Hemisphere:  Northern  ·  Constellation: Lynx (Lyn)
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II Hz 4, 


            Gary Imm
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II Hz 4

Getting plate-solving status, please wait...
II Hz 4, 


            Gary Imm
Powered byPixInsight

II Hz 4

Imaging telescopes or lenses: Celestron EdgeHD 11

Imaging cameras: ZWO ASI 1600MM Cooled Pro

Mounts: Astro-Physics Mach1GTO

Guiding cameras: ZWO ASI174MM

Software: Main Sequence Software Sequence Generator Pro  ·  PixInsight  ·  Stark Labs PHD2 2.6.3

Filters: Astrodon Green 31mm Gen2 I-Series  ·  Astrodon Blue 31mm Gen2 I-Series  ·  Astrodon Lum 31mm Gen2 I-Series  ·  Astrodon Red 31mm Gen2 I-Series

Accessory: ZWO EFW  ·  Celestron OAG  ·  MoonLite Focuser for EdgeHD 11

Dates:Feb. 26, 2020Feb. 27, 2020

Astrodon Blue 31mm Gen2 I-Series: 60x60" (gain: 139.00) -20C bin 1x1
Astrodon Green 31mm Gen2 I-Series: 60x60" (gain: 139.00) -20C bin 1x1
Astrodon Lum 31mm Gen2 I-Series: 60x120" (gain: 139.00) -20C bin 1x1
Astrodon Red 31mm Gen2 I-Series: 60x60" (gain: 139.00) -20C bin 1x1

Integration: 5.0 hours

Avg. Moon age: 3.01 days

Avg. Moon phase: 10.07%

Astrometry.net job: 3323913

RA center: 8h 58' 33"

DEC center: +37° 5' 16"

Pixel scale: 0.282 arcsec/pixel

Orientation: -5.685 degrees

Field radius: 0.147 degrees

Resolution: 2961x2300

Data source: Backyard


This rarely imaged object is a collisional ring galaxy system located 600 million light years away in the constellation of Lynx at a declination of +37 degrees. This object spans about 0.5 arc-minutes in our apparent view, which corresponds to an actual diameter of 100,000 light years. Much of the information below is taken from the 1976 Lynds and Toomre paper, "On the Interpretation of Ring Galaxies - The Binary Ring System II Hz 4".

I do not understand why Dr. Arp did not include this object in his catalog of peculiar galaxies. This object is certainly peculiar. Arp was aware of it - he actually named it in 1965 as the 4th entry in Emil Herzog's informal second list of peculiar galaxies. This object is also known as PGC 25211.

The bright foreground star in the center of this image is a double star, TYC 2491-465-1/2. I did not fix the bright star artifact (from using the ZWO 1600MM camera) as I usually do, since it was so close to the object that fixing the artifact would have compromised that part of the object image. Another foreground star - a small bright orange star - is superimposed over the object and should be ignored.

Galaxy ring systems are fairly common and can stem from different causes. Collisional ring systems, from a head-on collision between two galaxies, are extremely rare, consisting of 0.01% of all galaxies.

Our apparent view of the primary ring is likely at a slight angle, which causes the ring to look like a slight oval instead of a circle. Several bright blue star clusters are seen towards the bottom of the ring. Star formation such as this is often initiated by a gravitational galaxy encounter. Aside from the primary ring, several more subtle features are fascinating:

- The bright object inside of the ring is not a star but a remnant of the galaxy collision. Close inspection reveals that this remnant is connected to the ring via star streams and is likely the original core of the larger galaxy which generated the primary ring.

- Immediately above the primary ring is a very faint, almost identical secondary ring. This ring is easier to see on the inverted Astrobin view. The small object slightly above and left of the primary ring lies within (just barely) the faint secondary ring and is also likely a remnant of the collision. It is likely the original core of the smaller galaxy which generated the secondary ring.

Simulations show that this result of two rings and two off-center core remnants is consistent with two spiral galaxies (of mass M and M/2) which collide head-on. "Head-on" means that both disk planes are parallel and that the collision path is perpendicular to these disk planes. It is amazing to me that simulations confirm that the result of such a collision would be two rings and two off-center core remnants. Here is how it is described in the paper - "After a companion galaxy has fallen right through a given disk galaxy more or less along the axis, a transient but remarkably severe and ringlike density wave develops as that target disk rebounds from the short-lived extra pull toward its center".

The galaxy towards the top of the image is LEDA 25212, 800 million light years away and 110,000 light years in diameter.

This is the first image of this object on Astrobin.