The Triangulum Galaxy is the third largest member of the
Local Group of galaxies. It has a diameter measured through the D25 standard - the
isophote where the surface brightness of the galaxy reaches 25 mag/arcsec2, to be about , making it roughly 70% the size of the Milky Way. It may be a gravitationally bound
companion of the Andromeda Galaxy. Triangulum may be home to 40 billion stars, compared to 400 billion for the Milky Way and 1 trillion for Andromeda. The disk of Triangulum has an estimated mass of
solar masses, while the gas component is about 3.2 billion solar masses. Thus, the combined mass of all
baryonic matter in the galaxy may be 10 billion solar masses. The contribution of the
dark matter component out to a radius of is equivalent to about 50 billion solar masses.
Location – distance – motion ; above center) Estimates of the distance from the Milky Way to the Triangulum Galaxy vary depending on the measurement technique. The average of 102 distance estimates published since 1987 gives a
distance modulus of 24.69, or 0.883
Mpc (2.878 Mly). In general, measurements range from (or 2.38 to 3.07
Mly), with most estimates since the year 2000 lying in the middle portion of this range. This makes the Triangularum Galaxy slightly more distant than the Andromeda Galaxy (
at 2.54 Mly). The Triangulum Galaxy is around 0.75 Mly from the Andromeda Galaxy. Using the
Cepheid variable method, an estimate of was achieved in 2004. In the same year, the
tip of the red-giant branch (TRGB) method was used to derive a distance estimate of . In 2006, a group of astronomers announced the discovery of an
eclipsing binary star in the Triangulum Galaxy. By studying the eclipses of the stars, astronomers were able to measure their sizes. Knowing the sizes and temperatures of the stars, they were able to measure the
absolute magnitude of the stars. When the
visual and absolute magnitudes are known, the distance to the star can be measured. The stars lie at the distance of . Measurements of
blue supergiants in 2009 indicated a distance of 940 kpc (3.2 Mly) In 2023, the distance to M33 was measured to be 840 ± 11 kpc using Cephid photometry. The Triangulum Galaxy is a source of
H2O maser emission. In 2005, using observations of two water masers on opposite sides of Triangulum via the
VLBA, researchers were for the first time able to estimate the angular rotation and
proper motion of Triangulum. A velocity of relative to the Milky Way was computed, which means Triangulum is moving towards
Andromeda Galaxy and suggesting it may be a satellite of the larger galaxy (depending on their relative distances and margins of error). In 2004, evidence was announced of a clumpy stream of hydrogen gas linking the Andromeda Galaxy with Triangulum, suggesting that the two may have tidally interacted in the past. This discovery was confirmed in 2011. A distance of less than 300 kiloparsecs between the two supports this hypothesis. The
Pisces Dwarf (LGS 3), one of the small Local Group member galaxies, is located from the Sun. It is 20° from the
Andromeda Galaxy and 11° from Triangulum. As LGS 3 lies at a distance of from both galaxies, it could be a
satellite galaxy of either Andromeda or Triangulum. LGS 3 has a core radius of and 26 million solar masses. Pisces VII/Triangulum (Tri) III may be another satellite of Triangulum.
Structure In the French astronomer
Gérard de Vaucouleurs' revised Hubble Sandage (VRHS) system of
galaxy morphological classification, the Triangulum Galaxy is classified as type SA(s)cd. The
S prefix indicates that it is a disk-shaped galaxy with prominent arms of gas and dust that spiral out from the nucleus—what is commonly known as a
spiral galaxy. The
A is assigned when the galactic nucleus lacks a bar-shaped structure, in contrast to
SB class
barred spiral galaxies. American astronomer
Allan Sandage's "(s)" notation is used when the spiral arms emerge directly from the nucleus or central bar, rather than from an inner ring as with an (r)-type galaxy. Finally, the
cd suffix represents a stage along the spiral sequence that describes the openness of the arms. A rating of
cd indicates relatively loosely wound arms. This galaxy has an inclination of 54° to the line of sight from Earth, allowing the structure to be examined without significant obstruction by gas and dust. The disk of the Triangulum Galaxy appears warped out to a radius of about 8 kpc. There may be a halo surrounding the galaxy, but there is no bulge at the nucleus. This is an isolated galaxy and there are no indications of recent mergers or interactions with other galaxies, and it lacks the dwarf spheroidals or
tidal tails associated with the Milky Way. Triangulum is classified as unbarred, but an analysis of the galaxy's shape shows what may be a weak bar-like structure about the galactic nucleus. The radial extent of this structure is about 0.8 kpc. The nucleus of this galaxy is an
H II region, and it contains an
ultraluminous X-ray source with an emission of , which is the most luminous source of X-rays in the
Local Group of galaxies. This source is modulated by 20% over a 106-day cycle. However, the nucleus does not appear to contain a
supermassive black hole, as a best-fit value of zero mass and an upper limit of is placed on the mass of a central black hole based on models and the
Hubble Space Telescope (HST) data. This is significantly lower than the mass expected from the velocity dispersion of the nucleus and far below any mass predicted from the disk kinematics. This may suggest that supermassive black holes are associated only with galaxy bulges instead of with their disks. Assuming that the upper limit of the central black hole is correct, it would be rather an
intermediate-mass black hole. The inner part of the galaxy has two luminous spiral arms, along with multiple spurs that connect the inner to the outer spiral features. The main arms are designated IN (north) and IS (south).
Star formation , a star-forming region in the Triangulum Galaxy, as imaged by the
Hubble Space Telescope In the central 4′ region of this galaxy, atomic gas is being efficiently converted to molecular gas, resulting in a strong
spectral emission of
CO. This effect occurs as giant
molecular clouds condense out of the surrounding
interstellar medium. A similar process is taking place outside the central 4′, but at a less efficient pace. About 10% of the gas content in this galaxy is in the molecular form.
Star formation is taking place at a rate that is strongly correlated with local gas density, and the rate per unit area is higher than in the neighboring
Andromeda Galaxy. (The rate of star formation is about 3.4 solar masses Gyr−1 pc−2 in the Triangulum Galaxy, compared to 0.74 in Andromeda.) The total integrated rate of star formation in the Triangulum Galaxy is about . It is uncertain whether this net rate is currently decreasing or remaining constant. Based on analysis of the chemical composition of this galaxy, it appears to be divided into two distinct components with differing histories. The inner disk within a radius of has a typical composition gradient that decreases linearly from the core. Beyond this radius, out to about , the gradient is much flatter. This suggests a different star formation history between the inner disk and the outer disk and halo, and may be explained by a scenario of "inside-out" galaxy formation. This occurs when gas is accumulated at large radii later in a galaxy's life space, while the gas at the core becomes exhausted. The result is a decrease in the average age of stars with increasing radius from the galaxy core.
Discrete features Using infrared observations from the
Spitzer Space Telescope, a total of 515 discrete candidate sources of 24 μm emission within the Triangulum Galaxy have been catalogued as of 2007. The brightest sources lie within the central region of the galaxy and along the spiral arms. Many of the emission sources are associated with
H II regions of star formation. The four brightest HII regions are designated
NGC 588,
NGC 592,
NGC 595, and
NGC 604. These regions are associated with
molecular clouds containing 120,000 to 400,000 solar masses. The brightest of these regions, NGC 604, may have undergone a discrete outburst of star formation about three million years ago. This nebula is the second most luminous HII region within the Local Group of galaxies, at 45 ± 15 million times the
luminosity of the Sun. Other prominent HII regions in Triangulum include IC 132, IC 133, and IK 53. The northern main spiral arm contains four large
HII regions, while the southern arm has greater concentrations of young, hot stars. The estimated rate of
supernova explosions in the Triangulum Galaxy is 0.06
Type Ia and 0.62
Type Ib/
Type II per century. This is equivalent to a supernova explosion every 147 years, on average. As of 2008, a total of 100
supernova remnants have been identified in the Triangulum Galaxy, the majority of which lie in the southern half of the spiral galaxy. Similar asymmetries exist for H I and H II regions, plus highly luminous concentrations of massive,
O type stars. The center of the distribution of these features is offset about two arc minutes to the southwest. M33 being a local galaxy, the
Central Bureau for Astronomical Telegrams (CBAT) tracks
novae in it along with M31 and
M81. About 54
globular clusters have been identified in this galaxy, but the actual number may be 122 or more. The confirmed clusters may be several billion years younger than globular clusters in the Milky Way, and cluster formation appears to have increased during the past 100 million years. This increase is correlated with an inflow of gas into the center of the galaxy. The
ultraviolet emission of massive stars in this galaxy matches the level of similar stars in the
Large Magellanic Cloud. In 2007, a
black hole about 15.7 times the
mass of the Sun was detected in this galaxy using data from the
Chandra X-ray Observatory. The black hole, named
M33 X-7, orbits a companion star which it eclipses every 3.5 days. It is the largest
stellar mass black hole known. Unlike the Milky Way and Andromeda galaxies, the Triangulum Galaxy does not appear to have a
supermassive black hole at its center. This may be because the mass of a galaxy's central supermassive black hole correlates with the size of the galaxy's
central bulge, and unlike the Milky Way and Andromeda, the Triangulum Galaxy is a pure
disk galaxy with no bulge.
Relationship with the Andromeda Galaxy As mentioned above, M33 is linked to M31 by several streams of
neutral hydrogen and stars, which suggests that a past interaction between these two galaxies took place from 2 to 8 billion years ago, and a more violent encounter will occur 2.5 billion years in the future. The fate of M33 was uncertain in 2009 beyond seeming to be linked to its larger neighbor M31. Suggested scenarios include being torn apart and absorbed by the greater companion, fueling the latter with
hydrogen to form new stars; eventually exhausting all of its gas, and thus the ability to form new stars; or participating in the
collision between the Milky Way and M31, likely ending up orbiting the merger product and fusing with it much later. Two other possibilities are a collision with the Milky Way before the Andromeda Galaxy arrives or an ejection out of the Local Group. In 2019, astrometric data from
Gaia appears to rule out the possibility that M33 and M31 are in orbit. If correct, M33 is on its first infall proper into the Andromeda Galaxy (M31). ==See also==