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NGC 4242 A Low Surface Brightness Galaxy With No Core



 
 
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  #1  
Old September 7th 15, 07:04 AM
WA0CKY WA0CKY is offline
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Default NGC 4242 A Low Surface Brightness Galaxy With No Core

NGC 4242 is a very low surface brightness spiral galaxy in Canes Venatici about 1.75 degrees south of much better known M106. It may in fact be related to M106 as its distance is 33 million light-years based on cosmological redshift and 22 million light-years by an average of various non redshift measurements that are likely more accurate. This is about the same as the 23.5 million light-year distance often cited for M106. Assuming it is a companion they are only about 700,000 light-years apart much closer than we are to M32 (for now that is). In its center where I'd expect the core to be there is a starlike object. It's PSF is similar to a star indicating whatever it is is unresolved at my seeing. Is it the core or is it a star in our galaxy that happens to be perfectly aligned? Fortunately the HST has taken an image of the core and it seems to be just a field star. I've attached a crude composite of a near Uv and near IR image with the planetary camera in WFPC2. That appears to show it as just a star. Therefore the reports of no visible core are correct. It is classified as SAB(s)dm. It has little spiral structure. What passes for arms seem more radial than spiral and are mostly just star clouds and star clusters. While it is very blue little dust is seen so this may be its last gasp at star formation though radio observations show a HI disk that's 3 minutes across while the galaxy is 5 minutes on its major axis. That makes it's visual size only about 33,000 light-years, so it isn't a large galaxy. It was discovered April 10, 1788 by William Herschel. It's low surface brightness makes it a visual challenge, maybe that's why it isn't in either of the two Herschel 400 observing programs. Still its large angular size makes it a great galaxy that few seem to know about. It's low surface brightness would challenge those working from rather bright skies.

In the annotated image I did include three quasar candidates. NED's primary designation for these however is just a star. Only photographic redshifts are available for these and likely the cause of both their extreme distance estimates as well as possible quasar status. Watching these almost always turn into confirmed stars rather than quasars I tend not to include them in my annotations. Several known quasars with accurate spectroscopic redshifts are also noted in the image. Arp seemed to think these were nearby objects ejected from active galaxies. NGC 4242 isn't active in the way he seemed to think caused quasars so I wonder when he made of all them around this galaxy?

The annotated image shows quite a few galaxies at about 920 million light-years. While NED indicates the 4 south of NGC 4242 are SDSSCGB 39935 this leaves out the others scattered further out.

Seeing turned lousy during the collection of color data leading to some nasty color issues with halos of vivid red, green and blue around the brighter stars, especially those that are nearly white which are rather common in the image. I had a heck of a time dealing with these flares and halos. Seeing hit 6" at times compared to the less than 3" for the luminance data. I probably should have reshot the color but didn't realize this was a problem until I recently went to process it and it was lost in the sun's glare. I managed to process out all but a hint of the problem but it took way too much time. If it hadn't have been such an interesting galaxy I'd likely have reshot it next year.

14" LX200R @ f/10, L=5x10' RGB=2x10', STL-11000XM, Paramount ME

Rick
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Last edited by WA0CKY : September 7th 15 at 07:06 AM.
  #2  
Old September 8th 15, 08:52 PM posted to alt.binaries.pictures.astro
Stefan Lilge
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Posts: 2,269
Default NGC 4242 A Low Surface Brightness Galaxy With No Core

Rick,

that looks really good for a low surface brightness galaxy.
Nice object, but as you mention it might be hard to image under bright
skies.

Stefan



"WA0CKY" schrieb im Newsbeitrag ...


NGC 4242 is a very low surface brightness spiral galaxy in Canes
Venatici about 1.75 degrees south of much better known M106. It may in
fact be related to M106 as its distance is 33 million light-years based
on cosmological redshift and 22 million light-years by an average of
various non redshift measurements that are likely more accurate. This
is about the same as the 23.5 million light-year distance often cited
for M106. Assuming it is a companion they are only about 700,000
light-years apart much closer than we are to M32 (for now that is). In
its center where I'd expect the core to be there is a starlike object.
It's PSF is similar to a star indicating whatever it is is unresolved at
my seeing. Is it the core or is it a star in our galaxy that happens to
be perfectly aligned? Fortunately the HST has taken an image of the
core and it seems to be just a field star. I've attached a crude
composite of a near Uv and near IR image with the planetary camera in
WFPC2. That appears to show it as just a star. Therefore the reports of
no visible core are correct. It is classified as SAB(s)dm. It has
little spiral structure. What passes for arms seem more radial than
spiral and are mostly just star clouds and star clusters. While it is
very blue little dust is seen so this may be its last gasp at star
formation though radio observations show a HI disk that's 3 minutes
across while the galaxy is 5 minutes on its major axis. That makes it's
visual size only about 33,000 light-years, so it isn't a large galaxy.
It was discovered April 10, 1788 by William Herschel. It's low surface
brightness makes it a visual challenge, maybe that's why it isn't in
either of the two Herschel 400 observing programs. Still its large
angular size makes it a great galaxy that few seem to know about. It's
low surface brightness would challenge those working from rather bright
skies.

In the annotated image I did include three quasar candidates. NED's
primary designation for these however is just a star. Only photographic
redshifts are available for these and likely the cause of both their
extreme distance estimates as well as possible quasar status. Watching
these almost always turn into confirmed stars rather than quasars I tend
not to include them in my annotations. Several known quasars with
accurate spectroscopic redshifts are also noted in the image. Arp
seemed to think these were nearby objects ejected from active galaxies.
NGC 4242 isn't active in the way he seemed to think caused quasars so I
wonder when he made of all them around this galaxy?

The annotated image shows quite a few galaxies at about 920 million
light-years. While NED indicates the 4 south of NGC 4242 are SDSSCGB
39935 this leaves out the others scattered further out.

Seeing turned lousy during the collection of color data leading to some
nasty color issues with halos of vivid red, green and blue around the
brighter stars, especially those that are nearly white which are rather
common in the image. I had a heck of a time dealing with these flares
and halos. Seeing hit 6" at times compared to the less than 3" for the
luminance data. I probably should have reshot the color but didn't
realize this was a problem until I recently went to process it and it
was lost in the sun's glare. I managed to process out all but a hint of
the problem but it took way too much time. If it hadn't have been such
an interesting galaxy I'd likely have reshot it next year.

14" LX200R @ f/10, L=5x10' RGB=2x10', STL-11000XM, Paramount ME

Rick


--
WA0CKY

 




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