Local Galaxies
Local Galaxies
| Group Name | local |
| Reference | Revision on the Nearby Galaxies Catalogue, Cambridge University Press (Tully, 1988; revised 2003) |
| Prepared by | R. Brent Tully (U Hawaii)
Stuart Levy (NCSA/U Illinois) |
| Labels | Yes |
| Files | g8local.speck, g8local.label |
| Dependencies | none |
| Census | 247 galaxies |
The galaxies in the local data group represent the best picture we have of the galactic neighborhood. These 247 galaxies compose a collection of galaxies that is more complete than other galaxy surveys, which typically show only larger and brighter galaxies.
These galaxies were compiled by Brent Tully at the University of Hawaii. Known for discovering the Tully-Fisher relation, a correlation between the luminosity and the rotation period of a spiral galaxy, Brent dedicates his time toward understanding the structure of the Universe.
Data Variables
The nearby galaxies have data variables that describe their brightness, size, and orientation. We list these variables in the following table.
| Data Variables for the Local Galaxies | |||
| Number | Name | Description | Units |
| 0 | type | Galaxy Morphological Type | -- |
| 1 | diamkpc | Galaxy diameter | kpc |
| 2 | Mblueneg | Negative absolute B magnitude | mag |
| 3 | lum | Galaxy luminosity scaled to Mblueneg | -- |
| 4 | txno | Texture number | -- |
| 5 | cloud | The cloud (grouping) number | -- |
| 6 | pgc | Principal Galaxies Catalogue number | -- |
| 7 | hicnt | References for a grouping catalog | -- |
| 8-13 | orient0 | Unit vectors for the polygon orientation | Mpc |
The galaxy morphological type variable is an integer that reflects the type of galaxy classified first by Edwin Hubble (1889-1953) in the 1930s. The classification scheme has four main groups: elliptical galaxies (E), barred spiral galaxies (SB), unbarred spiral galaxies (S), and irregular galaxies (Irr). The integers assigned to these types are decoded in the table below. In this numbering system, barred and unbarred spiral galaxies (S & SB) are merged, since data on bars are often inconclusive. For an overview of galaxy classification, see “Galaxies” in the extragalactic overview.
| Galaxy Morphology Codes and Census | |||
| Number | Hubble Type | Galaxy Type | Census |
| -5 | E | Elliptical | 20 |
| -3 | E/SO | Elliptical/Lenticular (classification uncertain) | 3 |
| -2 | SO | Lenticular | 2 |
| 0 | SO/a | Lenticular/Spiral | 39 |
| 1 | Sa | Spiral | 0 |
| 2 | Sab | Spiral | 3 |
| 3 | Sb | Spiral | 3 |
| 4 | Sbc | Spiral | 1 |
| 5 | Sc | Spiral | 8 |
| 6 | Scd | Spiral | 9 |
| 7 | Sd | Spiral | 12 |
| 8 | Sdm | Spiral | 12 |
| 9 | Sm | Spiral/irregular | 15 |
| 10 | Irr | Irregular | 117 |
| 12 | S | Spiral/irregular (classification uncertain) | 0 |
| 13 | P | Peculiar | 0 |
This table also describes the population of galaxies in the data set. Most of them are irregular galaxies; SO/a spirals (mostly dwarfs) are the second-highest population in the data set. Most of the galaxies around us are small, but is this true elsewhere in the Universe? Turns out the answer is probably yes, and we happen to live in a large galaxy.
Other data variables describe the galaxy's size, brightness, and orientation. diamkpc is the diameter of the galaxy in kiloparsecs. Mblueneg is the negative value of the intrinsic B magnitude of the galaxy. The luminosity is a linear scaling of the intrinsic B magnitude, with lum = 1 corresponding to Mblueneg = 19.
In this data set, a galaxy is often a member of a group, a cluster, a cloud, and a supercluster. Many groups and clusters of galaxies form a cloud, and many clouds form a supercluster. For example, the Milky Way belongs to the Local Group, which is in the Coma-Sculptor Cloud (cloud number 140) and in the larger Virgo Supercluster. Another, numbered 190 and named the Pavo-Ara Cloud, includes the large elliptical galaxy Centaurus A (NGC 5128), M83, and its bevy of galaxies. There are also galaxies present from cloud 650, called the Pegasus Spur. These clouds take the name of the constellations they appear in. Our home cloud, Coma-Sculptor, connects galaxies that are, from our perspective on Earth, in the direction of the constellation Coma Berenices and stretch toward the opposite side of the Sun in the direction of the constellation Sculptor.
The pgc variable is the Principal Galaxies Catalogue number, and the hicnt is a cross-referencing number for a galaxy grouping catalog. Following that, the next six columns beginning with orient0 specify the orientation of the galaxy.
Structure Within the Local Galaxies
Each of these green points is a Galactic neighbor. We have defined three boxes (see table below) that highlight the galaxies in this data group.
| Boxes for the Local Galaxies | ||
| Box | Dimensions (Mpc) | |
| Number | (xcen,ycen,zcen xlen,ylen,zlen) | Description |
| 1 | 0, 0, 0 0.85, 0.85, 0.85 | Local Group box |
| 2 | 0, 0, 0 3.75, 3.75, 3.75 | Box around the entire data set |
| Clipbox | 0, 0, 0 0.18, 0.18, 0.18 | Milky Way group of galaxies |
The smallest box encloses those galaxies around the Milky Way. This is a clip box and is activated with the command cb on in the Partiview Command Line. Within this box are 12 galaxies, our nearest neighbors.
To see those galaxies that live around the block, turn off the clip box (cb off) and turn on the normal boxes by pressing the Box Toggle Button. Inside this dark yellow box are the three dozen or so Local Group galaxies. We discussed this group in the Local Group and in “Beyond the Milky Way” in the Milky Way Atlas, but now we have a greater appreciation for the context of these galaxies among the other nearby galaxies in the local Universe.
Within the Local Group, the two major players are the Andromeda Galaxy (M31) and the Milky Way. These are large spiral galaxies that have a bevy of smaller galactic companions clustered around them. Another, less concentrated grouping is found near the galaxy NGC 3109, on the outskirts of the group.
Membership in the Local Group is based on two criteria: the mutual gravitational attraction of the galaxies and their distance from us. While the former seems sensible and the latter somewhat arbitrary, this is the historical evolution behind selecting the galaxies of the Local Group.
Finally, we have the adjacent neighborhoods that, in this data set, surround the Local Group. Upon inspection, you may see that these galaxies tend to clump into groups that rival the Local Group in size and number. Most of these groups have dominant galaxies that tend to influence their evolution, just as Andromeda and the Milky Way influence the smaller dwarf galaxies in the Local Group. These include NGC 5128, or Centaurus A, a large (500-kpc-diameter), peculiar elliptical galaxy in Centaurus with a large, irregular dust lane and strong radio-emitting gas lobes emanating from its nucleus (not shown in the Atlas). These peculiar features are likely the result of a galaxy collision some 500 million years ago. Also, M81, the large spiral in Ursa Major, dominates the Ursa Major Group, and NGC 253, a large spiral near the South Galactic Pole, is the largest member of the Sculptor Group.
Some of these adjacent groups around the Local Group appear more concentrated than others, but they all tend to have similar properties. Dominated by one or a few large galaxies, the majority of galaxies tend to be small dwarf and irregular galaxies.
© 2002-2005 American Museum of Natural History
Last Modified: 2007-12-19 by Brian Abbott