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Designations: IC1613, Caldwell C51, The Scarecrow
Object Type: Irregular Barred Dwarf Galaxy (IBm)
Constellation: Cetus
RA: 01h 4.8m (Epoch 2000)
Dec: 2 deg 07m (Epoch 2000) 
Size: 18.8 x 17.3 min
Distance: 2.3 million light years
Discoverer: Max Wolf 1906

Visual Description:  The irregular dwarf galaxy IC 1613 (Caldwell 51) is the most challenging object to see in the Caldwell Catalog.  It hides in the north-wetern quadrant of Cetus, just 1/2 degree south of the Pisces border, where it escaped detection until Max Wolf discovered it photographically in 1906 with the Bruce 16-inch refractor at the Astrophysical Observatory in Heidelberg, Germany.  

That Messier and the Herschels failed to spot this loose assemblage of dim suns is not surprising.  Although IC 1613 shines at 9th magnitude, its integrated brightness is deceiving.  The galaxy's light is spread across an area of sky about the size of a quarter Moon, so each square arcminute of its surface shines with an appaent magnitude of 15.5 - more than six times fainter than Pluto's maximum brightness.  This breath of extragalactic light was far beyond the visual grasp of Messier and his contemporaries, and it was too dim and large for the powerful but narrow-field Herschel telescopes.  Many amateur astronomy guides pass over IC1613, claiming it is too dim and difficult for amateur telescopes, but this is not necessarily the case.  Astronomers often refer to IC1613 as the dimmest member of our Local Group of galaxies.  

IC 1613 lies 2.3 million light-years distant, so it is as far as M31, the Great Andromeda galaxy.  It measures 12,400 light-years across, which is comparable to the sizes of NGC147 and 185 (Caldwell 17 and 18), two of the dwarfs attending M31.  But the Andromeda dwarfs are much easier than IC 1613 to see, because they have higher surface brightness.  IC 1613 has the intrinsic luminosity of a mere 80 million Suns, making it one of the least luminous galaxies known.  Like the Andromeda dwarfs, IC 1613 is one of the few galaxies known to be approaching our Milky Way.  And though its approach speed of 234 km per second is comparable to that of NGC 147, it is not part of the Andromeda Galaxy subsystem. To understand why, we have to travel in our minds above the plane of our Local Group. Imagine a circle whose radius equals 3 million light-years. If we place the Milky Way at the center of this circle, we see the Local Group dominated by two groupings of galaxies. One huddles around the Milky Way at the center; the other lies at the 9:30 position, a little more than two-thirds of the way out from the center. (The latter group comprises the Andromeda Galaxy, its four brightest dwarf attendants, and M33, the Great Spiral in Triangulum.) IC 1613 would be in the same concentric circle as the Andromeda Galaxy grouping, but 40° away from it, in the 7:30 position; the spatial separation between the Andromeda Galaxy and IC 1613 — nearly 2 million light-years — is far too great for the objects to be considered closely associated. In fact, IC 1613 is but one of dozens of dwarf galaxies that populate the Local Group. Barnard's Galaxy (Caldwell 57) is another, and astronomers believe that many more of these diminutive galaxies have yet to be discovered. One of the problems, of course, is that if a loosely structured dwarf galaxy is seen against the plane of the Milky Way, its stars are hard to differentiate between those of our own galaxy.

Fortunately, IC 1613 lies well away from the stellar madness of the Milky Way. We see it against a relatively star-poor backdrop in Cetus. In photographs taken with large telescopes, IC 1613 looks, ironically, like a close-up shot of the galactic center taken with a 100-mm lens, but with one major difference: while the system has an oval appearance over­all, its interior looks stretched, fragmented, shorn, and tattered. In other words, IC 1613 looks like extragalactic road kill. This analogy is not far from the truth. Throughout their his­tories irregular galaxies have taken severe gravitational beatings from passing galaxies or larger neighboring ones. Left alone, the irregu­lars would rearrange themselves into more symmetrical systems, which is what IC 1613 is trying to do. That's why I call IC 1613 the Scarecrow. In a remote way it reminds me of the scarecrow in the "Wizard of Oz"; after the witch's winged monkeys tore the straw man apart, he started to piece himself back together.  

Indeed, IC 1613 may be a highly flattened spiral system seen tilted 27° from face on. But its spiral structure, if any, is certainly different than that of classical spirals like the Pinwheel Galaxy (M33) or the Whirlpool Galaxy (M51). IC 1613's features are more like those of the Large Magellanic Cloud, for both systems dis­play a prominent central mass, slight spiral structure, and a well-resolved axial bar. Actually, astronomers find that IC 1613 most closely resembles the transition-type dwarf galaxy DDO 216 in Pegasus as well as Barnard's Galaxy. So irregular galaxies may not be so dis­organized as first impressions imply. 

IC 1613's brightest stars are concentrated in the galaxy's northeastern quadrant, which is separated from its core by an oval swath of dust that is rimmed by what appears to be a spiral arm. This quadrant also contains a huge association of giant blue stars and may be a cradle of star formation. Indeed, the galaxy is riddled with star-forming regions. Spectroscopic observations conducted with the Canada-France-Hawaii Telescope atop Mauna Kea have revealed 20 stellar associations in the galaxy's core and its northeastern and north­western quadrants. Studies of the northeast­ern quadrant have shown that its youngest stars are between 5 and 20 million years old, while its oldest stars have ages up to 10 billion years. A total of 43 star-cluster candidates have also been located, of which 18 are probable open star clusters. (IC 1613 contains no bright globular clusters.) Hubble Space Telescope observations of roughly 700 square light-years in the galaxy's core support the notion that IC 1613 is still birthing stars. For the past 250 to 350 mil­lion years, the galaxy has had a roughly constant star-formation rate of about 35,000 suns per year, which is about 50 percent lower than it was 400 to 900 million years ago.

The galaxy is also rich in supergiant stars, and spectroscopic observations with the William Herschel Telescope on La Palma in the Canary Islands have revealed six interlocking interstellar shells in IC 1613. The shells are gigantic, with diameters ranging from 220 to 1,000 light-years, and they are expanding at speeds up to 50 km per second. Stellar winds associated with supernova explosions seem to drive this expansion. The region could be the first stage in the formation of a supergiant shell with a diameter greater than 3,000 light-years. Others have found that expanding "super-bubbles" cover the whole of the galaxy. Each superbubble contains at least one stellar association, and the more massive stars in these associations seem to be quite important in shaping the interstellar medium of the galaxy.

HST also imaged Cepheid variable stars in IC 1613, and those stars confirmed R. Brent Tully's distance determination of 2.3 million light-years, which was derived by more indirect means.  

Telescope: TMB 152 mm APO Refractor
Focal Length: 800mm  - 1200 * 0.75 (AP Focal Reducer )
Mount: Takahashi NJP 160
Camera: SBIG ST-10XME
Exposure: 156 minutes of luminance
Other: SBIG ST-402ME autoguider

Image Processing: CCDStack and Photoshop CS2

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Copyright(c) 2007 Doug Sanqunetti All rights reserved.