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Arp 273 UGC 1810 UGC 1813 Colliding Galaxies, 


            Jerry Macon
Arp 273 UGC 1810 UGC 1813 Colliding Galaxies
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Arp 273 UGC 1810 UGC 1813 Colliding Galaxies

Technical card

Dates:Dec. 1, 2019Dec. 2, 2019

Astrodon Gen 2 L 36mm: 126x200" (gain: 99.00) -12C bin 1x1
Astrodon Gen 2 RGB 36mm: 124x200" (gain: 99.00) -16C bin 1x1

Integration: 13.9 hours

Avg. Moon age: 5.32 days

Avg. Moon phase: 28.87%

Bortle Dark-Sky Scale: 4.00

Temperature: -10.00

Basic astrometry details job: 3100992

RA center: 2h 21' 31"

DEC center: +39° 22' 26"

Pixel scale: 0.500 arcsec/pixel

Orientation: 358.282 degrees

Field radius: 0.252 degrees

Resolution: 2772x2330

Locations: Dark Star Observatory, Taos, New Mexico, United States

Data source: Own remote observatory

Remote source: Non-commercial independent facility


Images from the following two scopes (piggybacked) contributed to this image:
AG12+ASI1600MM at .70 asec/pix (RGB)
TV127is+ASI183MM at .75 asec/pix. (L)
They were all registered to the best R image taken on the AG12.

Arp273 is a pair of interacting galaxies, 300 million light years away in the constellation Andromeda. It was first described in the Atlas of Peculiar Galaxies, compiled by Halton Arp in 1966. The larger of the spiral galaxies, known as UGC1810, is about five times more massive than the smaller galaxy. It has a disc that is tidally distorted into a rose-like shape by the gravitational pull of the companion galaxy below it, known as UGC1813. The smaller galaxy shows distinct signs of active star formation at its nucleus, and "it is thought that the smaller galaxy has actually passed through the larger one."

A series of uncommon spiral patterns in the large galaxy is a tell-tale sign of interaction. The large, outer arm appears partially as a ring, a feature seen when interacting galaxies actually pass through one another. This suggests that the smaller companion actually dived deep, but off-center, through UGC 1810. The inner set of spiral arms is highly warped out of the plane with one of the arms going behind the bulge and coming back out the other side. How these two spiral patterns connect is still not precisely known.

The larger galaxy in the UGC 1810 - UGC 1813 pair has a mass that is about five times that of the smaller galaxy. In unequal pairs such as this, the relatively rapid passage of a companion galaxy produces the lopsided or asymmetric structure in the main spiral. Also in such encounters, the starburst activity typically begins in the minor galaxies earlier than in the major galaxies.

Arp 273 lies in the constellation Andromeda and is roughly 300 million light-years away from Earth. The image shows a tenuous tidal bridge of material between the two galaxies that are separated by tens of thousands of light-years from each other.



Jerry Macon
License: Attribution Creative Commons

Sky plot

Sky plot


Arp 273 UGC 1810 UGC 1813 Colliding Galaxies, 


            Jerry Macon