Astronomy & Astrophysics

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$\begin{figure} \par\includegraphics[width=7.6cm,clip]{2631f1.eps} \end{figure}$ Figure 1: Radial variation of A₁, the normalized Fourier amplitude of the stellar density, and phase $\phi _1$ , for the lopsided galaxy NGC 1637, measured from the near-infrared image of the OSUBGS sample. The disk scale-length is 2.45 kpc and the value of $\langle A_1\rangle=0.19$ was measured between 3.7 kpc and 6.1 kpc. The near-IR image of the galaxy from the OSUBGS catalog is shown in Fig. 18. This galaxy shows a high degree of lopsidedness, but has no interacting companion and no evidence of a recent merger.
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In the text

$\begin{figure} \par\includegraphics[width=7.6cm,clip]{2631f2.eps} \end{figure}$ Figure 2: Histogram of m=1 Fourier amplitude in the stellar density deduced from the near-infrared images of the OSUBGS sample of galaxies. The $\langle A_1\rangle$ parameter is the average of the normalized Fourier amplitude for m=1, over the radial range between 1.5 to 2.5 disk scalelengths.
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In the text

$\begin{figure} \par\includegraphics[width=7.7cm,clip]{2631f3.eps} \end{figure}$ Figure 3: Cumulative functions of $\langle A_1\rangle$ (fraction of galaxies with $\langle A_1\rangle$ larger than the x-axis value) for galaxies seen below a certain cut-off in the inclination angle i; for $i < 30\hbox {$^\circ $ }, 50\hbox {$^\circ $ },$ and $70\hbox {$^\circ $ }$ . The curves are nearly identical, hence the highest value of the cut-off $70\hbox {$^\circ $ }$ is chosen, as it gives the largest number of galaxies in the sample. Galaxies more inclined than $70\hbox {$^\circ $ }$ no longer follow the same statistical distribution and have been rejected.
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In the text

$\begin{figure} \par\includegraphics[width=7.6cm,clip]{2631f4.eps} \end{figure}$ Figure 4: Histogram of m=1 Fourier amplitude in the gravitational potentials (Q₁) for the OSUBGS sample of galaxies.
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In the text

$\begin{figure} \par\includegraphics[width=7.6cm,clip]{2631f5.eps} \end{figure}$ Figure 5: HI velocity profile of NGC 150, showing the two horns, the center of the profile and the extremity of the two horns. F₁ and F₂ are the integrated flux of each horn, and their ratio gives an estimate of the HI asymmetry.
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In the text

$\begin{figure} \par\includegraphics[width=7.6cm,clip]{2631f6.eps} \end{figure}$ Figure 6: Histogram of the E₁ parameter measured on HI velocity profiles.
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In the text

$\begin{figure} \par\includegraphics[width=7.6cm,clip]{2631f7.eps} \end{figure}$ Figure 7: Cumulative function of $\langle A_1\rangle$ , for three groups of Hubble types of spiral galaxies: the early-types ( 0 < T < 2.5), the intermediate-types ( 2.5 < T < 5), and the late-types ( 5 < T < 7.5). The late-type spirals are more lopsided than the early-type ones.
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In the text

$\begin{figure} \par\includegraphics[width=7.5cm,clip]{2631f8.eps} \end{figure}$ Figure 8: Cumulative function of $\langle A_1\rangle$ for galaxies with values of $\langle A_2\rangle$ higher than the median value of $\langle A_2\rangle$ over our sample (dashed line), and $\langle A_2\rangle$ smaller than this threshold (solid line). Galaxies with large m=2 asymmetries are clearly more lopsided than galaxies with small m=2 asymmetries. Here $\langle A_2\rangle$ is the average value of A₂ between 1.5 and 2.5 disk scalelengths. The correlation between A₁ and A₂ remains if we take the average value of A₂ between 0.5 and 1.5 scalelengths (which would rather correspond to a central bar), or between 2.5 and 3.5 scalelengths (which would rather correspond to outer spiral arms).
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In the text

$\begin{figure} \par\includegraphics[width=7.5cm,clip]{2631f9.eps} \end{figure}$ Figure 9: Correlation between the $\langle A_1\rangle$ parameter and the tidal parameter $T_{{\rm P}}$ defined in text, for the 35 most lopsided galaxies with $i<70\hbox {$^\circ $ }$ . No correlation is found. Strong lopsidedness in very isolated galaxies ( $T_{\rm P}<-6$ ) is frequent, and is not the privilege of galaxies with companions.
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In the text

$\begin{figure} \par\includegraphics[width=7.5cm,clip]{2631f10.eps} \end{figure}$ Figure 10: Simulation of distant interaction between two galaxies of comparable mass (Run 9). Top: temporal evolution of $\langle A_1\rangle$ (solid line) and the tidal parameter $T_{{\rm P}}$ (dashed, defined in Sect. 3). A strong lopsidedness is triggered during the interaction. It is observed for about 4 Gyrs, but is much lower once the system is isolated. Bottom: Q₁ parameter in the disk plane for the total potential (solid line), the internal target galaxy potential only (dashed curve), and the external potential of the companion only (dotted curve). During the interaction, a strong lopsidedness results from the potential of the companion, but is not totally supported by the potential of the target galaxy itself. The maximal lopsidedness in Q₁ can be decomposed as an external contribution from the companion (short-lived) and an internal lopsidedness supported by the target galaxy potential (long-lived). This explains why there is a strong peak in Q₁ and A₁ during the interaction, and a long-lived but weaker lopsidedness after the interaction.
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In the text

$\begin{figure} \par\includegraphics[width=7.5cm,clip]{2631f11.eps} \end{figure}$ Figure 11: Largest value of $\langle A_1\rangle$ as a function of the tidal parameter $T_{{\rm P}}$ for all the simulations of tidal galaxy interactions. All the results of the simulations lie below the solid line in this (A₁; $T_{{\rm P}}$ ) plane. This scenario does not explain the presence of strong lopsidedness ( $\langle A_1\rangle$ larger than 0.1) in isolated galaxies (tidal parameter <-5), while there are many observed cases.
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In the text

$\begin{figure} \par\includegraphics[width=7.5cm,clip]{2631f12.eps} \end{figure}$ Figure 12: Temporal evolution of $\langle A_1\rangle$ in two minor mergers ( top: Run 4b, bottom: Run 6c). The solid line is when stars from the companion (or formed in gas from the companion) are included. The dashed line corresponds only to stars from the target galaxy (or formed in gas clouds present in the target before the merger). When the two curves have significantly different values, this means that the system is highly disturbed, for instance at t₁ (see the corresponding snapshot for Run 4b in Fig. 13). Tidal tails, streams, and other merger indicators, are still seen at t₂. The system can be classified as an "isolated galaxy'' after the two curves have reached similar values, for instance at t₃ (see Fig. 13). We then consider that the maximal lopsidedness for an "isolated'' galaxy is found when the two curves reach similar values, at t₀: corresponding values of $\langle A_1\rangle$ are given for each simulation in Table 3.
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In the text

$\begin{figure} \par\includegraphics[width=15.4cm,clip]{2631f13.eps} \end{figure}$ Figure 13: Snapshots of the minor merger scenario, Run 4b (see the temporal evolution of $\langle A_1\rangle$ and the definition of t₁, t₂, t₃, in Fig. 12). At t₁, there is a strong lopsidedness, but the ongoing merger is clearly visible. Even at t₂, when the stars from the companion are more "mixed'' in the system, and no longer dominate the value of $\langle A_1\rangle$ , there are still obvious signs of the recent merger (like arcs and tidal debris in the outer parts, that are even more visible in the gas component). Only at t₃, the system is an "isolated'' galaxy without sign of a past merger, but the amplitude of the lopsidedness is much lower, as shown by Fig. 12.
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In the text

$\begin{figure} \par\includegraphics[width=7.6cm,clip]{2631f14.eps} \end{figure}$ Figure 14: Cumulative function of $\langle A_2\rangle$ for late-type, intermediate-type, and early-type spiral galaxies (a similar figure for $\langle A_1\rangle$ has been shown in Fig. 7). Here $\langle A_2\rangle$ is the average value of A₂ between 1.5 and 2.5 disk scalelengths, but the anti-correlation of $\langle A_2\rangle$ with T remains if we consider the average values between 0.5 and 1.5, or 2.5 and 3.5 disk scalelengths. Our sample obeys the general rule that m=2 bars/arms are stronger in early-type spiral galaxies. This is the opposite of what we found for lopsidedness, which is stronger in late-type galaxies.
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In the text

$\begin{figure} \par\includegraphics[width=7.5cm,clip]{2631f15a.eps}\par\vspace*{3mm} \includegraphics[width=7.5cm,clip]{2631f15b.eps} \end{figure}$ Figure 15: Same as Fig. 8 for early-type spiral galaxies only ( left, 0<T<4) and late-type galaxies only ( right, 4<T<8). As discussed in the text, this shows that the correlation between m=1 and m=2 asymmetries found for the whole sample in Sect. 3 does not just mean that they are stronger in the same types of galaxies, but really means that they are preferentially formed simultaneously. Indeed, this correlation is still found when we limit the sample to early- or late-type galaxies.
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In the text

$\begin{figure} \par\includegraphics[width=7.6cm,clip]{2631f16.eps} \end{figure}$ Figure 16: Configuration of accretion simulations with one filament. The spatial offset of the filament with respect to the disk center is given in Table 4 for each simulation. The size of the filament is 25 kpc.
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In the text

$\begin{figure} \par\includegraphics[width=7.6cm,clip]{2631f17.eps} \end{figure}$ Figure 17: Top: temporal evolution of A₁ in a simulation of accretion (Run 2). A strong lopsidedness is triggered, even in the stellar component (through star formation), in a galaxy that looks completely isolated. Then the value of $\langle A_1\rangle$ decreases slowly while the disk re-orients itself, but remains larger than 0.1 during more than 3 Gyrs. Bottom: same simulations with no accretion after t=2 Gyr. The lopsidedness is long-lived (about 3 Gyrs), but slightly shorter-lived than in the galaxy interaction/merger scenario, probably because perturbations to the halo are weaker in this case.
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In the text

$\begin{figure} \par\includegraphics[width=11.2cm,clip]{2631f18.eps} \end{figure}$ Figure 18: Snapshot of a simulation of asymmetrical gas accretion ( left), giving an overall aspect rather similar to NGC 1637 ( right: NIR map from the OSUBGS data, after deprojection). A strong lopsidedness is present in this very isolated galaxy. We have used two "filaments'' to provide a more realistic case: the main filament shown in Fig. 16 is present, with an accretion rate of 4 $M_{\odot }$ yr^-1, and a second filament, symmetric of this first one with respect to the galaxy center, has an accretion rate of 2 $M_{\odot }$ yr^-1. These accretion rates are for a galactic mass of 10¹¹ $M_{\odot }$ , and correspond to doubling the mass of the galaxy in about one Hubble time.
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In the text

$\begin{figure} \par\includegraphics[width=7.6cm,clip]{2631f2.eps} \end{figure}$	Figure 2: Histogram of m=1 Fourier amplitude in the stellar density deduced from the near-infrared images of the OSUBGS sample of galaxies. The $\langle A_1\rangle$ parameter is the average of the normalized Fourier amplitude for m=1, over the radial range between 1.5 to 2.5 disk scalelengths.
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$\begin{figure} \par\includegraphics[width=7.6cm,clip]{2631f4.eps} \end{figure}$	Figure 4: Histogram of m=1 Fourier amplitude in the gravitational potentials (Q₁) for the OSUBGS sample of galaxies.
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$\begin{figure} \par\includegraphics[width=7.6cm,clip]{2631f5.eps} \end{figure}$	Figure 5: HI velocity profile of NGC 150, showing the two horns, the center of the profile and the extremity of the two horns. F₁ and F₂ are the integrated flux of each horn, and their ratio gives an estimate of the HI asymmetry.
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$\begin{figure} \par\includegraphics[width=7.6cm,clip]{2631f6.eps} \end{figure}$	Figure 6: Histogram of the E₁ parameter measured on HI velocity profiles.
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$\begin{figure} \par\includegraphics[width=7.6cm,clip]{2631f7.eps} \end{figure}$	Figure 7: Cumulative function of $\langle A_1\rangle$ , for three groups of Hubble types of spiral galaxies: the early-types ( 0 < T < 2.5), the intermediate-types ( 2.5 < T < 5), and the late-types ( 5 < T < 7.5). The late-type spirals are more lopsided than the early-type ones.
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$\begin{figure} \par\includegraphics[width=7.5cm,clip]{2631f9.eps} \end{figure}$	Figure 9: Correlation between the $\langle A_1\rangle$ parameter and the tidal parameter $T_{{\rm P}}$ defined in text, for the 35 most lopsided galaxies with $i<70\hbox {$^\circ $ }$ . No correlation is found. Strong lopsidedness in very isolated galaxies ( $T_{\rm P}<-6$ ) is frequent, and is not the privilege of galaxies with companions.
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$\begin{figure} \par\includegraphics[width=7.6cm,clip]{2631f16.eps} \end{figure}$	Figure 16: Configuration of accretion simulations with one filament. The spatial offset of the filament with respect to the disk center is given in Table 4 for each simulation. The size of the filament is 25 kpc.
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