The Galactic Barbecue

M81 and M82 are among the most studied galaxies due to their proximity, size and physical phenomena. This image is an opportunity to explore the subject of gravitational interaction between galaxies a little further. The first part of the story of galactic interaction you can find here: https://www.astrobin.com/ru40fg/H

Of course you can just enjoy the image and leave it at that. You can also come along on a journey to a deeper understanding of what is happening in the image. From events on an inter-galactic scale all the way through to some fundamental physics on the atomic scale.I have included many links to imagery from spaced based observatories in other parts of the spectrum, because those images do support the reasoning so much.

Acquistion and post-processing

This image was taken from my Bortle 5 backyard in the prime f/3 focus of a 32cm Hypergraph. I captured 7 hours of RGB data and 8.5 hours of Ha data using a multi-narrowband filter of which only the Ha result was used. Calibration and stacking was done in AstroPixelProcessor and further processing was done in PixInsight. In post-processing I used the continuum subtraction method to bring out the Ha signal, which is so important to tell the story.

In this Image

(For reference, see annotated version here https://www.astrobin.com/full/fd5sy9/0/ )

Galactic Cirrus

Throughout the image (but not overly emphasized) is a haze of dusty clouds: galactic cirrus (also known as Integrated Flux Nebula or IFN). This is a foreground element. They are clouds of dust that envelope our own Milky Way and are reflecting light of the Milky Way as a whole.

M81

Resembling our Milky Way in appearance, size and mass, M81 is a Grand Design Spiral (a fancy way of saying that it has well defined spiral arms) galaxy with a 70 million solar mass black hole in the center (as evidenced by vast amounts of X-ray radiation from the core, a very compact radio source in the center and high orbital velocities very close to the nuclues as measured by spectroscopy). It has active star formation, but only in the spiral arms. The core of the galaxy is devoid of this.

Holmberg IX

The blue patch above M81 is the dwarf irregular galaxy Holmberg IX. It is located along a tidal debris path close to M81 (see 21cm image later), suggesting it has been formed out of denser clumps of the tidal stream (a so called tidal dwarf) and it has a large population of stars that are younger than 200 million years, giving it the youngest stellar population of any nearby galaxy.

Arp’s loop

Going further to the right, there is a double arches structure called “Arp’s loop”. There is a fair bit of controversy around the nature of this. The area in the direction of M81 contains a lot of galacitic cirrus. Arp’s loop was originally thought to be a tidal stream originating in M81. However, studies in Infrared and comparisions to the color of the feature show a complex intermingling of foreground galactic cirrus with stars and extended disk features of M81. Radio observations show clumps of HI gas at the top position of Arp’s loop and UV images show small clusters of young stars there. However, the large loop structure we see in visual wavelengths as all but galactic cirrus.

A multi-wavelength view

The availability of data in several parts of the spectrum gives us a much better understanding of the physical processes at play. So let’s explore:

Radio: Radio observations of the 21cm line of neutral hydrogen (HI) show bridges of material between M81 and M82, indicative of tidal disruption due to close encounters. https://www.skatelescope.org/wp-content/uploads/2014/07/Messier-81-Messier-82-and-NGC3077.jpg In this radio picture one can clearly see HI gas streams between M81 and M82 (and NGC 3077). Interestingly, one can see density peaks in the HI bridge at the site of Holmberg IX, hinting at the formation of this dwarf galaxy in the tidal stream itself. Futhermore, there is also a denser patch at the “arch” location of Arp’s loop.

Infrared: The Spitzer space telescope imaged M81 extensively and shows where dust and molecular gas shines and therefore where starbirth is prominent: http://www.spitzer.caltech.edu/images/6672-ssc2019-15a-Galaxy-M81-in-Infrared-Revisited

Ultraviolet: Look at this GALEX image in ultraviolet: https://sci.esa.int/s/w7evnx8 Here, we see the ultraviolet light of bright young stars. As seen earlier in infrared young star density clearly is very high in the spiral arms, but absent in the nucleus. Furthermore, we see young stars at the position of Holmberg IX and along the HI bridge from M81 to M82 roughly at the position of the double arches at the top op Arp’s loop. However, in UV there is no loop of any significant size.

The M81 / M82 system

M81, M82 and NGC3077 form a trio of interacting galaxies. As most of the research is on M82 (literally hundreds of research papers) and because NGC3077 is not in my image, I will concentrate on M81 and M82 exclusively. M81 is about half of the mass of our Milky Way and M82’s mass is about a tenth of that of M81. As noted earlier when looking in the 21cm line of HI gas, we see remnants of interaction between M81 and M82. The proper motions of M81 and M82 have been measured as well using Very Long Baseline Interferometry and have shown that M82 is gravitationally bound to M81: M81 en M82 are circling each other. More than that, they have had close encounters in the not so distant past (200 million years). There is ample evidence that the close encounter has resulted in the increased star formation rate in M82. How close this encounter was is a bit more elusive. One study postulates a head on collision where M82 penetrated into the disk of M81, losing much of it’s outer material in the process. The large HI halo in the M81/M82 system would be the remnants of that.

M82: The star of this show

All of the interaction history has resulted in a very high rate of star formation. We call this a starburst and M82 is a prime example of a so-called starburst galaxy. Within a few million years after the encounter the many young stars that have formed started dying off in supernovae and begin to pollute the galaxy with lost of dust. Starburst galaxis radiate up to 98% of their energy in the infrared (compare with our Milky Way 30%).

As galaxies go, M82 is a small galaxy. And yet, it is one of the brightest Infrared objects in the sky. Although M82 has long been classified as an irregular type of galaxy, the modern view is that it is a spiral galaxy (type SBc) with two open arms and the majority of its mass and virtually all of its gas and star formation confined to a small area around the nucleus. One of the theories of why this is so is the hypothesis that the M82 we see today is only the remaining core of what was once a much larger galaxy, with the outer parts ripped away by a close encounter or collision with M81. M82 has a rate of starbirth of a few tens of solar masses per year (our own galaxy about 3 solar mass per year). One of the most striking visual things about M82 that makes it picture worthy and the reason for so much research is the galactic superwind it hosts: gas and dust is literally blown out of the galaxy by the energy released from stellar winds, radiation pressure of hot young stars and blasts of fast moving material by supernovae at the sites of star formation deep in the galaxy. If that wind collides with other material, like gas and dust, it can cause a shock wave, making temperatures soar (evidenced by the x-ray emission and ionized gasses). Winds that are strong enough can break out of the galaxy, sending interstellar material into intergalactic space, like we are seeing here in M82.

The PAH universe

Again a look at another wavelength than visual: Have a look at this image in infrared: http://www.spitzer.caltech.edu/news/234-ssc2006-09-Galaxy-on-Fire-NASA-s-Spitzer-Reveals-Stellar-Smoke

Quite an explosion, isn’t it? The image shows infrared line emission around 8 micrometer. What are we seeing here? Light emitted by molecules. Just as atoms emit specific frequencies, molecules emits line emission as well. What kind of molecules? Something very familiar: Soot! Scientifically named Polycyclic Aromatic Hydrocarbon molecules. The same as you generate when you are black-burning meat on your barbecue. Not healthy because of their long term stability, but that stability is the exact reason why they survive in the harsh environment in space. In infrared, the emission of PAH is quite bright. After hydrogen, helium and oxygen, carbon is the fourth most abundant element in the universe and the basis of life as we know it. Carbons are formed by stellar evolution and because PAH’s light up in the presence of ultraviolet radiation that is emitted by young stars, PAH’s are important as tracers of stellar formation.

But why? A tiny little bit of quantum physics:

21cm Emission lines of neutral hydrogen (HI): An assignment to a bright student in WWII The Netherlands and a quantum wave equation in the floor of Westminster Abbey

As astrophotographers, we all are aware of Ha emission by the ionised hydrogen atom(HII). This is when an electron in the hydrogen atom has been brought into a higher energy state and falls back to a lower energy state. But visible light is highly obsured by dust. Would there also be hydrogen emission lines at radio wavelengths? This was a question that interested Hendrik Jan Oort (from the Oort Cloud). Oort had studied star velocities in order gain insight into our Milky Way, but frustratingly, visual light is heavily obscured by dust, limiting how far you can look.He had acquainted a bright young student called Henk van de Hulst and Oort gave van de Hulst the assignment to think about what mechanism could trigger line emission of HI in the radio spectrum and at what wavelength. Van de Hulst went to work on the question and after a few dead ends he found a promising answer by looking at electron spin reversal. Neutral hydrogen consists of one proton and one electon and both particles have a quantummechanical property called “spin”. This spin can be either up or down. In the HI atom, we have a proton and an electon. The state in which both the proton and the electron have their spin in the same direction has slightly more energy than when the spin direction is opposite. When changing from the high energy tot the low energy configuration, a photon is emitted with a wavelength of 21.1cm. Now, the thing is that the probability for this spontaneous spin flipping to go from a higher to a lower energy state is very small: once every 10 million years. It is because there is a massive amount of HI atoms in the universe (and the lack of inter HI collissions in space) that we can detect it. Note that the spin reversal mechanism was already known at the time. In 1928, Paul Dirac combined quantum wave/particle description with the theory of special relativity and wrote a relavistic wave equation for the electron. The mathematical solution of this equation contains the spin of the electron (and the existence of antimatter as an unexpected bonus). This Dirac equation (only 8 symbols) is of such a profound importance that it is engraved in the floor of Westminster Abbey, right in front of Isaac Newton’s tomb: https://www.atlasobscura.com/places/dirac-equation In 1944 Van de Hulst reported his prediction, noting that radio recivers of the time should be improved by a factor 100 to observe the emission. This theoretical prediction and the subsequent post-war detection in 1951 was the start of the discovery of the spiral structure and further mapping of our Milky Way and far beyond. Velocity measurements of HI were instrumental in measuring the rotation velocities of galaxies leading to the conclusion that there must be much more mass than we can (currently) see: One of the indirect signs of dark matter.

Molecular Emission lines: Quantum Chemistry

Dust particles in space radiate at all freqencies. The peak of this frequency is determined by the temperature. PAH molecules radiate at specific frequencies. But why not at all frequencies? To answer the question, we have to turn to the field of Quantum Chemistry (or Physical Chemistry or Computational Chemistry; these are all synonyms). Just like in atoms, the structure and interaction in and between molocules is governed by the laws of quantum physics. Quantum physics is very unintuitive. Most analogies to the classical world are leading nowhere. It is a field where the mathematical solution of underlying equations lead the way.

Starting in 1900 that in order to describe black body radiation over the full range of frequencies, light has to be considered to consist of discrete packages (quanta) of specific energy, which we now call photons. This evolved into the appreciation of the fact that both radiation and matter show behaviour of particles and of waves. Additionally, there is a only probability of particles being in a specific place, but not a certainty. This was a far cry from the well understood deterministic classic-mechanical view of the world in wich you can calculate these things exactly.

In classical mechanics we have Newton’s laws to describe the motion of matter and Maxwell’s laws to describe radiation. On the very small scale of atoms and molecules, quantum physics reigns and the equation to describe the (probability of) position and velocity of a particle (or photon) is called the Schrödinger wave equation. It is through the rigourous solution of this equation that we discover the behaviour of matter at the atomic scale.

Quantum Chemistry uses the Schrödinger wave equation to desribe the atoms and the electrons that are shared between those atoms in a molecule. Solving the wave equation learns that rotations and vibrations of molecules have discrete energy levels (in contrast to classical mechanics In the case of atoms light is emitted because of a transition of electrons to specific lower energy levels. In molecules, the energy level can change not only as result of electronic transitions but also because of changes in rotations and vibrations. Molecular spectra are therefore more complex than atomic spectra.

Emission by Polycyclic Aromatic Hydrocarbon molecules

Polycyclic Aromatic Hydrocarbon molecules are a family of planar (flat) molecules arranged in a characteristic honeycombed lattice structure of fused six-membered rings of carbon (C) atoms decorated at the edges by hydrogen (H) atoms. The following artist expression gives an idea of the molecular structures we are talking about. https://www.nasa.gov/multimedia/imagegallery/image_feature_398.html

Emission of PAH’s in the near infrared is because the electrons in PAH’s get energized by the ultraviolet light of young stars and the schock waves of supernovas. Molecules like PAH’s can absorb or emit radiation through the following energy changes: Electron re-distribution (like in atoms) at visible/UV/X-ray wavelengths, ), Rotation; at microwave wavelengths and through change of vibration at infrared wavelengths. By going from high energy to lower energy levels the PAH molecule goes through a number of small energy conversions by going from higher to lower vibrational states. Possible vibration changes are: Change in C-C and C-H bond length (stretching) or bending of the C-H bond in the plane of the PAH molecule and out of the plane of the PAH molecule. These all have different emission frequencies and studying them in different bands informs us about the conditions in space where we see them.

Further reading

Galaxies

Cosmic Collisions – The Hubble Atlas Of Merging Galaxies – Lars Lindberg Christensen, Raquel Yumi Shida and Davide de Martin (2009): A nice picture book of Hubble images of collicing galaxies with expanation of the dynamics of galaxy interactions.

Galaxy – Mapping the cosmos: James Geach (2014): An accessible journey into extragalactic space by an expert in the field. Lots of beautiful images.

An Introduction to Galaxies and Cosmology – Mark H. Jones and Robert J. A. Lambourne (2003): A fairly easy going undergraduate text.

More Things in the Heavens: How Infrared Astronomy Is Expanding Our View of the Universe:-Michael Werner and Peter Eisenhardt (2019): The story of the Spitzer Space Infrared Telescope and the application to a host of astronomical questions. I found the chapter on the hunt for exoplanets really fascinating, but it also includes a nice section on PAH’s.

Revealing the heart of the galaxy (The Milky Way and Its Black Hole), Robert H. Sanders: A historical account on the unraveling of mystery the structure of our Milky Way

Arp’s Loop

A Multi-wavelength analysis of M81: insight on the nature of Arp’s loop: ESO Astronomy & Astrophysics manuscript, April 12, 2010: You can find this directly or better still, go to the website of one of the contributors R. Jay GaBany: https://www.cosmotography.com/images/small_ngc3031.html

Galactic Cirrus:

A nice view of the galactic cirrus in ralationship to the plane of the Milky Way, you can find on the excellent site of Jay R. GaBany: https://www.cosmotography.com/images/galactic_cirrus.html

Chemistry of the Interstellar Medium

An Introduction to the Sun and Stars – Simon F. Green and Mark H. Jones (2003): A introductory undergraduate text containing a short but well illustrated section on molecular radiation.

Polycyclic Aromatic Hydrocarbons and dust in regions of massive star formation – Els Peeters:https://www.rug.nl/research/portal/files/3064834/c1.pdf This is the introduction of a thesis on the subject of PAH’s. It presents a relatively gentle introduction into the subject.

Physical Chemistry – Peter Atkins, Julio de Palma, James Keeler (2018): Going deep into physics, a well written university textbook on the classical and quantum physical fundamentals of chemistry with a lot of emphasis on molecular spectra. Although full of mathematical formulae, it is illustrated throughout with a lot of high quality artwork that helps to get a better understanding of the subject and it has breakouts to how this all relates to what we observe, including some astrophysical cases.