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1. Introduction

Non-axisymmetric distortions (bars, ovals) in inner parts of disk galaxies are recognized, due to N-body simulations including gas (e.g. Friedli & Benz 1993; Combes 1994), to be an efficient mechanism for driving the interstellar medium (ISM) into the nuclear region. Numerous observations complete the picture by showing that various kinds of central activity, like Seyfert nuclei (e.g. Hummel et al. 1987), LINERs and starbursts (e.g. Devereux 1989; Telesco et al. 1993) are often correlated with the presence of bars (note however counter-examples of many Seyferts (McLeod & Rieke 1995), as well as galaxies with strong IR excess (Pompea & Rieke 1990), that do not show any non-axisymmetric deviation).

While a large-scale bar is probably sufficient to fuel a starburst occuring inside a few hundred parsecs, it seems unable to bring the ISM down to the scale governed by a central blackhole (<10 pc) that presumably powers active galactic nuclei (AGNs). The "bar-within-bar'' scenario was proposed by Shlosman et al. (1989) to overcome the problem: first, a large-scale bar accumulates gas in a sub-kpc nuclear disk, that undergoes, when becoming massive enough, a secondary bar-forming instability, susceptible to funnel the ISM down to the BH region. The possibility to create such a double-bar, with the inner component rotating at a higher angular rate, was demonstrated in simulations of Friedli & Martinet (1993).

In turn, searches for inner isophotal twists that would observationally confirm the existence of double-bar configurations were initiated. Preexisting detections of twists in spiral galaxies (de Vaucouleurs 1974; Kormendy 1979, 1982; Jarvis et al. 1988; Pompea & Rieke 1990; Buta 1990; Buta & Crocker 1993, BC93 hereafter) were substantially multiplied due to near-IR observations of Shaw et al. (1993, 1995) and BVRI survey of Wozniak et al. (1995, W95 hereafter). Further twists in the near-IR were reported by Elmegreen et al. (1996, E96 hereafter) who summarized the preceding surveys: 51 isophote twists were discovered amongst 80 barred spirals and lenticulars (the frequency is insignificant since galaxies where the twist was expected were observed preferentially). The last authors also examined blue plates in the Sandage & Bedke atlases and found 18 additional galaxies displaying the twist. Finally, Rauscher (1995) adds 5 other examples in the near-IR, raising the number of twist detections to 74.

A (considerable) part of twists can be due to projection effects on triaxial structures - bars, bulges or combination of both - with varying excentricity but no intrinsic (i.e. face-on viewed) variation of isophotal position angle.

For the intrinsic twists, a scenario competitive with the bar-within-bar hypothesis was suggested by Shaw et al. (1993) on the grounds of the orbital structure inside a bar: a gaseous ring between two inner Lindblad resonances (ILRs), phase-shifted with respect to the main bar due to its association with x2 orbits, perturbs gravitationally the stellar component thus causing its isophote twist. In contrast to the double-bar of Friedli & Martinet, the perturbed region can be tilted only towards the leading side of the main bar, both components rotating at the same pattern speed.

Recently, Davis & Hunter (1995) and Friedli (1996) extended the panorama of double-bar dynamics by considering counter-rotating nuclear bars.

There are numerous open questions concerning the twists in disk galaxies: What is the frequency of the twist phenomenon? What is the fraction of intrinsic twists? How frequent are triaxial bulges? Is there a significant correlation between the intrinsic twists and the presence of nuclear activity? Are the intrinsic twists correlated with inner/nuclear rings? What mechanism is responsible for the intrinsic twists: bar-within-bar instability or gas perturbing stars between the ILRs, or both? How the twist properties vary along the Hubble sequence?

The above questions have no definitive answers mainly because of incompleteness of existing surveys, their bias towards galaxies with enhanced nuclear activity, insufficient resolution close to galactic centers where the twists occur, and projection effects.

This survey is intended to enlarge the set of disk galaxies showing the nuclear isophote twist and to quantify it for future statistical purposes. The paper is organized as follows: Sect. 2 summarizes observations and data reduction procedures including the ellipse fitting on isophotes. Section 3 deals, on a qualitative level, with projection and deprojection of bars and double bars, since this problem is crucial for establishing meaningful statistics and conclusions about the nature of twists. Individual galaxies are shortly described in Sect. 4, conclusions are outlined in Sect. 5. The contour plots as well as profiles of ellipticity, position angle and surface brightness along bars are given in the appendix.

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