ASTRO: Arp 72 -- Going faint -- past 24th magnitude

Arp 72 is two interacting galaxies in the constellation Serpens Caput.
It is in Arp's class "spiral galaxies with small high surface brightness
companions on arms". He said of it: "...faint material from arm to and
around companion. Opposite arm faint, sweeps around east of galaxy"

It is composed of two galaxies NGC 5994 (small possibly spiral galaxy)
and NGC 5996, a starburst galaxy classed as SBc. They are about 160
million light-years away: A NED note says; "The UV spectrum of this
starburst (classified as such by Balzano 1983 on the basis of optical
data) is similar to the one of the prototype NGC 7714. Here also Si IV
and C IV show P Cygni profiles, a signature of Wolf-Rayet stars. A high
far- infrared luminosity confirms the presence of vigorous star
formation (Deutsch & Willner 1987). This activity is possibly triggered
by the strong interaction with the companion NGC 5994 (Bushouse 1987).

Limiting magnitude of this image is about 24.5! That means the dimmest
object in the image is almost 16 million times dimmer than the faintest
naked eye star visible from my observing deck below the observatory
(magnitude 6.5). See the ID image for details. This is the faintest
I've ever gone in only 40 minutes. Usually my limit is more like 22.5
unless I really let the noise come through. In this case the noise
level was so low the original FITS goes no deeper than this processed
image. I sure wish I knew what I did differently!

NED lists over 1700 galaxies within a 23 minute circle centered on my
image (not Arp 72 which is a bit high as I wanted to catch the possibly
interacting pair at the very bottom. Since all but a very few, less
than 10, are dimmer than 24.5 and the field is 33 minutes wide, not 23,
I probably caught more than 2500 galaxies in this image but you may have
to blow up the image to see them all. Not bad for only 40 minutes of
luminosity data! Math says 3038 so I'm being conservative. In the ID
image the two galaxies marked with their distances are magnitude 20.2
and 20.6, left to right. This was a night of better than average seeing
and transparency. I assume sky glow was unusually low this night though
I never went out to look up. Appears I should have. Most of the stars
in this image are really galaxies per the SDSS survey data. This is why
most of the stars in the enlarged ID image look odd. They aren't stars!

I moved Arp 72 high to catch the two possibly interacting galaxies at
the bottom of the image west of center (right). The big galaxy is SDSS
J154629.02+173914.4 at magnitude 17.2. Unfortunately there is no
redshift data on it. The small round "companion" is SDSS
J154629.94+173918.9 at magnitude 18.2. Redshift data is available for
it and puts it at 1.2 billion light-years. Both seem to have about the
same intergalactic reddening so probably are about the same distance.
But whether the smaller galaxy is just superimposed over the halo of the
larger or they are really interacting I don't know. The very blue
elongated galaxy to the NW is SDSS J154625.91+174017.7 at magnitude 18
and a redshift distance of 650 million light-years. The reddish round
galaxy SE of the possibly interacting pair is SDSS J154634.61+173850.2
at magnitude 17.3 and a redshift distance of 1.3 billion light-years,
about the same as the nearby pair.

The round blue galaxy above and a bit west of Arp 72 is SDSS
J154649.41+175902.0 at 620 million light-years. Someone else doing Arp
galaxies wanted to know what this one was a couple years ago. I found
her question searching the net for images of this guy. No one answered
her. I wonder if she ever figured it out. It's listed as PGC 1546998
in The Sky 6's data base. The somewhat smaller blue "bug splat" of a
galaxy above and east of Arp 72 is SDSS J154712.85+175727.2. It is far
smaller than the former galaxy being only 160 million light-years
distant. Thus it is likely related to Arp 72 and judging by the vivid
blue color likely interacted with it in the past, triggering massive
star formation that is still going on.

East of Arp 72 level with the companion is a "small spiral with a very
dense core and faint disk without obvious arms. It is SDSS
J154714.35+175153.1 which is 1.7 billion light-years away. Above it and
further east is a double galaxy, the eastern one being nearly starlike.
The main one is SDSS J154722.89+175309.7 at 1.4 billion light-years.
I can't find the eastern one in NED! Checking the POSS 2 blue plates
the second galaxy is plainly seen. Why the SDSS missed it I don't know.

Due west of Arp 72 is a small slightly oval blue galaxy. It is SDSS
J154641.50+175337.2 at 620 million light-years. Further west is the
more elongated blue galaxy cataloged as SDSS J154633.14+175300.4. It is
shown at 630 million light-years so likely related to its nearby
companion. They are surprisingly blue for their distance.

At the eastern edge, near the center are three rather bright galaxies.
The western and northern most is a double blue galaxy. It is listed in
both the SDSS catalog and the MCG catalog. The catalogs can't seem to
agree on its position nor its red shift. It is either SDSS
J154757.80+174921.5 with a red shift of 600 million light-years or MCG
+03-40-041 at 500 million light-years. Take your pick! It could be
that the are measuring the red shift of different galaxies in the pair.
If they aren't really related this difference could be real.
Further east and south is the apparently much larger yellowish
elliptical galaxy known as CGCG 107-038 or ARK 485. The latter
indicates it is an emission line object. Something you don't expect of
an elliptical galaxy that should have used up all its gas long ago and
thus not have strong emission lines. It may be bright and interesting
but I couldn't find a red shift value for it. I was hoping notes at NED
would help me but there was only one and it told me only what was
obvious from my image; "Elliptical red object with an envelope." Below
it is another very blue spiral, SDSS J154802.94+174538.4 at 440 million
light-years. Above these three, nearly directly north of the mystery
double blue galaxy is a reddish galaxy with what appears to be a very
disturbed outer envelope. It is SDSS J154756.71+175216.1 at 1.2 billion
light-years. There are several nearly star-like galaxies around it,
could one of them be responsible for so disturbing this galaxy.

There are hundreds of other galaxies I could mention, thousands
actually, such as the tiny reddish but bright galaxy SE of the bright
blue star below Arp 72. It is SDSS J154704.34+174833.0 at 1.3 billion
light-years yet shines at 18th magnitude. But I have to stop somewhere
or write a book on this field so I'll stop with the comment that oddly,
NED lists no quasars in this field. For going so deep I found that
surprising.

I've also attached a 2x blow up of the image showing Arp 72 and pointing
out a 23.8 and 24.5 magnitude galaxies as well as some rather bright,
20th magnitude galaxies 3.8 billion light-years away. Two SDSS galaxies
are marked with their magnitude. They are too faint for collecting red
shift data. One is magnitude 23.8 and should show on most monitors.
The one at 24.5 will test how well you can dig into the dark regions and
pull out detail. It is there but you may have to play with your monitor
if it isn't well calibrated to find it.

Arp's image with the 200" telescope is at:
http://nedwww.ipac.caltech.edu/level...big_arp72.jpeg

SDSS image:
http://casjobs.sdss.org/ImgCutoutDR4...00&height=1000

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

the faint stars are so faint I cannot see them on my monitor.

the larger galaxy reminds me of my bluetooth earpiece!

what a cool pair of galaxies and a very nice image Rick



"Rick Johnson" wrote in message
ter.com...
Arp 72 is two interacting galaxies in the constellation Serpens Caput.
It is in Arp's class "spiral galaxies with small high surface brightness
companions on arms". He said of it: "...faint material from arm to and
around companion. Opposite arm faint, sweeps around east of galaxy"

It is composed of two galaxies NGC 5994 (small possibly spiral galaxy)
and NGC 5996, a starburst galaxy classed as SBc. They are about 160
million light-years away: A NED note says; "The UV spectrum of this
starburst (classified as such by Balzano 1983 on the basis of optical
data) is similar to the one of the prototype NGC 7714. Here also Si IV
and C IV show P Cygni profiles, a signature of Wolf-Rayet stars. A high
far- infrared luminosity confirms the presence of vigorous star
formation (Deutsch & Willner 1987). This activity is possibly triggered
by the strong interaction with the companion NGC 5994 (Bushouse 1987).

Limiting magnitude of this image is about 24.5! That means the dimmest
object in the image is almost 16 million times dimmer than the faintest
naked eye star visible from my observing deck below the observatory
(magnitude 6.5). See the ID image for details. This is the faintest
I've ever gone in only 40 minutes. Usually my limit is more like 22.5
unless I really let the noise come through. In this case the noise
level was so low the original FITS goes no deeper than this processed
image. I sure wish I knew what I did differently!

NED lists over 1700 galaxies within a 23 minute circle centered on my
image (not Arp 72 which is a bit high as I wanted to catch the possibly
interacting pair at the very bottom. Since all but a very few, less
than 10, are dimmer than 24.5 and the field is 33 minutes wide, not 23,
I probably caught more than 2500 galaxies in this image but you may have
to blow up the image to see them all. Not bad for only 40 minutes of
luminosity data! Math says 3038 so I'm being conservative. In the ID
image the two galaxies marked with their distances are magnitude 20.2
and 20.6, left to right. This was a night of better than average seeing
and transparency. I assume sky glow was unusually low this night though
I never went out to look up. Appears I should have. Most of the stars
in this image are really galaxies per the SDSS survey data. This is why
most of the stars in the enlarged ID image look odd. They aren't stars!

I moved Arp 72 high to catch the two possibly interacting galaxies at
the bottom of the image west of center (right). The big galaxy is SDSS
J154629.02+173914.4 at magnitude 17.2. Unfortunately there is no
redshift data on it. The small round "companion" is SDSS
J154629.94+173918.9 at magnitude 18.2. Redshift data is available for
it and puts it at 1.2 billion light-years. Both seem to have about the
same intergalactic reddening so probably are about the same distance.
But whether the smaller galaxy is just superimposed over the halo of the
larger or they are really interacting I don't know. The very blue
elongated galaxy to the NW is SDSS J154625.91+174017.7 at magnitude 18
and a redshift distance of 650 million light-years. The reddish round
galaxy SE of the possibly interacting pair is SDSS J154634.61+173850.2
at magnitude 17.3 and a redshift distance of 1.3 billion light-years,
about the same as the nearby pair.

The round blue galaxy above and a bit west of Arp 72 is SDSS
J154649.41+175902.0 at 620 million light-years. Someone else doing Arp
galaxies wanted to know what this one was a couple years ago. I found
her question searching the net for images of this guy. No one answered
her. I wonder if she ever figured it out. It's listed as PGC 1546998
in The Sky 6's data base. The somewhat smaller blue "bug splat" of a
galaxy above and east of Arp 72 is SDSS J154712.85+175727.2. It is far
smaller than the former galaxy being only 160 million light-years
distant. Thus it is likely related to Arp 72 and judging by the vivid
blue color likely interacted with it in the past, triggering massive
star formation that is still going on.

East of Arp 72 level with the companion is a "small spiral with a very
dense core and faint disk without obvious arms. It is SDSS
J154714.35+175153.1 which is 1.7 billion light-years away. Above it and
further east is a double galaxy, the eastern one being nearly starlike.
The main one is SDSS J154722.89+175309.7 at 1.4 billion light-years.
I can't find the eastern one in NED! Checking the POSS 2 blue plates
the second galaxy is plainly seen. Why the SDSS missed it I don't know.

Due west of Arp 72 is a small slightly oval blue galaxy. It is SDSS
J154641.50+175337.2 at 620 million light-years. Further west is the
more elongated blue galaxy cataloged as SDSS J154633.14+175300.4. It is
shown at 630 million light-years so likely related to its nearby
companion. They are surprisingly blue for their distance.

At the eastern edge, near the center are three rather bright galaxies.
The western and northern most is a double blue galaxy. It is listed in
both the SDSS catalog and the MCG catalog. The catalogs can't seem to
agree on its position nor its red shift. It is either SDSS
J154757.80+174921.5 with a red shift of 600 million light-years or MCG
+03-40-041 at 500 million light-years. Take your pick! It could be
that the are measuring the red shift of different galaxies in the pair.
If they aren't really related this difference could be real.
Further east and south is the apparently much larger yellowish
elliptical galaxy known as CGCG 107-038 or ARK 485. The latter
indicates it is an emission line object. Something you don't expect of
an elliptical galaxy that should have used up all its gas long ago and
thus not have strong emission lines. It may be bright and interesting
but I couldn't find a red shift value for it. I was hoping notes at NED
would help me but there was only one and it told me only what was
obvious from my image; "Elliptical red object with an envelope." Below
it is another very blue spiral, SDSS J154802.94+174538.4 at 440 million
light-years. Above these three, nearly directly north of the mystery
double blue galaxy is a reddish galaxy with what appears to be a very
disturbed outer envelope. It is SDSS J154756.71+175216.1 at 1.2 billion
light-years. There are several nearly star-like galaxies around it,
could one of them be responsible for so disturbing this galaxy.

There are hundreds of other galaxies I could mention, thousands
actually, such as the tiny reddish but bright galaxy SE of the bright
blue star below Arp 72. It is SDSS J154704.34+174833.0 at 1.3 billion
light-years yet shines at 18th magnitude. But I have to stop somewhere
or write a book on this field so I'll stop with the comment that oddly,
NED lists no quasars in this field. For going so deep I found that
surprising.

I've also attached a 2x blow up of the image showing Arp 72 and pointing
out a 23.8 and 24.5 magnitude galaxies as well as some rather bright,
20th magnitude galaxies 3.8 billion light-years away. Two SDSS galaxies
are marked with their magnitude. They are too faint for collecting red
shift data. One is magnitude 23.8 and should show on most monitors.
The one at 24.5 will test how well you can dig into the dark regions and
pull out detail. It is there but you may have to play with your monitor
if it isn't well calibrated to find it.

Arp's image with the 200" telescope is at:
http://nedwww.ipac.caltech.edu/level...big_arp72.jpeg

SDSS image:
http://casjobs.sdss.org/ImgCutoutDR4...00&height=1000

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

Richard Crisp wrote:
the faint stars are so faint I cannot see them on my monitor.

the larger galaxy reminds me of my bluetooth earpiece!

what a cool pair of galaxies and a very nice image Rick

As my old monitor was dying the bottom two levels of the gray scale
chart attached were just black and barely darker than the third level.
Now with the new monitor calibrated I see each step as equals at both
ends not just the middle. I'd never have seen the 24.5 one even in the
FITS with the old monitor. Now they show rather easily. Brighten the
image until it is no longer black and they should show up. I normally
set my darkest pixel about 15. It appears to be 14 on this image.
Second level on the attached image is 11-12.

Using CCDSoft and The Sky to plate solve the FITS I was able to find
every galaxy NED listed in the SDSS survey that was magnitude 24.5 and
above. I could put the cursor on any little spot and sure enough there
was a SDSS galaxy at that position. I found several at 24.6 but they
were scattered on the image well away from Arp 72. I needed the 2x
scale to see them easily. Had to really work to see them at 1x but then
the monitor has really tiny pixels beyond what my eyes can resolve
compared to the old one. The survey stopped at 24.6 and I tried to find
them all. Not many at that level so wasn't hard. I found all but 10 of
them, that's out of well over 2000. Nearly every faint spot, especially
those that look the least bit odd, are galaxies not stars. Survey
listed most stars as well but the galaxies outnumbered the stars by a
huge margin.

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".

"Rick Johnson" wrote in message
ter.com...
Richard Crisp wrote:
the faint stars are so faint I cannot see them on my monitor.

the larger galaxy reminds me of my bluetooth earpiece!

what a cool pair of galaxies and a very nice image Rick

As my old monitor was dying the bottom two levels of the gray scale
chart attached were just black and barely darker than the third level.
Now with the new monitor calibrated I see each step as equals at both
ends not just the middle. I'd never have seen the 24.5 one even in the
FITS with the old monitor. Now they show rather easily. Brighten the
image until it is no longer black and they should show up. I normally
set my darkest pixel about 15. It appears to be 14 on this image.
Second level on the attached image is 11-12.

Using CCDSoft and The Sky to plate solve the FITS I was able to find
every galaxy NED listed in the SDSS survey that was magnitude 24.5 and
above. I could put the cursor on any little spot and sure enough there
was a SDSS galaxy at that position. I found several at 24.6 but they
were scattered on the image well away from Arp 72. I needed the 2x
scale to see them easily. Had to really work to see them at 1x but then
the monitor has really tiny pixels beyond what my eyes can resolve
compared to the old one. The survey stopped at 24.6 and I tried to find
them all. Not many at that level so wasn't hard. I found all but 10 of
them, that's out of well over 2000. Nearly every faint spot, especially
those that look the least bit odd, are galaxies not stars. Survey
listed most stars as well but the galaxies outnumbered the stars by a
huge margin.



Matthias Kronberger determined that my M33 had a limiting magnitude in the
19.71 range

that was under a near full moon but with pretty good transparency

I am eager to try some unfiltered luminance under a new moon

here's what Matthias wrote:

Good question. The limiting magnitude for M 33 (I didn't check M 31) is
practically the same as for the DSS red plate. I could identify a faint open
cluster (V=19.70) as a barely visible smudge so that's probably the limit.

Rick,

amazing what objects you are digging out. I just wanted to put this one on
my list, but it was already there :-)
I'll never reach mag 24.5 though, best I ever got in 12 hours or so was
around mag 20. That seems to be the magical barrier for my skies and
equipment, I can't seem to get past it no matter how long I expose.

Stefan


"Rick Johnson" schrieb im Newsbeitrag
ter.com...
Arp 72 is two interacting galaxies in the constellation Serpens Caput.
It is in Arp's class "spiral galaxies with small high surface brightness
companions on arms". He said of it: "...faint material from arm to and
around companion. Opposite arm faint, sweeps around east of galaxy"

It is composed of two galaxies NGC 5994 (small possibly spiral galaxy)
and NGC 5996, a starburst galaxy classed as SBc. They are about 160
million light-years away: A NED note says; "The UV spectrum of this
starburst (classified as such by Balzano 1983 on the basis of optical
data) is similar to the one of the prototype NGC 7714. Here also Si IV
and C IV show P Cygni profiles, a signature of Wolf-Rayet stars. A high
far- infrared luminosity confirms the presence of vigorous star
formation (Deutsch & Willner 1987). This activity is possibly triggered
by the strong interaction with the companion NGC 5994 (Bushouse 1987).

Limiting magnitude of this image is about 24.5! That means the dimmest
object in the image is almost 16 million times dimmer than the faintest
naked eye star visible from my observing deck below the observatory
(magnitude 6.5). See the ID image for details. This is the faintest
I've ever gone in only 40 minutes. Usually my limit is more like 22.5
unless I really let the noise come through. In this case the noise
level was so low the original FITS goes no deeper than this processed
image. I sure wish I knew what I did differently!

NED lists over 1700 galaxies within a 23 minute circle centered on my
image (not Arp 72 which is a bit high as I wanted to catch the possibly
interacting pair at the very bottom. Since all but a very few, less
than 10, are dimmer than 24.5 and the field is 33 minutes wide, not 23,
I probably caught more than 2500 galaxies in this image but you may have
to blow up the image to see them all. Not bad for only 40 minutes of
luminosity data! Math says 3038 so I'm being conservative. In the ID
image the two galaxies marked with their distances are magnitude 20.2
and 20.6, left to right. This was a night of better than average seeing
and transparency. I assume sky glow was unusually low this night though
I never went out to look up. Appears I should have. Most of the stars
in this image are really galaxies per the SDSS survey data. This is why
most of the stars in the enlarged ID image look odd. They aren't stars!

I moved Arp 72 high to catch the two possibly interacting galaxies at
the bottom of the image west of center (right). The big galaxy is SDSS
J154629.02+173914.4 at magnitude 17.2. Unfortunately there is no
redshift data on it. The small round "companion" is SDSS
J154629.94+173918.9 at magnitude 18.2. Redshift data is available for
it and puts it at 1.2 billion light-years. Both seem to have about the
same intergalactic reddening so probably are about the same distance.
But whether the smaller galaxy is just superimposed over the halo of the
larger or they are really interacting I don't know. The very blue
elongated galaxy to the NW is SDSS J154625.91+174017.7 at magnitude 18
and a redshift distance of 650 million light-years. The reddish round
galaxy SE of the possibly interacting pair is SDSS J154634.61+173850.2
at magnitude 17.3 and a redshift distance of 1.3 billion light-years,
about the same as the nearby pair.

The round blue galaxy above and a bit west of Arp 72 is SDSS
J154649.41+175902.0 at 620 million light-years. Someone else doing Arp
galaxies wanted to know what this one was a couple years ago. I found
her question searching the net for images of this guy. No one answered
her. I wonder if she ever figured it out. It's listed as PGC 1546998
in The Sky 6's data base. The somewhat smaller blue "bug splat" of a
galaxy above and east of Arp 72 is SDSS J154712.85+175727.2. It is far
smaller than the former galaxy being only 160 million light-years
distant. Thus it is likely related to Arp 72 and judging by the vivid
blue color likely interacted with it in the past, triggering massive
star formation that is still going on.

East of Arp 72 level with the companion is a "small spiral with a very
dense core and faint disk without obvious arms. It is SDSS
J154714.35+175153.1 which is 1.7 billion light-years away. Above it and
further east is a double galaxy, the eastern one being nearly starlike.
The main one is SDSS J154722.89+175309.7 at 1.4 billion light-years.
I can't find the eastern one in NED! Checking the POSS 2 blue plates
the second galaxy is plainly seen. Why the SDSS missed it I don't know.

Due west of Arp 72 is a small slightly oval blue galaxy. It is SDSS
J154641.50+175337.2 at 620 million light-years. Further west is the
more elongated blue galaxy cataloged as SDSS J154633.14+175300.4. It is
shown at 630 million light-years so likely related to its nearby
companion. They are surprisingly blue for their distance.

At the eastern edge, near the center are three rather bright galaxies.
The western and northern most is a double blue galaxy. It is listed in
both the SDSS catalog and the MCG catalog. The catalogs can't seem to
agree on its position nor its red shift. It is either SDSS
J154757.80+174921.5 with a red shift of 600 million light-years or MCG
+03-40-041 at 500 million light-years. Take your pick! It could be
that the are measuring the red shift of different galaxies in the pair.
If they aren't really related this difference could be real.
Further east and south is the apparently much larger yellowish
elliptical galaxy known as CGCG 107-038 or ARK 485. The latter
indicates it is an emission line object. Something you don't expect of
an elliptical galaxy that should have used up all its gas long ago and
thus not have strong emission lines. It may be bright and interesting
but I couldn't find a red shift value for it. I was hoping notes at NED
would help me but there was only one and it told me only what was
obvious from my image; "Elliptical red object with an envelope." Below
it is another very blue spiral, SDSS J154802.94+174538.4 at 440 million
light-years. Above these three, nearly directly north of the mystery
double blue galaxy is a reddish galaxy with what appears to be a very
disturbed outer envelope. It is SDSS J154756.71+175216.1 at 1.2 billion
light-years. There are several nearly star-like galaxies around it,
could one of them be responsible for so disturbing this galaxy.

There are hundreds of other galaxies I could mention, thousands
actually, such as the tiny reddish but bright galaxy SE of the bright
blue star below Arp 72. It is SDSS J154704.34+174833.0 at 1.3 billion
light-years yet shines at 18th magnitude. But I have to stop somewhere
or write a book on this field so I'll stop with the comment that oddly,
NED lists no quasars in this field. For going so deep I found that
surprising.

I've also attached a 2x blow up of the image showing Arp 72 and pointing
out a 23.8 and 24.5 magnitude galaxies as well as some rather bright,
20th magnitude galaxies 3.8 billion light-years away. Two SDSS galaxies
are marked with their magnitude. They are too faint for collecting red
shift data. One is magnitude 23.8 and should show on most monitors.
The one at 24.5 will test how well you can dig into the dark regions and
pull out detail. It is there but you may have to play with your monitor
if it isn't well calibrated to find it.

Arp's image with the 200" telescope is at:
http://nedwww.ipac.caltech.edu/level...big_arp72.jpeg

SDSS image:
http://casjobs.sdss.org/ImgCutoutDR4...00&height=1000

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

Rick,

amazing what objects you are digging out. I just wanted to put this one on
my list, but it was already there :-)
I'll never reach mag 24.5 though, best I ever got in 12 hours or so was
around mag 20. That seems to be the magical barrier for my skies and
equipment, I can't seem to get past it no matter how long I expose.

Stefan


"Rick Johnson" schrieb im Newsbeitrag
ter.com...
Arp 72 is two interacting galaxies in the constellation Serpens Caput.
It is in Arp's class "spiral galaxies with small high surface brightness
companions on arms". He said of it: "...faint material from arm to and
around companion. Opposite arm faint, sweeps around east of galaxy"

It is composed of two galaxies NGC 5994 (small possibly spiral galaxy)
and NGC 5996, a starburst galaxy classed as SBc. They are about 160
million light-years away: A NED note says; "The UV spectrum of this
starburst (classified as such by Balzano 1983 on the basis of optical
data) is similar to the one of the prototype NGC 7714. Here also Si IV
and C IV show P Cygni profiles, a signature of Wolf-Rayet stars. A high
far- infrared luminosity confirms the presence of vigorous star
formation (Deutsch & Willner 1987). This activity is possibly triggered
by the strong interaction with the companion NGC 5994 (Bushouse 1987).

Limiting magnitude of this image is about 24.5! That means the dimmest
object in the image is almost 16 million times dimmer than the faintest
naked eye star visible from my observing deck below the observatory
(magnitude 6.5). See the ID image for details. This is the faintest
I've ever gone in only 40 minutes. Usually my limit is more like 22.5
unless I really let the noise come through. In this case the noise
level was so low the original FITS goes no deeper than this processed
image. I sure wish I knew what I did differently!

NED lists over 1700 galaxies within a 23 minute circle centered on my
image (not Arp 72 which is a bit high as I wanted to catch the possibly
interacting pair at the very bottom. Since all but a very few, less
than 10, are dimmer than 24.5 and the field is 33 minutes wide, not 23,
I probably caught more than 2500 galaxies in this image but you may have
to blow up the image to see them all. Not bad for only 40 minutes of
luminosity data! Math says 3038 so I'm being conservative. In the ID
image the two galaxies marked with their distances are magnitude 20.2
and 20.6, left to right. This was a night of better than average seeing
and transparency. I assume sky glow was unusually low this night though
I never went out to look up. Appears I should have. Most of the stars
in this image are really galaxies per the SDSS survey data. This is why
most of the stars in the enlarged ID image look odd. They aren't stars!

I moved Arp 72 high to catch the two possibly interacting galaxies at
the bottom of the image west of center (right). The big galaxy is SDSS
J154629.02+173914.4 at magnitude 17.2. Unfortunately there is no
redshift data on it. The small round "companion" is SDSS
J154629.94+173918.9 at magnitude 18.2. Redshift data is available for
it and puts it at 1.2 billion light-years. Both seem to have about the
same intergalactic reddening so probably are about the same distance.
But whether the smaller galaxy is just superimposed over the halo of the
larger or they are really interacting I don't know. The very blue
elongated galaxy to the NW is SDSS J154625.91+174017.7 at magnitude 18
and a redshift distance of 650 million light-years. The reddish round
galaxy SE of the possibly interacting pair is SDSS J154634.61+173850.2
at magnitude 17.3 and a redshift distance of 1.3 billion light-years,
about the same as the nearby pair.

The round blue galaxy above and a bit west of Arp 72 is SDSS
J154649.41+175902.0 at 620 million light-years. Someone else doing Arp
galaxies wanted to know what this one was a couple years ago. I found
her question searching the net for images of this guy. No one answered
her. I wonder if she ever figured it out. It's listed as PGC 1546998
in The Sky 6's data base. The somewhat smaller blue "bug splat" of a
galaxy above and east of Arp 72 is SDSS J154712.85+175727.2. It is far
smaller than the former galaxy being only 160 million light-years
distant. Thus it is likely related to Arp 72 and judging by the vivid
blue color likely interacted with it in the past, triggering massive
star formation that is still going on.

East of Arp 72 level with the companion is a "small spiral with a very
dense core and faint disk without obvious arms. It is SDSS
J154714.35+175153.1 which is 1.7 billion light-years away. Above it and
further east is a double galaxy, the eastern one being nearly starlike.
The main one is SDSS J154722.89+175309.7 at 1.4 billion light-years.
I can't find the eastern one in NED! Checking the POSS 2 blue plates
the second galaxy is plainly seen. Why the SDSS missed it I don't know.

Due west of Arp 72 is a small slightly oval blue galaxy. It is SDSS
J154641.50+175337.2 at 620 million light-years. Further west is the
more elongated blue galaxy cataloged as SDSS J154633.14+175300.4. It is
shown at 630 million light-years so likely related to its nearby
companion. They are surprisingly blue for their distance.

At the eastern edge, near the center are three rather bright galaxies.
The western and northern most is a double blue galaxy. It is listed in
both the SDSS catalog and the MCG catalog. The catalogs can't seem to
agree on its position nor its red shift. It is either SDSS
J154757.80+174921.5 with a red shift of 600 million light-years or MCG
+03-40-041 at 500 million light-years. Take your pick! It could be
that the are measuring the red shift of different galaxies in the pair.
If they aren't really related this difference could be real.
Further east and south is the apparently much larger yellowish
elliptical galaxy known as CGCG 107-038 or ARK 485. The latter
indicates it is an emission line object. Something you don't expect of
an elliptical galaxy that should have used up all its gas long ago and
thus not have strong emission lines. It may be bright and interesting
but I couldn't find a red shift value for it. I was hoping notes at NED
would help me but there was only one and it told me only what was
obvious from my image; "Elliptical red object with an envelope." Below
it is another very blue spiral, SDSS J154802.94+174538.4 at 440 million
light-years. Above these three, nearly directly north of the mystery
double blue galaxy is a reddish galaxy with what appears to be a very
disturbed outer envelope. It is SDSS J154756.71+175216.1 at 1.2 billion
light-years. There are several nearly star-like galaxies around it,
could one of them be responsible for so disturbing this galaxy.

There are hundreds of other galaxies I could mention, thousands
actually, such as the tiny reddish but bright galaxy SE of the bright
blue star below Arp 72. It is SDSS J154704.34+174833.0 at 1.3 billion
light-years yet shines at 18th magnitude. But I have to stop somewhere
or write a book on this field so I'll stop with the comment that oddly,
NED lists no quasars in this field. For going so deep I found that
surprising.

I've also attached a 2x blow up of the image showing Arp 72 and pointing
out a 23.8 and 24.5 magnitude galaxies as well as some rather bright,
20th magnitude galaxies 3.8 billion light-years away. Two SDSS galaxies
are marked with their magnitude. They are too faint for collecting red
shift data. One is magnitude 23.8 and should show on most monitors.
The one at 24.5 will test how well you can dig into the dark regions and
pull out detail. It is there but you may have to play with your monitor
if it isn't well calibrated to find it.

Arp's image with the 200" telescope is at:
http://nedwww.ipac.caltech.edu/level...big_arp72.jpeg

SDSS image:
http://casjobs.sdss.org/ImgCutoutDR4...00&height=1000

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