Astronomers have discovered an unusual gravitational lensing configuration, dubbed ‘Hamilton’s Object,’ consisting of two images of a clumpy spiral background galaxy in a fold configuration and a third resolved image of the same spiral galaxy, which together create a tangential cusp configuration.
In images obtained by the NASA/ESA Hubble Space Telescope, Shawnee State University astronomer Timothy Hamilton and colleagues discovered that the immense gravity of an intervening, and uncatalogued, foreground cluster of galaxies was warping space, magnifying, brightening, and stretching the image of a distant galaxy behind it.
Though Hubble surveys reveal a lot of these funhouse-mirror distortions caused by gravitational lensing, this object was uniquely perplexing.
In this case, a precise alignment between a background galaxy and a foreground galaxy cluster produces twin magnified copies of the same image of the remote galaxy.
This rare phenomenon occurs because the background galaxy straddles a ripple in the fabric of space.
This ripple is an area of greatest magnification, caused by the gravity of dense amounts of dark matter, the unseen glue that makes up most of the Universe’s mass.
As light from the faraway galaxy passes through the cluster along this ripple, two mirror images are produced, along with a third image that can be seen off to the side.
“We were really stumped,” said Dr. Hamilton, co-author of a paper published in the Monthly Notices of the Royal Astronomical Society.
The rare phenomenon wasn’t well-known when he spotted strange linear features in 2013.
As he looked through quasar images, he spotted the mirrored images and parallel streaks. He had never seen anything like it before, and neither had other team members.
So the astronomers began their quest to solve the mystery of these tantalizing straight lines, later dubbed Hamilton’s Object for its discoverer.
Using data from the Sloan Digital Sky Survey (SDSS), they found that a foreground cluster that’s causing the lensing resided in the same area as the magnified images, but it did not show up in any catalogued survey.
Nevertheless, the fact that the strange images were at the center of a cluster made it clear to the team that the cluster was producing the lensed images.
Their next step was in determining whether the three lensed images were at the same distance, and therefore were all the distorted portraits of the same faraway galaxy.
Spectroscopic measurements with the Gemini and W.M. Keck observatories in Hawaii helped the researchers make that confirmation, showing that the lensed images were from a galaxy located more than 11 billion light-years away.
The remote galaxy, based on a reconstruction of the third lensed image, appears to be an edge-on, barred spiral with ongoing, clumpy star formation.
They also found that the foreground cluster, known as SDSS J223010.47-081017.8, resides more than 7 billion light-years away.
“This gravitational lens is very different from most of the lenses that were studied before by Hubble, particularly in the Hubble Frontier Fields survey of clusters,” said lead author Dr. Richard Griffiths, an astronomer at the University of Hawaii.
“You don’t have to stare at those clusters for long to find many lenses. In this object, this is the only lens we have. And we didn’t even know about the cluster at first.”
According to the team, because the fold-configuration shows highly distinctive surface brightness features, follow-up observations of microlensing or detailed investigations of the individual surface brightness features at higher resolution can further shed light on properties of mysterious dark matter.
“It’s great that we only need two mirror images in order to get the scale of how clumpy or not dark matter can be at these positions,” said co-author Dr. Jenny Wagner, an astrophysicist at the University of Heidelberg.
“Here, we don’t use any lens models. We just take the observables of the multiple images and the fact they can be transformed into one another. They can be folded into one another by our method. This already gives us an idea of how smooth the dark matter needs to be at these two positions.”
Richard E. Griffiths et al. 2021. Hamilton’s Object – a clumpy galaxy straddling the gravitational caustic of a galaxy cluster: constraints on dark matter clumping. MNRAS 506 (2): 1595-1608; doi: 10.1093/mnras/stab1375