Wolf–Lundmark–Melotte

Wolf–Lundmark–Melotte galaxy
File:The WLM galaxy on the edge of the Local Group.jpg Wolf–Lundmark–Melotte galaxy
Observation data (J2000 epoch)
Constellation	Cetus
Right ascension	00h 01m 58.1s
Declination	−15° 27′ 39″
Redshift	-122 ± 2 km/s
Distance	3.04 ± 0.11 Mly (930 ± 30 kpc)
Type	IB(s)m
Apparent dimensions (V)	11′.5 × 4′.2
Apparent magnitude (V)	11.0
Notable features	-
Other designations
WLM, DDO 221, UGCA 444, PGC 143

The Wolf–Lundmark–Melotte (WLM) is an irregular galaxy discovered in 1909 by Max Wolf, and is located on the outer edges of the Local Group. The discovery of the nature of the galaxy was accredited to Knut Lundmark and Philibert Jacques Melotte in 1926. It is in the constellation Cetus.

Star formation

In 1994, A. E. Dolphin used the Hubble Space Telescope to create a color–magnitude diagram for WLM. It showed that around half of all the star formation in this galaxy occurred during a starburst that started ~13 Gyr ago. During the starburst, the metallicity of WLM rose from [Fe/H] ~ −2.2 to [Fe/H] −1.3. There being no horizontal-branch population, Dolphin concludes that no more than ~20 M_☉ per Myr of star formation occurred in the period from 12 to 15 Gyr ago. From 2.5 to 9 Gyr ago, the mean rate of star formation was 100 to 200 M_☉ per Myr. Being at the edge of the Local Group has also protected WLM from interactions and mergers with other galaxies, giving it a "pristine" stellar population and state that make it particularly useful for comparative studies.

Globular cluster

WLM has one known globular cluster (PGC 910901) at 00^h 01^m 29.5^s −15° 27′ 51″ that Hodge et al. (1999) determined has M_v = −8.8 and [Fe/H] = –1.5 with an age of ~15 Gyr. This cluster has a luminosity that is slightly over the average for all globulars. The seeming lack of faint low-mass globular clusters cannot be explained by the weak tidal forces of the WLM system.

References in fiction

In E. E. Smith’s Lensman novels, the "Second Galaxy" is identified as "Lundmark's Nebula". However, some believe the "Second Galaxy" may not be the Wolf–Lundmark–Melotte galaxy, since the first chapter of the first novel in the series (Triplanetary) and the series-establishing material appearing at the beginning of subsequent novels states that the "Second Galaxy" and the "First Galaxy" (the Milky Way) collided and passed through each other "edge-on" during the "planet-forming era"—implying that the "Lundmark's Nebula" of the series must necessarily be obscured from view by the Milky Way; however, according to others, it could have passed through at an angle and thus be identified with the galaxy described in this article; some have stated that this is the galaxy that E.E. Smith was thinking of when he wrote the series. However, the distance to Lundmark's nebula is defined quite precisely in Grey Lensman and is much larger than the distance to Wolf–Lundmark–Melotte (approximately ten times greater). Additionally, in Second Stage Lensmen multiple references are made to the spiral arms of Lundmark's Nebula. Wolf–Lundmark–Melotte does not possess such structures. At the time of writing of these books, the name of Lundmark was associated with such classifications and Smith may have elected to use this as a "believable" name for an entirely fictional galaxy.

At the time the Lensman series was written, most astronomers favored the tidal theory of Solar System formation, which required that planets be formed by the close approach of another star. In order to produce the massive numbers of planets necessary to evolve into the galactic civilizations in both the Milky Way and Lundmark's Nebula portrayed in the Lensman series, E.E. Smith felt it would have been necessary for another galaxy to have passed through the Milky Way in the past to produce the large number of close encounters necessary to form so many planets.