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Accueil > Anciennes pages d’équipe > Dynamique des galaxies > DAGAL > DAGAL@LAM > Signatures of bar/peanut bulges in 2D kinematic maps

 Signatures of bar/peanut bulges in 2D kinematic maps

The presence of a bar and a peanut bulge in a disk galaxy is visible in the morphology as well as in the kinematics of stars (and gas, which we will not discuss here). The latter is generally studied in terms of the first four moments of the line-of-sight velocity distribution (LOSVD), namely : velocity (V), velocity dispersion (sigma), h3 and h4.
The imprints of a bar/peanut on stellar kinematics have been studied by Chung&Bureau (2004) and Bureau&Athanassoula (2005) in 1D, “long-slit” cases. Their analysis on edge-on, simulated disks lead them to identify the following features as a signature of the presence of a bar/peanut :

  • A steep, quasi-exponential central light profile with a shoulder or plateau at moderate radii ;
  • A double-hump rotation curve, possibly showing a local maximum and minimum at moderate radii ;
  • A broad central peak in sigma, with a shoulder or plateau at moderate radii ;
  • A correlation between V and h3.

The last point, in particular, was found to be the best indicator for the presence of bar/peanut non-axisymmetries. They also note that all these signatures become less and less evident going from an end-on to a side-on view, as well as for weaker bar/peanut features. This makes the bar/peanut position angle and the bar/peanut strength degenerate quantities, as far as the kinematic imprints are concerned.
In what follows I will show how the picture evolves when going from 1D to 2D stellar kinematics.

The simulations I am using belong to the GTR series (for the movies, click here) and consist of a stellar/gaseous disk embedded in a core-like, spherical dark-matter halo. I have been extracting kinematic information out of these simulations using the 2D-Voronoi binning technique (Cappellari & Copin 2003) and its application to simulations (Brown et al. 2013), which I eventually customised and optimised for my scope.

Comparison to 1D results

In Figure 1 you can see an example of the 2D maps for the four kinematic moments mentioned above. Isodensity contours are overplotted in black. The object is shown in an edge-on/end-on projection, where the bar/peanut signatures are supposed to be the strongest. The simulation is taken at a time where the bar/peanut are well into the secular-evolution phase. The disk is entirely made of dissipationless material, which makes this simulation ideal for a comparison with the N-body models of Bureau & Athanassoula (2005). In Figure 2 you can see what the 1D kinematic would look like for this galaxy. Shown are the values of the kinematic moments along an horizontal slit passing through the centre of the object. The four signatures of the presence of a bar/peanut can all be appreciated in this plot.

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Figure 1
Kinematic maps for a strong bar and peanut bulge seen in an edge-on, end-on projection.
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Figure 2
1D kinematics for the same system as in Fig. 1. The slit is placed along the minor axis of the bar (y=0 in Fig. 1).

What more is there in 2D ?

Figure 1 shows potentially interesting features off the plane. Note, in particular, the “X”-shaped region of h4 minima or the extended regions of h3-V correlation in the h3 maps. How general are these ? How do they depend on the strength of the bar/peanut structure and on its position angle ? Below we will inspect more simulations at different times and from different viewing angles. The aim is to understand if a bar/peanut leaves characteristic imprints on 2D kinematic maps which go beyond those already investigated in 1D studies.

In Figure 3 you can see how the kinematic maps for the simulation shown above vary with the position angle. Again, the snapshot under consideration is taken from the secular evolution phase of bar/peanut growth. The top row shows the results for the side-on case, where the line of sight is perpendicular to the bar major axis. The bottom row shows the end-on case, where the line of sight is parallel to the bar major axis. The middle rows show intermediate position angles. Going from top to bottom, the effect of the viewing angle on the kinematic quantities is quite remarkable. The isovelocity contours evolve from an almost vertical and parallel set to one where the lines shrink towards the centre of the projected density distribution. The sigma maps become richer and explore a wider range of magnitudes. The h3-V correlation becomes stronger and develops "wings" offset from the major axis of the plot. Similarly prominent regions of h4 minima develop both along and away from z=0.

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Figure 3
Variation of the kinematic maps with viewing angle.

In Figure 4 you can see an example of how the kinematic maps vary with the strength of the bar/peanut component. Each row shows the results from a different simulation in the end-on projection. All the simulations are taken at the same time and ordered by peanut strength. Again, going from top to bottom the maps change considerably. The velocity dispersion decreases considerably in magnitude (the simulation in the second row has a lower velocity dispersion in the initial set up, which may explain why the sequence in sigma is not perfect). The h3-V correlation becomes less striking, but it remains visible even at very low peanut strengths. The X-shaped regions of h4 minima gradually disappear, to leave room to two bland blobs encompassing the major axis of the plot.

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Figure 4
Variation of the kinematic maps with bar/peanut strength.

The potential of 2D maps as opposed to long-slit studies is evident for velocity, h3 and h4 maps. As for the sigma maps, the 1D behaviour along either the major or minor axis of the bar is reasonably representative of the global behaviour of the map. The variation of the kinematic properties with bar/peanut strength and viewing angle are different : the former tends to act on the magnitude of the features, the latter on their morphology. This is promising in the perspective of disentangling the effect of the two. Work on this and other aspect of the study is in progress. Details will be given in a forthcoming paper (Iannuzzi et al., in preparation).

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