Hercules, or Her, is a dwarf spheroidal galaxy situated in the Herculesconstellation and discovered in 2006 in data obtained by the Sloan Digital Sky Survey.[4] The galaxy is located at a distance of about 140 kpc from the Sun and moves away from the Sun with a velocity of about 45 km/s.[4][6] It is classified as a dwarf spheroidal galaxy (dSph). It has a noticeably elongated (ratio of axes ~ 3:1) shape with a half-light radius of about 350 pc.[4][7][note 1] The cause of this elongation is under debate.[5] Some studies have found a gradient of velocities across the galaxy's body[8] and a faint stellar stream associated with the galaxy,[3] suggesting that the elongation may be caused by tidal forces acting from the Milky Way galaxy.[3] However, other studies failed to find a velocity gradient or an associated stellar stream, suggesting that it is not being tidally disrupted.[5]
Her is one of the smallest and faintest satellites of the Milky Way—its integrated luminosity is about 30,000 times that of the Sun (absolute visible magnitude of about −6.6),[note 2] which is comparable to the luminosity of a typical globular cluster.[4][7] However, its total mass is about 7 million solar masses, which means the galaxy's mass to light ratio is around 330. A high mass to light ratio implies that Her is dominated by dark matter.[6][note 3]
The stellar population of Her consists mainly of old stars formed more than 12 billion years ago.[3] The metallicity of these old stars is also very low at [Fe/H] ≈ −2.58 ± 0.51,[note 4] which means that they contain 400 times less heavy elements than the Sun.[9] The stars of Her were probably among the first stars to form in the Universe. Currently there is no star formation in Her.[3] Measurements have so far failed to detect neutral hydrogen in it—the upper limit is 466 solar masses.[10]
Notes
^From other sources the half-radius is around 230 pc.[3]
^From other sources the absolute magnitude is around −5.3.[3]
^It is difficult to estimate the mass of such faint galaxies due to significant foreground contamination, which inflates the velocity dispersion. A paper published in 2009 arrived at a lower mass estimate of about 2 million solar masses within the half-radius; the total mass within 433 pc was estimated at 4 million solar masses.[8]
^From other sources the metallicity is around −2.3.[2][6]
^ abcdefgBelokurov, V.; Zucker, D. B.; Evans, N. W.; Kleyna, J. T.; Koposov, S.; Hodgkin, S. T.; Irwin, M. J.; Gilmore, G.; Wilkinson, M. I.; Fellhauer, M.; Bramich, D. M.; Hewett, P. C.; Vidrih, S.; De Jong, J. T. A.; Smith, J. A.; Rix, H. -W.; Bell, E. F.; Wyse, R. F. G.; Newberg, H. J.; Mayeur, P. A.; Yanny, B.; Rockosi, C. M.; Gnedin, O. Y.; Schneider, D. P.; Beers, T. C.; Barentine, J. C.; Brewington, H.; Brinkmann, J.; Harvanek, M.; Kleinman, S. J. (2007). "Cats and Dogs, Hair and a Hero: A Quintet of New Milky Way Companions". The Astrophysical Journal. 654 (2): 897–906. arXiv:astro-ph/0608448. Bibcode:2007ApJ...654..897B. doi:10.1086/509718. S2CID18617277.
