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Count the arms in this superb image of NGC1232 by ANTU - the new 8.2 metre telescope of ESO’s VLT in Chile.

The Milky Way - Detailed Structure

Swinburne Online Education Exploring Galaxies and the Cosmos

© Swinburne University of Technology

Activity:

Galactic Spiral Arms

• Spiral arms in the Milky Way and other galaxies.

• Star forming regions highlighting the spiral features.

• Mechanisms for formation and persistence of spiral arms.

ANTU image showing how dark rifts and bright clusters of stars delineate what we see as ‘spiral arms’.

SummaryThis Activity aims to provide examples, theories and understanding in the following areas:

Spiral shapes in NatureRegular shapes and patterns in nature invite a search for a mechanism for their formation.

Your author for this section has tried to find Planet Earth examples to introduce each Activity. (It will get harder as we journey further out in the Universe!)

In this case, this ‘one-armed’ spiral shape, both beautiful and mysterious, does not suggest any ‘rotational motion’ to us. It challenges life scientists to understand its formation and its change to . . .

In science, organised structures such as occur in biology, in atomic structure, and in the solar system, lead to

theories - enabling predictions.

Regularities provide clues to understanding

Amongst the seeming chaos of stars and dust, the spiral arms of galaxies stand out, both in beauty and in inviting explanation.

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Ironically we probably know more about the dynamics of distant galaxies than we know about the mystery of life, as in this simple tree fern frond.

More familiar spiralsRotating fireworks and rotating garden water sprinklers also produce ‘spiral arm’ effects.

In these cases the ‘mechanism’ is the outward directed jet effect of the gunpowder or water, respectively.

The jet effect certainly implies such arms trail the rotation direction. Will we find galaxy arms trail or might they at times lead the rotation direction of the galaxy?

We will be searching for a ‘mechanism’ for the generation and sustaining of galaxy spiral arms.

Spirality and rotationWe will see that symmetric and spiral galaxy features do not imply rotation in the way we might at first expect.

We have seen that, early in our own century, and due to telescope limitations of the day, we had no evidence that the spiral nebulae were not part of the entire Milky Way ‘universe’.

In fact, the spiral nebulae were suspected, by some, as being embrionic solar systems according to the ‘nebula hypothesis’ - and the suggestion of rotation of the spiral shape must have added to the incorrect idea.

In the previous Activity we saw how the V-R velocity curve was generally flat away from the central region. Hence the period of a star orbit, P=2R/V, increases with Radius. Consider two orbits of the inner star of a sample of four:

Radii and periods are in the ratios 1:2:3:4

After 1 orbit After 2 orbits

The Winding ProblemIf the spiral arms rotated with the stars that comprise them, how would their appearance change with time?

Within a few rotations, the arms would wind up tightly - and could not persist as observed.

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Do spiral arms (appear to) trail or lead ?Establishing the rotation direction of a galaxy depends on knowing the orientation of the plane of the galaxy.

The previous Activity showed that the right side of NGC253 is receding from us (faster than the centre of the galaxy). But . . .

This tilt would mean the galaxy is rotating clockwise.The dark rifts on the near side of the galaxy establishes that clockwise is the correct interpretation.

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In most cases, where the tilt of the plane can be determined, arms appear to trail.

Leading or trailing spiral arms?

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For example, where, in the one image (eg NGC4622 at right), we appear to have arms in both directions, leading arms must, at least, be possible.

There may be some exceptions to the rule.

Really, a lot of astronomy involves applying logic and puzzle solving to the problem at hand and can be great fun. Certainly a little physics and maths helps (and we can still get it wrong!) . . .

The winding problem demonstrates that the spiral arms do not rotate with the stars comprising the arms. (Indeed, do the arms rotate at all! What observations could we make to decide if they do?)

As we now proceed to review the actual nature of spiral arms and the phenomena associated with them, bear the winding problem in mind to anticipate a solution to it.

Winding program

For PC users.

What do we see as ‘arms’?To investigate spiral arms in our own Galaxy, what do we search for and what difficulties are involved?

Taking our clues from other galaxies, what objects (in visible light first) seem to indicate the positions of spiral arms?

NGC5238 M83 AAT 008

• Dark dust rift lanes• Bright, clustering, stars• Pink (hydrogen) nebulae

We can detect such markers in the Milky Way (with one proviso - what is it?). So far, so good . . .

Not so easyNot all galaxies show prominent spiral arms.

NGC300 AAT 057

This image shows the same star, dust and gas spiral arm markers as in the NGC5238 image in the previous frame, but the arms are not as well delineated.

The task of identifying armsin our own Galaxy will be difficult if it is a galaxy similar to NGC300.

And there’s one further problem . . .We have to try to identify spiral arm markers from our viewpoint within the Galactic disk itself!

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This image of an edge-on spiral galaxy shows the extent of absorbing dust and gas that may also hamper our observations within our own Galaxy.

It also highlights the fact that spiral arms are not evident when ‘viewed from the side’ - the problem will be same as we try to find arms in our own Galaxy.

The distance problemLocating the spiral arm markers from within the plane of our own Galaxy:Though we can identify the star, gas and dust spiral arm markers, when they are all superimposed in our side-on view, how do we:a) identify each marker along with its distance,b) view through to arm markers at large distances.This image toward the centre of our Galaxy shows stars, hydrogen nebulae and dust lanes. But how can we identify spiral arms and view through the confusion to large distances?

NGC Milky Way! AAT 028

Markers to spiral arms: i) dustThe following is to remind us of the value and difficulty in the various markers to spiral arms: This image of the Orion Nebula clearly indicates the dense clouds of dust on this side and the far side of the ‘trapezium’ cluster of young stars.And recall that, even away from such clouds, the average interstellar extinction is about one magnitude per kiloparsec toward the centre of the Galaxy.For our Galaxy therefore, dust serves only to hamper visual observations to greater distances.

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Markers to spiral arms: ii) stars

Young open clusters of stars, including very luminous O and B main-sequence stars, are the main visible features that dominate spiral arms.Since massive OB stars are short lived objects, spiral arms must correspond to regions of active star formation.This also suggests that even the spiral arm may be a transient phenomena as far as its location is concerned.

NGC1232 ESO 8.2m ANTU

AAT 012 M20 Trifid Nebula - distance about 2Kpc

Markers to spiral arms: iii) hydrogen gasEmission nebulae result from excitation by intense UV radiation from hot young O and B stars.

Amongst many radiation mechanisms, the pink colour of the Hydrogen alpha emission is usually predominant (HII regions).

Though visible in nearby galaxies, the difficulty of observation to large distances in our Galactic plane limits their usefulness as spiral markers in our own Galaxy.

21cm emission from HI cloudsAs described in the last Activity, 21cm emissions can be received from large distances - - from throughout our Galaxy.

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21 cm observations are made in a given direction.A radial velocity plot is made from Doppler shifted signals. Positive (recession) velocities should come from regions interior to the Sun’s orbit. The highest velocity peak identifies cloud C at Rmin.

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21cm spiral arm tracingIdentifying gas clouds from 21cm radiation enables spiral arms to be ‘traced’.

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The negative (approach) velocity at point A identifies a cloud exterior to the Sun’s orbit.

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Cloud B shares the Sun’s velocity.

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Cloud at D orbits between the Sun and the cloud at C.

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Giant Molecular CloudsThe best current spiral tracers.

Giant molecular clouds (GMC’s), from which massive stars are born, can be located in a way similar to that of HI regions.

Different wavelengths and molecules (such as CO) are used to produce the Doppler shifted radial velocity diagrams.Survey’s of GMC’s strongly delineate some of the Galaxy’s spiral arms shown in the next frame . . .

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The spiral features of our GalaxyWhat is the outcome from the various forms of arm tracing for our Galaxy?

Carina arm Sagittarius arm

Perseus arm- based on various types of tracers, at points (not shown) along the arms. Evidence is not available for regions inside the sector masked by the Galactic centre.

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What causes spiral arms?What do regions of gas and dust and young stars imply?

They are regions where young stars are forming from the gas and dust.

The collapse of gas and dust to form stars requires a trigger - either slow contraction under gravity or compression by factors such as supernovae* explosions. But if it is compression, why does it seem to trace out spiral arms? M16 Eagle Nebula AAT 047

*Click here to be reminded about supernovae

Density WavesRecall that spiral arms do not share the rotation rate of the stars comprising them. (The winding problem).

Could the arms themselves be providing the increased density? Are they ‘density waves’ moving through a uniform gas, dust and star field, producing the star-forming regions which in turn delineate the arms?

Link the factors a) that star forming regions require a cause for compression to increased densities and b) that the arms do not share the stellar rotation about the galactic centre.

Star velocity

Arm velocity

Various ModelsA number of density wave models lead to the generation of spiral patterns.

In the mid-1960’s the Lin and Shu model proposed that in the inner disk the stars overtake the spiral arm; in the outer disk the spiral pattern moves faster than the stars.

Stars in the central region also drift closer to the centre, pulled backward by the stars further out in the spiral - if, as is generally observed, the spiral trails the galactic rotation.

Two questions arise - and remain under investigation: 1. Will the spiral pattern eventually die out? 2. What causes the density wave initially?

Computer ModelsThe advances in computer speed has enabled galaxies of tens of thousands of stars to be simulated.

Resulting galaxies bear remarkable similarity to real galaxies - with arms of varying looseness and even including barred spirals that are included in Hubble’s tuning fork diagram classification of galaxies.

Elliptical orbitsMaterial orbiting a galaxy in slowly precessing elliptical orbits also produces regions of increased density.

The mathematics of the model cannot be included here but a visual simulation can be achieved as follows:

1. Take an ellipse.2. Enclose an ellipse reduced (here 90%) and rotated (here 10o) from the previous ellipse.

3. Repeat.

The spiral effect beginning to appear would be an area of increased density if the ellipses represented the orbits of stars, gas and dust. This is shown more completely in the

next frame.

Here we show 46 orbits (all with same eccentricity) nested so each is 95% of the size of, and rotated by 10o from, the next larger orbit.

NGC2997 is superimposed, showing remarkable coincidence, especially along the inner arms, and inviting a reason for the ‘extra arms’ (dotted).

NGC2997 AAT 017

The two dark spirals have not been added -

they are an effect of line closeness (density).

Spirals from ellipses

Inclination of galaxiesQuery posed. The match in the previous frame also involved the

deliberate tilting of the resulting nested ellipses (shrinking the vertical size) to match NGC2997’s spiral arms.

For example, the ‘face-on’ spiral galaxy shown at left is a deliberately distorted image of . . .the real M81 galaxy at right!

This suggests we should be very careful when assessing the true inclination of galaxies to the ‘plane of the sky’.

NOAO

Summary• Spiral tracers that are evident in external galaxies can be used within our own Galaxy (apart from the sector masked by the Galactic Center) to trace spiral arms.These include:

Young galactic clusters Regions of absorbing gas & dustHII regions HI regionsGiant Molecular Clouds• Spiral arms do not rotate at the same rate as the stars that comprise them, but can be explained as density waves moving relative to the background material.• Our Galaxy appears to be a two (plus two) armed spiral of intermediate pitch angle or winding looseness (type Sb) - perhaps similar to NGC 2997 in the earlier frame. The brightest part of the Milky Way is our view of the Sagittarius arm

between the Sun and the Galactic centre.

The Galactic Centre.

In any spiral galaxy, two features stand out:• the spiral arms, and• the brighter, bulging centre.

What lies in the innermost region is obscured by gas, dust and the increased star density - and this is especially so for our own Galaxy from our viewpoint out in the disk.

Portion of NRAO VLA Radio Image of the Galactic Centre

The observing tools for overcoming these difficulties, and what has been found to lie in the inner regions, are the subjects of the next Activity.

AAT/IAC/RGO/UKS images © David Malin (used with permission):http://www.aao.gov.au/local/www/dfmIndividual Malin images (© David Malin (used with permission)), shown with a 6 character code - such as AAT028, - are found at the website ending with that code; eg:http://www.aao.gov.au/local/www/dfm/aat028.html

NOAO (National Optical Astronomy Observatories) image © Association of Universities for Research in Astronomy Inc. (AURA), all rights reserved. Credit AURA/NOAO/National Science Foundation.http://www.noao.edu/image_gallery/galaxies.html

Hubble Space Telescope images indexed by subject:http://oposite.stsci.edu/pubinfo/subject.html

ESO (European Southern Observatory) VLT images:http://www.eso.org/outreach/info-events/ut1fl/astroimages.html

Image Credits

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Introducing supernovaeA supernova is the catastrophic collapse of a star to form a black hole or a neutron star.

Left: The Crab Nebula is the remains of a supernova observed by astronomers in 1054AD.

We learnt that supernovae occur when massive stars reach the end of their life. Once a massive star has exhausted its nuclear fuel, it collapses in a brilliant explosion, releasing vast amounts of energy and violently ejecting its outer layers.

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If a supernova occurred within 50 light years of Earth, the gamma rays and high energy particles emitted during the explosion would kill most life-forms and cause severe genetic damage in others. A local supernova explosion is a possible explanation for the extinction of the dinosaurs.

More amazing facts

The supermassive star Eta Carinae. 150 years ago, Eta Carinae suffered a violent outburst which gave the star its present strange appearance. Miraculously, the star survived the outburst - for now. Astronomers believe that Eta Carinae may soon explode in a supernova.

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