Object of the Week November 5th, 2023 – NGC 918 and its surrounding Molecular Clouds

[Uwe Glahn]

NGC 918 (UGC 1888, MCG+03-07-011)
Galaxy in Aries

RA: 02 25 20.8
DEC: +18 29 45
Mag: 12.4v (13.1b)
Size: 3.5' x 2.0'

Since Mel Bartels publish his observations of the faint background nebula, the topic and the new field of activity in the Deep Sky fascinated me. After some years of follow-up I have to assume - yes, theses nebula are doable. And you don't need Bortle 1 skies or an f/3 reflector.

Quite special are "background objects of the background nebula", say, galaxies behind the nebula which forms an exciting pair and a possibility to experience the field and discover the depth of the Deep Sky.

One of the Gems is NGC 918 with its surrounding Molecular Clouds MBM 7 and 8.

Lets start with NGC 918. The history is quickly told. The barred spiral galaxy was discovered by John Herschel on Jan 11, 1831. It is also an active nucleus galaxy (AGN). Two supernovae were discovered. According to this, a 15vmag star is lying only 13" NW of the nucleus. Modern measurements gave distances of about 67 million light years from the Milky Way.

Zooming out to a typical telescope field of around 0.5°, we find brighter background nebula directly east and west of NGC 918. These nebula are already visible in the DSS plates. They are named PGCC G152.27-38.90 and PGCC G152.05-39.03. PGCC stands for "Planck Galactic Cold Clump" [2016A&A...594A..28P]. They are molecular clouds - a dense concentration of gas and dust within our Milky Way. These clouds consist of molecular hydrogen. The satellite "Planck" uses his three highest bands (857, 454, and 353GHz) to detect the very cold clumps. There are no distances for this cloud, but more further below.

Zooming further out to a binocular field of around 4°, other brighter nebula north of NGC 918 coming along. They are named as MBM 7 and 8. MBM stands for "Magnani-Blitz-Mundy" [1985ApJ...295..402M]. This was the first systematic survey and catalogue of molecular content. The authors found 57 clouds, two of them north of NGC 918. In addition to this we have a distance of around 700 light years for MBM 7 and 850 light years for MBM 8 [2021ApJS..256...46S].

The important question is - is it visible for us? Yes it is. After my experience molecular clouds are much easier to observe than equally (at the DSS) shining red HII regions or blue refection nebula. There are simply large, which makes it difficult to frame the objects. NGC 918 itself is a target for an 4-inch telescope. With this aperture and reachable field the MBM clouds are in range of visibility. Larger telescopes shows the faint arms of the galaxy and the PGCC clouds with lower magnification.

image: Jim Thommes, FSQ 106N
LBN707_FSQ_2PM_PI3_AF4_crp_med.jpg
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annotated image

sketch: Uwe Glahn, 4" binocular telescope, 14x, NELM 6m5+
NGC918_IFN_4.jpg
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sketch: Uwe Glahn, 27" telescope, 113x, NELM 6m5+
NGC918_IFN_27.jpg
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GIVE IT A GO AND LET US KNOW!

[akarsh]

The important question is - is it visible for us? Yes it is. After my experience molecular clouds are much easier to observe than equally (at the DSS) shining red HII regions or blue refection nebula. There are simply large, which makes it difficult to frame the objects.

Very fascinating, Uwe! Thanks for a great OOTW, this reminds me of another such galaxy NGC 7497. I'll put this on my list.

[ScottH]

Wow, thanks for the early post, Uwe, because it allowed me to chase the molecular cloud(s) last night under 21.3 mpsas skies. At 37x in my 6" reflector, my view was EXACTLY what you drew with your 4" binocular telescope at 14x. Spot on! I love the drawings of such things with your 4" binocular telescope because unlike Mel's drawings of diffuse nebulae, you and I see pretty much the same amount. Nice choice for "RFT" owners.

Scott H.

[Erik68]

Fantastic!
I'll have to give it a shot too!

[akarsh]

Thanks for featuring this excellent target, Uwe. I took a look at both NGC 918 and NGC 7497 during two nights of excellent transparency near Death Valley National Park in California, about 8 hours drive from where I live.

918.jpg
18" f/4.5 + 31mm Nagler