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Previous image Up to Astr111 Index Next image ASTR 110 Photography Projects, Fall 2015

Messier 110 (NGC 205) & Caldwell 18 (NGC 185)
Caleb Vroegop & Jay De Man

NGC205 NGC185
NGC 205 Dwarf Elliptical Galaxy - Caleb Vroegop NGC 185 Dwarf Elliptical Galaxy- Jay De Man

You perhaps may be asking what a galaxy is: A galaxy is a gravitationally bound collection of stars, leftovers from stars, gas, dust, and dark matter. Both galaxies shown are dwarf ellipticals. These differ from what you may know as the traditional galaxy, with its winding arms and warmly glowing center. Instead, elliptical galaxies represent populations of stars which are older, have disorganized orbits around the galaxy's center-of-mass, and are not spread out into a disc but have a primarily ellipsoidal shape. Dwarf ellipticals are a particular type of elliptical galaxy, smaller, and almost always in some sort of enormous orbit around a much larger galaxy, or group of galaxies.

NGC 205 was first observed on August 10, 1773 by Charles Messier in the constellation Andromeda. NGC 205 has a number of unique features. Unlike most ellipticals, it appears to have hints of active star forming regions, characterized by cool, coalescing gas. It is a satellite of M31, the Andromeda Galaxy, and shows evidence of tidal distortion caused by interaction with Andromeda's enormous gravity. Most galaxies, of any size, have a black hole at the center. This exotic, extremely massive object (though physically very small in the cosmological scheme) forms at the center of supermassive collapsed stars, and continues to accrete material throughout its existance, increasing its mass and gravitational attraction. NGC 205, however, may not have one at all (Valluri, M. et al.: 2005). This indicates a potentially unique intragalactic environment, wherein stars rotate about their mutual center of mass, instead of a supermassive object at their center.

NGC 185, a dwarf elliptical with quite a bit of spunk, was discovered by William Herschel on November 30, 1787, in the constellation Cassiopia. NGC 185 also orbits M31, Andromeda Galaxy, and its unique composition gives clues to its early history. Spectroscopy and detailed observation indicate quite recent star formation (not a normal feature of ellipticals, which typically have older stars), which may have occurred due to recycled gas near the center of the galaxy. Though there's very recent star formation, the rest of the galaxies stellar population is very, very old. It is hypothesized by Geha, M. et al (2015) that NGC 185 was captured very early in its formation, and through gravitational interaction with Andromeda, star formation was slowed. Lots of gas was retained in the dwarf elliptical as a result. This also lends itself nicely by way of cause for NGC 185's more recent stellar formation. NGC 185 is also incredibly active, so much so that it was hypothesized NGC 185 contained an Active Galactic Nucleus, a supermassive black hole energizing infalling gas to such a degree jets of gas and light are emitted. Under further examination, it was discovered that NGC 185 does not possess such a powerful object, but instead has active supernova remnants near its center (Martinus: 2012).

Structural similarites are evident between the two objects, as they both rightly fall under similar classification. They each are ellipsoidal, have generally old stellar populations, and are full of wacky orbits following no obvious trend. They each have very bright centers, with more diffuse outer boundaries, and though color varies with image process, both share a generally yellow hue. This indicates the similar age of the stellar populations in each, as new stars are typically blue, whereas old ones are yellow.

Differences between the two are few, but interesting. NGC 185 is much more active, retains some gas at its center, and (though not visible in the given images) has a few brand-new stars. NGC 205 not only has a generally older stellar population, it also is nearly 5x smaller than NGC 185. NGC 185 is brighter than NGC 205. Though both orbit Andromeda, NGC 205 shows more evidence of tidal interaction, whereas NGC 185 has stayed more stable over time. NGC 185 also has a much higher relative concentration of stars at its center, causing Baade to remark, "There is no nucleus, and the intensity distribution in the central region is very similar to that observed in rich Globular Clusters." (Baade: 1944)


Geha, M. et al. (2015) "HST / ACS Direct Ages of the Dwarf Elliptical Galaxies NGC 147 and NGC 185." The Astrophysical Journal, 811:114

Howley, Kristen M. (2011) "Dynamics of NGC 205 and M32, Andromeda's Distorted Inner Satellites." Proquest Dissertations Publishing

Martius, L. P. (2012) "The Ionization Mechanism of NGC 185: How To Fake A Seyfert Galaxy?" Mor. Not. R. Astron. Soc., 419:3159-3166

Baade, W. (1944) "NGC 147 and NGC 185, Two New Members of the Local Group of Galaxies." Astrophysical Journal, 100:147

Valluri, M. et al. (2005). "The Low End of the Supermassive Black Hole Mass Function: Constraining the Mass of a Nuclear Black Hole in NGC 205 via Stellar Kinematics". Astrophysical Journal, 638:137-152

Object NGC 205 NGC 185
Right Ascension (J2000) 0:42:22 00:38:58
Declination (J2000) +41:41:118 +48:20:15
Filters used B (Blue), R (Red), V (Green) B (Blue), R (Red), V (Green)
Exposure time per filter B and C (180s), V and R (120s) B (180s), R (90s), V and C (120s)
Image dimension 300x250 pixels; 21.9x11 arcminutes 417x250 pixels; 11.7x10 arcminutes
Date/time observed September 28, 2015, 3:03 UT September 29, 2015, 2:39 UT



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