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Object of the Week June 6, 2021 – NGC 5694 A Foster Son of the Galactic Halo
NGC 5694
Type: Globular Cluster
Constellation: Hydra
RA: 14h 39m 36.6s
Dec: -26° 32’ 18”
Mag: 10.2
Diam: 4.3’
A rather visually non-descript globular cluster has played a key role with the on-going work being done by astronomers and physicists to reverse-engineer the Milky Way in order to better understand how our home galaxy has become what it is today. That globular cluster is today’s Object of the Week NGC 5694, located in the constellation Hydra.
NGC 5694 was cataloged by William Herschel as H II.196 with this notation, “Pretty bright, small, nearly round, bright in the middle, resolvable,” following its discovery on May 22, 1784. Herschel cataloged it as a "faint nebula." NGC 5694 was first resolved and recognized as a globular cluster by C.O. Lampland and Clyde W. Tombaugh at Lowell Observatory in 1932. Its ten brightest stars were measured at about mag 16.5. At its distance of about 113,000 light-years, this cluster's apparent diameter of 3.6 minutes of arc corresponds to a linear extension of almost 120 light-years. Its apparent visual brightness of 10.17 mag corresponds to an absolute visual magnitude of -7.81, or an intrinsic luminosity of about 120,000 times that of our Sun.
This distant globular cluster has been recognized for many years as an object which might not be gravitationally bound to the Milky Way. Measurements of radial velocity and distance from the galactic center taken in the 50’s and 60’s demonstrated that virtually all the known globular clusters could consistently be assumed to have bound orbits; by far the single most outstanding exception was NGC 5694, which from the data appeared to be almost in a hyperbolic orbit. The caveat was the analyses of orbital status were hampered by large uncertainties in both numbers, because the only radial-velocity measurements available at the time were based on low-resolution spectrograms and the distance data was derived old brightest star data.
In 1976 new observations of both NGC 5694’s distance and velocity enable the redetermination of its orbit. From that analysis came this conclusion: that if the conventional galactic mass models are more or less correct, then NGC 5694 is in a hyperbolic orbit which make it an extragalactic object which is simply passing through the Milky Way or it is a normal galactic globular cluster that has somehow been pushed into higher-energy orbit, i.e., with an orbital velocity that enables it to escape from the Milky Way. What could have pushed NGC 5694 into a high-energy, escape velocity orbit? The hypothesis presents the Magellanic Clouds to be only obvious candidates with the right distance and sufficient mass to throw a halo globular cluster into a hyperbolic orbit. If the Clouds are indeed responsible for its present orbit, then it is also possible that NGC 5694 belonged originally to the cluster system of the Magellanic Clouds and not the Milky Way galaxy. The refinement of NGC 5694’s orbit only deepened the mystery of its origin.
Jump forward nearly 40 years. There is Gaia measured proper motions with distances and line-of-sight velocities available. There is high-resolution spectra available from the Very Large Telescope measuring metallicity to determine chemical abundance, the proportion of the material of a star or other celestial object that is in elements other than hydrogen or helium. The spectra of six red giant branch stars of NGC 5694 are obtained. NGC 5694’s abundance pattern is significantly different from the other globular clusters of similar metallicity, as well as from the bulk of halo field stars. The chemical analysis confirmed the orbital findings that NGC 5694 is a globular cluster of extra-galactic origin. The study concluded that the cluster most likely originated in a dwarf galaxy having a slower chemical evolution than the Milky Way. In consideration of the overall chemical composition of NGC 5694, the study further concluded that it is by far the most metal-poor globular cluster, and it is as old or even slightly older than the previous known oldest globular cluster NGC 6397.
It is interesting to note that the chemical oddities that make NGC 5694 such a peculiar object are mostly shared by Ruprecht (Rup) 106, a globular cluster whose extragalactic origin has also been suspected. The hypothesis of a common origin for the two clusters, in the same (now disrupted) dwarf satellite of the Milky Way, cannot be easily dismissed.
Visually the globular cluster reveals itself as very small, compact glow in instruments 10 inches or less. Several experienced observers report that even in instruments from 12 to 20 inches they failed to resolve any of the cluster’s stars. Small globular clusters can take magnification well on steady nights. Good dark skies are certainly advantageous.
Walter Scott Houston notes that this cluster is only 2' in diameter and may be mistaken for an 11th mag star in a 4-inch at low power. He adds that a 10-inch will reveal it clearly. He includes this globular in his Hydra Hysteria. He writes: "With a diameter of more than 3' and shining at 10th mag, it should be relatively easy to find." He notes that it is "smaller, fainter and more concentrated than NGC 5897. About 4' in diameter, it shines with the total light of a 10th mag star. None of its stars could be resolved in a 10-inch." In 1972 he called it one of two globulars worth-while looking up in Hydra (the other being NGC 4590). He estimated the magnitude as 10th or 11th: "though only 2' in diameter, it can be seen in my 5-inch binoculars after its exact place has been noted relative to nearby stars."
Brian Skiff notes his observations:
- 25cm - sim to N5634. core more strongly concen. 2' diam @ 180x, circ, no res
- 30cm - well & broadly [strong broad] concen, 1'.8 diam w/50" fairly distinctcore. core and inner halo gran. two *s S
Steve Coe (1992, The Deep-Sky Observer, Webb Society, Issue 1) observing with a 17.5-inch f/4.5 at 100x notes: "pB, pL, R, bM; 165x and 320x very gainy, no resolution on a night I rated 5/10. At a much better site on an evening I rated 8/10 the 17.5-inch would show 6 stars superimposed on a hazy outer corona."
Now it is your turn. Give it a go and let us know.
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Credits: NASA, ESA, G. Fahlman (Dominion Astrophysical Observatory), R. Ibata (Université de Strasbourg), and F. Ferraro (Università di Bologna); Processing: Gladys Kober (NASA/Catholic University of America)
