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Object of the Week October 23, 2016 – Arp 13 The Paramecium Galaxy and the NGC 7448 Galaxy Group
Object of the Week October 23, 2016 – Arp 13 The Paramecium Galaxy and the NGC 7448 Galaxy Group
Arp 13, NGC 7448, UGC 12294
Type: Spiral SA(rs)bc
RA: 23h 00m 3.6s
Dec: +15° 58’ 49
Size: 2.3’x1.0’
Magnitude: 12.2
Companion Galaxies
NGC 7454, UGC 12305
Type: Elliptical edge on
RA: 23h 01m 6.6s
Dec: +16° 23’ 18
Size: 1.5’x0.8’
Magnitude: 12.8
NGC 7463, UGC 12316
Type: Spiral edge on
RA: 23h 01m 51.8s
Dec: +15° 58’ 57
Size: 1.9’x0.5’
Magnitude: 13.8
NGC 7464, UGC 12315
Type: Elliptical face on
RA: 23h 01m 53.7s
Dec: +15° 58’ 26
Size: 31”x28”
Magnitude: 14.4
NGC 7465, UGC 12317
Type: Barred lenticular 1.1’x0.7’ 13.4
RA: 23h 02m 01.0s
Dec: +15° 57’53
Size: 1.1’x0.7’
Magnitude: 13.4
NGC 7437, UGC 12270
Type: Spiral face on
RA: 22h 58m 10.1s
Dec: +14° 18’ 32
Size: 1.7’x1.7’
Magnitude: 13.9
UGC 12313
Type: Dwarf
RA: 23h 01m 43.8s
Dec: +16° 04’ 04
Size: 1.1’x0.3’
Magnitude: 15.4
UGC 12321
Type: Spiral edge on
RA: 23h 02m 19.0s
Dec: +16° 01’ 42
Size: 46”x9”
Magnitude: 16.0
This week’s Object of the Week is the thirteenth object from a list of 338 bizarre and unusual galaxies. This object, labeled Arp 13 by Dr. Halton Arp, is the 12th magnitude galaxy NGC 7448, located in the constellation Pegasus. NGC 7448 was first discovered by William Herschel in 1784. It is one of six galaxies that Dr. Arp classified as a spiral galaxy with detached segments. The most notable Arp galaxy of this class is M66 (Arp 16). NGC 7448 has a diameter of 75,000 light years and has more star formation going on than in most spirals. It has a stellar mass of about 24 billion suns.
When Arp published his Atlas of Peculiar Galaxies in 1966 little was known about the physical processes that shaped these galaxies, including Arp 13. Now over a half century later Arp 13 has slowly yielded at least some of its many secrets.
Perhaps most surprising, or then not really for an Arp galaxy, is that there is a magnitude 19.2 quasar located 18.5” ENE of the nucleus of the galaxy. This quasar, 2257+157, is located within the outer arm; by the theory espoused by Dr. Arp it would have been ejected from the nucleus. In the photo from the Atlas of Peculiar Galaxies there appears to be a faint stellar object at the position charted for the quasar in MegaStar 5. However, this quasar isn’t the only secret lurking in and around Arp 13.
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In 2001 Dr. Arp and David Russell published a paper in The Astrophysical Journal titled "A Possible Relationship Between Quasars and Clusters of Galaxies." Here are the key points the authors make:
- The distribution in the sky of galaxy clusters shows significant association with relatively nearby, large, active galaxies.
- The pattern is that of clusters paired equidistant across a central galaxy with the apparent magnitudes and redshifts of their constituent galaxies being closely matched.
- The clusters and the galaxies in them tend to be strong X-ray and radio emitters.
- The central, low-redshift galaxies often show evidence of ejection in the direction of the higher redshift clusters.
- In all these respects the clusters closely resemble quasars which have been shown to be similarly associated with active parent galaxies.
- Empirically, the quasars are ejected from active galaxies. They evolve to lower redshift with time, forming stars, and fragmenting at the end of their development into clusters of low-luminosity galaxies.
- The cluster galaxies can be at the same distance as their lower redshift parents because they still retain a component of their earlier, quasar intrinsic redshift.
So it shouldn’t be surprising to find there are a number of companion galaxies in the vicinity of NGC 7448. This is a very loose grouping of galaxies, associated from the spatial association and similarity of velocities (redshift). It is the namesake of the NGC 7448, a group of galaxies which include NGC 7454, NGC 7463, NGC 7464 and NGC 7465 that can readily be observed within 1/2 ° of NGC 7448, as well as NGC 7437 located 1.8° south. NGC 7463/4/5 are a compact subgroup and NGC 7464 and NGC 7465 are in the process of merging.
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Gravitational interactions are clearly present in this group, with a tail of atomic hydrogen gas extending from the tight NGC 7463/4/5 subgroup toward NGC 7448, and another tail extending off of NGC 7448. Other galaxies in the group, albeit more difficult to visually observe are UGC 12313, UGC 12321, and two recently discovered Dwarf galaxies from the Arecibo Galaxy Environment Survey AGES J230051+161122 and J230200+140944.
Start with transparent dark skies and steady seeing. In the 8” to 10” range Arp 13 should be visually detectable as a faint smudge, perhaps a very slight hint of elongation detectable, but no discernible brightening. Moving up to the 12” to 16” range begins to bring out some detail –elongation is detectable with brightening towards the middle. The next bump up in detail begins with 18” of aperture or more. Now spiral structure is readily discernible as are brightness gradients throughout the galaxy. For some this view of the galaxy may resemble the microscopic unicellular protozoan, the Paramecium. For those of you who have a 25” or large scope you should be able detect the broken arms of NGC 7448. Is anyone able to ferret out the 19th magnitude quasar in the galaxy?
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Now after focusing on NGC 7448 at high power, bring the power and widen the field of view. How many of the companion galaxies in the cluster are you able to observe?
Give it a go and let us know!
