:disk:bulge). As we can see the initial mass ratio between disk and bulge is 2:1. Note that our models have a somewhat larger bulge than Naab & Burkert (2001). The disk-disk merger model has a mass ratio 3:1, the initial disks are placed on elliptical inter-penetrating orbits to ensure merging. Both systems are placed initially with the z-component of the spin angular momentum parallel to the orbital angular momentum. The spin-orbit coupling is thus larger, although it is not maximum. For simulations involving two elliptical galaxies we use Jaffe (1983) spherical non-rotating models as input conditions. The mass ratio of the initial models is 3:1 and the systems are placed on a parabolic orbit with impact parameter equal to the half mass radius of the smaller system. We use non-dimensional units throughout with G=1, M=1 and R=1. Each simulation consists of 106 particles in total. We have run these simulations with the parallel version of GADGET (Springel et al. 2001) on a Beowulf Cluster with 16 CPU’s. Each run takes of the order of 5 × 10 5 seconds of CPU time. Results Modern kinematic diagnostics from high-S/N spectroscopy provide useful tests on the collisionless merger hypothesis for the formation of elliptical galaxies. Bender, Saglia & Gerhardt (1994) show that the skewness of the LOSVDs (given by the h3 parameter) has opposite sign to the rotation velocity, and follows a distinct pattern. Previous N-body experiments (Bendo & Barnes 2000, Naab & Burkert 2001) disagree as to whether the relation found by Bender et al. is reproduced by the models. We have obtained the LOSVD for each merger remnant from 100 points of view, by binning a slit placed along the projected major axis. Then we fitted the LOSVD by a Gauss-Hermite polynomial Modeling galaxies and their environment 3 Figure 1. h3 vs. V /σ for the remnant of a 3:1 disk merger (left) and for the 3:1 merger between two ellipticals (right). in the way done by Bender et al. (1994). The results for the two experiments exposed here are presented in Figure 1. Disk mergers (see Figure 1, left) yield trends similar to observations for large values of V /σ , but with overall lower h 3 values in the models than in real galaxies. And we fail to reproduce high h 3 values for low V /σ . E-E mergers (see Figure 1, right) show large values of h 3 for low V /σ but do not show the wings of values at large V /σ. Conclusions 3:1 disk-disk mergers reproduce the trend found by Bender et al. (1994) for large V /σ, in disagreement with Naab & Burkert (2001). Our initial models have a larger bulge. This helps stabilize the disk during the merger (see Gonzalez-Garcia & Balcells 2004). We are not able to find large values of h3 for the inner parts. 3:1 E-E mergers show large values of h 3 for low V /σ, but do not show values of h3 for large V /σ. In the outer parts of the 3:1 disk merger remnant we can find material from the disk of the large system. Thus, the observed h 3 vs. V /σ relation at large V /σ might be the signature of the rotation of the initial large disk. The observed values for low V /σ may show the structure of the remnants of mergers between spheroid dominated galaxies. 4 2. Effects of Dark Satellites on Disks CDM cosmological simulations show more substructure than satellites are observed in the Local Group (Moore et al. 1999, Klypping et al. 1999). This presents a problem for the CDM scenario. One may think of these haloes as shallow dark matter haloes that were not able to keep their barionic content, or were not able to form stars, therefore we could name them as dark satellites (Stoehr et al. 2002). These satellites may interact with the visible matter. Then the effects of the interactions could perhaps be detected (Font et al. 2001, Johnston et al. 2002). Font et al. (2001) do not find evidence for any disk heating due to collisions in their simulations. Johnston et al. (2002) use the tidal streams of dwarf satellites to put constrains on the number of these dark matter satellites. Abadi et al. (2003) performed a number of simulations to build a disk out of cosmological conditions and find no evidence of structures in the disk revealing an interaction with the dark satellites. However these simulations lack particle resolution in the disk and look for very specific results, overlooking other possible effects. Model details Our simulations involve the interaction of a disk galaxy, modeled by a disk, a bulge and a halo, with several dark haloes placed around the disk. The disk galaxy was built following Kuijken & Dubinski (1995). Our initial models have a bulge a disk and a halo. The masses of the different components in our initial disk model are in the ratios: 19:2:1 (halo:disk:bulge). The dark satellites are modeled by Jaffe (1983) non-rotating spheroids. We used 10 satellites, placed on orbits with parameters taken to follow results from cosmological simulations (Tormen 1997). We have 10 satellites in total, 4 with masses 1/100 of the large galaxy and 6 with masses 1/200. Our model has 1.5 × 106 particles and the evolution was run using the parallel version of the GADGET code (Springel et al. 1999) on a Beowulf cluster. Results We present preliminary results from one of the simulations performed so far. We have looked for observable features produced by the interaction with the dark satellites. In principle, such features could be seen directly on the disk (e.g. as spiral arms), on the light profile or on the kinematics of the stars. We present here mainly results for the features directly observable on the disk. We have obtained diagnostics on the shape and velocity evolution of the disk by binning the particle distribution on logarithmically spaced bins. For each bin the mean of the positions and velocities is calculated. Modeling galaxies and their environment 5 Figure 2. Observed features on the disk. Left panels show a snapshot after several satellites have passed close to the disk. Top panel shows a face on view, bottom panel shows an edge-on view. Right panels show the same model 3 time units afterwards. Top panel shows a face-on view, bottom panel an edge-on view. 6 Figure 2 left presents the results for one of our snapshots. Top panel shows how the galaxy, seen face-on, has developed a spiral structure. Bottom panel shows a contour plot taken from a point of view along the x-axis (edge-on view). We find a warp developing at the edge. Figure 2 right shows results for the same simulation 3 time units after the previous one. The warp has evolved, more satellites have hit the disk and different structures have developed. A different warp can be appreciated. Future work Analyze the evolution of the warp. Analyze the internal kinematics for possible signatures of the interactions. Follow the development of spiral arms and bar distortions. We plan to run a number of simulations to allow us to draw general conclusions on observable effects due to the dark satellites. References Abadi M.G., Navarro J.F., Steinmetz M., Eke V.R., 2003, ApJ, 597, 21 Bender R., Saglia R. P., Gerhard O. E., 1994, MNRAS, 269, 785 Bendo G., Barnes J. E., 2000, MNRAS, 319, 315 Font A.S., Navarro J.F., Stadel J., Quinn T., 2001, ApJ, 563, L1 Gonzalez-Garcia A. C., Balcells M., 2004, MNRAS, in press Gonzalez-Garcia A. C., Balcells M., 2005, in preparation Jaffe W., 1983, MNRAS, 202, 995 Johnston K.V., Spergel D.N., Haydn C., 2002, ApJ, 570, 656 Klypin A., Kravtsov A., Valenzuela O., Prada F., 1999, ApJ, 522, 82 Kuijken K, Dubinski J., 1995, MNRAS, 277,1341 Moore B., et al., 1999, ApJ, 524, 19 Naab T., Burkert A., 2001, ApJ, 555, L91 Springel V., Yoshida N., White S. D. M., 2001, NewAst., 6, 79 Stoehr F., White S.D.M., Tormen G., Springel V., 2002, MNRAS, 335, L84 Toomre A., Toomre J., 1972, ApJ, 178, 623 Tormen G., 1997, MNRAS, 290, 411
© Copyright 2025 Paperzz