ASTRO Holmberg IV

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

Rick

Rick,

if this is near M101 I will have to put it on my list as I am always in
search of galaxies for the second half of the night in spring.

Stefan

"Rick Johnson" schrieb im Newsbeitrag
...

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

Rick