Low Resolution (204
KB)
High Resolution (889
KB) |
|
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
|
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.
|
Figure 1. 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. 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. |
