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Hercules (dwarf galaxy)

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Not to be confused with Hercules A.
Hercules Dwarf Galaxy[1]
Hercules, showing its elongated shape
Observation data (J2000 epoch)
ConstellationHercules
Right ascension16h 31m 02s[1]
Declination+12° 47′ 30″[1]
Distance479+26
−23 kly (147+8
−7 kpc),[2]
430+20
−20 kly (133+6
−6 kpc[3])
Apparent magnitude (V)14.7 ± 0.5[4]
Characteristics
TypedSph[4]
Apparent size (V)16.8′[4]
Half-light radius (physical)704 ± 65 ly (216 ± 20 pc)[5]
Half-light radius (apparent)5.83 ± 0.65′[5]
Other designations
Her,[1] PGC 4713560

Hercules, or Her, is a dwarf spheroidal galaxy situated in the Hercules constellation 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

[edit]
  1. ^ From other sources the half-radius is around 230 pc.[3]
  2. ^ From other sources the absolute magnitude is around −5.3.[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]
  4. ^ From other sources the metallicity is around −2.3.[2][6]

References

[edit]
  1. ^ a b c d "NAME Her Dwarf Galaxy". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 2010-02-22.
  2. ^ a b Adén, D.; Feltzing, S.; Koch, A. (2009). "A photometric and spectroscopic study of the new dwarf spheroidal galaxy in Hercules. Metallicity, velocities, and a clean list of RGB members". Astronomy and Astrophysics. 546 (3): 1147–1168. arXiv:0908.3489. Bibcode:2009A&A...506.1147A. doi:10.1051/0004-6361/200912718. S2CID 7851529.
  3. ^ a b c d e f g Sand, David J.; Olszewski, Edward W.; Willman, Beth (2009). "The Star Formation History and Extended Structure of the Hercules Milky Way Satellite". The Astrophysical Journal. 704 (2): 898–914. arXiv:0906.4017. Bibcode:2009ApJ...704..898S. doi:10.1088/0004-637X/704/2/898. S2CID 42754875.
  4. ^ a b c d e f g Belokurov, 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. S2CID 18617277.
  5. ^ a b c d Longeard, Nicolas; Jablonka, Pascale; Battaglia, Giuseppina; Malhan, Khyati; Martin, Nicolas; Sánchez-Janssen, Rubén; Sestito, Federico; Starkenburg, Else; Venn, Kim A. (2023). "The Pristine dwarf galaxy survey–V. The edges of the dwarf galaxy Hercules". Monthly Notices of the Royal Astronomical Society. 525 (2): 3086–3103. arXiv:2304.13046. Bibcode:2023MNRAS.525.3086L. doi:10.1093/mnras/stad2227.
  6. ^ a b c Simon, J. D.; Geha, M. (2007). "The Kinematics of the Ultra-faint Milky Way Satellites: Solving the Missing Satellite Problem". The Astrophysical Journal. 670 (1): 313–331. arXiv:0706.0516. Bibcode:2007ApJ...670..313S. doi:10.1086/521816. S2CID 9715950.
  7. ^ a b Martin, N. F.; De Jong, J. T. A.; Rix, H. W. (2008). "A Comprehensive Maximum Likelihood Analysis of the Structural Properties of Faint Milky Way Satellites". The Astrophysical Journal. 684 (2): 1075–1092. arXiv:0805.2945. Bibcode:2008ApJ...684.1075M. doi:10.1086/590336. S2CID 17838966.
  8. ^ a b Adén, D.; Wilkinson, M.I.; Read, J.I. (2009). "A new low mass for the Hercules dSph: the end of a common mass scale for the dwarfs?". The Astrophysical Journal Letters. 706 (1): L150–L154. arXiv:0910.1348. Bibcode:2009ApJ...706L.150A. doi:10.1088/0004-637X/706/1/L150. S2CID 118367297.
  9. ^ Kirby, E. N.; Simon, J. D.; Geha, M.; Guhathakurta, P.; Frebel, A. (2008). "Uncovering Extremely Metal-Poor Stars in the Milky Way's Ultrafaint Dwarf Spheroidal Satellite Galaxies". The Astrophysical Journal. 685 (1): L43–L46. arXiv:0807.1925. Bibcode:2008ApJ...685L..43K. doi:10.1086/592432. S2CID 3185311.
  10. ^ Grcevich, J.; Putman, M. E. (2009). "H I in Local Group Dwarf Galaxies and Stripping by the Galactic Halo". The Astrophysical Journal. 696 (1): 385–395. arXiv:0901.4975. Bibcode:2009ApJ...696..385G. doi:10.1088/0004-637X/696/1/385.
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