johnnydeep wrote: ↑Tue Oct 05, 2021 1:11 pm
neufer wrote: ↑Tue Oct 05, 2021 12:21 am
johnnydeep wrote: ↑Mon Oct 04, 2021 10:54 pm
Yes, I’d say it does look “soft” and slightly out of focus. I take it Hubble’s optics are fine and this is simply the best it can do with something over 100 times farther away than Andromeda!
It “looks soft” because it's all puffy/swollen from the collision
(Observe how sharp the background galaxies appear.)
I did indeed observe them before I posted and concluded that the background galaxies didn't look particularly sharp to me either. Are those in the pic below sharp? Maybe, and the largest in this view would be about 10 times farther away (assuming it is about the same absolute size).
On the other hand, ARP 273 is another pair of interacting galaxies, also about 300 mly away, and I suppose it looks similarly "soft"...or does it?:
So, to what do we attribute the "puffy and swollen" appearance? More gas and less clumping due to being stirred up by tidal forces?
There are at least two reasons for the different degrees of "softness" in the Mice galaxies versus UGC 1810.
UGC 1810 is a pair of galaxies where the arms, particularly in the upper, larger galaxy, are particularly well defined. They are undoubtedly a log brighter than the underlying disk. In the Mice galaxies, by contrast, there aren't really any arms. These galaxies are basically all disks with tidal tails attached. Neither the disks nor the tails are extremely well defined.
Second, consider the filters used for the two images. For the Mice galaxies image, the filter used
were 475 nm, 606 nm and 814 nm. Of these, both the 606 nm and the 814 nm filters are good at detecting the small red stars that undoubtedly form soft "halos" or at least "puffed-up outer disks" around the main disks.
For the UGC 1810 image, the filters were 390 nm, 475 nm and 600 nm. Both the 390 nm and the 475 nm filters are very good at detecting the the bright hot stars in the arms. The 600 nm will do an okay, but in no way an outstanding job, at detecting an outer halo of old red stars around these galaxies.
So in short: The filters used for UGC are well designed for pickiing out the bright well-formed arms dominated by hot bright stars in these galaxies. The filters used for the Mice galaxies are well designed for picking out the "fog" of old cool stars that are undoubtedly surrounding the disks of these galaxies.
To see what I mean, take a look at this Hubble picture of NGC 4214:
NGC 4214. Image: NASA, ESA and the Hubble Heritage Team. Acknowledgement: R O'ConnelL
As you can see, NGC 4214 contains some brilliantly bright sites of star formation, which have given birth to a number of extremely luminous supermassive blue stars. But NGC 4214 also contains a "fog" of millions and millions of small red stars.
It is the same kind of small red stars that make the disks of the Mice galaxies look diffuse.
I think it is also true that astrophotographers apply "sharpening" to their images, whatever that is and however that is done.
So I guess it is possible that those who created the picture of UGC 1810 applied more sharpening to their image than those who created the picture of the Mice galaxies.
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