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تسجيل مستخدم جديدThe Influence of Interactions and Minor Mergers on the Structure of Galactic Disks: I.Observations and Disk Models
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This paper is the first part in our series on the influence of tidal interactions and minor mergers on the radial and vertical disk structure of spiral galaxies. We report on the sample selection, our observations, and data reduction. Surface photometry of the optical and near infrared data of a sample of 110 highly-inclined/edge-on disk galaxies are presented. This sample consists of two subsamples of 61 non-interacting galaxies (control sample) and of 49 interacting galaxies/minor merging candidates. Additionally, 41 of these galaxies were observed in the near infrared. We show that the distribution of morphological types of both subsamples is almost indistinguishable, covering the range between 0 <= T <= 9. An improved, 3-dimensional disk modelling- and fitting procedure is described in order to analyze and to compare the disk structure of our sample galaxies by using characteristic parameters. We find that the vertical brightness profiles of galactic disks respond very sensitive even to small deviations from the perfect edge-on orientation. Hence, projection effects of slightly inclined disks may cause substantial changes in the value of the disk scale height and must therefore be considered in the subsequent study.
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We present the second part of a detailed statistical study focussed on the effects of tidal interactions and minor mergers on the radial and vertical disk structure of spiral galaxies. In the first part we reported on the sample selection, observatio
ns, and applied disk models. In this paper the results are presented, based on disk parameters derived from a sample of 110 highly-inclined/edge-on galaxies. This sample consists of two subsamples of 49 interacting/merging and 61 non-interacting galaxies. Additionally, 41 of these galaxies were observed in the NIR. We find significant changes of the disk structure in vertical direction, resulting in ~1.5 times larger scale heights and thus vertical velocity dispersions. The radial disk structure, characterized by the cut-off radius and the scale length, shows no statistically significant changes. Thus, the ratio of radial to vertical scale parameters, h/z0, is ~1.7 times smaller for the sample of interacting/merging galaxies. The total lack of typical flat disk ratios (h/z0)>7 in the latter sample implies that vertical disk heating is most efficient for (extremely) thin disks. Statistically nearly all galactic disks in the sample (93%) possess non-isothermal vertical luminosity profiles like the sech (60%) and exp (33%) distribution, independent of the sample and passband investigated. This indicates that, in spite of tidal perturbations and disk thickening, the initial vertical distribution of disk stars is not destroyed by interactions or minor mergers.
We investigate the influence of stellar migration caused by minor mergers (mass ratio from 1:70 to 1:8) on the radial distribution of chemical abundances in the disks of Milky Way-like galaxies during the last four Gyr. A GPU-based pure N-body tree-c
ode model without hydrodynamics and star formation was used. We computed a large set of mergers with different initial satellite masses, positions, and orbital velocities. We find that there is no significant metallicity change at any radius of the primary galaxy in the case of accretion of a low-mass satellite of 10$^9$ M$_{odot}$ (mass ratio 1:70) except for the special case of prograde satellite motion in the disk plane of the host galaxy. The accretion of a satellite of a mass $gtrsim3times10^9$ M$_{odot}$ (mass ratio 1:23) results in an appreciable increase of the chemical abundances at galactocentric distances larger than $sim10$ kpc. The radial abundance gradient flattens in the range of galactocentric distances from 5 to 15 kpc in the case of a merger with a satellite with a mass $gtrsim3times10^9$ M$_{odot}$. There is no significant change in the abundance gradient slope in the outer disk (from $sim15$ kpc up to 25 kpc) in any merger while the scatter in metallicities at a given radius significantly increases for most of the satellites initial masses/positions compared to the case of an isolated galaxy. This argues against attributing the break (flattening) of the abundance gradient near the optical radius observed in the extended disks of Milky Way-like galaxies only to merger-induced stellar migration.
We use hydrodynamic simulations of minor mergers of galaxies to investigate the nature of surface brightness excesses at large radii observed in some spiral galaxies: antitruncated stellar disks. We find that this process can produce the antitruncati
on via two competing effects: (1) merger-driven gas inflows that concentrate mass in the center of the primary galaxy and contract its inner density profile; and (2) angular momentum transferred outwards by the interaction, causing the outer disk to expand. In our experiments, this requires both a significant supply of gas in the primary disk, and that the encounter be prograde with moderate orbital angular momentum. The stellar surface mass density profiles of our remnants both qualitatively and quantitatively resemble the broken exponentials observed in local face--on spirals that display antitruncations. Moreover, the observed trend towards more frequent antitruncation relative to classical truncation in earlier Hubble types is consistent with a merger-driven scenario.
Multiple, sequential mergers are unavoidable in the hierarchical build-up picture of galaxies, in particular for the minor mergers that are frequent and highly likely to have occured several times for most present-day galaxies. However, the effect of
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ence on the initial orbital and internal properties of the accreted objects. We find that mergers with 10-20% mass of the mass of the host lead to the formation of thick disks whose characteristics are similar, both in morphology as in kinematics, to those observed. Despite the relatively large mass ratios, the host disks are not fully destroyed by the infalling satellites: a remaining kinematically cold and thin component containing ~15-25% of the mass can be identified, which is embedded in a hotter and thicker disk. This may for example, explain the existence of a very old thin disk stars in the Milky Way. The final scale-heights of the disks depend both on the initial inclination and properties of the merger, but the fraction of satellite stellar particles at ~4 scale-heights directly measures the mass ratio between the satellite and host galaxy. Our thick disks typically show boxy isophotes at very low surface brightness levels (>6 magnitudes below their peak value). Kinematically, the velocity ellipsoids of the simulated thick disks are similar to that of the Galactic thick disk at the solar radius. The trend of sigma_Z/sigma_R with radius is found to be a very good discriminant of the initial inclination of the accreted satellite. In the Milky Way, the possible existence of a vertical gradient in the rotational velocity of the thick disk as well as the observed value of sigma_Z/sigma_R at the solar vicinity appear to favour the formation of the thick disk by a merger with either low or intermediate orbital inclination.
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