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NGC 5238
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NGC 5238
NGC 5238 is an irregular galaxy in the constellation Canes Venatici. Located at a comoving distance of 4.51 Mpc, it is 64.4 arcseconds in diameter. It has sometimes been classified as a blue compact dwarf galaxy. Although some authors have hypothesized it to be a member of the M101 Group of galaxies, it is currently believed to be an isolated galaxy.
At an inclination of 39° with respect to Earth, NGC 5238 has a total mass of 117 million solar masses, with a star formation rate of 0.01 solar masses per year. Of the total mass, HI gas appears to account for 26 million solar masses.
In 1977, NGC 5238 was hypothesized to not be a single galaxy, but rather a pair of interacting galaxies. It was not until ten years later that a dedicated study of the galaxy's rotation curve was undertaken, showing that the galaxy is indeed a single galaxy. One of the two regions that was thought to be the nucleus of a galaxy was instead shown to be simply a large HII region around 100 pc in diameter.
The morphological type of NGC 5238 has been the subject of some controversy. In 1979, the galaxy was classified as a barred spiral galaxy. Soon after, in 1984, the galaxy was included in a study of blue compact dwarf galaxies, incompatible with the classification of a barred spiral. However, the barred spiral classification was considered the correct classification for years. It was not until the mid 1990s that the galaxy was first recognized as a dwarf irregular galaxy. Even after this, the majority of studies recognized the galaxy as a spiral galaxy until 2015, when the classification of irregular finally became widely accepted.
As it appears to us, NGC 5238 is tilted at an inclination of 39°. This 2013 estimate follows previous estimates of 30° in 1987, 37 ± 5° in 1992, and 47° in 1999 In the Spitzer 3.6 μm band, the semimajor axis of its angular size is 64.4", with an ellipticity of 0.201.
NGC 5238 has two stellar streams: one umbrella-shaped one to its north, and another sparser overdensity of stars to its south.
The distance estimate to NGC 5238 has been brought down considerably since first calculated. The first published distance estimate was 7 Mpc, derived using redshift. This remained the predominant estimate until 1996, when the distance was found to be much less, estimated at 5.18 Mpc. Subsequently, using spectral data from the HI 21cm line, the distance was calculated to be 4.7 Mpc in 1999, although an updated HI study found a slightly higher value at 5.20 Mpc in 2002. Five years later, in 2007, the distance estimate was lowered even further to 4.50 Mpc, extremely close to today's accepted value.
One way to determine distance unambiguously is by standard candles. The tip of the red giant branch is such a method; every galaxy's brightest red giant stars must have exactly the same known luminosity. When combined with corrections for interstellar reddening, this allows for accurate determination of a galaxy's distance. By 2009, a Hubble Space Telescope image of NGC 5238 had become available, resolving the individual stars within the galaxy. Using this method, the distance modulus was calculated at 28.27 magnitudes, corresponding to a distance of 4.51 Mpc, today's accepted value.
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NGC 5238
NGC 5238 is an irregular galaxy in the constellation Canes Venatici. Located at a comoving distance of 4.51 Mpc, it is 64.4 arcseconds in diameter. It has sometimes been classified as a blue compact dwarf galaxy. Although some authors have hypothesized it to be a member of the M101 Group of galaxies, it is currently believed to be an isolated galaxy.
At an inclination of 39° with respect to Earth, NGC 5238 has a total mass of 117 million solar masses, with a star formation rate of 0.01 solar masses per year. Of the total mass, HI gas appears to account for 26 million solar masses.
In 1977, NGC 5238 was hypothesized to not be a single galaxy, but rather a pair of interacting galaxies. It was not until ten years later that a dedicated study of the galaxy's rotation curve was undertaken, showing that the galaxy is indeed a single galaxy. One of the two regions that was thought to be the nucleus of a galaxy was instead shown to be simply a large HII region around 100 pc in diameter.
The morphological type of NGC 5238 has been the subject of some controversy. In 1979, the galaxy was classified as a barred spiral galaxy. Soon after, in 1984, the galaxy was included in a study of blue compact dwarf galaxies, incompatible with the classification of a barred spiral. However, the barred spiral classification was considered the correct classification for years. It was not until the mid 1990s that the galaxy was first recognized as a dwarf irregular galaxy. Even after this, the majority of studies recognized the galaxy as a spiral galaxy until 2015, when the classification of irregular finally became widely accepted.
As it appears to us, NGC 5238 is tilted at an inclination of 39°. This 2013 estimate follows previous estimates of 30° in 1987, 37 ± 5° in 1992, and 47° in 1999 In the Spitzer 3.6 μm band, the semimajor axis of its angular size is 64.4", with an ellipticity of 0.201.
NGC 5238 has two stellar streams: one umbrella-shaped one to its north, and another sparser overdensity of stars to its south.
The distance estimate to NGC 5238 has been brought down considerably since first calculated. The first published distance estimate was 7 Mpc, derived using redshift. This remained the predominant estimate until 1996, when the distance was found to be much less, estimated at 5.18 Mpc. Subsequently, using spectral data from the HI 21cm line, the distance was calculated to be 4.7 Mpc in 1999, although an updated HI study found a slightly higher value at 5.20 Mpc in 2002. Five years later, in 2007, the distance estimate was lowered even further to 4.50 Mpc, extremely close to today's accepted value.
One way to determine distance unambiguously is by standard candles. The tip of the red giant branch is such a method; every galaxy's brightest red giant stars must have exactly the same known luminosity. When combined with corrections for interstellar reddening, this allows for accurate determination of a galaxy's distance. By 2009, a Hubble Space Telescope image of NGC 5238 had become available, resolving the individual stars within the galaxy. Using this method, the distance modulus was calculated at 28.27 magnitudes, corresponding to a distance of 4.51 Mpc, today's accepted value.
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