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Holmberg IV A very strange blue galaxy

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#1 Rick J

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Posted 01 December 2013 - 02:19 PM

Holmberg IV is an barred irregular galaxy of the Magellanic class that is considered to be a member of the M81 group. Though it is located only 1.5 degrees west southwest of M101. Redshift puts it about 13 million light-years distant which is about right for the M81 group. However Tully Fisher measurements put it almost twice as far at 21 million light-years. This is more in line with the distance to M101. Holmberg did apparently consider it part of the M81 group. Wikipedia considers it part of the M81 group as well. Though I found SEDS considers it part of the M101 group as did one paper listed in the notes at NED. For now I'll go with its location and non redshift distance that put it much closer to M101 than M81 both as we see it projected in the sky and as it really is in 3D space if the non redshift distance is more accurate. Redshift is usually very inaccurate for close in galaxies like this. While Tully Fisher measurements are open to several interpretations they tend to be more accurate at close distances than redshift. To argue the other side; if it is 21 million light-years away its size is a bit over 25,000 light-years. Toward the large end of dwarf status. Moving it to only 13 million light years puts it at 16,000 light-years more typical of a dwarf. But if it is part of the M81 group it lies a minimum of about 7 million light-years from M81 given its distance of 30.6 degrees from M81. That's too far in my opinion to be part of its group. It is a minimum of 477,000 light-years from M 101 assuming the same distance or about 2 to 2.5 million light-years assuming M101 is 23 million light-years away and it is 21 million light-years distant. Thus I'm back to saying it belongs to M101 not M81. If the closer redshift distance is right then it belongs to neither being too far from both.

The galaxy is of low surface brightness so made the DDO catalog of such galaxies as entry DDO 185. NED classes it as IB(s)m. Most sources consider it a dwarf galaxy. It is very blue so contains a lot of relatively new stars. A possible indication of interaction with another galaxy in the recent past. I'll pick M101 as the likely culprit. The galaxy has no obvious nucleus that I can see. Though there is a very minor condensation toward the middle of the galaxy just down from the northern, rather bright amorphous region that might be a core. Radio observations indicate it is likely a disk galaxy tilted rather close to edge on. Apparently its star formation has used up most of its dust with the portion not turned to stars being ejected by the interaction. At least that's one way to interpret this galaxy.

For some reason redshift data is available for only parts of my image. Much of the lower left has no redshift information and other regions has very little. So the annotated image appears rather odd with those blank areas. While some faint fuzzies around 86 Ursa Major at the lower right were lost in processing out the halo (see below) I was surprised that the all with redshift data survived surprisingly unhurt including one that is listed as fainter that 21st magnitude and 5 billion light-years distant. Apparently my technique worked better than I thought except for exceeding faint fuzzies.

This is another object in which I collected the luminance on a night of not awful seeing but the color data was collected on a night with poor seeing. I'd hoped the color data would have been usable but it seems to be only barely usable. Seeing varied so much the stars took on rather bloated and slightly randomly distorted shapes that were different on every sub. This caused some color flare issues when a flare was especially strong. A problem I've been fighting thanks to the horrid imaging skies we've been having. Also the bright star to the lower right is 86 Ursa Majoris, a 5.7 magnitude A0V star. It created a horrid reflection that covered a good quarter of the image. These apparently come from the corrector plate. I spent a lot of time removing it. In doing so some of the faint objects in its area suffered. My normal methods of subtracting the halo out by using a halo made by aligning the halos of several similar stars then combining with data rejection so the other stars all vanish leaving just the halo, didn't work because for reasons I don't understand the halo contained a partial image of the internal baffle in the scope. Something I couldn't recreate. So I had to do the job manually. I'm not any good at that. I had to call in the wife who has a bit more artistic ability to help. Now she expects I owe her big time in chores for her. It might have been easier to have retaken it with the star out of the image and the galaxy low in the frame. But with the rotten weather that might never happen.

14" LX200R @ f/10, L=4x10' RGB=2x10', STL-11000XM, Paramount ME

Full image at my normal 1" per pixel

Annotated

Attached cropped image at 0.67" per pixel

Rick

Attached Thumbnails

  • 6227209-HOLMBERG_IV_L4X10RGB2X10CROP150.JPG


#2 David Ault

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Posted 01 December 2013 - 02:29 PM

Very nice image Rick and a fantastic write up! I always enjoy learning about everything visible in an image.

Regards,
David

#3 coinboy1

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Posted 01 December 2013 - 05:01 PM

Rick,

I have always admired your images and information. Is your image cropped? I think your star images across the field is excellent. That makes me think highly of the ACF design and I cant believe your images are so crisp @F10.

#4 Rick J

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Posted 02 December 2013 - 02:31 PM

When I say "full image" it is just that, the full frame of the CCD. But the stars do suffer in the corners. While this is the full frame that is 4008x2672, I post these half size as 4008x2672 is too big. That reduces them from 0.5" per pixel down to 1" per pixel hiding some of the issues with the corners.

For reasons I've never been able to figure out sometimes one side (always the left) will elongate and other times it doesn't. Even if it stays round it does grow in size. Typically if I have 2.5" FWHM stars in the center they will grow to 3.25" in the corners. If they stay round it tends to make them just look brighters. If they elongate then it is very obvious. At first I thought the elongation was a focus issue. But when they do this there's no focus point that they turn round. The next night they may be round again. I did have an issue with a lose set screw that allowed the camera to tilt causing a different kind of elongation (both sides elongate with tilt). I've not tracked down the cause of it when tilt isn't the cause. I've talked to several ACF users, some have the problem, others don't. Those that do are using 12" and larger. None I've talked with have seen it in the 10" or smaller size. Also today's version is f/8 and a bit different than my old f/10 version. I've not heard of the issue with the f/8 version.

One other issue with the SCT design seems to be that you get nasty reflections from bright stars (Mag 5 or brighter in my 14") that are very difficult to remove and down to mag 8 are an issue to remove (this image is an example of one right at the edge of being able to remove it). So imaging around very bright stars is sometimes just not possible with this design due to this reflection. It covers nearly all the frame and has fine structure so can't be removed like a gradient. I have to image similar stars, remove the stars and subtract that reflection from the one in the image after very careful alignment leaving the star itself, just not the reflection. It is a challenging process, at least for me. Even the pros have this issue with the design. See the image with the 48" Schmidt for example at this link. Their reflection is half my size as they put the corrector at the radius of curvature while today's SCTs but the corrector at the focal point, half as distant. Also they have no filters or optical window for the CCD so I get multiple rings of nearly but not exactly the same size. The micro lenses on the CCD then multiply this into many rings that overlap. It can be a real mess. Some using SCT's to image the flame have posted their nasty results from Alnitak for example.

Rick

#5 jshalpha

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Posted 02 December 2013 - 06:45 PM

Great image and nice write up.

Jim S.

#6 Man in a Tub

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Posted 02 December 2013 - 10:41 PM

Great image and nice write up.

Jim S.


+1

#7 Louietheflyisme

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Posted 03 December 2013 - 04:50 AM

Wow, that's a lot of info there and a great image to compliment it.


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