wvreeven
May 23rd, 2016, 07:18 AM
Arp 313 - NGC 3991, 3994 and 3995
Three interacting galaxies
Constellation: Ursa Major
RA: 11 57 36.852
DEC: +32 16 40.00
Mag: 13.5, 13.4 and 12.7
Size: 1'.0 x 0'.3, 51" x 29" and 2'.6 x 0'.9
These three galaxies were discovered by Heinrich Louis d'Arrest on February 5, 1864. They were included by Arp as number 313 in the fourth major category of his atlas where "group character is the most important consideration".
We all know d'Arrest as one of the two discoverers of Neptune, the other being Johann Gottfried Galle. Fascinating as the story of the discovery of Neptune may be, this is not it ;-)
At the time of the Neptune discovery, d'Arrest was a 2nd assistant astronomer at the Berlin Observatory. In 1848 he moved to the Leipzig Observatory where he saw (but not discovered) NGC 607 and NGC 7005 using the 1830 4.6" F/16.8 Fraunhofer refractor. in Leipzig he also discovered the comet 6P/d'Arrest. In 1857 d'Arrest started at the Copenhagen Observatory, where he stayed until 1875. There he would discover 338 new NGC-objects and observe another 64 already known deep sky objects using the 11" F/17.5 Merz refractor. D'Arrest won the Gold Medal of the Royal Astronomical Observatory in 1875 and died that same year on June 14.
I came upon this group three weeks ago while working on my list of "bright and interesting galaxies". I compiled this list mostly based on The Night Sky Observer's Guide, but also a few other sources. One flaw of this list is that it only contains NGC and IC IDs and not Arp or other IDs. When I looked at the region of NGC 3994 and 3995 in SkyTools I noticed the galaxy NGC 3991 which, oddly enough, is not on my list. The three galaxies are amazing to behold and I immediately decided these were to be my next OOTW. I used magnifications between 256 x and 692 x to try and squeeze out as many details as I could. Even at 692 x the three galaxies fit in one field of view of 9'.5 (Ethos 3.7 mm).
2104
NGC 3991 is a very intriguing galaxy that consists of several knots that lie more or less in a row. The northern most knot is biggest and brightest and is elongated with a stellar nucleus which lies towards the southern edge of the knot. Right next to it lies another knot. Three more knots lie further south. These three lie in a straight line and this line is parallel to the line between the first two knots but all five do not lie in a straight line.
NGC 3994 is an oval glow with a stellar nucleus. I have the feeling a bar runs through the galaxy along the minor axis. However, this bar is not visible in photographs so it was probably my imagination.
NGC 3995 is the most complex of all three and also the hardest to see due to its large size and low surface brightness. At the center lies a bright region that splits in two towards the north east. In the western most of these lies a small, slightly brighter region. To the south from the central region runs a bar that ends in a small hook. The bar is slightly bent. Occasionally I see a detached, exceedingly faint glow light up east from the bar.
Here is a crude drawing I made. NGC 3994 is drawn too large and the split part of NGC 3995 as well but the rest I think is pretty accurate:
2105
These galaxies and their structure should be visible in smaller telescopes, perhaps as small as 6". Larger telescopes may try to see nearby LEDA 1988828 (mag 17.30 B) and LEDA 1992366 (mag 18.10 B).
As always,
"Give it a go and let us know!
Good luck and great viewing!"
Three interacting galaxies
Constellation: Ursa Major
RA: 11 57 36.852
DEC: +32 16 40.00
Mag: 13.5, 13.4 and 12.7
Size: 1'.0 x 0'.3, 51" x 29" and 2'.6 x 0'.9
These three galaxies were discovered by Heinrich Louis d'Arrest on February 5, 1864. They were included by Arp as number 313 in the fourth major category of his atlas where "group character is the most important consideration".
We all know d'Arrest as one of the two discoverers of Neptune, the other being Johann Gottfried Galle. Fascinating as the story of the discovery of Neptune may be, this is not it ;-)
At the time of the Neptune discovery, d'Arrest was a 2nd assistant astronomer at the Berlin Observatory. In 1848 he moved to the Leipzig Observatory where he saw (but not discovered) NGC 607 and NGC 7005 using the 1830 4.6" F/16.8 Fraunhofer refractor. in Leipzig he also discovered the comet 6P/d'Arrest. In 1857 d'Arrest started at the Copenhagen Observatory, where he stayed until 1875. There he would discover 338 new NGC-objects and observe another 64 already known deep sky objects using the 11" F/17.5 Merz refractor. D'Arrest won the Gold Medal of the Royal Astronomical Observatory in 1875 and died that same year on June 14.
I came upon this group three weeks ago while working on my list of "bright and interesting galaxies". I compiled this list mostly based on The Night Sky Observer's Guide, but also a few other sources. One flaw of this list is that it only contains NGC and IC IDs and not Arp or other IDs. When I looked at the region of NGC 3994 and 3995 in SkyTools I noticed the galaxy NGC 3991 which, oddly enough, is not on my list. The three galaxies are amazing to behold and I immediately decided these were to be my next OOTW. I used magnifications between 256 x and 692 x to try and squeeze out as many details as I could. Even at 692 x the three galaxies fit in one field of view of 9'.5 (Ethos 3.7 mm).
2104
NGC 3991 is a very intriguing galaxy that consists of several knots that lie more or less in a row. The northern most knot is biggest and brightest and is elongated with a stellar nucleus which lies towards the southern edge of the knot. Right next to it lies another knot. Three more knots lie further south. These three lie in a straight line and this line is parallel to the line between the first two knots but all five do not lie in a straight line.
NGC 3994 is an oval glow with a stellar nucleus. I have the feeling a bar runs through the galaxy along the minor axis. However, this bar is not visible in photographs so it was probably my imagination.
NGC 3995 is the most complex of all three and also the hardest to see due to its large size and low surface brightness. At the center lies a bright region that splits in two towards the north east. In the western most of these lies a small, slightly brighter region. To the south from the central region runs a bar that ends in a small hook. The bar is slightly bent. Occasionally I see a detached, exceedingly faint glow light up east from the bar.
Here is a crude drawing I made. NGC 3994 is drawn too large and the split part of NGC 3995 as well but the rest I think is pretty accurate:
2105
These galaxies and their structure should be visible in smaller telescopes, perhaps as small as 6". Larger telescopes may try to see nearby LEDA 1988828 (mag 17.30 B) and LEDA 1992366 (mag 18.10 B).
As always,
"Give it a go and let us know!
Good luck and great viewing!"