ASTRO: Arp 318/Hickson 16

Arp 318 is a group of 4 galaxies also known as Hickson 16. It is in the
constellation of Cetus and is about 170 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 (16B), NGC 835 (16A), NGC 838
(16C) and NGC 839 (16D). Hickson lettered them brightest to dimmest.
NGC 833 is classed as (R')Saec;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. It's 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. It's 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 super
massive 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.

The 5th galaxy is in the lower left corner of my image. It is NGC 848
and has a red shift putting it at 171 million light years, the same as
the other 4 so it is most certainly part of the group. It carries the
uncertain classification of (R')SB(s)abpec?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.)

Arp's comment about this 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 red shift 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 red shifted 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 red shift 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.

SDSS coverage is only of the upper part of the image so the SDSS image
is cut off and only shows the northern 3 galaxies of the group. I've
included it anyway at the same scale as my cropped image 0.67" per pixel.

Arp's image:
http://nedwww.ipac.caltech.edu/level...ig_arp318.jpeg

14" LX200R @ f/10, L=3x10' (clouds cost me 2) RGB=2x10'x3, STL-11000XM,
Paramount ME

Rick
--
Correct domain name is arvig and it is net not com. Prefix is correct.
Third character is a zero rather than a capital "Oh".

all four galaxies show wind blown outer structures - interesting.



On Sat, 11 Sep 2010 11:18:00 -0500, Rick Johnson
wrote:

Arp 318 is a group of 4 galaxies also known as Hickson 16. It is in the
constellation of Cetus and is about 170 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 (16B), NGC 835 (16A), NGC 838
(16C) and NGC 839 (16D). Hickson lettered them brightest to dimmest.
NGC 833 is classed as (R')Saec;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. It's 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. It's 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 super
massive 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.

The 5th galaxy is in the lower left corner of my image. It is NGC 848
and has a red shift putting it at 171 million light years, the same as
the other 4 so it is most certainly part of the group. It carries the
uncertain classification of (R')SB(s)abpec?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.)

Arp's comment about this 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 red shift 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 red shifted 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 red shift 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.

SDSS coverage is only of the upper part of the image so the SDSS image
is cut off and only shows the northern 3 galaxies of the group. I've
included it anyway at the same scale as my cropped image 0.67" per pixel.

Arp's image:
http://nedwww.ipac.caltech.edu/level...ig_arp318.jpeg

14" LX200R @ f/10, L=3x10' (clouds cost me 2) RGB=2x10'x3, STL-11000XM,
Paramount ME

Rick