BDanielMayfield wrote: ↑
Tue Aug 04, 2020 7:02 pm
No. To find the bright white blob that Boomer and I think might be the core of a second interacting galaxy draw a line from your green circle though the center of the main galaxy and continue down about the same distance to the bottom edge of your cropped image. (Your image has cropped off most of it.) This 'blob' is much larger than what you circled, and it is crossed by the dust band of the larger galaxy's lower spiral arm. In fact this blob of white starlight is larger than the yellower extended core of the main galaxy. The shape of this blob is very elliptical. It could even be an Elliptical galaxy with zero star formation going on. A possible dim white tidal tail of stars from this proposed intruder could be behind and mostly above the main lower arm.
That's all 3D speculation based on a 2D snapshot though. What's really needed is a view from Andromeda and/or Triangulum!
I see what you mean, Bruce, but I don't agree.
The diffuse white "arc" that smoothly follows the bend of the spiral arm of NGC 2442 is the remnant of extremely vigorous star formation in this spiral arm in the past. Compare the white arc of NGC 2442 with the very similar (but even more extreme) white arc in galaxy NGC 5395.
Take a look at the picture at right of M51, and take a good look at the spiral arm near the bottom of the image.
You can see that red emission nebulas surrounding massive hot newborn stars of spectral class O and B have been born typically near the outer edge of the massive dark dust lane of this spiral arm. But note that outside this line of red emission nebulas, we can see scattered supergiant stars of different colors set against a relatively smooth background of bluish stars of (I'd say) late B, A and F-type stars, mixed with a huge number of fainter stars of spectral class G (and still more still fainter stars that are even cooler).
Note that this "outer arc" of stars follows the shape of the inner, starforming dust lane perfectly. Note that the color of it is much less blue than the bluest, youngest stars in the bright star clusters, but it is much bluer than the brownish "inter-arm" regions near the center of the galaxy.
This is the remnant of vigorous star formation in the past. The brightest, hottest products of that burst of star formation have died since then, and cooler, less blue and ultraviolet stars remain. In time, this remnant of vigorous star formation will grow ever less blue and also (over time) fainter.
So, in short, that white arc in NGC 2442 is the remnant of past star formation and not the remnant of another galaxy.
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