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Galaxy M108 (NGC 3556)
Matthew de Wit

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Galaxies are nearly the largest structures in our universe. They contain stars, dust clouds, planets, and large star forming gas clouds. The galaxy that we live in is called the Milky Way, a spiral galaxy. M108 is a bar spiral galaxy located 13.1 MPc away from us. It has a size of around 76,000 ly across, which is about 3/4 the size of our own galaxy. M108 is a slightly peculiar galaxy because of how obscured the core region of this galaxy is by dust, and because of how much gas and dust exist in its arms.

Historically speaking M108 was discovered in 1781 by Pierre Mechian. Pierre Mechain did not record his discovery so the galaxy was rediscovered by William Herschel in 1789.

The light from the galaxy is produced by the stars that reside in the galaxy, as well as by ionized clouds of hydrogen. The colour of this light gives clues as to how old the galaxy and what types of stars are present in the galaxy. The arms have a distinctive blue colour, this means that there are more hot blue stars in these regions. The core is very yellow meaning that most of the hot stars have died off and the older cooler stars are producing the light. The fact that there is so much blue light present in this galaxy means that the galaxy is relatively young at the edges as the hot stars, which have a shorter lifespan, have not died off yet. A pair of features that are unique to M108 are the very bright HII regions that glow red. The can be seen more clearly in the image below.

M108 in Halpha Halpha with Apache Point Observatory 3.5m telescope (via NED on M108).






This image was taken at 6580 Angstroms shows the H alpha regions in M108. H alpha emission is given off by ionized hydrogen atoms. This corresponds to HII regions, which are regions of hydrogen gas energized by young, hot stars. The bright region circled in red on left corresponds to the bright red region in the same position in the optical image. This is one of the large HII regions, the other region that can be seen clearly in the optical image corresponds to the other region circled in red. By mapping these regions one can find the region of the galaxy that are undergoing new star birth and one can find the younger regions of the galaxy.

M108 in Near InfraredNear infrared from 2 Micron All Sky Survey (via NED on M108).










The official classification of M108 is SB(s)cd. This means that it is a spiral galaxy with a bar through the middle of it. This bar is very difficult to see at optical wavelengths, but becomes more prominent at near infrared wavelengths as in the image above. NGC 3556 does not have a ring around the central core. Its core is very small and there is almost no central bulge present. This is denoted by the "cd" classification.

The dust that resides in M108 causes light scattering and blocks light from stars that may be behind this dust. By observing the galaxy in near infrared light, a wavelength of 2 microns, we can see the stars and that lie behind the dust. The reason for this is because the dust does not scatter red light nearly as much as blue light, and so observing at a slightly shorter wavelength allows for us to see behind the dust. The features that become the most pronounced in this image compared to the optical image is the core of the galaxy. The core region of this galaxy is very small and there is almost no central bulge present. This agrees with the classification of the galaxy.

In 2003 a study of M108 was conducted by Wang, Chaves, & Irwin using the Chandra X-ray telescope to investigate what x-ray sources resided in and around the galaxy. A total of 33 distinct objects were found to be emitting x-rays in the galaxy itself. Wang et al did a multi-wavelength comparison of the x-ray data with optical, 6580 Angstroms spectral line, and 2MASS. The image below shows optical and X-ray data. Two of the marked x-ray sources, 24 and 36, correspond to the two large HII regions mentioned earlier. These x-ray sources correspond to a supershell created from supernova. This means that in these regions there were extremely large and hot that created the HI super shells when they erupted in supernova at the end of their life cycle. These super shells can be seen as diffuse x-ray radiation emanating in large bubble out of the disk of the galaxy.

Image and caption from Wang et al: "HST WFPC2 image, compared with the diffuse X-ray emission contours covering the central region of NGC 3556...the three sources with possible optical counterparts are labeled. The center of the galaxy is marked as a plus sign."








Frommert, Hartmut, and Christine Kronberg. "M108." Students for the Exploration and Development of Space. Accessed 2 May. 2011. <>

Wang, Daniel, Chaves, Tara, and Irwin, Judith. "A Chandra Observation of the Edge-on Galaxy NGC 3556 (M108): Violet Galactic Disk-Halo Interaction Revealed". (2003) Astrophysical Journal. 598, 969. <>

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) 11h 11m 30s
Declination (J2000) +55:40'00"
Filters used blue(B), green(V), red(R), and clear(C)
Exposure time per filter 28x120s Clear, 10x300s Blue, 4x300s V, 3x300s R
Date observed

February 8, 2011 (CBVR)
February 28, 2011 (BC)
March 2, 2011 (BRV)
March 4, 2011 (C)

The raw data for this report was calibrated and combined using Maxim by removing the noise and camera vignetting from the images. This was done by calibrating each image according to data and filter type with an appropriate, bias, dark and flat. Maxim auto calibrates to the best possible fit. Once the images were properly calibrated they were first combined according to filter and so four images were produced, one for each filter. These images were then combined to form the colour calibrated image.



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