NGC 474 = UGC 864 = PGC 4801 = Arp 227 (one of the pair) = CGCG 385-71
Galaxy in Pisces
RA: 01:20:07
Dec: +03:24:55
Size: 7.9' × 6.3'
Mag (V): 11.51
ngc474.jpg
[From the NGC/IC project]
Why would a "boring" elliptical galaxy feature as an OOTW? Perhaps the Arp designation provides a clue, but this innocuous-looking elliptical is anything but boring. A deeper image immediately shows why:
ngc474.png
[POSS II Blue from DSS]
Science
NGC 474 stands out as one of the best-known examples of shell galaxies: elliptical or S0 galaxies with ripple-like structures around them that are likely 3D shell structures. These shells are huge, spanning typically > 100kpc in some cases¹. Other examples that showed up in a cursory internet search are NGC 3923, NGC 1344, NGC 7600, ESO 381-12 and Centaurus A. However, apparently as many as 20% of elliptical galaxies may have shells!² I imagine they're just not as spectacular and well-defined as they are in NGC 474 or NGC 3923.
The natural question that follows immediately: what causes the shells? The Bad Astronomer Phil Plait has a very nice and readable article from 2016 on the subject. Quoting from there, "One idea on how these form is if a big elliptical collides with a smaller galaxy, and the collision is head-on (the smaller galaxy plunges right through the center of the elliptical). The gravitational interaction can cause ripples of material to compress and spread outward, a bit like ripples on a pond." He further explains that the resulting density waves trigger star formation in shell-like regions, and this is also consistent with why these shells are only seen around elliptical and S0 galaxies and not spirals — spiral structures would not survive such a large collision.
Shell galaxies have been classified into three types depending on the morphology of their shells. Type 1 have interleaved shells aligned along the major axis (like NGC 3923), whereas type 2 have shells randomly distributed all over the galaxy (like ESO 202-G 015), and type 3 are those that cannot be classified into either type 1 or type 2 because the shells don't appear concentric or there are too few shells [4] (e.g. NGC 7135 [5]). NGC 474 is classified as a type 2 system³.
NGC 474, which has as many as 10 concentric shells², lies an estimated 30 Mpc from us³. While you're looking at NGC 474, don't miss nearby NGC 470. Together they form Arp 227. From the proximity (5.4') and similar velocity, it is clear that the two galaxies are interacting; however, the origin of the shells and its relation to the interaction is still up for debate³. Couple recent (2020) papers²³ conclude that the shells were most likely formed from the merging of a galaxy with somewhere between a 3rd to a 100th of the main galaxy's mass (such a wide spread!). One of the papers³ postulates that this happened about 2 billion years ago, and that since then, NGC 474 has been accreting cold gas from the outskirts of gas-rich NGC 470.
Visual Observation
This is an early evening object this time of the year, so it's best to catch it right after dark.
Visually, I've only observed this with the Leviathan of Fort Davis (Jimi's 48"), so I'm copy-pasting Steve's observing notes with a 13.1" to first declare that at least the core isn't only for the largest amateur scopes:
"fairly bright, small, round, small bright core. Forms a pair with NGC 470 6' W. NGC 467 lies 15' SW and NGC 479 is 30' NE."
— Steve Gottlieb, 1984-08-24
I was inspired by this really deep APOD image, as I posted earlier here on DSF, to put this on the list for Jimi's scope in October 2014. There were 5 observers that night — Jimi Lowrey, Steve Gottlieb, Alan Agrawal, Bob Douglas and me — and everyone saw at least some shell structures.
NGC474_CfhtCoelum_1500.jpg
[APOD link]
The resulting report is on the aforementioned DSF post, but I repeat it here for completeness
474ann.jpg
[Annotated DSS image]
I found the object rather difficult. A dark patch (marked 2 in the above annotation) in the direction of NGC 470 from the core, was detected on multiple occasions with averted vision. Two "inner" shells (marked 1 and 3) popped in and out with averted vision. I was able to clearly discern the outermost ripple (marked 4) by moving the core of NGC 474 out of the field-of-view, thereby isolating a curved bright region in a dark background.
Epilog
So now that we know shell galaxies aren't all that rare, surely there must be many? David Malin (of color photography and Malin 1 fame!) and D. Carter compiled a catalog of 137 shell galaxies, but they're all below a declination of -17°!
So this winter when you come out of your shell,
GIVE IT A GO AND LET US KNOW!
References
[1] Ron Buta's lectures notes on galaxy morphology: https://ned.ipac.caltech.edu/level5/...uta/Buta7.html
[2] Fensch et. al. (2020): https://doi.org/10.1051/0004-6361/202038550
[3] Alabi et. al. (2020): https://doi.org/10.1093/mnras/staa1992
[4] R C Thomson (1991): https://adsabs.harvard.edu/pdf/1991MNRAS.253..256T
[5] Marino et. al. (2009): https://doi.org/10.1051/0004-6361/200911819
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