Messier 81 (NGC 3031)

Observatory / Course Archives / ASTR 212 Spring 2019 / Leon

New Year’s Eve 1774, astronomer Johann Elert Bode looks up to the sky and discovers two nebulous objects very close together in the northern night sky. Several years after Bode recorded his findings, French astronomer Charles Messier looks at the objects and catalogues them in his infamous "Messier Catalogue", a catalogue in which he recorded unexplained comet-like objects observed in the sky. Messier gives them the names M81 and M82; the greater of the two, M81 is also referred to as “Bode’s Galaxy” since, Bode was the one who made the initial discovery. M81 is an unbarred spiral galaxy, categorized as a SA(s)ab type galaxy, located in the Ursa Major constellation. And because M81 has such a high luminosity, it is easily visible to an observer gazing at the night sky with binoculars. A black hole about 15 times the mass of the Milky Way’s is located at the central of M81. Although the galaxy can be seen fairly well, the distance to M81 it is approximately 11.74 million light years away; this corresponds to a linear size of about 90 thousand light years.

M81 is a Grand Design spiral and by studying its' structure and the visible light emitted, we can learn about what is going on in the galaxy. In the arms, we see that there are more purple/violet/blue tones. These areas are where one is most likely to find younger-hotter stars. Since the stars emmitt hot blue light, it also means that they will burn out more quickly, thus, the stars had to have been formed more recently and are young. The cloudy purple colors that hug the arms is due to those new stars forming and heating up the gas around it. When ionized, it produces the beautiful colors we see in the arms. In the center of the galaxy we see more white/yellow colors due to older stars residing there. Older stars are lower in temperature, which also corresponds to longer wavelengths. As a result, we see more red/yellow in the interior. There is also an Active Galactic Nucleus (AGN) in the center of the galaxy that makes the galaxy so luminous. The high luminosity is a result of a constsant devour of matter closest to the center. One other feature of M81 are the noticeable brown streaks following the arms of the spiral. These are made up of interstellar material like dust due to galactic collisions with nearby galaxies. This dust is a major component because it provides the materials needed for star formation, which we noted took place in those outer edges of the galaxy.

Non-Optical Observations:

Figures 1 and 2: The first image is taken by the National Radio Astronomy Observatory’s Very Large Array Sky Survey (NVSS). It shows the radio continuum of M81 and a very noticeable second bright spot that is just as bright as the AGN of the galaxy. The second image is taken by Roentgen Satellite (ROAST) and shows the x-ray emission of another more faint object nearby.

Optical wavelengths are very useful since they are collecting the visible light of an object in space and returning a focused image that we can see and study. However, sometimes there can be hidden information because the visible light spectrum doesn’t reveal the whole story. Above are images taken by two non-optical wavelength sources: the National Radio Astronomy Observatory’s Very Large Array Sky Survey (NVSS) and the Roentgen Satellite (ROAST). These images reveal that a mysterious object illuminates within Bode’s Galaxy. The first image in Figure 1, was taken by the radio NVSS and converted to a “radio continuum” map. This kind of image is made by radio emission that is passed through a filter and collects ionized hydrogen over time, thus giving it a more broad look. Not only does this radio map show us that M81 puts out radio waves, but it also shows us something peculiar.

In the center of M81 is the AGN. This AGN produces bright radio waves which can be seen in the center of each image. However, another point just as bright as the AGN is seen south of the galaxy's nucleus. When looking at the Rehoboth image, there isn’t anything that bright interrupting the galaxy. The only things we see are three typical bright stars. So, what could it be? In order to figure this out, we turn again to another non-optical wavelength telescope: x-rays. In Figure 2 we see that instead of two bright points, there are three. Not only then does the object emit radio waves, but it also emits x-rays, so putting that together, the object must be a supernova. Supernova emit large amounts of radio waves and x-rays, which is why we see a large radio point and another x-ray point. As a result, if we were only to observe the optical wavelengths of M81 (i.e. the Rehoboth image) then this mysterious object would literally not have been illuminated. Today the supernova is cataloged as SN1993J and in order to find this supernova in the optical image, it is easier to locate the three stars mentioned earlier that lay just south of the AGN. The three stars create a triangle and the one on the farthest most right of the triangle is the supernova!

References:

“Messier 81– the Bode Galaxy” <https://www.universetoday.com/47054/messier-81-1/>

“Messier 81” <https://www.nasa.gov/feature/goddard/2017/messier-81>

“M81 -- Spiral Galaxy (Type Sb)” ,http://coolcosmos.ipac.caltech.edu/cosmic_classroom/multiwavelength_astronomy/multiwavelength_museum/m81.html>

NED info: <http://ned.ipac.caltech.edu/cgi-bin/nph-objsearch?objname=M81&img_stamp=YES&list_limit=9&extend=no>

Right Ascension (J2000)	09:55:33.5
Declination (J2000)	+69:04:02
Filters used	B (Blue), C (Clear), R (Red), V (Green)
# of Exposures	B = 13, C = 30, R = 13, V = 4
Exposure time per filter	B (240 sec), V, R, C (120 sec)
Date/time observed	February 13, 2019

Data reduction:

The initial image of Messier 81 was taken using the Calvin-Rehoboth Robotic Telescope in New Mexico. The telescope uses a charge-coupled device (CCD) camera, which is more sensitive to blue light, to collect data from exposures with four different filters (Red, Green, Blue and Clear) over a span of time. Since the CCD camera is more red-sensitive, different numbers of exposures for each filter were needed. Before being able to use the images effectively, a calibration, bias frame and dark image are created so to get rid of any instrumental effects, overscan, bias, thermal noise and read-out noise that comes from the CCD. A flat-image is also taken to measure the effective response of each pixel due to dust, sensitivity variations and vignetting. After, removing all the imperfections, a clear and colorful image is created by combining all the images into one. However, that isn’t the final image. Tools such as color balance, saturation and gamma are used to emphasize the feature of the image. In the initial image for M81, the color balance was set to Red(R): 2, Green(V): 1, and Blue(B): 3.5. The luminance weight was set to 0% and the saturation was at 165%. The final image used in this page was compressed to a JPEG using a linear only fit.