This field-of-view in Boötes features two targets that have rarely been imaged, with only a handful of Astrobin posts to date of either one: NGC 5557 and Arp 199. The image spans about 29'x26', with a plate scale of about 0.47"/pixel, and is the result of 19 hours of integration, split equally between luminance and RGB colour, taken over the course of seven nights in May of 2022. A further 8 hours of exposures were rejected based on seeing and sky-background cuts.

The field is dominated by NGC 5557, a massive elliptical galaxy at a distance of about 125 million light years [†], surrounded by a gigantic low-surface brightness tidal complex. The tidal structures include three outer shells in the galactic disk, a variety of tidal streams and clumps, and three tidal dwarf galaxies (TDGs). Only one of the TDGs is contained in this field, and is the small blue cloud to the left of the galactic disk, known as MATLAS-1824; the two other TDGs are off the field to the left [‡]. Mousing over the image, or selecting Revision "C", will bring up an inverted-grayscale starless version, for a clearer display of the tidal complex.

The other rarely-imaged object in this field, Arp 199, is a pair of interacting galaxies near the upper-right corner of the frame, at a distance of about 140 million light years, with the nearly edge-on blue spiral NGC 5545 appearing to make an almost direct bullseye strike on the nucleus of the nearly-face on spiral NGC 5544. It's no wonder that this peculiar configuration is part of Halton Arp's Atlas of Peculiar Galaxies! Not incidentally, a fantastic Arp resource is @Gary Imm's Complete Arp Catalogue! 

The tidal complex surrounding NGC 5557 was first discovered in 2011 from ultra-deep imaging studies using the wide-field MegaCam camera on the Canada-France-Hawaii telescope, by astronomers from the ATLAS^3D international collaboration [§]. I was surprised to find that my image is almost as deep as the MegaCam image, which has features down to about 29 mag/arc-sec^2 (in g-band). Comparison with an ATLAS^3D surface-brightness map shows that my image goes down to about 28 mag/arc-sec^2 (to set the scale, the easily-seen blue TDG left of the galactic disk has an average surface brightness of 25.1 mag/arc-sec^2). 

The portion of the tidal structure imaged here covers about 22', equivalent to about 800,000 light years at the distance of NGC 5557. There is another 15' of tidal tail off the field to the left, with the whole complex spanning about 1.2 million light-years, making it one of the largest stellar structures ever observed around a galaxy (assuming that none of it is Milky Way cirrus, which cannot be completely ruled out, but is not thought to be the case).

Astronomers from the ATLAS^3D collaboration argued that NGC 5557, and another elliptical, NGC 680, with similar tidal features, present a challenge to the conventional "hierarchical" bottom-up model of galaxy formation, which implies that massive ellipticals are among the oldest galaxies, with ages of between 7 and 10 billion years based on their stellar populations. They estimated that NGC 5557 (and NGC 660) originated only 1-3 billion years ago from a merger of two large spiral galaxies. This follows in part from the fact that tidal streams of this complexity are known to survive for no more than a few billion years, while the lower bound on the age is implied by the fact that any merger-triggered starburst activity has ceased. The huge size of the tidal structures on both sides of NGC 5557 suggests that it was produced by a merger of two large progenitors, and the presence of gas revealed by HI mapping studies of this region suggests that the merger was of two spirals. 

I find it hard not to "see" a connection between Arp 199 and the NGC 5557 tidal complex, the right side of which points roughly in the direction of the interacting pair, and ends just 2' from it (which would be equivalent to only 80,000 light years at that distance). Although I haven't found any claims of a physical connection between these systems in the literature, the redshifts of all three galaxies are very similar (averaging about 3,100 km/sec, roughly 1% of the speed of light), with relative speeds about 20 times smaller and comparable to the relative speed between the Andromeda Galaxy and the Milky Way (which is only 110 km/sec). It is also interesting to note that the galaxies in Arp 199 have rather small physical diameters, only about 50,000 light-years, and even look like the inner portions of more typical spirals. Could it be that these are the remnants of spirals that were disrupted by a close encounter with NGC 5557, with material that was stripped from them ending up as part of the NGC 5557 tidal complex on that side? If this wild conjecture has any connection with reality, it would seem unlikely that the spirals in Arp 199 would be related to the progenitors that created the full tidal complex ... but now I've done much too much idle speculating for my own good! 

[†] It is worth noting that more recent estimates put the distance to NGC 5557 at about 150 million light-years.

[‡] The blue cloud beyond the upper-right edge of the disk (and to the right of a bright red star), known as MATLAS-1811, is an ultraviolet source that as far as I can tell has not been associated with NGC 5557 (its redshift has not been measured, unlike that of MATLAS-1824, which is almost identical to the redshift of NGC 5557). 

[§] It took some doing to find out what ATLAS^3D actually stands for, and though it is clever, it's definitely not what you'd call a simple acronym! ATLAS is short for the unwieldy "Assembly of early-Type gaLAxies with their fine Structures", and 3D refers to the fact that it is a volume-limited survey (a representative local volume with a radius of about 42 Mpc that avoids a region around the dusty plane of the Milky Way).