Messier 101 (Pinwheel Galaxy)

Observatory / Course Archives / ASTR 212 Spring 2019 / Whitten

Messier 101 (M101) is a face-on spiral galaxy that was discovered by Pierre Méchain in 1781. Also known as the Pinwheel Galaxy, it is a spiral galaxy located 6.6 megaparsecs (21.5 million light-years) from Earth and is located in the constellation Ursa Major. Spiral galaxies have multiple arms that extend from the central bulge and appear as spirals because of their rotation. There are two main types of spiral galaxies: normal (S) and barred (SB). Barred spiral galaxies have a bar-shaped central bulge, while normal ones don’t. Other methods of classification are the size of the bulge and the tightness of the spiral arms. M101 is classified as SAB(rs)cd. The “SAB” means that it’s a spiral galaxy with a mildly barred bulge, “rs” signifies that there’s a slight ring around the center, and “cd” means that its spirals are not very tightly bound. This spiral galaxy is also called a flocculent spiral, which means that the spiral arms aren't very well defined.

Other than old stars and a black hole, there isn't much in the galaxy's central bulge. There is little to no star formation because there isn't very much gas and dust located there. The galactic bulge is very yellow in comparison to the galactic disk because it contains older stars. These stars appear yellow or white because they are not as hot and therefore have much longer lifetimes than blue stars. The arms of this galaxy appear blue because they contain many young, blue stars. Spiral galaxy arms have many young stars due to clouds of high density molecular hydrogen and a high rate of star formation. The light we see is being emitted by stars in these regions. There are also some regions where dust is blocking us from seeing what's behind it. This is visible in the image below.

The second image, taken by the Hubble Space Telescope, is a true-color image of the Pinwheel Galaxy. Dust can clearly be seen in the spiral arms and just outsideof the central bulge.

The Pinwheel Galaxy is a grand design galaxy, which means we see it face-on. Unlike many other face-on galaxies, this one isn’t symmetrical. When spiral galaxy arms form, they are pretty uniform due to their rotation. Something that can disrupt this symmetry is interaction with surrounding matter. M101 is asymmetrical due to tidal interactions with neighboring galaxies. Nearby is a galaxy that was likely distorted due to tidal interactions with M101. This is NGC 5474, which is similarly asymmetrical, but much smaller in size. These tidal interactions cause the cloud of interstellar hydrogen gas in the Pinwheel Galaxy's spiral arms to compress, which in turn causes star formation activity. These areas of star formation activity can be detected in ultraviolet and x-ray images, as shown below.

The third image is a composite image that combines infrared data taken by the Spitzer Space Telescope, visible data taken with the Hubble Space Telescope, and x-ray data taken with the Chandra X-Ray Observatory.

The image above is not a true-color image, but is colored in a way to contrast the different areas of the galaxy. Infrared data from the Spitzer Space Telescope allows us to see past the dust that obscures some of the visible light from reaching us. X-ray data from the Chandra X-Ray Observatory allows us to see regions of star formation. Areas with x-ray emission indicate young star populations and star formation because young stars are very, very hot and as a result emit very high energy photons. The x-ray areas of the image above correspond to the bluer, brighter parts of the galaxy that we saw in the image taken with the Rehoboth telescope because young stars burn very bright.

The image taken with the Rehoboth telescope used 3x3 binning because the arms get pretty dim in some places, so there were 1.9 arcsec/pixel. The image showed the galaxy with a major axis of 645 pixels, which corresponds to 969 arcseconds or 0.0594 radians. Using this value and the published distance to the galaxy (6.6 Mpc), the major axis of M101 was measured to be 0.0392 Mpc (128,000 light-years). The major axis has been professionally measured as 0.0521 Mpc (170,000 light-years).This error can be attributed to the fact that the Rehoboth telescope can't register all of the light emitted by the galaxy, making it seem smaller. Another source of error could be that the edges of the galaxy are not a simple boundary. It's really up to observer interpretation where the "edge" is.

Right Ascension (J2000)	14:03:12.6
Declination (J2000)	+54:20:57
Filters used	B (Blue), V (Green), R (Red), C (Clear)
Exposure time per filter	B (300s x 6); V and R (300s x 3); C (240s x 3)
Image dimension	616x759 pixels; 18.5x22.77 arcminutes
Date/time observed	March 18, 2019, 09:22 UT

To obtain this image, I used Calvin's telescope in Rehoboth, New Mexico to image two overlapping fields. I needed to use two fields, one just above the other, because using just one didn't give me as much coverage of the galaxy as I wanted. In any image, there are unwanted artifacts that are left by the telescope.To remove these, I took a series of calibration images called bias, darks, and flats. The bias images are used to find each pixel's true zero-point offset. The dark image is used to remove thermal noise. The flat images are used to remove sensitivity variations, vignetting, and response to dust or other lens artifacts. I took multiple images for each of these, then stacked them with MaxIM to get the master (average) bias, dark, and flat images. Once the master images were created, the bias and dark were subtracted from the raw data. Then the raw data was divided by the flat image. Once the data was reduced, I made master color images from the Clear, Blue, Red, and Green images. For each field of view, the colored images were weighted and combined. Once a colored image was ready for each field of view, the mosaic tool was used to match overlapping stars and accurately combine the two images. For the top image, R was set to 2.5, V was 3, B was 22, and luminosity was weighted at 60%. For the bottom field, R was set to 1.4, V was 2.1, B was 8, and luminosity was weighted at 40%. I waited to combine the two fields together before changing the saturation to 200%.

References:

"Distance Results for Messier 101." NASA/IPAC Extragalactic Database, ned.ipac.caltech.edu/cgi-bin/nDistance?name=MESSIER+101.

Frommert, Hartmut, and Christine Kronberg. "Messier 101." The Messier Catalog, 4 Apr. 2016, www.messier.seds.org/m/m101.html.

"Pinwheel Galaxy." Wikipedia, 11 Apr. 2019, en.wikipedia.org/wiki/Pinwheel_Galaxy.

Kay, Laura, et al. 21st Century Astronomy. 5th ed., New York, W. W. Norton & Company, 2016.

"The Pinwheel Galaxy, M101, in the Infrared." Spitzer Space Telescope, Jet Propulsion Laboratory, www.spitzer.caltech.edu/images/1945-ssc2008-14b-The-Pinwheel-Galaxy-M101-in-the-Infrared.