Do the Two Outer Arms Lead in NGC 4622?

G. Byrd, R. Buta (Univ. of Alabama) T. Freeman (Bevill State College)

This website accompanies Abstract 52.08 presented at the 199th meeting of the American Astronomical Society in Washington, D. C. It also accompanies this press release.

Color Image of NGC 4622 based on new Hubble Space Telescope Observations. North is to the upper right and east is to the upper left.
The main conclusion of our study is that the two strong spiral arms seen in the above photograph have the leading sense (i.e., they spiral outward into the direction of rotation, which is clockwise in the image). How was this determined? Two pieces of information were needed: (1) a Doppler shift map, showing which half of the galaxy is receding from us and which half is approaching, due to the galaxy's rotation; and (2) a high resolution image to reveal the characteristic dust asymmetry that tells us which edge of the galaxy is closest to us (i.e., the near side of the disk).
A Doppler shift map, obtained by R. Buta and G. B. Purcell in 1992 with the 4-meter telescope of Cerro Tololo Inter-American Observatory, showed that the north half of the galaxy recedes (see radial velocity map). The Doppler shift map alone cannot tell us the sense of winding of the arms because it only tells us which side is receding, not the sense of rotation.
To determine the sense of rotation, we need to know which side of the galaxy is tipped towards us. To do this, we take advantage of a characteristic dust asymmetry that occurs whenever a disk-shaped galaxy with a strong central bulge is viewed tipped to the line of sight. The following schematic shows that the light of the bulge is viewed mostly through the dust on the near side, while the dust is viewed mostly through the bulge on the far side. This allows the dust in the near half of the disk to be seen as clear thin filaments while those on the far side are less obvious. To determine the near side, all we do is find which side shows the strongest dust, especially near the center. This side will be the near side.

Previous ground-based observations of NGC 4622 did not show the characteristic dust asymmetry expected because the galaxy is very distant (200 million light years) and the Earth's atmosphere blurred the details enough to render them invisible. However, the superior resolution of the Hubble Space Telescope has allowed the dust lanes to be detected for the first time. The color image above shows them on the east (left) side of the galaxy. To make them more obvious, the following image shows a closeup of the central region where the light of the bright bulge has been muted. The dust lanes appear as the whitish filaments on the left side of the image. These lanes are seen close to the center, and demonstrate unambiguously that the east edge is the near side of the galaxy's rotating disk. The solid line in this image is the line of nodes, which represents the intersection between the plane of the galaxy's disk and the plane of the sky.

An image of the inner parts of NGC 4622, based on HST observations, showing the dust lanes (thin light arcs) on the east (left) side. The galaxy is tipped around the line of nodes shown. N stands for the direction of north while E stands for the direction of east. 5" refers to an angle of 5 arcseconds on the sky. The light areas near the bright star are instrumental in nature. Many of the very faint star-like objects in this picture are globular clusters associated with the strong bulge.
We confirm that the whitish areas in the above map are due to dust because they appear redder than their surroundings. This is shown in the following black and white map where redder areas are light and bluer areas are dark. Again, most of the clear dust lanes lie on the east side of the line of nodes. Thus, there is no ambiguity - the east side of the galaxy is tipped towards us. If the north side recedes (circle with + inscribed), then the galaxy is rotating clockwise. In this circumstance, the two outer arms must be leading. This is the first time two strong and obvious spiral arms in a galaxy have been shown unambiguously to be have the leading sense.

This map, also based on the HST observations, highlights dusty zones of reddening in NGC 4622, which are seen as the thin light arcs east of the line of nodes (two solid white lines). The receding (circle with +) and approaching (circle with -) halves are indicated.
The following picture provides a summary of our results in the form of a labeled ground-based image where the bulge has been muted for illustrative purposes.

The leading outer arms and trailing single arm labeled in this picture, which is not an HST picture but a ground-based, processed image. North is to the top and east is to the left, unlike the HST pictures above where north is more to the upper right. The HST dust silhouette locations are around the indicated line, but are too blurred to be seen in this image.
The authors acknowledge the support of NASA/STScI grant GO 8707 for this research. We also thank Z. Levay of the Space Telescope Science Institute for help in producing the color image of NGC 4622.