Hubble took high resolution images of M33 showing a bright blue appendage, somewhat isolated. This squiggly bright blue object is located roughly near the end of a spiral arm.
This object is not in other M33 images like the one in Wikipedia.
Striking areas of star birth glow bright blue throughout the galaxy, particularly in beautiful nebulas of hot, ionized hydrogen gas like star-forming region NGC 604 in the upper left.
They call this object a nebula. In my opinion, the zoomed image looks like a luminous continous twisted filament, not a shapeless nebula.
The glaxy itself has several bright blue stars which could be type O, the hottest star type. Perhaps they are the sign of some "star birth" but they are not in one area.
This squiggly object is unusual. This could be called an "area" but it has apparent structure; while an "area" implies none.
Its location at the end of the spiral arm suggests some explanations.
This squiggly shape has multiple bends.
The Birkelund current pair could be bending here at the end of the arm to complete its path back to the split back at the galaxy core.
Each bend becomes a possible source of synchrotron radiation.
There is no Chandra image of this end of the galaxy so it must not be generating X-ray, so it lacks the velocity of current required for X-ray.
With a lower velocity current, then the maximum frequency will be in ultraviolet or blue. When in between, it would be "bright" blue.
The end of the path of the Birkelund current pair could be leaking protons and electrons. The result of their mutual capture is a hydrogen atom which immediately emits an emission line. The Lyman-alpha line is in ultraviolet. This emission line is the signature of "hot, ionized hydrogen gas."
There must be a magnetic field distorting the path of the plasma to result in the observed squiggly shape.
The images were taken with the Hubble Advanced Camera for Surveys
ACS is a highly versatile instrument that became the primary imaging instrument aboard HST. It offered several important advantages over other HST instruments: three independent, high-resolution channels covering the ultraviolet to the near-infrared regions of the spectrum.
Perhaps, this instrument shows ultraviolet as bright blue.
The Wikipedia image from VLT is only optical so with no UV capability that explains its omission if the object is only emitting UV.
The M33 appendage could be: a plasma filament of protons and electrons, is twisting and causing hydrogen atoms to result at each twist.
That is a possible alternate scenario.
If the object's spectrum were all strong emission lines that wold be confirmation of that scenario.
If the spectrum were synchrotron radiation with ultraviolet and no emission lines that wold be confirmation of that scenario.
As often occurs, just a visual image misses important details found in other wavelengths.
Wikipedia's image of NGC 604 is in optical and just an amorphous nebula.
That mess of gas and dust would block optical but not shorter wave lengths of UV.