NGC 3675 a flocculent galaxy

[RickJ]

NGC 3675 is a beautiful but rarely imaged flocculent galaxy in Ursa Major. By redshift it is about 46 million light-years distant. Other measurements, mostly Tully-Fisher, range from 41 to 65 million light-years. The one claimed to be most accurate in a 2009 paper says 54 million light-years. I found little agreement beyond this. It is classed as SA(s)b;HII LINER. I'm a bit puzzled over the HII as the only note at NED mentioning HII said the regions were less than 2" across and indicated they considered them a minor feature. It is tilted rather strongly toward an edge on view. This means we see the dust lanes on the near side much better than those of the far side. In fact the galaxy has a large halo that further hides the far side from view. I've applied considerable processing to this region to bring out detail usually hidden by the halo in the few images of this galaxy I found on the net. Even the Sloan Survey image has difficulty with detail of the far side. The core seems to have no real bulge. It even appears recessed as if the flocculent arms and dust lanes extend further above and below the disk's plane than does the core. This is likely an illusion due to my processing to bring out spiral structure right to the very center; but one I can't seem to shake.

My reason for imaging it is the odd outer dust lane on the east side as well as the extensive outer faint regions. Are they plumes? Most images fail to show the outer regions with many stopping at the dust band not even realizing it is a band rather than the edge of the galaxy yet it is bright enough to show color rather well.

There's a really strange galaxy above NGC 3675. It is SDSS J112622.06+434124.4. It has one arm on the east side mostly detached from the core. There's nothing on it in the literature. I'd love to see what Hubble's view of it would be. There are several galaxy clusters in the image. The one due east of NGC 3675 is WHL J112649.0+433441. The BCG is listed at 4.05 billion light-years but the cluster with the same location has a photographically determined redshift of 4.25 billion light-years. I consider the former more likely the more reasonable estimate. To the west and a bit north is the 22 member group WHL J112459.4+433810. Right near the northwest end of NGC 3675 is the small 7 member group WHL J112557.7+433919. Virtually my entire field is covered by ZwCl 1122.8+4351. It is listed as being 19 minutes across and containing 99 members. It's morphology is listed as open which means it lacks any pronounced concentration of galaxies. Yet another cluster is centered just off the east edge of my image. Several of its members are in the frame so I drew a line to indicate where the center is just a few pixels beyond the edge of the image. The cluster is GMBCG J171.91396+43.71478. The BCG is listed at 2.5 billion light-years with the cluster's photographic distance at 2.6. Again, the photographic determination is usually less accurate so I went with the 2.5 figure in the image.

The vast majority of galaxies in the field had no redshift values at NED. All that did are listed in the annotated image.

I see by the current Astronomy Magazine the term "flocculent galaxy" was coined by Debra Meloy Elmegreen the current president of the American Astronomical Society. I didn't realize it was such a new term. I'd been using it for these galaxies for decades, likely before she was even born. But then I don't publish in astronomical science literature.

I've also included the Sloan Survey image of this galaxy. It was the only image I found on the net that indicated there was more to the galaxy than all the other images I found were showing.

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

Rick

[Jairo]

Hi, I noticed the stars are a little blue on the left side. What causes that?

[RickJ]

Actually it varies across the image. At the far right it is the top that has the blue fringe and lower left has no fringe at all. Several factors are at work here. A SCT scope like I use has a single non-achromatic corrector. This cause the blue to be slightly out of focus so the stars in the blue image are larger than in red and green. The atmosphere also tends to separate the colors though the effect is small when the object is high in the sky. Another is my scope is very sensitive to temperature. Even a 1C temperature change causes stars to change by about 2 pixel in separation across an image. Two stars were centered at pixel 5,5 and 2000, 1330 after a 1 C temperature change they are at either about 4,4 and 1999, 1329 or 6,6 2001,1331. All this puts a premium on aligning the colors. Standard alignment software can't handle all of these image scale changes as they handle only positional changes and assume the scale stays the same. In this case my notes show a 4.8C temperature drop during this exposure. This means stars across the image, using a standard alignment tool, would result in a misalignment of the colors (Blue was taken first then L then red then green) that was up to 9 pixels if the center was aligned or 18 if a corner was aligned. Not good. So I use a program called Registar. It handles the image scale issue rather well but often takes two passes to be dead on when it is severe as in this case. Still the blue fringe will be there and some elongation from atmospheric dispersion. In this case I was lazy and used only one pass rather than two. Each pass uses up several hundred megabytes of disk space so I use two only in severe cases.

At least they found older images. Many are still not showing up since the move. I can't even get them to reload as it says they are there. I can't delete them because they aren't there. Maybe they'll get it straightened out. This happened the last time they updated to a new server. It drove away most of the better imagers. Not many left any more.

Rick