Galaxies Part 1

By Duncan Lunan

Magellanic Bridge

As previously noted, our Local Group of galaxies consists of dwarf galaxies, small irregular ones, small elliptical ones, and three large spirals:  the Milky Way, M31  (the Great Nebula in Andromeda), which is bigger than the Milky Way, and M33 in Triangulum.   M33 is smaller than either the Milky Way or M31, and has apparently had an encounter with M31, 2 to 8 billion years ago, creating a bridge of material between them.  There’s a similar bridge between the two largest irregular galaxies, the Magellanic Clouds in the southern hemisphere, and they may be parts of a small spiral which collided destructively with the outer arms of the Milky Way.  Several dwarf galaxies are doing that right now, and the resulting rings of dust, gas and new stars have helped to map the distribution of dark matter around the Milky Way. 

M51 whirlpool galaxy, 23 m. ly distant, ripples from NGC 5195 passing behind

A. Bertram Chandler’s ‘Tales of the Galactic Rim’ assumed it was a place where strange things could happen and normal laws of space-time could break down, and in some cases it seems real-life events could be drastic.  One of the nearer galaxies to the Local Group is the Whirlpool Galaxy, M51, whose spiral form was recognised in 1845 by the Third Earl of Rosse, using the giant telescope which he built at Parsonstown in southern Ireland. 

M51 by William Parsons, 3rd Earl of Rosse (Lord Rosse) in 1845.

Its nearby companion is linked to it by a thick bridge of stars and dust, showing that there’s been a major near-collision in the recent galactic past, and x-ray emission and supernovae show that there’s still turbulence on the rim.  It appears that radiation from new stars forming on the rim has destroyed the organic materials in interstellar space which are thought to be the precursors of life, perhaps rendering those regions permanently sterile. 

Turbulent dark matter has also been detected there.  It’s been known for a long time that there had to be some form of ‘missing mass’, around the Milky Way and other galaxies, because the stars in the outer parts of the spiral arms were orbiting too rapidly to be explained by the collective gravity of the stars which could be seen.  At first, attempted explanations centred around MACHOS  (Massive Compact Halo Objects, a ring of stray planets surrounding the galaxy) and WIMPS, Weakly Interacting Massive (sub-atomic)  Particles, e.g. a very small rest mass for the neutrino.  It subsequently was found to have one, but not enough to account for the Missing Matter.  The Royal Observatory in  Edinburgh was particularly involved in the search for MACHOS:  by 1970 the ROE had accumulated a vast archive of astronomical plates, and built a scanner called GALAXY to search them automatically for evidence of MACHOS in the halos of other galaxies.  The survey turned up many events such as exploding stars which had previously been missed, but not the multiple occultations which would be caused by huge numbers of stray planets. 

Galaxy scanner ROE, with 16-inch Schmidt camera and Elliot 4130 computer, 1970, dark matter not MACHOS

Wandering planets do exist, and several dozen have been discovered, but they are within galaxies, not outside, and not in the huge numbers which would be needed to explain the Missing Mass.  If neither sub-atomic particles nor dark planetary bodies could explain it, it had to be accepted that there was another kind of matter, invisible but with strong gravitational attraction, and more of it than the total amount of normal matter we could see.  Not everyone accepts that  (Kulvinder Singh Chadha, ‘Cosmology on the Rocks?’, Astronomy Now, June 2009), and there are competing hypotheses such as MOND  (Modified Newtonian Dynamics)  which attempt to explain the Missing Mass by tweaking Newtonian or Einsteinian gravitational theory, but so far none of them have won acceptance.

Another major effort was to pin down the precise value of the Hubble Constant, which determines the rate of expansion of the Universe.  Different measurements produced worryingly different results, and it came to be recognised that the Hubble Constant isn’t constant:  it’s slowly increasing with time, having overcome gravitational attraction 7.5 to 5 billion years ago.  The unknown force which is doing it is known as Dark Energy, and since energy and mass are equivalent in Relativity Theory, it’s been determined that there’s more of it than everything else combined – 71.4% of the total content of the Universe.

Ratios of Dark Matter, Dark Energy and visible matter, pie chart.

In 1981, Prof. Martin Harwit had published Cosmic Discovery, the Search, Scope and Heritage of Astronomy, in which he argued that more than 90% of what could be discovered had already been discovered – little knowing that more than 95% of the Universe had yet to be discovered in the first place!  When Stephen Hawking said that his ambition was to describe the entire Universe in a single equation which could be printed on a T-shirt, Prof, Freeman Dyson had replied with a book called Infinite in All Directions  (1988), in which he argued that there were no limits to knowledge and the process of discovery would be infinite.   Seldom has a philosophical argument produced such evidence in its favour, so fast.

In the early 1960s the estimated distance to the nearest galaxy, M31 in Andromeda, was doubled from 750,000 to 1.5 million light-years;  I remember the amazement with which I learned that as a teenager, from a monthly astronomy item on the BBC Third Programme, as it then was.  The distance to M31 has since been increased to 2 million l.y., with distances to everything else increased accordingly.  Beyond the boundary of the Local Group, from about 10 million light-years away out to about 50 million light-years.  Within that range lie many spiral galaxies, at different angles to us, such as the Pinwheel Galaxy, at 11.9 million light-years.  That too has had a destructive encounter, with the smaller M82 which is now a starburst galaxy, with intense star formation going on a result.  But there are still more violent events going on out there:  Centaurus A, a powerful radio source 12 million light-years away, is a near-spherical elliptical galaxy, formed by previous collisions, which has now hit a spiral edge-on.  The Fireworks Galaxy NGC 6946, at 12 mly, is undergoing powerful interactions with its nearby satellites.  The Sombrero Hat galaxy, at 29 mly, has a satellite, SUCDI, which is much brighter than its companions, and a source of intense x-rays being generated as a neutron star or black hole strips the outer layers from a companion star.  The object appears to be an ultra-compact dwarf galaxy, containing about 10 million stars and about 10 billion years old, possibly a massive cluster which has formed independently rather than a small spiral which has been stripped of its outer arms.  (Julian Cribb, ‘Astronomers Discover Galactic “Missing Link”, Swinburne, March 2009.)

Centaurus A has jets expanding in two directions from its central black hole, but another source discovered early in the history of radio astronomy was the elliptical galaxy M87, which turned out to have a very powerful jet issuing from its nucleus.  In The Scientist Speculates  (ed. I.G. Good, 1962), Arthur C. Clarke compared it to the launch of a globular cluster from the Milky Way in his 1955 novel The City and the Stars.  The jet is travelling at a high fraction  of the speed of light, fast enough for parts of it to appear to be travelling faster than light due to relativistic effects.  The jet is one of a pair being emitted in opposite directions from the poles of an ‘accretion disc’ of matter falling into a supermassive black hole, up to 7 billion times the mass of the Sun.  Similar, less powerful jets are beamed out from neutron stars, pulsars and stellar-mass black holes, and even more powerful ones from the quasars  (quasi-stellar objects), very massive back holes in galaxies of the early Universe.  The M87 black hole is one of the largest known, and its accretion disc has been imaged in radio waves by the Event Horizon Telescope, whose team hope to image the one at the heart of the Milky Way, though it’s much smaller, only 4 million solar masses.

First black hole image, M87, Event Horizon Telescope Collaboration, April 2019

In 2009 an article in Astronomy Now entitled ‘Black Holes Came First’ revealed a hitherto unsuspected relationship between the masses of galaxies’ central bulges and the masses of the central black holes in those galaxies.  Nobody knew what mechanism could explain that, but observations by the Very Large Array of radio telescopes in the USA showed that in the oldest visible galaxies, 13 billion light-years away, the black holes are relatively more massive.  The implication was that whatever the interaction mechanism is, it cut in as the galaxies take form and therefore the black holes must have formed first – which is very hard to understand.

By April 2009, a possible explanation was presented at the European Week of Astronomy and Space Sciences at the University of Hertfordshire.  Dr. Curtis Saxton and Prof. Kinwah Wu suggested that the formation of proto-galaxies could trigger a ‘dark gulp’ of infalling dark matter, forming the supermassive black holes before the main structure of the galaxies.   At the same conference, other researchers argued that dark matter and dark energy don’t exist!    But if the supermassive black holes form first, it could help to explain why the rates of star formation in early galaxies, within a billion years of the Big Bang, are unexpectedly high.   (Nancy Atkinson, ‘New Findings Challenge Galaxy Formation Ideas’, Universe Today, April 21st, 2009.)

Hubble Tuning Fork

Edwin Hubble devised a classification system for galaxies with an accompanying diagram called ‘the ‘Hubble tuning fork’.  Assumed to be an evolutionary sequence, it began on the left with elliptical galaxies, then split into an upper ‘Sa’ arm for spiral galaxies and a lower ‘Sb’ one for barred spirals.  Now we know that, while the ‘Hubble tuning fork’ goes back at least 11 billion years, and while there were some elliptical galaxies in the very early Universe, they were probably formed by collisions between the first large spirals;  and the barred spirals result from collisions between dwarf galaxies and spirals.  Our Milky Way is at position B1 on the left of the bottom arm of the present-day ‘fork’:  it has a short central bar with clear spiral structure coming off from the ends of it and tidily (as well as tidally)  wrapping around it.  After the Milky Way collides with the Andromeda spiral 3-5 billion years from now, it’s thought they’ll end up as a giant elliptical galaxy on the ‘handle’ of the fork, with most of the dust and gas driven off and minimal new star formation.

We can already see more than 13 billion years back, to around 400 million years after the Big Bang, and back there we’re seeing galaxies forming, typically by convergence of three huge streams of gas– too early for dust.  Supermassive black holes form early and the energy from their emission discs blows gas out of the nuclei, putting a cap on star formation there and expelling everything that doesn’t get trapped in the discs.  That’s why galaxy nucleus mass is related to central black hole mass. 

This image shows “slices” of the Universe at different times throughout its history (present day, and at 4 and 11 billion years ago).

There hadn’t yet been time for the heavier elements to be synthesised in supernovae and scattered in sufficient quantities for stars like the Sun and planets like the Earth to form – but now that we know the heavier elements were synthesised faster than that, by colliding neutron stars, how early were the first of those?  In the discs, back then, star formation was generally violent and sporadic.  These are called ‘starburst galaxies’ – it still happens now, usually as an immediate aftereffect of galactic collisions. 350-650 million years after the Big Bang, they’re so violent they’re being called ‘superstarburst galaxies’.  Their cores would have been blue, rather than the reddish-yellow of galactic nuclei today, because back then they would have been dominated by new, hot giant and supergiant stars.

In the 1960s, as Big Bang theory began to win the contest with the Steady State, one thing bothered me.  In astrophysics lectures, we were told that in stars with more than 1.3 times the mass of the Sun, energy production was dominated by the ‘Phoenix Reaction’, catalysed by carbon, nitrogen and oxygen.  If the early Universe consisted only of hydrogen and helium, what would happen to stars of that mass?  Would they ever stabilise, or would they continue to contract, burning ever more fiercely until they went supernova?  Could multiple supernovae in early galaxies explain the apparent faster-than-light flickering seen in quasars?

I tried this idea of ‘pressure-cooker stars’ on astrophysicists at the time, and occasionally since, and it’s safe to day that it aroused no interest whatever.  The quasar effects were explained by relativistic jets from supermassive black holes  (see above), and no doubt early massive stars would stabilise somewhere above the Main Sequence, but nobody thought it worth checking on.  Now that we can see back to the earliest galaxies, however, they do look different, and one reason for that is that my ‘pressure-cooker stars’ did exist.  They did stabilise, but they accumulated a lot more mass than their present-day Main Sequence counterparts, burned more brightly, went supernova sooner, and gave birth to the supermassive black holes at the cores of galaxies today – even small ones.   (Fraser Cain, ‘Supermassive Black Holes or Their Galaxies?   Which Came First?’, Universe Today, 6th September 2017.)  I didn’t go far enough in astrophysics to publish anything, and couldn’t get anyone else interested, so I can’t claim any credit;  but it’s nice to know I wasn’t completely wrong. 

First stars ‘pressure-cookers’

The James Webb Space Telescope should be able to see the very first galaxies taking shape, and it’s pretty certain there will be more surprises to come..  As of , Thursday Feb 22nd, the telescope is safely in orbit around the Earth-Sun L2 point, after successfully deploying all the parts of its complex structure on the way out there.  Cooling down of the optics is continuing, in the shadow of the giant 5-layered sunshade, and once all the mirror segments had been freed from the clamps, a first test showed that the images from all 18 segments were grouped within a single image, centred on  the target star in Ursa Major.  That meant they were already close to their final positions, when all 18 images will combine into one with very high resolution.  Moving them very slightly, one at a time, each image was identified with the mirror segment that produced it, and at the same time they were moved into a hexagon which is an expansion of the final configuration.  ‘Segment Align Step 2’ has been complete since, bringing them still closer.  Cooling will continue for weeks before the final configuration is reached, but the JWST is expected to be operation by July.

(To be continued). 

Duncan Lunan has written as series of Astronomy articles for The Orkney News which you can find by tying into the search engine

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