Terzan 7 is a sparse and youngglobular cluster that is believed to have originated in theSagittarius Dwarf Spheroidal Galaxy (Sag DEG) and is physically associated with it. It is relativelymetal rich with[Fe/H] = -0.6[6] and an estimated age of 7.5Gyr.[4] Terzan 7 has low levels ofnickel ([Ni/Fe] = -0.2) which supports its membership in the Sag DEG system since it has a similar chemical signature.[7] It has a rich population ofblue stragglers that are strongly concentrated toward the center of Terzan 7.[8] It has an average luminosity distribution of Mv = -5.05.[9] It has ahalf-light radius (Rh) of 6.5pc.[10]
Nearly all globular clusters of theMilky Way'sgalactic halo formed at the same time (12-15 Gyr). Even the far situatedNGC 2419 (~100 kpc fromGalactic Center) has a similar age. This trend also applies to the age of globulars found in theLarge Magellanic Cloud andFornax Dwarf (~140 kpc from Galactic Center). However, a few globulars seem to be significantly younger than the rest; these includePalomar 1,Palomar 3,Palomar 4,Palomar 12,Palomar 14,Ruprecht 106,IC 4499,Arp 2,Eridanus,Fornax 4, and Terzan 7. In particular, the ones associated with the Sag DEG appear to have formed more recently. The data suggests that all the present outer halo globulars may have originally formed in dwarf spheroidals.[10]
Alternatively, a hierarchical galaxy formation model is hypothesized under which a portion, possibly large, of the Milky Way's globular clusters would have originated in theaccretion of otherdwarf spheroidals like Sag DEG. The best candidate for this idea isPalomar 12.[6][13]
^abcGeisler, Doug; Wallerstein, George; Smith, Verne V.; Casetti-Dinescu, Dana I. (September 2007), "Chemical Abundances and Kinematics in Globular Clusters and Local Group Dwarf Galaxies and Their Implications for Formation Theories of the Galactic Halo",The Publications of the Astronomical Society of the Pacific,119 (859):939–961,arXiv:0708.0570,Bibcode:2007PASP..119..939G,doi:10.1086/521990,S2CID119599242
^Sbordone, L.; Bonifacio, P.; Marconi, G.; Buonanno, R. (2004), "Chemical abundances in Terzan 7",Memorie della Societa Astronomica Italiana,75: 396,Bibcode:2004MmSAI..75..396S
^Held, Enrico V.; Rosenberg, Alfred; Saviane, Ivo; Momany, Yazan (March 12–16, 2001), written at Pucon, Chile, Geisler, D.; Grebel, E.K.; Minniti, D. (eds.), "The Globular Cluster Terzan 7 in the Sagittarius Dwarf Spheroidal Galaxy",Extragalactic Star Clusters,207, San Francisco: Astronomical Society of the Pacific (published 2002): 165,Bibcode:2002IAUS..207..165H
^abGottlieb, Steve (August 1, 2000), "Sky and telescope",Sky and Telescope,100 (2): 112,ISSN0037-6604
^Terzan, Agop (1968), "Six nouveaux amas stellaires (Terzan 3-8) dans la region DU centre de la Voie lactee et les constellations DU Scorpion et DU Sagittaire",C. R. Acad. Sci.,267 (Ser. B):1245–1248,Bibcode:1968CRASB.267.1245T
^Briley, Michael M.; Martell, S.; Smith, G. H. (December 2007), "The Homogeneity of Light Elements in the Sagittarius Elliptical Dwarf Galaxy Globular Clusters Terzan 7 and Arp 2",American Astronomical Society, AAS Meeting #211, #58.31,211: 58.31,Bibcode:2007AAS...211.5831B
