Holmberg I A Blue Dwarf Galaxy

Holmberg I is the first of 9 dwarf galaxies in Holmberg's list of dwarf galaxies, most of which lie somewhat in the area of M 81. Holmberg 1 lies about 2.5 degrees northwest (330°) of M81. It is a very blue, low surface brightness dwarf with little HII emission according to papers I read. That surprised me as the HST image shows some nice nebula in it (see my crudely processed attached image). Then I noticed the filters used by the HST, Green (centered on the same frequency as most green lasers, 555nm) was assigned to blue with near infra red, 814nm, was assigned to red. Thus the blue nebulae were seen in green light rather than H alpha red. They show as blue fuzzy "stars" in my image and are quite weak in red light. I need to revisit this one in H alpha sometime.

The redshift distance is 10 million light-years but this is highly unreliable so close in. Still the non redshift values show it only slightly further. They range from 12.5 to 19.2 million light-years. Three values were derived from the HST images, some of which I used for my rendering. Those are in quite close agreement with the latest having the value of 13.0 million light-years which I show on the annotated image.

I got quite a shock when I had NED list the objects of all types in the field with redshift data. It returned only Holmberg I. So I asked for all galaxies or galaxy clusters and quasars or quasar candidates. That returned only three others! One is barely visible on the FITS stack but almost invisible on the processed TIFF file. After JPG compressing it is gone unless you hype the brightness greatly. It is listed as being possibly a dwarf spherical galaxy. It's position is somewhere in a 5 arc second circle but that does include the faint fuzz patch I've marked. The other galaxy right near Holmberg I is bright but near starlike until you look at its PSF. One star is listed as a strong ultraviolet source by the Galex satellite and is considered a candidate quasar though with no red shift and little else I could find on which to make this claim.

When I asked for all objects it did return 3 HII regions. Two agree with tiny blue "stars" in the galaxy in my image. Only one is in the HST image as I cropped it and it is nearly star-like even in the HST image but there might be a hint of fuzz there. Same with the one just out of my processed version of the HST image. It was cropped out due to blank regions nearby not covered by the HST image. The third one listed is not in the galaxy but is east of it and appears just like a star in both my and the HST image. I can't see any hint of nebulosity in either.

I didn't expect so much confusion when I took this one but as is rather often the case I ended up with questions and not much agreement on anything.

Of the 9 Holmberg galaxies I'm yet to catch VI and VII. The former is at -21.3° so too low unless I have a one in a thousand night. VII is at +6.3° but there are a lot on my to-do list at its R.A. and it just hasn't been selected as yet. I had it at high priority this spring but the weather never cooperated.

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

Rick

This is the one I meant to say was a member of the M81 group and then didn't correct it when I corrected the prior post. Also I forgot to mention this paper http://iopscience.iop.org/1538-3881/...122_6_3070.pdf . It is a 2001 paper and the distance to the galaxy is now considered a bit greater than the article states it but otherwise it has a lot of good information on the galaxy though it gets a bit deep going.

Rick
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Originally Posted by WA0CKY

Holmberg I is the first of 9 dwarf galaxies in Holmberg's list of dwarf galaxies, most of which lie somewhat in the area of M 81. Holmberg 1 lies about 2.5 degrees northwest (330°) of M81. It is a very blue, low surface brightness dwarf with little HII emission according to papers I read. That surprised me as the HST image shows some nice nebula in it (see my crudely processed attached image). Then I noticed the filters used by the HST, Green (centered on the same frequency as most green lasers, 555nm) was assigned to blue with near infra red, 814nm, was assigned to red. Thus the blue nebulae were seen in green light rather than H alpha red. They show as blue fuzzy "stars" in my image and are quite weak in red light. I need to revisit this one in H alpha sometime.

The redshift distance is 10 million light-years but this is highly unreliable so close in. Still the non redshift values show it only slightly further. They range from 12.5 to 19.2 million light-years. Three values were derived from the HST images, some of which I used for my rendering. Those are in quite close agreement with the latest having the value of 13.0 million light-years which I show on the annotated image.

I got quite a shock when I had NED list the objects of all types in the field with redshift data. It returned only Holmberg I. So I asked for all galaxies or galaxy clusters and quasars or quasar candidates. That returned only three others! One is barely visible on the FITS stack but almost invisible on the processed TIFF file. After JPG compressing it is gone unless you hype the brightness greatly. It is listed as being possibly a dwarf spherical galaxy. It's position is somewhere in a 5 arc second circle but that does include the faint fuzz patch I've marked. The other galaxy right near Holmberg I is bright but near starlike until you look at its PSF. One star is listed as a strong ultraviolet source by the Galex satellite and is considered a candidate quasar though with no red shift and little else I could find on which to make this claim.

When I asked for all objects it did return 3 HII regions. Two agree with tiny blue "stars" in the galaxy in my image. Only one is in the HST image as I cropped it and it is nearly star-like even in the HST image but there might be a hint of fuzz there. Same with the one just out of my processed version of the HST image. It was cropped out due to blank regions nearby not covered by the HST image. The third one listed is not in the galaxy but is east of it and appears just like a star in both my and the HST image. I can't see any hint of nebulosity in either.

I didn't expect so much confusion when I took this one but as is rather often the case I ended up with questions and not much agreement on anything.

Of the 9 Holmberg galaxies I'm yet to catch VI and VII. The former is at -21.3° so too low unless I have a one in a thousand night. VII is at +6.3° but there are a lot on my to-do list at its R.A. and it just hasn't been selected as yet. I had it at high priority this spring but the weather never cooperated.

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

Rick