Messier 99
A grand design spiral galaxy with a distorted arm and a hydrogen bridge.
Messier 99 (also cataloged as NGC 4254 and known as St. Catherine's Wheel) is a grand design spiral galaxy located in the northern constellation Coma Berenices, about 49 million light-years (15 million parsecs) from the Milky Way. Pierre Méchain spotted it on March 17, 1781, and passed the find along to Charles Messier, who added it to his catalog of comet-like objects. It ranks among the first galaxies where a spiral structure was recognized—Lord Rosse first noted this pattern in the spring of 1846.
The galaxy is classified as SA(s)c, meaning it is a pure spiral with loosely wound arms. Its shape is unusual: one arm appears normal, while the other is extended and more loosely coiled. It tilts 42° relative to our line of sight, with its major axis positioned at an angle of 68°.
A bridge of neutral hydrogen gas connects NGC 4254 to VIRGOHI21, an HI region that may be a dark galaxy. The gravitational pull of this object might have warped M99 and pulled out the gas bridge, suggesting the two galaxy-sized bodies had a close encounter before drifting far apart. Alternatively, VIRGOHI21 could be tidal debris left over from an interaction with the lenticular galaxy NGC 4262 about 280 million years ago. Once the encounter ends, the drawn-out arm is expected to relax and match the normal arm.
Although M99 is not classified as a starburst galaxy, its star formation rate is three times higher than that of other galaxies of similar Hubble type—likely triggered by the encounter. It appears to be entering the Virgo Cluster for the first time, bound to the cluster's periphery at a projected separation of about 3.7° (roughly one megaparsec) from the cluster center at Messier 87. As it moves through the intracluster medium, much of its interstellar medium is being stripped away by ram pressure.
Four supernovae have been observed in M99: SN 1967H (Type II, magnitude 14.0), discovered by Fritz Zwicky on July 1, 1967; SN 1972Q (type unknown, magnitude 15.8), found by Leonida Rosino on December 14, 1972; SN 1986I (Type II, magnitude 14), spotted by Carlton Pennypacker and colleagues on May 17, 1986; and SN 2014L (Type Ic, magnitude 17.2), detected by the THU-NAOC Transient Survey on January 26, 2014. One luminous red nova, PTF 10fqs (Type LRN, magnitude 20.1), was discovered in M99 by the Palomar Transient Factory on April 16, 2010.
Quick Facts
- Constellation Name
- Coma Berenices
- Epoch
- J2000
- Group Cluster
- Virgo Cluster
- Ra
- 12 · 18 · 49.625
- Dec
- +14 · 24 · 59.36
- Z
- 0.008029
- Dist Ly
- 13.87 Mpc
- Appmag V
- 9.9
- Names
- Coma Pinwheel Galaxy, Virgo Cluster Pinwheel, M99, NGC 4254, PGC 39578, UGC 7345
Facts from the source article.
Lore & Background
Messier 99 has a morphological classification of SA(s)c, indicating a pure spiral shape with loosely wound arms. It has a peculiar shape with one normal looking arm and an extended arm that is less tightly wound. The galaxy is inclined by 42° to the line-of-sight with a major axis position angle of 68°. A bridge of neutral hydrogen gas links NGC 4254 with VIRGOHI21, an HI region and a possible dark galaxy. The gravity from the latter may have distorted M99 and drawn out the gas bridge, as the two galaxy-sized objects may have had a close encounter before parting greatly. However, VIRGOHI21 may instead be tidal debris from an interaction with the lenticular galaxy NGC 4262 some 280 million years ago. It is expected that the drawn out arm will relax to match the normal arm once the encounter is over. While not classified as a starburst galaxy, M99 has a star formation activity three times larger than other galaxies of similar Hubble type that may have been triggered by the encounter. M99 is likely entering the Virgo Cluster for the first time bound to the periphery of the cluster at a projected separation of 3.7°, or around one megaparsec, from the cluster center at Messier 87. The galaxy is undergoing ram-pressure stripping of much of its interstellar medium as it moves through the intracluster medium.
Reader's Guide
Messier 99 is notable as one of the first galaxies in which a spiral pattern was recognized, identified by Lord Rosse in 1846. Its peculiar shape—one normal arm and one extended, less tightly wound arm—is likely the result of gravitational interactions, possibly with the dark galaxy candidate VIRGOHI21 or with the lenticular galaxy NGC 4262. A bridge of neutral hydrogen gas connects M99 to VIRGOHI21, and the encounter may have triggered star formation activity three times higher than typical for its Hubble type. The galaxy is entering the Virgo Cluster for the first time and is undergoing ram-pressure stripping of its interstellar medium. Four supernovae have been observed in M99: SN 1967H (Type II, mag. 14.0), SN 1972Q (type unknown, mag. 15.8), SN 1986I (Type II, mag. 14), and SN 2014L (Type Ic, mag. 17.2). Additionally, one luminous red nova, PTF 10fqs (Type LRN, mag. 20.1), was discovered on 16 April 2010. These events contribute to the galaxy's legacy as a site of transient astronomical phenomena.
Did You Know?
- M99 was discovered by Pierre Méchain on 17 March 1781 and reported to Charles Messier.
- It was one of the first galaxies in which a spiral pattern was seen, identified by Lord Rosse in the spring of 1846.
- A bridge of neutral hydrogen gas links M99 with VIRGOHI21, a possible dark galaxy.
- Four supernovae and one luminous red nova have been observed in M99.
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