Hercules (dwarf galaxy)

Not to be confused with Hercules A.

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]

Hercules Dwarf Galaxy[1]
Hercules, showing its elongated shape
Observation data (J2000 epoch)
Constellation	Hercules
Right ascension	16h 31m 02s[1]
Declination	+12° 47′ 30″[1]
Distance	479+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
Type	dSph[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

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

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]