Spiral Galaxy IC 342
Hannah Pagel

IC 342, also known as Caldwell C5, is a beautiful, face-on spiral galaxy. This galaxy was discovered in 1895 by W.F. Denning. The famous astronomer, Edwin Hubble, lived around the time of this discovery, and he believed that IC 342 was part of the group of galaxies closest to the Milky Way galaxy, known as the Local Group. However, further observations proved this idea false. It is actually part of the Maffei 1 group of galaxies. According to NASA’s Extra galactic Database, IC 342 is 3.82 Megaparsecs away, which is approximately 12.5 million light years. This means that this galaxy is close enough to have influenced the evolution of the Milky Way and the Local Group. Astronomers, such as Hubble, originally thought that it was not this far away because our view of it was blocked by dust, making it 2.4 magnitudes dimmer. Without this dust extinction, the galaxy could have been discovered earlier and be seen as one of the brightest galaxies in the night sky. IC 342 has a diameter of 17.9 kiloparsecs, as measured from this image. This is smaller than the Milky Way, which has a diameter of approximately 30 kiloparsecs. Currently, IC 342 is of interest to astronomers because it is an excellent target for studying star formation and astrochemistry.

Galaxies come in a variety of types. Their classification describes whether or not they have spiral arms, how tight those arms are wound, and if certain features, such as bars or rings, can be seen in the galaxy. IC 342 is classified as an SAB(rs)cd galaxy. The ‘S’ means that it is a spiral galaxy. ‘AB’ tells us that there is a hint of a bar in the galaxy--it is between having a bar and not having a bar through the center of the galaxy. The ‘rs’ describes in a similar way the hint of a ring around the center. Finally, the ‘cd’ describes how tight the arms are wound. These arms are wound fairly loosely, as type ‘a’ and ‘b’ galaxies have the tightest winding of arms. Spiral galaxies also have a central bulge around the core of the galaxy. However, because IC 342 is face-on, we see the bright core rather than the bulge around it. The individual stars you see in this image are stars from the Milky Way galaxy. There are many foreground stars because we are looking through the plane of the Milky Way to observe this galaxy. Even though it may be hard to see through the Milky Way, we can learn a lot about the IC 342 through this picture. We know that it must be rotating counterclockwise based on the orientation of the spiral arms. The blue regions represent young stars, which are blue in color. The dusty regions are regions of gas and dust which collapse to form the new stars. These areas contain many young, hot, blue stars. In these star forming regions, there are also giant molecular clouds, which are discussed in detail later. The spiral arm structure is due to what astronomers call "density waves." Basically, there are regions of high density (the spiral arms) that the stars move in and out of. The speed of this high density pattern is slower than the stars in the inner part of the disk, but faster than the stars in the outer part of the disk.

For this project, I took pictures of IC 342 in the optical wavelength. However, to gain more information about this galaxy, astronomers can take pictures in different wavelengths, such as infrared and x-ray.

By taking images in the infrared range of the electromagnetic spectrum, astronomers are able to see through dust. Dust blocks visible light, but not infrared. As mentioned, IC 342 is greatly blocked by the dust in our Milky Way galaxy, so we can get a better view of it by seeing through the dust with an instrument such as WISE - the Wide field Infrared Survey Explorer (Figure 1). For the blue regions, the data was taken at 3.4 and 4.6 microns, the regime for starlight, and for the red and green regions, the data was taken at 12 and 22 microns, which is emission from warm dust. In this image, the green and yellow regions represent dense areas of gas, the red represents the core, and the blue stars are stars that are within our own galaxy. The benefit of taking data in the infrared regime is to see these areas, such as the stars and structure seen here in green and red, that are typically blocked by dust. In the optical image above, these structural features cannot be seen, but since dust does not block IR light, they can be seen in the image below.

Figure 1: IC 342 in infrared wavelengths, taken by the WISE telescope. Red and green represent wavelengths of 12 and 22 microns. Blue represents wavelengths of 3.4 and 4.6 microns.

In order to gain information about black holes at the center and at other locations throughout this galaxy, astronomers have taken pictures in the X-ray regime, which show evidence of material gaining high energy as it falls toward the black hole. Figure 2 combines optical and x-ray data in order to show the locations of these black holes. The x-ray data (10-35 keV) was taken from NuSTAR, the Nuclear Spectroscopic Telescope Array, and is shown in this image as the color magenta. NuSTAR is the first telescope to take focused pictures of high-energy x-ray light. The black holes pictured here were first detected by the Chandra telescope, but NuSTAR allows astronomers to learn more about the properties of the black holes. These black holes are particularly of interest because they are brighter (in the x-ray regime, not the optical regime) than the black holes astronomers have observed in our galaxy. However, we know that they cannot be supermassive black holes, because supermassive black holes are only seen at the center of galaxies.

Figure 2: IC 342 in the optical and x-ray wavelengths. The two pink regions in this photograph, one located to the right of center and the other to the upper right of center, show the location of black holes.

Meier, Turner and Hurt (2000) studied 12CO and 13CO, which is carbon monoxide gas made with different isotopes of carbon, in IC 342. 12CO is typically used as a tracer for molecular gas in galaxies. However, they found out that 12CO and 13CO trace different components in the molecular gas. 12CO, which is optically thick, traces warmer regions on the outside “skins” of the molecular clouds, which are known as photodissociation regions (PDR), while 13CO traces the inner areas of the clouds, which tend to be cooler. Through studying these differences, Meier et al investigated the temperatures, densities and other general properties of molecular clouds. They found that 12CO is a good indicator of the morphology and the amount of gas present, but 13CO is better for examining physical properties, such as excitation temperatures, kinetic temperatures, and densities of the bulk of the molecular gas. In IC 342, the molecular gas properties can be described from examining 12CO and 13CO as follows: the molecular gas, primarily found at a temperature of 10 - 20 K, contains dense molecular clouds. In addition to being surrounded by a PDR "skin," these regions also are surrounded by a hot (> 50 K) intercloud medium. These different layers can be seen by looking at the different transitions of carbon monoxide. The layers of high temperatures were found to be associated with optically thick regions.

IC 342 has a very bright, prominent core. In order to examine this further, I measured the scale length of this galaxy. The scale length is defined by Elmegreen as "the distance over which light decreases by 1/e, or by about a factor of one-third." The brightness of the galaxy as a function of horizontal position can be seen in Figure 3. In order to find the scale length, I fitted the central peak to an exponential curve. The exponent goes as one over the scale length. This equalled 2.9 pixels in the picture. Converting to an angular distance, this became 34.22 arcseconds or 1.7*10^-4 radians. In terms of a linear diameter, this is approximately 0.63 kpc. This number shows how steeply the light drops off from the center of the galaxy. In comparison, the Milky Way has a scale length of approximately 3 kpc, or over 4.5 times longer than the scale length for IC 342.

Figure 3: The brightness of the core of IC 342 as a function of position. The brightness in counts relates to the horizontal position by the following equation: brightness = 4111*exp(0.3432*(position-538.6))+2977. While most of these numbers reflect the positional offset of the core and background level, the number 0.3432 gives insight into the scale length of IC 342.

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)

03:46:48

Declination
(J2000)

+68:05:44

Filters used

blue(B), green(V),
red(R), and clear(C)

Exposure time
per filter

4x60 seconds in
C

8x300 seconds in B

6x300 seconds in V

3x300 seconds in R

Date observed

March 19, 2013 (C)
March 21, 2013 (BVR)

In order to create this color image, I took images as described in the table above. I used calibration images (bias, dark, and flat), to calibrate the images. After aligning the calibrated images, I combined the images for each filter, so I had one image for each: clear, blue, green, red. In the combination process, I used the combination method, "median." Some of the bright stars were overexposed in some filters, so I used the "remove bloom" tool to fix this. Next, I combined those four pictures into one color picture using the following weights for the filters: 0.9 red, 1.2 green, and 7.5 blue, with a luminance weight of 10%. At this point, I removed the bad pixels from the image and put a Gaussian blur over the image to smooth out the galaxy. After this, I needed to adjust the brightness of the galaxy to get a better balance between the bright core and the faint arms. To do this, I used a gamma value of 0.5, with a minimum of 4307 and a maximum of 4500. As a final step to improve the color even more, I saturated the picture at 170% for values between 20-755.