Galaxies - The view from a distance

Online Astronomy Society Academy Short Course – Galaxies: The view from a distance Contents 1.

2.

3.

4.

5.

6.

Introduction Galaxy shape – a useful system of classification Our Earthly perspective The importance of distance to other galaxies Size matters The view brings structure to the Universe

What you will learn in the course -

The basis of galaxy classification: the Hubble sequence Key factors in the observation of galaxies How galaxy observation helps understand their formation and evolution The continuous changes that galaxies are undergoing Many references to more detailed background information, including references to original scientific papers

Links to other OAS Academy short courses -

Basic cosmology Galaxy structure The Measurement of Distance in Astronomy

About the author Hugh Allen is a passionate scientist specialising in chemistry. Growing up in Reading, he attended Reading Grammar School and studied Natural Sciences at Downing College, Cambridge. Hugh has spent more than 25 years in the chemical industry but has always enjoyed a fascination with astronomy and space exploration. Fascination became a passion when his wife bought him a big 8” aperture telescope!

Hugh is a member of the Online Astronomy Society and the William Herschel Society in Bath. As well as observing and imaging whenever cloud cover permits, he enjoys communicating his passion through writing with an aim to inspire a wider audience and bring astronomy to the forefront. http://www.facebook.com/groups/2217131413/#!/hugh.astrophotography

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Online Astronomy Society Academy Short Course – Galaxies: The view from a distance 1. Introduction Stars are not spread randomly around the Universe, they are found in galaxies.

Described by Edwin Hubble in 1926 as “island universes, scattered through the remote depths of space” (1), galaxies are gravitationally bound clumps of stars, gas, dust and dark matter. There are estimated to be hundreds of billions of galaxies stretching out to the edge of the observable universe. They range from a hundred million stars to many hundreds of billions.

From our earthly perspective galaxies have a seemingly bewildering array of shapes and sizes. Their real nature was not fully accepted until Hubble’s groundbreaking study of the Andromeda Galaxy published in 1925 (2). A deep understanding of the processes in their formation and evolution is still being developed. They present a vast topic and this course attempts to introduce them from a distant perspective. What does the view from the Earth tell us about their general nature? And what can we learn from their distribution in the sky? A separate course looks in detail at the structures inside galaxies.

Firstly, take in the view. Galaxies are beautiful objects and many are visible in amateur telescopes. All of the galaxy images in this course were captured through an 8” Meade LX90 reflector telescope. The white spots in the images are foreground stars in our own galaxy, the Milky Way:

(3)

NGC 2903

by Hugh Allen

NGC 4631 Whale Galaxy

by Hugh Allen

M87

(3)

by Hugh Allen

NGC 4449

by Hugh Allen M101 Pinwheel Galaxy

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by Hugh Allen

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2. Galaxy shape – a useful system of classification When faced with a complex set of objects, a good approach to understanding them is to classify them into groups. In the case of galaxies a system of classification was developed by Edwin Hubble in 1926, based on their observed shapes.(4) It became known as the Hubble sequence:

Original image from http://en.wikipedia.org/wiki/File:Hubble_sequence_photo.png Additional annotations by Hugh Allen

For spiral galaxies, the lower case letter designates the prominence of the central bulge of stars and the tightness of the winding spiral arms. For elliptical galaxies the number Ex measures the shape of the ellipse: x = 10(1 − b/a), where b = the length of the minor (shorter) axis and a = the length of the major axis.

Hubble also recognised that some of the galaxies he observed had no welldefined shape or symmetry at all, and these he simply called irregular galaxies (Ir). Galaxies have also been observed that are on the Hubble sequence but are distorted in some way, or produce unusual radiation. These are designated as peculiar galaxies (pec). How would you classify the galaxies in the Intro?

NGC2903: Sc (weak bar) M87: E1 pec (visible ‘jet’ emerging from nucleus) NGC 4631: Sc M101: Sc NGC 4449: Ir The Hubble sequence was extended by Gérard de Vaucouleurs in 1959, particularly in the number of classifications of spiral galaxies. But the basic principles have stood the test of time and form a very useful way to bring some order to the beautiful world of galaxies. Our own galaxy, the Milky Way, is a spiral galaxy classified as SBb on the Hubble sequence.

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3. Our Earthly perspective You may already have spotted a challenge in correctly classifying galaxies - the inclination of a galaxy to our line of sight.

The Hubble sequence was drawn to show spiral galaxies with a ‘face-on’ or 0° inclination that best displays their key features. However, spiral galaxies are formed in the shape of a thin, flat disk and they are randomly inclined to our line of sight. So some galaxies are closer to a 90° inclination, or ‘edge-on’. In these cases the spiral arms are not directly visible. The Hubble classification of edgeon spiral galaxies can however be estimated from the prominence of the central bulge. Here are a couple of examples: NGC 4565 Needle Galaxy by Hugh Allen Inclination angle 83° (5) Hubble classification Sb

NGC 5907 Splinter Galaxy by Hugh Allen Inclination angle 84° (5) Hubble classification Sc

The estimated inclination angles of the spiral galaxies in the Introduction are: NGC 2903 = 62°, NGC 4631 = 80° and M101 = 21° (4) The situation is more difficult for elliptical galaxies. Do elliptical galaxies really have intrinsically different shapes, or are we simply looking at the same shape from a variety of different angles? A statistical analysis of the frequency of occurrence of each type E0 to E7 suggests that the 3D shape of all elliptical galaxies is an almost oblate triaxial ellipsoid.(6) Don’t worry if you are not familiar with the technical language, this is what an oblate ellipsoid looks like:

An oblate ellipsoid would look circular when viewed from above. A triaxial ellipsoid would look oval from above.

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Copyright 2013 Š Online Astronomy Society - All rights researved http://www.onlineastronomysociety.com Twitter: OASoc

4. The importance of distance to other galaxies It is critical to know the distance to a galaxy. Without a measure of distance how can we know the true size and scale of what we are observing?

Galactic distances from Earth are described as ‘radial’ distances. They are measured in parsecs or light years (1 parsec = 3.26 light years). When we observe a distant galaxy we are therefore looking back in time. The light we observe from a galaxy 65 million light years away left the galaxy just as the dinosaurs became extinct 65 million years ago. Light travels at ~186,000 miles in a second, so 65 million light years equates to ~38,000 billion billion miles!

Measuring the radial distance to nearby galaxies relies on what are called standard candles. These are objects in the galaxy whose intrinsic brightness is known. The apparent brightness of the object is then measured from Earth using photometry. The radial distance to the object can then be estimated by applying the inverse square law of brightness variation with distance. Some examples of useful standard candles include Cepheid variable stars and Type 1a supernovae. A technique that exploits the intrinsic brightness of spiral galaxies is the Tully-Fisher Relationship. For more distant galaxies, the redshift in characteristic spectral lines is measured. All galaxies are receding from each other because of the expansion of the Universe. According to Hubble’s Law, the receding velocity of a galaxy (calculated from the redshift) is proportional to its distance from us.

The nearest large spiral galaxy to the Milky Way is the Andromeda Galaxy M31 which lies about 2½ million light years away. In winter months, the core of the galaxy is visible to the naked eye as a moderately faint star. In a telescope the much fainter spiral arms become visible and the whole galaxy fills an area of sky as large as three full moons! Along with the Milky Way and about 50 other galaxies, Andromeda is part of what is called the Local Group. Bound together by their mutual gravitational attraction the galaxies in the Local Group are orbiting around a common centre of mass. The Local Group is 10 million light years wide.

The close proximity of the galaxies in the Local Group means that their relative motions overwhelm the effect of the expansion of the Universe. As a result the Andromeda Galaxy is actually getting closer to the Milky Way. The very latest research based on measurements made with the Hubble Space Telescope suggests that the velocity of Andromeda along our line of sight is about 109km/s (nearly 250,000 miles per hour).(7) Using mathematical models, the same research group has predicted with a 41% likelihood that in about 4 billion years there will be a direct collision between our galaxy and Andromeda.(8) NASA has put together a fascinating web page describing the likely outcomes.(9) It should be remembered that intergalactic distances are estimates and are not made with pinpoint accuracy, particularly for more distant galaxies. For many reasons there is a significant margin of error.

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5. Size matters Galaxies come in a wide range of sizes, but we can work this out for ourselves.

The observed size of a galaxy is usually quoted as the angle it spans in the sky, typically in minutes of arc ‘ (1‘ = 1/60th of a degree = 0.01667°). The largest (major axis) and smallest (minor axis) angles are quoted. If the radial distance r is known then the angular size of the major axis can be converted into the diameter using simple trigonometry:

NGC 4565

dr

Angular size

The observer is on the Earth at M Galaxy diameter = length KL The distance r to the galaxy makes a right-angled triangle with the adjacent side of length half KL In a right-angled triangle, tangent (tan) of angle in ° = opposite length / adjacent length So tan (angular size/2) = half KL / r or Galaxy diameter KL = tan(angular size/2) x r x 2

Here are the estimated diameters of the galaxies featured already in this course: Distance in millions of parsecs Mpc (or in millions of light years Mly) 1 parsec = 3.26 light years 9.1 Mpc (29.5 Mly)

12.6’ (0.210°)

6 (0.1°)

0.001833

108,000

M87

16.8 Mpc (54.8 Mly)

8.3’ (0.138°)

6.6’ (0.110°)

0.001204

132,000

NGC 4631

6.1 Mpc (19.9 Mly)

15.5’ (0.258°)

2.7’ (0.045°)

0.002251

90,000

M101

6.9 Mpc (22.5 Mly)

28.8’ (0.480°)

26.9’ (0.448°)

0.004189

189,000

NGC4449

3.7 Mpc (12.1 Mly)

6.2’ (0.103°)

4.4’ (0.073°)

0.000899

22,000

NGC 5907

16.3 Mpc (53.1 Mly)

12.8’ (0.213°)

1.4 (0.023°)

0.001859

197,000

Galaxy reference

NGC 2903

Major diameter ‘ (or °)

Minor diameter ’ (or °)

Tan (major diameter °/2)

Estimated diameter in light years, ly (rounded to nearest ‘000)

12.9 Mpc 15.9’ 1.9’ 195,000 0.002313 (42.1 Mly) (0.265°) (0.032°) Note: Diameters and mean distance are taken from NASA/IPAC Extragalactic Database http://ned.ipac.caltech.edu/ NGC 4565

The smallest galaxies are described as dwarf galaxies. Dwarf galaxies are quite common, often gravitationally bound to larger galaxies as satellites. Take a close look at the image of NGC 4631 Whale Galaxy in the Introduction. The fuzzy, blue blob just below centre is a satellite galaxy with the catalogue number NGC 4627.

It is only ~11,000 light years in diameter and is a dwarf elliptical galaxy, E4 on the Copyright 2013 © Online Astronomy Society - All rights researved http://www.onlineastronomysociety.com Twitter: OASoc

Hubble sequence. Our Milky Way has its own family of dwarf satellites too.

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6. The view brings structure to the Universe The Universe is 13.7 billion years old and galaxies began to form just 0.5 billion years after the Big Bang. Since then their structure and distribution has been evolving. A useful way to help better interpret the view from a distance is to conduct surveys of ‘everything’ that we can see from our earthly vantage point.

This helps to avoid selection bias when choosing where and what to observe.

A good survey example is the Sloane Digital Sky Survey (SDSS). Since 2000, the SDSS has mapped a quarter of the sky. It uses a 2.5m aperture reflecting telescope at Apache Point Observatory in New Mexico. Its highly sensitive 120 megapixel camera allows it to make accurate brightness measurements to a distance of about 2 billion light years. A pair of spectographs are used to measure redshift. The SDSS had already captured almost 1 million galaxy images, and the latest release of data in August 2012 has taken that number to 1.5 million.

Here are three case studies which pull together some of our understanding of the view from a distance: Galaxies in clusters Many galaxies are gravitationally bound into groups and clusters. Galaxies within a cluster are moving relative to each other, orbiting the centre of mass of the cluster. Our own Local Group of galaxies is on the outer edge of the Virgo Cluster. Studies of the distribution of galaxies in clusters have shown that elliptical galaxies are more common at their centre whilst spiral galaxies become more common at their outer edge. Here is an image of the central area of the Coma Cluster, about 300 million light years away: Foreground stars from the Milky Way are indicated by tick marks _l One very bright star is marked but has been removed for clarity.

Every other object is a galaxy

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The view is dominated by two supergiant elliptical galaxies, NGC 4889 (E4 on the Hubble sequence) and NGC 4874 (E?). The bigger galaxy NGC 4889 (the upper bright galaxy in the image) is at least 250,000 light years in diameter, based on a visual diameter of 2.9’ at a distance of 333 million light years. The centre of the lower supergiant NGC 4874 is only just over 1’ from the centre of its lenticular neighbour NGC 4872, to the lower left. Both are at a distance of about 320 million light years which puts their centres only 94,000 light years apart. The high density of galaxies at the centre of clusters is thought to lead to the formation of elliptical galaxies from the interaction and merger of other galaxies.(10) The gravitational interaction of galaxies is a well established phenomenon. Here is an image of the two peculiar galaxies NGC 4676A and B nicknamed the Mice Galaxies. They lie nearly 300 million light years away. NGC 4676A proudly displays a beautiful tail of stars formed during their interaction. Spanning an angle of 1.8’ in the sky, the tail extends at least 160,000 light years:

The American astronomer Halton Arp drew up a catalogue of peculiar galaxies in the 1960’s to stimulate research into galaxy interaction and its contribution to the evolution of galaxies.(11) Mathematical models of galaxy interaction have been developed which can reproduce these beautiful forms. Follow the link in the reference list and have a go at generating a mouse’s tail yourself.(12)

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Galaxies then and now Data from the latest galaxy surveys has been used to compare the Hubble sequence 300 million years ago (galaxies 300 million light years away) to 6 billion years ago (6 billion light years away):(13) Abundance of galaxies on the Hubble sequence - past and present Data taken from R. Delgado-Serrano et al. (2010) ‘How was the Hubble Sequence 6Gyr ago?’

Astronomy & Astrophysics Vol 509 Presentation of data by Hugh Allen

4 13 3

10

15

~300 million light years

Elliptical % Lenticular %

Earth

52

Spiral % Peculiar/irregular %

~6 billion light years

31 72

Spiral galaxies seem to have formed over time from the interaction and merger of peculiar and irregular galaxies. The proportions of elliptical and lenticular galaxies seem to have remained almost constant.

Galaxy structures on a large scale Galaxies are gravitationally bound into groups and clusters. Using data from the SDSS, a plot of galaxy distribution with distance shows a web of filaments and sheets with huge voids in between.

This is a slice through the ‘sphere’ of galaxies radiating away from our location.

The distance scale is the redshift (there are many on-line calculators to convert redshift into distance).(14) A 3D movie has also been created that allows you to swoop through this beautiful structure. What a fitting way to end this course on galaxies and the ‘view from a distance’.(15) Image Credit: M. Blanton and the Sloan Digital Sky Survey Copyright 2013 © Online Astronomy Society - All rights researved http://www.onlineastronomysociety.com Twitter: OASoc

References (1) Hubble E. (1926) ‘A spiral nebula as a stellar system: Messier 33’ Astrophysical Journal, 63, p236-274 (quotation from p237) http://articles.adsabs.harvard.edu/full/1926ApJ....63..236H (2) Berendzen R & Hoskin M (1971) ‘Hubble’s announcement of Cepheids in spiral nebulae’ Astronomical Society of the Pacific Leaflets Vol 10 No. 504 p425440 http://adsabs.harvard.edu/full/1971ASPL...10..425B (3) Messier Catalogue (M) and New General Catalogue (NGC) http://messier.seds.org/ and http://en.wikipedia.org/wiki/New_General_Catalogue (4) Hubble E. (1926) ‘Extragalactic nebulae’ Astrophysical Journal, 64, p321-369 http://articles.adsabs.harvard.edu/full/1926ApJ....64..321H (5) Inclination angles calculated from diameter data in NASA/IPAC Extragalactic Database NED: cos-1(b/a) where b = minor diameter and a = major diameter http://ned.ipac.caltech.edu/ (6) Ryden, Barbara (1992) ‘The intrinsic shapes of elliptical galaxies’ Astrophysical Journal Part 1 vol 396 no. 2 p 445 – 452 http://adsabs.harvard.edu/abs/1992ApJ...396..445R (7) Roeland P. van der Marel et al. (2012) ‘The M31 velocity vector II: Radial orbit toward the Milky Way and implied Local group Mass’ The Astrophysics Journal vol 753 8 http://iopscience.iop.org/0004-637X/753/1/8 (8) Roeland P. van der Marel et al. (2012) ‘The M31 velocity vector !II: Future Milky Way M31-M33 orbital evolution, merging and fate of the Sun’ The Astrophysics Journal vol 753 9 http://iopscience.iop.org/0004-637X/753/1/9 (9) ‘NASA’s Hubble shows Milky Way is destined for head-on collision’ http://www.nasa.gov/mission_pages/hubble/science/milky-way-collide.html (10) Dressler A. (1980) ‘Galaxy morphology in rich clusters – Implications for the formation and evolution of galaxies’ Astrophysical Journal Part 1 vol 236, 351365 http://articles.adsabs.harvard.edu//full/1980ApJ...236..351D/0000355.000.html (11) Galaxy interaction simulation http://cas.sdss.org/dr5/en/proj/advanced/galaxies/collisions.asp Change the angles of the galaxies with respect to each other by clicking the dials with the left or right mouse buttons. Change their separation distance by changing the "Peri" variable, and change the mass of the red galaxy with respect to the green galaxy by changing "Red Galaxy Mass”.

(12) Halton Arp’s ‘Atlas of Peculiar Galaxies’ published by the California Institute of Technology in 1966 http://ned.ipac.caltech.edu/level5/Arp/frames.html (13) R. Delgado-Serrano et al. (2010) ‘How was the Hubble Sequence 6Gyr ago?’ Astronomy & Astrophysics Vol 509, http://www.aanda.org/index.php?option=com_article&access=doi&doi=10.1051/0 004-6361/200912704&Itemid=129 (14) Redshift calculator http://www.astro.ucla.edu/~wright/CosmoCalc.html (15) 3D universe video from the SDSS III data release August 2012 http://www.skyandtelescope.com/community/skyblog/newsblog/Fly-Through-a3D-Map-of-the-Universe-165686546.html Copyright 2013 © Online Astronomy Society - All rights researved http://www.onlineastronomysociety.com Twitter: OASoc