Dwarf Irregular Galaxies: Not So Pristine After All (Forwarded)

Subaru Telescope
National Astronomical Observatory of Japan
Hilo, Hawaii

August 05, 2004

Dwarf Irregular Galaxies: Not So Pristine After All

Astronomers have shown for the first time that even the smallest galaxies in the
Universe have complex structures that indicate a complex history. Using the
Subaru Telescope, a team of astronomers from the National Astronomical
Observatory of Japan, the Institute of Physics in Lithuania, the University of
Durham, Paris Observatory, Kyoto University, Gunma Astronomical Observatory, and
the University of Tokyo have discovered an extended halo of stars with a sharp
cutoff in the dwarf irregular galaxy Leo A, a member of the Local Group of
galaxies that includes the Milky Way. The discovery challenges current scenarios
of galaxy formation by showing that instead of being the preservers of pristine
building blocks that combined to form larger galaxies, dwarf irregular galaxies
have their own history of build-up.

Understanding galaxy formation and evolution on time scales comparable to the
age of the Universe is one of astronomy's greatest challenges. In the scenarios
of standard cosmology (Note 1), galaxies are built up via hierarchical merging:
small primordial density fluctuations in the smooth distribution of matter in
the early Universe grow and combine to form larger structures like the Milky
Way. The most numerous type of galaxies in the universe -- dwarf irregular
galaxies (Note 2) -- are supposed to preserve their properties unchanged over
billions of years and represent pristine primeval building blocks. This is one
reason why astronomers have recently been studying dwarf irregular galaxies with
great interest.

The team led by Professors Nobuo Arimoto (National Astronomical Observatory of
Japan) and Vladas Vansevicius (Institute of Physics, Lithuania) has studied Leo
A -- an isolated and extremely gas rich dwarf irregular galaxy with only 0.01%
of the mass of the Milky Way and a low fraction chemical elements produced by
earlier generations of stars. These characteristics suggest that this galaxy has
been evolving without significant interaction with other galaxies. This galaxy
has been believed to have quite a simple structure, in contrast to large disk
galaxies like the Milky Way. However, this view needs to be changed due to deep
imaging of the outer regions of this galaxy with the Subaru Telescope.

Prior to these observations, Leo A was already known to have a large angular
size (7' x 5'; Note 3) and Subaru Telescope equipped with its Prime Focus Camera
(Suprime-Cam) was an ideal instrument to study the stars at the galaxy's outer
limits (Fig. 1). A single exposure with Suprime-Cam covers a field of view of
34' x 27' (pixel size 0''.2 x 0''.2) with high sensitivity. The team acquired
optical images of the dwarf irregular galaxy Leo A with three broad band filters
in November 2001. In order to trace the entire extent of the old stars in Leo A,
the team employed red giant branch (RGB) stars which are evolved low-mass stars
with very high luminosity and are expected to represent well the extended
structures of galaxies. They investigated inside an ellipse of semi-major axis a
= 12' which fully covers the galaxy, and detected 1394 RGB stars distributed
symmetrically and smoothly within this field.

Fig. 2 shows the radial profile of the surface number density of the red giant
stars. The team found significantly larger disk structure (with a semi-major
axis of 5.5') than previously known (3.5'). Moreover, the deep observations
permitted the discovery of a new stellar component in dwarf irregular galaxies,
which the team calls a "halo" (5.5'-7.5'), which has a less steep slope in the
number density of RGB stars. The halo component ends at 8' from the center of
the galaxy with a sharp cutoff in the RGB star distribution. The existence of
such a halo structure in dwarf irregular galaxies had been unconfirmed before
these observations.

The size of Leo A revealed by these new observations is twice as large as its
previously accepted size, suggesting that even in the nearby universe we see
galaxies only as "tips of icebergs" that are actually a few times more extended.

The newly discovered halo with a sharp stellar cutoff and the disk of the dwarf
irregular galaxy Leo A closely resembles the structure as well as stellar and
gaseous content found in large full-fledged disk galaxies like the Milky Way.
The complicated structure of large massive galaxies has been believed to be a
result of the merging of less massive galaxies like dwarf irregular ones.
However, this study clearly reveals that the dwarf irregular galaxy Leo A
already has disk and halo components, and suggests complex build-up histories
for even very low mass galaxies like Leo A, which are supposed to form directly
from the primordial density fluctuations in the early universe (Note 1), and
challenges contemporary understanding of galaxy evolution. Professors N. Arimoto
and V. Vansevicius believe Leo A is a "Rosetta stone" (Note 4) for understanding
the process of galaxy formation and evolution.

The scientific paper on this research has been accepted for publication in the
August 20, 2004, Astrophysical Journal Letters (Volume 611, Number 2, L93).

Note 1: The standard cosmological model postulates that galaxy formation is
governed by the gravity of the so-called cold dark matter that fills the
universe, even though galaxies currently observed consist mainly of normal matter.

Note 2: Dwarf irregular galaxies are small galaxies with an irregular
distribution of stars and gas. These galaxies do not have strong spiral
structures like disk galaxies such as the Milky Way.

Note 3: At the distance of Leo A, 2.6 million light years, 1' in extent
corresponds to 750 light years.

Note 4: The Rosetta Stone is a stone discovered in an Egyptian village, Rosetta,
with writing on it in two languages (Egyptian and Greek), using three scripts
(hieroglyphic, demotic and Greek). This stone played a crucial role in the
deciphering of ancient Egyptian hieroglyphs.

IMAGE CAPTIONS:

[Low Resolution
http://www.subaru.naoj.org/Pressrele...eoA_BVI_72.jpg (204KB)
Hi Resolution:
http://www.subaru.naoj.org/Pressrele...oA_BVI_300.jpg (889KB)]

Object Name: Dwarf Irregular Galaxy Leo A
Telescope: Subaru Telescope / Prime Focus
Instrument: Suprime-Cam
Filter: B (0.45 microns), V (0.55 microns), I (0.80 microns)
Color: Blue (B), Green (V), Red (I)
Date: UT 2001 November 20-21
Exposure Time: 50 min (B), 30 min (V), 20 min (I)
Field of View: Approx. 13.4 arcmin x 10.7 arcmin
Orientation: North up, East left
Position: RA (J2000.0) = 9h 59m 26.5s, Dec (J2000.0) = + 30d 44m 47s (Leo)
Distance: Approx. 2.6 million light years

[Figure 1:
http://www.subaru.naoj.org/Pressrele...08/05/Fig1.gif (167KB)]
The Suprime-Cam V-band (0.55 micron) image of the dwarf irregular galaxy Leo A.
The ellipses (b/a = 0.6, a -- semi-major axis) indicate: previously measured
size of the galaxy, a = 3'.5 (cyan); the radial distance where the discovered
halo becomes prominent, a = 5'.5 (blue); the newly established size of Leo A, a
= 8'.0 (red); the zone used for background source surface number density
determination, a = 12'.0 (green).

[Figure 2:
http://www.subaru.naoj.org/Pressrele.../05/Fig2_E.gif (25KB)]
The radial profile of the RGB star surface number density in Leo A. The lines
fitted to the old disk, 2'.0 a 5'.5 (blue), the halo, 5'.5 a 7'.5 (red),
and the background, 8'.0 a 12'.0 (green) radial profiles are shown.