NGC 3184 (Spiral Galaxy in Ursa Major)

Observatory / Course Archives / ASTR 212 Spring 2019 / Wierenga

Introduction

NGC 3184, a face-on spiral galaxy, lies approximately 40 million light-years away in the constellation of Ursa Major. This galaxy is unique in more ways than one!

But before we dig deeper into this amazing galaxy, let's review spiral galaxies and how their spiral arms form. Spiral galaxies typically contain a disk of stars, gas, and dust which undergoes differential rotation; a central bulge that can be barred or unbarred; and a surrounding stellar halo. The disk has spiral arms which are formed as star explosions produce pressure waves which, when taken with the differential rotation, causes the materials in the arms to build up behind where density is already high.

Now on to how NGC 3184 is such an interesting spiral galaxy ...

First, NGC 3184 is a difficult galaxy to classify as its appearance does not have enough definite characteristics to put it in any given category. According to the NASA Extragalactic Database (NED), it has a complex morphology of SAB(rs)cd, meaning it is weakly barred and is in "transition" towards having more ring-like structures. Another name for this kind of classification is an "intermediate spiral galaxy." Basically, this galaxy barely has a bar across its central bulge and barely has rings!

Second, There was a magnitude 14 Type II supernova detected in NGC 3184 by Reiki Kushida at the Yatsugatake South Base Observatory in Japan on December 9, 1999. Kushida discovered the supernova by using a 0.40-m reflector telescope.

Finally, NGC 3184 is known for its high abundance of heavy elements. So cool!

Star Color

Pink patches in the spiral arms of spiral galaxies signify emission nebulae and star formation while blue patches typically point to the presence of hot, young, and massive stars. You can see these young blue stars in the above picture of NGC 3184. A description of NGC 3184 when it was the subject of NASA's Astronomy Picture of the Day says that "although [it] contains hundreds of billions of stars, the blue color of its spiral arms comes mostly from relatively few bright young blue stars." These "bright stars...were created in huge density waves that circle the center."

It is also possible that the brightest of the blue stars seen in the above image indicate the presence of NGC 3184's known HII regions which, as "[clouds] of glowing gas and plasma [with a] large amount of ionised atomic hydrogen within them," are the areas of star formation (University of Manitoba student).

Linear Size Calculation

But just how big is this galaxy? With an estimated distance of 40 million light-years, or 12.25 +/- 0.89 mpc (NED), we can find a maximum angular size of 6.534 arcminutes, which corresponds to a linear size of 79,908.31 light-years. The angular size used in this calculation was found by drawing a line along the major axis of the galaxy (the long direction) and using this line to get the length of the galaxy in pixels. Pixels are then converted to angular size using a scale of 1.32 arcsec/px.

Surface Brightness Profile

NGC 3184's bright blue stars are very prominent in my image, but the rest of the image does not appear super bright. In order to determine the brightness of my galaxy, I calculated its "surface brightness profile," which shows how my galaxy's brightness drops with distance from the center according to a simple exponential model. Similar to the process for calculating linear size, I started by drawing a line across the center of my galaxy (see second picture above).

I then used a line to export a light profile of my galaxy (as seen in the second image) which could be plotted with radial distance in pixels as the independent variable (x-axis) and the light intensity in counts as the dependent variable (y-axis). The resulting graph reflected a Gaussian function, though it was influenced by the brightness of the spiral arms, so I adjusted the fit-line accordingly. The resulting graph is below. The fit value returned, which represents a scale radius (where the brightness drops off by a factor of 1/e) of 9.590 +/- 0.4782. This can be converted to a physical value of 0.752 kpc. This value seems to make sense, given that the brighter stars seem very close to the center of the galaxy, and it doesn't take moving very far-only about 3,262 light-years (or 4% of the galaxy's linear size!)-from the center of the galaxy for brightness to begin to drop off exponentially.

References:

"Alert Notice 267: 1012+41 Supernova 1999gi in NGC 3184." American Association of Variable Star Observers, https://www.aavso.org/aavso-alert-notice-267.

"Detailed Information for a Named Object: NGC 3184." NASA/IPAC Extragalactic Database (NED), California Institute of Technology, LINKHERE.

"Intermediate Spiral Galaxy." Wikipedia, https://en.wikipedia.org/wiki/Intermediate_spiral_galaxy.

Nemiroff, Robert and Jerry Bonnell. "Astronomy Picture of the Day: Gangly Spiral Galaxy NGC 3184." NASA, https://apod.nasa.gov/apod/ap000920.html.

"NGC 3184." Wikipedia, https://en.wikipedia.org/wiki/NGC_3184.

Quiring, David. A Project on NGC 3184, University of Manitoba, http://umanitoba.ca/faculties/science/astronomy/jwest/projects/umquirid/.

Right Ascension (J2000)	10:18:16.8
Declination (J2000)	41:25:27
Filters used	B (Blue), C (Clear), R (Red), V (Green)
Exposure time per filter	B (300s x 5), V (300s x 4), and R (60s x 5); C (300s x 14)
Image dimension	1092x736 pixels; 23.66x15.95 arcminutes
Date/time observed	March 1, 2019, 6:55 UT

In order to reduce the raw data received from observation and remove the effects of CCD bias, thermal noise, and nonuniform CCD sensitivity/illumination, I used MaxIm to calibrate my raw data files of bias, dark, and flat field frames. I first inspected my raw data files to ensure that they looked like typical biases, darks, and flats. I then combined all of the biases, darks, and flats into a Master Bias, Master Dark, and Master Flats, respectively. These master frames were similar to the individual frames but now less influenced by the aforementioned effects. Using the master frames, I could run a final calibration to produce clear images for each filter. I finally could combine the filters to produce the image that you see at the top of this page. (Though, not quite). I also had to adjust my images color balance, luminance weight, and saturation. I set my color balance to 3 for red, 1 for green, and 3 for blue and my luminance weight to 70%. My color saturation % was 440%. I chose these values as they allowed the blue stars in my image to be prominent, as they are a key feature of my galaxy.