Hercules (dwarf galaxy)

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
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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]

  1. ^ a b c d "NAME Her Dwarf Galaxy". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 2010-02-22.
  2. ^ a b Cite error: The named reference Aden2009 was invoked but never defined (see the help page).
  3. ^ a b c d e f g Cite error: The named reference Sand2009 was invoked but never defined (see the help page).
  4. ^ a b c d e f g Cite error: The named reference Belokurov2007 was invoked but never defined (see the help page).
  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 Cite error: The named reference Simon2007 was invoked but never defined (see the help page).
  7. ^ a b Cite error: The named reference Martin2008 was invoked but never defined (see the help page).
  8. ^ a b Cite error: The named reference Aden2009b was invoked but never defined (see the help page).
  9. ^ Cite error: The named reference Kirby2008 was invoked but never defined (see the help page).
  10. ^ Cite error: The named reference Grcevich2009 was invoked but never defined (see the help page).


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