The Nature of Boxy/Peanut Spiral Galaxies: Overview and New Results

M. Bureau, K. C. Freeman, PASA, 14 (2), in press.

Next Section: Preliminary Results
Title/Abstract Page: The Nature of Boxy/Peanut
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Contents Page: Volume 14, Number 2

Observational Program


The main goal of our observational programs is to determine which mechanism is at the origin of boxy/peanut-shaped bulges, particularly to test the accretion and bar-buckling instability scenarios which were discussed in the previous section (§2). In addition, if most boxy/peanut galaxies prove to be edge-on barred spirals, the data will be used to study the vertical structure of bars and seriously test 3D bar models observationally for the first time. We will briefly describe our sample of galaxies and then discuss each type of observations (optical long-slit spectroscopy, HI line-imaging, near-infrared imaging, and optical imaging) and the goals we are trying to achieve with each of them.

Finding the Objects: The Galaxy Sample


Our sample of galaxies is composed of 32 edge-on spirals, of which about 2/3 have boxy or peanut-shaped bulges and 1/3 have other various types of bulges for comparison purposes (spheroidal, ellipsoidal, and exponential). Most of the galaxies were selected from existing catalogues (Jarvis 1986; Shaw 1987; de Souza & dos Anjos 1987; Karachentsev, Karachentseva, & Parnovsky 1993) and they are all accessible from the south (tex2html_wrap_inline399). In order to get enough spatial resolution in the long-slit spectroscopy but still have objects small enough to be practical to image with our near-infrared camera, we have selected objects with bulges larger than tex2html_wrap_inline401 and total (disk) size smaller than about tex2html_wrap_inline403 (at the 25 B mag arcsectex2html_wrap_inline407 level).

Testing the Bar Hypothesis: Optical Long-Slit Spectroscopy


Although some features of an edge-on spiral galaxy can give an indication that the galaxy is barred (such as a plateau in the projected light distribution), the usual photometric criteria used to identify bars in spiral galaxies are useless when the galaxy is seen edge-on. But, Kuijken & Merrifield (1995) (see also Merrifield 1996) have shown that it is possible to identify a bar in those edge-on systems based on the particular dynamics of barred spirals. They show that the gas in an edge-on barred spiral galaxy has a different position-velocity diagram (PVD) than the gas in an axisymmetric disk, and that the ``figure-of-eight'' seen in the PVD is a signature of triaxiality (see Fig. 1a). The origin of this feature is qualitatively easy to understand. The collisional nature of the gas implies that it follows closed non-overlapping and non-intersecting orbits (the gas would shock otherwise). In a barred potential, around corotation, there are no such orbits: the orbits are elongated and change orientation by tex2html_wrap_inline409 at corotation, therefore crossing each other. Thus, a gap will appear in the gas PVD of a barred spiral galaxy (compared to an axisymmetric spiral) due to the unavailability of orbits around corotation (see Fig. 1 in Kuijken & Merrifield 1995 and/or Fig. 3 in Merrifield 1996). Stellar absorption lines should show a weaker but similar behaviour. This feature can then be used to identify barred galaxies among edge-on spirals.

Figure 1: Structure and kinematics of some sample galaxies. Top panel: Blue image of the galaxy (DSS). Bottom panel: Position-velocity diagrams (PVDs) of the ionised gas along the major axis of the galaxy. (a) NGC 5746, (b) NGC 6722, (c) IC 4767, (d) IC 5096, (e) ESO 240-G 11, (f) NGC 4703.



Consequently, the most important part of our observational program is optical medium-resolution long-slit spectroscopy. By taking long-slit emission-line spectra along the major axis of the sample galaxies, we can identify which of the sample galaxies are barred and indirectly test the validity of the bar-buckling formation scenario for boxy/peanut bulges. If no emission lines (e.g.\ Htex2html_wrap_inline411, [N II]) are detected, we have to rely on stellar absorption lines. Because of the nature of our sample, we will be able to say if a boxy/peanut-shaped bulge necessarily implies the presence of a bar, and vice-versa.

The long-slit spectroscopic results will also shed light on other aspects of the sample galaxies' nature. For example, we will detect gas or stellar counter-rotation, we should be able to detect the presence of a nuclear bar, and we will be able to look for evidence of bar destruction (e.g. high velocity dispersion in the plane) in the galaxies where no bar is detected and hopefully learn about those destruction mechanisms. In addition, by covering a large wavelength range, we can obtain ratios of gas emission lines useful in diagnosing the physical conditions in the interstellar medium (ISM) of our galaxies. This is particularly interesting because we get information on small scales (e.g. nuclear activity) and large scales (e.g. shock excitation of gas in bars), especially since bars are often considered an efficient way to feed material to the central part of active galaxies.

Section 4.1 presents preliminary optical long-slit spectroscopy results for some galaxies in our sample. The observations are discussed in § 5.

Penetrating the Dust Barrier: HI Line-Imaging


Ideally, to study the vertical structure of bars/bulges, one wants to work with systems as edge-on as possible. This reduces the problems of deprojecting the light distribution of slightly inclined systems and removing the contamination of the bulge light by the superposed disk, an operation which is model dependent. It is therefore very important to have a few ``perfectly'' edge-on boxy/peanut-bulge galaxies in our sample. But, the absorption in the plane affects the observed profile of the emission lines in perfectly edge-on and dusty systems, and prevents the use of the Kuijken-Merrifield method to identify which systems are barred. The solution to this problem is to go to regions of the spectrum where the disk is optically thin, e.g. near-infrared or radio wavelengths.

In the near-infrared, the H Brtex2html_wrap_inline419 emission line is well suited for this kind of work as the disk is probably optically thin at the K-band. Unfortunately, the line is quite weak in most galaxies and near-infrared spectrographs with adequate resolution are not common instruments. A more attractive solution to the absorption problem is to use line-imaging with a radio synthesis telescope. The HI (tex2html_wrap_inline42321 cm) line is ideal as even the very dusty edge-on spiral galaxies are probably optically thin at 21 cm. We can then get the velocity distribution through the dust lane all along the disk, and with a spectral resolution much higher than possible optically. Because of the 2D spatial coverage, we can also study the behaviour of the gas as a function of height above the plane. In addition, the HI data allow to directly look for evidence of accretion events (e.g. debris or tidal tails). Thus, information on the likelihood of both of the two formation mechanisms proposed for boxy/peanut bulges is obtained simultaneously. Section 4.2 presents preliminary results of HI line-imaging for one galaxy in our sample (IC 2531). Those observations are also discussed in § 5.

Although the optical spectroscopic data contain less information than the radio synthesis data, they remain our primary tool because radio synthesis observations are useful only for the largest and most HI-rich galaxies in our sample (limited spatial resolution and sensitivity).

Studying the Vertical Structure of Bars: Near-Infrared Imaging


Once the edge-on galaxies with a bar are identified by the Kuijken-Merrifield method, near-infrared K-band imaging allows the study of the vertical structure of the bars and the investigation of the problem of their thickness without being significantly affected by dust absorption. For the first time, the 3D models of barred spiral galaxies constructed over the years will be seriously tested. For example, we will be able to derive the shape and vertical scaleheight of the bar and disk separately, and study how those vary with radius and Hubble type. In a model where a peanut bulge grows out of the thickening of a bar, Sellwood (private communication) obtains scaleheights for the inner part of the bulge/bar, the end of the bulge/bar, and the outer disk in the approximate ratios 2:3:1. Very little is known observationally about those issues at the moment (Andredakis & Sanders 1994). It will also be possible to compare directly the lengths of the boxy/peanut bulges, in units of disk scalelength, with the predicted lengths of bars from simulations. As mentioned previously, this is one of the possible problems of the bar-buckling formation scenario. Because the light distribution obtained from near-infrared photometry is relatively unaffected by dust, it is also much better than the usual optical photometry when used to compare with the density distributions predicted by various models, as it is possible to extend the observations/models comparison into the inner disk region where absorption is usually a problem.

Although we are not presenting any near-infrared imaging results in this paper, the observations are well underway and look promising.

Testing the Accretion Hypothesis: Optical Imaging


Information on the stellar populations of the bulge, disk, and bar, and their approximate ages is very helpful in order to discriminate between the different scenarios proposed for the formation of boxy/peanut bulges. In particular, this allows to discriminate between the scenario where the boxy/peanut shape arises from the thickening of a bar (homogeneous stellar population and identical ages for the disk and bar/bulge) and the scenario where it arises from the accretion of satellites (different stellar populations and ages for the disk and bar/bulge). The necessary information can be obtained with multi-color photometry. Imaging in many bands is also needed to study the structure of the galaxies as the colors carry information on the M/L ratio of each component of the light distribution, and allow the contribution of each component of the galaxy to the mass distribution to be evaluated. We are at the moment doing B, V, I, and K-band photometry of the sample galaxies, and adding the U-band is being considered as it would help us break the age-metallicity degeneracy. An interesting application of this data is to study the vertical distribution of stellar populations, and try to understand how the disk stars map into bar/bulge stars when/if the bar buckling instability occurs. Obviously, the major problem faced when using the optical multi-color photometry is that it is strongly affected by dust absorption and can therefore be used reliably only in dust-free systems or far from the disk.

No optical imaging results are presented in this paper but again the observations are underway.

Next Section: Preliminary Results
Title/Abstract Page: The Nature of Boxy/Peanut
Previous Section: Boxy/Peanut Galaxies Formation Scenarios
Contents Page: Volume 14, Number 2

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