Galaxies known
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Overview of Known Galaxies
Early Star-Forming Galaxies and Reionization
The earliest known galaxies, discovered through the NASA Hubble Space Telescope, date back to when the Universe was approximately 800 million years old. These early star-forming galaxies played a crucial role in cosmic history by emitting intense ultraviolet radiation, which likely triggered the reionization of intergalactic hydrogen1.
Satellite Galaxies of the Milky Way
The Milky Way hosts at least twenty-three known satellite galaxies, with luminosities ranging from a thousand to a billion times that of the Sun. Recent measurements indicate that these galaxies share a common mass scale, particularly within their central 300 parsecs, making them some of the most dark-matter-dominated galaxies known2.
Optically Luminous Galaxies
A catalog from the Sloan Digital Sky Survey lists 1525 of the most optically luminous galaxies at redshift z < 0.3. This includes super spirals, super lenticulars, super post-mergers, and giant ellipticals. Super spirals and lenticulars, with stellar masses between 10^11.3 and 10^12 solar masses, are the most massive disk galaxies currently known. These galaxies have likely formed through minor mergers and cold accretion, maintaining high star formation rates throughout their history3.
Hyperluminous Infrared Galaxies
There are 39 known hyperluminous infrared galaxies, identified through various surveys. These galaxies, with far-infrared luminosities exceeding 10^13 solar masses, are undergoing star formation at rates greater than 1000 solar masses per year. They are considered primeval galaxies in the midst of significant star formation episodes4.
Old Galaxies in the Young Universe
Massive spheroidal galaxies, which contain more than half of all stars in the local Universe, were identified at redshifts between 1.6 and 1.9. These galaxies, fully assembled and massive, existed when the Universe was only about a quarter of its current age. Their early formation challenges existing models of galaxy formation, suggesting a faster build-up of massive early-type galaxies than previously expected5.
The Nearest Dwarf Galaxy: Sagittarius
The Sagittarius dwarf galaxy, the nearest known galaxy, is located 24 kpc from the Sun and 16 kpc from the Milky Way's center. It is undergoing strong tidal disruption and is expected to be integrated into the Milky Way. This galaxy contains a well-populated red horizontal branch, a substantial carbon star population, and evidence of a metallicity spread6.
Optically Invisible Massive Galaxies
Recent submillimetre observations have revealed 39 massive star-forming galaxies at redshifts greater than 3, which are invisible in the ultraviolet to near-infrared spectrum. These galaxies, with star formation rates of 200 solar masses per year, represent the bulk population of massive galaxies missed in previous surveys. They are likely the progenitors of the largest present-day galaxies in clusters7.
Galaxies with Known Dynamical Parameters
For galaxies with known rotation curves, various integral properties such as morphological type, photometric radius, and absolute luminosity have been compiled. The analysis indicates that star formation rates are not solely dependent on interstellar density, highlighting the complexity of galaxy formation and evolution8.
Ultra-Faint Galaxies at the Epoch of Reionization
Spectroscopic observations have confirmed the existence of ultra-faint galaxies from the epoch of reionization. These galaxies, detected through gravitational lensing, provide critical constraints on how the Universe was reionized9.
Conclusion
The study of known galaxies spans a wide range of types and epochs, from the earliest star-forming galaxies to the nearest dwarf galaxies. Each discovery provides valuable insights into the processes of galaxy formation and evolution, challenging and refining our understanding of the Universe.
Sources and full results
Most relevant research papers on this topic
Early star-forming galaxies and the reionization of the Universe
The Hubble Space Telescope has discovered the earliest galaxies, which likely induced the reionization of intergalactic hydrogen during the universe's early days.
Highly CitedA common mass scale for satellite galaxies of the Milky Way
The Milky Way's satellite galaxies have a common mass of about 107 within their central 300 parsecs, indicating they are the most dark-matter-dominated galaxies known.
Highly CitedA Catalog of the Most Optically Luminous Galaxies at z < 0.3: Super Spirals, Super Lenticulars, Super Post-mergers, and Giant Ellipticals
Super spirals and lenticulars are the most massive disk galaxies, with super post-mergers potentially being the precursors of giant elliptical galaxies in low-density environments.
Hyperluminous infrared galaxies
Hyperluminous infrared galaxies with emission peaking at rest frame wavelengths 50 m are undergoing star formation at rates >103 M yr1 and are strong candidates for being primeval galaxies in the process of a major episode of star formation.
Highly CitedOld galaxies in the young Universe
Massive early-type galaxies were built up much faster in the early Universe than previously expected, with four fully assembled systems identified at l.6 z 1.9.
Highly CitedSagittarius: the nearest dwarf galaxy
The Sagittarius dwarf galaxy is the nearest known Galactic satellite galaxy, with a similar size and luminosity to the largest dwarf spheroidal, and is currently undergoing strong tidal disruption before integration into the Galaxy.
Highly CitedA dominant population of optically invisible massive galaxies in the early Universe
Submillimetre-wavelength observations reveal a large population of optically invisible massive galaxies in the early Universe, challenging our understanding of massive-galaxy formation.
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