The canonical Einstein Cross, the quadruply lensed quasar Q2237+0305, is
seen in this Hubble image. A new study has found two additional Einstein
Crosses created by the lensing of compact galaxies. [NASA, ESA, and
STScI]. Hi-res image Diagram illustrating how light from a distant source can be bent by a
foreground object to produce four identical images of the source. Click
to enlarge. [NASA/ESA/D. Player (STScI)] Detection and confirmation of the Einstein crosses, KIDS J232940-34092
(top two rows) and KIDS J122456+005048 (bottom two rows), via the KiDS
survey and the MUSE integral field spectrograph on the VLT. Click to
enlarge. [Napolitano et al. 2020]. Hi-res image
Seeing quadruple? In a rare phenomenon, some distant objects can
appear as four copies arranged in an “Einstein cross”. A new study has
found two more of these unusual sights — with an unexpected twist.
Searching for Rare Crosses
Gravitational lensing — the bending of light by the gravity of
massive astronomical objects — can do some pretty strange things. One of
lensing’s more striking creations is the Einstein cross, a
configuration of four images of a distant, compact source created by the
gravitational pull of a foreground object (which is usually visible in
the center of the four images).
The canonical example of this phenomenon is the Einstein Cross, a
gravitationally lensed object called QSO 2237+0305, seen in the cover
image above. In this case, as with the majority of known Einstein
crosses, the background source is a distant quasar — the small and
incredibly bright nucleus of an active galaxy. But other sources can be
lensed into Einstein crosses as well, under the right circumstances.
In a new study led by Nicola Napolitano (Sun Yat-sen University
Zhuhai Campus, China), a team of scientists presents confirmation of two
new Einstein crosses discovered within the 1,000 square degrees imaged
in the Kilo-Degree Survey using the Very Large Telescope (VLT) in Chile.
Einstein crosses are unusual enough to begin with, but these two
discoveries are especially rare: the lensed sources are not quasars. Instead, they’re entire galaxies.
Galaxies, but Bite-Size
When a distant galaxy is lensed by a foreground object, it’s commonly
smeared out into an an arc or a ring; this is because galaxies are
large, extended objects. But if a galaxy is compact enough, the entire
galaxy can be lensed into quadruple images instead of a smeared-out
ring. Such is the case with Napolitano and collaborators’ new
discoveries, KIDS J232940-340922 and KIDS J122456+005048: they’re both
quadruply lensed compact galaxies known as post-blue nuggets.
What’s a blue nugget? This adorable categorization applies to a type of
galaxy found only in the early universe. Blue nuggets are extremely
small, quite massive, and undergoing a violent burst of star formation
that produces lots of large, bright, blue stars.
Blue nuggets are thought to be suddenly quenched — their star
formation is cut off — early in their evolution. As their star
population then evolves, these post-blue nuggets then transition into
red nuggets, compact collections of red stars that are theorized to
become the cores of today’s large elliptical galaxies.
A Bright Future
By spectroscopically following up their Einstein cross discoveries,
Napolitano and collaborators show that the two sources are both very
compact, massive galaxies with low specific star formation rates. Their
properties are consistent with post-blue nuggets currently undergoing
quenching — which means that these Einstein crosses are excellent
sources to study to learn about galaxy evolution.
The authors predict that, with future observatories like the Vera Rubin Observatory, Euclid, or the China Space Station Telescope, we may be able to find many thousands of Einstein crosses like these. There’s a lot to learn ahead!
Citation
“Discovery of Two Einstein Crosses from Massive Post-blue Nugget Galaxies at z > 1 in KiDS,” N. R. Napolitano et al 2020 ApJL 904 L31. doi:10.3847/2041-8213/abc95ba
Source: American Astronomical Society - NOVA
