COMMON SHAPING MECHANISMS OVER SCALES OF
MILLIONS
On the left is The Owl
planetary nebula shown
in optical light, and on the right is the Perseus cluster of galaxies taken in
X-ray by the Chandra X-ray Observatory. At the center of both objects pairs of
fainter bubbles can be seen. The right image covers a region more than ten
thousand times as large as the region covered by the left image, and about a quadrillion (
) as massive. The cloud of gas in the planetary nebula (left)
is at about ten thousands degrees Kelvin (or Celsius), while the cluster’s gas
(right) is at a temperature of tens of millions of degrees Kelvin. The bubbles were
formed by fast jets, one jet per bubble, which inflated the bubbles by filling
it with hot, tenuous gas.
Noam Soker is working on the common processes
operating in these two vastly different types of objects. The colors in these
images are not real, but rather represent the physical state of the gas.
INTRODUCTION
The
main research areas of Noam Soker are the shaping of planetary nebulae and the
evolution of hot gas at the center of clusters of galaxies. These seem to be
two research areas without any connection between them, as a planetary nebulae's
typical size is one light year, while the centers of galaxy clusters reach
sizes of more than a hundred thousand light years.
The
typical temperature of the gas in planetary nebulae is ten thousand degrees
Kelvin, while that in cluster of galaxies is more than ten million degrees. A
typical galaxy clusters is about quadrillion () as massive as a typical planetary nebulae. However, in a
series of papers, Noam Soker presents an interesting connection between these
two classes of objects. This is summarized in a short paper
In Nature magazine.
Planetary
nebulae are clouds of gas
blown by sun-like stars during its final stages of nuclear burning. The
envelope of the star turns into an expanding cloud, while the stellar core
shrinks, heats up, eventually becoming a small (the size of Earth, or about one
hundredth the
size of the present Sun) hot star called a white dwarf. During a period of about fifty thousand years
the hot central star heats the envelope to tens of thousands of degrees, making
it glow in spectacular shapes. To see these many shapes, as revealed by the Hubble Space Telescope, click here, and for more
explanation and links click here. Note: The colors
in these images are not real, but rather represent the physical state of the gas.
Namely, the color coding corresponds to the element which contributes the most
to the emitted light.
Clusters of
galaxies are groups of hundreds
to thousands of galaxies. Hot gas at a temperature of tens of millions of
degrees Kelvin fills the space between the galaxies emitting radiation in the
X-ray band (the scientific name for the Roentgen radiation). As the X-ray
radiation is absorbed by our atmosphere, telescopes in space must be used to
reveal this gash such as the Chandra X-ray Observatory. Individual
galaxies, on the other hand, can be seen in optical light. In many clusters two
fast jets launched in opposite directions by the central black hole at speeds
close to the speed of light, inflate a pair of bubbles inside the hot gas. If
the process repeats itself, up to several pairs of bubbles can be formed. The density inside the jets are too low to
emit efficiently in the X-ray; these jets are seen in radio emission, emitted
by ultra-fast (relativistic) electrons moving under the influence of strong
magnetic fields. See the optical, radio, and X-ray images of the Hydra A cluster of galaxies.
NOAM’s RESEARCH
It
is widely accepted that faint X-ray bubbles in the hot gas in clusters of
galaxies are inflated by fast jets. These jets are blown by a super-massive
black hole at the center of the cluster. It is less widely accepted in the
planetary nebulae community that faint optical bubbles are blown by jets. Jet
shaping of planetary nebulae was suggested more than twenty years ago (Mark
Morris 1987; Noam Soker 1990; Raghvendra Sahai 1998).
In his research, Noam Soker uses the common morphologies of hot gas in clusters
of galaxies and the morphologies of many (but not all) planetary nebulae, to
strengthen the model for jet shaping of many planetary nebulae (not all
planetary nebulae are shaped by jets). Moreover, the jets in planetary nebulae
are most likely blown by a binary companion accreting mass from the progenitor.
An accretion disk is formed around the companion, which causes the launching of
two oppositely ejected jets.
Taking
the opposite direction, Noam Soker tries to use known properties of planetary
nebulae to learn more about clusters of galaxies.
EXAMPLES OF SIMILARITIES
The
figure below shows pairs of bubbles in two planetary nebulae (Hu 2-1 and V 171) and two clusters of galaxies (NGC 62,
which
A BINARY MASSIVE BLACK HOLE?
Fabio Pizzolato (a
postdoctoral fellow at the Technion) and Noam Soker have used the similarity in
morphology between the planetary nebulae Hb 5 (in
optical) and the structure of the hot gas in the MS 0735.6+7421
cluster of galaxies (as revealed in
X-ray by the Chandra X-ray Observatory), to speculate that a binary
black hole resides at the center of the cluster. In planetary nebulae this type
of structure (called point-symmetric) is thought to be caused by a binary
companion leading to jets’ precession (a periodic change in the direction of
the jets’ axis). Pizzolato and Soker suggest that similar point-symmetric structures in the
X-ray cavities of galaxy clusters might be associated with the presence of
massive binary black holes. The X-ray image is of lower resolution; hence it is
compared to the lower resolution image of the planetary nebulae.
The planetary nebula Hb 5. Left panel: a Hubble Space
Telescope high-resolution optical image (from Y. Terzian
and A. R. Hajian, 2000). Right panel: a
low-resolution image of the same object from the catalogue of Schwarz et al.
(1992), and the original resolution was degraded by Gaussian smoothing (R. Corradi, private communication). The edge-to-edge linear
scale is about one light year. Note the similarity of the low resolution image
(right) to the X-ray image of the MS 0735.6+7421
cluster (in red in the images below).
The galaxy cluster MS
0735.6+7421: An X-ray image (red), and the radio image (blue) added in
the left panel (From Brian McNamara and collaborators). The edge-to-edge linear scale is
about one million light year.
RIPPLES
IN THE PERSEUS CLUSTER
Arcs of enhanced X-ray
emission (image below) in the Perseus cluster signal arcs of higher density.
These were interpreted by Andy Fabian and collaborators as sound waves (for
more detail click here)
Enhanced X-ray emission resulting from arcs of somewhat
higher density caps. While some researchers interpret these as sound waves
Pizzolato and Soker suggest these are rims of bubbles.
Fabio
Pizzolato and Noam Soker suggest that the
X-ray ripples observed in the Perseus
cluster are not sound waves, but rather the rims of radio-faint weak bubbles
which are only slightly hotter than their environment. These bubbles were
formed by several episodes of jets launching. This is based in part on
similarities to arcs in planetary nebulae, as seen in infrared with the Spitzer
space Telescope (first image below) and in optical with the the
Hubble space Telescope ( three images below; from R. Sahai and J. T. Trauger 1998). In planetary nebulae these arcs are not
sound waves.
There
are several differences between the ripples in planetary nebulae and clusters.
The most important is that in clusters the jets are blown by the central
massive black hole. The same type of jets that inflate bubbles, can inflate
weaker (lower contrast to their environment) bubbles that form ripples (caps of
higher density) at their leading front as they move outward. In planetary nebulae jets, probably blown by
an accreting companion, inflate bubbles pairs. The ripples observed by the
infrared Spitzer telescope in the halo of the planetary nebula M 57 (below) may
have been formed by local faster (sporadic) ejection events from the red giant progenitor of the planetary
nebula. For example, magnetic flares on the red giant surface, or large moving
blobs in the giant envelope (convective cells), may have caused such faster local
ejection events. In any case, Soker proposes that both in the Perseus cluster
of galaxies and in the planetary nebula M 57 the ripples are caused by lower
density bubbles formed by faster and directional streams.
NASA's Spitzer Space Telescope image in infrared of the planetary nebula Messier 57 (the Ring Nebula). The halo with the ripples composed of molecular gas. The inner colored (not real) region is ionized gas observed also in optical. For more detail on this image click here
He 2-131
He 2-138
He 2-277
Optical images obtain with the Hubble pace Telescope reveal ripples in planetary nebulae (from R. Sahai and J. T. Trauger 1998).
THE FILLING OF BUBBLES IN PLANETARY NEBULAE
In
clusters of galaxies bubbles are filled with very hot gas which because of its
very low density, emits too weakly in the x-ray to be detected. This gas is
revealed by the radio emission of relativistic (ultra-fast) electrons
accelerating via magnetic forces. The
surrounding neighborhood of the bubbles strongly emit in X-rays.
In
planetary nebulae the surrounding neighborhood of bubbles emit in optical,
while the bubbles are filled with gas at several millions of degrees, which
strongly emits in X-ray. An example of a planetary nebula with X-ray emission is
NGC 6543 (Cat's Eye Nebula). For credits and detail click here.
The planetary nebula NGC 6543.
Left: X-ray image (from Y.-H.
Joel Kastner
(Rochester Institute of Technology) and I are involved in a long project to
detect and understand the X-ray emission from planetary nebulae. One of these planetary
nebulae is presented below.
