Aquarius IV

Aquarius IV
A Rubin image centered on Aquarius IV, combining images in the g, r, and z filters. The yellow star marks a likely blue horizontal branch member.
Observation data (J2000 epoch)
Constellation	Aquarius
Right ascension	22h 01m 53.76s
Declination	−02° 34′ 30.0″
Distance	109 kpc (0.36 Mly)
Absolute magnitude (V)	−1.9+0.6 −1.0
Characteristics
Type	dSph
Half-light radius (physical)	62+13 −20 ly (19+4 −6 pc)
Half-light radius (apparent)	0.60′+0.14′ −0.17′
Notable features	satellite galaxy of Milky Way
Other designations
Rubin J2201−0234

Aquarius IV (also known as Rubin J2201−0234) is an ultra-faint dwarf spheroidal galaxy located in the outer halo of the Milky Way in the constellation of Aquarius, at a distance of 109 kiloparsecs from the Sun. It is the first cosmic object discovered using the preliminary data from the Vera C. Rubin Observatory in Chile.

Discovery

The galaxy was first identified by an international research team led by astronomer William Cherny, utilizing photometric data from the Early Data Preview 2 (EDP2) release of the Vera C. Rubin Observatory. The researchers detected a localized overdensity of stars within the Milky Way’s stellar halo, with the initial discovery reaching a statistical significance of approximately 8σ.

To verify the structure, the team conducted a follow-up analysis using archival deep-field imaging from the Dark Energy Camera (DECam), mounted on the 4-meter Víctor M. Blanco Telescope at the Cerro Tololo Inter-American Observatory. This reanalysis confirmed the stellar overdensity with a significance of 6σ. The object had remained uncatalogued until now due to the extremely low luminosity of its constituent stars. Furthermore, the distribution of stars on a color-magnitude diagram revealed a clear alignment with a single theoretical isochrone, confirming the system is gravitationally bound.

A preprint of the scientific paper detailing these findings was published on arXiv on August 3, 2026.

Physical characteristics

Aquarius IV is classified as an ultra-faint dwarf spheroidal galaxy with an effective half-light radius of only 19+4
−6 parsecs (approximately 62+13
−20 light-years) and an angular half-light radius of 0.60+0.14
−0.17 arcminutes. The upper limit of the galaxy's ellipticity is ϵ < 0.61 with 95% confidence. The system's absolute magnitude in the visible band is −1.9+0.6
−1.0, classifying it as an object of extremely low luminosity. The total luminous flux of the galaxy is provided by only a small, sparse number of individual faint stars.

The distribution of stars on the color-magnitude diagramis consistent with an ancient isochrone with an extremely low metallicity of Z = 0.0001. The age of the stellar population is estimated to be approximately 13 billion years, placing its formation in the early stages of the Universe's evolution. Photometric analysis identified a probable blue horizontal branch star at a distance of 0.8 arcminutes from the center of the system, which confirms the ancient nature of the stellar population.

See also

• Satellite galaxies of the Milky Way
• Dwarf spheroidal galaxy (dSph)
• Milky Way