The Wolf-Lundmark-Melotte Galaxy, WLM

Wolf-Lundmark-Melotte (WLM, DDO 221), type Ir+,

Right Ascension	0 : 02.0 (h : m)
Declination	-15 : 28 (deg : m)
Distance	4200.0 (kly)
Visual Brightness	10.9 (mag)
Apparent Dimension	12×4 (arc minutes)

This faint member (of only -14.7 mag absolute magnitude) galaxy of the

Local Group

was discovered in 1909 by Max Wolf, but its nature as a galaxy was figured

out much later, by Knut Lundmark and P.J. Melotte in 1926.

More recent investigations have fixed its

distance at about 4.2 million light years, somewhat farther away than

the Andromeda galaxy (2.9 million light years) and placing it at the

very edge of the region occupied by the Local Group [this value has

eventually been corrected for Hipparcos data]. WLM is one

of the most isolated members; its nearest neighbour dwarf galaxy, IC

1613, is more 1 million light years away.

The ESO astronomers recently obtained deep CCD images of WLM

with the 3.5-m ESO New Technology Telescope (NTT) and the ESO

Multi-Mode Instrument (EMMI). With the excellent imaging capabilities

of this facility, they were able to see large numbers of individual

stars in this galaxy.

In this image, WLM appears to be quite

elongated, and surprisingly, its largest extension is about 8000 light

years or more than twice as large as thought before. Nevertheless,

this is still 12 times smaller than the Milky Way galaxy.

There is a clear excess of very faint stars that extends to the

edge of the sky field covered by the NTT exposures. Some of these are

probably foreground stars inside the Milky Way, but there are many

more than expected, and their numbers increase closer to WLM.

These stars look quite different from the ones in the central region

of that galaxy: they are much redder and relatively faint; this is a

strong indication that they are much older.

The distant stars thus appear to belong to an old halo around this

dwarf galaxy. This shows that WLM must be very old, perhaps as

old as the Milky Way, and in any case much older than indicated by the

ages of the stars near its centre. The present discovery furthermore

proves that a halo may also form around a dwarf galaxy, and it is not

a feature exclusively reserved for large spiral galaxies.

This photo is a reproduction from a combination of two CCD frames

observed with the 3.5-m New Technology Telescope (NTT) and the ESO

Multi-Mode Instrument (EMMI). The field covers an area of approx. 7.8

x 11.5 arcmin.

In the lower part is seen the WLM dwarf galaxy, fully

resolved into individual stars. From these observations, this galaxy

has now been found to possess an extended halo of which many stars can

be perceived in the upper part of the field. A few background galaxies

that are unrelated to WLM are also visible. At the distance of

WLM, about 3 million light years, the field corresponds to a

projected area of about 10,000 x 6,000 light years.

The frames were observed in the near-infrared I-band and each

exposure lasted about 45 minutes. The seeing was about 0.9 arcsec,

and the faintest stars that are visible have an I-band magnitude of

24. In this reproduction, North is to the left and East is down.

This photo was obtained by the European Southern Observatory (ESO).

It may be reproduced, if credit is given to ESO.

Last Modification: 8 Mar 1998, 20:10