It is now the general doctrine that many (estimated at more than 50%) galaxies have been involved in one or more "collisions" with other galaxies, or even merged with galaxy clusters. This was more common in early Universe time when galaxies were then much closer. The word "collision" is put in quotes to emphasize the actual nature of the event. In effect, two galaxies that meet 'interact' in the sense that they join into one with only a few stars actually bumping together in a destructive manner. The term "merge" is probably more descriptive of the process involved.
A release on October 21, 1997 from one of the HST research teams describes important new information on star formation within an evolving galaxy system that is undergoing collision. Examine this pair of telescope images of the Antennae galaxy:
The left one is a ground-based telescope view of two colliding
galaxies which together make up what is called the Antennae galaxy, so-named
from the long wisps of luminous gas extending like an insect's antennae. Because
of its proximity (63 million light years away), it has been a prime candidate for a closer look. The right image is a much higher resolution HST view of the central galactic mass (green box outline) of merged stars from the two once separated galaxies. More information about the central interior of this developing supergalaxy, and about regions of active star formation appears in this image:
The surprise is the numerous clusters of blue stars. Each appears to be groups of up to a million young (hence bright and hot) individual stars. The clusters likely are still developing, as cold Hydrogen gas in giant molecular clouds (typically 100s of light years across) distributed in pockets through each galaxy are being squeezed during the collision process. They contract and heat up into individual stars as this goes on, often collapsing rapidly enough for many of the stars to explode almost like "firecrackers". Other pre-existent stars are likely to be destroyed as the collision continues. The two orange centers are the older surviving parts of each galaxy.
Mayall's Object, ARP 148, is an example of a collision which has drawn out one of the galaxies:
Still another spectacular collision seen by HST is that just beginning between NGC2007 and IGC2163. As the two merge, most individual stars will not collide with another star, since in any such galaxy the distance between stars is actually huge, lessening the changes of direct collisions (although as the pair of galaxies pass through, any given star must always face the possibility of encounter with a star somewhere along their mutual paths):
As galaxies approach, materials (gases; dust) are exchanged between them, new stars are born, and Black Holes grow. This is clearly going on between two (unidentified) galaxies that were imaged by Chandra:
Another example is galaxy 3C321. A jet of gas/dust is being expelled to a smaller nearby galaxy (about 20000 l.y. away), but some of that is also deflected into outer space. The actual Chandra image is in the inset at lower right; an artist's rendering is the main illustration (which portrays the intergalactic jet that is poorly displayed in the Chandra view):
These observations support the growing view that collisions
were a more common process in the early Universe (but still happen even now).
Perhaps as many as one-third of the ancient galaxies collided during the lengthy
period when galaxies were much closer, i.e., Big Bang expansion was less far
along. Sometimes more than two galaxies are involved in the colliding process. One solid indication of collisions is the notable irregularity of the galaxy composite, with irregular center(s) and distorted spiral arms. These three HST examples of multiple collisions, imaged in the infrared, illustrate that:
In this recent image, made by combining images obtained by the NICMOS and ACS sensors, 4 galaxies can be resolved individually. As a pair merge, the increased gravitational attraction of the composite can draw in nearby galaxies to foster further enlargement of a galactic grouping. Another well known grouping of close-spaced galaxies that appear to be headed for some kind of amalgamation is Stephan's Quintet, with three of the five seen in this HST view: One type of galaxy with a distinct Active Galactic Nucleu is the Seyfert class, mentioned on page 20-3. Seyfert galaxies have strongly ionized Hydrogen, helium, Oxygen, and nitrogen in their central region. Interaction between infalling gases and a Black Hole is the probably cause. Here is the Circinus galaxy:
The Seyfert Sextet is a group of six galaxies, 190 million light years away in the constellation Serpens. They consist of 3 ellipticals and 3 spirals (only five visible in this orientation; the small spiral galaxy seen face on is not in this group, being much more distant). None of the galaxies is more than 35000 l.y. across. This configuration has been interpreted as a congregation of galaxies in the process of colliding and being ripped apart by gravitational interaction. The elongate bright central areas in two regions of the cluster may be the cores of merging pairs of galaxies. Unlike some colliding galaxies, there is no visible evidence of bright new stars being formed in these phases of collision.
This next pair of spiral galaxies are also starting their collision interactions. Note the stream of gas and dust between them. Stars are forming in this bridge.
The usual end product of the merging of two spiral galaxies is an elliptical galaxy; many elliptical galaxies formed this way. Collisions can also give rise to spiral structure. Some globular clusters also presumably originated from interactive collision.
One of the more visually intriguing results of a collision is the Cartwheel Galaxy (below), another Ringed Galaxy, in which the passage of one galaxy through another generated shock waves traveling at high velocities. As these waves moved outward, they condensed Hydrogen into a huge collection of new stars that lie along the front of the advancing waves:
An image released in January, 2006 appears to locate one or two galaxies that may be the ones that passed through the Cartwheel galaxy, producing shock waves responsible for several rings in which the new stars are forming. The prominent outer ring contains the largest number of resulting stars:
Galaxy collisions can release copious amounts of energy. The Chandra X-Ray telescope has detected a huge release of X-rays stemming from the elliptical galaxy NGC 1700, located 160 million light years from Earth:
At 90000 light years in diameter, this is the largest X-ray source near Earth yet discovered in the Universe. The emissions come from a vast spinning cloud of Hydrogen gas excited to temperatures in excess of 8 million degrees. Astronomers studying this cloud surmise that the collision was between a spiral and an elliptical galaxy.
But the biggest collision by far is Abell754, found within the Hydra constellation (but more distant), in which two small galaxy clusters have been colliding over the last half billion years. This yields a tremendous source of X-rays as detected by ESA's XMMM-Newton Observatory. Below is a view produced from data received, but with the image touched up by an artist for emphasis. And beneath that is a plot of energy variations within the colliding clusters that will eventually produce one huge galaxy (supporting the prevailing concept that most galaxies in the Universe have built up by the collision process). Although now less common as the Universe expands, drawing galaxies apart ovrall, galaxy collisions are still taking place throughout
the Universe, as illustrated by recent HST observations of the Centaurus A (NGC5128),
the closest active galaxy to our own Milky Way galaxy. Centaurus A, itself much larger than the Milky Way, is a
known radiation "hot spot", being the source of intense X-rays and Radio waves. This galaxy is only 10 million light years away; what we see now represents its condition at 10,000,000 years prior to today.
The circular inset shows Centaurus A as seen optically through
a ground-based telescope. The detailed view to the right was acquired by
HST's Wide Field Camera. An elongate disc, marked by dark dust, is spread
across a large white glow that is identified as an elliptical galaxy. This
pairing is interpreted to be an intermingling of a spiral galaxy in collision
with this elliptical galaxy. The Infrared Camera on HST can penetrate the
dust to reveal a hot, turbulent mass of stars, dust and gas from the spiral
galaxy falling into the core of the elliptical one, as seen in the larger
view.
The Chandra X-ray telescope (page 20-3) captured an unsuspected feature of Centaurus A - namely, a jet
of material ejected to a distant of 25000 l.y. from the core. This single jet of intense X-ray energy is roughly
at right angles to the plane of the disc.
A Black Hole (see page 20-6) is postulated to occur towards the center
of the two interacting Centaurus systems. This B.H. may be as massive as 10 billion
solar masses, occupying a volume similar to our Solar System. The Black
Hole is "sucking" matter from both galaxies into its growing body. This
set of observations is the most detailed yet of the consequences of galactic
collisions.
One model of future Universe expansion paths indicates that
the nearby Andromeda spiral galaxy could come close to our Milky Way galaxy and might even collide with us, several billion years in the future. There is considerable recent evidence that
a small galaxy is presently passing through the Milky Way. Known as the Sagittarius
dwarf spheroidal galaxy (or Sgr), its presence has been deduced from motions
of certain stars that do not fit the motions of M.W. stars in the spiral arms;
also star "tails" stretch out in the galactic halo, suggesting that
Sgr is in a broad orbit that has caused it to intersect the Milky Way before.
The vast distances between stars keeps interactions to a
minimum. Sagittarius is the closest small galaxy to the M.W. Here is an artist's idea of Sagittarius as it is presently encroaching on the M.W. Beneath is a radio telescope image of this Dwarf. (Another image is shown on page 20-3a.)
The role of galaxy destruction by merging, leading to a new supergalaxy, has been underappreciated until recently. In the early Universe, proximity of galaxies before expansion had separated them must have led to frequent collisions as the norm. Some astronomers think that most galaxies as seen today were products of earlier collisions. (Note: this subsection was added in January 2002, in response to information summarized in an article for that month in Scientific American by Ronald J. Reynolds, entitled "The Gas between the Stars".)
Seemingly empty regions of space - both within and between galaxies -actually contain variably small quantities of matter. The population of identifiable matter (molecules; protons, photons, cosmic rays, etc.) can be very small in space between galaxies and their halos. But, because the totality of intergalactic space is by far the largest volumetrically in the Universe, the small population, expressed as density, is actually the bulk of all known matter. This is particularly true because Dark matter (described on page 20-10), of presently unknown nature, is also present and is predominant. Among the elements, the most abundant species is Hydrogen, occurring as several states; helium is present at about 10% and the higher atomic number element species together constitute only a fraction of 1%. Within galaxies, these same elements make up what has been termed an "atmosphere" to describe the gases and particles not in the associated stars. The dominance of Hydrogen within galaxies is evident from this NICMOS Hubble Space Telescope image of NGC 4013, taken at a infrared wavelength in which Hydrogen appears to glow red:
Galactic Hydrogen occurs in the following states within the nebulae present in both the central midplane and to a much smaller extent in the halo : 1) neutral Hydrogen (HI), found mainly in the central midplane of a galaxy, which has a temperature ~120°K; it is responsible for giving off the 21 cm radiation (1420 MHz) used by radio astronomers to map its distribution; 2) molecular Hydrogen (H2), with a temperature around 15°K, which, although it comprises only 18% of all Hydrogen in the galaxy, is concentrated mostly in the gas nebulae in the central plane and is the principal material that organizes into stars; its distribution is mapped both in the 2.2 µm and the far UV regions of the EM spectrum. Outside the gaseous nebulae and predominantly in the halo, the Hydrogen exists in three states, brought about by their higher kinetic temperatures: 1) warm H(I), T = less than 3000°K, about 30% of all the Hydrogen in the galaxy, extending out to 33000 light years (ly); 2) warm H(II), ionized (loss of electron), T =3000-10000+°K, 35% of the Hydrogen, extending out to about 65000 ly; 3) hot H(II), also ionized, T = upwards of 1 million degree K, present at low densities out to about 180000 ly, and making up about 45% of all Hydrogen. Thus, it is now known that nearly all ionized galactic Hydrogen is located beyond the central plane, i.e., in the halo, but make up about 22% of the mass; the stars themselves account for only about 2% by volume but 30% of the mass; the remaining mass is represented by H(I), 35% of which is in the halo and 0.1% in the clouds. To restate this: a spiral galaxy is mostly Hydrogen with some present in stars concentrated in the central plane but also found in smaller numbers in the halo which itself is composed of neutral and ionized Hydrogen whose temperatures are much higher than the Hydrogen gases within the central plane.
Clouds of Hydrogen in the halo region of the Milky Way have been imaged in the infrared. In this next illustration, a view of these clouds is shown in the vertical strip to the right; the side-on image of the Milky Way is an artist's rendition rather than an actual observation. An idea of just how far a galaxy's gas extends into a halo is evident in this illustration of NGC4325 in which a visible light image of the galaxy itself (center, elliptical) is registered to an X-ray image that displays the tenuous gas (which can reach temperatures here up to 1000000° K) extending to great distances outward diameter to purple edge = 1.9 million l.y.). The purple and green indicate two arbitrarily-divided levels of X-ray emission. Indications are that (mostly ionized) Hydrogen is expelled as a plasma from the central plane region after supernova events. This occurs in streamers that are given the descriptive name "columns" and resemble the flares jetted out from the Sun (page 20-3a). They may start as bubbles which rise thermally and draw in Hydrogen gas as they pass into the halo. These move out to distances in excess of 10000 ly, but tend to break up with some material returning to the plane as more diffuse "fountains". Their outward pathways may be controlled by a galactic magnetic field. This predicted phenomenon has not yet been imaged (so its existence is still not proved) but here is an artist's depiction of what it might look like in our Milky Way galaxy (compare this to the illustrations of the Milky Way imaged at various wavelengths on page 20-3): A survey of gas densities around the Sun in our part of the Milky Way is being carried out by ground telescopes, by FUSE (Far Ultraviolet Spectrometer Experiment), and by CHIPS (Cosmic Hot Interstellar Plasma Spectrometer; also operating in the far UV). In May of 2003, a group at the University of California, Berkeley, and French colleagues, produced maps of the densities out to 1000 light years, as follows: The top map is a section perpendicular to the plane of the M.W.; the bottom map is a cross-section through the top map in the plane labeled Chimney Axis. The light-toned areas are regions of very hot gases (kinetic temperatures in the 1 million degrees Kelvin range) with lower than average intragalactic densities. The dark-toned areas are higher density, lower temperature regions. The reason for the low density "bubble", actually roughly chimney-shaped with protuberances, is still uncertain. One hypothesis turns to a supernova explosion in the past that may have produced "galactic winds" that blew intragalactic material out of the resulting low density region. At such low densities, atoms of Hydrogen can move greater distances without collisions and thus attain higher kinetic temperatures. These studies of the processes involving galactic atmospheres are opening up new insights into the factors that lead to star formation during the lifetime of a galaxy. Some examples of chimneys in nearby galaxies have been imaged by the Hubble Space Telescope and other observatories. Research along the lines described above is likely to improve our understanding of the activities within galaxies that determine their history. An article entitled The Galactic Odd Couple by Astronomer Kimberly Weaver in the July 2003 issue of Scientific American contains so much information new to the writer that I have decided to add this subsection, presented as a synopsis in much the order followed in her paper. Weaver's main thesis is that both Starbursts and Active Galactic Nucleus's (AGNs) are commonplace in galaxies and each seems interdependent in several possible ways on the other and on Black Holes. Furthermore, the two represent the most powerful release of energy (strong in every segment of the EM Spectrum) at galactic scales (hypernovae, quasars, and gamma ray bursts are tops in power output at individual star scales) now known in the Universe. An AGN is thus characterized by strong emissions of energy beyond that of starlight, especially in the radio wavelengths, and by expulsion of matter as radio jets. This diagram shows an artist's model of an AGN, the jets, and starbursts: Let's commence by looking at various manifestations of starbursts. We will concentrate on M82, the "Cigar" galaxy, which has been classified as irregular but tends to be lenticular. This starburst galaxy is small (about 16000 light years across) and is located about 11 million l.y. from Earth. It is rated as one of the brightest galaxies ever found and is visible to the naked eye in the constellation Ursa Major. Because so much data and imagery have been collected at different wavelengths, this is a good moment to look at how this galaxy appears in various parts of the spectrum. First, let's set it in context with its surroundings. M82 is part of a triad of galaxies that includes the larger spiral M81. Many astronomers interpret this proximity to indicate that M82 may have collided (or may eventually) with M81 and has spun off the third smaller galaxy (NGC3077) as a result. Here is a HST optical range (Visible) image of M82, showing why it was named the "Cigar Galaxy". There is no single central core of dense stars as is customary with spiral galaxies. Instead of spiral arms the outer regions seem to contain formless clouds of dust and gas. And in this rendition the central region seems no brighter than the outer parts. This image of M82 was taken through the Japanese Subaru ground telescope using the H-alpha excitation line as one of the color input bands. It shows a hotter interior with a central jet emerging normal to the plane of the galaxy. The Chandra X-ray Telescope (page 20-4) presents this image of M82, again showing some of the starbursts. A very different appearance is displayed in this Ultraviolet image of M82 This radio telescope image of M82 shows a number of bright areas which are interpreted to be starburst regions. This is a timely moment to think about M82 in the context of the general figure illustrating Galaxy Anatomy shown near the middle of page 20-3. The continuous central yellow area, which contains a much denser collection of stars, an accretion disk, a Black Hole, and an outward jet, is the Active Galactic Nucleus. Various blue stars surround the AGN. A few more examples of starbursts should affix their appearance and characteristics in your mind. The first image shows a central starburst cluster in galaxy NGC 3603. Below it is a ring of stars in M94, envisioned in the UV. The third image shows a prominent ring of starburst objects just out from the center of galaxy NGC 4314. This image of NGC1808 shows a well-defined AGN that may contain unresolved starburst stars as well. Beneath it is NGC9812, showing a mix of AGN excited gas and some starbursts. Starbursts can occur in galaxies that don't have obvious AGNs. These proliferations of active star formation happen when two galaxies collide. The Antennae galaxy is a good example:
Galactic Gas
This atmosphere is somewhat analogous to that on Earth (decreasing in density upward) in that its Hydrogen constituents drop in numbers as the outer reaches of the halo are approached. Like the atmosphere of Earth, and similar also to that surrounding the Sun, the galactic atmosphere is dynamically and continuously active. Both within the star-forming region of the central plane and, to a lesser extent in the inner halo, this gas is perturbed by shock waves and expelled particles in stellar winds, generated mainly by supernovae explosions (page 20-6). Many of these explosions are associated with massive, short-lived stars (the O and B types described on page 20-2). The result is "turbulence" driving some matter into denser clots (initiating regions of new star formation but also producing volumes of much lower Hydrogen concentrations called "bubbles"). Conceptually, the galactic atmosphere could be visualized as looking a bit like Swiss cheese, with holes where the bubbles occur.
From Scientific American, January 2002
Starbursts and the Active Galactic Nucleus
Now to the meat of Dr Weaver's paper.
While Black Holes come in all sizes, the larger ones (supermassive Black Holes; which nevertheless contain about 0.1% of the total galactic mass) are involved in both AGNs and starbursts. Furthermore, B.H.'s may also be responsible for star formation at slower rates (non-bursts). Suupermassive B.H.'s contain a compacted mass up to a trillion times that of the Sun but are only less than 1000 times its diameter. AGNs, as stated earlier, are superbright (although not notably in Visible wavelengths), often outshining the rest of its galaxy. AGNs are superrich in quasars which individually give off huge quantities of radiation. Many starbursts are as brilliant (in terms of energy given off) as the AGNs. A starburst period of activity is usually about 10 million years in length, during which the rate of star formation is up to 1000 times that of normal creative activity in a galaxy. One likely cause of a starburst is intermingling of gases when two galaxies collide (pass through each other), and the density of gases needed to make the stars increases. Both starbursts and AGN formation were much more prevalent in the first 1-2 billion years of Universe history.
The question arises as to whether starbursts and major activity within an AGN occur simultaneously. Observations show that AGNs are often associated with young, bright, massive stars. However, it is often difficult to demonstrate the presence of starbursts within an AGN since the latter is marked by large amounts of obscuring dust (some of which is tied to the starburst process). The AGN can be "dormant" (no conspicuous visibility) until its gases begin to flow abundantly into the supermassive B.H. Starbursts commonly occur at an early stage of this process. Both starbursts and AGNs can be verified as active by examining their spectral - ionized Oxygen and other elements show brighter individual spectral lines.
Four possibilities for the interrelation between starbursts and AGNs - which came first and what causal connections exist between them and between B.H.'s - are cited by Dr. Weaver. We reproduce two schematic diagrams from her Scientific American paper that speak to this:
In the first scenario, AGNs are just manifestations (equivalent to) of starbursts and are possibly not set up by Black Holes. The second case postulates coincidence, i.e., no direct causal relation between starbursts and AGNs, even though both exist as separate phenomena. The third option considers starbursts to be caused by the supermassive B.H. pulling enough gas rapidly towards it to provide a denser environment in which many more than average star formation events take place. Thus the starbursts result during the period in which gases also react around the Black Hole to form quasar-rich AGNs. The activity controlled by the B.H. that causes the AGN releases shock waves that aid in star burst formation beyond the inner zone. The fourth mechanism argues that starbursts precede the inception and growth of their associated supermassive Black Hole. As each massive star dies, it evolves into a Black Hole or a Neutron star. In time, these many B.H.'s coalesce to form the supermassive B.H. found at or near the center of the galaxy (this subject is treated on pages 20-5 and 20-6). A variant involves collisions of smaller stars that then speed up their lives before exploding into dense cold matter. Generally, this last mode first yields Black Holes of intermediate size (as has been observed in some galaxies) but usually the final outcome is a single large B.H.
But the bottom line in Dr. Weaver's paper is that supermassive Black Holes, AGNs, and Starbursts are a common, perhaps prevalent, set of components in spiral, and perhaps elliptical and also irregular galaxies.
