NGC 3398
DSS image of NGC 3398
Overlaid DSS image of NGC 3398, 30' x 30' with north at top and west to the right

Aladin viewer for the region around NGC 3398
IC 644, MCG+09-18-038, UGC 5954, PGC 32564, SDSS J105131.42+552327.4

Type  Galaxy
Magnitude  14.4
Size  0.833' x 0.3' @ 80°
Right Ascension  10h 51' 31.4"  (2000)
Declination  55° 23' 28" N
Constellation  Ursa Major
Description  vF, S, E, er
Classification  SABa
Observing Notes

Harold Corwin

NGC 3398 = IC 644. Considerable confusion has surrounded the identification of this object and its neighboring galaxies. The original observation is due to William Herschel, who found a "vF, S, E 20deg sp nf, er" nebulae on 17 April 1789 2m 11s preceding, 0d 50' north of 44 UMa. Reducing these offsets, taking the proper motion of 44 UMa into account, gives the position (for 1950) 10 48 24, +55 41.1.

There are four galaxies in the area that might be the one that Herschel saw. Here are data for them:
   RA  (1950.0) Dec     B_t   PA   Type         MCG         CGCG    UGC   IC
Notes
10 48 28.4 +55 39 23 14.55 73 SA:(rs:)ab? +09-18-038 267-18 5954 0644
* superposed 0.55 sp
10 48 32.1 +55 43 53 15.6 130 SA:(rs?)0^+ +09-18-039 267-19 -- 0646
10 48 44.8 +55 39 04 -- 55: E2/S0^-: +09-18-041 -- -- --
10 48 59.9 +55 51 56 14.82 20 SAB(s)cd III +09-18-043 267-22 5976 --
Sev F sts, knots, comps near
On the face of it, UGC 5976 is the most likely candidate: it is second brightest, the position angle is correct, it is knotty, and it is the largest of the galaxies in the area. However, its position is well off of Herschel's, and it has the lowest surface brightness of any of the galaxies here. I think it is doubtful that Herschel would have picked it up while sweeping.

Instead, Herschel's position falls near UGC 5954, the brightest galaxy of the four, and also the one with the highest surface brightness. This means that it is the one that Herschel would be most likely to see. The position angle is at least in the correct quadrant, and Herschel's note "extremely mottled" could well be due the presence of the star near south-preceding combined with the galaxy's bright nucleus and pseudo-ring of uneven brightness.

Still, visual verification of this theory would be nice to have.

Finally, the identity of the two IC objects in the area -- IC 644 and IC 646 -- is unambiguous. While Swift's positions are often none too good, they are at least adequate in this case. The offset in RA is about 12 time-seconds for both, while the declinations are within a minute of arc. IC 644 turns out to be identical to NGC 3398, while IC 646 is MCG +09-18-039.
NGC Notes by Harold Corwin
Other Data Sources for NGC 3398
Associated objects for NGC 3398
Nearby objects for NGC 3398
Credits...

Drawings, descriptions, and CCD photos are copyright Andrew Cooper unless otherwise noted, no usage without permission.

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NGC 3398