Galaxies are fascinating.The first attempt to decipher one – ours – was Herschel’s, done through one of those “Arrrr, matey” telescopes way back in the 1700s. It was done purely by optical observations. Assuming a uniform distribution of stars we would see an “Amoeboid” galaxy, shapeless and formless, containing a given number of stars. It assumes a sun in a center and looking out over a flat plane – the “grindstone model”.
But there are better strategies to explore the size and structure of the Milky Way.
1. select bright objects you can see throughout the galaxy and trace directions and distances
2. observe objects at wavelengths other than visible to circumvent problem of optical obscuration) and catalog directions and distances – more useful to utilize wavelength like radio or x ray or IR to get more information
3. trace orbital velocities in different directions relative to our position.
But structure hard to determine when we are INSIDE the object being studied; time and distance measurements can be difficult and on a purely visual level the center of the galaxy is obscured by gas and dust.
One way of measuring distances is by using the Cepheid method using Cepheid variable stars (more about this – and it’s fascinating – here and here.
In a nutshell, there is a region called “the instability strip” where stars pulsate because of changing environments within. All massive stars go through a Cepheid phase during their evolution as they age. The more luminous a Cepheid the longer its pulsation period – and observing the period of pulsation yields a measure of luminosity and thus the distance. The Cepheid method allows us to measure distances to star clusters throughout the Milky Way.
Type 1 (classical) Cepheids have simple pulsation frequencies, and are relatively linear; type II Cepheids have pulsation oscillation overtones, like a “beat frequency”- much more complicated, and their presence, before their type was established, served to confuse the Cepheid issue somewhat in the early days.
We can also explore the galaxy using clusters of stars. There are two types – open clusters, which are young clusters of recently formed stars within the disk of the galaxy, not evenly distributed, and globular clusters, old and centrally concentrated clusters of stars mostly in the halo around the galaxy. The distribution of globular clusters is not centered on the sun but on a location which is heavily obscured from direct (visual) observation = center of galaxy.
Structure of Milky Way: 75000 light years across with a nuclear bulge in the middle, an outer disk (arms, halo (around the galaxy in space) and globular clusters on the outskirts – our Sun is about 20 000 LY away from centre out on of the arms about 2/3 away from the center. In the disk we have open clusters – younger stars (O/B associations). In IR images (clearer; free from obstructing dust – interstellar dust (absorbing optical light) emits mostly infra red) shows that most of the dust in the Milky Way is in the centre but there is quite a lot of it also in the halo around the galaxy. Disk stars have nearly circular orbits in the disk(plane) of the galaxy; halo stars have highly elliptical orbits, randomly oriented, passing through the disk plane as they travel.
Total mass in milky way approx 200 billion solar masses. Additional mass in halo, total approx 1 trillion solar masses – most of the mass not emitting any radiation => dark matter!
DARK MATTER
Proposed in 1933 by Fritz Zwicky – checked out Coma Cluster – galaxies flying around too fast (as measured by Doppler effect) for their visible mass to keep them together a those speeds so he proposed dark matter was present.
A few decades later Vera Rubin (“mother of dark matter”) started to notice FLAT rotation curves. In spiral galaxies the flatness tells us that matter is distributed uniformly throughout space but the LIGHT is not, it is obviously concentrated, so there must be something else out there – and it’s the MAJORITY of the something that is out there.
If this was normal matter then mass would mostly come from protons and neutrons = baryons. The density of baryons right after the big bang leaves a unique imprint in the abundances of deuterium and lithium. The density of baryonic matter is only about 4% of critical density, total is 30% – so most dark matter must be non-baryonic in nature.
So what’s there? Believe it or not, WIMPs and MaCHOs.
Trying to explain the flat rotation curves by invoking things like black holes, brown dwarfs, etc, we have a couple of whimsical baryonic particles, made from conventional Periodic Table stuff.
A WIMP is a Weakly Interacting Massive Particle They could be either neutrinos (seem to have mass but are too small – they are a component but not the dominant component)or hypothetical elementary particles called axions – as yet, not detected, but predicted to change to and from photons in the presence of strong magnetic fields)
A MaCHO is a Massive Compact Halo Object, normal baryonic matter which emits little or no radiation and is unassociated with any solar system, drifting through space unanchored, as it were. SInce they emit no light of their own they are very hard to detect – but can be detected because of a phenomenon known as gravitational lensing, where a distant star is “lensed” by the passage of a MaCHO between the observer and the distant star.
All clear?
Good, because we go on with the startling question, “Are we sure that Dark Matter is real?” Sure, there are effects that we can measure and see – but could it simply be that we got the principle of gravity wrong on a very large scale? This alternative, going under the name of MoND or Modified Newtonian Dynamics) was a viable alternative – right until 2006 when gravitational lensing was dragged into the model. In a nutshell, light from a distant quasar was observed to be bent around a foreground galaxy – can actually see the double image of the lensed quasar in the pictorial evidence, and you can also see the lensing object, i.e. the galaxy. So light from a quasar, or a more distant galaxy, is distorted by lensing, sometimes into just arcs of light at the edge of the “lens” – this can be used to probe the distribution of matter in the cluster.
Space between galaxies is not empty but filled with hot gas (observable in X rays) this gas remains gravitationally bound to the dark matter – provides further evidence for dark matter.
The smoking gun for dark matter did not arrive until the evidence provided by the Bullet Cluster.
Gravitational lensing will trace total mass; hot X ray gass will trace baryonic mass. Lensing of background galaxies seen in the optical images lets the mass distribution be mapped; X rays trace the hot gas, dominant source of baryons in cluster merger. But in the image of the Bullet Cluster collision, these two don’t line up in the image! WHy is that? Well, Dark Matter does not seem to interact with itself the way a diffuse gas does during a cluster collision. In the Bullet CLuster post-collision image, we can clearly see that the gas experienced drag during the collision, with the two clusters having a definite effect on one another. The dark matter did not slow down in the least, and is now AHEAD of the gas cloud – it doesn’t interact with itself or affect itself. More on the BUllet Cluster, including a link to a animation of the collision illustrating the above point, can be found here and here.
Traditional:
Spherical cloud of turbulent gas = first stars and star clusters = rotating cloud of gas begins to contract to an equatorial plane = stars and clusters left behind in halo as gas cloud flattens = new generations of stars have flatter distributions restricted to disk of the galaxy = the disk of the galaxy is now very thin
But in a revision of the traditional structure, modification to the traditional model appears to be a recently observed large ring at about 120 000 LY diameter – perhaps remnants of some earlier collision? The distribution of stars and neutral hydrogen mostly in spiral; but in IR images the distribution of dust gives a bar across the center of the galaxy.
The spiral arms are usually full of young stars – this is where new star formation goes on (older stars could be anywhere). Stationary shock waves trigger star formation in the spiral arms – shocks initiate this but then it’s self-sustaining through O/B ionization fronts and supernova shock waves. The star-forming regions get elongated due to differential rotation.
Grand design spiral galaxies – two dominant spiral arms (e.g. M100); Flocculent (woolly) galaxies also have spiral patterns but no dominant pair of spiral arms (NGC 300)
Whirlpool Galaxy (M 51) self sustaining star forming regions along spiral arm patterns clearly visible (see the picture in previous post!)
The galactic center contains a 3.6 million solar mass supermassive black hole, determined by following orbits of individual stars near the centre of Milky Way as seen in the IR images – more here.
Hubble law: the more distant the galaxy the faster it is moving away from us (red-shift, Doppler effect)
Many galaxies typically millions or billions of years of parsecs from our galaxy – typical units
Mpc = megaparsec= 1 million parsecs
Gps = gigaparsecs= one billion parsecs
Supermassive black holes at the centre of every massive galaxy.
Clusters of galaxies – they generally exist in clusters not in isolation.
Rich clusters contain 1000 or more galaxies, boast a diameter of approx 3 megaparsecs, and are condensed around a large central galaxy. Poor clusters are less than 1000 galaxies in number, maybe just a handful or a few parsecs across. Our own galaxy cluster consists of our “local group” – three spiral galaxies (Andromeda, M33, Milky Way), two irregular galaxies (the Magellanic Clouds) and the rest (Leo, Fornax, Canis Major) are the most common galaxy type, dwarf elliptical galaxies. They may be the most common, but it is by far easier to observe the big bright galaxies.
Particularly in rich clusters, galaxies can collide or interact leaving particular patterns behind – can produce ring galaxies (Cartwheel Galaxy) and tidal tails. Astronomer Bob Berrington has simulations of some of this stuff at his website
Galaxies with extremely violent energy release in nuclei – active galactic nuclei (AGN) up to many thousand times more luminous than the entire Milky Way.
Seyfert galaxies a particular kind of AGN where cores are specially bright light is coming not just from stars but from something else too – AGNs tend to be interacting systems, something about interactions makes them massively bright. They have a supermassive black hole core and a thin accretion disk surrounded by a dense dust torus, gas clouds of ionized high velocity gas – emits particle jets (electrons, protons) from poles, fast hot moving gas at VERY HIGH temperatures. Hot gas radiates very brightly, and can outshine the entire galaxy (torus typically about a light year across)
Radio galaxies (centaurus A closest AGN to us) jet in X ray and radio. Jets powered by accretion of matter onto a supermassive black hole – twisted magnetic fields confine material in jet and produce synclotron radiation.
Supermassive black holes at center of most galaxies – fed and fuelled by stars and gas from near central environment – galaxy interactions may enhance the flow of matter onto central black holes.
Quasars – active nuclei in elliptical galaxies with even more powerful central sources than Seyferts – quasars live in galaxies, and they show extreme red shifts in their spectral lines so they are very far away. We don’t see them locally, they are a reminder of galactic history – but the study of quasars allows investigation of
*Large scale structure of universe
*Early history of universe
*Galaxy evolution
*Dark matter
Quasars show red shifts of up to 6, which can shift UV into the visible spectrum…
Pant, pant, pant…
Okay. That is QUITE enough for one night. Past midnight now, local time. I better get to bed. The rest of today I will catch up on tomorrow, which is also the last day of Launchpad.
But before I go – we got a book recommendation today, apparently one of the worst books ever written, with a “read it if you dare” warning. Now, of course, I have to go find that sucker. The title is apparently “Galaxy 666”. I will have to hunt for that one – it was recommended by such GUSTO that I want to at least have a taste of it now, especially when I know all this stuff about galaxies now and can gleefully pick holes in the thing…
Bedtime.
