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

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

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Preliminary Results


We will present here preliminary results illustrating the status of our project (as of September 1996). More galaxies have been observed, but not all data have been analysed yet.

Optical Emission Line Observations


We present new long-slit spectroscopic results for a few galaxies in our sample. The data were taken using the Double Beam Spectrograph on the 2.3 m telescope at Siding Spring Observatory. The spectra were centered on the Htex2html_wrap_inline411 emission line in the galaxies (tex2html_wrap_inline4236563 Å at rest). The galaxies NGC 5746, NGC 6722, IC 4767, IC 5096, and NGC 4703 were observed with a tex2html_wrap_inline443 SITE ST-D06A thinned CCD, while the observations of ESO 240-G 11 used a less sensitive tex2html_wrap_inline445 Loral CCD. All galaxies were observed with a tex2html_wrap_inline447 slit aligned with the major axis (just above the dust lane when present). The spectral resolution is about 1.1 Å FWHM (0.55 Å pixeltex2html_wrap_inline449), and the spatial resolution is tex2html_wrap_inline451 pixeltex2html_wrap_inline449.

The data were reduced in the standard manner within IRAF and rebinned to a logarithmic scale. In order to isolate the emission lines, the combined exposures were sky-subtracted and continuum-subtracted to produce the final spectra (some work remains to be done to get a better subtraction of strong sky lines, see e.g. Fig. 1a).

In the next few paragraphs, we will comment on our observational results for each of the galaxies analysed so far. A more general discussion concerning the implications of the observations will be given in §5. In the figures, we show only the region of the spectrum around Htex2html_wrap_inline411, including the two [N II] lines (tex2html_wrap_inline4236548 Å and tex2html_wrap_inline4236584 Å at rest). Our goal here is to illustrate the range of dynamical features visible in our spectra, and to show what the signature of a bar in an edge-on spiral galaxy looks like (the ``figure-of-eight'' of Kuijken & Merrifield 1995). Each spectrum is accompanied by an image of the corresponding galaxy from the Digitized Sky Survey on the same scale to illustrate the range of galaxy types and morphologies we look at and to allow for a connection to be made between certain dynamical features and galaxy morphologies (e.g.\ ``figure-of-eight'' spectrum and boxy/peanut-shaped bulge).

  • NGC 5746 (Fig. 1a):

    NGC 5746 is without doubt our most spectacular result so far. This is one of the two original galaxies used by Kuijken & Merrifield (1995) to test the signature of a bar on the PVD of an edge-on spiral galaxy. We confirm here their results, but with higher S/N data. NGC 5746 is an intermediate-type spiral and a prototype peanut-shaped bulge galaxy. It is nearby (1720 km stex2html_wrap_inline449) so the amount of spatial detail in the spectrum is quite exciting. For example, just outside the steeply rising inner part of the PVD, one can clearly see a dip in the upper part of the profile. The origin of this dip is not known. The line-splitting (``figure-of-eight'') in the PVD is obvious, extending to about twice the peanut-length, and probably denotes the presence of a bar seen partially side-on. It is seen in all three lines (although superposed on the stellar absorption in the case of Htex2html_wrap_inline411). In fact, it is also easily visible in the two [S II] lines at tex2html_wrap_inline4236717 Å and tex2html_wrap_inline4236731 Å (rest wavelengths). In addition, one can see the line ratios (e.g. ([N II] tex2html_wrap_inline4236584 Å)/(Htex2html_wrap_inline411 tex2html_wrap_inline4236563 Å)) varying with position along the major axis, suggesting that the physical conditions of the ISM are different in the bulge and disk regions.

  • NGC 6722 (Fig. 1b):

    NGC 6722 is a peanut-bulge galaxy very similar to NGC 5746, but more distant (4626 km stex2html_wrap_inline449). Nevertheless, one can still see the ``figure-of-eight'' clearly, especially in the redder [N II] line. Again, the split in the lines extends to about two peanut-lengths, and the rise is very steep in the inner part of the bulge. We also see many HII regions in the disk.

  • IC 4767 (Fig. 1c):

    IC 4767 also has a peanut-shaped bulge and is relatively distant (3600 km stex2html_wrap_inline449). Contrary to NGC 5746 and NGC 6722, it is an early-type spiral. No dust lane is seen and in fact no emission lines are readily visible in the spectrum before subtracting the bright continuum. After continuum-subtraction, however, the spectrum shows strong gaseous line emission in the very inner part of the bulge. No emission is detected in the outer part of the bulge or in the disk. While Htex2html_wrap_inline411 is strongly suppressed by the stellar absorption, [N II] (tex2html_wrap_inline4236584 Å) shows very rapidly increasing rotation in the center. The [N II] line does not show any features and it is not clear where the line emission originates, although an inner disk like that seen in NGC 128 is the most probable explanation (Emsellem 1997).

  • IC 5096 (Fig. 1d):

    IC 5096 is another intermediate-type spiral at moderate distance (3087 km stex2html_wrap_inline449). Although the bulge does not display a peanut shape, it does possess boxy isophotes. It also displays a clear, strongly split PVD in the [N II] line at 6584 Å. The extent of the line-splitting seems again to be about two bulge-lengths. This result is exciting and probably indicates the presence of a bar seen end-on. The line ratios in the inner bulge region also seem different than in the rest of the galaxy.

  • ESO 240-G 11 (Fig. 1e):

    ESO 240-G 11 is a nearby (2843 km stex2html_wrap_inline449) late-type spiral with an extended disk and a very small flattened bulge. In fact, the bulge shape is hard to determine, as it blends with the disk. The PVD displays a typical late-type galaxy behaviour over most of its length: the peak (rotation curve) is slowly-rising in the inner parts and completely flat in the outer parts. But, again, the redder [N II] line shows structure in the bulge region and, despite the low S/N, a split in the line is seen (at least, the PVD does not behave as it should in an axisymmetric disk). Although no spatial variation in the line ratios is seen in the inner parts, the feature seen in the PVD remains a surprising and unexpected result.

  • NGC 4703 (Fig. 1f):

    NGC 4703 is another relatively distant late-type spiral (4340 km stex2html_wrap_inline449). It has a flattened bulge embedded in an almost edge-on dusty disk. NGC 4703 is perfect to demonstrate the effect of dust absorption on our long-slit spectroscopy. The PVD shows a featureless slowly rising solid-body rotation curve. This is exactly what is expected from an optically thick rotating disk, as one can only see the light from an outer disk annulus (see e.g. Bosma et al. 1992). In galaxies with a strong dust lane like NGC 4703, the need to observe emission lines in a region of the spectrum where the disk is optically thin is obvious. Otherwise, the dynamics in the central regions of the galaxies is simply hidden from view.

HI Observations


We present here new results of HI radio synthesis observations of the galaxy IC 2531. The observations were carried out using the Australia Telescope Compact Array (ATCA) in the 6.0A configuration for a 12 hr period (1996 April 6). Observations in other configurations are planned to increase the S/N and get a better uv coverage. We used a 8 MHz bandwidth with 512 channels in each polarisation centered on the HI line in the rest frame of the galaxy. This yielded a velocity resolution of 3.3 km stex2html_wrap_inline449 channeltex2html_wrap_inline449, more than seven times better than the optical spectroscopy. The spatial resolution was about tex2html_wrap_inline495 (tex2html_wrap_inline497).

The data presented here are extremely preliminary. They have not been edited, calibrated or cleaned: after subtracting the continuum, the data were directly imaged. Nevertheless, qualitative features are probably reliable.

Fig. 2 is similar to Fig. 1, showing a Digitized Sky Survey image of IC 2531 accompanied by its HI major axis position-velocity diagram. IC 2531 is a nearby (2477 km stex2html_wrap_inline449) late-type spiral with a small peanut-shaped bulge, an extended disk, and a strong dust lane. The Htex2html_wrap_inline411 spectroscopy shows strong absorption effects. The HI PVD in Fig. 2 extends to the limit of the optical disk. The rotation curve appears solid-body in the inner parts, but rises rapidly and flattens out in the outer parts, where there is significant structure: line-splitting is visible at least on one side of the galaxy. HI velocity profiles along the disk show the doubly-peaked nature of the profiles clearly. This feature is not visible in our optical spectrum and thus illustrate the usefulness of using radio synthesis imaging when dealing with galaxies with strong dust lanes. It should be noted that here, the split in the HI line starts at the end of the bulge and extends to almost five times its length. One should then ask if the origin of this feature is the same in IC 2531 as it is in galaxies like NGC 5746.

Figure: Structure and kinematics of IC 2531. Top pabel: Blue image of the galaxy (DSS). Bottom panel: HI position-velocity diagram along the major axis of the galaxy. The contours correspond to tex2html_wrap_inline503, tex2html_wrap_inline505, tex2html_wrap_inline507, tex2html_wrap_inline509, tex2html_wrap_inline511, and tex2html_wrap_inline513 (not flux calibrated).

Next Section: Discussion
Title/Abstract Page: The Nature of Boxy/Peanut
Previous Section: Observational Program
Contents Page: Volume 14, Number 2

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