Intro

Spectacular professional images of exotic gravitational lenses produced by massive galaxy clusters are almost commonplace [†]. The gravitational field of a massive cluster can generate a dramatic panorama with many highly-distorted images of distant galaxies that lie far behind the cluster. The most prominent lensed galaxies take the form of so-called giant luminous arcs, which are found near the centres of many massive clusters, where the gravitational fields are strongest, while many smaller "arclets" appear further out. But resolving gravitational lensing is extremely challenging for amateur imagers, even in the most accessible cases, since these features are very faint, and cover very small angular scales. Only a handful of lensing examples have been posted on Astrobin. 

Thanks in part to Russell Croman's powerful AI deconvolution module BlurXTerminator, I've managed to resolve many lensing features in a very deep image that I took of the massive galaxy cluster Abell 2218, which contains thousands of galaxies at a distance of 2.1 billion light years. Abell 2218 was one of the first gravitational lenses of its kind to be discovered (initially reported in 1986), and for many years was considered to be one of the most spectacular [‡]. The cluster was imaged several times by the Hubble Space Telescope: a greyscale image was released in 1995 (and was featured in APOD only a few weeks after that website's launch!), followed by a full-colour image in 2000, with a final image release in 2008 covering a somewhat larger region. 

Image 

My image of Abell 2218 is displayed above with a dotted rectangle that identifies the region imaged by the Hubble in 2008, along with two insets: the upper inset shows the dotted region of my image upsampled by 150% and rotated to match the position angle of the Hubble image (and with a bit extra contrast enhancement), while the bottom inset shows the Hubble image at the same scale (which is down by a factor of about 6.5 from its full resolution). Some twenty-five of the arc(lets) in the Hubble image are visible in mine, identified with green arrowheads in the upper inset, to be compared with the same features in the Hubble inset [§]. The properties of some of these arcs are discussed in detail below. 

Mousing over the image displayed above, or selecting Revision "J", will show my image of the cluster without the insets, while Revision "K" has the insets but omits the green arrowheads. 

The image of the cluster spans about 28'x24' at a plate scale of 0.47"/pixel, and is the result of 34 hours of integration, with about 21 hours in luminance and 13 hours in RGB colour, taken over the course of thirteen nights in the summer of 2021. The cluster's centre is located near the image centre, inside the dotted rectangle, and is dominated by its brightest galaxy, a giant elliptical (cD) catalogued as LEDA 140648. The full cluster covers most of the field, and many galaxies of comparable size to those in the centre are plainly visible, sprinkled throughout the frame. The most prominent galaxies in the image are much closer than Abell 2218, the largest of which is a spiral with prominent blue arms near the image centre, which is catalogued as UGCA 413, and is at a distance about 350 million light years. 

There is also a striking object along the right side of the frame, about half-way up from the image centre, with a curious shape, and a remarkable 3D relief against the background sky; here is a closeup at full resolution (with some additional enhancement):



This enigmatic object does not appear to have been catalogued. SIMBAD shows that there is an infrared source with a position near the top-centre edge of the object, identified by the IRAS Infrared Satellite, but the only available data on the source is its flux (no optical counterpart is given). So nothing is known about this "mystery" object (which makes me think of a blue tomato sprinkled with salt), despite the fact that all of the galaxies in this field of comparable size and brightness to this apparition have been catalogued (and some that are much less prominent)!

Analysis

Returning to the gravitational lensing in Abell 2218, most of the arc(lets) are centred on the giant elliptical galaxy at the centre of the cluster, but two large arcs and several arclets are centred instead on the 2nd-brightest galaxy, towards the bottom-left corner of the insets. The presence of two arc systems is one of several indications that Abell 2218 is not one galaxy cluster, but two clusters that are still in the process of merging!

The three most luminous arcs in the Hubble image are prominently resolved in mine, and their colours are also reproduced fairly well. Two of these are centred on the cD galaxy: a bluish arc between the 12 and 1 o'clock positions, and an orange arc at about 7 o'clock; the other is centred on the 2nd-rank galaxy: a light-magenta arc that runs between about 12 and 5 o'clock, which I've bounded with two arrowheads. The other large arc centred on the 2nd-rank galaxy in the Hubble image can also be seen in mine (bounded by arrowheads at about 6 and 7 o'clock), although it is poorly resolved; nonetheless it's arguable that my image shows that two arc systems are present.

The colour of a lensed galaxy is a function of its distance and type, and the redshifts of many of the arcs in Abell 2218 have been measured, which establishes their distances (distance at large redshift is ambiguous, so it is more appropriate to quote the look-back time). For example, the orange arc below the cD galaxy is an image of a galaxy at a redshift of 0.7 (look-back time of about 6 billion years), while the blue arc above it has a redshift of around 2.5 (look-back time about 11 billion years!). By comparison, the redshift of the galaxies in Abell 2218 is about 0.17, with a look-back time of 2.1 billion years (usually quoted as an equivalent distance of 2.1 billion light-years). 

Finally, I'd like to acknowledge again how essential BlurXTerminator (BXT) was to resolving so many of the lensing features in the cluster. I used BXT fairly aggressively, along with some sharpening at the end of the workflow, but there are few if any noticeable artifacts. BXT is revolutionary!

Notes

[†] Gravitational lensing is a major branch of astrophysical research, with applications that include estimates of the abundance of dark matter in galaxy clusters, determinations of cosmological parameters like the Hubble constant, and their use as "natural" telescopes to detect galaxies at record-setting distances. 

[‡] Abell 2218 was one of three galaxy clusters with giant luminous arcs  that were discovered almost simultaneously; the other two were Abell 370 and CL2244-02. The very first gravitational lens to be detected was the so-called Twin Quasar Q0957+561, reported in 1979. While Einstein himself realized that gravitational lenses were possible, he thought they would never be seen (basing his calculations on lensing by a single star), but Fred Zwicky suggested in 1937 that galaxies might produce detectable images.

[§] The arc(lets) have been extensively analyzed, notably in a 1996 paper on the first Hubble image, and I used a detailed map in the latter reference to assist in identifying many of the lensing features in the larger 2008 Hubble image.