Messier 109 (NGC 3996)

Observatory / Course Archives / ASTR 212 Spring 2019 / Muyskens

On March 12, 1781, Pierre Mechain discovered a "nebula near Beta in the Great Bear." This object was independently cataloged as by Charles Messier as M 109 and by William Herschel as NGC 3992. Although Messier knew about it first, Hercshell was the first to catalogue this object. Before it was known to be a galaxy Adrimal Smyth described it saying, "From every inference this nebula is a vast and remote globular cluster of worlds." This "fine object" is more truly a vast cluster of worlds.

Messier 109 is an easlily locateable barred spiral galaxy and can be found less than a degree away from the star Phecda, the bottom left star making on the dipper of the big dipper in the constellation Ursa Major. M109 has three small satelite galaxies one of which is indicated in the figure on the right.

M109 is the most distant Messier object from our galaxy. The distance M109 is estimated to be 18.10 ± 1.28  (Lelli); we find a maximum angular size of 6.265 arcminutes, which corresponds to a linear size of 57220 light years.

Messier 109 is a barred spiral galaxy. About half of all spiral galaxies have bars, which are made up of many stars. The bars act like a funnel for gas toward the center and acts like a stellar nursery. Bars are thought to form from a self perpetuating wave from the center of the galaxy that reshapes the orbits of stars. M109 has been used in various studies on how bars effect how gas moves around a galaxy. It is also thought that the satelite galaxies have an effect on the brightness of the bar.

The yellowing center of the galaxy is indicative of an older star population. As seen in the image, the center is very bright which could be a result of the satellite galaxies or other effects in the galaxy. In the center of the galaxy, stars are the main source of light. Moving outward, the spiral arms are bluer in color. This purplish-blue color indicates that star formation is taking place. The purple color comes from emission nebula, which is caused by gas being ionized by extremely hot stars. The blue color is caused by reflection nebula. These nebulae show up blue because the blue light coming from hot stars is scattered by the dust. The presence of hot stars means that these are newly formed stars and indicated recent star formation. The darker areas in between these spiral arms is made up of gas a dust that is not compressed as much as in the brighter areas, so less star formation is taking place. Visible in the image is the satellite galaxy UGC 6969.

The caculation of the size galaxy M109 was done through an analysis of the clear filter image. The major axis (shown in the figure below) length in pixels was estimated to the edge of the galaxy as 284 px, the minor axis as 152.7 px. Using the pixel scale of the CCD (sensor) that was used to capture the image, the length in arcseconds was calculated. From this, it can be converted into radians. The value in radians and the distance is used to calculate the size of hte galaxy using the equation θ = l / d. From this equation and the size in radians, size of the object in light years can be determined. For this calculation of M109, the angular size of the major axis was 375.9 arcseconds corresponding to a size of 56220 lightyears, whereas the angular size of the minor axis was 201.6 arcseconds corresponding to a size of 23164 lightyears.

Messier 109 is not a face-on spircal galaxy which means that the galaxy is at an angle, or inclination in respect to our viewing "plane." The inclination for a galaxy is given in degrees; 0 for a face-on galaxy and 90 for an edge-on galaxy.

To calculate the angle of inclination for messier 109, the major and minor axies must be found as indicated on the figure to the left. The equation for the inclination is

Because the major (a) and minor (b) axies are a ratio, it does not matter what units they are being compared in, be it distance, angular size, or pixels on an image. Using the eqaution and values for the size of the major and minor axies in lightyears, the inclination for M109 is 66°.

References:

Lelli, Federico, et al. "The Small Scatter of the Baryonic Tully-Fisher Relation." The Astrophysical Journal       Letters, Jan. 2016. IOPP, doi:10.3847/2041-8205/816/1/L14. 

"Plotner, Tammy. “Messier 109.” Universe Today, 24 Dec. 2015, <www.universetoday.com/50202/messier-109/>.

Bournaud, F., and F. Combes. “Gas Accretion on Spiral Galaxies: Bar Formation and Renewal.” Astronomy & Astrophysics, vol. 392, no. 1, EDP Sciences, Aug. 2002, pp. 83–102. Crossref, doi:10.1051/0004-6361:20020920.

Tate, Jean. “Barred Spiral Galaxy.” Universe Today, 25 Dec. 2015 <www.universetoday.com/40300/barred-spiral-galaxy/>.

Right Ascension (J2000)	11:57:35
Declination (J2000)	+53:22:25
Filters used	B (Blue), C (Clear), R (Red), V (Green)
Exposure time per filter	B (300s x 16); V (120s x 7); R (120s x 6); C (120s x 5)
Image dimension	783x478 pixels; 17.2x10.5 arcminutes
Date/time observed	March 5, 2019, 10:00 pm