M33

M33
Name	M33
Epoch	J2000
Type	Sc
Constellation	Triangulum

M33 is a nearby spiral galaxy in the Local Group notable for its prominent H II regions and relatively low mass compared with other large spirals. It lies near Triangulum (constellation), is visible to amateur astronomers, and has been a frequent target of studies by observatories and space missions investigating galactic structure, star formation, and dark matter. Astronomers have compared it with Andromeda Galaxy, Milky Way, and dwarf companions such as Large Magellanic Cloud and Small Magellanic Cloud in efforts to understand disk evolution and satellite interactions.

Nomenclature and identification

The galaxy is cataloged in legacy catalogs including the Messier catalog, New General Catalogue, and other surveys; it is commonly referred to by its Messier number from Charles Messier and appears in compilations by William Herschel and John Herschel. Historical designations also relate to photographic and radio surveys by facilities such as Palomar Observatory, Arecibo Observatory, and the Lowell Observatory. Modern identifiers derive from multiwavelength campaigns including the Two Micron All Sky Survey and the Sloan Digital Sky Survey, plus entries in the NASA/IPAC Extragalactic Database and catalogs maintained by the European Southern Observatory.

Physical characteristics

This spiral exhibits properties measured across electromagnetic bands by instruments on Hubble Space Telescope, Spitzer Space Telescope, and the Chandra X-ray Observatory, with complementary radio mapping from Very Large Array and James Clerk Maxwell Telescope. Photometric studies link its absolute magnitude to calibrations from the Cepheid variable distance scale refined with observations from Hipparcos and Gaia (spacecraft). Spectroscopic analyses by teams using the Keck Observatory, Very Large Telescope, and Subaru Telescope have constrained metallicity gradients and rotational velocities critical to comparisons with results from Sloan Digital Sky Survey galaxy samples.

Structure and morphology

The galaxy's disk and spiral arm pattern have been mapped by optical surveys like the Palomar Observatory Sky Survey and ultraviolet imaging from Galaxy Evolution Explorer; infrared structure emerges from data by Spitzer Space Telescope and WISE (satellite). Studies of its spiral dynamics reference theories developed by Bertil Lindblad and C. C. Lin with simulations run on computational facilities at NASA Ames Research Center and Max Planck Institute for Astrophysics. Morphological classification is compared against catalogs from the Hubble Sequence and analyses by researchers affiliated with Harvard–Smithsonian Center for Astrophysics and European Southern Observatory.

Stellar populations and star formation

Resolved stellar photometry with Hubble Space Telescope and ground-based instruments at Keck Observatory and Gemini Observatory has enabled color–magnitude diagram studies of populations from young clusters studied in programs by Carnegie Institution for Science to old halo stars examined by teams from the Institute of Astronomy, Cambridge. Star formation rates inferred using Hα emission and far-infrared fluxes rely on calibrations from work by Robert K. Kennicutt and comparisons to starburst systems cataloged by Infrared Astronomical Satellite. Investigations into initial mass function variations reference analyses by Edwin Salpeter and subsequent revisions by groups at Max Planck Institute for Astronomy.

Interstellar medium and H II regions

Prominent H II complexes comparable to 30 Doradus in scale have been cataloged in surveys led by the Royal Astronomical Society and mapped in radio by Effelsberg 100-m Radio Telescope. The brightest nebular region, subject to spectroscopic campaigns by European Southern Observatory instruments and ultraviolet observations by International Ultraviolet Explorer, informs studies of massive stellar feedback pioneered by groups at University of California, Berkeley and Princeton University. Molecular gas mapped with IRAM 30m Telescope and Atacama Large Millimeter/submillimeter Array provides data for comparisons with molecular cloud studies at Harvard–Smithsonian Center for Astrophysics and theories advanced by researchers from California Institute of Technology.

Kinematics, mass, and dark matter

Rotation curves derived from H I mapping by Westerbork Synthesis Radio Telescope and optical spectroscopy using Keck Observatory instruments constrain total mass and dark matter halo profiles analyzed with models from Navarro–Frenk–White and alternatives discussed in literature by researchers at Princeton University and Institute for Advanced Study. Dynamical studies compare satellite interactions informed by studies of Local Group dynamics involving Andromeda Galaxy and simulations run on supercomputers at Lawrence Livermore National Laboratory and National Center for Supercomputing Applications. Estimates of baryonic mass consider stellar mass functions calibrated by work at University of Cambridge and gas mass measurements from Arecibo Observatory surveys.

Observational history and research spacecraft studies

Historical observations trace to telescopic work by Giovanni Battista Hodierna and later cataloging by Charles Messier and William Herschel; photographic era contributions from Isaac Roberts and mapping projects at Palomar Observatory advanced morphological knowledge. Space-based campaigns by Hubble Space Telescope programs, ultraviolet studies by Galaxy Evolution Explorer, infrared mapping by Spitzer Space Telescope and WISE (satellite), X‑ray surveys by Chandra X-ray Observatory and XMM-Newton, and radio investigations using Very Large Array and Arecibo Observatory have each produced major datasets. Ongoing and planned observations by facilities such as James Webb Space Telescope and coordinated programs with European Space Agency instrumentation continue to refine distance, structure, and star formation histories examined by international collaborations including teams from National Aeronautics and Space Administration, European Southern Observatory, Japan Aerospace Exploration Agency, and university consortia at University of Chicago and University of California, Santa Cruz.

Category:Local Group galaxies