Object name: HCG18

Designation(s): ARP318, NGC0833, NGC0835, NGC0838, NGC0839, NGC0848, HCG18,

Arp 318 is a group of 4 galaxies also known as Hickson 16. It is in the constellation of Cetus and is about 160 million light years distant. Hickson 16 consists of 4 galaxies but Arp 318 may consist of 5. Depends on your source. Arp\'s image only includes 4 of the 5. The 5th is too far for the 200\" to image it with the others. With other large objects, Arp went with the 48\" Schmidt photo, however. Since he didn\'t here I\'ll assume 4 is correct but the 5th is in my image so won\'t escape my covering it.

The 4 galaxies, right to left are, NGC 833 (HCG16B), NGC 835 (HCG16A), NGC 838 (HCG16C) and NGC 839 (HCG16D). Hickson lettered them brightest to dimmest. NGC 833 is classed as (R\')Sa:pec;Sy2 LINER so has an active nucleus likely caused by its interaction with NGC 835. NGC 835 shows tidal plumes from the interaction. It is classed SAB(r)ab: pec LINER with some sources saying it too is a Seyfert 2 galaxy. Its ring like arm and overall shape is somewhat akin to some galaxies thought to have taken a direct center hit by another galaxy. Usually, the ring is further out and more complete. Something has sure clobbered it, Maybe NGC 833.

NGC 838 is also disturbed showing some rather strong dust features. It is classed as SA(rs)0^0 pec: Sbrst. Its classification as a starburst galaxy indicates it too has been altered by an encounter. I\'m leaning toward it being the bullet that hit NGC 835 if indeed it was hit by one. With those huge dark dust clouds, it could be in starburst mode for some time. The final galaxy of the four, NGC 839 is classed as Spec sp; LINER Sy2. It too has an active nucleus. It appears to have a bright ring structure as well with a very dark cloud. It\'s not as red as those in NGC 838 probably indicating less UV light from young stars is hitting it. The UV light causes dust to give off a reddish light called Extended Red Emission. The lack of UV hitting it likely is due to it not being a starburst galaxy and thus not having sufficient supermassive stars to give off the unneeded light. Or the cloud is located such that it is blocked from such light. It\'s hard to tell. In any case, it, like the other 3, has obviously had a close encounter of the galactic kind. All four were discovered by William Herschel on November 28, 1785. None are in either H400 observing program.

The 5th galaxy is in the lower left corner of my image. It is NGC 848 and has a redshift putting it at 171 million light years, about the same as the other 4 so it is most certainly part of the group. It carries the uncertain classification of (R\')SB(s)ab pec? Sbrst. Note the large plumes, especially to the north. So it has also been rather strongly jostled about by one or more of the other 4 galaxies but seems to be making a run for it rather than take further beatings. (I\'m typing this with the help of a 12-year-old granddaughter, the anthropomorphic slant is hers though she doesn\'t know the meaning of the word.) This galaxy was discovered by Ormond Stone In December 1885. While I\'ve seen it included as part of HCG 16 it is too far away to meet Hickson\'s compact group definition. Its inclusion is an error.

Arp\'s comment about this 4 galaxy entry is: \"Faint, diffuse streamers, peculiar galaxies.

I\'ve included an annotated image showing distances in billions of light years to those objects that NED has redshift data for. G is for galaxy and Q for quasar. QC means quasar candidate and P after the distance means it was determined photographically. This is done by noting the point at which the redshifted UV light suddenly drops off. Theory says this happens at a particular frequency for a resting source. By how far this point is shifted toward the red its redshift and thus distance can be estimated. Of course, if something unforeseen alters the UV spectrum this could be in error though it is usually considered a valid indicator of distance and is how most of Hubble\'s deep field images are evaluated for their galactic distances. The label is immediately right of the object when possible. When that would cover up something the label is moved to a clear space and a line is drawn to the object.

Usually, distant galaxies are quite reddened but this image has a surprising number of galaxies over a billion light years distant that are still blue to white in color.

This image doesn\'t go nearly as deep as normal as two subframes were lost to clouds and I didn\'t realize it at the time. This data loss makes going deep too noisy due to under sampling of the photons I was collecting.

Arp\'s image:
http://ned.ipac.caltech.edu/level5/Arp/Figures/big_arp318.jpeg

14\" LX200R @ f/10, L=3x10\' RGB=2x10\'x3, STL-11000XM, Paramount ME