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Astr212 Galaxy Projects, Spring 2009

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The Splinter or Knife Edge Galaxy (NGC 5907), Luke Leisman

NGC 5907

A shimmering dagger in the night sky, the Knife Edge or Splinter galaxy is a sight to behold from a small telescope as it hangs, poised, in the heart of Draco the Dragon. At a distance of around 200 million trillion miles (approximately 10 Megaparsecs or 40 million light years) from earth, the Splinter Galaxy measures over 180 thousand trillion miles (linear diameter of 31 kpc) across and shares a lot in common with the Milky Way Galaxy (approximately 50 kpc across) where we live.

One similarity is the galaxy's disk-like shape. Galaxies are generally classified into two main types, ellipticals, that look like big blobs on the sky, and spirals, which look like giant swirling disks. The Splinter Galaxy and the Milky Way are both spirals; from earth we see the disk of the Splinter Galaxy almost edge on, like a giant CD tilted away from the viewer. Unlike a CD however, both the Milky Way and the Splinter Galaxy contain a bright bulge in the center that contains a super massive black hole.

Professional astronomers go further and classify NGC5907 as a SA(s)c galaxy. This means that it has loose and clumpy spiral arms which, while not visible in the above photo, are still present. The classification also reveals that compared with other spiral galaxies, NGC5907 has a small central bulge. In addition, the (s) designation tells us that the NGC5907 does not contain any rings of stars.

Another way that the Splinter Galaxy is similar to the Milky Way is in its color. We can see that, especially near its outer edges, the galaxy has a definite bluish hue. This blue light comes from hot, bright giant stars whose light dominates the light of other stars in the galaxy. Hot blue stars burn faster than colder, red stars, so they have much shorter life times, several million years verses several billion. Thus, the bluish color indicates that stars are actively forming, since the blue stars haven't yet burned up.

In addition to indicative colors, the Knife Edge displays a slight warp - rather than being completely flat it bends up or down at the edges like a floppy pancake. NGC 5907 radioThis warp is probably best seen by looking at radio waves from the galaxy rather than light we can see with our eyes. The figure at the left from Shang et al. 1998 maps the radio emissions detected by the Very Large Array onto a visual picture of the Splinter Galaxy. Picturing the map in three dimensions, one can see how the galaxy bends up near the bottom of the image and down near the top. This warp is important to astronomers in determining how the galaxy formed (see below).

Why we look at the galaxy in radio rather than in optical has to do with physical processes going on in the galaxy itself. Radio waves are emitted by cold gas clouds between stars, which gives astronomers a way of mapping cold materials in galaxies that would not otherwise be visible. Cold atomic hydrogen has one electron spinning as it moves around a central proton. When that electron reverses its spin it emits a radio wave, which we can detect back on earth. Beyond just mapping hydrogen clouds, radio emissions are important in determining rotation curves of the galaxy (how fast it is rotating at different distances from the center) and how much dark matter is in the galaxy.

While this bent dagger, like most spirals, is a treat for the eye, the Splinter Galaxy is more than just a pretty sight. Discovered by William Herschel, the discoverer of Uranus, in 1788, the galaxy has been important in the study of supernova, and in galaxy structure. Now the galaxy is of special interest to astronomers not so much for its warped structure, but rather because of a recently discovered faint trail of stars surrounding the galaxy, as we can see in this enhanced image from R Jay Gabany of the Blackbird Observatory:

NGC 5907 with tails

This trail is of interest because it gives clues into how galaxies form and into how this galaxy formed. Astronomers think that the wispy, "ghostly" trail is the remnant of a smaller dwarf galaxy that smashed through the middle of NGC5907. In the process the Knife Edge Galaxy ripped the smaller galaxy apart, causing it to be stretched into the wisps we see today. This process of galaxy collisions and of galaxy stripping is important in determining how galaxies form. David Martinez-Delgado of the Instituto de Astrofisica de Canarias in Spain and his research group show that these wisps of low-density stars are all that is left of the smaller dwarf galaxy which has lost most of its mass to the Splinter Galaxy, and even more to star formation and globular clusters beyond the galaxy. They connect this evidence of the merger of the two galaxies as evidence of the current "Hierarchical" model of galaxy formation. what this model basically says is that structures in space form on all scales at the same time. In other words, galaxies can be built by the combination of smaller galaxies while also forming out of larger clouds. Galaxies then evolve over time as the continue to collide with other galaxies, and as larger galaxies like the Splinter digest smaller dwarf galaxies.

Also of interest is a distinct dust lane blocking some of the light from part of the galaxy. Calculations from the Calvin images reveal that the dust lane blocks enough green light that the galaxy appears slightly more than 2.5x fainter (approximately 1.1 magnitudes of extinction) and that the light is reddened by about half of a magnitude, or just under 2x redder from the dust. We can further see the impact of the dust by looking at the light that the galaxy emits at near infrared wavelengths (just redder than the eye can see). The composite image below of 1.11-2.32 micron light from NGC 5907 near infraredthe 2 Micron All Sky Survey (2MASS) does not show any dark lanes, indicating that the extinction was indeed dust, since dust does not block near infrared light in significant amounts. Rather, the near infrared image shows the light from redder, older stars in the Knife Edge Galaxy.

From the information contained in the pictures we can also calculate the optical depth and column density of the galaxy. The optical depth gives an indication of how easily light can pass through the dust, while the column density indicates how closely packed the dust is. Calculations give an optical depth of 1.03 which is around average for edge on spiral galaxies, and a basic assumption of the average size of a space dust particle to be 200nm gives a column density of 5.5*10^12 m^-2.

Much of this information about dust come from the Knife Edge galaxy's light profile. A light profile is like a cross sectional slice down the middle of the galaxy graphing how bright the galaxy is at all different points along it.

NGC 5907 light profile

The dip of the red line (the right half of the galaxy) under the blue line shows clearly that less light is coming from the right half of the galaxy due to dust. furthermore, we can see that this dust lane occurs at a radius of around 0.4 kpc on the sky. Light profiles are useful not only in determining dust extinction, but also in determining many physical features of the galaxy including the ratio of the bulge light to disk light and structural information for classifying the galaxy. One can see the bulge in the quick rise in the amount of light on the left side of the light profile graph (the center of the galaxy).

The Calvin images in this web page were reduced using Maxim, with standard bias and dark subtraction, and flat division. The images in each filter were aligned and median combined into composite images. These images (one in each of the 4 bands) were then color combined in the ratio of 1 red (R filter) to 1.5 green (V filter) to 9.5 blue (B filter). The color was then saturated to 200%. The composite file was then saved using a gamma stretch of .5 to bring out some of the fainter detail of the galaxy.

References:

Elmegreen, Debra Meloy. Galaxies and Galactic Structure. Upper Saddle River, NJ: Prentice-Hall, 1998.

Kutner, Marc L. Astronomy: A Physical Perspective. 2nd ed. Cambridge, UK: University Press, 2003.

Martinez-Delgado, David, et al. "The Ghost of a Dwarf Galaxy: Fossils of the Hierarchical Formation of the Nearby Spiral Galaxy NGC 5907."The Astrophysical Journal, 689:184-193, 10 December 2008 <http://adsabs.harvard.edu/abs/2008ApJ...689..184M>.

Nemiroff, Robert and Jerry Bonnell "The Star Streams of NGC5907." NASA Astronomy Picture of the Day. 19 June 2008. <http://apod.nasa.gov/apod/ap080619.html>.

Shang, Zhaohui, et al. "Ring Structure and Warp of NGC5907: Interaction with Dwarf Galaxies." THE ASTROPHYSICAL JOURNAL, 504:L23–L26, 1 September 1998 <http://www.iop.org/EJ/article/1538-4357/504/1/L23/985308.html>.

This research has made use of the NASA/IPAC Extragalactic Database (NED) which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration.

Right Ascension (J2000) 15:15:53
Declination (J2000) +56:19:33
Filters used blue(B), green(V), red(R), and clear(C)
Exposure time per filter 18x300 seconds in C, 9x300 seconds in B, 5x300 seconds in V and 3x300 seconds in R
Date observed

March 2, 2009 (C)
March 7, 2009 (CBVR)