Instituto de Astrofisica
University of La Laguna
La Laguna, Tenerife, Spain
21/12/99
IAC RESEARCHERS FIND YOUNG BLUE STARS IN A DWARF ELLIPTICAL GALAXY OF THE LOCAL GROUP
Investigators David Martinez Delgado and Antonio Aparicio Juan, of the Stellar Populations group at
the Instituto de Astrof’sica de Canarias (IAC), have found evidence for the existence of young stars in
NGC 185, a dwarf elliptical galaxy in the Local Group, of which the Milky Way is also a member. These
results, based on observations carried out with the telescopes at the Roque de los Muchachos
Observatory on La Palma, have been published in the Astronomical Journal. The observations in
question offer the first solid demonstration that recent star formation has taken place in a dwarf
elliptical galaxy and seriously challenge the commonly held view that this type of galaxy is populated
only by old stars.
“Dwarf elliptical galaxies, such as NGC 185, pose a complex and little understood problem that might
even be said to have a ‘social’ guise among astrophysicists,” explains Aparicio, who heads the IAC’s
Stellar Populations group, which has carried out the study. This is because these galaxies were directly
involved in the construction of one of the key paradigms of the astrophysics of this century: the
concept of stellar populations, introduced by Walter Baade (1893-1960), an astronomer at the
Carnegie Institute in Pasadena (California, USA). In the 1940s, Baade carried out a major synthesis of
that imposed order on the chaos of apparently unconnected data in circulation among astronomers at
the time concerning the properties of various types of stars in the Milky Way and other galaxies. As a
result of this work, he introduced the concepts of Populations I and II. The latter essentially included the
oldest stars, which were characterized by being very red. “One interesting aspect, which seemed at
the time to be perfectly clear,” says Aparicio, “was that there appeared to be objects that were
composed exclusively of Population II stars, and that among these objects were the dwarf elliptical
galaxies.”
In 1944, Baade himself had realized that there were some blue (young) “stars” in the nuclei of some
dwarf elliptical galaxies, and specifically in that of NGC 185. According to Aparicio, this presence was
seen to be at odds with a Population II object and was a “disagreeable” exception to the ordered
scheme of populations. Baade explained it away as an “impurity” and left it at that. Even stranger, the
astronomers who succeeded him in the study of these galaxies continued in the same manner without
paying much attention to this “impurity”.
Researchers at the IAC began to study NGC 185 in 1996, using the Jakobus Kapteyn Telescope (JKT)
and Nordic Optical Telescope (NOT) at the Roque de los Muchachos Observatory. “We identified
Baade’s supposed blue ‘stars’,” explains Martinez Delgado, “but there was a problem. If these objects
were really stars, they had to be very luminous, very massive and therefore very young (less than 10
million years old). That implied two associated phenomena: first there had to be a great quantity of
gas, which, however, was not the case, and second these massive stars had to be accompanied by a
great number of intermediate-mass stars, which were nowhere to be seen. These two puzzles
remained unsolved and resulted in the problem being temporarily shelved.
What has become clear from the new study is that the blue “stars” in NGC 185 are in fact star
clusters, which, given their distance from us, appear practically point-like. According to the IAC team,
this confused Baade, in whose photographic plates, the clusters were indistinguishable from stars.
“And so we have solved the problem,” says Aparicio. “The clusters are made up of thousands of stars,
which no longer need be so luminous or therefore so massive or young: their combined luminosity is
now sufficient to account for the total luminosity of the clusters, which is what we observe. Each star
can be thousands of times less luminous than Baade supposed. Consequently, their ages turn out to be
some hundreds of millions of years and their masses moderate. Neither is the presence of large
quantities of gas necessary, since this would have been expelled long ago.
On the one hand, this result confirms the suspicion that dwarf elliptical galaxies do not merely comprise
very old stars, and that star formation has therefore recently taken place. On the other hand, adds
Aparicio, “the stellar clusters we have found are a trace of the last star formation episode that
occurred in NGC 185, hundreds of millions of years ago (in comparison, the age of the galaxy and its
oldest stars is some 10 000 to 15 000 million years). They are a reliable indication that star formation
in these galaxies has continued into recent epochs and therefore that the Populations I and II scheme is
over simplified.”
MORE INFORMATION:
Antonio Aparicio
Tel.: 922605245, 922273541 or 696462597.
BETACAM ANIMATIONS AVAILABLE – telephone: 922605206 and 922605371
IMAGE CAPTION: [http://www.iac.es/gabinete/noticias/iacnotas/21dic.gif]
Formation in the galaxy NGC 185
The observation of recent star formation in the galaxy NGC 187 contradicts the scheme established in
the 1940s by the distinguished astronomer Walter Baade according to which dwarf elliptical galaxies
are populated only by old stars.
