Object of the Week, January 24 2021 — Holmberg II

[akarsh]

Name: Holmberg II = UGC 4305 = Arp 268
RA: 08 19 05
Dec: +70 43 12 (J2000)
Constellation: Ursa Major
Size: ~ 7' x 5'
Mag: 11.4
SBr: ~15 mag/arcmin² (23.9 MPSAS)

Holmberg II is a faint irregular dwarf galaxy in Ursa Major, about 10 Mly away from us. The galaxy was first described by Swedish astronomer Erik Holmberg of Lund Observatory in his 1950 "Holmberg dwarfs" paper [1]. It is a member of the nearby M81--NGC 2403 group, likely the closest one to us in that group [3].

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[DSS POSSII Red plate. Credit: MAST/STScI]

In the professional world, Holmberg II seems to be rather well-studied mostly for its gas-rich interstellar medium [2], star formation [3,4], and an ultra-luminous X-ray source [5]. Here is a beautiful HST image of the HII regions in this galaxy:
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[Credit: HST/NASA/ESA]

In the amateur world, Holmberg II is perhaps best known for being part of a "short" observing program of the 9 Holmberg dwarf galaxies [6], and also of the Arp catalog.

I first observed this object in 2013 with my 18" from Bortle 2 skies near Pontotoc, TX. At 200x, I was able to identify mottling and two brighter regions marked in this rough sketch:
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My meager notes from an observation in Dec 2015 from the darker Commanche Springs Astronomy Campus in Crowell, TX say that the object was rather easy with my 18", and I was able to pick out the central bright region of the object and also hold two brightenings within that region. The HII regions could not be identified. The notes further say that it was also visible in my friend's 5" Takahashi refractor, and while it was certainly not easy to see, it was not as indistinct as I had presumed it would be.

I would love to give this object another shot to see if I can see the HII knots. Hodge, Strobel and Kennicutt [7] describe 82 HII regions in this galaxy! [7] and [3] also point out that an unusually large number of HII regions in this galaxy show ring structures. Perhaps the brightest of these HII regions is cataloged as Holmberg II [HK82] 11 in NED (SIMBAD has it listed as HK Ho II 11) and appears to be well within the range of our amateur telescopes. Here is the position data for this HII knot:
RA: 08 19 13
Dec: +70 43 08 (J2000)
This is the HII region seen immediately below the center in the HST image posted above.

Finally, let me end with some biographical facts about Prof. Holmberg [8,9]:

Holmberg was one of the first to consider determining galaxy masses from rotation curves.
He discovered the Holmberg effect – significantly more number of satellite galaxies lie in projection above the poles of spiral galaxies than in the equatorial planes.
He is famous for his work on interacting galaxies. In 1941, he constructed an analog computer with light bulbs and photocells to simulate the collisions of galaxies!
His waltz with Russian astronomer Anna Massevitch was one of the highlights of the closing banquet of the IAU General Assembly in Brighton in 1970!


As always,

"Give it a go, let us know"!


References:
[1] http://adsabs.harvard.edu/pdf/1950MeLuS.128....5H
[2] http://adsabs.harvard.edu/pdf/1992AJ....103.1841P
[3] https://iopscience.iop.org/article/10.1086/308241/pdf
[4] https://iopscience.iop.org/article/10.3847/1538-4357/aab041/meta
[5] https://phys.org/news/2015-06-unexpectedly-black-hole-monsters-rapidly.html
[6] https://www.springer.com/gp/book/9781852337520

PS: High-res HST image to enjoy! https://archive.stsci.edu/cgi-bin/hla/display?config=stdatu&image=hlsp_legus_hst_acs_ugc4305_f814w_v1_drc%2Chl sp_legus_hst_acs_ugc4305_f555w_v1_drc&size=2371,22&izoom=1.000000&detector=ACS%2FWFC&aperture=multi&title=hlsp_legus%20ACS%2FWFC%20F814W%2FF555W%20(co lor)%20UGC-4305-2

[Clear Skies]

Logged a single observation for this galaxy but was unable to pick out any significant detail with my aperture.

26 February 2019, 14" SCT @ 168x/29' from the French Ardennes:

A faint, irregular, NNE-SSW elongated glow, brighter in the WSW part. On the eastern edge is a small, elongated triangle of mag. 11 and fainter stars pointing ESE. The galaxy is slightly wider on the NNE side.

[Steve Gottlieb]

With my old 17.5" and 20 years back, I logged Holmberg II as

"Immediately picked up at 140x as a large, low surface brightness glow mostly west of a distinctive triple star (mag 11.5-12.5), which is situated along on the eastern edge of the galaxy! Diffuse and elongated ~3:2 N-S, ~4'x2.5', although the edges generally fade into the background so the size was difficult to estimate. It appeared slightly brighter in the vicinity of the triple star [probably refers to HII region HSK 45]. The visibility of Holmberg II was surprisingly easy and it was quite obvious although it appeared more like a faint galactic nebula than a galaxy."

With Jimi's 48" I recorded the following HII regions --

"Just west of the brighter superposed mag 12 star is a small, fairly bright HII region (HSK 45 = HK 10), ~15" diameter. A second, much fainter HII knot (HSK 70 = HK 4) with a diameter of ~10" is 30" following the mag 13 star (SE vertex of the triangle). A similar third HII knot (HSK 73 = HK 1) lies 30" N. Just off its north edge is a fourth extremely small knot (HSK 71 = HK 2). The HSK designations are from "HII Regions of Holmberg II", PASP, 106, 309 (1994) and HK designations from Hodge and Kennicutt "At Atlas of HII regions in 125 galaxies" in AJ, 88, 296 (1983)."

Steve

[Howard B]

Excellent OOTW Akarsh, and I especially like the links to all the research papers - this little dwarf galaxy is more interesting than I suspected! I have only one observation to share, from 2015, with my 28-inch:

"This irregular dwarf galaxy was bigger and fainter than I expected, but several clumps were visible with extended averted vision. This could be much more interesting on a really dark and transparent night! 253x, SQM 21.21."

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I'm not sure what I meant by "extended averted vision" - probably that it took me a long time using averted vision to see everything in my sketch. There's over an hour between my notes for Arp 268 and the previous object in my notebook, so that makes sense.

[Uwe Glahn]

Hi Akarsh,

welcome as a new OOTW contributor and all the best for the future.
You started with a very interesting object. I just have one entry with my older 16-inch under rural skies. I noted: large glow wit low surface brightness but easy visible even in the searching eyepiece; brightest part west from a small triangle; this part is around 2'x3' large with North-south elongation and diffuse edges to the north; east of the triangle, a faint north-west elongated line is visible without showing single (HII) spots

sketch: 16", 129x, NELM 6m5+
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home (http://www.deepsky-visuell.de/Zeichnungen/Holmberg2.htm)

[Robin]

Hi Akarsh,

I logged two observations of this galaxy with different telescopes:

With my 12" Dobsonian I observed it three years ago at 49x - 168x magnification and it looked the best at 84x. I saw a large, faint and diffuse brightening near three 12-14 mag stars. It was very hard to detect at 168x. Please find attached my sketch.

With my 8" Dobsonian I observed it in 2007 at a dark site in the Northern Colorado Rocky Mountains. I used 48x magnification, but noted that a larger exit pupil might help. Very faint and diffuse glow.

Clear skies

Robin