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Spiral Galaxy NGC 2841
Christopher Ver Hoef

Spiral Galaxy NGC2841

Although most spiral galaxies, such as the Milky Way, have fairly well-defined arms, not all of them do. NGC 2841 is one of the latter. A type of galaxy known as a flocculent spiral galaxy, its spiral arms are very poorly-defined and blurry; as you can see above, the arms are nearly impossible to make out. It has been calculated to be about 14 megaparsecs away, or about 46 million light years away. From the image above, its diameter is calculated to be about 40 kiloparsecs across, which is slightly larger than the Milky Way's 30 kiloparsecs.

As a flocculent spiral galaxy, notable features are hard to make in NGC 2841. They commonly merge together, creating a haze. However, some features are still discernible. If one looks closely, they can see a slight gap between the arms and the much brighter core. The galaxy has a slight blue haze throughout, indicating lots of star formation. Hot, blue stars burn out and collapse faster than red ones, so the presence of so much blue means that blue stars are constantly being produced, and in large numbers as well. According to the NASA/IPAC Extragalactic Database, or NED, NGC 2841 is a type SA(r)b galaxy. This means that it is a spiral galaxy, it has no bar, it has a ring at the center, and the arms are wound moderately tightly. Although it is difficult to make out, the ring is nearly visible in the above image, with the inside edges almost obscured by the glare of the core. Dust lanes, large swaths of dust between stars, can be seen as faint rings in the galaxy. These dust lanes can also help us figure out which side of the galaxy is closer to us; the lanes in the top left of the galaxy are slightly clearer than those in the bottom right, which means they are most likely closer to us.

NGC2841 in X-rayNGC2841 in X-ray and optical

Pictured above are two images of NGC 2841 in X-rays, taken by the Chandra X-ray Observatory. The right image is the left with an optical image superimposed; X-rays are represented in blue, while gray and white is light we can see. X-rays are emitted from things left behind after supernovas, such as pulsars and disks of dust around black holes or neutron stars. The large amount of blue means that plenty of stars have already died in the galaxy, even though it is constantly producing new ones.

NGC2841 InfraredTo the left is an infrared, or IR, image of NGC 2841 taken by the Spitzer Space Telescope between wavelengths of 3.6 and 8 micrometers. This continuum of infrared is emitted by dust warmed from ultraviolet rays from hot stars. As mentioned before, young blue stars are very hot, and so would emit a lot of ultraviolet rays. The large amount of IR emission spread throughout the galaxy confirms that there are hot stars across the galaxy. The gap between the red and blue light clearly shows the ring at the center of the galaxy.

In 2001, a team of scientists including L. M. Macri, P. B. Stetson, and G. D. Bothun discovered several Cepheid variable in NGC 2841. Cepheids are highly useful for calculating distances to galaxies, as not only are they all about the same brightness, but they are bright enough to be seen from very large distances. A little over twenty Cepheids were found, but the group also had to make corrections for various astronomical conditions, such as dust between the stars and the earth. Using the brightness of the Cepheids, the group calculated NGC 2841 to be about 14.1 megaparsecs away, or 46.0 million light years. Due to the expansion of space and Hubble law, this means that the galaxy is traveling away from us at a speed of about 638 km/s.

In spiral galaxies, the brightness of the galaxy is strongest in the center and drops off exponentially as we move away from the center. This drop in brightness is commonly given by e^(r/rs), where rs is the scale length. The scale length is the distance over which brightness decreases by a factor of 1/e, which is approximately equal to 1/3. I calculated this length to be about 9.4 kiloparsecs, or 31 thousand light years. This is significantly larger than the Milky Way's 3 kiloparsecs; a certain amount of this is to be expected, as NGC 2841 is not only larger than the Milky Way but also constantly producing bright stars, although the full degree of difference is unexpected. A graph of the brightness dropoff relative to the center is shown below.

NGC2841 light profile

 

References:

Boquien, Médéric. NGC 2841; 3.6, 5.8, 8.0 microns; Spitzer. 2006. Wikimedia Foundation, St. Petersburg. http://en.wikipedia.org/wiki/File:NGC2841_3.6_5.8_8.0_microns_spitzer.png

Elmegreen, Debra M. Galaxies and Galactic Structure. Upper Saddle River, NJ: Prentice Hall, 1998. 93-95.

Macri, L. M., P. B. Stetson, G. D. Bothun, W. L. Freedman, P. M. Garnavich, S. Jha, B. F. Madore, and M. W. Richmond. "The Discovery of Cepheids and a New Distance to NGC 2841 Using the Hubble Space Telescope." 2001 The Astrophysical Journal 559, 243

Wang, Q D. "NGC 2841: Galactic Chimneys Turn Up the Heat," Chandra X-ray Observatory. March 05, 2006. Accessed April 2013. <http://chandra.harvard.edu/photo/2006/n2841/>

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) 09:22:02
Declination (J2000) -50:58:35
Filters used Blue (B), green (V), infrared (I), and clear (C)
Exposure time per filter

8x300 seconds in B, 7x300 seconds in C, 3x300 seconds in V, 2x300 seconds in I

Date observed

March 21, 2013 (CIVB)

Background visual noise is a large problem in astronomical observing. To get around this, several images were taken without any target. The bias image is an image taken by the camera with zero exposure time. Because of this, it only records noise caused by reading the pixels off of the chip that records the image, and so this noise can be taken off. The dark image is taken with the shutter of the camera closed; any "light" recorded will be just background noise caused by differences in temperature that can be taken out of the final image. Finally, the flat image is a picture taken of a uniform light source, such as the sky before any stars appear. The pixels are not uniformly sensitive, so the flat allows us to calibrate how sensitive a pixel is. Once all of these images have been taken, they can be combined in a computer with the actual image of the galaxy in order to get the clearest image possible.

 

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