Hubble Resolves a Blaze of Stars in a Galaxy’s Core

The central region of the small galaxy NGC 1705 blazes with
the light of thousands of young and old stars in this image,
taken by NASA’s Hubble Space Telescope.

At 17 million light-years away, the individual stars of the
dwarf irregular galaxy NGC 1705 are out of range of all but
the sharp eye of Hubble. NGC 1705 is an ideal laboratory to
conduct investigations on star formation history. Young,
blue, hot stars are strongly concentrated toward the galaxy’s
center, while older, red, cooler stars are more spread out.
This galaxy has been forming new stars throughout its lifetime,
but a burst of star-formation activity occurred as recently
as 26 to 31 million years ago. This “starburst” is responsible
for many of the young stars on the outskirts of the galaxy’s
core, as well as the central giant star cluster.

NGC 1705 is classified as a dwarf irregular because it is small
and lacks any regular structure. Many astronomers now believe
that dwarf galaxies, like NGC 1705, were the first systems to
collapse and start forming stars in the early universe. They
represent the building blocks from which more massive objects
(spiral and elliptical galaxies) were later formed through
mergers and accretion. Nearby small galaxies are thought to
be the leftovers of the galaxy-formation process.

Dwarf irregulars are similar in many ways to very young
galaxies, but they are much nearer and easier to study. These
galaxies seem to have consumed only a tiny percentage of their
reservoir of gas. Their stars have a much lower fraction of
heavy elements than does the Sun. These are all indications
that only a few generations of stars have formed there over
time. Current star formation is taking place at a fairly high
rate in starburst episodes. All these characteristics make
dwarf irregular galaxies the ideal local analogues to young
galaxies from the early universe. Understanding their evolution
is extremely useful and sheds light on the many processes
related to galaxy formation and evolution.

Dwarf irregulars play a key role in astronomers’ attempts to
unravel the history of galaxies in the early universe. These
galaxies are probably best described as fairly old stellar
systems whose chemical and physical properties can be ascribed
to a process of slow evolution. The Hubble observations of the
stars in NGC 1705 and other close irregulars have demonstrated
that these galaxies are several billion years old. NGC 1705
could be as old as 13.5 billion years.

This image was taken in March 1999 and November 2000 by an
international science team led by Monica Tosi at Italy’s
National Institute of Astrophysics (INAF) at the Osservatorio
Astronomico di Bologna. Other team members include
Alessandra Aloisi (JHU), Mark Clampin (STScI), Laura Greggio
(INAF, Osservatorio Astronomico di Padova), Claus Leitherer
and Antonella Nota (STScI). Hubble’s Wide Field Planetary
Camera 2 observed the galaxy in ultraviolet, blue, visible, and
infrared light. Although not included in this image, NICMOS
(Near Infrared Camera and Multi-Object Spectrometer)
observations were also made of the galaxy’s central core.

Image Credit: NASA, ESA, and The Hubble Heritage Team (STScI/AURA)
Acknowledgment: M. Tosi (INAF, Osservatorio Astronomico di Bologna)

NOTE TO EDITORS: For additional information, please contact
Alessandra Aloisi, Johns Hopkins University, Dept. of Physics
and Astronomy, 3400 N. Charles St., Baltimore, MD 21218, (phone)
410-516-8520, (fax) 410-516-5096, (e-mail) aloisi@pha.jhu.edu or

Monica Tosi, National Institute of Astrophysics (INAF),
Osservatorio Astronomico di Bologna, Italy (phone) +39-051-2095759,
(e-mail) tosi@bo.astro.it or

Keith Noll, Hubble Heritage Team, Space Telescope Science
Institute, 3700 San Martin Drive, Baltimore, MD 21218, (phone)
410-338-1828, (fax) 410-338-4579, (e-mail) noll@stsci.edu or

Lars Lindberg Christensen, Hubble European Space Agency
Information Centre, Garching, Germany (phone) +49-89-3200-6306
(089 within Germany), (cell): +49-173-3872-621 (0173 within Germany),
(e-mail) lars@eso.org.

Electronic image files and additional information are available at
http://hubblesite.org/news/2003/07 and
http://heritage.stsci.edu/2003/07.

The Space Telescope Science Institute (STScI) is operated by the
Association of Universities for Research in Astronomy, Inc. (AURA),
for NASA, under contract with the Goddard Space Flight Center,
Greenbelt, MD. The Hubble Space Telescope is a project of international
cooperation between NASA and the European Space Agency (ESA).