Figure 1. Image of Cheshire Cat gravitationally lensed galaxies produced
by combining Chandra X-ray data with Hubble Space Telescope optical
data. Gemini spectroscopy provided critical spectroscopic data for
characterizing the foreground lensing cluster and understanding its
history and likely fate as a "fossil group." Image Credit: Hubble Space Telescope and Chandra X-ray Observatory
Figure 2. Color–magnitude diagram of all galaxies detected in the GMOS
images with r' brighter than 24.5 mag (595 galaxies, black dots). Blue
circles and red squares are the spectroscopically confirmed members of
the G1 and G2 groups respectively, with filled and open symbols
representing which of these galaxies lie inside and outside of 0.5 r200
of the group. Red short dash and blue long dash vertical lines
represent a two magnitude gap from the E and W eye galaxies,
respectively. The black triangles represent spectroscopically confirmed
background and foreground galaxies, while black dots represent objects
without spectroscopic redshifts. The two central galaxies of the groups,
SDSS J103843.58+484917.7 and SDSS J103842.68+484920.2 (the "eyes" of
the Cheshire Cat) are represented by a letter (E and W) in the CMD.
Gemini observations provide a key scientific context for a striking new
image of the gravitationally lensed galaxy group popularly known as the
Cheshire Cat. The image, released by NASA’s Chandra X-ray Observatory,
combines X-ray data with optical images from the Hubble Space Telescope
to paint a remarkable likeness to the famously devious smiling cat in
Lewis Carrol’s Alice in Wonderland.
Behind the scenes, Gemini played a critical role in the image’s scientific story by taking the spectral fingerprints of many of the galaxies that make up the foreground cluster which bends the distant galactic light. For more details on the image, and how gravitational lenses work, see the Chandra press release here.
Gemini South astronomer Rodrigo Carrasco, a co-author on the paper which
spawned the image, adds that Gemini provided the core spectroscopic
data that allowed the team to fully characterize the cluster of
foreground galaxies which serves as the lens. "With the Gemini data we
were able to study the individual members of the cluster which we expect
will eventually become fossil groups," says Carrasco. Fossil groups
consist of a giant elliptical central galaxy which is the end-product of
interactions with other galaxies over billions of years, and surrounded
by smaller, less bright galaxies. "Our results indicate that merging of
fossil groups – in this case, the Cheshire Cat – could be one of the
ways that a large fraction of fossil groups in the nearby Universe
formed."
The paper, published in The Astrophysical Journal, is available at: http://xxx.lanl.gov/abs/1505.05501.
Paper Abstract:
Abstract: The Cheshire Cat is a relatively poor group of galaxies
dominated by two luminous elliptical galaxies surrounded by at least
four arcs from gravitationally lensed background galaxies that give the
system a humorous appearance. Our combined optical/X-ray study of this
system reveals that it is experiencing a line of sight merger between
two groups with a roughly equal mass ratio with a relative velocity of
∼1350 km s-1. One group was most likely a low-mass fossil
group, while the other group would have almost fit the classical
definition of a fossil group. The collision manifests itself in a
bimodal galaxy velocity distribution, an elevated central X-ray
temperature and luminosity indicative of a shock, and gravitational arc
centers that do not coincide with either large elliptical galaxy. One of
the luminous elliptical galaxies has a double nucleus embedded
off-center in the stellar halo. The luminous ellipticals should merge in
less than a Gyr, after which observers will see a massive 1.2 − 1.5 x
1014 M⊙ fossil group with an Mr = −24.0
brightest group galaxy at its center. Thus, the Cheshire Cat offers us
the first opportunity to study a fossil group progenitor. We discuss the
limitations of the classical definition of a fossil group in terms of
magnitude gaps between the member galaxies. We also suggest that if the
merging of fossil (or near-fossil) groups is a common avenue for
creating present-day fossil groups, the time lag between the final
galactic merging of the system and the onset of cooling in the
shock-heated core could account for the observed lack of well-developed
cool cores in some fossil groups.
Source: Gemini Observatory
