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Register a new userLopsidedness in WHISP galaxies: II. Morphological lopsidedness
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The distribution of stars and gas in many galaxies is asymmetric. This so-called lopsidedness is expected to significantly affect the dynamics and evolution of the disc, including the star formation activity. Here, we measure the degree of lopsidedness for the gas distribution in a selected sample of 70 galaxies from the Westerbork HI Survey of Spiral and Irregular Galaxies. This complements our earlier work (Paper I) where the kinematic lopsidedness was derived for the same galaxies. The morphological lopsidedness is measured by performing a harmonic decomposition of the surface density maps. The amplitude of lopsidedness A_1, the fractional value of the first Fourier component, is typically quite high (about 0.1) within the optical disc and has a constant phase. Thus, lopsidedness is a common feature in galaxies and indicates a global mode. We measure A_1 out to typically one to four optical radii, sometimes even further. This is, on average, four times larger than the distance to which lopsidedness was measured in the past using near-IR as a tracer for the old stellar component, and will therefore provide a new, more stringent constraint on the mechanism for the origin of lopsidedness. Interestingly, the value of A_1 saturates beyond the optical radius. Furthermore, the plot of A_1 vs. radius shows fluctuations which we argue are due to local spiral features. We also try to explain the physical origin of this observed disc lopsidedness. No clear trend is found when the degree of lopsidedness is compared to a measure of the isolation or interaction probability of the sample galaxies. However, this does not rule out a tidal origin if the lopsidedness is long-lived. Additionally, we find that the early-type galaxies tend to be more morphologically lopsided than late-type galaxies. Both results together indicate a tidal origin for the lopsidedness.
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The frequently observed lopsidedness of the distribution of stars and gas in disc galaxies is still considered as a major problem in galaxy dynamics. It is even discussed as an imprint of the formation history of discs and the evolution of baryons in dark matter haloes. Here, we analyse a selected sample of 70 galaxies from the Westerbork HI Survey of Spiral and Irregular Galaxies. The HI data allow us to follow the morphology and the kinematics out to very large radii. In the present paper, we present the rotation curves and study the kinematic asymmetry. We extract the rotation curves of receding and approaching sides separately and show that the kinematic behaviour of disc galaxies can be classified by five different types: symmetric velocity fields where the rotation curves of receding and approaching sides are almost identical; global distortions where the rotation velocities of receding and approaching side have an offset which is constant with radius; local distortions which lead to large deviations in the inner and negligible deviations in the outer parts (and vice versa); and distortions which split the galaxies into two kinematic systems, visible in the different behaviour of the rotation curves of receding and approaching sides, which leads to a crossing and a change in side. The kinematic lopsidedness is measured from the maximum rotation velocities, averaged over the plateau of the rotation curves. This gives a good estimate of global lopsidedness in the outer parts of the sample galaxies. We find that the mean value of the perturbation parameter denoting the lopsided potential as obtained from the kinematic data is 0.056. 36% of all sample galaxies are globally lopsided, which can be interpreted as the disc responding to a halo that was distorted by a tidal encounter. In Paper II, we study the morphological lopsidedness for the same sample of galaxies.
Lopsidedness of the gaseous disk of spiral galaxies is a common phenomenon in disk morphology, profile and kinematics. Simultaneously, the asymmetry of a galaxys stellar disk, in combination with other morphological parameters, has seen extensive use as an indication of recent merger or interaction in galaxy samples. Quantified morphology of stellar spiral disks is one avenue to determine the merger rate over much of the age of the Universe. In this paper, we measure the quantitative morphology parameters for the HI column density maps from the Westerbork observations of neutral Hydrogen in Irregular and SPiral galaxies (WHISP). These are Concentration, Asymmetry, Smoothness, Gini, M20, and one addition of our own, the Gini parameter of the second order moment (GM). Our aim is to determine if lopsided or interacting disks can be identified with these parameters. Our sample of 141 HI maps have all previous classifications on their lopsidedness and interaction. We find that the Asymmetry, M20, and our new GM parameter correlate only weakly with the previous morphological lopsidedness quantification. These three parameters may be used to compute a probability that an HI disk is morphologically lopsided but not unequivocally to determine it. However, we do find that that the question whether or not an HI disk is interacting can be settled well using morphological parameters. Parameter cuts from the literature do not translate from ultraviolet to HI directly but new selection criteria using combinations of Asymmetry and M20 or Concentration and M20, work very well. We suggest that future all-sky HI surveys may use these parameters of the column density maps to determine the merger fraction and hence rate in the local Universe with a high degree of accuracy.
It has long been known that a large fraction of disc galaxies are lopsided. We simulate three different mechanisms that can induce lopsidedness: flyby interactions, gas accretion from cosmological filaments and ram pressure from the intergalactic medium. Comparing the morphologies, HI spectrum and m=1 Fourier components, we find that all of these mechanisms can induce lopsidedness in the gaseous component of disc galaxies. In particular, we estimate that flybys can contribute to ~20 per cent of lopsided galaxies. We compare our simulations with the observations of NGC 891, a lopsided, edge-on galaxy with a nearby companion (UGC 1807). We find that the main properties of NGC 891 favour a flyby event for the origin of lopsidedness in this galaxy.
An $m=1$ lopsided asymmetry is common in disc galaxies. Here, we investigate the excitation of an $m=1$ lopsidedness in host galaxies during minor mergers (mass ratio 1:10) while choosing a set of minor merger models (with varying orbital configurations, morphology of the host galaxy) from the GalMer library of galaxy merger simulations. We show that a minor merger triggers a prominent $m=1$ lopsidedness in the stars of the host galaxy. The strength of the $m=1$ lopsidedness undergoes a transient amplification phase after each pericentre passage of the satellite, in concordance with past findings of excitation of an $m=1$ lopsidedness due to tidal encounters. However, once the merger happens, and the post-merger remnant readjusts itself, the lopsidedness fades away in short time-scale ($sim 500-850$ Myr). Furthermore, a delayed merger can drive a prolonged ($sim 2$ Gyr) lopsidedness in the host galaxy. We demonstrate that the $m=1$ lopsidedness rotates with a well-defined pattern speed. The measured pattern speed is much slower than the $m=2$ bar pattern speed, and is retrograde with respect to the bar. This gives rise to a dynamical scenario where the Inner Linblad resonance (ILR) of the $m=1$ lopsidedness falls in between the corotation (CR) and the Outer Linblad resonance (OLR) of the $m=2$ bar mode. A kinematic lopsidedness also arises in the host galaxy, and the resulting temporal variation closely follows that of the density lopsidedness. The minor merger also triggers a transient off-centred stellar disc-dark matter halo configuration due to the tidal encounter with the satellite.
Lopsidedness is common in spiral galaxies. Often, there is no obvious external cause, such as an interaction with a nearby galaxy, for such features. Alternatively, the lopsidedness may have an internal cause, such as a dynamical instability. In order to explore this idea, we have developed a computer code that searches for self-consistent perturbations in razor-thin disc galaxies and performed a thorough mode-analysis of a suite of dynamical models for disc galaxies embedded in an inert dark-matter halo with varying amounts of rotation and radial anisotropy. Models with two equal-mass counter-rotating discs and fully rotating models both show growing lopsided modes. For the counter-rotating models, this is the well-known counter-rotating instability, becoming weaker as the net rotation increases. The m=1 mode of the maximally rotating models, on the other hand, becomes stronger with increasing net rotation. This rotating m=1 mode is reminiscent of the eccentricity instability in near-Keplerian discs. To unravel the physical origin of these two different m=1 instabilities, we studied the individual stellar orbits in the perturbed potential and found that the presence of the perturbation gives rise to a very rich orbital behaviour. In the linear regime, both instabilities are supported by aligned loop orbits. In the non-linear regime, other orbit families exist that can help support the modes. In terms of density waves, the counter-rotating m=1 mode is due to a purely growing Jeans-type instability. The rotating m=1 mode, on the other hand, grows as a result of the swing amplifier working inside the resonance cavity that extends from the disc center out to the radius where non-rotating waves are stabilized by the models outwardly rising Q-profile.
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