The Milky Way and Galaxies

Nicola Loaring, SAAO

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What is a galaxy?

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A massive collection of stellar and interstellar matter - stars, gas, dust, isolated in space and held together by its own gravity.

We reside within the Milky Way Galaxy, or just the Galaxy (note the capital G).

When you look up, all the individual stars you see are those in our own Galaxy.

Galaxies typically contain around 100 billion stars.

There are billions of other galaxies in the universe.

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A little history...

1610 Galileo Galilei used a telescope to study the Milky Way and discovered that it was composed of a huge number of faint stars.

1785 William Herschel counted the number of stars in different regions of the visible sky and produced a diagram of the shape of the Galaxy with the Solar System close to the centre!

1920s Edwin Hubble produced photographs that resolved the outer parts of some spiral “nebulae” as collections of individual stars outside of our Milky Way. (He also discovered the expansion of the universe!).

Sun takes 225 MY to orbit the Galaxy, we’re out in the suburbs!

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Our home: The Milky Way

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The Milky Way contains at least 100 billion stars and may have up to 400 billion stars.

The neighboring Andromeda Galaxy contains an estimated one trillion (1012) stars.

Filling the space between the stars is a disk of gas and dust called the interstellar medium.

A 2010 measurement of the radial velocity of halo stars finds the mass enclosed within 80 kiloparsecs is 7×1011 M☉

Mathematical models of the Milky Way suggests that the total mass of the entire Galaxy lies in the range 1-1.5×1012 M☉

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Stucture of the Milky Way

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The Milky Way system is a spiral galaxy consisting of over 400 billion stars, plus gas and dust arranged into three general components

•The halo

•The nuclear bulge and Galactic Centre

•The disk

Astronomers generally organise the interstellar medium and stars in the disk of the Milky Way in four spiral arms. All of these arms contain more interstellar gas and dust than the Galactic average as well as a high concentration of star formation, traced by H II regions and molecular clouds.

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Stucture of the Milky Way

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Astronomers generally organise the interstellar medium and stars in the disk of the Milky Way into spiral arms.

All of these arms contain more interstellar gas and dust than the Galactic average as well as a high concentration of star formation, traced by H II regions and molecular clouds.

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Our home: The Milky Way

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30 kpc ~ 96,000 LY across

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The Spiral Arm Problem

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The Spiral Arm Problem

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Orbital Motion

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We observe a "flat rotation curve" at large radii•Higher speeds at large radii mean you need to have more mass at large radii than is observed in the stars and gas alone...

Therefore, galaxies must have extended dark halos.

• Sagittarius A* discovered in 1974 is a radio source at the centre of our galaxy.

• Believed to be at the location of a 4.1 MSun black hole 27,000 light years from Earth.

• Stars in orbit around Sgr A* observed to move at breakneck speeds (up to 1400 km/s).

• Further observational evidence for a SMBH in the Milky Way's nucleus from detection of an X-ray source at the position of Sgr A*.

SGR A*

Galaxies

1pc =3x1013 km = 3.2 light years = 206,000AU

Size ~30,000 LY.

Mass ~10 Billion x Sun

70% spiral, 30% elliptical M33 - spiral galaxy M83 - Spiral galaxy

M32 - Elliptical galaxy

M33 spiral galaxy M83 – Spiral galaxy

Spiral Galaxies

Barred Spiral - NGC 6217

Spiral galaxy - M100

Galaxy classification

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Milky Way

The Closest Galaxy - Andromeda

2.5 million light years away

Current measurements suggest that the Andromeda Galaxy is approaching us at 100 to 140 km/s. The Milky Way may collide with it in 3 to 4 billion years. The two galaxies will merge to form a single elliptical galaxy over the course of about a billion years.

Interacting galaxies

Arp 148 is nicknamed Mayall’s object and is located in the constellation of Ursa Major, the Great Bear, approximately 500 million light-years away.

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Triple Galaxy Merger

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Triple Galaxy Merger

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Triple Galaxy Merger

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Milky Way, Andromeda and the Pinwheel are the 3 largest spiral galaxies, around 50 galaxies in total including dwarf galaxies.

The Local Group

Clusters:

Sizes 3.3-33 M LY.

Mass ~ 1014 -1015 SM.

Several 1000 galaxies.

Galaxy groups & clusters

•Groups:

•< 50 galaxies.

•Sizes 3 to 6 MLY.

•Mass ~ 1013 SM.

The Hubble Expansion

1929 Hubble found that galaxies are moving away from us and that their recessional velocity is proportional to their distance

This is the most distant optical image taken to date using the HST

The galaxies you see are nearly 13 billion light yrs away

These galaxies taken from 10 days of observing and are 4 billion times fainter than can be seen with the naked eye

Galaxy formation

Definition of an active galaxy

Over ten billion (10,000,000,000) galaxies are visible with modern telescopes.

Stars produce most of the light in these galaxies.

Galaxies with luminosities 1000x greater than normal galaxies are called active galaxies.

Extra light is confined to the central 1pc (~3.3 LY) (smaller distance than to our nearest star).

Taxonomy of AGN

Discovered by Carl Seyfert in 1943.

He found that a handful of spiral galaxies had very bright, point-like nuclei.

Optical spectra had broad emission lines

Called them Seyfert galaxies

If brighter than MV<-23 then called a Quasar (although local Seyferts where the galaxy is visible are classified as Seyferts even if MV<-23.

Most luminous objects in the Universe! Luminosities up to 1013 Lsun

Typical power of 1040W (~10 solar masses consumed per year).

More than 300,000 known from all sky surveys.

Despite being discovered at radio wavelengths, only 10% are radio loud, infact X-rays are the most efficient way to search for them!

Found to be variable in brightness on scales of days, weeks and months

Properties of Quasars

Standard model of Active Galactic Nuclei

NLR 10pc – 1kpc BLR ~1pc

Black holes in galaxies

• Strongest evidence for black holes comes from observations of the centres of galaxies, including our own.

• Stars and gas near the centres of many galaxies are moving extremely rapidly, orbiting a very massive unseen object.

• Masses inferred are millions to billions times the mass of the Sun.

• Intense energy and short-timescale fluctuations point to massive compact objects

• These objects have extended radio jets reaching beyond the galaxy

• Explained by a huge central supermassive black hole accreting stars and gas from their surroundings

Black Holes

Central engines of AGN and Quasars.

Fed by infalling stellar material surrounding the black hole.

Material forms an accretion disk before being sucked into the black hole

Supermassive black holes

• The giant elliptical galaxy NGC 4261 is one of the 12 brightest galaxies in the Virgo Cluster, located 45 million light-years away.

• The dark, dusty disk represents a cold outer region which extends inwards to an ultra hot accretion disk within a few hundred million miles of the suspected black hole.

Supermassive black holes

• The accretion disk weighs 100,000 times as much as our Sun.

• This object is about as large as our solar system, but weighs 1.2 billion times as much as our Sun.

• This means that gravity is about one million times as strong as on the Sun.

• Almost certainly this object is a black hole.

• NGC 1316

• The radio lobes span over one million light years

• 3C219

• Radio lobes span several hundred thousand light years

Jets and Lobes

M87.

This jet extends at least 5000 light years from the nucleus of M87

Quasar Jets The energy emitted from the jet in

3C273 probably comes from gas that falls toward a supermassive black hole at the centre of the quasar, but is redirected by strong electromagnetic fields into a collimated jet. Still not fully understood.

The amount of energy deposited at knot one in the optical is the same as for a typical Seyfert galaxy!

Jets can extend to several kpc, ie several 1000 LYs

Complimentary views

Centaurus A

Starburst galaxy M82 - IMBHs

• Chandra X-ray Observatory data in blue, optical data from the Hubble Space Telescope in green and orange, and infrared data from the Spitzer Space Telescope in red.

• First case where good evidence for more than one mid-sized black hole exists in a single galaxy. The evidence comes from how their X-ray emission varies over time and analysis of their X-ray brightness and spectra.

Chandra image that shows the central region of the galaxy containing two bright X-ray sources

X42.3+59 is located at a projected distance of 290 light years from the centre of M82. Mass is estimated to be between 12,000 and 43,000 times the mass of the Sun.

X41.4+60 is located 600 light years in projection away from the centre of M82. Mass between 200 and 800 times that of the Sun, and tilted at an angle between 60 and 80 degrees, meaning that the accretion disk is viewed almost side-on.

Summary

• The Universe is composed of billions of galaxies, which in turn are composed of billions of stars!

• “If it's true that our species is alone in the universe, then I'd have to say the universe aimed rather low and settled for very little”.

George Carlin