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Spiral Galaxy Messier 94 (NGC 436)
Nathan Jen

Previous imageUp to Astr212 IndexNext image M94 color (inner region)

Figure 1: M94 with slightly enhanced color, no outer disk shown

Messier 94 (M94) is one of the closest early type spiral galaxies to the sun with a distance of 5.0 Mpc and linear diameter of 8.3 kpc (as seen in figure 1) and can be found in the Canes Venatici I cloud. It is a spiral galaxy, just like almost 70% of the "ordinary" galaxies. However, M94 has some very interesting features that make it rather unique, some of which weren’t discovered until very recently (2009), despite being originally discovered in 1781. These recent discoveries are significant, challenging the commonly held classification and type, in addition to creating a spectacular image.  

Figure 1 shows a very bright bulge in the center with a relatively small disk containing some fairly tightly wound arms. The blue color of the arms indicates a lot of new star formation while the yellow bulge shows older stars. At the very edge of the disk, some red spots can be seen, indicating HII regions where hydrogen gas clouds are ionized.

M94 clear

Figure 2: M94 in clear filter only, black & white image only

Figure 2 is just what is seen in the Clear filter, showing more of the larger disk. A separate image is used to display this because of the washing out of the colors and arm features that otherwise occurs. To get a better image of this larger disk, a longer exposure would be needed. From slightly more advanced images, astronomers classified it as an (R)SA(r)ab galaxy, meaning it has a ring in the outer disk, arms that have spiraled to create a ring-like structure, and it has a large bulge with smooth, tight arms. The flocculent inner arm structure can be seen in figures 1 and 2, while figures 3 and 4 show the much larger and well defined arms.

M94 multiwavelength M94 large

Figure 3 (Left): M94 as shown by a combination of optical, long exposure optical, ultraviolet, and infrared at 24, 8 and 3.6 microns ( R Jay Gabany (Blackbird Obs.) Collaboration: I. Trujillo, I. Martinez-Valpuesta, D. Martinez-Delgado ( IAC); J. Penarrubia (IoA Cambridge); M. Pohlen (Cardiff))

Figure 4 (Right): M94 prior to multi-wavelength evaluation (Hillary Mathis & N.A.Sharp (NOAO), AURA, NSF) via Astronomy Picture of the Day

Recently, Trujillo, Martinez-Valpuesta, Martinez-Delgado, Penarrubia and Pohlen have compiled data from several non-optical wavelengths, resulting in figure 3. Previously, figure 4 was thought to be one of the best images of M94, resulting in the traditional classification. Specifically, the new images in UV and IR indicate that it is not a ring but instead has spiral arms in the outer region. Comparing images taken in UV (figure 5) and infrared (figure 6) and noting the unchanging features, they determined that the outer region is very active, based on shapes and the amount of mass in each region, and that stars of all ages are evenly distributed throughout the galaxy. The UV photo shows the newer stars whose light is not absorbed by the dust while the infrared photo highlights the dust grains that are warmed by the very young, hot stars nearby. Due to the discovery of arms in the outer region, the article also discusses some possible theories for the formation of the outer region. Because the outer disk is at a bit of an angle, some proposed a galaxy merger or interactions with nearby galaxies. However, the mass distribution makes these theories unlikely, and Trujillo et al. conclude that evidence of an oval distortion in the inner region is the most likely cause of the spiral arms seen in the outer region.

M94 in ultraviolet M94 in infrared

Figure 5: (Left): M94 in ultraviolet (via Cosmotography on M94 )

Figure 6: (Right): M94 in infrared (via Cosmotography on M94 )

References:
Kutner, Marc L. Astronomy: A Physical Perspective, 2nd ed. Cambridge: Cambridge University Press, 2003.

Leach, S. "M94 Galaxy" 2006 January 5. Accessed 9 May 2011. <http://www.sidleach.com/m94.htm>

Mathis, Hillary and N.A. Sharp. "Starburst Galaxy M94" Astronomy Picture of the Day. 2002 November 21. Accessed 2 May 2011. <http://apod.nasa.gov/apod/ap021121.html>

Trujillo, I, Martiez-Valpuesta, I, Martinez-Delgado, D, Penarrubia, J, Gabany, R. J., Pohlen, M. "Unveiling the Nature of 94's (MGC4736) Outer Region: A Panchromatic Perspective." Astrophysical Journal 704:618-628, (2009) <http://adsabs.harvard.edu/abs/2009ApJ...704..618T>

Trujillo, I, Martiez-Valpuesta, I, Martinez-Delgado, D, Penarrubia, J, Gabany, R. J., Pohlen, M. "M94 (NGC4736): a Panchromatic Perspective". Cosmotography. 2006. Web. <http://www.cosmotography.com/images/small_new_m94.html >

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) 12:50:54
Declination (J2000) +41:07:10
Filters used blue(B), green(V), red(R), and clear(C)
Exposure time per filter 55x30 sec. in C, 8x300 sec. in B, 7x150 sec. in V, 14x60 sec. in R
Date observed

March 2, 2011 (BC)
March 3, 2011 (BRV)

Bias filters 110228-bias.fit and 110303-bias.fit were used according to which date data was taken on. Similarly, 110228-dark-180s.fit and 110303-dark-180s.fit were used. Flat filters for B, C, R, and V from 110210 were used, chosen for their respective colors. Manual removal of some bad pixels was necessary, and after combining the images, a Low-Pass Kernel filter was used to smooth out the pixels.

 

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