Proceedings of the Workshop
"The Magellanic Clouds and Other Dwarf Galaxies"
of the Bonn/Bochum-Graduiertenkolleg

A Relation between Box/Peanut Bulges

and their Satellite Systems

Rainer Lütticke and Ralf-Jürgen Dettmar

Astronomisches Institut der Ruhr-Universiät Bochum

Received 13th March 1998
Abstract. We present new statistics of bulges of disk galaxies that reveals a high frequency of box- or peanut shaped (b/p) bulges of up to nearly 50%. This requires very common physical processes to explain such a high fraction. In an analysis of a possible connection between this internal structure and the presence of satellite systems in otherwise isolated disk galaxies we find a correlation between b/p bulges and the presence of satellites. Together with investigations of the environment of galaxies showing a b/p bulge we conclude that b/p bulges preferentially occur in disk galaxies with companions.

1. Introduction

Box and peanut shaped (b/p) bulges are not really that peculiar as it seemed in the past and very common processes are required to explain the high frequency. At present several mechanisms for their origin are discussed. Binney & Petrou (1985) and Whitemore & Bell (1988) suggested that these structures may result from material accreted from infalling satellite companions (soft merging). An alternative mechanism for forming b/p bulges are instabilities or resonances animated by bars (Combes et al. 1990; Raha et al. 1991). N-body simulations for stars in barred potentials have demonstrated that with regard to the shape of bulges this theory and observational evidence are consistent.

Within this framework, however, the question what may cause the bar becomes even more important. Dynamically cold disks can produce, through a global instability, a bar or galaxy interaction can drive bar formation in stable disks (Freeman 1996). However, a bar can also originate from an infalling satellite so that accretion and bar hypotheses for the formation of b/p bulges could be seen in a unified picture (Mihos et al. 1995). A sufficient mass concentration in the center of the bulge could finally cause the bar to dissolve (Sellwood 1996).

This proposed scenario would let galaxies evolve from SA over SB to SA, with bulges growing through a b/p phase either by disk instabilities, accreted material, or by both. Along the Hubble sequence, galaxies would eventually evolve from Sd to S0/Sa (Pfenniger 1993). More recent results from N-body simulations support this evolutionary view of the morphology (Friedli & Benz 1995; Walker et al. 1996; Norman et al. 1996).

2. Fraction of b/p bulges and barred disk galaxies

In a complete sample of edge-on disk galaxies selected from the RC3 (de Vaucouleurs et al. 1991) with D25>2 arcmin (∼1350 galaxies) we characterized bulges by their degree of b/p-shape (type 1, 2, and 3), as non-b/p, or unclassifiable using the "Digitized Sky Survey". The main result is that 46% of all classifiable galaxies have a b/p bulge. The frequency distributions of galaxies with b/p bulge and barred galaxies (all face-on disk galaxies in the RC3) binned by morphological type show the same general dependences for the later types (Sa - Sd). The maximum is at Sc. Early type galaxies (S0/S0a) with b/p bulge have a disproportionally high fraction.

3. Box/peanut bulges and satellite systems

A very surprising result comes from statistics of b/p bulges in isolated spiral galaxies (N=24) (Zaritsky et al. 1993; Zaritsky et al. 1997). These galaxies in the field have the advantage that gravitional forces of nearby massive galaxies can be neglected. The only possible interaction can happen with a small companion so that its influence can be investigated by comparison with isolated galaxies without satellites. We find that 90% of the classifiable galaxies with satellites have b/p bulges. In contrast, there is no galaxy from the control sample without satellites showing a b/p bulge.

4. Box/peanut bulges and the influence of environment

The mean density parameters - local density ("Nearby Galaxies Catalogue", Tully 1988), number of galaxies per sq degree, projected distance to the nearest, second nearest, and 10th nearest neighbour (Lauberts & Valentijn 1989) - are for galaxies with (type 1 + 2) and without b/p bulge with regard to the errors nearly the same. There is only a very weak tendency that the existence of b/p bulge is correlated with a higher density of galaxies.

The analysis of galaxies (N=18 resp. N=13) in the Virgo- and Ursa Major-Cluster shows that there the fraction of b/p bulges is the same as for the complete galaxy sample. However, for galaxies with a b/p bulge of type 1 we see that ∼2/3 have satellites within an area 2× diameter of the galaxy and ∼15% are interacting with larger galaxies.

Therefore we conclude that the small scale environment is more important than the large scale mass distribution for the existence of b/p bulges.

5. Conclusion

Box/peanut bulges preferentially occur in disk galaxies with companions which can be detected in ∼2/3 of the most distinct b/p bulges. The favorable model scenarios for the development of b/p bulges therefore are resonances at a bar triggered by galaxy interaction or by an infalling satellite in an otherwise stable disk. However, currently it can not totally be excluded that some b/p-structures result from bars produced by dynamically cold disks through a global instability or directly from material accreted from infalling satellite companions (soft merging).

References

For further reference see the homepage of the first authors PhD Thesis.
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First version: 07thAugust,1998
Last update: 08thOctober,1998
Jochen M. Braun   &   Tom Richtler
 (E-Mail: jbraun|richtler@astro.uni-bonn.de)